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13 The Peripheral Nervous System and Reflex Activity
Peripheral Nervous System (PNS)
• All neural structures outside the brain
   • Sensory receptors
   • Peripheral nerves and associated ganglia
   • Motor endings
Sensory Receptors
• Specialized to respond to changes in their environment (stimuli)
• Activation results in graded potentials that trigger nerve impulses
• Sensation (awareness of stimulus) and perception (interpretation of the meaning of the
  stimulus) occur in the brain

Classification of Receptors
• Based on:
   • Stimulus type
   • Location
   • Structural complexity
Classification by Stimulus Type
• Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch
• Thermoreceptors—sensitive to changes in temperature
• Photoreceptors—respond to light energy (e.g., retina)
• Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry)
• Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive
  pressure, inflammatory chemicals)

Classification by Location
  • Respond to stimuli arising outside the body
  • Receptors in the skin for touch, pressure, pain, and temperature
  • Most special sense organs
Classification by Location
2.Interoceptors (visceroceptors)
   • Respond to stimuli arising in internal viscera and blood vessels
   • Sensitive to chemical changes, tissue stretch, and temperature changes
Classification by Location
  • Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective
    tissue coverings of bones and muscles
  • Inform the brain of one’s movements
Classification by Structural Complexity
1.Complex receptors (special sense organs)
  • Vision, hearing, equilibrium, smell, and taste (Chapter 15)
2.Simple receptors for general senses:
  • Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle
  • Unencapsulated (free) or encapsulated dendritic endings
Unencapsulated Dendritic Endings
• Thermoreceptors
   • Cold receptors (10–40ºC); in superficial dermis
   • Heat receptors (32–48ºC); in deeper dermis
Unencapsulated Dendritic Endings
• Nociceptors
   • Respond to:
      • Pinching
      • Chemicals from damaged tissue
      • Temperatures outside the range of thermoreceptors
      • Capsaicin
Unencapsulated Dendritic Endings
• Light touch receptors
   • Tactile (Merkel) discs
   • Hair follicle receptors
Encapsulated Dendritic Endings
• All are mechanoreceptors
   • Meissner’s (tactile) corpuscles—discriminative touch
   • Pacinian (lamellated) corpuscles—deep pressure and vibration
   • Ruffini endings—deep continuous pressure
   • Muscle spindles—muscle stretch
   • Golgi tendon organs—stretch in tendons
   • Joint kinesthetic receptors—stretch in articular capsules

From Sensation to Perception
• Survival depends upon sensation and perception
• Sensation: the awareness of changes in the internal and external environment
• Perception: the conscious interpretation of those stimuli
Sensory Integration
• Input comes from exteroceptors, proprioceptors, and interoceptors
• Input is relayed toward the head, but is processed along the way
Sensory Integration
• Levels of neural integration in sensory systems:
   1. Receptor level—the sensor receptors
   2. Circuit level—ascending pathways
   3. Perceptual level—neuronal circuits in the cerebral cortex
Processing at the Receptor Level
• Receptors have specificity for stimulus energy
• Stimulus must be applied in a receptive field
• Transduction occurs
   • Stimulus energy is converted into a graded potential called a receptor potential
Processing at the Receptor Level
• In general sense receptors, the receptor potential and generator potential are the same
                      receptor/generator potential in afferent neuron
                          action potential at first node of Ranvier

Processing at the Receptor Level
• In special sense organs:
                            receptor potential in receptor cell
                               release of neurotransmitter
                     generator potential in first-order sensory neuron
                        action potentials (if threshold is reached)

Adaptation of Sensory Receptors
• Adaptation is a change in sensitivity in the presence of a constant stimulus
   • Receptor membranes become less responsive
   • Receptor potentials decline in frequency or stop
Adaptation of Sensory Receptors
• Phasic (fast-adapting) receptors signal the beginning or end of a stimulus
  • Examples: receptors for pressure, touch, and smell
• Tonic receptors adapt slowly or not at all
  • Examples: nociceptors and most proprioceptors
Processing at the Circuit Level
• Pathways of three neurons conduct sensory impulses upward to the appropriate brain
• First-order neurons
   • Conduct impulses from the receptor level to the second-order neurons in the CNS
• Second-order neurons
   • Transmit impulses to the thalamus or cerebellum
• Third-order neurons
   • Conduct impulses from the thalamus to the somatosensory cortex (perceptual level)

