THE SPINAL CORD AND THE SPINAL NERVES:
Introduction:
- Spinal cord and nerves contain neural circuits that control some of the quickest reactions
to environmental changes
- Spinal cord reflex: a quick, automatic response to certain kinds of stimuli that involves
neurons only in the spinal nerves and spinal cord
- Spinal cord is site of integration of neuronal stimulation that arises locally or is triggered
by nerve impulses form the PNS
- Spinal cord is the highway for sensory nerve impulses headed for the brain and motor
nerve impulses from brain to skeletal muscles
17.1 Spinal Cord Anatomy
- Protective Structures:
o First layer of protection or the CNS is hard bony skull or vertebral column
 Strong protective defence
o Second layer of protection is the meninges
 3 membranes that lie between the bony encasement and the nervous tissue
in both the brain and spinal cord
o Space between two of the meninges membranes contain cerebrospinal fluid that
suspends the CNS tissue in a weightless environment and is shock absorbing and
cushioning
- Vertebral Column
o Spinal cord is in the vertebral canal of the vertebral column
o Vertebral foramina of all the vertebral stacked on top of each other form the
vertebral canal
o Vertebral ligaments provide additional protection
- Spinal Cord Compression
o Certain disorders may put pressure on the vertebral column and disrupt its normal
functions
o May result from fractured vertebral, herniated intervertebral discs, tumors,
osteoporosis, or infections
o If the source of the compression is found than spinal cord function usually returns
to normal is no neural tissue is destroyed
o Weakness, pain, paralysis, decreased or loss of sensation below the level of injury
- Meninges
o Three protective, connective tissue coverings that encircle the spinal cord and
brain
o From superficial to deep they are
 Dura mater
 Arachnoid mater
 Pia mater
o Leptomeninges are the arachnoid and pia mater together
o Between the pia and arachnoid mater is a space (subarachnoid space) which
contains shock absorbing cerebrospinal fluid
o Spinal meninges surround the spinal cord and are continuous with the cranial
meninges (Encircle the brain)
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o All 3 spinal meninges cover the spinal nerves up to the point where they exit the
spinal column through the intervertebral foramina
o Epidural space: fat tissue and connective tissue that protects the spinal cord
o Dura mater:
 Most important of the 3 spinal meninges
 Thick strong layer composed of dense irregular connective tissue called
dura mater
 Mater means mother
 Believed that all tissue in the body came from dura mater
 Forms a sac from the level of the foramen magnum in the occipital bone
where it is continuous with the meningeal dura mater of the brain to the
second sacral vertebrae
 Also continuous with the epineurium (outer covering of spinal and cranial
nerves)
o Arachnoid mater:
 Middle of the meningeal membranes
 Thin avascular covering made of cells and thin loose arrays of collagen
 Spider web arrangement of delicate collagen fibers and some elastic fibers
 Deep to the dura mater and is continuous through the foramen magnum
with the arachnoid mater of the brain
 Between the dura and arachnoid mater is the subdural space
 Contain interstitial fluid
o Pia mater:
 Innermost meninx
 Thin transparent connective tissue layer that adheres to the surface of the
spinal cord and brain
 Consists of thin squamous to cuboidal cells within interlacing bundles of
collagen fibers and some fine elastic fibers
 Within the pia mater there are many blood vessels that supply oxygen and
nutrients to the spinal cord
 Triangular shaped membranous extensions of the pia mater suspend the
spinal cord in the middle of its dural sheath
 Denticulate ligament: thickenings of the pia mater (the extensions)
o Protect laterally and fuse with the arachnoid mater and
inner surface of the dura mater between the anterior and
posterior nerve roots of spinal nerves on either side
o Extending along the entire length of the spinal cord the
ligaments protect the spinal cord against sudden
displacement that could result in shock
External Anatomy of the Spinal Cord:
o Spinal cord: oval in shape and flattened in the anterior posterior axis
o Extends from the medulla oblongata (most inferior part of the brain) to the
superior border of the second lumbar vertebra
 In adults
o In infants it extends form the 3rd or 4th lumbar vertebra
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o Both the spinal cord and the vertebral column grow longer as part of overall body
