ANSWER THE ESSAYS AND THE BULLET (-) QUESTIONS

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APLECTURE21PeripheralNervousSystem.ppt

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Peripheral Nervous System

Spinal Cord, Spinal Nerves, & Somatic Reflexes

spinal cord

spinal nerves

somatic reflexes

Spinal cord

Spinal nerve rootlets

Spinal nerve

Subarachnoid space

Posterior median sulcus

Posterior root ganglion

Rib

Dura mater

Arachnoid mater

Epidural space

Vertebra (cut)

Functions of the Spinal Cord

conduction
bundles of fibers passing information up & down spinal cord, connecting different levels of the trunk with each other and with the brain

locomotion
walking involves repetitive, coordinated actions of several muscle groups
central pattern generators are pools of neurons providing control of flexors & extensors that cause alternating movements of the lower limbs

reflexes
involuntary, stereotyped responses to stimuli
involves brain, spinal cord and peripheral nerves

Surface Anatomy

spinal cord – nervous tissue cylinder arising from brainstem at

the skull’s foramen magnum

passes through the vertebral canal
Upper 2/3; average 1.8cm thick, 45cm long

gives rise to 31 pair of spinal nerves
first pair passes between the skull and C1
rest pass through intervertebral foramina

segment – part of the spinal cord supplied by each pair of

spinal nerves

Surface Anatomy

longitudinal grooves on anterior and posterior sides
anterior median fissure and posterior median sulcus

spinal cord regions: cervical, thoracic, lumbar, & sacral

two areas of the cord are thicker than elsewhere
cervical enlargement – nerves to upper limb
lumbar enlargement – nerves to pelvic region and lower limbs

medullary cone (conus medullaris) – cord tapers to a point inferior to lumbar enlargement

cauda equina – bundle of nerve roots occuping vertebral canal in

glower lumbar & sacral regions

Anatomy of Lower Spinal Cord

Spinal cord

Spinal nerve rootlets

Spinal nerve

Subarachnoid space

Posterior median sulcus

Posterior root ganglion

Rib

Dura mater

Arachnoid mater

Epidural space

Cauda equina

(a)

(b)

Col

T12

Cervical

enlargement

Dural

sheath

Subarachnoid

space

Lumbar

enlargement

Medullary

cone

Terminal

filum

Cervical

spinal

nerves

Thoracic

spinal

nerves

Lumbar

spinal

nerves

Sacral

spinal

nerves

Vertebra (cut)

Meninges of the Spinal Cord

meninges – 3 fibrous connective tissue membranes enclosing

brain & spinal cord

separate soft tissue of central nervous system from bones of cranium and vertebral canal

from superficial to deep –
dura mater- tough, collagenous membrane surrounded by epidural space filled w/ fat, blood vessels, & loose C.T.

arachnoid mater- epithelial layer with C.T. fibers lining dura mater;

subarachnoid space- filled w/ CSF fluid

pia mater- delicate, translucent membrane following spinal cord

contours

Meninges of Vertebra and Spinal Cord

Fat in epidural space

Spinous process of vertebra

Spinal nerve

Posterior root ganglion

Spinal cord

Denticulate ligament

Subarachnoid space

(a) Spinal cord and vertebra (cervical)

Posterior

Meninges:

Dura mater (dural sheath)

Arachnoid mater

Pia mater

Vertebral body

Cross-Sectional Anatomy of the Spinal Cord

central area of gray matter shaped like a butterfly & surrounded by white matter in 3 columns
gray matter - neuron cell bodies with little myelin
site of information processing – synaptic integration
white matter – abundantly myelinated axons
carry signals from one part of the CNS to another

Gray matter:

White matter:

Anterior median fissure

(b) Spinal cord and meninges (thoracic)

Posterior horn

Lateral column

Gray commissure

Anterior column

Central canal

Posterior column

Posterior root ganglion

Spinal nerve

Lateral horn

Anterior horn

Pia mater

Arachnoid mater

Meninges:

Dura mater (dural sheath)

Posterior root of spinal nerve

Posterior

median sulcus

Anterior root

of spinal nerve

c: Sarah Werning

Spinal Tracts

ascending tracts – carry sensory information up the spinal cord
descending tracts – carry motor information down the spinal cord
all nerve fibers in a given tract have a similar origin, destination, & function

Ascending

tracts

Descending

tracts

Posterior column:

