Chapter 13 - The Peripheral Nervous System

Function of the Peripheral Nervous System

links the central nervous system to the outside world

• nerves extend from every area of the body → provide the brain with sensory input and allows for motor output to reach effector organs

Structures of Peripheral Nervous System

any nervous system tissue that is not the brain or spinal cord:

• sensory receptors

• afferent fibers

• efferent fibers

Afferent Division

carry impulses from the body to the central nervous system

• afferent fibers

• sensory information carried by this division

• impulses allow CNS to interpret information and send out a response

Efferent Division

carry impulses from CNS to the effector organs

• efferent fibers

• motor information carried by this division

• impulses activate muscle or glands to carry out response

Somatic Division (subsection of efferent division)

skeletal muscle tissue is the effector

Autonomic Division (subsection of efferent division)

cardiac muscle, smoot muscle, and glands are effectors

Function and Classification of Sensory Receptors and Sensation 

specialized structures that respond to changes in the environment (internal or external), called stimuli

• classified into 3 ways

  • stimulus type

  • location

  • receptor structure

Stimulus Type (classification of sensory receptors and sensation)

what change in the environment activates the receptor

Location (classification of sensory receptors and sensation)

either location in the body or the location of the source of the stimulus (inside or outside the body)

Receptor Structure (classification of sensory receptors and sensation)

nonencapsulated vs. encapsulated

Mechanoreceptors (stimulus type)

respond to mechanical force

• must physically deform sensory receptor → change its shape

Thermoreceptors (stimulus type)

respond to temperature changes

Photoreceptors (stimulus type)

respond to light or different wavelengths of color

• only found in eyes → generating vision

Chemoreceptors (stimulus type)

respond to chemicals in solution

Nociceptors (stimulus type)

respond to damaging or harmful stimuli

Exteroceptor (location)

mostly found at or near the body surface → skin

• receptor is sensitive to stimuli that arise outside the body

Interoceptors (location)

found deeper in body

• organs deep → visceral organs

• respond to stimuli that arise deep in the body

Proprioceptors (location)

found in skeletal muscle, tendon, joints, ligaments

• similar to interoceptor

• responds to changes in body movement or position

  • prevention from tripping or falling

Receptors of the General Senses

composed mostly of nerve endings → modified dendritic endings of sensory neurons

• non-encapsulated (free) nerve ending

• encapsulated nerve ending

Non-Encapsulated (free) Nerve Endings

dendritic ends of sensory receptors have no capsule or covering

• mostly abundant in epithelia and connective tissue

• respond mostly to pain (nociceptors) and temperature (thermoreceptors) 

Encapsulated Nerve Endings

dendritic endings are enclose in a capsule

• most mechanoreceptors are encapsulated

  • ex) muscle spindles, tendon organs

Sharp Pain

usually occurs at onset of injury

• impulses carried by small, myelinated fibers

  • ex) paper cut

Burning Pain

occurs after sharp pain

• impulses carried by small, nonmyelinated fibers

• extends for longer periods of time

How is pain measured?

no real objective way of measuring pain

• pain is interpreted different by different people

Suppression of Pain Perception

occurs most often during times of stress → coping/escape mechanism

• endorphins and enkephalins suppress the feeling of pain

• important in an emergency and stressful situations

  • ex) bear is chasing you after you twisted your ankle

Visceral Pain

noxious stimulation of receptors of the thorax and abdominal cavity

• stimuli that result in visceral pain:

  • extreme stretching of tissue, ischemia, muscle spasms/cramps, irritating chemicals

Referred Pain

pain stimuli arising from one part of the body are perceived as coming from another part

• brain is “confused” by the signal → cannot determine the exact location of source

  • ex) an early sign of a heart attack is your left arm hurting

Nerve

cordlike organ that is part of the peripheral nervous system, consisting of parallel bundles of peripheral axons enclosed by connective tissue wrappings

• sensory or motor 

Afferent Nerves

carry information toward CNS

• sensory information

Efferent Nerves

carry information away from CNS

• motor information

Mixed Nerves

carry information to and from the CNS

• sensory and motor information

Endoneurium (structure of a nerve)

single axon is surrounded by this

Perineurium (structure of a nerve)

groups of axons are bundled together by this

• forms a fascicle

Epineurium (structure of a nerve)

encloses all other structures

• bundles of fascicles form the actual nerve

• also has blood vessels and lymphatic vessels

Nerve Damage and Regeneration in CNS

severed/damaged axons do NOT regenerate

• when damages, the neuron dies

Nerve Damage and Regeneration in PNS

severed/damages axons are capable of regeneration

• for regeneration to occur:

