PNSChapter 13

Peripheral Nervous System (PNS)

• Provides links from and to world outside body• All neural structures outside brain

– Sensory receptors– Peripheral nerves and associated ganglia– Efferent motor endings

Sensory Receptors

• Specialized to respond to changes in environment (stimuli)

• Activation results in graded potentials that trigger nerve impulses

• Sensation (awareness of stimulus) and perception (interpretation of meaning of stimulus) occur in brain

Classification of Receptors

• Based on– Type of stimulus they detect– Location in body– Structural complexity

Classification by Stimulus Type

• Mechanoreceptors—respond to touch, pressure, vibration, and stretch

• 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

• Exteroceptors– Respond to stimuli arising outside body– Most special sense organs

• Interoceptors (visceroceptors)– Respond to stimuli arising in internal viscera and

blood vessels• Proprioceptors

– Inform brain of body movements

Classification by Receptor Structure

• General senses (simple receptors)– Tactile sensations (touch, pressure, stretch,

vibration), temperature, pain, and muscle sense– Modified dendritic endings of sensory neurons

• Non-encapsulated (free nerve endings)• Encapsulated

• Special senses– Vision, hearing, equilibrium, smell, and taste

(Chapter 15)

Sensory Integration• Somatosensory system – part of sensory

system serving body wall and limbs– Receives inputs from Exteroceptors,

proprioceptors, and interoceptors– Input relayed toward head, but processed

along way– Levels of neural integration in sensory

systems:1. Receptor level—sensory receptors2. Circuit level—processing in ascending pathways3. Perceptual level—processing in cortical sensory areas

Adaptation of Sensory Receptors

• Adaptation is change in sensitivity in presence of constant stimulus– Phasic (fast-adapting) receptors signal beginning

or end of stimulus• Examples - receptors for pressure, touch, and smell

– Tonic receptors adapt slowly or not at all• Examples - nociceptors and most proprioceptors

Processing at the Perceptual Level

• Interpretation of sensory input depends on specific location of target neurons in sensory cortex

• Aspects of sensory perception:– Perceptual detection—ability to detect a stimulus

(requires summation of impulses)– Magnitude estimation—intensity coded in

frequency of impulses– Spatial discrimination—identifying site or pattern of

stimulus (studied by two-point discrimination test)

Perception of Pain• Warns of actual or impending tissue damage

protective action• Impulses travel on fibers that release

neurotransmitters glutamate and substance P• Some pain impulses are blocked by inhibitory

endogenous opioids (e.g., endorphins) • All perceive pain at same stimulus intensity, but pain

tolerance varies– "Sensitive to pain" means low pain tolerance, not low pain

threshold– Pain tolerance, and response to pain medication, is rooted

in genetics.

Homeostatic Imbalance

• Hyperalgesia (pain amplification), – NMDA receptors-allow spinal cord to "learn"

hyperalgesia• Phantom limb pain – felt in limb no longer

present– epidural use during anesthesia to prevent

Visceral and Referred Pain

• Stimulation of visceral organ receptors– Felt as vague aching, gnawing,

burning– Activated by tissue stretching,

ischemia, chemicals, muscle spasms

• Referred pain– Pain from one body region

perceived from different region – Visceral and somatic pain fibers

travel in same nerves; brain assumes stimulus from common (somatic) region

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?

• Peripheral nerves classified as cranial or spinal nerves

• Ganglia– Contain neuron cell bodies associated with nerves

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 regeneration

Figure 13.4 (1 of 4)

Endoneurium

Dropletsof myelin

Fragmentedaxon

Schwann cells

Site of nerve damage

The axonbecomesfragmented atthe injury site.

1

Figure 13.4 (2 of 4)

Schwann cell Macrophage Macrophagesclean out thedead axon distalto the injury.

2

Figure 13.4 (3 of 4)

Fine axon sproutsor filaments

Aligning Schwann cellsform regeneration tube

3 Axon sprouts,or filaments,grow through aregeneration tubeformed bySchwann cells.

Figure 13.4 (4 of 4)

Schwann cell Site of newmyelin sheathformation

4 The axonregenerates anda new myelinsheath forms.

Single enlargingaxon filament

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”

Figure 13.5 (a)

Frontal lobe

Temporal lobe

InfundibulumFacialnerve (VII)Vestibulo-cochlearnerve (VIII)Glossopharyngealnerve (IX)Vagus nerve (X)Accessory nerve (XI)

Hypoglossal nerve (XII)

(a)

Filaments ofolfactory nerve (I)

Olfactory bulb

Olfactory tract

Optic chiasma

Optic nerve(II)