Gracile fasciculus

Cuneate fasciculus

Posterior spinocerebellar tract

Anterior spinocerebellar tract

Anterolateral system

(containing

spinothalamic

and spinoreticular

tracts)

Anterior corticospinal tract

Lateral

corticospinal tract

Lateral reticulospinal tract

Tectospinal tract

Medial reticulospinal tract

Lateral vestibulospinal tract

Medial vestibulospinal tract

Ascending Tracts

ascending tracts carry sensory signals up the spinal cord

sensory signals travel across three neurons from origin in receptors to the destination in the sensory areas of the brain

first order neurons – detect stimulus and transmit signal to spinal cord or brainstem

second order neurons – continues to the thalamus at the upper end of the brainstem

third order neurons – carries the signal the rest of the way to the sensory region of the cerebral cortex

Major Ascending Tracts

gracile fasciculus

carries signals from midthoracic & lower body parts

to sense vibration, visceral pain, & proprioception from lower limbs

cuneate fasciculus

carries signals from upper limbs & up

the same type of sensory signals as gracile fasiculus

spinothalamic tract

carry signals for pain, pressure, temperature, light touch, tickle, and itch

spinoreticular tract

carries pain signals resulting from tissue injury

posterior (dorsal) and anterior (ventral) spinocerebellar tracts-carry proprioceptive signals from limbs and trunk up to the cerebellum

Descending Tracts

descending tracts - carry motor signals down the brainstem and spinal cord

involves two neurons

upper motor neuron originate in cerebral cortex or brainstem and terminates on a lower motor neuron

lower motor neuron in brainstem or spinal cord
axon of lower motor neuron leads the rest of the way to the muscle or other target organ

Descending Motor Tracts

corticospinal tracts- cerebral cortex
finely coordinated limb movements

tectospinal tract –midbrain (tectum)
reflex turning of head in response to sights & sounds

lateral & medial reticulospinal tract- reticular formation brainstem
controls muscles of upper & lower limbs (posture & balance)
contain descending analgesic pathways

lateral & medial vestibulospinal tract- brainstem vestibular nuclei
begins in brainstem vestibular nuclei
receives impulses for balance from inner ear
controls extensor muscles of limbs for balance control

Poliomyelitis and ALS- both destroy motor neurons resulting
in skeletal muscle atrophy

poliomyelitis
caused by the poliovirus
destroys motor neurons in brainstem & anterior horn of spinal cord
signs of polio include muscle pain, weakness, and loss of some reflexes
followed by paralysis, muscular atrophy, and respiratory arrest

amyotrophic lateral sclerosis (ALS) – Lou Gehrig disease
destruction of motor neurons & muscular atrophy
also sclerosis (scarring) of lateral regions of the spinal cord
astrocytes fail to reabsorb the neurotransmitter glutamate from the tissue fluid (accumulates to toxic levels)
early signs- muscular weakness, difficulty speaking, swallowing, & use of hands
sensory & intellectual functions remain unaffected

Anatomy of a Nerve- spinal cord communicates w/ body
via spinal nerves

Rootlets

Posterior root

Epineurium

Fascicle

Myelin

Perineurium

Endoneurium

Unmyelinated nerve fibers

Myelinated nerve fibers

(a)

Posterior root

ganglion

root

Spinal

nerve

Blood

vessels

(b)

Fascicle

Epineurium

Perineurium

Endoneurium

Nerve

fiber

Blood

vessels

Classification of Nerve Fibers

sensory (afferent) nerves
carry signals from sensory receptors to the CNS

motor (efferent) nerves
carry signals from CNS to muscles and glands

mixed nerves
consists of both afferent and efferent fibers
conduct signals in two directions

both sensory and motor fibers can also be described as:
somatic or visceral
general or special

Anatomy of Ganglia in the PNS

ganglion - cluster of neurosomas outside the CNS
enveloped in an endoneurium continuous with that of the nerve

among neurosomas are bundles of nerve fibers leading into and out of the ganglion

posterior root ganglion associated with spinal nerves

Sensory nerve fibers

Posterior root

Anterior root

Spinal cord

Anterior root

Motor nerve fibers

To spinal cord

Spinal nerve

Posterior root ganglion

Epineurium

Blood vessels

Posterior root

ganglion

Sensory

pathway

Motor

pathway

Somatosensory

neurons

To peripheral

receptors and effectors

Direction of

signal transmission

Spinal Nerve Roots and Plexuses

Cervical nerves (8 pairs)

