Basic Anatomy of the Somatic Nervous System

Question 1

What is the axon hillock?

Answer 1

The initial initiation segment of an axon where membrane potentials are summated.

Question 2

What is the effector organ of the somatic nervous system?

Answer 2

Skeletal muscle.

Question 3

Is muscle tone controlled by the somatic or autonomic nervous system?

Answer 3

Somatic.

Question 4

What is the zone that forms the boundary between the CNS and PNS?

Answer 4

The Redlich-Orbersteiner zone.

Question 5

What is another term for the Redlich-Orbersteiner zone?

Answer 5

The root entry zone.

Question 6

What change can be seen in the Redlich-Orbersteiner zone?

Answer 6

Change in type of myelin produced on both sides; oligodendrocytes produce myelin in the CNS whereas Schwann cells produce myelin in the PNS.

Question 7

Why is the Redlich-Orbersteiner zone the site for most nerve-based neurological syndromes?

Answer 7

The Redlich-Orbersteiner zone is often the place of neuro-vascular compression.

Question 8

Name some examples of nerve-based neurological syndromes that may occur in the Redlich-Orbersteiner zone.

Answer 8

1 - Trigeminal neuralgia

2 - Vestibular schwanomas

3 - Acoustic neuromas

4 - Facial nerve compression

5 - Vago-glossopharyngeal compression

6 - Cerebello-pontine angle nerve compressions

Question 9

What is the difference in tumours in the CNS as opposed to the PNS?

Answer 9

In the CNS, malignant tumours exist (as an exclusive feature of glia), whereas in the PNS, all tumours are benign.

Question 10

What is the morphology of a somatic efferent neurone (motoneurone)?

Answer 10

Multipolar

Question 11

What is the morphology of most afferent neurones?

Answer 11

Pseudounipolar

Question 12

Where are the somata of somatic motoneurones located?

Answer 12

• In the ventral horn of the spinal cord

* In cranial nerve motor nuclei of the brain

Question 13

What gives somatic motoneurones Aα classification?

Answer 13

They innervate extrafusal muscle.

Question 14

List the subtypes of motor neurones.

How do they differ in terms of myelination?

Answer 14

•Heavily myelinated:
–Aα
–Aβ
•Lightly myelinated:
–Aδ

Question 15

What is a motoneurone?

Answer 15

An efferent that innervates skeletal muscles to:

• Displace the limbs
• Set muscle tone

Question 16

What is the range of thickness of myelin in axons?

Answer 16

0.5-2.5 um

Question 17

Name 4 demyelinating diseases.

Answer 17

Demyelinating diseases:

1 - Multiple Sclerosis

2 - Guillain Barre

3 - Diabetes

4 - Polyneuropathies

Question 18

Other than increasing the velocity of axonal conduction of nerve impulses, what other function is served by the myelin sheath?

Answer 18

If a nerve axon is cut and it dies away, the myelin sheath left behind maintains the tunnel where the old axon ran. This creates a path taken by remyelinating axons

Question 19

What are the 3 characteristic neuroglia of the PNS? What are their functions?

Answer 19

• Schwann cells:
  • 1 axon myelinated per Schwann cell.
  • Insulates peripheral nerve axons.
• Satellite cells:
  •Physical support of neurones in the PNS.
• Microglia:
  •Immune and inflammatory functions.

Question 20

What are the membranous envelopes of nerves in the PNS from superficial to deep?

Answer 20

• Epineruium:
  •Enseaths the entire nerve.
  •Interfasicular bands attach adjacent nerve fascicles.
• Perineurium:
  •Enseaths a single fascicle (a collection of axons).
• Endoneurium:
  •Enseaths a single axon.

Question 21

What are the three classes of muscle?

Answer 21

Skeletal, cardiac and smooth.

Question 22

Which of the three classes of muscle are striated?

Answer 22

Skeletal and cardiac.

Question 23

To which area of the muscle is the neuromuscular junction confined to?

Answer 23

The nerve entry point. Also known as the neruovascular hilum.

Question 24

What are the important features of the nerve entry point?

Answer 24

• It is the geographical centre of all muscles.
• It is the site of entry of a motoneurone into the substance of a muscle.
• It is the site of entry of arterial supply to a muscle.
• It is the site of exit of venous drainage of a muscle.

Question 25

What are the 4 characteristic neuroglia of the CNS? What are their functions?

Answer 25

• Astrocytes:
  •Axon guidance
  •Synaptic support
  •Control of BBB
• Microglial cells:
  •Immune and inflammatory function
• Ependymal cells:
  •Give rise to epithelial tissue that produce CSF.
• Oligodendrocytes:
  •Many axons myelinated per oligodendrocyte
  •Insulates central nerve axons.