Chapter 13
The Spinal Cord & Spinal Nerves
Lecture Outline
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INTRODUCTION
• The spinal cord and spinal nerves mediate reactions to
environmental changes.
• The spinal cord has several functions.
• It processes reflexes.
• It is the site for integration of EPSPs and IPSPs that arise
locally or are triggered by nerve impulses from the periphery
and brain.
• It is a conduction pathway for sensory and motor nerve
impulses.
• The size of the vertebral canal varies in different regions of
the vertebral column and affects spinal cord injuries.
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Chapter 13
The Spinal Cord & Spinal Nerves
• Together with brain forms the CNS
• Functions
– spinal cord reflexes
– integration (summation of inhibitory and excitatory) nerve
impulses
– highway for upward and downward travel of sensory and
motor information
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SPINAL CORD ANATOMY
• The spinal cord is protected by two connective tissue
coverings, the meninges and vertebra, and a cushion of
cerebrospinal fluid.
• The vertebral column provides a bony covering of the spinal
cord (Figure 13.1c).
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Spinal Cord Protection
By the vertebral column, meninges, cerebrospinal
fluid, and vertebral ligaments.
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Meninges
• The meninges are three coverings that run continuously
around the spinal cord and brain (Figures 13.1a, 14.4a).
– The outermost layer is the dura mater.
– The middle layer is the arachnoid.
– The innermost meninx is the pia mater, a thin,
transparent connective tissue layer that adheres to the
surface of the spinal cord and brain
• Denticulate ligaments are thickenings of the pia mater that
suspend the spinal cord in the middle of its dural sheath.
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Structures Covering the Spinal Cord
• Vertebrae
• Epidural space filled with fat
• Dura mater
– dense irregular CT tube
• Subdural space filled with
interstitial fluid
• Arachnoid = spider web of
collagen fibers
• Subarachnoid space = CSF
• Pia mater
– thin layer covers BV
– denticulate ligs hold in place
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Structures Covering the Spinal Cord
• Vertebrae
• Epidural space filled with fat
• Dura mater
– dense irregular CT tube
• Subdural space filled with interstitial fl
• Arachnoid = spider web of collagen
fibers
• Subarachnoid space = CSF
• Pia mater
– thin layer covers BV
– denticulate ligs hold in place
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Applications
• The subarachnoid space is between the arachnoid
mater and pia mater and contains cerebrospinal fluid
(CSF).
• Inflammation of the meninges is known as meningitis.
• Removal of cerebrospinal fluid from the subarachnoid
space is called a spinal tap (lumbar puncture). This
procedure is used to diagnose pathologies and to
introduce antibiotics, contrast media, anesthetics, and
chemotherapeutic drugs.
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External Anatomy of the Spinal Cord
• The spinal cord begins as a continuation of the medulla
oblongata and terminates at about the second lumbar
vertebra in an adult (Figure 13.2).
– It contains cervical and lumbar enlargements that
serve as points of origin for nerves to the extremities.
• The tapered portion of the spinal cord is the conus
medullaris, from which arise the filum terminale and
cauda equina.
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External Anatomy of Spinal Cord
• Flattened cylinder
• 16-18 Inches long &
3/4 inch diameter
• In adult ends at L2
• In newborn ends at L4
• Growth of cord stops at age 5
• Cervical enlargement
– upper limbs
• Lumbar enlargement
– lower limbs
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Inferior End of Spinal Cord
• Conus medullaris
– cone-shaped end of spinal cord
• Filum terminale
– thread-like extension of pia mater
– stabilizes spinal cord in canal
• Caudae equinae (horse’s tail)
– dorsal & ventral roots of lowest spinal
nerves
• Spinal segment
– area of cord from which each pair of
spinal nerves arises
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Spinal tap or Lumbar Puncture
• Technique
– long needle into subarachnoid space
– safe from L3 to L5
• Purpose
– sampling CSF for diagnosis
– injection of antibiotics, anesthetics or chemotherapy
– measurement of CSF pressure
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Spinal nerves
• The 31 pairs of spinal nerves are named and numbered
according to the region and level of the spinal cord from
which they emerge (Figure 13.2).
– 8 pairs of cervical nerves,
– 12 pairs of thoracic nerves,
– 5 pairs of lumbar nerves,
– 5 pairs of sacral nerves, and
– 1 pair of coccygeal nerves.
• Spinal nerves are the paths of communication between the
spinal cord and most of the body.
• Roots are the two points of attachment that connect each
spinal nerve to a segment of the spinal cord (Figure 13.3a).
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Spinal Cord & Spinal Nerves
• Spinal nerves begin as roots
• Dorsal or posterior root is incoming sensory fibers
– dorsal root ganglion (swelling) = cell bodies of sensory
nerves
• Ventral or anterior root is outgoing motor fibers
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Internal Anatomy of the Spinal Cord
• The anterior median fissure and the posterior median sulcus
penetrate the white matter of the spinal cord and divide it
into right and left sides (Figure 13.3b).
