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The Peripheral Nervous System. Chapter 14. The Peripheral Nervous System (PNS). The nervous system outside the brain and spinal cord (CNS) Provides vital links to the body and outside world Nerves allow the CNS to receive information and initiate action

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the peripheral nervous system pns
The Peripheral Nervous System (PNS)
  • The nervous system outside the brain and spinal cord (CNS)
    • Provides vital links to the body and outside world
    • Nerves allow the CNS to receive information and initiate action
  • Sensory inputs and motor outputs categorized as
    • Somatic or visceral
    • General or special
  • Autonomic nervous system (ANS): general visceral motor part of the PNS
    • Two divisions: parasympathetic and sympathetic
functional organization of the pns

Central nervous system (CNS)

Peripheral nervous system (PNS)

Sensory (afferent) division

Motor (efferent) division

Somatic sensory

General: Touch, pain,

pressure, vibration,

temperature, and

proprioception in

skin, body wall, and

limbs

Visceral sensory

General: Stretch,

pain, temperature,

chemical changes,

and irritation in

viscera; nausea and

hunger

Somatic nervous

system

Autonomic

nervous system

(ANS)

Motor innervation of

all skeletal muscles

Motor innervation

of smooth muscle,

cardiac muscle,

and glands

Special: Hearing,

equilibrium, vision

Special: Taste, smell

Sympathetic

division

Parasympathetic

division

Functional Organization of the PNS

Figure 14.1

basic structural components of the pns
Basic Structural Components of the PNS
  • Sensory receptors: pick up stimuli from inside or outside the body
  • Nerves: bundles of peripheral axons
  • Ganglia: clusters of peripheral neuronal cell bodies
  • Motor endings: axon terminals of motor neurons
    • Innervate effectors (muscle fibers and glands)
peripheral sensory receptors
Peripheral Sensory Receptors
    • Structures that pick up sensory stimuli that initiate signals in sensory axons
  • General sensory receptors – widely distributed nerve endings of sensory neurons monitor:
    • Touch, Pressure, Vibration, Stretch, Pain, Temperature, Proprioception
  • Two main categories of sensory receptors
    • Free nerve endings of sensory neurons: monitor general sensory information
    • Complete receptor cells: specialized epithelial cells or small neurons that monitor most types of special sensory information
  • Sensory receptors also classified according to
    • Location, type of stimulus detected, and structure
classification by location
Classification by Location
  • Exteroceptors: sensitive to stimuli arising from outside the body
    • Located at or near body surfaces
    • Include receptors for touch, pressure, pain, and temperature
  • Interoceptorsreceive stimuli from internal viscera
    • Located in digestive tube, bladder, and lungs
    • Monitor a variety of stimuli: changes in chemical concentration, taste stimuli, stretching of tissues, temperature
  • Proprioceptors located in skeletal muscles, tendons, joints, and ligaments
    • Monitor degree of stretch
    • Send inputs on body movement to the CNS
classification by stimulus detected
Classification by Stimulus Detected
  • Mechanoreceptors: respond to mechanical forces
    • Touch, pressure, stretch, vibration, and itch
    • Baroreceptors monitor blood pressure
  • Thermoreceptors: respond to temperature changes
  • Chemoreceptors: respond to chemicals in solution
  • Photoreceptors: respond to light, located in the eye
  • Nociceptors: respond to harmful stimuli that result in pain
classification by structure
Classification by Structure
  • General sensory receptors divided into two groups:
    • Free nerve endings: abundant in epithelia and underlying connective tissue
      • Respond to pain and temperature
      • Monitor affective senses
    • Encapsulated nerve endings: consist of one or more end fibers of sensory neurons
      • Enclosed in connective tissue
structure of free and encapsulated general sensory receptors
Structure of free and encapsulated general sensory receptors

Tactile corpuscle

(touch, light pressure)

Epithelial tactile

complexes (light touch)

Free nerve

endings

(pain and

temperature)

Epidermis

Dermis and

hypodermis

Lamellar

corpuscle

(deep pressure)

Hair follicle receptor

(hair movement)

Bulbous corpuscle

(pressure)

free nerve endings
Free Nerve Endings
  • Two specialized types of free nerve endings
    • Epithelial tactile complexes (Merkel discs): consist of tactile epithelial cell innervated by sensory nerve ending
      • Slowly adapting receptors for light touch
    • Hair follicle receptors: wrap around hair follicles, rapidly adapting receptors
unencapsulated nerve endings
Unencapsulated Nerve Endings