growth during early childhood
 Elongation of the spinal cord stops at 4-5 but growth of the vertebral
column continues
 This is why spinal cord doesn’t extend full length of the vertebral
column
o 42-45 cm in adults and mx diameter is 1.5 cm in the lower cervical region and is
smaller in the thoracic region and its inferior tip
o Cervical enlargement (superior enlargement): extends from the 4th cervical
vertebra to the 1st thoracic vertebra
 Nerves to and from upper limbs arise from the cervical enlargement
o Lumbar enlargement (inferior enlargement): extends form the 9th to 12th thoracic
vertebra
 Nerves to and from the lower limbs arise from the lumbar enlargement
o Conus medullaris: below the lumbar enlargement where the spinal cord terminates
as a tapering, conical structure
 Ends between L1-L2
o Filum terminale: comes from the conus medullaris, extension of the pia mater that
extends inferiorly and fuses with the arachnoid mater and dura mater to anchor
the spinal cord to the coccyx
o Spinal cords branch off and exit laterally through the intervertebral foramina
between adjacent vertebrae
o Because spinal cord is shorter than the vertebral column nerves that arise from
lumbar, sacral and coccygeal regions of the spinal cord do not leave the vertebral
column at the same level they exit cord
 Roots of these lower spinal nerves angle inferiorly alongside the filum
terminale in the vertebral canal like wisps of hair
 Roots of these nerves are called cauda equine “horse tail”
Spinal Tap:
o Long hallow needle inserted into the subarachnoid space to withdraw CSP for
diagnostic purposes
o Patient lies on their side and the vertebral column is flexed
 Flexion of the column increases the distance between the spinous
processes of the vertebrae allowing for easy access to the subarachnoid
space
o Spinal cord ends at L2 but the meninges and circulating CSF extend to S2
 Between L2 and S2 the spinal meninges are present but the spinal cord is
not
o Spinal tap is between L3 and L4 or L4 and L5 because these regions provide safe
access to the subarachnoid space without the risk of damaging the spinal cord
Internal Anatomy of the Spinal Cord:
o H or butterfly shape of gray matte that is surrounded by white matter
o Two grooves in the white matter and divide it into the right and left sides
o Anterior median fissure: wide groove on the anterior (ventral) side
o Posterior median fissure: a narrow groove on the posterior (dorsal) side
o Gray matter consists mostly of cell bodies of neurons, neuroglia, unmyelinated
axons, and dendrites of interneurons and motor neurons
o White matter consists of bundles of myelinated axons of sensory neurons,
interneurons, and motor neurons
o Gray commissure: forms the crossbar H
 In the center is a space called the central canal
 Central canal extends the entire length of the spinal cord and
contains cerebrospinal fluid
o Its superior ends is continuous with the fourth ventricle in
the medulla oblongata of the brain
o Anterior white commissure is anterior to the gray commissure which connects the
white matter of the right and left sides of the spinal cord
o Functions of the spinal cord:
 White tracts propagate sensory impulses from receptors to the brain and
motor impulses from the brain to effectors
 Gray matter receives and integrates incoming and outgoing information
o In gray matter of the CNS there are clusters of neuronal cell bodies that form
functional groups called nuclei
 Sensory nuclei receive input from receptors via sensory neurons
 Motor nuclei provide output to effector tissues via motor neurons
o Gray matter on each side of the spinal cord is divided into regions called horns
o Anterior gray horn: contains somatic motor nuclei
 Clusters of cell bodies of somatic motor neurons that provide nerve
impulses for contraction of skeletal muscles
o Posterior gray horn: contain cell bodies and axons of interneurons as well as
axons of incoming sensory neurons
o Lateral gray horns: between the anterior and posterior gray horns
 Present only in thoracic, upper lumbar, and sacral segments of the spinal
cord
 Contain the cell bodies of autonomic motor nuclei that regulate activity of
smooth muscle, cardiac muscle, and glands
o The anterior and posterior gray matter horns divide the white matter on each side
into 3 columns
 Anterior white column (ventral)
 Posterior white column (dorsal)
 Lateral white columns
o Each column directs distinct bundles of axons having a common origin or
destination and carrying similar information
