Chapter 13 The Spinal Cord Spinal Nerves Together

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 1

Spinal Cord Protection By the vertebral column, meninges, cerebrospinal fluid, and vertebral ligaments. 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 3 – denticulate ligs hold in place

External Anatomy of Spinal Cord • Flattened cylinder • 16 -18 Inches long & 3/4 inch diameter • In adult ends at L 2 • In newborn ends at L 4 • Growth of cord stops at age 5 • Cervical enlargement – upper limbs • Lumbar enlargement – lower limbs 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 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 6

Spinal tap or Lumbar Puncture • Technique – long needle into subarachnoid space – safe from L 3 to L 5 • Purpose – sampling CSF for diagnosis – injection of antibiotics, anesthetics or chemotherapy – measurement of CSF pressure 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 4 th ventricle of brain 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 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 10

Location of Tracts inside Cord • Motor tracts – pyramidal tract (corticospinal) – extrapyramidal tract – Sensory tracts ---spinothalamic tract ---posterior column ---spinocerebellar 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 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

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 14 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 15

Illustration of the Stretch Reflex 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 17

Illustration of Tendon Reflex 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 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 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) 21

Spinal Nerves • 31 Pairs of spinal nerves • Named & numbered by the cord level of their origin – 8 pairs of cervical nerves (C 1 to C 8) – 12 pairs of thoracic nerves (T 1 to T 12) – 5 pairs of lumbar nerves (L 1 to L 5) – 5 pairs of sacral nerves (S 1 to S 5) – 1 pair of coccygeal nerves • Mixed sensory & motor nerves 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 23

Endoneurium Perineurium Epineurium 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 &25 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 – T 7 to T 12 supply abdominal wall as well 26

Cervical Plexus • Ventral rami of spinal nerves (C 1 to C 5) • Supplies parts of head, neck & shoulders • Phrenic nerve (C 3 -C 5) keeps diaphragm alive • Damage to cord above C 3 causes respiratory arrest 27

Phrenic Nerve 28

Brachial Plexus • Ventral rami from C 5 to T 1 • Supplies shoulder & upper limb • Passes superior to 1 st rib & under clavicle • Axillary n. = deltoid & teres m. • Musculocutaneous n. = elbow flexors • Radial n. = shoulder & elbow extensors • Median & ulnar nn. = flexors of wrist & hand 29

Branches off Brachial Plexus 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 31

Lumbar Plexus • Ventral rami of L 1 to L 4 • 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 32

Branches of Lumbar Plexus • Notice: Femoral and Obturator nerves • Found anterior and medial to hip joint 33

Sacral Plexus • Ventral rami of L 4 -L 5 & S 1 -S 4 • Anterior to the sacrum • Supplies buttocks, perineum & part of lower limb • Sciatic nerve = L 4 to S 3 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 34 foot)

Branches of Sacral Plexus • Notice: Sciatic nerve origins 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 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 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 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 39