1. 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: Arachnoid mater: Pia 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-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
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