Download The Central Nervous System

The Peripheral Nervous System
Chapter 13
Introduction
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The CNS would be useless without a
means of sensing our own internal as well
as the external environments
In addition, we need a means by which
we can effect our external environment
The peripheral nervous system provides
these links to the CNS
Introduction

The peripheral nervous system includes
all the neural structures outside the brain
and spinal cord
– Sensory receptors
– Peripheral nerves and their ganglia
– Efferent motor endings
Introduction

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Basic components of
the PNS
Sensory components
provide the
information
interpreted by the
CNS
Motor components
stimulate the
effectors of the CNS
The CNS commands;
the PNS acts
Sensory Receptors

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Sensory receptors are structures that are
specialized to respond to changes in their
environment
Such environmental changes are called
stimuli
Typically activation of a sensory receptor
by an adequate stimulus results in
depolarization or graded potentials that
trigger nerve impulses along the afferent
fibers coursing to the CNS
Sensory Receptors

Sensory receptors are classified by
– The type of stimulus they detect
– Their location in the body
– The relative complexity of their structure
Classification by Stimulus Detected

Mechanoreceptors
– general nerve impulses when they, or adjacent
tissues, are deformed by mechanical forces
•
•
•
•

Touch
Pressure
Vibration
Stretch
Thermoreceptors
– Sensitive to temperature changes
Classification by Stimulus Detected

Photoreceptors
– Respond to light energy

Chemoreceptors
– Respond to chemicals in solution
• Smell
• Taste
• Blood chemistry

Nociceptors
– Respond to potentially damaging stimuli that
result in pain
Classification by Stimulus Detected

Note that the overstimulation of any of the
aforementioned receptors is painful and
thus virtually all receptors function as
nociceptors at one time or another
Classification by Location

Receptors are recognized according to
their location or the location of the stimuli
to which they respond
– Externoceptors
– Internoceptors or visceroceptors
– Proprioceptors
Classification by Location

Externoceptors
– Sensitive to stimuli arising from outside of the
body
– Typically located near the surface of the body
– Include receptors for
•
•
•
•
•
Touch
Pressure
Pain
Temperature
Special sense receptors
Classification by Location

Internoceptors or visceroceptors
– Respond to stimuli arising from within the
body organs, internal viscera and blood
vessels
– Monitor a variety of internal stimuli
•
•
•
•
Feel pain
Discomfort
Hunger
Thirst
– However, we are usually unaware of the
working of our internal visceroceptors
Classification by Location

Proprioceptors
– Respond to internal stimuli in skeletal
muscles, tendons, joints, ligaments, and
connective tissue coverings of bones and
muscles
• Monitor the degree of stretch of the tissue in which
they are located
Classification by Structure

Based on structural complexity there
simple and complex receptors
– Simple are equivalent to modified dendritic
endings of sensory neurons
• Found in skin, mucous membranes, muscles and
connective tissue
– Monitor general sensory information
– Complex receptors are associated with the
special senses
• Located in the special sensory organs
– Specific sensory information (sight, hearing, etc)
General Sense Organs

The widely distributed general sensory
receptors are involved in
– Tactile sensation
• Touch / pressure / stretch / vibration
– Temperature monitoring
• Heat / cold
– Pain
– Muscle stretch
Free Dendritic Endings
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Simple, widely dispersed receptors
present everywhere in the body
Particularly abundant in epithelia and
connective tissue
Most are unmyelinated
Free Nerve Endings
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Free nerve endings
have small knoblike
swellings
Respond to pain,
temperature, and
possible mechanical
pressure
Merkel Cells

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Merkel cells attach
to the deeper layers
of the skin
epidermis
Functions as light
touch receptors
Root Hair Plexuses


Root hair plexuses
are free dendritic
endings that
entwine the hair
follicles
They function as
light touch
receptors that detect
slight bending of
hairs
Root Hair
Plexus
Encapsulated Dendritic Endings



