The Spinal Cord and Spinal Nerves

1
Chapter 14

• The Spinal Cord and Spinal Nerves

2
Spinal Cord

• Begins at foramen magnum extends down to L1/L2.
• Contiguous with the medulla of the brain stem.
• Passes through the vertebral foramen of all
  vertebrae.
• Consists of cervical, thoracic and lumbar
  divisions same as vertebral column.
• PNS - 31 paired spinal nerves one at each level
  C1-C8, T1-T12 and L1-L5, S1-S5 and Co-1.
• Contains gray and white matter, however gray
  matter is central and white matter surrounds gray
  matter.
• At L1/L2 level cona medullaris tapers into cauda
  equina

3

• Spinal cord levels
• C1-C8
• T1-T12
• L1-L5
• Cauda eqiuna
• S1-S5

4
The Meninges

• Consists of 3 layers around the brain and spinal
  cord
• Dura mater- tough outer fibrous connective tissue
  layer two layers periosteal (periosteum) and
  meningeal.
• Arachnoid mater- inner layer attached to dura
  mater - subarachnoid space between arachnoid and
  pia mater filled with CSF.
• Pia mater- thin connective tissue layer closely
  adhered to brain surface and spinal cord. Pia
  mater goes deep into the sulci (grooves) of the
  brain.

5
Vertebral column (x-section)

6
Spinal cord (x-section)

7
Gray matter of spinal cord

• Shaped like a butterfly with central canal.
• Wings contain anterior, and posterior horns.
• Thoracic and Lumbar levels also have lateral
  horns.
• Contains neuron cell bodies, unmyelinated axons,
  dendrites and neuroglia.
• Gray commissure has fibers that communicate
  between right and left halves of spinal cord.

8
Gray matter of spinal cord

9
White matter of spinal cord

• Contains myelinated ascending and descending
  spinal tracts.
• Divided into columns or funiculi each column
  consists of tracts or fasciculi.
• Also has white commissural tracts that cross thru
  cord
• Ascending tracts carry sensory information from
  body to brain
• Descending tracts carry motor information from
  brain to motor nerves
• Tracts named according to where they originate
  and terminate e.g. Lateral spinothalamic tract

10
Tracts in white matter of spinal cord

11
Ascending spinal tracts

• Transmit sensory information from peripheral
  body.
• Receptors relay information to spinal cord via
  sensory nerve in dorsal root and may synapse in
  dorsal horn of gray matter, decussate or ascend
  to a higher level.
• Tracts are named according to their origin and
  destination, e.g. Lateral spinothalamic tracts
  anterior spinothalamic posterior
  spinalcerebellar tract, etc.

12

• Ascending tracts
• Three neuron chain
• First-order
• Receptor to afferent
• nerve into spinal cord.
• Second order
• Interneuron to relay
• nucleus in thalamus.
• Third order
• Thalamus projects
• information to cortex.

13
Descending spinal tracts

• Take information from brain and descend in gray
  columns to appropriate level to a lower motor
  neuron.
• Motor innervation is contralateral i.e. the left
  side of the brain controls the right side of the
  body and vice versa. Cross over at pyramidal
  decussation in medulla.
• Tracts are named according to their origin and
  destination, e.g. Lateral corticospinal tract
  anterior corticospinal tract reticulospinal
  tract, etc.

14

• Descending tracts
• Two neuron chain
• Upper motor neuron
• From cortex or brain
• stem to LMN
• Lower motor neuron
• From spinal cord to
• muscle or target organ
  

15
Nerve structure

• Nerves are only in the periphery
• Cable-like organs in PNS cranial and spinal
  nerves
• Consists of 100s to 100,000s of myelinated and
  unmyelinated axons (nerve fibers).
• Endoneurium surrounds each axon (nerve fiber).
• Axons are grouped into bundles of fascicles
• Perineurium surrounds each fascicle
• Epineurium surrounds each nerve bundle
• Conduction is saltatory (i.e. jumps node to node)
  in myelinated nerves and continuous in
  nonmyelinated.

16
Nerve anatomy

17

18
Spinal Nerves

• There are 31 pairs of spinal nerves. Spinal
  nerves are all mixed nerves (i.e. sensory and
  motor). Each forms by combining the dorsal and
  ventral roots at a segment of the spinal cord.
• As spinal nerves exit vertebral foramen they
  branch into a meningeal nerve and a dorsal and
  ventral ramus.
• The meningeal branch re-enters the spinal column
  to supply the meninges, spinal ligaments and
  vertebrae.
• After leaving the spinal cord, the ventral ramus
  of select nerves combine to form nerve plexuses
  that innervate the appendages and other parts of
  the body.