• The gray matter of the spinal cord is shaped like the letter H
or a butterfly and is surround by white matter.
– The gray matter consists primarily of cell bodies of
neurons and neuroglia and unmyelinated axons and
dendrites of association and motor neurons.
– The gray commissure forms the cross bar of the Hshaped gray matter.
• The white matter consists of bundles of myelinated axons of
motor and sensory neurons.
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Gray Matter
of the Spinal
Cord
• Gray matter is shaped like the letter H or a butterfly
– contains neuron cell bodies, unmyelinated axons &
dendrites
– paired dorsal and ventral gray horns
– lateral horns only present in thoracic spinal cord
– gray commissure crosses the midline
• Central canal is continuous with 4th ventricle of brain
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Internal Anatomy of the Spinal Cord
• Anterior to the gray commissure is the anterior white
commissure, which connects the white matter of the right
and left sides of the spinal cord.
• The gray matter is divided into horns, which contain cell
bodies of neurons.
• The white matter is divided into columns.
– Each column contains distinct bundles of nerve axons
that have a common origin or destination and carry
similar information.
– These bundles are called tracts.
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White Matter of the Spinal Cord
• White matter covers gray matter
• Anterior median fissure deeper than Posterior median sulcus
• Anterior, Lateral and Posterior White Columns contain axons
that form ascending & descending tracts
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SPINAL CORD PHYSIOLOGY
• The spinal cord has two principal functions.
• The white matter tracts are highways for nerve impulse
conduction to and from the brain.
• The gray matter receives and integrates incoming and
outgoing information.
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Sensory and Motor Tracts
• Figure 13.12 shows the principal sensory and motor tracts in
the spinal cord. These tracts are discussed in detail in
Chapter 16 summarized in tables 16.3 and 16.4.
• Sensory (ascending) tracts conduct nerve impulses toward
the brain.
– the lateral and anterior spinothalamic tracts and the
posterior column tract.
• Motor (descending) tracts conduct impulses down the cord.
– Direct pathways include lateral and anterior corticospinal
and corticobulbar tracts.
– Indirect pathways include rubrospinal, tectospinal, and
vestibulospinal tracts.
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Tracts of the Spinal Cord
• Function of tracts - highways for sensory & motor
information
– sensory tracts ascend
– motor tracts descend
• Naming of tracts
– indicates position & direction of signal
– example = anterior spinothalamic tract
• impulses travel from spinal cord towards brain
(thalamus)
• found in anterior part of spinal cord
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Location of Tracts inside Cord
• Motor tracts
Sensory tracts
– pyramidal tract (corticospinal) ---spinothalamic tract
– extrapyramidal tract
---posterior column
–
---spinocerebellar
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Function of Spinal Tracts
• Spinothalamic tract
– pain, temperature, deep pressure & crude touch
• Posterior columns
– proprioception, discriminative touch, two-point
discrimination, pressure and vibration
• Direct pathways (corticospinal & corticobulbar)
– precise, voluntary movements
• Indirect pathways (rubrospinal, vestibulospinal)
– programming automatic movements, posture & muscle
tone, equilibrium & coordination of visual reflexes
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Reflexes and Reflex Arcs
• The spinal cord serves as an integrating center for spinal
reflexes. This occurs in the gray matter.
– A reflex is a fast, predictable, automatic response to
changes in the environment that helps to maintain
homeostasis.
– Reflexes may be spinal, cranial, somatic, or autonomic.
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Reflex Arc
• Specific nerve impulse pathway
• 5 components of reflex arc
– receptor
– sensory neuron
– integrating center
– motor neuron
– effector
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Reflex Arc
• A reflex arc is the simplest type of pathway; pathways are
specific neuronal circuits and thus include at least one
synapse.
• Reflexes help to maintain homeostasis by permitting the
body to make exceedingly rapid adjustments to homeostatic
imbalances.
• Somatic spinal reflexes include the stretch reflex, tendon
reflex, flexor (withdrawal) reflex, and crossed extensor
reflex; all exhibit reciprocal innervation.
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Stretch Reflex (patellar reflex)
• It operates as a feedback mechanism to control muscle
length by causing muscle contraction.
– Prevents injury from over stretching because muscle
contracts when it is stretched
• Monosynaptic,ipsilateral reflex arc
• Events of stretch reflex
– muscle spindle signals stretch of muscle
– motor neuron activated & muscle contracts
• Brain sets muscle spindle sensitivity as it sets muscle tone
(degree of muscle contraction at rest)
• Reciprocal innervation (polysynaptic- interneuron)
– antagonistic muscles relax as part of reflex
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Illustration of the Stretch Reflex
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Tendon Reflex
• It operates as a feedback mechanism to control muscle
tension by causing muscle relaxation when muscle
force becomes too extreme.