 Bundles may extend long distances up and down the spinal cord called
tracts
o Sensory (ascending) tracts: consist of axons that conduct nerve impulses from the
spinal cord toward the brain
o Motor (descending) tracts: consists of axons that carry nerve impulses away from
the brain down the spinal cord
o Sensory and motor tracts of the spinal cord are continuous with motor and sensory
tracts in the brain
o More white matter in the cranial end of the spinal cord than in the caudal end
o All ascending tracts are heading to the brain they become thicker as they progress
from caudal to cranial
o Descending tracts are also thicker at the cranial
o Gray matter (especially the anterior horn) is largest at lower cervical levels and at
lower lumbar upper sacral level
 Levels correspond with upper and lower limb anatomy respectively
o Large among of skeletal muscle tissue in the limbs require
motor innervation
Comparison of Various Spinal Cord Segments:
- Cervical:
o Large diameter, large amounts of white matter, oval shape
o C1-C4 posterior gray horn is large, anterior gray horn is small
o C5 and below the posterior horns are enlarged and the anterior horns are well
developed
- Thoracic:
o Small diameter due to small amount of gray matter
o T1 has more gray matter
o Small lateral gray horn is present
o Anterior and posterior gray horns are small
- Lumbar:
o Nearly circular
o Very large anterior and posterior gray horns
o Less white matter than cervical segments
- Sacral:
o Relatively small with large amounts of gray matter
o Small amounts of white matter
o Anterior and posterior gray horns are large and thick
- Coccygeal
o Resemble lower sacral spinal segments, but much smaller
17.2 Spinal Nerves:
- Spinal nerves are nerves associated with the spinal cord and are parallel bundles of axons
and their associated neuroglial cells wrapped in several layers of connective tissue
- Connect the CNS to sensory receptor, muscles, and glands in al parts of the body
- 31 pairs of spinal nerves
- Spinal cord is segmented because the 31 pairs of spinal nerves emerge at regular intervals
from the spinal cord through intervertebral foramina
o Each pair is said to arise from a spinal segment
- 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
- I pair of coccygeal nerves C01
- First cervical pair emerges between the atlas and occipital bone
o All other ones emerge from the vertebral column through the intervertebral
foramina between adjoining vertebrae
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Not all spinal cord segments are aligned with their corresponding vertebrae because of
the differential growth between the spinal cord and the vertebral column
o Roots of lumbar, sacral. And coccygeal nerves descend at an angle to reach their
respective foramina before emerging from the vertebral column
 Constitutes the cauda equine
Structure of a Single Nerve
- Neurons are conductive cells of the nervous tissue
- Nerves are bundles of axons and their associated neuroglial cells wrapped in layers of
connective tissue
o Consist of long cells
- Muscles and nerves have different functions but structures are similar
o Long muscle cells have a loos connective tissue surrounding the endomysium
which distributes capillaries throughout the muscle tissue
- Within nerves there are many axons of neurons with their surrounding neurilemmal and
myelin sheaths
- Nerve fiber: axon and its associated glial cells
- Endometrium: loose connective tissue covering that the nerve fiber sits in
o Mesh of collagen fibers, fibroblasts, and macrophages
- Perineurium: thicker sheath of connective tissue, holds many nerve fibers together into
bundles called fasciculi
o Diffusion barrier that maintains the osmotic environment and fluid pressure
within the enpineurium
 Blood-nerve barrier
- Outer connective tissue sheath that completes the structure of the nerve- epineurium
Spinal Cord Injuries:
- Due to trauma as a result of factors like car crash, falls, contact sports, diving
- Effects of the injury depend on the extent of direct trauma to the spinal cord or
compression of the cord by fractured or displaced vertebrae or blood clots
- Most common sites of injury are the cervical, lower thoracic, and upper lumbar regions
- Monoplegia: is paralysis of one limn only
- Diplegia: paralysis of both upper limbs or both lower limbs
- Paraplegia: paralysis of both lower limbs
- Hemiplegia: paralysis of the upper limb, trunk, an lower limb on one side
- Quadriplegia: paralysis of all 4 limbs
- Complete transection: means that the cord is severed from one side to the other, cutting
