Chapter 13

The PNS

Peripheral Nervous System (PNS)

• All neural structures outside the brain

• Sensory receptors

• Peripheral nerves and associated ganglia

• Motor endings

Copyright © 2010 Pearson Education, Inc. Figure 13.1

Central nervous system (CNS) Peripheral nervous system (PNS)

Motor (efferent) divisionSensory (afferent)division

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision

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)

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

• 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

Copyright © 2010 Pearson Education, Inc. Figure 13.2

1

2

3

Receptor level(sensory receptionand transmissionto CNS)

Circuit level(processing inascending pathways)

Spinalcord

Cerebellum

Reticularformation

Pons

Musclespindle

Jointkinestheticreceptor

Free nerveendings (pain,cold, warmth)

Medulla

Perceptual level (processing incortical sensory centers)

Motorcortex

Somatosensorycortex

Thalamus

Processing at the Receptor Level

• In general sense receptors, the receptor potential and generator potential are the same thing

stimulus

receptor/generator potential in afferent neuron

action potential at first node of Ranvier

Processing at the Receptor Level

• In special sense organs:

stimulus

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

Processing at the Circuit Level

• Pathways of three neurons conduct sensory impulses upward to the appropriate brain regions

• 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)

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

Copyright © 2010 Pearson Education, Inc. Figure 13.3b

Bloodvessels

Fascicle

Epineurium

Perineurium

Endoneurium

AxonMyelin sheath

(b)

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

Ganglia

• Contain neuron cell bodies associated with nerves

• Dorsal root ganglia (sensory, somatic) (Chapter 12)

• Autonomic ganglia (motor, visceral) (Chapter 14)

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

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

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2

Table 13.2

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

Table 13.2

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

Table 13.2

Table 13.2

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 receptors

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2

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

Table 13.2

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)

Figure 13.6

CervicalnervesC1 – C8

ThoracicnervesT1 – T12

LumbarnervesL1 – L5

Sacral nervesS1 – S5

Coccygeal nerve Co1

Cervical plexus

Intercostalnerves

Cervicalenlargement

Lumbarenlargement

Cauda equina

Brachial plexus

Lumbar plexus

Sacral plexus

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

Figure 13.7 (a)

Dorsal rootganglion

Gray matterWhite matterVentral rootDorsal root

Dorsal andventral rootlets of spinal nerve

Dorsal ramusof spinal nerveVentral ramusof spinal nerve

Sympathetic trunkganglion

Spinal nerve

Rami communicantes

Anterior view showing spinal cord, associated nerves, and vertebrae. The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve.

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 region

Figure 13.7 (b)

Dorsal ramus

Ventral ramus

Intercostal nerve

Spinal nerve

Rami communicantes

Dorsal rootganglion Dorsal rootVentral root

Sympathetic trunkganglion

Sternum

(b) Cross section of thorax showing the main roots and branches of a spinal nerve.

Branches of intercostalnerve

• Lateral cutaneous• Anterior cutaneous

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)

Figure 13.8

Hypoglossalnerve (XII)

C1

C2

C3

C4

C5

Segmentalbranches

Lesser occipitalnerveGreater auricularnerve

Ansa cervicalis

Phrenic nerve

Supraclavicularnerves

Accessory nerve (XI)

Transversecervical nerve

Ventralrami:

Ventral rami

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

Figure 13.9 (a)

Upper

Middle Trunks

Lower

Roots (ventral rami):

Upper subscapular

Lower subscapular

Thoracodorsal

Medial cutaneousnerves of the armand forearm

Long thoracic

Medial pectoral

Lateral pectoral

Nerve tosubclaviusSuprascapular

Dorsal scapular

Posteriordivisions

Anteriordivisions

Lateral

PosteriorCords

Medial

Axillary

Musculo-cutaneousRadial

Median

Ulnar

Posteriordivisions

Trunks Roots

C4

C5

C6

C7

C8

T1

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

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 forearm

• 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 limb

Figure 13.9 (c)

Median nerve

Musculocutaneous nerve

Radial nerveHumerus

Ulna

Ulnar nerveMedian nerve

Radius

Radial nerve (superficial branch)

Superficial branch of ulnar nerveDorsal branch of ulnar nerve

Digital branch of ulnar nerveMuscular branchDigital branch

(c) The major nerves of the upper limb

Axillarynerve

Anteriordivisions

Posteriordivisions

Trunks Roots

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 leg

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

Figure 13.10

(a) Ventral rami and major branches of the lumbar plexus

Iliohypogastric

L1

L2

L3

L4

L5

Ilioinguinal

Genitofemoral

Lateral femoralcutaneous

Obturator

Femoral

Lumbosacraltrunk

Lateral femoralcutaneous

Anterior femoralcutaneousSaphenous

Obturator

IliohypogastricIlioinguinalFemoral

Ventral rami Ventralrami:

(b) Distribution of the major nerves from the lumbar plexus to the lower limb

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

Figure 13.11 (a)

SuperiorglutealLumbosacraltrunkInferiorgluteal

CommonfibularTibialPosteriorfemoralcutaneousPudendal

Sciatic

Ventral rami and major branches of the sacral plexus

L4

L5

S1

S2

S3

S4

S5

Co1

Ventral rami Ventral rami:

Figure 13.11 (b)

Superior gluteal

Inferior gluteal

Common fibular

Deep fibular

Superficial fibular

Plantar branches

Tibial

Sural (cut)

Posterior femoralcutaneous

Pudendal

Sciatic

(b) Distribution of the major nerves from the sacral plexus to the lower limb

Reflexes

• 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 organ

5. Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

Figure 13.14

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

1

2

3

4

5

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

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