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NERVOUS SYSTEM

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					NERVOUS SYSTEM: NERVOUS TISSUE
CHAPTER OBJECTIVES
When you have completed this chapter you should be able to:
   Describe the anatomy of a neuron.


      Classify the different types of neuron according to their structure and direction of the action
       potential.

      Describe the types and functions of neuroglia.

      Understand and describe how a nerve impulse is generated and propagated.


The nervous system along with the endocrine system work together to coordinate all of the body
systems. It does this by detecting, storing, transmitting and responding to information or stimui.

The nervous system can be anatomically subdivided into the central nervous system (CNS) and the
peripheral nervous system (PNS). The central nervous system consists of the brain and spinal cord
and the peripheral nervous system consists of the spinal nerves and ganglia.

Nervous tissue
Neurons or nerve cells are the basic components of the nervous system and our bodies contain
billions of them. Supporting and protecting the neurons are neuroglia cells which form a type of
connective tissue around the nerve cells.

Neurons

Neurons come in various shapes and sizes but they all contain a cell body and usually two processes;
a dendrite and an axon. Dendrites are short, thin branched projections (the word dendrite is derived
from the Greek word "dendron", which means tree) that receive signals and transmit them towards
the cell body. They form synapses with other neurons and respond to neurotransmitters. Axons are
long straight projections which transmit signals (action potentials) away from the cell body. Their
ends branch to form presynaptic terminals which contain neurotransmitters to send signals away
from the cell.

Components of a
                Description
Neuron
Cell body          Contains a large nucleus and granular protoplasm.
                                Axons are long straight processes which transmit signals (action potentials) away
Axon                            from the cell body. Their ends branch to form presynaptic terminals which
                                contain neurotransmitters.
                                Dendrites are short, thin branched processes which receive signals and transmit
Dendrite
                                them towards the cell body.

Neurons can be classified according to their structure or by the direction in which the action
potentials travel.
Structure


Neuron Types Neuron Structure
Unipolar                Has an axon but no dendrites.               Mostly sensory fibres.
Bipolar                 Has an axon and a dendrite.                 Part of specialised sensory organs.
Multipolar              Has one axon and numerous dendrites. Motor and interneurons.

Direction of Action Potential


Neuron Types                                     Direction of the Action Potential
Sensory (Afferent) Neurons                       Conduct signals to the CNS.
Motor (Efferent) Neurons                         Conduct signals from the CNS to the muscles.
Interneurons (Association Neurons) Conduct signals from one neuron to another.

Neuroglia

Neuroglia are essential for the normal functioning of the nervous system. They have a number of
supporting roles throughout the nervous system and there are 5 different types of neuroglia cells
which carry out these functions.

Neuroglia                       Location Function
                                         Star shaped cells that help keep the neurons in place as well as
Astrocytes                      CNS
                                         regulating the composition of the surrounding extracellular fluid.
Ependymal cells CNS                       Secrete and move the cerebral spinal fluid.
                                          They engulf unwanted tissue in the CNS, e.g. microorganisms and
Microglia                       CNS
                                          damaged tissue.
Oligodendrocytes CNS                      Each cell forms myelin sheaths around multiple axons in the CNS.
Schwann Cells    PNS                      Each cell forms a myelin sheath around a single axon in the PNS.

Myelin Sheaths
The lipid rich membrane of the oligodendrites or schwann cells tightly wrap around a section of an
axon several times like a swiss roll. It is this tightly packed membrane that forms the myelin sheath
around an axon, which is now known as a myelinated fibre. Cells line up in rows along the axon and
between each adjacent oligodendrite or schwann cell is a tiny gap called a node of Ranvier. The
myelin sheath acts like as an insulator between the nodes of Ranvier, only allowing the action
potential to leap from node to node rather than to travel along the entire length of the axon. This
means that axons with a myelin sheath conduct action potentials quicker along their length than
unmyelinated axons. The myelin sheath also prevents the action potential from being passed to
adjacent neurons as well as protecting the fibre.

Clinical Considerations
          It is unclear exactly what causes multiple sclerosis, but it is thought to be an autoimmune
          disease. The immune system attacks its own cells (oligodendrocytes, schwann cells)
          resulting in the demyelination of axons throughout the nervous system as well as the
Multiple formation of scar tissue. Demyelination interferes with the ability of the nerve to send
Sclerosis signals, and the scarring can cause damage to the nerves themselves.

            Multiple sclerosis causes muscle weakness, double vision, problems with balance and
            coordination, and problems with memory and problem solving.

Action Potentials

In every part of our body are electrically charged particles known as ions which can be positively or
negatively charged. Neurons rely on these differently charged ions to create and conduct electrical
impulses (action potentials).

Name             Symbol Electrical Charge
Sodium ions      (Na+) Plus 1 positive charge.
Potassium ions (K+) Plus 1 positive charge.
Calcium ions (Ca++) Plus 2 positive charge.
Chloride ions (Cl-)       Minus 1 negative charge.

All cells have a 'resting potential', meaning when at rest the overall charge of ions inside the cell are
negative compared to the ions outside the cell in the extracellular fluid. The difference in charge
across the cell membrane of a neuron creates a potential electrical difference of about -70 milivolts
(mV). The cell membrane maintains this resting potential by selectively allowing some ions to pass
into the cell via special channels or gates and by blocking the entry of other ions. Due to the
electrochemical gradients Na+ slowly diffuses into the neuron and K+ slowly diffuses out of the
neuron. Because of this natural diffusion the resting neuron must actively pump Na+ out of the cell
and take K+ in to maintain its resting potential of -70 mV.