Table 14.1 (1 of 4)

encapsulated nerve endings
Encapsulated Nerve Endings
  • Mechanoreceptors include four main types
    • Tactile (Meissner’s) corpuscles
    • Lamellar (Pacinian) corpuscles
    • Bulbous corpuscles (Ruffini endings)
    • Proprioceptors
tactile corpuscles
Tactile Corpuscles
  • Tactile (Meissner’s) corpuscles
    • Spiraling nerve ending surrounded by Schwann cells
    • Occur in the dermal papillae
    • Rapidly adapting receptors for discriminative touch
    • Occur in sensitive, hairless areas of the skin

Table 14.1 (2 of 4)

encapsulated nerve endings1
Encapsulated Nerve Endings
  • Lamellar Corpuscles: single nerve ending surrounded by layers of flattened Schwann cells
    • Occur in the hypodermis
    • Sensitive to deep pressure—rapidly adapting receptors
  • Bulbous Corpuscles: located in the dermis and respond to pressure
    • Monitor continuous pressure on the skin—adapt slowly
  • Proprioceptors: monitor stretch in locomotory organs
    • Three types of proprioceptors
three types of proprioceptors
Three Types of Proprioceptors
  • Muscle spindles: measure the changing length of a muscle
    • Imbedded in the perimysium between muscle fascicles
  • Golgi tendon organs: located near the muscle-tendon junction
    • Monitor tension within tendons
  • Joint kinesthetic receptors: sensory nerve endings within the joint capsules
structure of proprioceptors

Secondary sensory

endings (type II fiber)

 Efferent (motor)

fiber to muscle spindle

 Efferent

(motor) fiber

to extrafusal

muscle fibers

Primary

sensory

endings

(type Ia

fiber)

Extrafusal

muscle

fiber

Muscle

spindle

Intrafusal

muscle

fibers

Connective

tissue capsule

Sensory

fiber

Tendon

organ

Tendon

Structure of Proprioceptors

Figure 14.3

proprioceptors
Proprioceptors

Table 14.1 (4 of 4)

cranial nerves
Cranial Nerves
  • Attach to the brain and pass through foramina of the skull
  • Numbered from I–XII
  • Cranial nerves I and II attach to the forebrain
    • All others attach to the brain stem
  • Primarily serve head and neck structures
    • The vagus nerve (X) is the only cranial nerve that extends into the abdomen
the cranial nerves

Filaments of

olfactory nerve (I)

Frontal lobe

Olfactory bulb

Olfactory tract

Optic nerve (II)

Temporal lobe

Optic chiasma

Optic tract

Oculomotor

nerve (III)

Infundibulum

Trochlear

nerve (IV)

Facial nerve (VII)

Vestibulocochlear

nerve (VIII)

Trigeminal

nerve (V)

Abducens

nerve (VI)

Glossopharyngeal

nerve (IX)

Vagus nerve (X)

Cerebellum

Accessory nerve (XI)

Medulla oblongata

Hypoglossal nerve (XII)

The Cranial Nerves

Figure 14.4a

the cranial nerves1

Cranial nerves

Sensory function

Motor function

Somatic

sensory

(SS)

Visceral

sensory

(VS)

Somatic

motor

(SM)

Visceral motor:

parasympathetic

(VM)

I

Olfactory

Smell

II

Optic

Vision

III

Oculomotor

SM

VM

IV

Trochlear

SM

General

V

Trigeminal

SM

VI

Abducens

SM

(b)

The Cranial Nerves

Figure 14.4b (1 of 2)

the cranial nerves2

Cranial nerves

Sensory function

Motor function

Somatic

sensory

(SS)

Visceral

sensory

(VS)

Somatic

motor

(SM)

Visceral motor:

parasympathetic

(VM)

VII

Facial

General

General;

taste

SM

VM

VIII

Vestibulocochlear

Hearing;

equilibrium

Some

IX

Glossopharyngeal

General

General;

taste

SM

VM

X

Vagus

General

General;

taste

SM

VM

XI

Accessory

SM

XII

Hypoglossal

SM

(b)

The Cranial Nerves

Figure 14.4b (2 of 2)

olfactory nerves
Olfactory Nerves
  • Sensory nerves of smell

Table 14.2 (1 of 12)

ii the optic nerves
II The Optic Nerves
  • Sensory nerve of vision

Table 14.2 (2 of 12)

iii the oculomotor nerves
III The Oculomotor Nerves
  • Innervates four of the extrinsic eye muscles