all sensory and motor tracts
o Loss of all sensations and voluntary movement below the level of transection
 Permanent loss of all sensation in dermatomes below the injury because
ascending nerve impulses cannot propagate past the transection to reach
the brain
- Dermatome: is an area of skin that provides sensory input to the CNS via a pair of spinal
nerves
- All voluntary muscle contractions will be lost below the transection because nerve
impulses descending from the brain can’t pass
- Closer the injury is to the head the more of the body that may be affected
-
Muscle functions that may be retained at progressively lower levels of spinal cord
transections
o C1-C3: no function maintained from the neck down, ventilator needed for
breathing, wheelchair with head, shoulder and breath device needed
o C4-C5: diaphragm, allows breathing
o C6-C7: some arm and chest muscles, allows for feeding, dressing, and manual
wheelchair not electric
o T1-T3: intact arm function
o T4-T9: control of trunk above the umbilicus
o T10-L1: most thigh muscles, allows walking with long leg braces
o L1-L2: most leg muscles, allows for walking with short braces
- Hemisection: partial transection of the cord on either the left or right side
o Brown-Sequard syndrome (the 3 symptoms) occur below the level of injury
 Damage of the posterior column causes loss of proprioception and fine
touch sensations on the ipsilateral (same) side as the injury
 Damage of the lateral cortico spinal tract (motor) tract causes ipsilateral
paralysis
 Damage of the spinothalamic tract (sensory tract) causes loss of pain and
temperature sensations on the contralateral side
- Spinal shock occurs after transection and sometimes Hemisection
o An immediate response to spinal cord injury characterized by temporary areflexia
(loss of reflex function)
o Signs of acute spinal shock include: slow heart rate, low blood pressure, flaccid
paralysis of skeletal muscles, loss of somatic sensations, urinary bladder
dysfunction
Organization of Spinal Nerves:
- Spinal nerves are designed like trees
- Spinal nerve root damage
o Most common cause is a herniated intervertebral disc
o Damage to vertebrae as a result of osteoporosis, osteoarthritis, cancer, or trauma
can also damage spinal nerve roots
o Symptoms of spinal nerve root damage include: muscle weakness, pain, loss of
feeling
o Rest, manual therapy, pain medications, and epidural injections are the most
widely used conservative treatments
- Spinal nerves arise from the spinal cord as a series of small rootlets
- Two types of rootlets
o Anterior (ventral) and posterior (dorsal)
 Anterior emerge from 2 or 3 irregular rows
 Contain axons of multipolar motor neurons arising from cell
bodies in the anterior regions of the spinal cord gray matter
- Projecting from the posterolateral sulcus of the spinal cord is another series of rootlets
o Posterior rootlets
 Contain the central processes of the sensory unipolar neurons
 Transmit action potential from peripheral receptor organs to the
CNS
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Each series of anterior rootlets converge to form larger anterior (ventral) roots
Each series of posterior rootlets converges to form larger posterior (dorsal) roots
Each posterior root has a swelling
o Posterior (dorsal) root ganglion
 Contains the cells bodies of sensory neurons
- The anterior and posterior roots on each side of the spinal cord correspond to one
developmental segment or level of the body
o As they project laterally from the spinal cord they converge to form a mixed nerve
called the spinal nerve trunk
 A mixed nerve has both sensory and motor axons
 Spinal nerve trunk runs a short distance before branching into 2
large branches and a variable series of smaller branches
Branches of Spinal Nerves:
- Each large spinal nerve branch named a ramus follows a specific course to different
peripheral regions
- 2 largest branches are somatic branches that run in the musculoskeletal wall of the body
- Posterior ramus serves the deep muscles and skin of the posterior surface of the trunk
- Anterior ramus serves the muscles and structures of the upper and lower limbs and the
muscles and skin of the lateral and anterior regions of the trunk
- Smaller visceral branches like meningeal branch and communicating rami, form the
autonomic pathways to smooth muscle and glandular tissue
- Meningeal branch renters the vertebral canal through the intervertebral foramen and
supplies the vertebrae, vertebral ligament, blood vessels of the spinal cord, and meninges