When a neuron is stimulated a section of its membrane becomes depolarised by the exchange of ions
across it. A section of the cell membrane opens its sodium channels allowing sodium ions to move
inside the cell. The sodium ions are positively charged and are attracted into the cell by the
negatively charged ions inside, as well as the lower sodium concentrations. The influx of positive
ions reverses (depolarises) the resting potential and the inside of the neuron becomes more
positively charged. When depolarisation reaches a certain level or threshold, i.e. the voltage inside
the cell reaches at least -55 milivolts, it triggers the opening of more sodium channels which in turn
triggers the opening of sodium channels in the adjacent cell membrane. Thus depolarisation is spread
along the entire cell membrane in a wave; this is an action potential and conducts the nerve impulse
along the axon. Once the action potential reaches the end of the axon the action potential is
converted to a chemical signal by the release of a neurotransmitter. The inside of the cell continues
to depolarise until the voltage peaks at about +35 milivolts, at which point the cell membrane closes
its sodium channels so that no more Na+ can enter, and opens its K+ channels to allow the positively
charged K+ ions to leave the cell. This process reverses the depolarisation (repolarisation) and
allows the neuron to return to its original resting potential of -70 milivolts.

SELF-TEST
Complete the following questions before you go onto the next section:
   Draw and label a typical neuron.


      What is the significance of myelin sheaths.

      Describe how an action potential is propagated.



NERVOUS SYSTEM: CENTRAL NERVOUS
SYSTEM
CHAPTER OBJECTIVES

When you have completed this chapter you should be able to:

      Describe the difference between the CNS and PNS.

      Locate and identify the forebrain, midbrain and hindbrain .

      Identify the major gyri and sulci of the brain.

      Describe the main parts of the forebrain and describe its function.

      Describe the main parts of the midbrain and describe its function.

      Describe the main parts of the hindbrain and describe its function.
      Describe the main parts of the spinal cord.


The central nervous system (CNS) consists of the brain and spinal cord and the peripheral nervous
system (PNS) consists of the spinal nerves and ganglia.

Brain
The brain occupies the cranial cavity and can be divided into three main parts. The forebrain,
midbrain and hindbrain. The midbrain and hindbrain are collectively know as the brain stem and
contain the nuclei from which the cranial nerves originate.

Name                         Description             Important named parts

                                                           Telencephalon (cerebrum).
Forebrain                    Largest part of the
                                                           Diencephalon (thalamus, hypothalamus, pineal
Prosencephalon               brain.                         body).

Midbrain
Mesencephalon (brain         1.5 cm in length.             Quadrigeminal bodies - cerebral peduncles.
stem)

Hindbrain                                                  Pons.
Rhombencephalon (brain                                     Medulla oblongata.
stem)                                                      Cerebellum.



Forebrain

Telencephalon (Cerebrum)

The cerebrum is the largest part of the brain and is divided into left and right hemispheres by a
longitudinal fissure that runs along the median sagittal plane. Inferiorly the hemispheres are
connected together by a band of white matter called the corpus collosum. The outer layer of the
cerebrum is composed of grey matter and called the cerebral cortex. It is responsible for the analysis
of sensory input, memory, learning and cognitive thought. Each hemisphere is greatly folded
forming gyri (folds) and sulci (grooves) which increases the surface area of the cerebral cortex.
Although the exact location of the sulci and gyri varies between different individuals, there are a
number of large gyri and deep sulci which can be identified as constant landmarks. The main ones
have been listed below;

Name             Description
Longitudinal       A large fissure running from back to front along the median sagittal plane; it divides the
fissure            cerebrum into left and right cerebral hemispheres.

                   Descending downwards and forwards from the top of the hemisphere. It divides the
Central sulcus
                   frontal and parietal lobes.

Parietal-occipital Descending downwards and forwards mainly inside the longitudinal fissure, it divides the
sulcus             parietal and occipital lobes.

                 This is found at the posterior border of the frontal lobe, in front of the central sulcus. It
Precentral gyrus descends downwards and forwards from the top of the hemisphere. Forms the primary
                 motor area (cortex).

                   This is found at the anterior border of the parietal lobe, behind the central sulcus. It
Postcentral
                   descends downwards and forwards from the top of the hemisphere. Forms the primary
gyrus
                   sensory area (cortex).

                   Found on the lateral side of the brain it ascends almost horizontally from the front of the
Lateral sulcus
                   brain to the angular gyrus and separates the temporal lobe from the frontal lobe above.


Each hemisphere can be further divided into lobes, their names of which correlate with the
surrounding bones that protect them.

Lobe of the
            Description                                                   Function
Cerebrum

                                                                                 The primary motor area
                                                                                  (cortex).
             The largest lobe found at the front of the brain                    Motor association area
                                                                                  (motor control).
             undercover of the frontal bone. It contains the
                                                                                 Brocha's area - motor speech
Frontal lobe precentral gyrus posteriorly. It is separated from the
                                                                                  (production)
             parietal lobe posteriorly by the central sulcus and from            Cognitive thought and
             the temporal lobe inferiorly by the lateral sulcus.                  memory.
                                                                                 Personality
                                                                                 Primary olfactory cortex.

                                                                                 Primary auditory area
               Found at the side of the brain undercover of the                   (hearing).
Temporal
               temporal bone. It is separated above from the frontal             Auditory association area
lobe                                                                              (hearing).
               lobe by the lateral sulcus.
                                                                                 Wernicke area (speech
                                                                                  comprehension).
                                                                            Special senses (hearing,
                                                                             smelling).
                                                                            Learning and memory
                                                                             (retrieval).
                                                                            Emotions

                                                                            Primary sensory area
            Found at the top of the brain undercover of the parietal         (cortex).
            bone. Anteriorly it contains the postcentral gyrus and is       Sensory association area
Parietal
            separated from the frontal lobe by the central sulcus.           (general senses).
lobe                                                                        Body orientation.
            Posteriorly it is separated from the occipital lobe by the
                                                                            Primary gustatory cortex
            parietal-occipital sulcus.
                                                                             (taste).

                                                                            Primary visual area (cortex).
Occipital   Found at the back of the brain undercover of the                Visual association area
lobe        occipital bone.                                                  (vision) - visual
                                                                             interpretation.

                                                                            Special senses (taste,
            The smallest lobe of the brain found deep in the
Insula                                                                       hearing).
            cerebrum between the lips of the lateral sulcus.                Visceral sensation.