Consist of one or more terminal fibers of
sensory neurons enclosed in a connective
tissue capsule
Virtually all are mechanoreceptors
Vary greatly in shape, size, and
distribution in the body
Messner’s Corpuscles



Small receptors in
which a few
spiraling dendrites
are surrounded by
Schwann’s cells and
then by an egg
shaped sheath of
connective tissue
Found in the dermal
papillae of sensitive
areas of the body
Light touch, tactile
receptor
Krause End Bulbs
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
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A variation of
Meissner’s
corpuscles
Receptors for
discriminative touch
Most abundant in
mucous membranes
Pacinian Corpuscles
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Pacinian corpuscles
are scattered deep in
the dermis, and in
the subcutaneous
tissue under the skin
It contains a single
dendrite surrounded
by up to 60 layers of
Schwann cells and is
enclosed by connective tissue
Respond to pressure
Rapidly adapting
Ruffini’s Corpuscles



Found in the dermis
subcutaneous tissue,
and joint capsules
Contains a spray of
dendritic endings
enclosed by a
flattened capsule
Monitor deep and
continuous pressure
Muscle Spindles


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Proprioceptors found
throughout skeletal
muscle
Each muscle spindle
consists of a bundle of
modified skeletal muscle
fibers called intrafusal
fibers enclosed in a
connective tissue capsule
The infrafusal fibers
detect when a muscle is
stretched and initiate a
reflex to resist the stretch
Golgi Tendon Organs



Proprioceptors located in
tendons, close to the
point of skeletal muscle
insertion
They consist of small
bundles of tendon fibers
enclosed in a layered
capsule with dendrites
coiling around the fibers
When GTO activate, the
contracting muscle is
inhibited, which causes it
to relax
Joint Kinesthetic Receptors
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Proprioceptors that monitor stretch in
the capsules of synovial joints
Includes Pacinian and Ruffini corpuscles,
free dendritic endings and receptors
resembling Golgi tendon organs
Joint position and motion
Sensory Receptor Potentials
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Sensory stimuli reaches us as many
different forms of energy
Sensory receptors associated with sensory
neurons convert the energy of the
stimulus into electrical energy
The energy changes the action potential of
the receptor
Action potential are generated as long as
the stimulus is applied
Stimulus strength is determined by the
frequency of impulse transmission
Adaptation of Sensory Receptors
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Adaptation occurs in certain sensory
receptors when they are subjected to an
unchanging stimulus
As a result, the receptor potentials
decline in frequency or stop
Some receptors adapt quickly (pressure,
touch and smell)
Nocioceptors and proprioceptors adapt
slowly or not at all as they serve a
protective function
Nerves and Associated Ganglia

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A nerve is a cordlike organ
that is part of the peripheral
nervous system
Every nerve consists of
parallel bundles of peripheral
axons enclosed by successive
wrappings of connective tissue
Nerves and Associated Ganglia
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Within a nerve, each axon is
surrounded by a delicate layer
of loose connective tissue
called endoneurium
The endoneurium layer also
encloses the fiber’s associated
myelin sheath
Nerves and Associated Ganglia
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Groups of fibers are bound
into bundles or fascicles by a
courser connective tissue
wrapping called the
perineurium
All the fascicles are enclosed
by a tough fibrous sheath
called the epineurium to form
a nerve
Nerves and Associated Ganglia
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Neuron are actually only a
small fraction of the nerve
The balance is myelin, the
protective connective tissue
wrappings, blood vessels, and
lymphatic vessels
Nerves and Associated Ganglia

Nerves are classified according to the
direction in which they transmit impulses
– Nerves containing both sensory and motor
fibers are called mixed nerves
– Nerves that carry impulses toward the
CNS only as sensory (afferent) nerves
– Nerves that carry impulses only away from
the CNS are motor (efferent) nerves

Most nerves are mixed as purely sensory
or motor nerves are extremely rare
Nerves and Associated Ganglia

Since mixed nerves often carry both
somatic and autonomic (visceral) nervous
system fibers, the fibers within them may
be classified further according to the
region they innervate as
–
–
–
–
Somatic afferent
Somatic efferent
Visceral afferent
Visceral efferent
Nerves and Associated Ganglia