- Rami communicants: are components of the autonomic nervous system
- Intercostal muscles:
o Anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses
and are known as intercostal or thoracic nerves
o These nerves connect directly to structures they supply in the intercostal spaces
and are mainly distributed to a single body segment
o Ramus of T2 (anterior) innervates the intercostal muscles of the second intercostal
space and supplies the skin of the axilla and posteromedial aspect of the arm
o Nerves T3-T6 extend along the costal grooves of the ribs and then to the
intercostal muscles and skin of the anterior an lateral chest wall
o Nerves T7-T12 supply the intercostal muscles, the abdominal muscles, and the
overlying skin
o Posterior rami of the intercostal nerves supply the deep back muscles and skin of
the posterior aspect of the thorax
- Plexuses:
o Axons from the anterior rami of spinal nerves (except T2-T12) do not go directly
to the body structures they supply
 They form networks on both eh left and right sides of the body by joining
with various numbers of axons from anterior rami of adjacent nerves
 These networks are called plexuses
o Principle spinal nerve plexuses are the cervical plexus, brachial plexus, lumbar
plexus, sacral plexus
Dermatomes vs. Cutaneous Fields:
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Skin all over the body is supplied by somatic sensory neurons that carry nerve impulses
from the skin into the spinal cord and brain stem
Somatic motor neurons that carry impulses out of the spinal cord innervate the underlying
skeletal muscles
Each spinal nerve contains sensory neurons that serve a specific, predictable segment of
the body
Trigeminal nerve (V) serves the skin of the face and the scalp
The area of the skin that provides sensory input to the CNS via one pair of spinal nerves
or the trigeminal nerve (V) is called a dermatome
Knowing each spinal cord segments supply each dermatome makes it possible to locate
damaged regions of the spinal cord
Cutaneous fields: are regions of skin supplied by a specific nerve arising from a plexus
o Median nerve from the brachial plexus has a distinct cutaneous field
 Overlaps multiple dermatomes because the median nerve contains neuron
from multiple spinal nerve levels
 Nerves arising from plexus can contain neurons form more than one spinal
level damage within a cutaneous field typically alerts a clinician to
peripheral nerve damage rather than spinal root or spinal cord damage
Need to assess whether sensation in a region of skin is from a dermatome or cutaneous
region to properly diagnose the site
Regions where there is a lot of overlap of dermatomes loss of sensation may result from
only one the nerves supplying the region
o Means that deliberate production of a region of complete anesthesia may require
at least three adjacent spinal nerves be cut or blocked by the drug
Shingles:
o Acute infection of the PNS caused by herpes the virus that causes chicken pox
o After a person recovers from chicken pox the virus retreats to a posterior root
ganglion
o If the virus is reactivated the immune system usually prevents it from spreading
o Sometimes it is reactivated during a time of weakened immune system
 Leaving the ganglion and travelling down sensory neurons of the skin
 Result is pain, discoloration and blisters of the skin
 Blisters are in a line that mark the distribution of a dermatome of a
particular cutaneous sensory nerve belonging to the infected
posterior root ganglion
Cervical Plexus
o Formed by the roots of anterior rami of the C1-C4 with contribution form C5
 One plexus on each side of the neck alongside the first 4 vertebrae
o Supplies the skin and muscles of the head, neck, and superior part of the shoulder
and chest
o Phrenic nerves arise from this plexus and supply motor fibers to the diaphragm
o Branches of cervical plexus also run parallel to 2 cranial nerves (X1) accessory
and (X11) hypoglossal
o Superficial sensory branches of the cervical plexus:
 Lesser occipital
 Origin: C2

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Distribution
o Skin of the scalp posterior and superior to the ear
 Greater auricular
 Origin: C2-C3
 Distribution:
o Skin anterior, inferior, and over ear, and over parotid
glands
 Transverse cervical
 Origin C2-C3
 Distribution:
o Skin over anterior and lateral aspect of neck
 Supraclavicular:
 Origin: C3-C4
 Distribution
o Skin over superior portion of chest and shoulder
o Deep (largely motor) braches of cervical plexus:
 Ansa cervicalis:
