1. Anatomy & Physiology
Chapter 13
The Spinal Cord
SPR 08

2. The Spinal Cord 3 Functions: Conduction – bundles of nerve fibers
(white matter) conduct information
Sensory info to brain
Motor commands to effectors
Input at one level affects output at another level
Locomotion
Central pattern generators
Reflexes

3. Spinal Cord Anatomy Physical landmarks
Foramen magnum to L1-L2 in adult
Early in fetal development extends full-length of vertebral column
Protected by bone, meninges, and CSF
31 pairs of Spinal Nerves
a. Intervertebral foramina
b. Cord not visibly segmented, but each part supplied by a pair of spinal nerves is called a segment

4. Spinal Cord Anatomy
Ventral (anterior) median fissure and dorsal (posterior) median sulcus
a. Longitudinal grooves visible on cord
b. Partially divide into right and left
Medullary cone – cord tapers to a point
Terminal filum - extension of pia mater to coccyx, functions to anchor cord
Cauda equina – bundle of nerve roots, L2 – S5, lower nerve roots “chase” exit points through vertebral canal

5. Spinal Cord Regions
Regions named for where spinal nerves emerge, not for corresponding vertebrae
Cervical
Cervical enlargement – due to increased gray matter of ventral horn (somatic motor neurons)
Thoracic
Lumbar
Lumbar enlargement – also more gray
Sacral

7. Meninges of Spinal Cord
Dural sheath
Single layer around spinal cord
Tough collagenous membrane
Not attached to bone of vertebral column
Epidural space
Between vertebrae and dural sheath
Contains padding of fat, loose CT, & blood vessels
Epidural anesthesia

8. Meninges of Spinal Cord
Arachnoid mater
Simple squamous epithelium attached to inner dura
Loose mesh of collagen and elastic fibers span gap to attach to pia mater – “cobweb” appearance
Extends to level of S2 with dural sheath

9. Meninges of Spinal Cord
Subarachnoid space
Between arachnoid membrane and pia mater
Filled with CSF

10. Meninges of Spinal Cord
Pia mater
Delicate, translucent “shrink wrap” of cord
Terminal filum – continuation of pia beyond the medullary cone
Denticulate ligaments – saw-toothed shelves of pia that extend through arachnoid to dura to anchor cord

12. Cross-Sectional Anatomy
Gray matter
Contains little myelin, mostly somas and dendrites
Site of synaptic integration
Makes up central core of spinal cord
White matter
Abundance of myelinated axons
Tracts = bundles of axons

13. Gray Matter Central H or butterfly of spinal cord
Dorsal (posterior) horns
Almost entirely interneurons
Ventral (anterior) horns
Some interneurons, but mainly house large somas of somatic motor neurons
Somatic neurons send out axons via ventral root to skeletal muscle
Amount of ventral gray matter at any given level reflects amount of skeletal muscle innervated at that level

14. Gray Matter Lateral horn – thoracic & lumbar regions only
Contains neurons of sympathetic nervous system
Their axons also exit via ventral roots
Ventral roots serve both motor divisions of PNS (somatic & autonomic)
Gray commissure – cross bar that encloses central canal
Central canal – mostly collapsed in adults
Open in young children & limited places in adults, contains ependymal cells and CSF

15. Spinal Nerves Dorsal root Sensory or afferent nerve fibers
Cell bodies in dorsal root ganglia
Ventral root
Motor neurons of somatic and autonomic systems
Dorsal and ventral roots fuse to form spinal nerve

17. White Matter Bundles of myelinated axons
Provide avenues of communication between different levels of CNS
Ascending – sensory fibers
Descending – motor fibers
Transverse – commissural fibers
Bundles arranged into three pairs of Columns = Funiculi
Dorsal (Posterior) column
Lateral column
Ventral (anterior) column

19. White Matter Subdivisions of each column = Tracts (Fasciculi)
Ascending tracts = sensory
Descending tracts = motor
Each tract made up of neurons with similar destinations and functions
Decussation
Contralateral vs. ipsilateral

20. Spinal Tracts
All major spinal tracts are part of multineuron pathways that connect brain to periphery of body
Most pathways decussate at some point
Most consist of a chain of neurons
Most exhibit somatopy, a precise spatial relationship among tract fibers that reflects orderly mapping of body
All pathways are paired (right and left)

22. Ascending tracts Each tract is repeated on right & left sides
of spinal cord
All fibers in a given tract have similar origin, destination and function
Three-neuron pathway
First-order neuron – receptor to spinal cord
Second-order neuron – continues to thalamus “gateway”
Third-order neuron – to sensory region of cerebral cortex