Peripheral nerves are generally classified
on whether they arise from the brain or
spinal cord as
– Cranial nerves / brain and brain stem
– Spinal nerves / spinal cord

Ganglia are collections of neuron cell
bodies associated with nerves in the PNS
– Ganglia associated with afferent nerve fibers
contain cell bodies of sensory neurons
– Ganglia associated with efferent nerve fibers
contain cell bodies of autonomic neurons, as
well as a variety of integrative neurons
Regeneration of Nerve Fibers
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Damage to nervous tissue is serious
because mature neurons do not divide
If the damage is severe or close to the cell
body, the entire neuron may die, and
other neurons that are normally
stimulated by its axon may die as well
However, in certain cases, cut or
compressed axons on peripheral nerves
can regenerate successfully
Regeneration of Nerve Fibers

Almost immediately
after a peripheral axon
has been cut, the
separated ends seal
themselves off and swell
as substances being
transported along the
axon begin to
accumulate
Regeneration of Nerve Fibers

Wallerian degeneration
spreads distally from the
injury site completely
fragmenting the axon
Regeneration of Nerve Fibers
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Macrophages that
migrate into the trauma
zone from adjacent
tissues, phagocytize the
disintegrating myelin
and axonal debris
Generally, the entire
axon distal to the injury
degrades within a week
However, the nucleus and
neurilemma remain
intact with the
endoneurium
Regeneration of Nerve Fibers
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Schwann cells then
proliferate and migrate
to the injury site
They release growth
factors that encourage
axon growth
Additionally, they form
cellular cords that guide
the regenerating axon to
their original contacts
Regeneration of Nerve Fibers

The same Schwann cells
then protect, support,
and remyelinate the
regenerating axons
Regeneration of Nerve Fibers
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
Axons regenerate at a rate of 1 to 5 mm a
day
The greater the distance between the
severed nerve endings, the greater the
time for regeneration
Greater distances also lessen the chance
of successful regeneration because
adjacent tissues often block growth by
protruding into larger gaps
Regeneration of Nerve Fibers
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CNS nerve fibers never regenerate under
normal circumstances
Brain and spinal cord damage is
considered as irreversible
The difference in regenerative capacity is
largely due to the support cells of the CNS
Macrophage invasion in the CNS is much
slower than in the PNS
Oligodendrocytes surrounding the
damaged axon die and thus cannot guide
axon regeneration and growth
Cranial Nerves
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Twelve pair of cranial nerves are
associated with the brain and pass through
various foramina of the skull
The first two attach to the forebrain, while
the rest originate from the brain stem
Cranial nerves serve only the head and
neck structures with the exception of the
vagus nerves
In most cases, the nerve are named for the
structures they serve or their primary
functions
Location of Cranial Nerves

The cranial nerves as they emerge from the
brain and spinal cord
Cranial Nerves
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The cranial nerves
are numbered from
the most rostal to
the most caudal
Some cranial nerves
are exclusively
sensory and others
are exclusively
motor and still
others are mixed
The differences are
due to the functions
the nerves serve
Olfactory Nerve: I
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Fibers arise from
olfactory epithelium
of nasal cavity
Synapse with
olfactory bulb which
extends as olfactory
tract
Purely sensory;
carries afferent
impulses for sense of
smell
Optic Nerves: II
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Fibers arise from
retina to form
sensory nerve
Converge to form
optic chiasma with
partial crossover
Enter thalamus and
synapse there
Thalamic fibers
runs as optic
radiation to visual
cortex for
interpretation
Oculomotor Nerve: III
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
Fibers extend
from midbrain
to eye
Mixed nerve
that contains a
few proprioceptors, but is
chiefly motor
Supplies four
of six extrinsic
muscles that
move the eye in
its orbit
Trochlear Nerves: IV
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Fibers emerge
from midbrain
to enter orbits
Mixed nerve;
primarily
motor
Innervates
extrinsic
muscles in the
orbit
Trigeninal Nerves: V
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
Extends from
pons to face
Forms three
divisions
– Ophthalmic
– Maxillary
– Mandibular