Processing at the Perceptual Level
• Identification of the sensation depends on the specific location of the target neurons in
  the sensory cortex
• Aspects of sensory perception:
   • Perceptual detection—ability to detect a stimulus (requires summation of impulses)
   • Magnitude estimation—intensity is coded in the frequency of impulses
   • Spatial discrimination—identifying the site or pattern of the stimulus (studied by the
     two-point discrimination test)

Main Aspects of Sensory Perception
• Feature abstraction—identification of more complex aspects and several stimulus
• Quality discrimination—the ability to identify submodalities of a sensation (e.g., sweet
  or sour tastes)
• Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g., the
  melody in a piece of music)

Perception of Pain
• Warns of actual or impending tissue damage
• Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and
• Impulses travel on fibers that release neurotransmitters glutamate and substance P
• Some pain impulses are blocked by inhibitory endogenous opioids
Structure of a Nerve
• Cordlike organ of the PNS
• Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue
Structure of a Nerve
• Connective tissue coverings include:
  • Endoneurium—loose connective tissue that encloses axons and their myelin sheaths
  • Perineurium—coarse connective tissue that bundles fibers into fascicles
  • Epineurium—tough fibrous sheath around a nerve
Classification of Nerves
• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic
  (visceral) fibers
• Pure sensory (afferent) or motor (efferent) nerves are rare
• Types of fibers in mixed nerves:
   • Somatic afferent and somatic efferent
   • Visceral afferent and visceral efferent
• Peripheral nerves classified as cranial or spinal nerves

• Contain neuron cell bodies associated with nerves
   • Dorsal root ganglia (sensory, somatic) (Chapter 12)
   • Autonomic ganglia (motor, visceral) (Chapter 14)
Regeneration of Nerve Fibers
• Mature neurons are amitotic
• If the soma of a damaged nerve is intact, axon will regenerate
• Involves coordinated activity among:
   • Macrophages—remove debris
   • Schwann cells—form regeneration tube and secrete growth factors
   • Axons—regenerate damaged part
• CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber

Cranial Nerves
• Twelve pairs of nerves associated with the brain
• Most are mixed in function; two pairs are purely sensory
• Each nerve is identified by a number (I through XII) and a name
        ―On occasion, our trusty truck acts funny—very good vehicle anyhow‖

I: The Olfactory Nerves
• Arise from the olfactory receptor cells of nasal cavity
• Pass through the cribriform plate of the ethmoid bone
• Fibers synapse in the olfactory bulbs
• Pathway terminates in the primary olfactory cortex
• Purely sensory (olfactory) function
II: The Optic Nerves
• Arise from the retinas
• Pass through the optic canals, converge and partially cross over at the optic chiasma
• Optic tracts continue to the thalamus, where they synapse
• Optic radiation fibers run to the occipital (visual) cortex
• Purely sensory (visual) function
III: The Oculomotor Nerves
• Fibers extend from the ventral midbrain through the superior orbital fissures to the
  extrinsic eye muscles
• Functions in raising the eyelid, directing the eyeball, constricting the iris
  (parasympathetic), and controlling lens shape

IV: The Trochlear Nerves
• Fibers from the dorsal midbrain enter the orbits via the superior orbital fissures to
  innervate the superior oblique muscle
• Primarily a motor nerve that directs the eyeball
V: The Trigeminal Nerves
• Largest cranial nerves; fibers extend from pons to face
• Three divisions
   • Ophthalmic (V1) passes through the superior orbital fissure
   • Maxillary (V2) passes through the foramen rotundum
   • Mandibular (V3) passes through the foramen ovale
• Convey sensory impulses from various areas of the face (V1) and (V2), and supplies
  motor fibers (V3) for mastication