 This nerve divides into superior and inferior roots
 Superior root
 Origin: C1
 Distribution
o Infrahyoid and geniohyoid muscles of the neck
 Inferior root
 Origin: C2-C3
 Distribution
o Infrahyoid muscles of neck
 Phrenic
 Origin: C3-C5
 Distribution
o Diaphragm
 Segmental branches
 Origin C1-C5
 Distribution:
o Prevertebral (deep) muscles of neck, levator scapulae, and
middle scalene muscles
Clinical Connection (cervical plexus)
o Phrenic nerves originate from C3.C4, C5 and supply the diaphragm
o Complete severing of the spinal cord above the origin of the phrenic nerves
causes repository arrest
o Breathing stops because phrenic nerves are no longer sending impulses to
diaphragm
o Phrenic nerve may also be damaged due to pressure from malignant tumours in
the mediastinum like tracheal or esophageal tumors
Brachial Plexus
o Anterior rami of spinal nerves C5-C8 and T1 form the roots of the brachial plexus

o
o
o
o
o
o
o
o
Extends inferiorly and laterally on the other side of the last four cervical
and first thoracic vertebra
 Passes above the first rib posterior to the clavicle and then enters the axilla
Roots of the brachial plexus are the anterior rami of the spinal nerves
Roots of the spinal nerves untie to form trunks in the inferior part of the neck
 Superior, middle, and inferior trunks
 Trunks are posterior to the clavicle
Trunks then divide into division called anterior and posterior division
In the axilla the divisions unite to form cords
 Lateral, medial, and posterior cords
 Cords are named in relation to their position relative to the axillary artery
Branches of the brachial plexus form the principal nerves of the brachial plexus
Brachial plexus provides almost the entire nerve supply of the shoulders and
upper limbs
The terminal branches arise from the brachial plexus
 Axillary nerve
 Supplies the deltoid and teres minor muscles
 Musculocutaneous nerve
 Supplies the anterior muscles of the arm
 Radial nerve
 Supplies the muscles on the posterior aspect of the arm and
forearm
 Median nerve
 Supplies most of the muscles of the anterior fore arm (6 ½ out of
8) and some muscles of the hand
 Ulnar nerve
 Supplies the anteromedial muscles of the forearm (the other 1 1/2)
and most muscles of the hand
Nerves of the brachial plexus:
 Dorsal scapular
 Origin C5
 Distribution:
o Levator, scapulae, rhomboid major, and rhomboid minor
muscles
 Long thoracic:
 Origin: C5-C7
 Distribution
o Serratus anterior muscle
 Nerve to subclavius
 Origin: C5-C6
 Distribution: subclavius muscle
 Suprascapular
 Origin: C5-C6
 Distribution:
o Supraspinatus and infraspinatus muscles
 Musculocutaneous
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Origin: C5-C7
Distribution
o Coracobrachialis, biceps brachii, and brachialis muscles
Lateral pectoral
 Origin: C5-C7
 Distribution
o Pectoralis major muscles
Upper subscapular
 Origin: C5-C6
 Distribution:
o Subscapularis muscle
Thoracodorsal
 Origin: C6-C8
 Distribution:
o Latissimus dorsi muscle
Lower subscapular
 Origin: C5-C6
 Distribution:
o Subscapularis and teres major muscles
Axillary
 Origin: C5-C6
 Distribution
o Deltoid and teres minor muscles, skin over deltoid and
superior posterior aspect of arm
Median nerve
 Origin: C5-T1
 Distribution
o Flexors of the arm, expect flexor carpi ulnaris
o Ulnar half of the flexor digitorum profundus
o Some muscles of the hand (lateral palm)
o Skin of lateral 2.3 of palm of hands and fingers
Radial
 Origin: C8-T1
 Distribution
o Triceps brachii
o Extensors muscles of forearm
o Skin of posterior arm and of forearm
o Lateral two-thirds of dorsum of hand and finger over
proximal and middle phalanges
Medial pectoral
 Origin: C8-T1
 Distribution
o Pectoralis major and pectoralis minor muscles
Medial cutaneous nerve of arm
 Origin: C8-T1

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Distribution
o Skin of medial and posterior aspects of distal third of arm
Ulnar
 Origin: C8-T1
 Distribution
o Flexor carpi ulnaris, ulnar half of the flexor digitorum
profundus
o Most muscles of the hand
o Skin of medial side of hand, little finger
o Medial half of ring finger
Clinical Connection (Brachial Plexus)
o Injury to superior roots of brachial plexus (C5-C6) may result from forceful
pulling away of the hand form the shoulder
 Upper limb shoulder is adducted the arm is medially rotated, elbow is
extended, forearm is pronated, wrist is flexed
 This condition is called Erb-Duchemme palsy or waiter’s tip positon
 Loss of sensation along lateral side of the arm
o Radial nerve injury can be caused by improperly administered intramuscular