Mixed nerve
innervating the
face, forehead
and muscle of
mastication
Abducens Nerves: VI
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Fibers leave
inferior pons and
enter orbit to run
to eye
Mixed nerve; but
primarily motor
This nerve
controls the
extrinsic eye
muscles that
abduct the eye
(turn it laterally)
Facial Nerves: VII
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Fibers issue from the
pons, enters temporal
bone, emerges from
inner ear cavity to run
to the lateral aspect of
the face
Mixed nerve with five
major branches
– Temporal, zygomatic,
buccal, mandibular,
and cervical

Innervates muscles of
facial expression
Vestibulocochlear Nerves: VIII
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Fibers arise
from hearing
and equilibrum
apparatus to
enter brain
stem at pons
medulla border
Purely sensory
This nerve
provides for
hearing and
balance
Glossopharyngeal

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Fibers emerge
from medulla
and run to
throat
Mixed nerve
provide motor
control of
tongue and
pharynx
Sensory fibers
conduct taste
and general
sensory info
Vagus Nerves: X
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Fibers emerge from
medulla and descend
into neck, thorax and
abdomen
Mixed nerve; fibers are
parasympathetic except
those serving muscles of
pharynx and larynx
Parasympathetic fibers
supply heart, lungs,
abdominal viscera
Accessary Nerves: XI
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Unique in that it
is formed by
branches of
cranial and spinal
nerves
Mixed nerve, but
primarily motor
in function
supplying fibers
to innervate the
trapezius and
sternoclediomastoid
Hypoglossal Nerves: XII
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Fibers arise
from the
medulla to
travel to
tongue
Mixed nerve
but primarily
motor
Innervates
muscles that
move the
tongue
Distribution of Spinal Nerves
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There are 31 pairs of
spinal nerves each
containing thousands of
nerve fibers
All arise from the spinal
cord and supply all parts
of the body except the
head and neck
All are mixed nerves
Spinal nerves are named
according to where they
exit the spinal cord
Distribution of Spinal Nerves

The distribution of
spinal nerves
–
–
–
–
–
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Cervical (8)
Thoracic (12)
Lumbar (5)
Sacral (5)
Coccyx (1)
C1-C7 nerves exits
above the vertebrae
C8 and below nerves exit
below the vertebrae
Innervation of the Back
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Each
spinal
nerve
connects
to the
spinal
cord by
two roots
Each root
forms
from a
series of
rootlets
Innervation of the Back
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Ventral roots contain motor (efferent) fibers
Dorsal roots contain sensory (afferent) fibers
Innervation of the Back

The spinal root pass laterally from the cord, and unite
just distal to the dorsal root ganglion, to form a spinal
nerve before emerging from the vertebral column
Dorsal & ventral rami

A spinal nerve is
short (1-2 cm)
because it
divides almost
immediately
after emerging
to form a small
dorsal ramus, a
larger ventral
ramus, and a
tiny meningeal
branch
Dorsal & ventral rami
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In the thoracic
region there is
also a rami
communicantes
joined to the
base of the
ventral rami
These rami
contain autonomic (visceral)
nerve fibers
Rami are both
motor & sensory
Innervation of Body Regions

Except for T2-T12, all
ventral rami branch
and join one another
lateral to the vertebral
column forming nerve
plexuses
–
–
–
–

Cervical
Brachial
Lumbar
Sacral
Note that only ventral
roots form plexuses
Innervation of Body Regions

Within plexuses the different ventral rami
crisscross each other and become
redistributed so that
– Each branch of the plexus contains fibers from
several different spinal nerves
– Fibers from each ventral ramus travel to the body
periphery via several different routes or branches