VI: The Abducens Nerves
• Fibers from the inferior pons enter the orbits via the superior orbital fissures
• Primarily a motor, innervating the lateral rectus muscle
VII: The Facial Nerves
• Fibers from the pons travel through the internal acoustic meatuses, and emerge through
  the stylomastoid foramina to the lateral aspect of the face
• Chief motor nerves of the face with 5 major branches
• Motor functions include facial expression, parasympathetic impulses to lacrimal and
  salivary glands
• Sensory function (taste) from the anterior two-thirds of the tongue

VIII: The Vestibulocochlear Nerves
• Afferent fibers from the hearing receptors (cochlear division) and equilibrium receptors
  (vestibular division) pass from the inner ear through the internal acoustic meatuses, and
  enter the brain stem at the pons-medulla border
• Mostly sensory function; small motor component for adjustment of sensitivity of

IX: The Glossopharyngeal Nerves
• Fibers from the medulla leave the skull via the jugular foramen and run to the throat
• Motor functions: innervate part of the tongue and pharynx for swallowing, and provide
    parasympathetic fibers to the parotid salivary glands
•   Sensory functions: fibers conduct taste and general sensory impulses from the pharynx
    and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors

X: The Vagus Nerves
• The only cranial nerves that extend beyond the head and neck region
• Fibers from the medulla exit the skull via the jugular foramen
• Most motor fibers are parasympathetic fibers that help regulate the activities of the
  heart, lungs, and abdominal viscera
• Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors,
  chemoreceptors, and taste buds of posterior tongue and pharynx

XI: The Accessory Nerves
• Formed from ventral rootlets from the C1–C5 region of the spinal cord (not the brain)
• Rootlets pass into the cranium via each foramen magnum
• Accessory nerves exit the skull via the jugular foramina to innervate the trapezius and
  sternocleidomastoid muscles

XII: The Hypoglossal Nerves
• Fibers from the medulla exit the skull via the hypoglossal canal
• Innervate extrinsic and intrinsic muscles of the tongue that contribute to swallowing
  and speech

Spinal Nerves
• 31 pairs of mixed nerves named according to their point of issue from the spinal cord
   • 8 cervical (C1–C8)
   • 12 thoracic (T1–T12)
   • 5 Lumbar (L1–L5)
   • 5 Sacral (S1–S5)
   • 1 Coccygeal (C0)
Spinal Nerves: Roots
• Each spinal nerve connects to the spinal cord via two roots
• Ventral roots
   • Contain motor (efferent) fibers from the ventral horn motor neurons
   • Fibers innervate skeletal muscles)
Spinal Nerves: Roots
• Dorsal roots
   • Contain sensory (afferent) fibers from sensory neurons in the dorsal root ganglia
   • Conduct impulses from peripheral receptors
• Dorsal and ventral roots unite to form spinal nerves, which then emerge from the
 vertebral column via the intervertebral foramina

Spinal Nerves: Rami
• Each spinal nerve branches into mixed rami
   • Dorsal ramus
   • Larger ventral ramus
   • Meningeal branch
   • Rami communicantes (autonomic pathways) join to the ventral rami in the thoracic

Spinal Nerves: Rami
• All ventral rami except T2–T12 form interlacing nerve networks called plexuses
  (cervical, brachial, lumbar, and sacral)
• The back is innervated by dorsal rami via several branches
• Ventral rami of T2–T12 as intercostal nerves supply muscles of the ribs, anterolateral
  thorax, and abdominal wall

Cervical Plexus
• Formed by ventral rami of C1–C4
• Innervates skin and muscles of the neck, ear, back of head, and shoulders
• Phrenic nerve
   • Major motor and sensory nerve of the diaphragm (receives fibers from C3–C5)
Brachial Plexus
• Formed by ventral rami of C5–C8 and T1 (and often C4 and T2)
• It gives rise to the nerves that innervate the upper limb
• Major branches of this plexus:
   • Roots—five ventral rami (C5–T1)
   • Trunks—upper, middle, and lower
   • Divisions—anterior and posterior
   • Cords—lateral, medial, and posterior

