Session 10 ILOs - Nervous tissue, CNS and PNS

Question 1

Describe the structure of a neuron and a nerve cell body (perikaryon)

Answer 1

A neurone has a cell body with an axon projecting from it towards to target tissue (with arborisations are the end) and dendrites branching out from the cell body (perikaryon).

As a basic structure, a neurone has one axon and many dendrites. The cell body, dendrites and the proximal axon are in the CNS, whereas the distal axon and aborisations are within the PNS. Generally, the axon is myelinated.

Question 2

Differentiate between myelinated and unmyelinated nerves and explain their general distribution
and function in the spinal cord

Answer 2

Myelinated have layers of myelin sheath covering them with spaces between each sheath, forming the nodes of Ranvier; myelination occurs around each individual fibres. Myelinated fibres are generally found in the CNS, visceral fibres, motor neurones (not sensory neurones)

Non-myelinated fibres do not have individual Schwann cell sheaths around each nerve. Instead, they have Schwann cell around a bundle of nerves and this is continuous so there are no nodes of Ranvier.

Question 3

Relate demyelination to a slowing of conduction velocity within a nerve

Answer 3

Demyelination disrupts/eliminates the nodes of Ranvier which significantly slows down the conduction velocity, as saltatory conduction cannot take place because the action potential cannot ‘jump’.

Question 4

Explain the function of Schwann cells and the node of Ranvier

Answer 4

One of the most important functions of the Schwann cell is to myelinate axons in the PNS. Myelin is a fatty layer that insulates the axon and helps to increase the saltatory conduction of the neuron by forming nodes of Ranvier. This increases the conduction velocity of the neurone.

Question 5

Explain the role of glial cells and list the 5 main types of glial cell and their function

Answer 5

Glial cells (or neuroglia), are non-neuronal cells in the CNS and PNS that do not produce electrical impulses. They maintain homeostasis, form myelin, and provide support and protection for neurones.

5 types of glial cells:

1. Oligodendrocytes - surround axons in the CNS with myelin
2. Astrocytes - control the flow of nutrients in the CNS and feet contribute to the blood brain barrier
3. Microglial cells - macrophages of the brain, remove damaged nerve cells and immune function
4. Ependymal cells - synthesise CSF in the ventricles
5. (PNS) Satellite cells - sit around cell body and look after it

Question 6

Describe the structure of grey and white matter (in CNS and PNS)

Answer 6

Grey matter - grey because it contains lots of cell bodies. Present around the periphery of the brain, but centre of the spinal cord in a butterfly shape

White matter - white because it contains lots of myelinated axons. Present in the centre of the brain, but around the outside of the spinal cord

Question 7

Describe the 5 main structures of neurones (e.g. multipolar neurones) and where you find them

Answer 7

1. Multipolar = most common:
   - One axon, multiple dendrites
   - Found in CNS
2. Bipolar:
   - One axon and one dendrite
   - Found in sensory organs (olfactory cells, retina, inner ear)
3. Pseudo-unipolar:
   - One axon that has split into two branches (one heading to CNS, one heading to PNS)
   - Sensory neurons of the dorsal root ganglia
4. Unipolar
   - One process leading away from the soma
   - Sensory from skin and organs, to the spinal cord
5. Anaxonic
   - Many dendrites but NO cell body
   - Helps with visual processes

Question 8

Describe the structure and function of the synapse (and its 4 variations)

Answer 8

The synapse is the junction between two nerve cells, consisting of a minute gap across which impulses pass by diffusion of a neurotransmitter

Variations:

1. Axosomatic
   - Directly to the cell body of another neuron
2. Axodendritic
   - Axon terminal synapses with a dendritic spine
3. Axoaxonic
   - Axon terminal synapses directly onto another anoxic terminal
4. Dendro-dendritic
   - Connections between the dendrites of two different neurons

Question 9

Describe the structure and function of Schwann cells and satellite cells (in the PNS)

Answer 9

Schwann cells cover individual neurones in the PNS which forms nodes of Ranvier, or surround bundles of neurones to form an unmyelinated neurone.

Satellite cells sit around the cell body of the peripheral nerve cells - they resemble the astrocytes of the CNS and assist in regulating the external chemical environment.

Question 10

Describe the signs, symptoms and cause of multiple sclerosis

Answer 10

Multiple Sclerosis

Signs and symptoms: Fatigue, slurred speech, vision problems, numbness and tingling sensations, mobility issues, urinary retention and constipation

Cause: An autoimmune disease, where the myelin sheath is degraded which causes a loss of condition velocity - thought that the target of the antibodies is the Epstein-Barr virus (EBV)

Question 11

Describe the structure and function of: astrocytes and the blood brain barrier

Answer 11

Astrocytes contribute to the blood brain barrier. They are a star-like shape with little foot processes called perineural feet, which help to form tight gap functions. The function is to transport nutrients from blood to the nerve cells.
In the blood brain barrier, the astrocytes provide biophysical support for endothelial cells that form the blood brain barrier (establish the link between the endothelial blood flux and neurons)