Spinal nerves, cervical, lumbar and sacral plexus

1. Spinal nerves, cervical, lumbar and sacral plexus

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 • Conusmedullaris – terminal portion of the spinal cord • Filumterminale – 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 • Caudaequina – 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 Figure 13–7a

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-S4and 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 Figure 13–8

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 Figure 13–13a

28. 5 Patterns of Neural Circuits in Neuronal Pools • Serial processing: • moves information in single line • Parallel processing: • moves same information along several paths simultaneously Figure 13–13c

29. 5 Patterns of Neural Circuits in Neuronal Pools • Reverberation: • positive feedback mechanism • functions until inhibited Figure 13–13e

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

44. Spinal Nerves • Spinal nerves attach to the spinal cord via roots • Dorsal root • Has only sensory neurons • Attached to cord via rootlets • Dorsal root ganglion • Bulge formed by cell bodies of unipolar sensory neurons • Ventral root • Has only motor neurons • No ganglion - all cell bodies of motor neurons found in gray matter of spinal cord

45. Spinal Nerves • 31 pair • each contains thousands of nerve fibers • All are mixed nerves have both sensory and motor neurons) • Connect to the spinal cord • Named for point of issue from the spinal cord • 8 pairs of cervical nerves (C1-C8) • 12 pairs of thoracic nerves (T1-T12) • 5 pairs of lumbar nerves (L1-L5) • 5 pairs of sacral nerves (S1-S5) • 1 pair of coccygeal nerves (Co1)

46. Formation of Rami • Rami are lateral branches of a spinal nerve • Rami contain both sensory and motor neurons • Two major groups • Dorsal ramus • Neurons innervate the dorsal regions of the body • Ventral ramus • Larger • Neurons innervate the ventral regions of the body • Braid together to form plexuses (plexi)

47. Dermatomal Map • Spinal nerves indicated by capital letter and number • Dermatomal map: skin area supplied with sensory innervation by spinal nerves

48. Introduction to Nerve Plexuses • Nerve plexus • A network of ventral rami • Ventral rami (except T2-T12) • Branch and join with one another • Form nerve plexuses • In cervical, brachial, lumbar, and sacral regions • No plexus formed in thoracic region of s.c.

49. The Cervical Plexus • Buried deep in the neck • Under the sternocleidomastoid muscle • Formed by ventral rami of first four cervical nerves • Most are cutaneous nerves • Some innervate muscles of the anterior neck • Phrenic nerve – the most important nerve of the cervical plexus

50. Branches of Spinal Nerves • Dorsal Ramus • Neurons within muscles of trunk and back • Ventral Ramus (VR) • Braid together to form plexuses • Cervical plexus - VR of C1-C4 • Brachial plexus - VR of C5-T1 • Lumbar plexus - VR of of L1-L4 • Sacral plexus - VR of L4-S4 • Coccygeal plexus -VR of S4 and S5 • Communicating Rami: communicate with sympathetic chain of ganglia • Covered in ANS unit