• ipsilateral polysynaptic reflex
• Golgi tendon organs are in tendon
– activated by stretching of tendon
– inhibitory neuron is stimulated (polysynaptic)
– motor neuron is hyperpolarized and muscle relaxes
• Both tendon & muscle are protected
• Reciprocal innervation (polysynaptic)
– causes contraction of ipsilateral muscle group
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Illustration of Tendon Reflex
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Flexor and Crossed Extensor Reflexes
• The flexor (withdrawal) reflex is ipsilateral and is a protective
withdrawal reflex that moves a limb to avoid pain (Figure
13.16).
• This reflex results in contraction of flexor muscles to
move a limb to avoid injury or pain.
•
• The crossed extensor reflex, which is contralateral, helps to
maintain balance during the flexor reflex.
• This is a balance-maintaining reflex that causes a
synchronized extension of the joints of one limb and flexion
of the joints in the opposite limb (Figure 13.17).
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Flexor (withdrawal) Reflex
• Step on tack (pain fibers send
signal to spinal cord
• Interneurons branch to different
spinal cord segments
• Motor fibers in several
segments are activated
• More than one muscle group
activated to lift foot off of tack
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Crossed Extensor Reflex
• Lifting left foot requires
extension of right leg to
maintain one’s balance
• Pain signals cross to opposite
spinal cord
• Contralateral extensor muscles
are stimulated by interneurons
to hold up the body weight
• Reciprocal innervation - when
extensors contract flexors
relax, etc
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Clinical Considerations
• Checking a patient’s reflexes may help to detect
disorders/injury
• Plantar flexion reflex -- stroke the lateral margin of
the sole
– normal response is curling under the toes
– abnormal response or response of children under
18 months is called Babinski sign (upward fanning
of toes due to incomplete myelination in child)
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SPINAL NERVES
• Spinal nerves connect the CNS to sensory receptors,
muscles, and glands and are part of the peripheral nervous
system.
– The 31 pairs of spinal nerves are named and numbered
according to the region and level of the spinal cord from
which they emerge (Figure 13.2).
– Roots of the lower lumbar, sacral, and coccygeal nerves
are not in line with their corresponding vertebrae and
thus form the cauda equina (Figure 13.2).
• Spinal nerves connect to the cord via an anterior and a
posterior root (Figure 13.3a). Since the posterior root
contains sensory axons and the anterior root contains motor
axons, a spinal nerve is a mixed nerve.
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Spinal Nerves
• 31 Pairs of spinal nerves
• Named & numbered by the cord level of
their origin
– 8 pairs of cervical nerves
(C1 to C8)
– 12 pairs of thoracic nerves
(T1 to T12)
– 5 pairs of lumbar nerves
(L1 to L5)
– 5 pairs of sacral nerves
(S1 to S5)
– 1 pair of coccygeal nerves
• Mixed sensory & motor nerves
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Connective Tissue Covering of Spinal Nerves
• Spinal nerve axons are grouped within connective tissue
sheathes (Figure 13.4a).
– A fiber is a single axon within an endoneurium.
– A fascicle is a bundle of fibers within a perineurium.
– A nerve is a bundle of fascicles within an epineurium.
• Numerous blood vessels are within the coverings.
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Connective Tissue Coverings
• Endoneurium =
wrapping of each
nerve fibers
• Perineurium =
surrounds group
of nerve fibers
forming
a
fascicle
• Epineurium =
covering of entire
nerve
– dura mater
blends into it at
intervertebral
foramen
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Endoneurium – Perineurium -- Epineurium
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Branching of
Spinal Nerve
• Spinal nerves formed from dorsal & ventral roots
• Spinal nerves branch into dorsal & ventral rami
– dorsal rami supply skin & muscles of back
– ventral rami form plexus supply anterior trunk & limbs
– meningeal branches supply meninges, vertebrae & BV
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A Nerve Plexus
• Joining of ventral rami of spinal
nerves to form nerve networks or
plexuses
• Found in neck, arm, low back &
sacral regions
• No plexus in thoracic region
– intercostal nn. innervate
intercostal spaces
– T7 to T12 supply abdominal
wall as well
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Plexuses
• The cervical plexus supplies the skin and muscles of the
head, neck, and upper part of the shoulders; connects with
some cranial nerves; and supplies the diaphragm (Figure
13.6, Exhibit 13.1).
– Damage to the spinal cord above the origin of the phrenic
nerves (C3-C5) causes respiratory arrest.
– Breathing stops because the phrenic nerves no longer
send impulses to the diaphragm.