LOBES AND MAJOR LANDMARKS OF THE CEREBRUM
Diencephalon

The diencephalon consists of two thalami, two hypothalami and a single pineal body.
Thalamus

The thalami are the largest parts of the diencephalon and are located in the centre of the brain in the
outer walls of the third ventricle. They are often connected to each other across the third ventricle by
a small interthalamic adhesion.

The thalamus receives sensory and motor input as well as influences mood. It receives mostly
sensory input including auditory and visual input and relays the signals to the cerebral cortex.

Sensory Nuclei of the Thalamus Sensory input from

Medial geniculate nucleus       Auditory

Lateral geniculate nucleus      Visual

Ventral posterior nucleus       Other sensory input


Pineal body

This is a small pine-cone shaped gland projecting from the posterior of the third ventricle by a mid
line stalk. Its role is not fully understood but it is thought to be involved in the sleep-wake cycle and
the onset of puberty.

Hypothalamus

The hypothalamus is located at the very bottom of the diencephalon below the thalamus and behind
the optic chiasma. It is very important and is often referred to as the 'master gland' as it controls a
large number of bodily functions, one of the most important being that of homeostasis. Homeostasis
is the maintenance of the bodies physiology, i.e. the maintenance of blood pressure, body
temperature, weight and the chemical composition of the body's fluids. Other regulatory roles of the
hypothalamus are control of our mood and emotions, autonomic functions, food and water intake,
sleep wake cycle and endocrine function.

Name             Description                                Function

Mammillary       A pair of small white bodies protruding
                                                            Emotional responses to smells.
bodies           from the front of the hypothalamus.

                 A stalk which connects the hypothalamus    Through its connection the hypothalamus
Infundibulum
                 with the pituitary gland (hypophysis).     regulates the function of the pituitary gland.



Mesencephalon (midbrain)
The smallest part of the brainstem measuring 1.5 cms it consists of the tectum, tegmentum, cerebral
peduncles and the substantia nigra. It is responsible for the visual and gustatory response as well as
the coordination of movement.

Name            Description

                Roof of the midbrain, consisting of four nuclei which form 4 mounds, collectively
                know as quadrigeminal bodies, on the dorsal surface of the brain stem.
Tectum
                The 2 superior nuclei are called the superior colliculi the 2 inferior nuclei are called
                the inferior colliculi. The superior colliculi control the visual response and the
                inferior colliculi control the auditory response.
                Is the floor of the midbrain and consists of ascending tracts from the spinal cord to the
Tegmentum
                brain. It controls motor functions.

Substantia      A pigmented lamina located between the tegmentum and cerebral peduncles which helps
nigra           to coordinate movement.

Cerebral        Located inferior to the tegmentum and consist of descending (motor) tracts from the
peduncles       cerebrum to the spinal cord and cerebellum.

Cranial nerve
                The nuclei of the trochlear and oculomotor cranial nerves are located in the midbrain.
nuclei



Brainstem

The midbrain and hindbrain are collectively know as the brain stem. It is the lowest part of the brain
and is continuous inferiorly with the cervical spinal cord at the foramen magnum. Its fibres connect
the peripheral nervous system (spinal nerves and cranial nerves) to the central nervous system (brain
and spinal cord).

The brain stem is extremely important because it contains the nuclei from which most cranial nerves
originate as well as the vital centres necessary for survival; breathing, digestion, heart rate, blood
pressure and for consciousness (being awake and alert).

Retinacular Formation

The retinacular formation is a series of important nuclei that span the brainstem and receive the
majority of the sensory information from the body and the motor signals from the cerebrum. The
nuclei also play an integral role in the maintenance of the conscious state.

Rhombencephalon (hindbrain)
The hindbrain consists of the pons superiorly, the cerebellum posteriorly and the medulla oblongata
inferiorly. The medulla oblongata is continuous inferiorly with the spinal cord.

Pons

Located in front of the cerebellum, the pons is only 2.5 cm in length and bulges anteriorly. It consists
of descending fibres travelling to the spinal cord and ascending fibres to the cerebellum. It also
contains the nuclei of four of the cranial nerves and the respiratory centre which controls expiration.

Name                Description

                    Located anteriorly in the pons they connect the cerebrum to the cerebellum and
Pontine nuclei
                    coordinate voluntary movement.

Respiratory
                    Controls respiratory (expiration) movements.
centre

                    The nuclei of the following cranial nerves are located in the posterior part of the pons;

Cranial nerve              Trigeminal (V)
nuclei                     Abducens (VI)
                           Facial (VII)
                           Vestibular cochlear (VIII)



Medulla Oblongata

The medulla oblongata is only 3 cm in length and is the most inferior portion of the brainstem being
continuous with the spinal cord inferiorly. It consists of the pyramids and olives and contains
ascending and descending nerve tracts, several nuclei and importantly the 'vital centres', which
regulate heart rate, respiration and blood vessel diameter. It also contains some non-vital centres
involved in swallowing, vomiting, sneezing and coughing.

Name          Description                                                                Function

              Two enlargements on the anterior surface of the length of the medulla;
              they taper towards the spinal cord. Here the descending nerve tract    Conscious voluntary
Pyramids
              fibres (corticospinal fibres) cross over to the other side to form the movements.
              'pyramidal decussation'.

              Two protrusions found on the anterolateral side of the medulla just        Balance Coordination
Olive
              lateral to the pyramids. It consists of an olivary complex of nuclei.      of sound from the ear.
            The medulla is the centre for several important regulatory reflexes;
                                                                                    Heart rate.
Vital                                                                               Respiratory
                   Cardiac centres
Centres                                                                             (inspiration). Blood
                   Respiratory centres
                   Vasomotor centres                                               vessel diameter.