  • the cell body must remain undamaged

  • the distance between severed ends must be short

• regeneration axons do not “behave” as before the injury

Axon Regeneration in the PNS (step 1)

injury occurs

• axon is damaged/torn during the injury

• proximal portion of the axon will seal itself off

  • prevents more damage from occurring

• the distal portion of the cell will degenerate

  • damaged portion of the axon is not attached anymore, with nothing keeping it alive

Axon Regeneration in the PNS (step 2)

clean-up

• macrophages invade area of damaged axon

  • dead/dying portion of neuron is destroyed

  • necessary to clear up space and would get in the way of regeneration

Axon Regeneration in the PNS (step 3)

axon regeneration begins

• schwann cells release growth factor to stimulate axon growth

  • proximal end of axon grows filaments

• schwann cells form a “tube”

  • allows for filaments to travel in the correct direction of regeneration

Axon Regeneration in the PNS (step 4)

completion of regeneration

• axon filaments continue to grow → forms a complete axon

• schwann cells form a new myelin sheath around a new axon

  • the new regenerated axon has a smaller diameter than the original

    • slight loss of function → slower sending of information

Cranial Nerves

12 pairs of cranial nerves extend from the brain

• serve head and neck structures

  • exception = the vagus nerve (X) extends into the abdomen

I: Olfactory Nerve

nerves associated with olfaction (smell)

• sensory nerve

II: Optic Nerve

nerves associated with vision

• sensory nerve

III: Oculomotor Nerve

supplies 4 of the 6 extrinsic eye muscles that move the eyeball

• motor nerve

IV: Trochlear Nerve

innervates extrinsic eye muscle that depressed eye and turns it laterally (called the superior oblique)

• motor nerve

V: Trigeminal Nerve

supplies sensory fibers to the face and motor fibers to the chewing muscles

• largest diameter

• mixed → sensory and motor nerve

VI: Abducens Nerve

controls extrinsic eye muscle that abducts the eye (the lateral rectus muscle)

• motor nerve)

VII: Facial Nerve

innervates muscle for facial expression, contributes to taste

• mixed → sensory and motor nerve

VIII: Vestibulocochlear Nerve

contributes to hearing and balance

• sensory nerve

IX: Glossopharyngeal Nerve

innervates tongue for taste and general senses, innervates pharynx for general senses, innervates muscles of pharynx for swallowing

• mixed → sensory and motor nerve

X: Vagus Nerve

fibers extend to and supply sensory and motor fibers to the organs of the thorax and abdomen

• mixed → sensory and motor nerve

XI: Spinal Accessory Nerve

supplies motor fibers to muscles that move the head and neck

• motor nerve

XII: Hypoglossal Nerve

innervates muscles of the tongue and under the tongue to allow movement of the tongue for chewing, speech, and swallowing

• “under the tongue”

• motor nerve

Spinal Nerves

31 pairs of spinal nerves branch from the spinal cord

• supply all parts of the body not served by the cranial nerves

• nerves connect to spinal cord via dorsal (sensory) and ventral (motor) roots

• spinal nerves divide to form dorsal ramus and ventral ramus

Dorsal Ramus

provides sensory and motor fibers to the skin and muscles of the back

Ventral Ramus

provides sensory and motor fibers to lateral and ventral body walls and to the upper and lower limbs

• branch to form nerve networks called nerve plexuses

• if one of the ventral rami is damaged → you still have 2 others and you don’t lose everything

How is a nerve plexus formed?

as ventral rami extend away from the spinal cord, they branch several times

• branches from neighboring rami join one another to form a “weaving network” of nerves

Effect of Nerve Plexus Formation

each branch of the plexus contains fibers from multiple spinal nerves

• fibers from each ramus travel to body via several routes

• 3 nerves plexuses in the body:

  • cervical plexus

  • brachial plexus

  • lumbosacral plexus

Cervical Plexus

cutaneous nerves that supply the neck, ear, back of the head, and shoulders

• mostly responsible for sending sensory information

• other branches serve skin to provide sensory information for the head, neck, shoulder, and clavicle regions

Brachial Plexus

provides fibers that supply the upper body limbs

• major branches of the brachial plexus:

  • median

  • ulnar

  • radial

• other branches supply muscle in chest, shoulders, and back for movement of arms and sensory information for same areas

Median Branch of Brachial Plexus

motor function to the arm, hand, and wrist

• sensory information from the lower arm and parts of the hand

Ulnar Branch of Brachial Plexus

motor function for hand muscles, flexion of wrist and fingers

• sensory information from the pinky and half of ring finger, medial side of hand and forearm