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Cervical Plexus
• Ventral rami of spinal nerves (C1
to C5)
• Supplies parts of head, neck &
shoulders
• Phrenic nerve (C3-C5) keeps
diaphragm alive
• Damage to cord above C3
causes respiratory arrest
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Phrenic Nerve
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Plexuses
• The brachial plexus constitutes the nerve supply for the
upper extremities and a number of neck and shoulder
muscles (Figures 13.7a and b, Exhibit 13.2).
– A number of nerve disorders may result from injury to the
brachial plexus.
– Erb-Duchene palsy or waiter’s tip palsy
– Klumphe’s palsy
– wrist drop
– carpal tunnel syndrome
– claw hand
– winged scapula
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Brachial Plexus
• Ventral rami from C5 to T1
• Supplies shoulder & upper limb
• Passes superior to 1st rib &
under clavicle
• Axillary n. = deltoid & teres m.
• Musculocutaneous n. = elbow
flexors
• Radial n. = shoulder & elbow
extensors
• Median & ulnar nn. = flexors of
wrist & hand
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Branches off Brachial Plexus
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Plexuses
• The lumbar plexus supplies the anterolateral abdominal
wall, external genitals, and part of the lower extremities
(Figure 13.9a and b, Exhibit 13.3).
– The largest nerve arising from the lumbar plexus is the
femoral nerve.
– Injury to the femoral nerve is indicated by an inability to
extend the leg and by loss of sensation in the skin over
the anteromedial aspect of the thigh.
– Obturator nerve injury is a common complication of
childbirth and results in paralysis of the adductor muscles
of the leg and loss of sensation over the medial aspect of
the thigh.
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Lumbar Plexus
• Ventral rami of L1 to L4
• Supplies abdominal wall,
external genitals &
anterior/medial thigh
• Injury to femoral nerve
causes inability to extend leg
& loss of sensation in thigh
• Injury to obturator nerve
causes paralysis of thigh
adductors
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Branches of Lumbar Plexus
• Notice: Femoral and Obturator nerves
• Found anterior and medial to hip joint
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Plexuses
• The sacral plexus supplies the buttocks, perineum, and
part of the lower extremities (Figure 13.10, Exhibit 13.4).
– The largest nerve arising from the sacral plexus (and
the largest nerve in the body) is the sciatic nerve.
– Injury to the sciatic nerve (common peroneal portion)
and its branches results in sciatica, pain that extends
from the buttock down the back of the leg.
– Sciatic nerve injury can occur due to a herniated
(slipped) disc, dislocated hip, osteoarthritis of the
lumbosacral spine, pressure from the uterus during
pregnancy, or an improperly administered gluteal
injection.
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Sacral Plexus
• Ventral rami of L4-L5 & S1-S4
• Anterior to the sacrum
• Supplies buttocks, perineum &
part of lower limb
• Sciatic nerve = L4 to S3 supplies
post thigh & all below knee
– Peroneal nerve injury
produces foot drop or
numbness
– Tibial nerve injury produces
calcaneovalgus (loss of
function on anterior leg &
dorsum of foot)
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Branches of Sacral Plexus
• Notice: Sciatic nerve origins
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Sciatic Nerve Branches
• Notice: Common Peroneal
nerve and Tibial nerve behind
the knee
• Notice: Sciatica pain extends
from the buttock down the leg
to the foot
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Dermatomes
• The skin over the entire body is supplied by spinal nerves
that carry somatic sensory nerves impulses into the spinal
cord.
– All spinal nerves except C1 innervate specific, constant
segments of the skin; the skin segments are called
dermatomes (Figure 13.11).
– Knowledge of dermatomes helps a physician to
determine which segment of the spinal cord or which
spinal nerve is malfunctioning.
– Skin on face supplied by Cranial Nerve V
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Dermatomes
• Damaged regions of the
spinal cord can be
distinguished by patterns
of numbness over a
dermatome region
• Infusing local anesthetics
or cutting roots must be
done over 3 adjacent
spinal nerves.
• Spinal cord transection
– injury that severs the
cord loss of sensation&
motor control below the
injury
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Disorders
• Neuritis
– inflammation of nerves
– caused by injury, vitamin deficiency or poison
• Shingles
– infection of peripheral nerve by chicken pox virus
– causes pain, skin discoloration, line of skin blisters
• Poliomyelitis
– viral infection causing motor neuron death and possible
death from cardiac failure or respiratory arrest
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Clinical Correlations
• Erb-Duchene palsy
– waiter’s tip position
– fall on shoulder
• Radial nerve injury
– improper deltoid
injection
or tight cast
– wrist drop
• Median nerve injury
– numb palm & fingers; inability to pronate & flex fingers
• Ulnar nerve injury (clawhand)
– inability to adduct/abduct fingers, atrophy of interosseus
• Long thoracic nerve injury (winged scapula)
– paralysis of serratus anterior, can’t abduct above horizontal
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end
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