Chapter 13The Spinal Cord & Spinal Nerves

Together with brain forms the CNS Functions spinal cord reflexes integration (summation of inhibitory and excitatory)

of nerve impulses highway for upward and downward travel of sensory

and motor information

We are only going to cover Pages 420-434 and 447

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Spinal Cord Protection

By the vertebral column, meninges, cerebrospinal fluid, and vertebral ligaments.

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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 with many BVs denticulate ligs hold in place

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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

External Anatomy of Spinal Cord

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Conus medullaris cone-shaped end of spinal cord

Filum terminale thread-like extension of pia mater stabilizes spinal cord in canal

Caudae equinae (horses tail) dorsal & ventral roots of lowest

spinal nerves Spinal segment area of cord from which each pair

of spinal nerves arises

Inferior End of Spinal Cord

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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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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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Gray Matter of the Spinal Cord

Gray matter is shaped like the letter H or a butterfly contains 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

Note: colors in reverse due to staining of tissue

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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 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

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Motor tracts!! ! ! Sensory tracts Direct (pyramidal) tract ---spinothalamic tracts Indirect(extrapyramidal) tract! ---posterior columns

see page 515 ---spinocerebellar

Location of Tracts inside Cord

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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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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 nerves13

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

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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

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Illustration of the Stretch Reflex

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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

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Illustration of Tendon Reflex

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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 ones 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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Endoneurium - around individual nerve fibers- Fascicles - a bundle of axons/nerve fibers Perineurium - around fascicles Epineurium - the superficial covering around the whole nerve

Connective Tissue Coverings of the Spinal Nerves

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Endoneurium - around individual nerve fibers- Fascicles - a bundle of axons/nerve fibers Perineurium - around fascicles Epineurium - the superficial covering around the whole nerve

Connective Tissue Coverings of the Spinal Nerves

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Clinical Considerations

Checking a patients 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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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

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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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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 nerves27

The End

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