Cervical enlargement

Vertebra T1

Thoracic nerves (12 pairs)

Vertebra L1

Medullary cone

Lumbar nerves (5 pairs)

Cauda equina

Sacral nerves (5 pairs)

Coccygeal nerves (1 pair)

T10

T12

Cervical plexus (C1–C5)

Brachial plexus (C5–T1)

Lumbar plexus (L1–L4)

Sacral plexus (L4–S4)

Vertebra C1 (atlas)

Sciatic nerve

Lumbar enlargement

Intercostal (thoracic)

nerves (T1–T12)

Coccygeal plexus

(S4–Co1)

T11

Spinal Nerves

31 pairs of spinal nerves (mixed nerves)
8 cervical
12 thoracic
5 lumbar
5 sacral
1 coccygeal

proximal branches forming spinal nerves
each spinal nerve has two points of attachment to the spinal cord
posterior (dorsal) root is sensory input to spinal cord
anterior (ventral) root is motor output out of spinal cord
these merge to form spinal nerve proper that enters intervertebral foramen

13-*

Spinal Nerves

distal branches of spinal nerves

distal to vertebral foramen, the nerve divides into:

anterior ramus – innervates the anterior and lateral skin and muscles of the trunk
gives rise to nerves of the limbs

posterior ramus – innervates the muscles and joints in that region of the spine and the skin of the back

meningeal branch – reenters the vertebral canal and innervates the meninges, vertebrae and spinal ligaments

Spinal Nerves

Anterior root

Posterior root

Spinal nerve

Posterior root ganglion

Sympathetic ganglion

Communicating rami

Posterior ramus

Anterior ramus

Meningeal branch

Vertebral body

Posterior

Deep muscles of back

Spinal cord

Spinous process

of vertebra

Transverse process

of vertebra

Rami of Spinal Nerves

Anterior root

Posterior root

Posterior root ganglion

Spinal nerve

Communicating rami

(a) Anterolateral view

Thoracic cavity

Spinal nerve

Posterior ramus

Anterior ramus

Intercostal nerve

(b) Cross section

Posterior and anterior rootlets

of spinal nerve

Anterior ramus

of spinal nerve

Sympathetic chain

ganglion

Posterior ramus

of spinal nerve

Sympathetic

chain ganglion

Communicating

rami

Lateral

cutaneous nerve

Intercostal

muscles

cutaneous nerve

Nerve Plexuses

anterior rami branch forming five nerve plexuses:
cervical plexus in the neck
supplies neck and phrenic nerve to the diaphragm

brachial plexus near the shoulder
supplies upper limb and some of shoulder and neck
median nerve – carpal tunnel syndrome

lumbar plexus in the lower back
supplies abdominal wall, anterior thigh and genitalia

sacral plexus in the pelvis
supplies remainder of lower trunk and lower limb

coccygeal plexus

somatosensory function – carry sensory signals from bones, joints, muscles, skin, proprioception (body position)

motor function – primarily to stimulate muscle contraction

The Cervical Plexus

Lesser occipital nerve

Anterior root

Roots

Great auricular nerve

Posterior root

Supraclavicular nerves

Transverse cervical nerve

Phrenic nerve

Ansa cervicalis:

Hypoglossal

nerve (XII)

The Brachial Plexus

Axillary nerve

Radial nerve

Median nerve

Ulnar nerve

Ulna

Radius

Lateral cord

Posterior cord

Medial cord

Posterior scapular nerve

Suprascapular nerve

Posterior cord

Musculocutaneous nerve

Axillary nerve

Lateral cord

Radial nerve

Median nerve

Medial cord

Ulnar nerve

Roots

Trunks

Posterior divisions

Cords

Anterior divisions

Musculocutaneous

nerve

Digital branch

of median nerve

Digital branch

of ulnar nerve

Superficial branch

of ulnar nerve

Long thoracic

nerve

The Lumbar Plexus

Hip bone

Sacrum

Pudendal nerve

Femoral nerve

Sciatic nerve

Femur

Common fibular nerve

Tibia

Fibula

Superficial fibular nerve

Deep fibular nerve

Medial plantar nerve

Lateral plantar nerve

Tibial nerve

Anterior view

Posterior view

Ilioinguinal nerve

Genitofemoral nerve

Obturator nerve

Lateral femoral cutaneous nerve

Posterior divisions

Roots

Anterior divisions

From lumbar plexus

From sacral plexus

Femoral nerve

Obturator nerve

Lumbosacral trunk

Iliohypogastric

nerve

Tibial nerve

Sacral & Coccygeal Plexuses

Superior gluteal nerve

Inferior gluteal nerve

Pudendal nerve

Co1

Posterior divisions

Roots

Anterior divisions

Sciatic nerve:

Common fibular nerve

Lumbosacral

trunk

Tibial nerve

Posterior cutaneous

nerve

Spinal Nerve Disease

sciatic nerve injury
sciatica – sharp pain that travels from gluteal region along the posterior side of the thigh and leg to ankle
ninety percent of cases result from herniated intervertebral disc or osteoporosis of lower spine

chickenpox – varicella-zoster virus, itchy rash
virus dormant in posterior root ganglia
kept in check by the immune system

shingles (herpes zoster) –virus travels down sensory nerves w/

compromised immune system (age > 50)

painful trail of skin discoloration & fluid-filled vesicles along path of nerve; usually chest & waist

Nature of Reflexes- reactions of glands or muscle to
stimulation

four important properties of a reflex
require stimulation
quick
involuntary
stereotyped

include glandular secretion & contraction of all three types of muscle
include some learned responses – conditioned reflexes

unlearned skeletal muscle reflexes mediated by brainstem and spinal cord

somatic reflexes – since they involve the somatic nervous system

Pathway of reflex arc

somatic receptors in skin, muscles, or tendons

afferent nerve fibers
carry information from receptors to posterior horn of spinal cord or the brainstem

integrating center
synaptic contact point between neurons in the gray matter of spinal cord or brainstem
determines whether the efferent neurons issue a signal to muscles

efferent nerve fibers
carry motor impulses to skeletal muscle

skeletal muscles- somatic effectors carry out response

The Stretch Relex

stretch (myotatic) reflex - when a muscle is stretched, it ‘fights back’ and contracts which maintains increased tonus, making it stiffer than unstretched muscle
helps maintain equilibrium and posture
head starts to tip forward as you fall asleep; muscles contract to raise head
stabilize joints by balancing tension in extensors & flexors smoothing muscle actions

stretch reflex is mediated primarily by the brain (not strictly a spinal reflex)

tendon reflex – reflexive contraction of a muscle when its tendon is tapped
knee-jerk (patellar) reflex is monosynaptic reflex
one synapse between the afferent and efferent neurons
testing somatic reflexes helps diagnose many diseases

reciprocal inhibition - reflex phenomenon that prevents muscles from working against each other by inhibiting the antagonist

Patellar Tendon Reflex Arc

To brain

Primary afferent

neuron stimulates

alpha motor neuron

to extensor muscle

Primary afferent

neuron excited

Muscle spindle

stimulated

Extensor muscle

stretched

Flexor muscle

(antagonist) relaxes

Alpha motor neuron

stimulates extensor

muscle to contract

Interneuron inhibits

alpha motor neuron

to flexor muscle

Primary afferent

neuron stimulates

inhibitory interneuron

The Flexor (Withdrawal) Reflexes

flexor reflex – the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus

requires contraction of the flexors and relaxation of the extensors in that limb

polysynaptic reflex arc – pathway in which signals travel over many synapses on their way back to the muscle

Contralateral

motor neurons

to extensor

excited

Sensory neuron

activates multiple

interneurons

Ipsilateral motor

neurons to flexor

excited

Ipsilateral flexor

contracts

Contralateral

extensor

contracts

Stepping on glass

stimulates pain receptors

in right foot

Extension of left leg

(crossed extension reflex)

ithdrawal of right leg

(flexor reflex)

Spinal Cord Trauma

10,000 -12,000 people paralyzed each year by spinal cord trauma
usually by vertebral fractures

55% occur in automobile or motorcycle accidents

complete transection – complete severance of cord
immediate loss of motor control below level of injury
above C4 poses the threat of respiratory failure
spinal shock
paralysis
paraplegia – paralysis of both lower limbs
quadriplegia – paralysis of all four limbs
hemiplegia – paralysis on one side of the body
paresis – partial paralysis or weakness of the limbs