Table 14.2 (3 of 12)

iv the trochlear nerves
IV The Trochlear Nerves
  • Innervates the superior oblique muscle (an extrinsic eye muscle)

Table 14.2 (4 of 12)

v the trigeminal nerves
V The Trigeminal Nerves

Table 14.2 (5 of 12)

the trigeminal nerves
The Trigeminal Nerves
  • Largest of the cranial nerves has three divisions
    • Ophthalmic division (V1)
    • Maxillary division (V2)
    • Mandibular division (V3)
  • Cell bodies of sensory neurons located in the trigeminal ganglion
  • Mandibular division contains motor fibers that innervate the chewing muscles
vi the abducens nerves
VI The Abducens Nerves
  • Abducts the eyeball—innervates lateral rectus muscle

Table 14.2 (6 of 12)

vii the facial nerves
VII The Facial Nerves
  • Innervates muscles of facial expression

Table 14.2 (7 of 12)

viii the vestibulocochlear nerves
VIII The Vestibulocochlear Nerves
  • Sensory nerve of hearing and balance

Table 14.2 (8 of 12)

ix the glossopharyngeal nerves
IX The Glossopharyngeal Nerves
  • Innervates structures of the tongue and pharynx

Table 14.2 (9 of 12)

x the vagus nerves
X The Vagus Nerves
  • A mixed sensory and motor nerve
    • “Wanders” into thorax and abdomen
    • Parasympathetic innervation of organs

Table 14.2 (10 of 12)

xi the accessory nerves
XI The Accessory Nerves
  • Unique among cranial nerves
  • Accessory nerves are formed from ventral rootlets of the spinal cord
  • Do not arise from the brainstem

Table 14.2 (11 of 12)

xii the hypoglossal nerves
XII The Hypoglossal Nerves
  • Runs inferior to the tongue innervates the tongue muscles

Table 14.2 (12 of 12)

spinal nerves
Spinal Nerves
  • 31 pairs—contain thousands of nerve fibers
  • Connect to the spinal cord
  • Named for point of issue from the spinal cord
    • 8 pairs of cervical nerves (C1–C8)
    • 12 pairs of thoracic nerves (T1–T12)
    • 5 pairs of lumbar nerves (L1–L5)
    • 5 pairs of sacral nerves (S1–S5)
    • 1 pair of coccygeal nerves (Co1)
spinal nerves posterior view

Cervical plexus

Cervical

nerves

C1 – C8

Brachial plexus

Cervical

enlargement

Thoracic

nerves

T1 – T12

Intercostal

nerves

Lumbar

enlargement

Lumbar

nerves

L1 – L5

Lumbar plexus

Sacral

nerves

S1 – S5

Sacral plexus

Coccygeal

nerve

Co1

Cauda equina

Spinal Nerves Posterior View

Figure 14.5

spinal nerves1
Spinal Nerves
  • Connect to the spinal cord by the dorsal root and ventral root
    • Dorsal root—contains sensory fibers
      • Cell bodies—located in the dorsal root ganglion
    • Ventral root—contains motor fibers arising from anterior gray column
  • Branch into dorsal ramus and ventral ramus
    • Dorsal and ventral rami contain sensory and motor fibers
    • Dorsal rami innervate back muscles follow a neat, segmented pattern
      • Innervate a horizontal strip of muscle and skin n iline with emergence point from the vertebral column
  • Rami communicantes connect to the base of the ventral ramus leading to the sympathetic chain ganglia
spinal nerves2

Sensory axon

and cell body

Dorsal root

ganglion

Dorsal root

Dorsal

ramus

Nerves

Spinal

nerve

Ventral

ramus

Ventral

root

Axon of

motor

neuron

Neuromuscular

junction

Sensory receptors in

skin (e.g., free nerve

endings of sensory

neuron)

Spinal Nerves

Figure 14.6

spinal nerves3
Spinal Nerves

White matter

Gray matter

Ventral root

Dorsal and ventral

rootlets of spinal

nerve

Dorsal root

Dorsal root

ganglion

Dorsal ramus

of spinal nerve

Ventral ramus

of spinal nerve

Spinal nerve

Rami communicantes

Sympathetic trunk

ganglion

(a) 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.