Brachial Plexus: Nerves
• Axillary—innervates the deltoid, teres minor, and skin and joint capsule of the shoulder
• Musculocutaneous—innervates the biceps brachii and brachialis and skin of lateral
• Median—innervates the skin, most flexors and pronators in the forearm, and some
  intrinsic muscles of the hand
• Ulnar—supplies the flexor carpi ulnaris, part of the flexor digitorum profundus, most
  intrinsic muscles of the hand, and skin of medial aspect of hand
• Radial—innervates essentially all extensor muscles, supinators, and posterior skin of
Lumbar Plexus
• Arises from L1–L4
• Innervates the thigh, abdominal wall, and psoas muscle
• Femoral nerve—innervates quadriceps and skin of anterior thigh and medial surface of
• Obturator nerve—passes through obturator foramen to innervate adductor muscles
Sacral Plexus
• Arises from L4–S4
• Serves the buttock, lower limb, pelvic structures, and perineum
• Sciatic nerve
   • Longest and thickest nerve of the body
   • Innervates the hamstring muscles, adductor magnus, and most muscles in the leg
     and foot
   • Composed of two nerves: tibial and common fibular

Innervation of Skin
• Dermatome: the area of skin innervated by the cutaneous branches of a single spinal
• All spinal nerves except C1 participate in dermatomes
• Most dermatomes overlap, so destruction of a single spinal nerve will not cause
  complete numbness

Innervation of Joints
• Hilton’s law: Any nerve serving a muscle that produces movement at a joint also
  innervates the joint and the skin over the joint

Motor Endings
• PNS elements that activate effectors by releasing neurotransmitters
Review of Innervation of Skeletal Muscle
• Takes place at a neuromusclular junction
• Acetylcholine (ACh) is the neurotransmitter
• ACh binds to receptors, resulting in:
   • Movement of Na+ and K+ across the membrane
   • Depolarization of the muscle cell
   • An end plate potential, which triggers an action potential
Review of Innervation of Visceral Muscle and Glands
• Autonomic motor endings and visceral effectors are simpler than somatic junctions
• Branches form synapses en passant via varicosities
• Acetylcholine and norepinephrine act indirectly via second messengers
• Visceral motor responses are slower than somatic responses
Levels of Motor Control
• Segmental level
• Projection level
• Precommand level
Segmental Level
• The lowest level of the motor hierarchy
• Central pattern generators (CPGs): segmental circuits that activate networks of ventral
  horn neurons to stimulate specific groups of muscles
• Controls locomotion and specific, oft-repeated motor activity
Projection Level
• Consists of:
   • Upper motor neurons that direct the direct (pyramidal) system to produce voluntary
     skeletal muscle movements
   • Brain stem motor areas that oversee the indirect (extrapyramidal) system to control
     reflex and CPG-controlled motor actions
• Projection motor pathways keep higher command levels informed of what is happening
Precommand Level
• Neurons in the cerebellum and basal nuclei
   • Regulate motor activity
   • Precisely start or stop movements
   • Coordinate movements with posture
   • Block unwanted movements
   • Monitor muscle tone
   • Perform unconscious planning and discharge in advance of willed movements
Precommand Level
• Cerebellum
   • Acts on motor pathways through projection areas of the brain stem
   • Acts on the motor cortex via the thalamus
• Basal nuclei
   • Inhibit various motor centers under resting conditions
• Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus
• Learned (acquired) reflexes result from practice or repetition,
   • Example: driving skills
Reflex Arc
• Components of a reflex arc (neural path)
   1. Receptor—site of stimulus action
  2. Sensory neuron—transmits afferent impulses to the CNS
  3. Integration center—either monosynaptic or polysynaptic region within the CNS
  4. Motor neuron—conducts efferent impulses from the integration center to an effector
  5. Effector—muscle fiber or gland cell that responds to the efferent impulses by
     contracting or secreting

Spinal Reflexes
• Spinal somatic reflexes
   • Integration center is in the spinal cord
   • Effectors are skeletal muscle
• Testing of somatic reflexes is important clinically to assess the condition of the nervous

Stretch and Golgi Tendon Reflexes
• For skeletal muscle activity to be smoothly coordinated, proprioceptor input is
   • Muscle spindles inform the nervous system of the length of the muscle
   • Golgi tendon organs inform the brain as to the amount of tension in the muscle and

