The Spinal Cord
• The spinal cord with its 31 pairs of spinal
nerves serves two important functions.
• It is the connecting link between the brain
and most of the body.
• It is involved in spinal reflex actions, both
somatic and visceral.
Basic Anatomy of the Spinal
• The spinal cord extends caudally from the brain for about
45 cm and has a width of ~14 mm. Its upper end is
continuous with the brain (medulla oblongata). The cord
is slightly thicker than a pencil.
• There are 31 pairs of spinal nerves:8 cervical, 12 thoracic,
5 lumbar, 5 sacral, and coccygeal. The roots of the
lumbar and sacral are called cauda equina.
• Surrounding and protecting the spinal cord is the
• The spinal cord is slightly flattened dorsally and
ventrally, with two enlargements-cervical and
lumbosacral from which the spinal nerves emerge
that innervate the upper and lower limbs.
Basic Anatomy of the Spinal
• The cervical enlargement supplies nerves
to the pectoral girdle and upper limbs.
• The lumbar enlargement supplies nerves
to the pelvis and lower limbs.
• Inferior to the lumbar enlargement, the
spinal cord becomes tapered and conical-
• Filum terminale-slender strand of fibrous
tissue that extends from conus medullaris.
• There are 8 cervical nerves(C), 12 thoracic(T), 5 lumbar
(L), 5 sacral (S), and 1 coccygeal (Co).
• Each pair of spinal nerves passes through a pair of
intervertebral foramina located between two successive
vertebrae. Each spinal nerve caudal to the first thoracic
vertebra takes its name from the vertebra immediately
• The nerves are then distributed to a specific pair of
segments of the body.
• The spinal cord and the roots of its nerves are protected by
the vertebral column, its ligaments, spinal meninges and
• The outer layer is called dura mater. This is a
tough, fibrous memebrane that merges with the
• The middle layer, the arachnoid, runs caudally to
the S2 vertebral level. This is delicate and
• The innermost is called, pia mater. It is highly
vascular and tightly attached to the spinal cord and
• Meningitis-bacterial or viral infection.
• Between the dura mater and periosteum of the
vertebrae is the epidural space that contains many
blood vessels and fat.
• Anesthetics can be injected here below the L3
vertebral level, from which it ascends to act upon
sensory neurons to help dull pain. This procedure
is called caudal block.(epidural block)
• Space between dura mater and archnoid-subdural
• Space between arachnoid and pia mater-
subarchnoid space-CSF, blood vessels, spinal
• This is a clear watery ultra filtrate solution
primarily derived from blood.
• The basic mechanism involves an active transport
system and passive diffusion into the four
• The CSF provides a cushion that protects the
delicate tissues of the spinal cord.
• It is also involved in the exchange of nutrients
between the blood and neurons of the brain and
• If the spinal cord is cut in X.S., a tiny central canal
is observed, which contains CSF.
• There is a dark portion of H-shaped or butterfly
shaped “gray matter”, surrounded by a larger area
of “white matter”.
• The spinal cord is divided into more or less
symmetrical halves by a deep groove called the
anterior(ventral) median fissure and a median
septum called posterior (dorsal) median sulcus.
• Extending from the spinal cord are the ventral and
dorsal roots of the spinal nerves.
• The gray matter of the spinal cord consists of nerve cell
bodies, dendrites and axon terminals(unmyelinated) and
neuroglia. It is pinkish-gray color because of a rich
network of blood vessels.
• The gray matter forms an H shape and is composed of
three columns of neurons-posterior, anterior and lateral
horns. The projections of gray matter toward the outer
surface of spinal cord are called horns.
• The two that run dorsally-posterior horns which function
in afferent input. The two that run ventrally-anterior
horns which function in efferent somatic output. The two
that extend laterally-lateral horns.
• The nerve fibers that form the cross of the H are known as
gray commisure-functions in cross reflexes.
• The white matter gets its name because it is
mainly composed of myelinated nerve fibers, and
myelin has a whitish color.
• The white matter is divided into three pairs of
columns or funiculi of myelinated fibers-anterior,
posterior, lateral and a commisure area.
• The bundles of fibers within each funiculus are
divided into tracts called fasciculi.
• Ascending tracts-sensory fibers carry impulse up
the spinal cord to the brain.
• Descending tracts-motor neurons transmit
• A series of connective tissue layer
surrounds each spinal nerve.
• Epineurium-outermost layer, consists of a
dense network of collagen fibers.
• Perineurium-extend inward from th
epineurium, dividing the nerve into a series
• Endoneurium-delicate connective tissue
Ventral and Dorsal Roots
• In the vicinity of the cord, each spinal nerve divides into a
ventral (anterior, motor) root and a dorsal (posterior,
• Ventral roots contain mostly efferent nerve fibers and
convey motor information.
• Dorsal roots contain afferent nerve fibers and convey
• The axons of motor neurons whose cell bodies are located
within the CNS in the ant. Horn emerge from the spinal
cord to form ventral roots (motor).
• Groups of sensory neurons , whose axons make up the
dorsal roots lie outside the cord in the dorsal root ganglia
or spinal ganglia of the PNS.
