Nervous System - Peripheral Nervous System

Question 1

Peripheral Nervous System

Answer 1

• all of the nervous tissue outside of the CNS
• primarily consists of nerves that function to communicate between the body and the CNS

Cranial nerves - PNS nerves that connect to the brain

Spinal Nerves - PNS nerves that connect to the spinal cord

Question 2

Cranial Nerves

Answer 2

• allow direct communication between the body and the brain
• one exception - accessory nerve (CN XI) arises from the spinal cord
• 12 pairs (CN I - CN XII)

Question 3

Spinal Nerves

Answer 3

• allow communication between the body and the spinal cord
• 31 pairs
• connected to the spinal cord by rootlets (dorsal root and ventral root)
• attached to spinal cord at regular intervals (reflection of segmental development)

8 Cervical

12 Thoracic

5 Lumbar

5 Sacral

1 Coccygeal

Question 4

Cranial Nerves and # Spinal Nerves

Answer 4

• 12 pairs cranial nerves
• 31 pairs spinal nerves

Question 5

Spinal Cord Segment

Answer 5

• each individual portion of the spinal cord that is derived from one somite (blocks of mesoderm that lead spinal cord to develop in segmental fashion)
• each spinal cord segment is associated with one spinal nerve pair (one nerve on right and one left)

Question 6

Somites

Answer 6

• derived from paraxial mesoderm
• repeating “blocks” of mesoderm that lie on either side of the neural tube
• develops into adult structures in a segmental pattern (i.e. vertebrae)

Question 7

Dorsal Root

Answer 7

• the posterior root (roots connect spinal nerves to spinal cord)
• sensory root - contains sensory neurons
• you’re “sensitive” when someone talks behind you “back”

Question 8

Ventral Root

Answer 8

• • the anterior root (roots connect spinal nerves to spinal cord)
• motor root - contains motor neurons
• you drive a “motor” vehicle facing “front”

Question 9

Spinal Nerve

Answer 9

• collection of axons in the PNS
• connected to the spinal cord by rootlets (dorsal and ventral roots)
• divides into two branches (dorsal ramus and ventral ramus)

Question 10

Dorsal Ramus

Answer 10

• one of two primary branches of the spinal nerve
• innervates the muscles and the skin of the back

Question 11

Ventral Ramus

Answer 11

• one of two primary branches of the spinal nerve
• innervates structures of the ventrolateral neck/trunk and limbs (everything the dorsal does not)
• typical - travels around the body wall and innervates muscle and skin of the trunk
• atypical - forms nerves plesuses

Question 12

Dorsal Root Ganglion

Answer 12

• a collection of neuron cell bodies in the PNS
• enclosed by a connective tissue covering
• each dorsal root has a ganglion associated with it (dorsal root ganglion) - contains cell bodies for all the afferent neurons that travel in a particular spinal nerve and its dorsal root

Question 13

Cauda Equina

Answer 13

• collection of dorsal and ventral roots in the inferior portion of the dural sac that have not exited the vertebral canal through intervertebral foramina
• named for its resemblance to a “horses tail”

Question 14

Distribution of cranial vs. spinal nerves

Answer 14

Cranial nerves - primarily innervate structures in the head and neck

Exceptions - CN X (Vagus Nerve) travels to the lower abdomen

Spinal nerves - primarily innervate structures below the head

Exceptions - the first few cervical nerves do send some branches to the head

Question 15

Dermatome

Answer 15

• the area of skin innervated by a particular spinal nerve pair
• due to segmental development from somites

Question 16

Nerve Roots and Rami

Answer 16

Question 17

Spinal Nerve Innervation

Answer 17

Dorsal Ramus - travels to the back region

Ventral Ramus - travels around the body wall

Thoracic Region (T2-T12) are “typical” spinal nerves because their ventral rami do not for plexuses

Question 18

Nerve Plexus

Answer 18

• a network of mixing nerves
• T2-T12 do not tend to form plexuses

Question 19

“Atypical” Spinal Nerves

Answer 19

• form plexuses
• only ventral rami
• some plexuses travel out into the limbs/extremities
• Spinal nerves that tend to form plexuses:

Cervical

T1

Lumbar

Sacral

Question 20

How spinal nerves exit the vertebral column

Answer 20

• through the intervertebral foramen
• Cervical spinal nerves exit superior to the vertebra they are named for (i.e. C4 nerve exits between C3 and C4 vertebrae)
• C8 passes inferior to C7 and superior to T1
• All remaining spinal nerves below pass through inferior to the vertebra they are named for (i.e. T4 nerve passes between T4 and T5)

Question 21

Distance bw spinal nerve and corresponding vertebra

Answer 21

• increases as you move inferiorly down the spinal column
• consequently, the spinal nerve roots must travel further to exit the vertebral canal
• creates cauda equina since the spinal cord ends at L2 (conus medullaris)