            The nuclei of the following cranial nerves are located in the medulla
            oblongata;
Cranial
nerve              Glossopharyngeal (IX)
nuclei             Vagus (X)
                   Accessory (XI)
                   Hypoglossal (XII)



Cerebellum

The cerebellum is the lobe of the brain situated in the posterior cranial fossa. Its surface is folded
into folia and consists of two hemispheres connected in the mid line by the vermis. It is separated
from the pons and medulla oblongata anteriorly by the fourth ventricle. The cerebellum is
responsible for coordination of movement and sends information to the thalamus and cortex.

Grey and white matter

The brain and spinal cord contain both grey and white matter.

In the Brain

The grey matter can be found in the cerebral cortex, the basal ganglia and the limbic system. It is
made up of the cell bodies, dendrites and synapses of the neurons and are grouped into functionally
important nuclei.

The white matter is made up of the myelinated fibres (axons) which connect the different parts of the
brain to each other as well as to the spinal cord.

In the Spinal Cord

The spinal cord is oval in cross section and consists of white and grey matter. The grey matter lies
centrally and is arranged into ventral, dorsal and lateral grey horns (anterior and posterior horns). It
consists of neurons and neurites, neuroglia and blood vessels. It appears grey because of the
abundance of neuronal cell bodies.

The white matter surrounds the grey mater and is white in colour due to the presence of myelin,
which insulates the nerve fibres.
Ventricles

Inside the brain are four interconnected cavities filled with cerebral spinal fluid; two lateral
ventricles, a single third ventricle and a single fourth ventricle. The two lateral ventricles are the
largest ventricles and lie one in each cerebral hemisphere. They are approximately C-shaped (wish
bone), each communicating with the thin mid line third ventricle via an intraventricular foramen.
The third ventricle communicates inferiorly with the fourth ventricle via the cerebral aqueduct and
descends in the mid plane through the midbrain. The fourth ventricle is a small, triangular chamber
found between the pons in front and the cerebellum behind. Inferiorly it narrows to form the central
canal which descends though the medulla oblongata and spinal cord.

Each ventricle contains a choroid plexus which secretes cerebral spinal fluid (CSF) into the
ventricles. The third ventricle contains the thalamus and hypothalamus in its lateral walls and the
infundibulum, tuber cinereum and the mammillary bodies in its floor. The corpus collosum forms the
roof of the lateral and third ventricles.

Cerebral Spinal Fluid (CSF)

The CSF is a clear fluid produced by the choroid plexuses of the ventricles. It circulates within the
ventricles as well as in the subarachnoid space between the pia mater and arachnoid mater
surrounding the brain and spinal cord. CSF baths the brain and spinal cord in a chemically stable
environment and provides it with nutrients. It also allows the brain to be buoyant and protects the
brain from jolting into the cranium.

Meninges

Surrounding the brain and spinal cord are three membranous layers;

In the Brain

Around the brain these three layers are collectively known as the meninges;
Outermost layer;


The outermost layer, the dura mater is dense and consists of two layers, a periosteal layer and a
meningeal layer. The periosteal layer adheres to the internal surface of the cranium and for the
majority of its course lies directly touching the meningeal layer. At certain locations the periosteal
and meningeal layers are pulled away from each other to create a space, a dural sinus. The sinuses
are filled with venous blood from the brain via the cerebral veins. They drain blood into the internal
jugular vein.

Dural Sinuses

Superior sagittal sinus
Cavernous sinus

Inferior petrosal sinus

Superior petrosal sinus

Transverse sinus

Sigmoid sinus

Occipital sinus


The meningeal layer folds inwards to form two double thickness sheets which help to hold the brain
in place. The falx cerebri is the fold of dura which projects vertically into the longitudinal fissure
between the cerebral hemispheres. The tentorum cerebelli is the fold of dura which projects
horizontally between the cerebellum below and the cerebrum above.
Middle layer;


The middle layer, the arachnoid mater, is thin and transparent and lines the inner surface of the dura
mater. It possesses arachnoid trabeculae (granulations) which project into the pia mater and villi
which project into the dura mata.
Innermost layer;


The innermost layer, the pia mater lies directly on the surface of the brain. This layer is very thin
and transparent and closely follows all of the gyri and sulci. Between the pia mater and the arachnoid
mater is the subarachnoid space in which the CSF is circulated. CSF is returned to the blood via the
arachnoid trabeculae (granulations).

In summary;
Name                      Location        Description

                          Innermost             Thin and transparent.
Pia Mater                                       Invests the surface of the brain and spinal cord.
                          layer.

                                                Thin and transparent.
Arachnoid Mater           Middle layer.         Project villi through the dura into the venous sinuses to
                                                 absorb CSF.

                          Outermost             Thick and fibrous.
Dura Mater
                          layer.                Made up of two layers; periosteal and meningeal.
                                                Contains the venous sinuses.
                          Folds inwards to form the falx cerebri and tentorum cerebelli.



CROSS SECTION OF THE SCALP
In the Spine

The spinal cord meninges are a continuation of the cranial meninges and are connected to the
foramen magnum. Like the cranial meninges the fibrous dura mater is the thick outermost layer and
is connected posteriorly to the posterior longitudinal ligament. The arachnoid mater lines the inner
surface of the dura mater and the pia mater lies directly on the spinal cord itself. The pia mater
attaches to the dura mater via the denticulate ligament.

DENTICULATE LIGAMENT




The spinal cord terminates at the level of L2, but the dura continues to the level of S2 creating a
cistern into which the the lower spinal roots hang.

SELF-TEST

Complete the following questions before you go onto the next section:
      Describe the important structures of the midbrain.

      Name three structures related to the third ventricle.

      Describe the functions of CSF.



Spinal cord
The spinal cord is continuous with the medulla oblongata at the foramen magnum, and descends in
the vertebral canal. It consists of 31 segments corresponding to the 31 spinal nerves; 8 cervical, 12
thoracic, 5 lumbar, 5 sacral and 1 coccygeal.

