Lecture 1: Introduction to the Peripheral Nervous System

Question 1

Describe the three fundamental dichotomies of the nervous system.

Answer 1

• Central v peripheral: CNS = brain and spinal cord; PNS = 12 pairs of cranial nerves, 31 pairs of spinal nerves, and the ganglia.
• Sensory v. motor: Sensory info is relayed in an afferent pattern, motor info is relayed in an efferent pattern. Different populations of neurons are used, as are different pathways.
• Somatic v. visceral: Somatic structures involve the body wall. Visceral structures and pathways concern the internal organs. Different populations and pathways are used.

Question 2

What are some somatic sensory stimuli?

Answer 2

Touch, pain (burning, cutting, tearing, crushing, etc), temperature, pressure, vibration

Question 3

In somatic motor and sensory innervation pathways, how many neurons connect CNS to the target muscle? How many neurons connect site of stimulus to CNS?

Answer 3

One

Question 4

Describe the functional segmentation of the spinal nerves. Include quantities and types of innervation associated with each.

Answer 4

There are 31 pairs of spinal nerves. 8 Cervical; 12 Thoracic; 5 Lumbar; 5 Sacral; 1 Coccygeal.
All have somatic sensory components.
S5 and Co1 do NOT have somatic motor; everything else does.
All nerves carry sympathetic axons.
Only S2-S4 carry parasympathetic axons.

Question 5

What are somites, and where do they come from?

Answer 5

The mesoderm of an embryo has a layer of paraxial mesoderm, near the axis/notochord. During the process of differentiation, the paraxial mesoderm undergoes segmentation, and becomes 37 pairs of somites – chunks of developmental tissue that will give rise to the body wall. Each somite has its own blood supply and innervation associated with it.

Question 6

Describe the divisions of tissue in each somite.

Answer 6

During differentiation, the innermost (closest to neural tube) tissue will become the sclerotome, which will give rise to axial bone. The outer layer of tissue will become the dermamyotome, which will give rise to skeletal muscle and dermis associate with that somite. The dermamyotome is further divided into epaxial demamyotome (dorsal) and hypaxial dermamyotome (ventral). The epaxial demamyotome gives rise to the intrinsic back muscles and the dermis that covers them. The hypaxial dermamyotome gives rise to all other dermis/muscle.

Question 7

Describe the pathway: burn hand on the stove.

Answer 7

Stimulus –> ventral ramus –> spinal nerve proper –> dorsal root ganglion (cell body) –> dorsal root –> dorsal horn –> CNS

Question 8

Describe the pathway: move intrinsic back muscle

Answer 8

Upper motor neruron –> cell body in ventral horn –> ventral root –> spinal nerve proper –> dorsal ramus –> intrinsic back muscle

Question 9

Describe the pathway: chiropractor “cracks” your back.

Answer 9

Stimulus –> dorsal ramus –> spinal nerve proper –> dorsal root ganglion (cell body) –> dorsal root –> dorsal horn –> CNS

Question 10

Describe the pathway: wave hello

Answer 10

Upper motor neuron –> cell body in ventral horn –> ventral root –> spinal nerve proper –> ventral ramus –> skeletal muscle