Thus, each muscle in a limb receives its nerve
supply from more than one spinal nerve
Damage to a single root cannot completely
paralyze any limb muscle
Innervation of the Back
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The innervation
of the posterior
body trunk is by
the dorsal rami
Each dorsal
ramus innervates
a narrow strip of
muscle and skin
Pattern follows a
neat, segmented
pattern in line
with emergence
from spinal cord
Innervation of Thorax & Abdomem
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
Only in the thorax
are the ventral rami
arranged in a simple
segmental pattern
corresponding to that
of the dorsal rami
Ventral rami of T1T12 course anteriorly
deep to each rib as
intercostal nerves
supplying the intercostal muscles & most
of abdominal wall
Cervical Plexus and the Neck

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The cervical plexus
lies deep under the
sternocleidomastoid
muscle
Plexus is formed by
the ventral rami of
the first 4 cervical
nerves
Most branches are
cutaneous nerve
that transmit
sensory impulses
from the skin
Cevical Plexus and the Neck
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The single most
important nerve of
the plexus is the
phrenic nerve
It receives its fibers
from C3 - C4
The phrenic nerve
runs inferiorly
through the thorax
and supplies motor
and sensory fibers
to diaphram
Breathing
Brachial Plexus and Upper Limb
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The large important brachial plexus is
situated partly in the neck and partly in
the axilla
It gives rise to virtually all the nerves that
innervate the upper limb
The brachial plexus is very complex and
is often referred to as the anatomy
student’s nightmare
Brachial Plexus and Upper Limb


The plexus is formed by the intermixing of the ventral
rami of the four inferior cervical nerves C5-C8 and
most of T1
It often receives fibers from C4 as well as T2
Brachial Plexus and Upper Limb

The terms used to describe the plexus from medial to
lateral are:
– Roots / Trunks / Divisions / Cords
Brachial Plexus and Upper Limb


The five roots (rami C5-T1) of the brachial plexus lie
deep to the sternocleidomastoid muscle
At the lateral border of that muscle, these nerves unite
to form the upper, middle, and lower trunks
Brachial Plexus and Upper Limb
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
Each of the three trunks divides almost immediately to
form anterior and posterior divisions
The divisions generally reflect which fibers will serve
the front or back of the limb
Brachial Plexus and Upper Limb


The divisions give rise to three large fiber bundles
called the lateral, medial, and posterior cords
All along the divisions and cords small nerve branch
off to supply muscles of the shoulder and arm
Brachial Plexus and Upper Limb


A summary of the differentiation of the brachial
plexus reveals how it gives rise to common nerves
The five peripheral nerves that emerge are the main
nerves of the upper limb
Brachial Plexus and Upper Limb

The main nerves
that emerge from
the brachial plexus
are
–
–
–
–
–
Axillary
Musculotaneous
Median
Ulnar
Radial
Roots
Axillary Nerve


The axillary nerve
branches off the
posterior cord and
runs posterior to the
surgical neck of the
humerous
It innervates the
deltoid and teres
minor muscles and
the skin and joint
capsule of the
shoulder
Axillary Nerve

Muscular branches
– Deltoid
– Teres minor

Cutaneous branches
– Some of the skin of shoulder region
Musculocutaneous Nerve


Musculocutaneous
nerve is the major
end of the lateral
cord, courses
inferiorly within the
anterior arm,
supplying motor
fibers to the elbow
flexors
Beyond the elbow it
provides for
cutaneous sensation
of lateral forearm
Musculocutaneous Nerve

Muscular branches
– Biceps brachii
– Brachialis
– Coracobrachialis

Cutaneous branches
– Skin on anterolateral aspect of forearm
Median Nerve



The median nerve
descends through
the arm without
branching
In the anterior
forearm, it gives off
branches to the skin
and most of the
flexor muscles
It innervates the five
intrinsic muscles of
the lateral palm
Median Nerve

Muscular branches
–
–
–
–
–
–
–

Palmaris longus
Flexor carpi radialis
Flexor digitorium superficialis
Flexor pollicus longus
Flexor digitorium profundus
Pronator
Intrinsic muscles of fingers 2 and 3
Cutaneous branches
– Skin of lateral two-thirds of hand, palm side
and dorsum of fingers 2 and 3
Ulnar Nerve



