MNSR 13 - Nervous system: Neurons, Myelin, Nerves

Question 1

What are the functions of the nervous system?

Answer 1

There are four functions of the nervous system:

1. coordinate muscle activity
2. Regulate the organs
3. Control input from the senses
4. Initiate actions

Question 2

What are receptors and effectors?

Answer 2

Receptors are cells that receive sensations, while effectors are cells that make the appropriate response to the sensations.

Question 3

What are the two subdivisions of the nervous system?

Answer 3

1. Central Nervous System (brain & spinal cord)
2. Peripheral Nervous System
   - Autonomic Nervous System (Sympathetic, Parasympathetic, Enteric)
   - Somatic Nervous System

Question 4

What is neurulation?

Answer 4

Neurulation is the formation of the Neural Tube from a specialized region of ectoderm called the neural plate.

1. Neural plate forms from the ectoderm and begins to fold.
2. Neural plate binds dorsally to form the neural crest.
3. Closure of the neural tube disconnects the neural crest from the epidermis.
4. Neural crest cells differentiate to form the PNS.

Question 5

What is a Neural Tube Disorder? Give examples.

Answer 5

Neural Tube Disorders are malformations of the neural tube. Tube defects include:

1. Anencephaly (head)
2. Encephalocele (head)
3. Spina Bifida (spine)

Question 6

Why do pregnant women take folic acid supplements?

Answer 6

Folic acid is the synthetic version of the vitamin B9 that all pregnant women and pre-pregnant women should take to prevent Neural Tube Defects for their baby. Folic acid:

1. is needed to synthesize DNA bases
2. reduces the chance of NTDs by 70% with 400mg/day preconceptional use and 12 weeks post-conceptional use
3. allieviates side-effects of some immunosuppressant drugs

Question 7

What does the somatic nervous system do and include?

Answer 7

1. Includes all neurons connected with muscles, sense organs, and skin
2. Deals with sensory information (afferent nerves: receptors to the spinal cord)
3. Controls the movement of the body (efferent nerves: spinal cord to muscles)

Question 8

What does the Neuron consist of?

Answer 8

1. Dendrites (less than 1mm in length)
2. Soma (contains nucleus, Nissl bodies, neurofilaments, neurotubules)
3. Axon (could be 1m in length)
4. Synaptic terminal

Question 9

What is the Sciatic nerve?

Answer 9

The longest nerve in the body.

Question 10

What are the three types of neurons?

Answer 10

1. Pseudounipolar (PNS):
   - connects sensory organs to the spinal cord
   - one long dendrite, one short axon
   - dendrite is structurally and functionally an axon
2. Bipolar (PNS):
   - specialized sensory neurons found in the retina, inner ear, and olfactory mucosa
   - one dendrite, one axon
3. Multipolar (CNS):
   - large amounts of information received
   - many dendrites, one axon that is long and branched
   - has motor neurons

Question 11

What are ganglia?

Answer 11

Ganglia is a mass of nerve cell bodies and dendrites. The brain is a fusion of many ganglia.

Question 12

What is a plexus?

Answer 12

A complex system of ganglia that provide relay points between different neurological structures in the body.

Question 13

What is Brachial Plexus Injury?

Answer 13

Brachial Plexus Injury is a paralyzed arm causing lack of sensation in the arm, hand, and wrist. Usually occurs during the birth process because a baby’s shoulder becomes impacted during the birth process, causing the brachial plexus to shear or tear.

Question 14

What do axons divide into, in the Central Nervous System?

Answer 14

Axons divide into two things:

• grey matter: contains somata, dendrites, and a few axons
• white matter: contains just axons

Question 15

What are glial cells?

Answer 15

Glial cells are non-nervous cells that surround nerve cells. In the CNS, they are called OLIGODENDROCYTES (neuroglial cells), while in the PNS they are called SCHWANN CELLS. They both wrap around axons to form layers of the myelin sheath that provide insulation to the axon.

Question 16

Explain myelination.

Answer 16

Myelination begins in the fourth month of fetal life and continues after birth. As the process continues, the cytoplasm from the Schwann cells is excluded, forming just layers of plasma membrane around the axon.

Question 17

What is found between each Schwann cell?

Answer 17

Between each Schwann Cell are gaps called the nodes of Ranvier, where the axon is not myelinated and the axolemma shows.

Question 18

What are non-myelinated nerve fibers?

Answer 18

Many axons are embedded in grooves of the cytoplasm of a Schwann Cell, which fuses to form a mesaxon.

Question 19

What are peripheral nerves? Explain its structure.

Answer 19

Anatomical structures consisting of many nerve fibers called fascicles.

1. The axons of a fascicle remain a matrix of loose collagen fibers and capillaries called the ENDONEURIUM.
2. Each fascicle is bound by dense collagen fibers called the PERINEURIUM.
3. All fascicles of a peripheral nerve remain in a layer of connective tissue called the EPINEURIUM.

Question 20

What is Multiple Sclerosis?

Answer 20

Multiple Sclerosis is the loss of the myelin sheath of the oligodendrocytes of the CNS. It can lead to partial or complete blindness, motor disorders of the limbs, and 80% of cases lead to total disability.

Question 21

What is neurogenesis?

Answer 21

Neurogenesis is the process by which neurons are generated from neural stem cells and progenitor cells. This process is most active while a baby is in the womb and is responsible for the production of the brain’s neurons.