Figure 14.7a

innervation of the back

Dorsal ramus

Ventral ramus

Spinal nerve

Rami communicantes

Intercostal nerve

Sympathetic trunk

ganglion

Dorsal root ganglion

Dorsal root

Ventral root

Branches of intercostal nerve

Lateral cutaneous

Anterior

cutaneous

Sternum

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

Innervation of the Back
innervation of the anterior thoracic and abdominal wall
Innervation of the Anterior Thoracic and Abdominal Wall
  • Thoracic region
    • Ventral rami arranged in simple, segmented pattern
    • Intercostal nerves—supply intercostal muscles, skin, and abdominal wall
      • Each gives off lateral and anterior cutaneous branches
introduction to nerve plexuses
Introduction to Nerve Plexuses
  • Nerve plexus—a network of nerves
  • Ventral rami (except T2–T12)
    • Branch and join with one another
    • Form nerve plexuses
      • In cervical, brachial, lumbar, and sacral regions
    • Primarily serve the limbs
    • Fibers from ventral rami crisscross
the cervical plexus

Ventral rami

Segmental

branches

Hypoglossal

nerve (XII)

Ventral

rami:

Lesser occipital

nerve

C1

Greater auricular

nerve

C2

Transverse

cervical nerve

C3

Ansa cervicalis

C4

Accessory nerve (XI)

C5

Phrenic nerve

Supraclavicular

nerves

The Cervical Plexus
  • Buried deep in the neck under the sternocleidomastoid muscle
  • Formed by ventral rami of first four cervical nerves (cn 1–4)
  • Most are cutaneous nerves
  • Some innervate muscles of the anterior neck
  • Phrenic nerve—the most important nerve of the cervical plexus

Figure 14.8

the brachial plexus and innervation of the upper limb

Major terminal

branches

(peripheral nerves)

Roots

(ventral

rami)

Cords

Divisions

Trunks

Anterior

Musculocutaneous

C5

Upper

Lateral

Posterior

Median

C6

Medial

Anterior

Ulnar

C7

Middle

Posterior

Radial

C8

Posterior

Anterior

Lower

Axillary

T1

Posterior

(c) Flowchart summarizing relationships within the brachial plexus

The Brachial Plexus and Innervation of the Upper Limb
  • Brachial plexus lies in the neck and axilla
  • Formed by ventral rami of C5–C8
  • Cords give rise to main nerves of the upper limb

Figure 14.9c

nerves from the lateral and medial cords

Axillary nerve

Humerus

Radial nerve

Musculocutaneous nerve

Ulna

Radius

Ulnar nerve

Median nerve

Radial nerve (superficial branch)

Dorsal branch of ulnar nerve

Superficial branch of ulnar nerve

Digital branch of ulnar nerve

Muscular branch

Median nerve

(a) The major nerves

of the upper limb

Digital branch

Nerves from the Lateral and Medial Cords
  • Musculocutaneous—main branch of the lateral cord
    • Innervates the biceps brachii and brachialis
  • Median—originates from both lateral and medial cords
    • Innervates anterior forearm muscles and lateral palm
  • Ulnar—branches from the medial cord
    • Innervates intrinsic hand muscles and skin of the medial hand

Figure 14.9a

nerves from the lateral and medial cords1

Roots (ventral rami)

Anterior

divisions

C4

Dorsal scapular

Posterior

divisions

C5

Nerve to

subclavius

Trunks

C6

Suprascapular

Roots

Upper

C7

Posterior

divisions

Trunks

Middle

C8

Lateral

Lower

T1

Cords

Posterior

Long thoracic

Medial pectoral

Medial

Lateral pectoral

Axillary

Upper

subscapular

Musculo-

cutaneous

Lower

subscapular

Radial

Thoracodorsal

Median

Medial cutaneous

nerves of the arm

and forearm

Ulnar

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

Nerves from the Lateral and Medial Cords

Figure 14.9b

nerves from the lateral and medial cords2
Nerves from the Lateral and Medial Cords

Major terminal

branches

(peripheral nerves)

Roots

(ventral

rami)

Cords

Divisions

Trunks

Anterior

Musculocutaneous

C5

Upper

Lateral

Posterior

Median

C6

Medial

Anterior

Ulnar

C7

Middle

Posterior

Radial

C8

Posterior

Anterior

Lower

Axillary

T1

Posterior

(c) Flowchart summarizing relationships within the brachial plexus

Figure 14.9c

nerves from the posterior cord

Musculocutaneous nerve

Axillary nerve

Branches of axillary nerve

Radial nerve

Ulnar nerve (cut)

Median nerve (cut)