The ulnar nerve
branches off the
medial cord of the
plexus
It descends along the
medial aspect of the
arm toward the elbow,
swings behind the
medial epicondyle,
then follows the ulna
along the forearm
Innervates most
intrinsic hand muscles
Ulnar Nerve

Muscular branches
– Flexor carpi ulnaris
– Flexor digitorium profundus (medial half)
– Intrinsic muscles of the hand

Cutaneous branches
– Skin of medial third of hand, both anterior
and posterior aspects
Radial Nerve



The radial nerve is a
continuation of the
posterior cord
The nerve wraps
around humerous,
runs anteriorly by the
lateral epicondyle at
the elbow
Divides into a superficial branch that
follows the radius and
a deep branch that
runs posteriorly
Radial Nerve

Muscular branches
–
–
–
–
–
–
–
–

Triceps brachii
Anconeus
Supinator
Brachioradialis
Extensor capri radialis
Extensor carpi brevis
Extensor carpi ulnaris
Muscles that extend fingers
Cutaneous branches
– Skin of posterior surface of entire limb
Lumbosacral Plexus



The sacral and lumbar plexuses overlap
substantially
Since many of the fibers of the lumbar
plexus contribute to the sacral plexus via
the lumbosacral trunk, the two plexuses
are often referred to as the lumbosacral
plexus
Although the lumbosacral plexus mainly
serves the lower limb, it also sends some
branches to the abdomen, pelvis and
buttocks
Lumbar Plexus and Lower Limb



The lumbar plexus
arises from the first
four spinal nerves
and lies within the
psoas major muscle
Its proximal branches
innervate parts of the
abdominal wall and
iliopsoas
Major branches of
the plexus descend to
innervate the medial
and anterior thigh
Femoral Nerve


The femoral nerve, the
largest of the lumbar
plexus, runs deep to
the inguinal ligament
to enter the thigh and
then divides into a
number of large
branches
The motor branches
innervate the anterior
thigh muscles while
the cutaneous branch
serves anterior thigh
Femoral Nerve

Muscular branch
– Quadiceps group
• Rectus femoris, vastus laterialis, vastus medialis,
vastus intermedius
– Sartorius
– Pertineus
– Iliacus

Cutaneous branches
– Anterior femoral cutaneous
• Skin of anterior and medial thigh
– Saphenous
• Skin of medial leg and foot, hip and knee joints
Obturator Nerve

The obturator nerve
enters the medial thigh
via the obturator
foramen and
innervates the
adductor muscles
Obturator Nerve

Muscular branch
–
–
–
–
–

Adductor magnus (part)
Adductor longus
Adductor brevis
Gracilis
Obturator externus
Cutaneous branches
– Sensory for skin of medial thigh and hip and
knee joints
Sacral Plexus and Lower Limb


The sacral plexus arises from spinal nerves L4-S4 and
lies immediately caudal to the lumbar plexus
The sacral plexus has about a dozen named nerves
Sacral Plexus and Lower Limb

Half the nerves serve muscles of the buttocks and
lower limb while others innervate pelvic structures
and the perineum
Sciatic Nerve





The sciatic nerve is the
thickest and longest
nerve in the body
The sciatic nerve leaves
the pelvis via the greater
sciatic notch
Actually the tibial and
common peroneal nerves
It courses deep to the
gluteus maximus muscle
It gives off branches to
the hamstrings and
adductor magnus
Sciatic Nerve

Muscular branch
–
–
–
–

Bicep femoris
Semitendinous
Semimembranous
Adductor magnus
Cutaneous branches
– Posterior thigh
Tibial Nerve


The tibial nerve through
the popliteal fossa and
supplies the posterior
compartment muscles of
the leg and the skin of
the posterior calf and
sole of foot
Important branches of
the tibial nerve are the
sural, which serves the
skin of the posterior leg
and the plantar nerves
which serve the foot
Tibial Nerve