At the level of the second lumbar vertebrae the spinal cord terminates by tapering to a conus
medullaris. From the conus medullaris is a long thin filament called the filum terminale. The
vertebral canal below the second lumbar vertebra is filled with the nerve roots from the lumbosacral
spine; this bunch of nerve roots resembles a horses tail and so is known as the cauda equina.

Along its course the spinal cord has two enlargements, the cervical enlargement and the lumbosacral
enlargement, in the cervical and lumbar regions respectively. These swellings are due to the large
spinal nerves which emerge from these parts of the cord to supply the upper and lower limbs.

The spinal cord is made up of a column of grey matter (contains cell bodies) surrounded by a
cylinder of white matter (myelinated neurons). The neurons of the grey matter are arranged into
ventral, dorsal and lateral horns. The fibres of the white matter travel longitudinally along the spinal
cord in designated columns.

SELF-TEST

Complete the following questions before you go onto the next section:

      How many segments does the spinal cord have?

      Which vertebral level does the spinal cord terminate?

      Why does the spinal cord have a number of enlargements along its course?



NERVOUS SYSTEM: PERIPHERAL
NERVOUS SYSTEM
CHAPTER OBJECTIVES

When you have completed his chapter you should be able to:

       Name all 12 cranial nerves and describe their functions.

       Describe the anatomy of a typical spinal nerve.

       Identify the cervical, lumbar, sacral and coccygeal plexuses.

       Name the main branches of each plexus and describe their course and function.

       Describe the difference between the parasympathetic and sympathetic nervous systems.


The peripheral nervous system (PNS) consists of the spinal nerves and ganglia and the cranial
nerves. The nerves of the PNS contain sensory fibres which relay signals to the central nervous
system (CNS) and motor fibres which relay signals from the CNS to the effector muscles/glands.

The PNS can be divided into sensory somatic and autonomic systems. The sensory somatic nervous
system is voluntary and relays sensory information, of which we are conscious, from the external
environment to the CNS and relays motor signals from the CNS to operate the muscles of the body.
The autonomic nervous system is involuntary and relays sensory information about the internals of
the body to the CNS and relays motor signals from the CNS to regulate the internal environment of
the body, e.g. vessel diameter.

Sensory somatic system
The sensory somatic system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Cranial nerves

The cranial nerves all originate or terminate in the brain stem. All cranial nerves, apart from the first
two, which are purely sensory, contain motor as well as sensory fibres and can be described as
'mixed' nerves. However, for descriptive terms each nerve is usually described in terms of its
predominant fibres. The motor (afferent) fibres originate in the brain stem and terminate in muscles
or glands, and the sensory (efferent) fibres originate in the sensory organs and receptors and
terminate in the brain stem.

Cranial Nerve       Fibres            Course                  Function

                                      Origin: mucosa of the
Olfactory (I)       Sensory                                            Smell.
                                      nasal cavity.
                                      Terminates: olfactory
                                  bulb.

                                  Origin: retina of the
                                  eyeball.
Optic (II)         Sensory        Terminates: lateral         Vision.
                                  geniculate body of the
                                  thalamus.

                                                              Extrinsic muscles of the eyeball
                                                               (superior, medial and inferior rectus
                                 Origin: midbrain.             and inferior oblique and levator
                   Predominantly
Oculomotor (III)                 Terminates: extrinsic         palpebrae superioris).
                   motor                                      Parasympathetic: intrinsic muscles of
                                 muscles of the eye.
                                                               the eyeball (sphincter of the pupil and
                                                               the ciliary muscle of the lens).

                                 Origin: midbrain
                   Predominantly                              Extrinsic muscle of the eyeball
Trochlear (IV)                   Terminates: extrinsic
                   Motor                                       (superior oblique).
                                 muscle of the eye.

                                  Origin: middle and          Sensory (ophthalmic, maxillary and
                                  upper face and the           mandibular nerves): scalp, face and
                                  pons.                        mouth.
Trigeminal (V)     Mixed                                      Motor (mandibular nerve): muscles of
                                  Terminates: pons and
                                  the muscles of               mastication (chewing) and soft palate
                                                               and the middle ear.
                                  mastication.

                                  Origin: pons.
                                                              Extrinsic muscle of the eyeball (lateral
Abducens (VI)      Motor          Terminates: extrinsic
                                                               rectus).
                                  muscle of the eye.

                                  Origin: taste buds and      Sensory: taste, external ear and palate.
                                  pons.                       Motor (temporal, zygomatic, buccal,
                                                               mandibular and cervical nerves):
                                  Terminates: thalamus
Facial (VII)       Mixed                                       muscles of facial expression and middle
                                  and muscles of facial        ear.
                                  expression and              Parasympathetic: salivary and lacrimal
                                  salivary glands.             glands.

                                Origin: cochlear and
Vestibulocochlear Predominantly semicircular canals of        Hearing.
(VIII)            sensory                                     Balance.
                                the inner ear.
                                Terminates: pons and
                                     medulla oblongata.

                                     Origin: pharynx,
                                     middle ear and                 Sensory: taste, tongue, pharynx, tonsils
                                     tongue and the                  and middle ear.
Glossopharyngeal
                 Mixed               medulla oblongata.             Motor: muscles of the pharynx
(IX)                                                                 (swallowing).
                                     Terminates: medulla
                                                                    Parasympathetic: parotid gland.
                                     oblongata, parotid
                                     gland and pharynx.

                                     Origin: viscera,               Sensory: pharynx, larynx, thoracic and
                                     tongue, pharynx and             abdominal organs and taste.
                                     larynx; medulla                Motor: soft palate, pharynx, intrinsic
                                                                     laryngeal muscles (voice) and extrinsic
                                     oblongata.
Vagus (X)           Mixed                                            tongue muscle.
                                     Terminates: medulla            Parasympathetic: to the thoracic and
                                     oblongata; viscera,             abdominal viscera - digestive tract,
                                     tongue, pharynx and             heart and lungs, kidneys, spleen liver
                                     larynx.                         and pancreas.

