Download Spinal Cord & Spinal Nerves

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
13-1
Spinal Cord Protection
By the vertebral column, meninges, cerebrospinal
fluid, and vertebral ligaments.
13-2
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
13-3
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
13-4
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
13-5
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
13-6
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
13-7
Gray Matter of the Spinal Cord
Note: colors in
reverse due to
staining of tissue
• 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
13-8
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
13-9
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
13-10
Location of Tracts inside Cord
• Motor tracts
– pyramidal tract (corticospinal)
– extrapyramidal tract
–
Sensory tracts
---spinothalamic tract
---posterior column
---spinocerebellar 13-11
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
13-12
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
13-13
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
13-14
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
13-15
Illustration of the Stretch Reflex
13-16
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
13-17
Illustration of Tendon Reflex
13-18
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
13-19
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 13-20
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)
13-21
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
13-22
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
13-23
Endoneurium
Perineurium
Epineurium
13-24
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
13-25
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
13-26
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
13-27
Phrenic Nerve
13-28
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
13-29
Branches off Brachial Plexus
13-30
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
13-31
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
13-32
Branches of Lumbar Plexus
• Notice: Femoral and Obturator nerves
• Found anterior and medial to hip joint
13-33
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)
13-34
Branches of Sacral Plexus
• Notice: Sciatic nerve origins
13-35
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
13-36
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
13-37
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
13-38
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
13-39