Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

1. Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

2. The spinal cord Gross anatomy 3 layers of meninges Epidural space (fat & vessels) CSF – subarachnoid space Terminates at L1/2 vertebral level (conus medullaris) Dura extends to S2 vertebral level Connects via filum terminale & denticulate ligaments (pia) 31 pairs of spinal nerves (mixed) cauda equina Cervical & lumbar enlargements

3. Lumbar Tap

4. Spinal Cord Anatomy Conus medullaris – terminal portion of the spinal cord Filum terminale – fibrous extension of the pia mater; anchors the spinal cord to the coccyx Denticulate ligaments – delicate shelves of pia mater; attach the spinal cord to the vertebrae Spinal nerves – 31 pairs attach to the cord by paired roots Cervical nerves are named for inferior vertebra All other nerves are named for superior vertebra Cervical and lumbar enlargements – sites where nerves serving the upper and lower limbs emerge Cauda equina – collection of nerve roots at the inferior end of the vertebral canal

5. Cross-Sectional Anatomy of the Spinal Cord Anterior median fissure – separates anterior funiculi Posterior median sulcus – divides posterior funiculi

6. The 3 Meningeal Layers Dura mater: outer layer of spinal cord subdural space: between arachnoid mater and dura mater Arachnoid mater: middle meningeal layer subarachnoid space: between arachnoid mater and pia mater filled with cerebrospinal fluid (CSF) Pia mater: inner meningeal layer

7. Structures of the Spinal Cord Paired denticulate ligaments: extend from pia mater to dura mater stabilize side-to-side movement Blood vessels: along surface of spinal pia mater within subarachnoid space

8. Cross-sectional anatomy Gray matter (cell bodies, neuroglia, & unmyelinated processes) Posterior horns (sensory, all interneurons) Lateral horns (autonomic, T1-L2) Anterior horns (motor, cell bodies of somatic motor neurons) Spinal roots Ventral (somatic & autonomic motor) Dorsal (DRG)

9. Cross-sectional anatomy White matter 3 funiculi (posterior, lateral, anterior) Ascending, descending, transverse Consist of “tracts” containing similarly functional axons All tracts are paired Most cross over (decussate) at some point Most exhibit somatotopy (superior part of the tracts are more lateral that inferior body regions) Most consist of a chain of 2 or 3 successive neurons

10. Gray Matter: Organization Dorsal half – sensory roots and ganglia Ventral half – motor roots Dorsal and ventral roots fuse laterally to form spinal nerves Four zones are evident within the gray matter – somatic sensory (SS), visceral sensory (VS), visceral motor (VM), and somatic motor (SM)

11. White Matter in the Spinal Cord Fibers run in three directions – ascending, descending, and transversely Divided into three funiculi (columns) – posterior, lateral, and anterior Each funiculus contains several fiber tracts Fiber tract names reveal their origin and destination Fiber tracts are composed of axons with similar functions Pathways decussate (cross-over) Most consist of two or three neurons Most exhibit somatotopy (precise spatial relationships) Pathways are paired (one on each side of the spinal cord or brain)

12. White Matter: Pathway Generalizations

13. 3 Connective Tissue Layers Epineurium: outer layer dense network of collagen fibers Perineurium: middle layer divides nerve into fascicles (axon bundles) Endoneurium: inner layer surrounds individual axons

14. Peripheral Distribution of Spinal Nerves Each spinal nerve connects to the spinal cord via two medial roots Each root forms a series of rootlets that attach to the spinal cord Ventral roots arise from the anterior horn and contain motor (efferent) fibers Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

15. Spinal Nerves: Rami The short spinal nerves branch into three or four mixed, distal rami Small dorsal ramus – to back Larger ventral ramus – to plexuses/intercostals Tiny meningeal branch – to meninges Rami communicantes at the base of the ventral rami in the thoracic region – to/from ANS

16. Nerve Plexuses All ventral rami except T2-T12 form interlacing nerve networks called plexuses Plexuses are found in the cervical, brachial, lumbar, and sacral regions Each resulting branch of a plexus contains fibers from several spinal nerves Fibers travel to the periphery via several different routes Each muscle receives a nerve supply from more than one spinal nerve Damage to one spinal segment cannot completely paralyze a muscle

17. Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall The back is innervated by dorsal rami via several branches The thorax is innervated by ventral rami T1-T12 as intercostal nerves Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall

18. The 4 Major Plexuses of Ventral Rami Cervical plexus Brachial plexus Lumbar plexus Sacral plexus

19. Cervical Plexus The cervical plexus is formed by ventral rami of C1-C4 (C5) Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders The most important nerve of this plexus is the phrenic nerve The phrenic nerve is the major motor and sensory nerve of the diaphragm