                                     Origin: medulla
                                                                    Neck muscles (sternocleidomastoid
                                     oblongata
                                                                     and trapezius).
Accessory (X)       Motor            Terminates: muscles
                                                                    Muscles of swallowing (pharynx and
                                     of the neck and                 soft palate).
                                     swallowing

                                     Origin: medulla
                                     oblongata                      Extrinsic and intrinsic muscles of the
Hypoglossal (XII)   Motor                                            tongue and hyoid muscles.
                                     Terminates: tongue
                                     and hyoid muscles.

SELF TEST

Complete the following questions before you go onto the next section:

       Name in order all 12 cranial nerves.

       Name the cranial nerves that innervate the extrinsic muscles of the eyeball.

       Name the cranial nerves that carry parasympathetic fibres.



Spinal nerves
There are 31 pairs of spinal nerves each arising from dorsal and ventral rootlets from the
corresponding 31 segments of the spinal cord. There are 8 pairs of cervical spinal nerves, the first set
arising above the atlas (C1) and the last arising below the seventh cervical vertebra (C7). There are
12 pairs of thoracic spinal nerves, 5 sets of lumbar spinal nerves, 5 sets of sacral and spinal nerves
and 1 set of coccygeal spinal nerves.

The dorsal roots arise from the posterolateral sides of the cord and carry sensory fibres from the
cord. The ventral roots arise from the anterolateral sides of the cord and carry motor fibres from the
cord. The dorsal and ventral roots unite to form the mixed (sensory and motor) spinal nerves. Just
before the ventral and dorsal roots join, there is a small swelling known as the dorsal root ganglion.
It is formed by the cell bodies of the sensory neurons.

With the exception of the 1st cervical, the sacral and the coccygeal spinal nerves, all of the spinal
nerves exit the vertebral canal via the intervertebral foramina. The intervertebral foramina are found
along the sides of the vertebral column, formed between the pedicles of adjacent vertebrae. The first
cervical spinal nerve exits above the atlas, between it and the occipital bone, and the sacral spinal
nerves exit via the ventral and dorsal sacral foramina.

After exiting through the intervertebral foramina the spinal nerves soon divide into dorsal and
ventral rami (branches). The dorsal branches supply the deep muscle and the skin of the back of the
trunk. The ventral branches supply the rest of the body. The ventral branches of the thoracic spinal
nerves form the intercostal nerves which supply the muscles of the thoracic cage. The ventral
branches of the rest of the spinal nerves interlink to form specialised nerve plexuses which supply
the rest of the body.

Cervical Plexus

The ventral branches of the 1st-4th cervical spinal nerves (C1-C4) interlink to form the cervical
plexus. The cervical plexus gives off many branches which supply the superficial muscles of the
neck and the skin over the neck and back of the head. An important branch of the cervical plexus is
the phrenic nerve which travels through the thorax to innervate the movement of the diaphragm;
without it we would not be able to breath.

Brachial Plexus

The ventral branches of the 5th-8th cervical and the 1st thoracic spinal nerves (C5-T1) interlink to
form the brachial plexus. The brachial plexus supplies the entire upper limb with motor and sensory
innervation. The brachial plexus divides into lateral, posterior and medial cords before dividing into
a number of large branches;

Brachial plexus (C5-
                     Origin            Course                             Innervates
T1)

Musculocutaneous     A branch of the   It descends between biceps and            Motor: flexors of the
Nerve (C5-C7)        lateral cord.                                                elbow joint;
                                       brachialis to the elbow where it
                                                                                  coracobrachialis,
                                         becomes the lateral cutaneous             biceps and brachialis
                                         nerve of the forearm.                     muscles.
                                                                                  Sensory (cutaneous):
                                                                                   Skin over the radial
                                                                                   (lateral) border of the
                                                                                   forearm.

                                                                                  Motor: deltoid and
                                                                                   teres minor muscles.
                                         Descends behind the axillary artery      Sensory: shoulder
Axillary Nerve (C5-   A branch of the
                                         winds around the surgical neck of         joint
C6)                   posterior cord.                                             Sensory (cutaneous):
                                         the humerus.
                                                                                   Skin over the shoulder
                                                                                   and lateral arm.

                                                                                  Motor: extensors of
                                       It exits the axilla under teres minor       the elbow, wrist and
                                       and runs around the back of the             hand.
                     A continuation of
                                       humerus in the radial groove with          Sensory: elbow, wrist
Radial Nerve (C5-T1) the posterior
                                       the arteria profunda brachii. From          and hand joints.
                     cord.                                                        Sensory (cutaneous):
                                       here it runs down the lateral side
                                                                                   Skin over the dorsum
                                       of the forearm to the wrist.
                                                                                   of the hand.

                                                                                  Motor: a few of the
                                                                                   flexors of the wrist
                                      Runs down the medial side of the             and hand and most of
                                                                                   the intrinsic muscles
                                      arm in front of the medial head of
                    A continuation of                                              of the hand.
Ulnar Nerve (C7-T1)                   the triceps to reach the elbow. It
                    the medial cord.                                              Sensory: hand joints.
                                      enters the flexor compartment of            Sensory (cutaneous):
                                      the forearm to travel to the wrist.          Skin of the ulnar
                                                                                   (medial) aspect of the
                                                                                   hand.

                                                                                  Motor: most of the
                                         It leaves the axilla with the             long flexors of the
                      Arises from the    brachial artery and travels to the        forearm and the
                                                                                   thenar muscles.
Median Nerve (C5-     medial and lateral elbow. In the forearm it travels to
                                                                                  Sensory (cutaneous):
T1)                   cords of the       the wrist where it enters the             Skin of the elbow,
                      brachial plexus.   carpal tunnel and divides into            wrist, and radial
                                         medial and lateral branches.              aspect of the palm of
                                                                                   the hand.
DIAGRAM OF THE BRACHIAL PLEXUS




Lumbar Plexus

The ventral branches of the 1st to 4th lumbar spinal nerves interlink to form the lumbar plexus. The
lumbar plexus supplies the thigh and skin of the buttocks and genitals;

Lumbar plexus
                Origin       Course                                       Innervates
(L1-L4)

                             The largest nerve of the lumbar plexus, it         Motor: iliopsoas,
Femoral nerve   Posterior
                             arises with in the substance of the psoas           sartorius and quadriceps
                divisions of
                             muscle. It passes into the thigh under the          femoris muscles.
                  L2-L4.        inguinal ligament, where it divides into          Sensory (cutaneous):
                                anterior and posterior branches.                   Skin over the anterior
                                                                                   and lateral thigh and the
                                                                                   medial leg and foot.