injections into the deltoid muscle
 Radial nerve may also be injured when a cast is applied too tightly around
the mid-humerus
 Radial nerve injury is indicated by wrist drop
 Inability to extend the wrist and finders
 Sensory loss is minimal die to overlap of sensory innervation by adjacent
nerves
o Median nerve injury may result in median nerve palsy
 Indicated by numbness, tingling, and pain in the palm and fingers
 Inability to pronate the forearm and flex the proximal interphalangeal
joints of all digits and the distal interphalangeal joints of the second and
third digits
 Wrist flexion is weak and thumb movements are weak
o Ulnar nerve injury may result in ulnar nerve palsy
 Indicated by an inability to abduct or adduct the fingers, atrophy of the
interosseous muscles of the hand, hyperextension of the
metacarpophalangeal joints and flexion of the interphalangeal joint
 Claw hand
 Also loss of sensation over the little finger
o Long thoracic nerve injury may result in paralysis of the serratus anterior muscle
 Medial border of the scapula protrudes giving the wing appearance
 When arm is raised the vertebral border and inferior angle of the scapula
pull away from the thoracic wall and protrude out
 Winged scapula
 Arm cannot be abducted beyond the horizontal position
Lumbar plexus:
o Anterior rami of spinal nerves L1-L4 form the roots of the lumbar plexus
o Less intricate intermingling of fibers than in brachial plexus
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o Iliohyprogastric nerve
 Origin: L1
 Distribution:
 Muscles of anterolateral abdominal wall, skin of inferior abdomen
and buttock
o Ilioinguinal
 Origin L1
 Distribution
 Muscles of anterolateral abdominal wall
 Skin of superior and medial aspect of thigh, roots of penis and
scrotum and labia majora and mons of pubis
o Genitofemoral
 Origin: L1-L2
 Distribution:
 Cremaster muscle, skin over middle anterior surface of thigh,
scrotum of male, labia majora
o Lateral cutaneous nerve of thigh
 Origin: L2-L3
 Distribution
 Skin over lateral, anterior, and posterior aspect of thigh
o Femoral
 Origin: L2-L4
 Distribution
 Largest nerve arising from the lumbar plexus
 Flexor muscles of hip joint and extensor muscles of knee joint,
skin over anterior and medial aspect of thigh, medial side of leg
and foot
o Obturator
 Origin: L2-L4
 Distribution
 Adductor muscle of hip joint, skin over medial aspect of thigh
Clinical connection (lumbar plexus)
o Largest nerve arising from the lumbar plexuses is the formal nerve
o Femoral nerve injury can occur in stab or gunshot wounds is indicated by an
inability to extend the leg and loss of sensation in the skin over the anteromedial
aspect of the thigh
o Obturator nerve injury
 Results in the paralysis of adductor muscles of the thigh and loss of
sensation over the medial thigh
 May result from pressure on the nerve by the fetal head during pregnancy
Sacral and Coccygeal Plexuses
o Anterior rami of L4-L5 and S1-S4
o Largely anterior to the sacrum
o Supplies the buttock, perineum, and lower limbs
o The roots (anterior rami) of spinal nerves S4-S5 and the coccygeal nerves form a
small coccygeal plexus

o
o
o
o
o
o
o
o
o
From this plexus arises the anococcygeal nerves
 Supply a small area of skin in the coccygeal region
Superior gluteal:
 Origin: L40L5
 Distribution:
 Gluteus minimum, gluteus medius, and tensor fascia latae muscles
Inferior gluteal
 Origin: L5-S2
 Distribution:
 Gluteus maximus muscle
Nerve to piriformis
 Origin S1-S2
 Distribution
 Piriformis muscle
Nerve to quadratus femoris and inferior gemellus
 Origin: L5-S2
 Distribution
 Obturator internus and superior gemellus muscles
Perforating cutaneous
 Origin: S2-S3
 Distribution
 Skin over inferior medial aspect of buttock
Posterior cutaneous nerve of thigh
 Origin: S1-S3
 Distribution
 Skin over anal region, inferior later aspect of buttock, superior
posterior aspect of thigh, superior part of calf, scrotum in male,
labia majora in female
Pudendal
 Origin: S2-S4
 Distribution
 Muscles of perineum, skin of penis and scrotum in male and
clitoris labia, majora, labia minora and vagina in female
Sciatic
 Origin: L4-S3
 Distribution
 Actually 2 nerves
o Tibial and common fibular
o Bound together by common sheath of connective tissue,
 Splits into its two divisions at the knee
 Descends through the thigh, sends branches to
hamstring muscles and the adductor Magnus
Tibial
 Origin: L4-S3
 Distribution


Gastrocnemius, plantaris, soleus, popliteus, tibialis posterior,
flexor digitorum longus, flexor halluces longus muscle