Posterior cutaneous nerve

Deep radial nerve

Superficial branch of radial nerve

Anterior

divisions

Posterior

divisions

Nerves from the Posterior Cord
  • Radial—continuation of the posterior cord
    • Largest branch of the brachial plexus
    • Innervates muscles of the posterior upper limb
  • Axillary innervates the deltoid and teres minor

Figure 14.12

axillary and radial nerves

Muscular innervation

Cutaneous innervation

Musculocutaneous

nerve

Median nerve

Ulnar nerve

C5

C5

C6

C6

C7

C7

C8

C8

T1

T1

Anterior view

Posterior view

Coraco-

brachialis

Biceps brachii

Brachialis

Medial cutaneous

nerves off

medial cord

Pronator teres

Flexor carpi radialis

Flexor

carpi

ulnaris

Palmaris longus

Flexor digitorum

superficialis

Flexor pollicis

longus

Musculocutaneous

nerve

Flexor digitorum

profundus

Pronator

quadratus

Adductor

pollicis

3 Thenar muscles

Ulnar nerve

3 Hypothenar

muscles

7 Interossei

Lumbricals to

digits 2,3

Lumbricals to

digits 4,5

Median nerve

Indicates variable contribution

Axillary and Radial Nerves

Figure 14.10

axillary and radial nerves1

Muscular innervation

Cutaneous innervation

Radial nerve

Axillary nerve

C5

C5

C6

C6

C7

C8

Anterior view

Posterior view

T1

Teres minor

Deltoid

Axillary

nerve

Axillary

nerve

Triceps brachii

(long head)

Triceps brachii

(lateral head)

Radial

nerve

Radial

nerve

Triceps brachii

(medial head)

Brachioradialis*

Extensor carpi radialis longus

Anconeus

Extensor carpi radialis brevis

Extensor digitorum

Extensor digiti minimi

Supinator

Extensor carpi ulnaris

Abductor pollicis longus

Extensor pollicis brevis

Extensor pollicis longus

Extensor indicis

Radial

nerve

Indicates variable contribution

*Functionally brachioradialis acts as a

flexor of the forearm. It develops from

the posterior compartment and thus is

innervated by a posterior division nerve.

Axillary and Radial Nerves

Figure 14.10 (continued)

the lumbar plexus and innervation of the lower limb
The Lumbar Plexus and Innervation of the Lower Limb
  • Lumbar plexus arises from L1– L4
    • Smaller branches innervate the posterior abdominal wall and psoas muscle
    • Main branches innervate the anterior thigh
      • Femoral nerve—innervates anterior thigh muscles
      • Obturator nerve—innervates adductor muscles
the lumbar plexus

Kidney

L3 vertebra

Ilioinguinal

nerve

Ureter

Iliohypogastric

Lateral femoral

cutaneous nerve

Ilioinguinal

Psoas major

Femoral

External iliac

artery

Lateral femoral

cutaneous

Femoral

nerve

Obturator

Urinary

bladder

Anterior femoral

cutaneous

Femoral

artery

Saphenous

(b) Nerves of the lumbar plexus, anterior view

(c) Distribution of the

major nerves from

the lumbar plexus

to the lower limb

The Lumbar Plexus

Ventral rami:

Ventral rami

L

1

Anterior division

Posterior

division

L

2

Iliohypogastric

Ilioinguinal

L

3

Genitofemoral

Lateral femoral

cutaneous

L

4

Obturator

L

5

Femoral

Lumbosacral

trunk

(a) Ventral rami and major branches of the

lumbar plexus

Figure 14.13

the sacral plexus
The Sacral Plexus
  • Arises from spinal nerves L4–S4 caudal to the lumbar plexus
  • Often considered with the lumbar plexus: Lumbosacral plexus
  • Sciatic nerve—the largest nerve of the sacral plexus
    • Actually two nerves in one sheath: tibial nerve—innervates most of the posterior lower limb and common fibular (peroneal) nerve—innervates muscles of the anterolateral leg
  • Superior and inferior gluteal nerves innervate the gluteal muscles
  • Pudendal nerve innervates muscles of the perineum
the sacral plexus1

Ventral rami:

Ventral rami

L4

Anterior division

Posterior division

Superior

gluteal

L5

Lumbosacral

trunk

S1

Inferior

gluteal

S2

Common

fibular

Tibial

S3

Posterior

femoral

cutaneous

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

S4

Pudendal

S5

Sciatic

Co1

Gluteus maximus

(medial portion

removed)