Muscular branch
–
–
–
–
–
–

Triceps surae
Tibialis posterior
Popliteus
Flexor digitorum longus
Flexor hallicus longus
Intrinsic muscle of the foot
Cutaneous branches
– Skin of the posterior surface of the leg and
the sole of the foot
Common Peroneal Nerve


The common peroneal
nerve descends the leg,
wraps around the head
of the fibula, and then
divides into superficial
and deep branches
These branches
innervate the knee joint,
the skin of the lateral
calf and dorsum of the
foot and the muscles of
the anterolateral leg
Common Peroneal Nerve

Muscular branch
–
–
–
–
–
–

Biceps foemoris (short head)
Peroneal muscles (longus, brevis, tertius)
Tibialis anterior
Extensor hallicus longus
Extensor digitorum longus
Extensor digitorum brevis
Cutaneous branches
– Skin of the anterior surface of leg and
dorsum of foot
Sarcal Plexus Nerves

Superior and inferior gluteal
– Innervate the gluteal muscles and tensor
fasciae latae

Pudendal
– Innervates the muscles of the skin of the
perineum
– Mediates the act of erection
– Voluntary control of urination
– External anal sphinter
Innervation of the Joints

Hilton’s law “. . . any nerve serving a
muscle producing movement at a joint
also innervates the joint itself and the
skin over the joint”
Innervation of Skin: Desmatomes



The are of skin that is innervated by the
cutaneous branch of a spinal nerve is
called a dermatome
All spinal nerves except C1 participate in
dermatomes
Adjacent dermatomes on the body trunk
are fairly uniform in width, almost
horizontal, and in direct line with their
spinal nerves
Innervation of Skin: Desmatomes


The skin of the
upper limbs is
supplied by C5-T1
The ventral rami of
the lumbar nerves
supply most of the
anterior muscles of
the thighs and legs
Innervation of Skin: Desmatomes

The ventral rami of
sacral nerves serve
most of the
posterior surfaces of
the lower limbs
Reflex Activity




Many of the body’s control systems
belong to the general category of stimulus
response consequences known as reflexes
A reflex is a rapid, predictable motor
response to a stimulus
It is unlearned, unpremeditated, and
involuntary
Basic reflexes may be considered to be
built into our neural anatomy
Reflex Activity


In addition to these basic, inborn types of
reflexes, there are many learned, or
acquired reflexes that result from
practice of repetition
There is no clear cut distinction between
basic and learned reflexes
Components of a Reflex Arc

All reflex arcs have five essential components
– The receptor
– The sensory neuron, afferent impulses to CNS
– Integration center
• Monosynaptic (one neuron)
• Polysynaptic (more than one chain of neurons)
– The motor neuron, efferent impulses to effector organ
– The effector, the muscle spindle or gland
Components of a Reflex Arc

Reflexes are classified functionally as
– Somatic reflexes
• (activate skeletal muscle)
– Visceral reflexes (autonomic reflexes)
• (activate smooth, cardiac muscle or visceral organs
Spinal Reflexes




Somatic reflexes mediated by the spinal
cord are called spinal reflexes
These reflexes may occur without the
involvement of higher brain centers
Other reflexes may require the activity of
the brain for their successful completion
Additionally, the brain is “advised” of
most types of spinal cord reflex activity
and can facilitate or inhibit them
Stretch and Deep Tendon Reflexes

If skeletal muscles are to perform
normally
– The brain must be continually informed of
the current state of the muscles
• Depends on information from muscle spindles
and Golgi tendon organs
– The muscles must exhibit healthy tone
• Depends on stretch reflexes initiated by the
muscle spindles

These processes are important to normal
skeletal muscle function, posture and
locomotion
Anatomy of Muscle Spindle


Each spindle
consists of 3-10
infrafusal
muscle fibers
enclosed in a
connective
tissue capsule
These fibers are
less than one
quarter of the
size of
extrafusal
muscle fibers
(effector fibers)
Anatomy of Muscle Spindle

The central
region of the
intrafusal fibers
which lack
myofilaments
and are
noncontractile,
serving as the
receptive
surface of the
spindle
Anatomy of Muscle Spindle


