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
Spinal Cord Protection
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
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
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
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
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
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 continuous with 4th ventricle of brain
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
Tracts of the Spinal Cord
Function of tracts
highway 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
Location of Tracts inside Cord
Motor tracts Sensory
tracts
pyramidal tract (corticospinal) ---spinothalamic tract
extrapyramidal
tract ---posterior
column
---spinocerebellar
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
Spinal Reflexes
Automatic response to change in environment
Integration center for spinal reflexes is gray matter of
spinal cord
Examples
somatic reflexes result in skeletal
muscle contraction
autonomic (visceral) reflexes
involve smooth & cardiac muscle and glands.
heart rate, respiration, digestion,
urination, etc
Note: cranial reflexes involve cranial nerves
Reflex Arc
Specific nerve impulse pathway
5 components of reflex arc
receptor
sensory neuron
integrating center
motor neuron
effector
4 important somatic spinal reflexes
stretch, tendon, flexor(withdrawal)
& crossed extensor reflexes
Stretch Reflex (patellar reflex)
Monosynaptic,ipsilateral
reflex arc
Prevents injury from over stretching because muscle
contracts when it is stretched
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
Illustration of the Stretch Reflex
Tendon Reflex
Controls muscle tension by causing muscle relaxation that
prevents tendon damage
Golgi tendon organs 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
Illustration of Tendon Reflex
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
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
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)
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
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
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
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
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
Phrenic Nerve
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
Branches off Brachial Plexus
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
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
Branches of Lumbar Plexus
Notice: Femoral and Obturator
nerves
Found anterior and medial to hip joint
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)
Branches of Sacral Plexus
Notice: Sciatic nerve origins
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
may be sign of herniated disc
Dermatomes & Myotomes
Each spinal nerve contains both sensory & motor nerve
fibers
Dermatome
area
of skin supplied by one spinal nerve
overlap prevents loss of sensation
if one damaged
sensory anesthesia requires 3
spinal nerves to be blocked
Skin on face supplied by Cranial Nerve V
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
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