Peripheral distribution of Spinal
• A typical spinal nerve has a white ramus(this contains
myelinated axons), and a gray ramus (unmyelinated fibers
that innervate glands and smooth muscles in the body wall
• A dorsal ramus(providing sensory and motor innervation
to the skin and muscles of the back), and a ventral ramus
(supplying the ventrolateral body surface, structures in the
body wall and the limbs).
• Each pair of nerves monitors a region of the body surface
called a dermatome.
• A complex, interwoven network of nerves is
a nerve plexus.
• The three large plexuses are the cervical
plexus, the brachial plexus and the
lumbosacral plexus. The latter can be
further divided into the lumbar plexus and
the sacral plexus.
Functional Roles of Pathways of
• Each pathway is composed of organized
sequences of neurons.
• Upper motor neurons in the brain influence the
activity of lower motor neurons in the cranial and
• Some neurons have long axons that terminate in
processing centers-called nucleus, ganglion, gray
matter of spinal cord or cortex of the brain.
General Somatic Efferent
• The brain exerts active influences on the activity
of skeletal muscles through descending motor
pathways that make up the upper motor neurons.
• These originate from the cell bodies in the cerebral
cortex and brainstem.
• These act by regulating and modulating the
activity of the lower motor neurons of the cranial
and spinal nerves.
Lower Motor Neurons
• These include alpha and gamma motor neurons.
• Alpha motor neurons have their cell bodied in
their CNS. Their axons course through cranial
and spinal nerves and terminate on the motor end
plates of skeletal muscle fibers (extrafusal muscle
fibers). Involved in stretch reflex.
• Gamma neurons also have cell bodied within the
CNS. Their axons pass through cranial and spinal
nerves to innervate the intrafusal muscle fibers
inside the neuromuscular spindles. Involved in the
gamma motor neuron reflex.
Lower Motor Neurons
• These are the only neurons that innervate the skeletal
muscle fibers, they function as the final common pathway,
the final link between the CNS and skeletal muscles.
• Axons are located both in the cranial and spinal nerves.
• Those in cranial nerves innervate the skeletal muscles
associated with the movements of the eyes, tongue,
chewing, swallowing, vocalizing.
• These are influenced by two sources: sensory receptors
that are integrated into reflexes and upper motor neurons
from the brain that form the “voluntary descending
Upper Motor Neurons
• This is entirely located in the CNS.
• Some of these have sequences that are made of
• They may be called first, second and third order
• A first-order neuron extends from sensory receptor
• A second-order neuron extends from the spinal
cord or brainstem to nucleus in the thalamus.
• A third-order neuron extends from the thalamus to
a sensory area of the cerebral cortex.
• A critical feature of many pathways is that they
cross over or decussate. By knowing where a
pathway crosses over, a physician can use this
information to help locate the site of an injury in
• Example is touch-pressure pathway that
decussates in the medulla oblongata.
• Many tracts are named after their nuclei of origin,
their termination as well as their location in the
spinal cord (eg. Lateral spinothalamic tract).
• Anterolateral System: this consists of the lateral
spinothalamic tract, spinoreticulothalamic tract
and anterior spinothalamic tract. This involves the
general sensatins of pain, temperature and light
• Posterior-column medial lemniscus pathway.
• A reflex is a predictable involuntary
response to a stimulus.
• A reflex involving the skeletal muscles is
called a somatic reflex.
• A reflex involving responses of smooth
muscle, cardiac muscle, or a gland is a
Classification of Reflexes
• Reflexes are classified according to :
• 1) their development : innate and acquired
• 2) site of information processing: cranial and
• 3)nature of resulting motor response : somatic and
• 4) the complexity of the neural circuit :
monosynaptic and polysynaptic reflexes.
• A reflex always starts with a sensory neuron and
ends with a motor neuron.
• In a monosynaptic (one synapse, two neuron)
reflex arc, the sensory and motor neurons synapse
• More often, however, one or more interneurons
synapse with the sensory and motor neurons in a
polysynaptc reflex arc.
• Most reflex actions never travel any higher than
spinal cord. Some maybe to brain stem.
Types of Reflexes
• Stretch (Myotatic Reflex)-monosynaptic
reflex arc. Maintains erect posture. Eg.
Knee-jerk or patellar reflex. Ipsilateral-
response and stimulus on same side.
• Gamma Motor Neuron Reflex Arc-this
acts to smooth out the movements of muscle
contractions or to sustain the contraction of
Types of Reflexes
• Plantar Reflex: this clinically tests the
integrity of the spinal cord from L4 to S2.
It is tested by drawing a blunt instrument
down the lateral aspect of the sole(plantar
surface) of the foot. A normal response is a
curling or downward flexion of the foot.
• Withdrawal reflex arc: involves sensory
receptors, afferent neurons, interneurons,
alpha motor neurons, skeletal muscles.
Some Diagnostic Reflexes
• Abdominal reflex
• Achilles Reflex
• Biceps Reflex
• Babinski’s reflex
• Hoffmann’s reflex
• Patellar Reflex
• Plantar reflex
• Triceps reflex