                                                                                  Motor: abdominal
                                Arises within the substance of psoas               muscles.
                  Anterior
                                emerging laterally to travel over quadratus       Sensory (cutaneous):
Iliohypogastric   division of
                                lumborum. It pierces transversus abdominis         Skin of the anterior
                  T12-L1                                                           buttocks and abdominal
                                to travel between it and internal oblique.
                                                                                   wall above the pubis.

                                Emerges from the superolateral border of
                                psoas major, passes over quadratus
                  Anterior                                                        Sensory (cutaneous):
                                lumborum and pierces transversus
Ilioinguinal      division of                                                      Skin of the superomedial
                                abdominis and internal oblique. It                 thigh and genital area.
                  L1
                                continues underneath external oblique and
                                enters the inguinal canal.

                                Emerges from the lateral aspect of psoas
                                                                                  Sensory (cutaneous):
                                major travels along iliacus to enter the
Lateral femoral                                                                    Skin over the lateral
                L2-L3           surface of the thigh under the inguinal
cutaneous                                                                          aspect of the thigh
                                ligament. Here it divides into anterior and        down to the knee.
                                posterior branches.

                               Passes through the substance of psoas              Sensory (cutaneous):
                  Anterior
                               major and emerges from its anterior                 Skin over the femoral
Genitofemoral     divisions of
                               surface where it divides into genital and           triangle and genital
                  L1-L2                                                            area.
                               femoral branches.

                                                                                  Motor: Adductors of the
                               It ascends through psoas major to emerge            thigh.
                                                                                  Sensory: hip and knee
                  Anterior     at the pelvic brim medial to the muscle. It
                                                                                   joints and cruciate
Obturator         divisions of descends along the lateral wall of the pelvis
                                                                                   ligaments.
                  L2-L4        to enter the thigh through the obturator           Sensory (cutaneous):
                               foramen.                                            Skin of the medial side
                                                                                   of the thigh.



Sacral Plexus

The ventral branches of the 4th lumbar to the 4th sacral spinal nerves (L4-S4) interlink to form the
sacral plexus. Because there is an overlap between the lumbar and sacral plexuses they are often
described together as the lumbosacral plexus. A large division known as the lumbosacral trunk
arises from the ventral branches of the L4-L5 and joins with the ventral branches of S1 to supply the
lower limb.

Sacral
             Origin           Course                                              Innervates
Plexus

                              The largest nerve in the body, it is really two
                              nerves bound together with connective tissue;
                              the medially placed tibial nerve and laterally
             Anterior
Sciatic                       placed common (fibular) peroneal nerve. It                Motor: hamstrings.
             divisions of
nerve                         exits the pelvis, through the greater sciatic             Sensory: hip joint.
             the L4-S3
                              foramen, underneath piriformis and runs down
                              the back of the leg to the knee where its two
                              components split from one another.

                                                                                        Motor: flexors of the
                              Arises as a separate division in the popliteal             leg.
             Larger of the    fossa, and descends into the flexor                       Sensory: knee and foot
             two divisions    compartment of the leg (calf) in which it travels          joints.
Tibial nerve
             of the sciatic   to the ankle. On reaching the ankle it divides            Sensory (cutaneous):
             nerve (L4-S3)    into medial and lateral plantar nerves which               skin over the back of
                                                                                         the leg and on the sole
                              enter the sole of the foot.
                                                                                         of the foot.

                                                                                        Motor: peroneal
                                                                                         muscles and extensors
                                                                                         of the ankle.
             Smaller of       Arises as a separate division in the popliteal            Sensory: knee, foot and
Common
             the two          fossa and travels laterally around the neck of             ankle joints.
(fibular)
             divisions of     the fibula to enter the peroneus longus muscle            Sensory (cutaneous):
Peroneal                                                                                 skin over the anterior,
             the sciatic      where it divides into deep and superficial
nerve                                                                                    lateral and posterior
             nerve (L4-S2)    branches.
                                                                                         aspect of the leg and
                                                                                         skin of the dorsum of
                                                                                         the foot and toes.

Superior     Posterior        It exits the pelvis through the greater sciatic           Motor: gluteus medius
Gluteal      divisions of     foramen, above piriformis to reach the gluteal             and minimus and
Nerve        L4-S1            region.                                                    tensor fasciae latae.

                                                                                        Motor: gluteus
Inferior     Posterior        It exits the pelvis through the greater sciatic
                                                                                         maximus.
Gluteal      divisions of     foramen, below piriformis to reach the gluteal
Nerve        L5-S2          region.

                            It exits the pelvis through the greater sciatic         Motor: perineal
             Anterior
Pudendal                    foramen, between piriformis and coccygeus                muscles.
             divisions of
Nerve                       and enters the pelvis again through the lesser          Sensory (cutaneous):
             S2-S4                                                                   skin of the perineum.
                            sciatic foramen.


Coccygeal Plexus

The ventral branches of the 4th to 5th sacral spinal nerves and the coccygeal spinal nerve (S4-Co1)
interlink to form the coccygeal plexus. It pierces the anococcygeal ligament to supply sensory
innervation to the skin of that region.

SELF TEST

Complete the following questions before you go onto the next section:

       Beginning at the spinal cord describe the formation of a spinal nerve.

       Which nerve roots contribute to each plexus?

       Name 3 nerves from each of the cervical, brachial and lumbar plexuses.