23. Ascending Tracts Gracile fasciculus
Carries signals for proprioception from legs, visceral pain and deep touch, from midthoracic and lower body
Cuneate Fasciculus
Carries same type of information, but from above T6
Medial lemniscus – gracile & cuneate systems decussate in medulla and form m. lemniscus that leads to thalamus

24. Ascending Tracts *Spinothalamic tract
Carries signals for pain, temp, pressure, light touch, tickle & itch
2nd order fibers decussate near point of entry into spinal cord
*Dorsal & ventral spinocerebellar tracts
Proprioception from limbs & trunk to ipsilateral cerebellum

26. Spinothalamic Tract

27. Descending tracts Descending tracts
Carry motor commands down from brain
Involves two neurons in pathway
Upper motor neurons – have somas in cerebral cortex or brainstem
Lower motor neurons - somas in brainstem or spinal cord

28. Upper Motor Neurons
Pyramidal cells (output neurons) of cerebral cortex or brainstem
Give rise to descending motor pathways
Controls muscles on contralateral side
Synapse with lower motor neurons in
brainstem or spinal cord
May facilitate or inhibit lower motor neuron

29. Lower Motor Neurons Anterior horn motor neurons
Innervate skeletal muscle
Only axon extends outside of CNS
Destruction or damage causes flaccid paralysis of affected motor unit

30. Descending Tracts *Corticospinal
Precise, finely coordinated limb movements
Fibers form ridges called pyramids on medulla oblongata (pyramidal tracts)
Control contralateral skeletal muscles
Tectospinal –
Begins in tectum of midbrain
Involved in reflex movements of head

31. Descending Tracts Lateral & medial reticulospinal tracts
Control muscles of limbs, maintain posture & balance
Also contain descending analgesic paths – reduce pain transmission to brain (Ch. 16)
*Vestibulospinal tract
Begins in vestibular nucleus of brainstem
Receives impulses for balance from inner ear, controls limb muscles that maintain balance & posture

32. Lateral & Ventral Corticospinal Tracts

33. Anatomy of Nerves
Nerve - bundle of nerve fibers (axons) bound together by connective tissue
Epineurium
Covers and protects nerves from stretching
Composed of dense irregular fibrous CT
Perineurium
Surrounds a fascicle
1 – 6 layers of squamous epithelium-like cells
Endoneurium - separates individual nerve fibers with thin sleeve of loose CT

34. Anatomy of Nerves Endoneurium External to neurilemma of Schwann cells
Surrounds individual nerve fibers with basal lamina and sleeve of loose CT
Nerves have very high metabolic rate
Blood vessels penetrate to perineurinium, O2 and nutrients diffuse from there through ECF

36. Anatomy of Ganglia in the PNS
Cluster of neuron cell bodies outside CNS
Enveloped in epineurium
Dorsal root ganglion is sensory cell bodies
Fibers pass through without synapsing (Unipolar)
Autonomic ganglion - does contain synapses
Preganglionic fiber onto postganglionic cell body

37. Ganglion

39. The Spinal Nerves 31 pairs spinal nerves 8 cervical 12 thoracic
5 lumbar
5 sacral
1 coccygeal
1st cervical emerges between skull and atlas, others exit at intervertebral foramina

41. Proximal Branches
Each spinal nerve has two points of attachment to spinal cord
Dorsal root
Divides into 6 – 8 rootlets that enter cord
Sensory input to spinal cord
Ventral root
6 – 8 rootlets that leave cord and converge
Motor output of spinal cord
Cauda equina - roots from L2 to Co of the cord

42. Distal Branches
Dorsal ramus - supplies dorsal body muscle, joints, & skin of back
Ventral ramus
In thoracic region – forms intercostal nerve to respiratory muscles
All other ventral rami form nerve plexuses
Meningeal branch - to meninges, vertebrae & spinal ligaments

45. Rami vs. Roots Roots Ramus
Lie medial to and form spinal nerves
Each is strictly sensory or motor
Ramus
Lie distal to and are lateral branches of spinal nerves
Carry both motor and sensory

47. Nerve Plexuses
Ventral rami - branch & anastomose repeatedly to form 5 nerve plexuses
Plexus – Latin for “braid”
Cervical (neck) - C1 to C5
Brachial (armpit) - C5 to T1
Lumbar (low back) - L1 to L4
Sacral (pelvis) - L4, L5 & S1 to S4
Coccygeal (adjacent to lower sacrum) - S4, S5 and C0

48. Structure of a Nerve Plexus
Notice the branching and merging of nerves in this example of a plexus

49. Nerve Plexus
Each individual nerve fiber that emerges from a plexus contains all the fibers that innervate a particular region of the body.
Because spinal nerves are sorted according to final destination, the plexus reduces the number of nerves needed to supply each body part.
Because each body region is innervated by by fibers that originate from several spinal nerves, damage to one spinal nerve does not result in complete loss of function in any one region.

