Neurology: Anatomy - Peripheral nervous system

Question 1

What are the components of the peripheral nervous system?

Answer 1

Consists of the nerve fibres and their nerve cells bodies that reside outside the CNS

Question 2

What are the three components of a nerve?

Answer 2

1. Bundle of nerve fibres outside the CNS (or a fascicle in the case of a larger nerve - a “bundle of bundled fibres”)
2. Connective tissue coverings that bind the nerve fibres and fascicles together
3. Vaso nervorum (blood vessels) that nourish the nerve fibres and their coverings

Question 3

What are the three layers of connective tissue binding nerve fibres and fascicles together?

Answer 3

Endoneurium: delicate connective tissue immediately surrounding neurolemma cells and axons
Perineurium: dense connective tissue layer enclosing fascicle
Epineurium: thick connective tissue surrounding and enclosing a bundle of fascicles, forming the outermost covering of the nerve (includes fatty tissue, blood vessels, and lymphatics)

Question 4

What are the three components of a nerve fibre?

Answer 4

1. Axon
2. Neurolemma
3. Surrounding endoneurial connective tissue

Question 5

What does the neurolemma consist of?

Answer 5

Consists of the cell membranes of Schwann cells that surround the axon, separating it from other axons

Question 6

What are the two possible forms of neurolemma?

Answer 6

1. In myelinated nerve fibres: Schwann cells specific to an individual axon, organised into a continuous series of enwrapping cells that form myelin
2. In unmyelinated nerve fibres: multiple axons are separately embedded within the cytoplasm of each Schwann cell - these Schwann cells do not produce myelin

Question 7

Are most fibres in cutaneous nerves myelinated or unmyelinated?

Answer 7

Unmyelinated

Question 8

What are the two different types of nerve fibres and their roles?

Answer 8

Afferent (sensory) fibres: neural impulses to the CNS from sensory organs
Efferent (motor) fibres: neural impulses from the CNS to effector organs

Question 9

What is the difference between the ventral and dorsal nerve roots?

Answer 9

Anterior (ventral) nerve root: consists of efferent fibres passing from nerve cell bodies in the anterior horn of spinal cord gray matter to effector organs
Posterior (dorsal) nerve root: consists of afferent fibres from nerve cell bodies in the spinal (sensory) or posterior (dorsal) root ganglion that extend peripherally to sensory endings and centrally to the posterior horn of spinal cord gray matter

Question 10

Describe the course of spinal nerves from the point at which they arise from the spinal cord

Answer 10

Initially arise from the spinal cord as rootlets which converge to form two nerve roots (ventral and dorsal, or anterior and posterior)
Anterior and posterior nerve roots unite, within or just proximal to the intervertebral foramina, to form a mixed (motor and sensory) spinal nerve, which then immediately divides into the posterior (dorsal) and anterior (ventral) rami (which also carry both motor and sensory fibres)

Question 11

Why is segmental (dermatomal) distribution so different to peripheral nerve distribution?

Answer 11

The majority of anterior rami merge with one or more adjacent anterior rami to form somatic nerve plexuses in which their fibres intermingle, and from which a new set of multisegmental peripheral nerves emerges