Autonomic nervous system
The autonomic nervous system (ANS) maintains our internal bodily functions without us being
aware of it. It consists of afferent (sensory) fibres that transmit sensory information from the
periphery (viscera) to the CNS, and efferent (motor) fibres that send motor signals from the CNS to
the periphery. This loop of autonomic sensory and motor fibres allows the body to monitor and
control such functions as heart rate, vessel diameter, smooth muscles surrounding organs and the
secretions of glands.

The ANS can be divided into two separate systems, sympathetic and parasympathetic, both of
which contain motor and sensory fibres. The fibres in both systems arise in the central nervous
system (brain and spinal cord) as preganglionic fibres. These synapse in ganglia with postganglionic
fibres, which then travel to the target organ.

Most organs are innervated by fibres from both divisions of the ANS; the sympathetic system
generally prepares the body for activity while the parasympathetic prepares it for rest.

Sympathetic
The sympathetic system controls our 'fight or flight' responses, i.e. quickly activates our body
preparing it for action by increasing the heart rate, dilating the pupils and vessels to the muscles,
increases skin sweating and suppresses digestion.

The motor (efferent) sympathetic fibres (preganglionic motor neurons) arise from the
intermediolateral column in the thoracic spinal cord and travel to the sympathetic ganglia listed
below, where they synapse with postganglionic neurons before reaching their target organs. The
sympathetic chain is found on either side of the vertebral column and consists of a number of
sympathetic ganglia strung together by sympathetic trunks.

Sympathetic Ganglia

Chain ganglia

Middle cervical ganglia

Superior cervical ganglia

Stellate ganglia

Aortic ganglia

Celiac ganglia

Renal ganglia

Mesenteric ganglia


White and grey rami connect the sympathetic trunks to the spinal nerves. Fibres emerge from the
chains to form sympathetic plexuses around the internal organs.

Sympathetic plexuses

Oesophageal plexus

Aortic plexus

Gastric plexus

Cardiac plexus

Superior hypogastric plexus
Inferior hypogastric plexus


The sensory (afferent) sympathetic fibres carry sensory information from the viscera to the CNS.
The afferent fibres enter the dorsal horn of the spinal cord alongside the sensory afferents from the
skin, this can cause confusion with referred pain.

Referred pain

Because the afferent fibres enter the dorsal horn of the spinal cord alongside the sensory afferents
from the skin, pain from the viscera can be perceived as originating from elsewhere in the body. This
is known as referred pain and must be understood when investigating the condition of a patient, for
example, when a person is experiencing a heart attack they will often feel pain in their left shoulder,
neck and arm. This is due to the sensory fibres from the heart and those areas of the arm entering the
spinal cord at the same level where the signals get confused.

Parasympathetic

The parasympathetic system controls the opposite responses to 'fight or flight' responses, i.e.
depresses the activity of the body preparing it for rest by decreasing the heart rate, constricting the
pupils and vessels to the skeletal muscles and stimulating digestion (peristalsis and salivary
production).

Parasympathetic cells are located in specialised nuclei throughout the brainstem and the sacral
spinal cord. Its fibres travel in four of the cranial nerves chiefly the vagus nerve as well as the
splanchnic and pelvic nerves. The majority of sensory (afferent) parasympathetic fibres return to the
medulla oblongata via the vagus nerve where the fibres then travel in the solitary tract to the
solitary nucleus which receives sensory information about the blood pressure, CO2/O2 levels and
digestive tract distension.

Parasympathetic nerves Targets

Vagus nerve               Heart, stomach, small intestine, parts of the large intestine.

Oculomotor                Sphincter of pupil and ciliary muscle.

Facial nerves             Lacrimal, nasal, submandibular and sublingual glands.

Glossopharyngeal nerve Parotid gland.

Pelvic splanchnic nerves Parts of the large intestine, rectum, bladder, penis or clitoris.


The motor (efferent) parasympathetic fibres travel to parasympathetic ganglia before reaching their
target organ.
Parasympathetic Ganglia Targets

Ciliary ganglia           Sphincter of pupil and ciliary muscle.

Pterygopalatine ganglia   Lacrimal and nasal glands.

Submandibular ganglia     Submandibular and sublingual glands.

Otic ganglia              Parotid gland.

Mesenteric ganglia        Parts of the large intestine and rectum.

Pelvic ganglia            Bladder, penis or clitoris.


Autonomic Reflexes

A reflex is a pathway with an afferent (sensory) signal that evokes an immediate efferent response
(motor) without the signal being transmitted to the brain.

e.g. afferent fibres from the viscera baroreceptors and chemoreceptors in the carotid sinus and arch
of the aorta are carried to the CNS by the major autonomic nerves where they control heart rate,
blood pressure and respiration.

Summary of ANS Function;
Structure            Sympathetic Action                                 Parasympathetic Action

Heart Rate           Increase.                                          Decrease.

Heart Contraction    Increase force.                                    Decrease force.

                     Directs blood to the skeletal muscles (away from   Directs blood to the to the skin and
Blood Vessels
                     the skin and viscera).                             viscera.

Sweat Production     Increases production.                              Decreases production.

Pupil Aperture       Dilates.                                           Constricts.

Trachea and
                     Dilates.                                           Constricts.
Bronchi

Blood Pressure       Increases.                                         Lowers.
Salivary Gland        Decreases production.                               Increase production.

Gastrointestinal                                                          Increases peristalsis, increases
                      Decreases peristalsis, reduced digestion.
Tract                                                                     digestion.

                                                                          Inhibits the conversion of glycogen
Liver                 Stimulates the conversion of glycogen to glucose.
                                                                          to glucose.

Kidney                Decreased urine production.                         Increased urine production.

Bladder               Wall relaxed, sphincter closed.                     Wall contracted, sphincter relaxed.




SELF TEST

Complete the following questions before you go onto the next section:

         Describe the functions of the sympathetic and parasympathetic nervous systems.

         Name and locate five sympathetic ganglia.

         Name and locate four parasympathetic nerves.

				
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