50. Cervical Plexus

51. Paralysis Quadriplegia – upper and lower limbs
Damage usually at C5 – C7
Cardiac and respiratory complications above C5 (Phrenic nerve)
“C3-4-5 keep you alive!”
Notable exceptions:
Christopher Reeves – C1-C2 fracture
Paraplegia – lower limbs
Injury below C7

52. Brachial Plexus

53. Lumbar Plexus

54. Sacral and Coccygeal Plexuses

55. Dermatomes
Each spinal nerve receives sensory input from a specific area of skin called dermatome
Except C1
Overlap at edges by 50%
A total loss of sensation requires anesthesia of 3 successive spinal nerves

56. Reflexes 4 properties of reflex: Require stimulation Quick Involuntary
Automatic responses to sensory input that occur without our intent or often even our awareness
4 properties of reflex:
Require stimulation
Quick
Involuntary
Predictable/stereotyped
Can include glandular secretion, all 3 muscle types, & can be learned (conditioned reflexes)

57. Somatic Reflexes Also called spinal reflexes
Unlearned skeletal muscle reflexes
Involve somatic nervous system and involuntary contraction of skeletal muscle
Pathway of somatic reflex arc
Stimulation of somatic receptors
Afferent fibers carry signal to dorsal horn of spinal cord
Interneurons integrate the information
Efferent fibers carry impulses to effectors
Skeletal muscles respond

58. The Muscle Spindle
Sense organs that monitor the length of skeletal muscles (proprioceptors)
More abundant in muscles of fine control
Provide cerebellum with feedback needed to regulate skeletal muscles
Intrafusal fibers – muscle fibers within a spindle
Other muscle fibers are extrafusal
Three types of nerve fibers in spindle:
Primary afferent
Secondary afferent
Gamma motor neurons

59. Muscle Spindles Muscle spindle = proprioceptor
Sensory nerve endings wrapped
around muscle cell (intrafusal fibers)
b. Contracted muscle fiber relaxes spindle
Muscle relaxation stretches muscle
fiber and muscle spindle, triggers impulse
d. Sensory impulse travels to spinal cord, and motor fibers lead back to same muscle to contract it
e. Helps maintain balance and posture

60. Muscle Spindle

61. The Stretch Reflex Also called myotatic reflex
Not a true spinal reflex, mediated by brain
When muscle stretched, it contracts & maintains increased tonus (stretch reflex)
Helps maintain equilibrium & posture
Stabilize joints by balancing extensors & flexors – feed back to set of synergists & antagonists
Smooth out muscle actions – not jerky

62. Tendon Reflex Sudden muscle stretch causes tendon reflex
Reflexive contraction of muscle when tendon tapped
Monosynaptic reflex – very prompt, little synaptic delay
Reciprocal inhibition – inhibits antagonists

63. The Patellar Tendon Reflex Arc

64. Flexor Reflex
Quick contraction of flexor muscles resulting in withdrawal of limb from injurious stimulus
Often involves reciprocal inhibition
Polysynaptic reflex arc – prolonged output from spinal cord due to different , more complex routes
Ipsilateral reflex

65. Crossed Extensor Reflex
Contraction of extensor muscles in opposite limb
Enables you to keep your balance
Afferent nerve fibers synapse with interneurons that communicate with motor neurons that supply muscles of contralateral limb - contralateral reflex
Intersegmental reflex arc

66. Flexor & Crossed Extensor Reflexes
Flexor (Withdrawal) reflex - withdrawal of foot
Crossed extensor reflex - maintaining balance by extending other leg

67. Golgi Tendon Organs In tendons, close to attachment to muscle
Connected to skeletal muscle fibers, and
innervated by sensory neuron
Stimulated by increased tension
Impulses produce reflex that inhibits or moderates
contraction
Golgi tendon organs have opposite effect as
muscle spindles
Also help maintain posture, protects muscles
from being overstretched & evens out workload
among muscle fibers

68. Golgi Tendon Reflex Proprioceptors in tendon near junction with muscle
Excessive tension on tendon inhibits motor neuron - muscle contraction decreased, protective reflex
Also functions when muscle contracts unevenly