19
Dorsal and ventral ramus nerves

• Dorsal ramus nerves innervates the ligaments,
  muscles and skin of the back.
• Ventral ramus nerves innervates the anterior and
  lateral muscles, skin and ligaments of the trunk.
  They also give rise to the nerve plexuses that
  innervate the appendages.

20

21

• Thoracic level
• -Dorsal ramus
• -Ventral ramus
• -Dorsal root
• ganglion
• -Sympathetic
• chain ganglion
• -white and gray
• rami communicantes

22

23
Spinal nerve plexuses from ventral ramus -
major nerves off of them.

• Plexus network of nerves
• Cervical plexus forms from C1 C4
• -Phrenic nerve is off of cervical plexus C3-C5
• Brachial plexus forms from C5 C8, and T1
• -Axillary, radial, median, ulnar nerves
• Lumbar plexus forms from L1 - L4
• -Femoral and obturator nerves
• Sacral plexus forms from L4 S4
• -Sciatic nerve

24

• Spinal nerve
• plexuses

25

• Cervical plexus
• -C1- C5
• Phrenic nerve
• -C3- C5

26

• Brachial plexus
• -C5-C8 T1
• Axillary nerve
• - C5 C6
• Median nerve
• - C5-C8 T1
• Radial nerve
• - C5-C8 T1
• Ulnar nerve
• - C8 T1

27

28

• Lumbar plexus
• L1- L4
• Femoral nerve
• -L2-L4
• Obturator nerve
• -L2-L4

29

• Sacral plexus
• -L4- S4
• Sciatic nerve
• -Tibial n. L4-S3
• -Comn fibular
• -L4-S2

30
DERMATOMES

• The area of skin that carries sensory information
  to the CNS from all of the spinal nerves (except
  C1) and cranial nerve V (facial).
• Entire body surface is supplied by a peripheral
  nerve.
• Knowing the distribution of these nerves is very
  helpful diagnostically in determining whether a
  nerve is affected at the central or peripheral
  level and at what level the injury may be.

31

32
Reflex Arc

• Consists of 1. sensory receptor, 2. afferent
  nerve, 3. interneuron, 4. efferent nerve and 5.
  an effector organ (muscle or gland).
• Occurs at the spinal level

33
Mono and Polysynaptic Reflexes

34
Receptors Classification by location

1. Exteroceptors- receptors sensitive to changes
   outside of body. Are found on or near the body
   surface pain, pressure, touch, temperature and
   special sense organs (eye, ear, nose, mouth).
2. Interoceptors- visceroceptors receive stimuli
   from viscera stretch, temperature, chemical,
   taste.
3. Proprioceptors- located in skeletal muscles and
   joints, tendons, ligaments perceive stretch in
   these organs

35
Proprioceptors

• Encapsulated receptors that monitor stretch in
  muscles and tendons.
• Ex. Muscle spindles
• Extrafusal fibers
• Intrafusal

36
Classification by Stimulus

1. Mechanoreceptors- respond to mechanical forces
   touch, pressure, stretch, vibration, and itch.
2. Thermoreceptors- respond to temperature changes
3. Chemoreceptors- respond to chemicals in solution
   and blood chemistry.
4. Photoreceptors- respond to changes in light-eye
5. Nociceptors- respond to pain and harmful stimuli
   leading to pain.

37
Classification by Structure

• Free nerve endings- in all tissues of body
  abundant in epithelial and connective tissue.
• - respond to pain, temperature, itch and light
  pressure
• Ex.- Merckels discs, hair follicle receptors
• Encapsulated nerve endings- enclosed in a capsule
  of connective tissue vary in shape and
  distribution.
• Ex.- Meissners, Pacinian and Ruffinis
  corpuscles

38
Reflex Arcs

• May be unilateral or contralateral
• Unilateral knee jerk Flexors contract extensor
  relax

39
Reflex Arcs

• Unilateral Excess tension on tendon causes
  inhibition of flexor and stimulation of extensor

40
Crossed-Extensor reflex arc

• Pain to one foot causes retraction of injured
  foot and extension of opposite foot

41
PNS to CNS