Muscle Spindles
• Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule
• Intrafusal fibers
   • Noncontractile in their central regions (lack myofilaments)
   • Wrapped with two types of afferent endings: primary sensory endings of type Ia
      fibers and secondary sensory endings of type II fibers

Muscle Spindles
• Contractile end regions are innervated by gamma () efferent fibers that maintain
  spindle sensitivity
• Note: extrafusal fibers (contractile muscle fibers) are innervated by alpha () efferent

Muscle Spindles
• Excited in two ways:
   1. External stretch of muscle and muscle spindle
   2. Internal stretch of muscle spindle:
       • Activating the  motor neurons stimulates the ends to contract, thereby stretching
         the spindle
• Stretch causes an increased rate of impulses in Ia fibers
Muscle Spindles
• Contracting the muscle reduces tension on the muscle spindle
• Sensitivity would be lost unless the muscle spindle is shortened by impulses in the 
    motor neurons
•   – coactivation maintains the tension and sensitivity of the spindle during muscle

Stretch Reflexes
• Maintain muscle tone in large postural muscles
• Cause muscle contraction in response to increased muscle length (stretch)
Stretch Reflexes
• How a stretch reflex works:
   • Stretch activates the muscle spindle
   • IIa sensory neurons synapse directly with  motor neurons in the spinal cord
   •  motor neurons cause the stretched muscle to contract
• All stretch reflexes are monosynaptic and ipsilateral
Stretch Reflexes
• Reciprocal inhibition also occurs—IIa fibers synapse with interneurons that inhibit the
   motor neurons of antagonistic muscles
• Example: In the patellar reflex, the stretched muscle (quadriceps) contracts and the
  antagonists (hamstrings) relax

Golgi Tendon Reflexes
• Polysynaptic reflexes
• Help to prevent damage due to excessive stretch
• Important for smooth onset and termination of muscle contraction
Golgi Tendon Reflexes
• Produce muscle relaxation (lengthening) in response to tension
   • Contraction or passive stretch activates Golgi tendon organs
   • Afferent impulses are transmitted to spinal cord
   • Contracting muscle relaxes and the antagonist contracts (reciprocal activation)
   • Information transmitted simultaneously to the cerebellum is used to adjust muscle

Flexor and Crossed-Extensor Reflexes
• Flexor (withdrawal) reflex
   • Initiated by a painful stimulus
   • Causes automatic withdrawal of the threatened body part
   • Ipsilateral and polysynaptic
Flexor and Crossed-Extensor Reflexes
• Crossed extensor reflex
   • Occurs with flexor reflexes in weight-bearing limbs to maintain balance
  • Consists of an ipsilateral flexor reflex and a contralateral extensor reflex
     • The stimulated side is withdrawn (flexed)
     • The contralateral side is extended
Superficial Reflexes
• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and cord-level reflex arcs
Superficial Reflexes
• Plantar reflex
   • Stimulus: stroking lateral aspect of the sole of the foot
   • Response: downward flexion of the toes
   • Tests for function of corticospinal tracts
Superficial Reflexes
• Babinski’s sign
   • Stimulus: as above
   • Response: dorsiflexion of hallux and fanning of toes
   • Present in infants due to incomplete myelination
   • In adults, indicates corticospinal or motor cortex damage
Superficial Reflexes
• Abdominal reflexes
   • Cause contraction of abdominal muscles and movement of the umbilicus in response
     to stroking of the skin
   • Vary in intensity from one person to another
   • Absent when corticospinal tract lesions are present
Developmental Aspects of the PNS
• Spinal nerves branch from the developing spinal cord and neural crest cells
   • Supply both motor and sensory fibers to developing muscles to help direct their
   • Cranial nerves innervate muscles of the head
Developmental Aspects of the PNS
• Distribution and growth of spinal nerves correlate with the segmented body plan
• Sensory receptors atrophy with age and muscle tone lessens due to loss of neurons,
  decreased numbers of synapses per neuron, and slower central processing
• Peripheral nerves remain viable throughout life unless subjected to trauma