Optic tractOculomotornerve (III)Trochlearnerve (IV) Trigeminalnerve (V) Abducensnerve (VI)CerebellumMedullaoblongata

Figure 13.5 (b)

*PS = parasympathetic(b)

Cranial nervesI – VI

I

II

III

IV

V

VI

Olfactory

Optic

Oculomotor

Trochlear

Trigeminal

Abducens

Yes (smell)

Yes (vision)

No

No

Yes (generalsensation)

No

No

No

Yes

Yes

Yes

Yes

No

No

Yes

No

No

No

Cranial nervesVII – XII

Sensoryfunction

Motorfunction

PS*fibers

Sensoryfunction

Motorfunction

PS*fibers

VII

VIII

IX

X

XI

XII

Facial

Vestibulocochlear

Glossopharyngeal

Vagus

Accessory

Hypoglossal

Yes (taste)

Yes (hearingand balance)

Yes (taste)

Yes (taste)

No

No

Yes

Some

Yes

Yes

Yes

Yes

Yes

No

Yes

Yes

No

No

Table 13.2

I: The Olfactory Nerves

• Purely sensory (olfactory) function• Afferent impulses for sense of smell• Injury diagnosis: partial or total loss of smell

(anosmia)

Table 13.2

II: The Optic Nerves

• Pass through the optic canals, converge and partially cross over at the optic chiasma

• Purely sensory (visual) function• Injury diagnosis: blindness

or partial loss of vision in affected eye

Table 13.2

III: The Oculomotor Nerves

• Functions in raising the eyelid, directing the eyeball, constricting the iris (parasympathetic), and controlling lens shape

• Injury: eye cannot be moved (up, down, inward) or rotates laterally when at rest, upper eyelid droops, double vision, and difficulty focusing on close objects

Table 13.2

IV: The Trochlear Nerves

• Primarily a motor nerve that directs the eyeball

• Injury: double vision, reduced ability to rotate eye inferolaterally

• Tested w/ III

V: The Trigeminal Nerves• Three divisions

– Ophthalmic (V1)– Maxillary (V2) – Mandibular (V3) – Convey sensory impulses from various

areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

• Injury: produces excruciating pain for a few seconds to a minute, recurring many times a day and by various causes (brushing teeth, breeze hitting the face)

VI: The Abducens Nerves

• Primarily a motor nerve, innervating the lateral rectus muscle

• Injury: eye cannot be moved laterally, at rest eyeball rotates medially

VII: The Facial Nerves

• Chief motor nerves of the face with 5 major branches– Temporal, zygomatic, buccal,

mandibular, cervical

• Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands

• Sensory function (taste) from the anterior two-thirds of the tongue

• Injury: Bell’s palsy

VIII: The Vestibulocochlear Nerves

• Mostly sensory function; small motor component for adjustment of sensitivity of receptors

• Injury: – Cochlear: deafness– Vestibular: dizziness,

rapid involuntary eye movements, loss of balance, nausea, vomiting

IX: The Glossopharyngeal Nerves• 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

• Injury: Impaired swallowing or taste

X: The Vagus Nerves

• 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

• Injury: Hoarseness or loss of voice, difficulty swallowing, impaired digestive system motility.

• Total destruction is incompatible with life

X: The Vagus Nerves

XI: The Accessory Nerves

• 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

• Injury: head turns toward side of injury, shrugging of that shoulder difficult

XII: The Hypoglossal Nerves

• Innervate extrinsic and intrinsic muscles of the tongue that contribute to swallowing and speech

• Injury: difficulties in speech/swallowing, problems with the tongue.

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

– Dorsal roots• Contain sensory (afferent) fibers from sensory neurons

in the dorsal root ganglia

• Dorsal + Ventral = spinal nerves

Spinal Nerves: Rami

• Each spinal nerve branches into mixed rami– Dorsal ramus– Ventral ramus - larger– Meningeal branch – tiny, reenters vertebral canal,

innervates meninges and blood vessels– Rami communicantes (autonomic pathways) join

ventral rami in thoracic region

Spinal Nerves: Rami

• All ventral rami (except T2–T12) form nerve plexuses (cervical, brachial, lumbar, and sacral)

• Back innervated by dorsal rami via several branches• Ventral rami of T2–T12 as intercostal nerves supply

muscles of ribs, anterolateral thorax, and abdominal wall

• Spinal roots get longer as move inferiorly in cord– Lumbar and sacral roots extend as cauda equina

Spinal Nerves: Plexuses

• Within plexus fibers criss-cross– Each branch contains fibers from several spinal

nerves– Fibers from as single ventral ramus go to body

periphery via several routes• Each limb muscle innervated by more than one spinal

nerve

Cervical Plexus and the Neck• Formed by ventral rami of C1–C4

• Most branches form cutaneous nerves– Innervate skin of neck, ear, back of head, and

shoulders– Other branches innervate neck muscles

Phrenic nerve• Major motor and sensory

nerve of diaphragm (receives fibers from C3–C5)

• Irritation hiccups

Brachial Plexus and Upper Limb

• Formed by ventral rami of C5–C8 and T1 (and often C4 and/or T2)

• Gives rise to nerves that innervate upper limb

© 2013 Pearson Education, Inc.

