Chapter 13 The Spinal Cord & Spinal Nerves

1. Chapter 13The 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 Tortora & Grabowski 9/e 2000 JWS

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 Tortora & Grabowski 9/e 2000 JWS

3. 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 Tortora & Grabowski 9/e 2000 JWS

4. 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 Tortora & Grabowski 9/e 2000 JWS

5. 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 Tortora & Grabowski 9/e 2000 JWS

6. 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 Tortora & Grabowski 9/e 2000 JWS

7. 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 Tortora & Grabowski 9/e 2000 JWS

8. Location of Tracts inside Cord • Motor tracts Sensory tracts • pyramidal tract (corticospinal) ---spinothalamic tract • extrapyramidal tract ---posterior column • ---spinocerebellar Tortora & Grabowski 9/e 2000 JWS

9. 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 Tortora & Grabowski 9/e 2000 JWS

10. 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 Tortora & Grabowski 9/e 2000 JWS

11. 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 Tortora & Grabowski 9/e 2000 JWS

12. 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 Tortora & Grabowski 9/e 2000 JWS

13. 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 Tortora & Grabowski 9/e 2000 JWS

14. 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 Tortora & Grabowski 9/e 2000 JWS

15. 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) Tortora & Grabowski 9/e 2000 JWS

16. 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 Tortora & Grabowski 9/e 2000 JWS

17. 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 Tortora & Grabowski 9/e 2000 JWS

18. Endoneurium Perineurium Epineurium Tortora & Grabowski 9/e 2000 JWS

19. 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 Tortora & Grabowski 9/e 2000 JWS

20. 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 Tortora & Grabowski 9/e 2000 JWS

21. 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 Tortora & Grabowski 9/e 2000 JWS

22. 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 Tortora & Grabowski 9/e 2000 JWS

23. Clinical Correlations • Erb-Duchene palsy • waiter’s tip position • fall on shoulder • Radial nerve injury • improper deltoid injectionor 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 Tortora & Grabowski 9/e 2000 JWS

24. 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 Tortora & Grabowski 9/e 2000 JWS

25. 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) Tortora & Grabowski 9/e 2000 JWS

26. 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 Tortora & Grabowski 9/e 2000 JWS

27. 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 Tortora & Grabowski 9/e 2000 JWS

28. 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 Tortora & Grabowski 9/e 2000 JWS

29. 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 Tortora & Grabowski 9/e 2000 JWS