Radial Branch of Brachial Plexus

motor function for extension of wrist and phalanges

• sensory information from back of arm, hand, and fingers

Phrenic Branch of Cervical Plexus

motor and sensory fibers to diaphragm

• only branch in the cervical plexus that descends into the chest

Lumbosacral Plexus

lumbar plexus and sacral plexus have large degree of overlap

Lumbar Plexus

innervates parts of abdominal wall muscle, major branches innervate anterior and medial thigh

• major branches:

  • femoral

  • obturator

Femoral Branch of Lumbar Plexus

motor function and sensory function for anterior and medial portion of leg

Obturator Branch of Lumbar Plexus

motor function and sensory function for medial portion of leg

Sacral Plexus

innervates buttocks and posterior aspect of lower limbs, pelvis structures and perineum

• major branches

  • sciatic

    • tibial

    • common fibular

Sciatic Branch of Sacral Plexus

longest nerve in human body

• serves posterior and lateral portion of thigh for sensory and motor function

Tibial Branch of Sacral Plexus

motor and sensory function to posterior portion of leg and food

• allows pointing foot downward (plantar flexion)

Common Fibular Branch of Sacral Plexus

motor and sensory function anterior and lateral portion of lower leg and foot

• allows pointing of foot upwards (dorsiflexion)

Ventral Rami in Anterolateral Thorax and Abdominal Wall

DO NOT FORM PLEXUSES, but are arranged in segmental pattern

• serve intercostal muscles between ribs, skin of anterolateral thorax, and most of abdominal wall

Dorsal Rami in Anterolateral Thorax and Abdominal Wall

innervate posterior body trunk in segmental pattern

• each dorsal ramus innervates narrow strip of muscle and skin at same area where it emerges from spinal column

Skin (Dermatomes) in Anterolateral Thorax and Abdominal Wall

area of skin innervated by a single spinal nerve

• provides sensory details (pressure, temperature, pain, etc.) of the skin to the brain

Reflex Arc

response that enables rapid and predictable responses by the body

Intrinsic Reflex

unlearned, unpremeditated, and involuntary → “built-in” response

• rapid, predictable motor response to a stimulus

• can still be modified according to circumstance

• ex) making fast adjustments to body posture after losing balance

  • can stop it from happening → getting a shot or tattoo

Acquired Reflex

results from practice or repetition → “learned” responses

• ex) driving a car, riding a bike, etc.

Receptor (component of a reflex arc)

site of stimulus action

Sensory Neuron (component of a reflex arc)

transmits afferent impulses to the central nervous system

Integration Center (component of a reflex arc)

synapses found in the central nervous system

Motor Neuron (component of a reflex arc)

conducts efferent impulses from the integration center to the effector organ

Effector (component of a reflex arc)

muscle fiber or gland that response to the efferent impulse

Spinal Reflex

any somatic reflex that is mediated by the spinal cord

• most occur without any higher central nervous system involvement

• makes the brain aware that the reflex is occurring → without actually controlling it

• types of spinal reflexes

  • stretch reflex

  • tendon reflex

  • flexor and cross-extension reflexes

Stretch Reflex (spinal reflexes)

prevents damage to muscle and tendons by causing a muscle to contract when the muscle is stretched too far

• all stretch reflexes are:

  • monosynaptic

  • ipsilateral

Monosynaptic

have only a single synapse between the sensory neuron and a motor neuron

Ipsilateral

the stimulus and response occur on the same side of the body

Muscle Spindles (stretch reflex)

provide information about the length of a particular muscle

• function as proprioceptors

• when a muscle is stretched the muscle spindle is activated

  • ex) the knee-jerk reflex

Tendon Reflex

prevents damage to muscle tendon by causing a muscle to relax when the muscle contracts too hard

• tendon reflexes are:

  • polysynaptic

  • ipsilateral

Polysynaptic

involves multiple synapses between a sensory neuron, interneuron, and motor neuron

Tendon Organs (tendon reflex)

provide information about the amount of tension in a muscle and its associated tendons

• are proprioceptors, sending information faster when the tendon is overstretched

• when a muscle contracts, it pulls on its tendon and the tendon organ is activated

  • ex) excessive contraction of quadricep muscle

Flexor Reflex (withdrawal reflex)

usually initiated by panful stimuli

• causes automatic withdrawal of body part from the stimulus source

• nociceptors are responsible for this

• ipsilateral and polysynaptic

  • can be overridden by consciously prevent the reflex from occurring

Crossed-Extensor Reflex

often accompanies flexor reflex

• prevalent in weight-bearing limbs to maintain balance during a flexor reflex

• contralateral

  • ex) stepping on a Lego

    • the leg/foot that steps on the Lego has an ipsilateral withdrawal reflex → you pick your foot up off the floor

    • opposite leg has contralateral extensor reflex → you shift your weight to the opposite side of your body

Contralateral

the stimulus occurs on one side of the body and the response occurs on the opposite side