Branches in foot are the medial plantar nerve and lateral plantar
nerve
o Medial plantar
 Abductor hallucis, flexor digitorum brevis, and flexor halluces brevis
 Skin over medial 2/3 of plantar surface of foot
o Lateral plantar
 Remaining muscles of foot not supplied by medial plantar nerve, skin over
lateral 1/3 of plantar surface of foot
o Common fibular
 Origin: L4-S2
 Distribution
 Divides into a superficial fibular and a deep fibular branch
o Superficial fibular
 Fibularis longus and fibularis brevis
 Skin over distal third of anterior aspect of leg and dorsum of foot
o Deep fibular
 Tibialis anterior, extensor hallucis longus, fibularis tertius, extensor
digitorum longus and brevis
 Skin on adjacent sides of great and second toes
17.3 Spinal Cord Functions
- Spinal cord has 2 principle functions in maintaining homeostasis
o Nerve impulse propagation and integration of information
- White matter tracts in the spinal cord are highways for nerve impulse propagation
o Sensory impulses from receptors flow toward the brain, and motor impulses flow
from the brain toward skeletal muscles and other effector tissues
- Gray matter of the spinal cord receives and integrates incoming and outgoing information
- Sensory and motor tracts
o Name of the tract indicates its position in the white matter of the spinal cord as
well as where it begins and ends
 Ex, anterior spinothalamic tract
 Located in anterior white column from spinal cord to the thalamus
of the brain
 Ascending information because nerve impulses go from spinal
cord to the brain
o Motor tracts are on one side of the spinal cord and sensory are on the other sides
 All tracts are present on both sides
o Nerve impulses from sensory receptors propagate up the spinal cord to the brain
along 2 main routes
 The spinothalamic tracts and the posterior columns
o The lateral and anterior spinothalamic tracts
 Convey nerve impulses for sensing pain, warmth, coolness, itching, and
deep pressure sense of touch (poorly localized)
o The left and right posterior columns carry nerve impulses for several types of
sensations

-
Proprioception (awareness of the positions and movements of muscles,
tendons, and joints
 Discriminative touch (ability to feel exactly what part of the body is
touched
 Two-point discrimination (ability to distinguish the touching of 2 different
points on the skin even though they are close
 Light pressure sensations
 Vibration sensations
o Through the activity of interneurons integration occurs in several regions of the
spina cord and brain
 Motor impulses to make a muscle contract or gland secrete can be initiated
at several levels
o Most autonomic responses originate in the brain stem and hypothalamus
o Cerebral cortex plays a major role in controlling precise, voluntary muscular
movements
o Motor output to skeletal muscles travel down the spinal cord in 2 types of
descending pathways
 The direct pathways are lateral corticospinal tract and anterior
corticospinal tract and the corticobulbar tract
 Each of these tracts convey nerve impulses that originate in the
cerebral cortex and are destined to cause precise, voluntary
movements of skeletal muscles
 The indirect pathways include the rubrospinal tract, tectospinal tract,
vestibuolspinal tract, lateral reticulospinal tract and medial reticulospinal
tract
 Tracts convey nerve impulses from the brain stem and other parts
that govern automatic movements and help coordinate body
movements with visual stimuli
 Maintain skeletal muscle tone, sustain contraction of postural
muscles, play role in equilibrium by regulating muscle tone in
response to movements of the head
Reflexes and Reflex Arcs:
o Spinal cord also promotes homeostasis by serving as an integrating center for
some reflexes
o Reflex is a fast involuntary unplanned sequence of actions that occurs in response
to a particular stimulus
 Some are inborn (pulling hand away from hot surface)
 Other are learned or acquired
 Slamming on brakes while driving
o When integration takes place in the spinal cord gray matter the reflex is a spinal
reflex
 Knee jerk
o If integration takes place at brain stem than it’s a cranial reflex
 Tracking movements of your eyes as you read a sentence
o Somatic reflexes: involves contraction of skeletal muscles
o Autonomic (visceral) reflexes: generally, are not consciously perceived

Responses of smooth muscle, cardiac muscle, glands
 Heart rate, digestion, defecation
o Pathway followed by nerve impulses that produce a reflex is a reflex arc (circuit)
 Sensory receptor, sensory neuron, integrating center, motor neuron,
effector