Anteriordivisions Roots (ventral rami):

Posteriordivisions

Trunks Roots

Dorsal scapular

Nerve tosubclaviusSuprascapular

Posteriordivisions

Lateral

Posterior

Medial

Axillary

Musculo-cutaneous

Radial

Median

Ulnar

Upper

Middle

Lower

Long thoracic

Medial pectoral

Lateral pectoral

Upper subscapular

Lower subscapular

Thoracodorsal

Medial cutaneousnerves of the armand forearm

C4

C5

C6

C7

C8

T1

Trunks

Roots (rami C5–T1), trunks, divisions, and cords

Cords

Figure 13.10a The brachial plexus.

Brachial Plexus: Five Important Nerves

• Axillary• Musculocutaneous• Median• Ulnar• Radial

Lumbar Plexus

• Arises from L1–L4

• Innervates thigh, abdominal wall, and psoas muscle

• Femoral nerve—innervates quadriceps and skin of anterior thigh and medial surface of leg

• Obturator nerve—passes through obturator foramen to innervate adductor muscles

Figure 13.11 The lumbar plexus.

Ventral rami

Iliohypogastric

Ilioinguinal

Genitofemoral

Lateral femoralcutaneous

Obturator

Femoral

Lumbosacraltrunk

IliohypogastricIlioinguinal

Femoral

Lateralfemoralcutaneous

Obturator

Anteriorfemoralcutaneous

Saphenous

Ventralrami:

Ventral rami and major branches of the lumbar plexus

Distribution of the major nerves from thelumbar plexus to the lower limb

L1

L2

L3

L4

L5

Sacral Plexus

• Arises from L4–S4

• Serves the buttock, lower limb, pelvic structures, and perineum

• Sciatic nerve– Longest and thickest nerve of body– Innervates hamstring muscles, adductor magnus,

and most muscles in 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 nerve

• All spinal nerves except C1 participate in dermatomes

• Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness

Figure 13.14

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

1

2

3

4

5

Reflex Arc

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 system

Stretch and Golgi Tendon Reflexes

• For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary – 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 tendons

Stretch Reflexes

• Maintain muscle tone in large postural muscles

• Cause muscle contraction in response to increased muscle length (stretch)

• How a stretch reflex works:– Stretch activates the muscle spindle– sensory neurons synapse directly with motor

neurons in the spinal cord– motor neurons cause the stretched muscle to

contract

Stretch Reflexes

• All stretch reflexes are monosynaptic and ipsilateral

• Reciprocal inhibition also occurs– 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

Figure 13.17 (1 of 2)

Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist.

When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.

The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.

The events by which muscle stretch is damped

Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.

Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).

Initial stimulus(muscle stretch)

Cell body ofsensory neuron

Sensoryneuron

Muscle spindleAntagonist muscle

Spinal cord

12

3a 3b

Figure 13.17 (2 of 2)

The patellar (knee-jerk) reflex—a specific example of a stretch reflex

Musclespindle

Quadriceps(extensors)

Hamstrings(flexors)

Patella

Patellarligament

Spinal cord(L2–L4)

Tapping the patellar ligament excitesmuscle spindles in the quadriceps.

The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.

Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.

The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.

Excitatory synapseInhibitory synapse

+

–

1

2

3a

3b

1

2

3a3b 3b

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 tension

Figure 13.18

+ Excitatory synapse– Inhibitory synapse

Quadriceps strongly contracts. Golgi tendon organs are activated.

Afferent fibers synapse with interneurons in the spinal cord.

Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.

Efferent impulses to antagonist muscle cause it to contract.

Interneurons

Spinal cord

Quadriceps(extensors)

Golgitendon

organHamstrings

(flexors)

1 2

3a 3b

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

Figure 13.19

Afferentfiber

Efferentfibers

Extensorinhibited

Flexorstimulated

Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.

Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.

Armmovements

Interneurons

Efferentfibers

FlexorinhibitedExtensorstimulated

+ Excitatory synapse– Inhibitory synapse