Intrafusal fibers
are wrapped by
two types of
afferent endings
that send
sensory inputs
to the CNS
Primary sensory
endings
– Type Ia fibers

Secondary
sensory endings
– Type II fibers
Anatomy of Muscle Spindle

Primary sensory
endings
– Type Ia fibers


Stimulated by
both the rate
and amount of
stretch
Innervate the
center of the
spindle
Anatomy of Muscle Spindle

Secondary
sensory endings
– Type II fibers

Associated with
the ends of the
spindle and are
stimulated only
by degree of
stretch
Anatomy of Muscle Spindle


The contractile
region of the
intrafusal
muscle fibers
are limited to
their ends as
only these areas
contain actin
and myosin
filaments
These regions
are innervated
by gamma ()
efferent fibers
The Stretch Reflex

Exciting a muscle spindle occurs in two
ways
– Applying a force that lengthens the entire
muscle (external stretch - either by weight or
by the action of an antagonist)
– Activing the  motor neurons that stimulate
the distal ends of the intrafusal fibers to
contact, thus stretching the mid-portion of
the spindle (internal stretch)
The Stretch Reflex

Whatever the
stimulus, when the
spindles are
activated their
associated sensory
neurons transmit
impulses at a higher
frequency to the
spinal cord
The Stretch Reflex

At spinal cord sensory neurons synapse directly (monosynaptically) with the  motor neurons which rapidly
excite the extrafusal muscle fibers of stretched muscle
The Stretch Reflex

The reflexive muscle contraction that follows (an
example of serial processing) resists further stretching of
the muscle
The Stretch Reflex

Branches of the afferent fibers also synapse with interneurons that inhibit motor neurons controlling the
antagonistic muscles inhibiting their contraction
The Stretch Reflex



Inhibition of the antagonistic muscles is
called reciprocal inhibition
In essence, the stretch stimulus causes the
antagonists to relax so that they cannot
resist the shortening of the “stretched”
muscle caused by the main reflex arc
While this spinal reflex is occurring,
impulses providing information on
muscle length and the velocity of
shortening are also being relayed to the
brain
The Stretch Reflex


The stretch reflex is most important in
large extensor muscles which sustain
upright posture
Contractions of the postural muscles of
the spine are almost continuously
regulated by stretch reflexes initiated
first on one side of the spine and then the
other
The Deep Tendon Reflex


Deep tendon reflexes cause muscle
relaxation and lengthening in response to
the muscle’s contraction
This effect is opposite of those elicited by
stretch reflexes
The Deep Tendon Reflex

When muscle tension
increases moderately
during muscle
contraction or
passive stretching,
GTO receptors are
activated and
afferent impulses are
transmitted to the
spinal cord
The Deep Tendon Reflex


Upon reaching the
spinal cord, information is sent to the
cerebellum, where it is
used to adjust muscle
tension
Simultaneously, motor
neurons in the spinal
cord supplying the
contracting muscle are
imhibited and
antagonistic muscle are
activated (activation)
The Deep Tendon Reflex

Golgi tendon organs help ensure smooth
onset and termination of muscle
contraction and are particularly
important in activities involving rapid
switching between flexion and extension
such as in running
The Flexor Withdrawal Reflex

The flexor, or withdrawal reflex is initiated by a
painful stimulus (actual or perceived) and causes
automatic withdrawal of the threatened body part
from the stimulus
The Crossed Extensor Reflex

The crossed extensor reflex is a complex spinal reflex
consisting of an ipsilateral withdrawal reflex and a
contralateral extensor reflex
The Crossed Extensor Reflex


The reflex is can occur when you step on a sharp object
There is a rapid lifting of the affected foot, while the
contralateral response activates the extensor muscles of
the opposite leg to support the weight shifted to it
Superficial Reflexes



Superficial reflexes are elicited by
cutaneous stimulation
These reflexes are dependent upon
functional upper motor pathways and
spinal cord reflex arcs
Babinski reflex
End of Chapter