20. Brachial Plexus Formed by C5-C8 and T1 (C4 and T2 may also contribute to this plexus) It gives rise to the nerves that innervate the upper limb

21. Trunks and Cords of Brachial Plexus Nerves that form brachial plexus originate from: superior, middle, and inferior trunks large bundles of axons from several spinal nerves lateral, medial, and posterior cords smaller branches that originate at trunks

22. Brachial Plexus: Nerves Axillary – innervates the deltoid and teres minor Musculocutaneous – sends fibers to the biceps brachii and brachialis Median – branches to most of the flexor muscles of forearm Ulnar – supplies the flexor carpi ulnaris and part of the flexor digitorum profundus Radial – innervates essentially all extensor muscles

23. Lumbar Plexus Arises from (T12) L1-L4 and innervates the thigh, abdominal wall, and psoas muscle The major nerves are the femoral and the obturator

24. Sacral Plexus Arises from L4-S4 and serves the buttock, lower limb, pelvic structures, and the perineum The major nerve is the sciatic, the longest and thickest nerve of the body The sciatic is actually composed of two nerves: the tibial and the common fibular (peroneal) nerves

25. Nerve plexuses - Summary Cervical – C1-C4 Phrenic nerve Brachial – C5 – T1 (roots/trunks/divisions/cords) Axillary, MC, median, ulnar, radial Lumbar – L1-L4 Femoral, obturator Sacral – L4-S4 Sciatic (common peroneal/tibial), pudendal

26. Dermatomes Area of skin innervated by the cutaneous branches of a single spinal nerve. All segments except C1 have dermotomal distribution UE typically from C5-T1 LE typically from L1-S1

27. 5 Patterns of Neural Circuits in Neuronal Pools Divergence: spreads stimulation to many neurons or neuronal pools in CNS Convergence: brings input from many sources to single neuron

28. Serial processing: moves information in single line Parallel processing: moves same information along several paths simultaneously 5 Patterns of Neural Circuits in Neuronal Pools

29. Reverberation: positive feedback mechanism functions until inhibited 5 Patterns of Neural Circuits in Neuronal Pools

30. Reflex activity 5 components of a reflex arc Receptor Sensory neuron Integration center (CNS) Motor neuron Effector

31. 4 Classifications of Reflexes By early development Innate or Acquired By type of motor response Somatic or Visceral By complexity of neural circuit Monosynaptic or Polysynaptic By site of information processing Spinal or Cranial

32. Spinal Reflexes Range in increasing order of complexity: monosynaptic reflexes polysynaptic reflexes intersegmental reflex arcs: many segments interact produce highly variable motor response

33. Monosynaptic Reflexes Have least delay between sensory input and motor output: e.g., stretch reflex (such as patellar reflex) Completed in 20–40 msec

34. Muscle Spindles The receptors in stretch reflexes Bundles of small, specialized intrafusal muscle fibers: innervated by sensory and motor neurons Surrounded by extrafusal muscle fibers: which maintain tone and contract muscle

35. Postural Reflexes Postural reflexes: stretch reflexes maintain normal upright posture Stretched muscle responds by contracting: automatically maintain balance

36. Polysynaptic Reflexes More complicated than monosynaptic reflexes Interneurons control more than 1 muscle group Produce either EPSPs or IPSPs

37. The Tendon Reflex Prevents skeletal muscles from: developing too much tension tearing or breaking tendons Sensory receptors unlike muscle spindles or proprioceptors

38. Withdrawal Reflexes Move body part away from stimulus (pain or pressure): e.g., flexor reflex: pulls hand away from hot stove Strength and extent of response: depends on intensity and location of stimulus

39. Reciprocal Inhibition For flexor reflex to work: the stretch reflex of antagonistic (extensor) muscle must be inhibited (reciprocal inhibition) by interneurons in spinal cord

40. Crossed Extensor Reflexes Occur simultaneously, coordinated with flexor reflex e.g., flexor reflex causes leg to pull up: crossed extensor reflex straightens other leg to receive body weight maintained by reverberating circuits

41. Integration and Control of Spinal Reflexes Though reflex behaviors are automatic: processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord Higher centers of brain incorporate lower, reflexive motor patterns Automatic reflexes: can be activated by brain as needed use few nerve impulses to control complex motor functions walking, running, jumping

42. Superficial reflexes Stroking of the skin elicits muscle contraction Involves functional upper motor pathways as well as cord level reflex arcs Plantar reflex (L4-S2)…Babinski is normal in infants Usually indicative of CNS damage in adults Abdominal reflex (T8-T12) Absent with corticospinal lesion

43. Spinal Cord Trauma: Transection Cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut Paraplegia – transection between T1 and L1 Quadriplegia – transection in the cervical region