Piriformis

Inferior gluteal

nerve

Pudendal nerve

Greater trochanter

of femur

Common fibular

nerve

Tibial nerve

Posterior femoral

cutaneous nerve

(b) Dissection of the gluteal region, posterior view

Sciatic nerve

Ischial tuberosity

The Sacral Plexus

Superior gluteal

Inferior gluteal

Pudendal

Sciatic

Posterior femoral

cutaneous

Common fibular

Tibial

Sural (cut)

Deep fibular

Superficial fibular

Plantar branches

(c) Distribution of the major nerves from

the sacral plexus to the lower limb

Figure 14.14

the sacral plexus2

Muscular innervation

of medial thigh

Muscular innervation

of anterior thigh

Cutaneous innervation

Femoral nerve

Obturator nerve

Anterior view

Posterior view

L2

L2

L3

L3

L4

L4

Iliacus

Sartorius

Obturator

externus

Pectineus

Rectus femoris

Adductor brevis

Vastus

intermedius

Vastus lateralis

Adductor

longus

Vastus

medialis

Lateral

femoral

cutaneous

nerve

Obturator

nerve

Adductor

magnus

Gracilis

Femoral

nerve

The Sacral Plexus

Figure 14.15

the sacral plexus3

Muscular innervation of

posterior thigh and leg

Muscular innervation of

lateral and anterior leg

Cutaneous innervation

Anterior view

Tibial nerve

Common fibular nerve

Posterior view

L4

L5

S1

S2

S3

Sciatic nerve*

Biceps femoris

(long head)

Biceps femoris

(short head)

Semitendinosus

Posterior

femoral

cutaneous

nerve

Adductor magnus

Semimembranosus

Plantaris

Gastrocnemius

Common

fibular

nerve

Gastrocnemius

Popliteus

Soleus

Tibialis

anterior

Fibularis

longus

Tibialis

posterior

Flexor

digitorum

longus

Extensor

hallucis

longus

Extensor

digitorum

longus

Flexor

hallucis

longus

Fibularis

brevis

Tibial

nerve

Fibularis tertius

Abductor hallucis

Flexor digitorum brevis

Extensor

digitorum

brevis

Flexor hallucis brevis

All other muscles

of the sole

Lumbricals

*Sciatic nerve formed by tibial and common

fibular nerves wrapped by common sheath.

The Sacral Plexus

Figure 14.15 (continued)

innervation of the skin dermatomes

C2

C3

C4

C5

T1

T2

T3

T4

T2

T2

T5

T6

T7

T8

C6

C6

T9

T10

C5

C5

T11

T12

L1

L1

C6

C6

S2

C7

C7

C8

S3

C8

L2

L2

L3

L3

L4

L4

L5

L5

S1

S1

(a) Anterior

view

Innervation of the Skin: Dermatomes
  • Dermatome: an area of skin innervated by cutaneous branches of a single spinal nerve
  • Upper limb: skin supplied by nerves of the brachial plexus
  • Lower limb
    • Lumbar nerves—anterior surface
    • Sacral nerves—posterior surface
map of dermatomes posterior view

C2

C3

C4

C5

C6

C7

C8

T1

T2

T3

T4

T5

T6

T7

T8

T9

T10

C6

C6

T11

T12

C7

C7

L1

S1

L2

C8

C8

L3

S2

L5

L4

S3

S4

S5

S1

S2

S2

S1

L1

L2

L5

L5

L3

L4

(b) Posterior

view

L4

L4

L5

L5

S1

Map of Dermatomes—Posterior View

Figure 14.16b

disorders of the pns
Disorders of the PNS
  • Shingles (herpes zoster): viral infection stems from childhood chicken pox
    • Often brought on by stress mostly experienced by those over 50
  • Migraine headache relates to sensory innervation of cerebral arteries
    • Arteries dilate and compresses and irritate sensory nerve endings
  • Myasthenia gravis progressive weakening of the skeletal muscles
    • An autoimmune disorder
    • Antibodies destroy acetylcholine receptors
the pns throughout life
The PNS Throughout Life
  • Spinal nerves form late in week 4
  • Each of the 31 pairs of spinal nerves:
    • Sends motor fibers to an individual myotome
    • Sends sensory fibers to the overlying band of skin
  • During week 5, nerves reach the organs they innervate
  • Embryonic muscles migrate to new locations
    • Some skin dermatomes become displaced
    • Muscles and skin always retain their original nerve supply
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