FAE: Nervous System

Question 1

What is the nervous system?

Answer 1

• An orderly arrangement of neurons and associated cells
• Its role is to control and adjust the activities of the systems of the body
• Sensory and motor neurones link the CNS to the peripheral structures of the body, travelling in peripheral nerves

Question 2

What do neurons have as they are very delicate cells?

Answer 2

They have a number of supporting cells that help them fulfill their role (4 types in CNS, 2 in PNS)

Question 3

What are the two main divisions of the nervous system?

Answer 3

1) CNS - brain and spinal cord

2) PNS - anything that leaves the CNS to peripheral side including axons, ganglia and supporting cells

Question 4

Describe the two cell types in the neural tissue (CNS)

Answer 4

1) neurons (nerve cells) - transmit electrical impulses along their cell membranes
- can transport material from the cell body, along the axon through microtubules
2) supporting cells - responsible for physical support (keeping things in the correct place) of neurons, regulating the interstitial fluid (fluid in which neurons sit) and bringing nutrients to neurons

Question 5

Describe neurons

Answer 5

• Highly differentiated cells with very limited capacity for cell division
• Have limited capacity to repair themselves after injury e.g. axons can often grow and find new connections
• They vary in length from a few mm (in CNS) to over a meter (in PNS), depending on height

Question 6

What is a multipolar neuron (axon)?

Answer 6

A neuron with a cell body (soma) which contains the nucleus and most of the organelles especially RER to make material that is transported along the axon to its terminations to release synaptic vesicles

Question 7

Describe the cell body of a multipolar neuron

Answer 7

• It contains numerous dendrites (processes) which are highly branched (with dendritic spines) where axons from other neurons will encroach
• It has lots of information coming in from synaptic knobs and it needs to decide if there is enough excitatory info to pass on the message to the next part of the CNS/PNS or if the message is inhibitory (nothing happens)
• The many connections allows humans varied behaviour

Question 8

Describe the axon of a multipolar neuron

Answer 8

• A long axon extends from the cell body which terminates in one or more synaptic terminals where the neuron comes into contact with another cell (neuron, receptor or effector organ e.g. muscle/gland)
• The axon also contains microtubules to allow transport of material to take place

Question 9

What is the name for the supporting cells of the CNS?

Answer 9

Neuroglial (glial) cells - non nervous support cells within the CNS, each with its own specific function
- four types of glial cell in the CNS

Question 10

What are the 4 types of glial cell in the CNS?

Answer 10

1) astrocytes - somewhat star shaped
2) microglia - macrophages of CNS
3) ependymal cells - line the ventricles of the brain
4) oligodendrocytes - support the axons, either through myelination or simply wrapping around them

Question 11

Describe astrocytes

Answer 11

• Astrocytes are the largest and most numerous of the glial cell types
• Their processes are in contact with the neurons (cell bodies and axons) and the walls of capillaries
• Act as a ‘net’ supporting the delicate neural tissues

Question 12

What are the 5 functions of astrocytes?

Answer 12

1) maintenance of the blood-brain-barrier, axons and cells bodies
2) contains microfilaments which give structural strength to the CNS
3) stabilises structure after injury ∴ producing scar tissue
- this is a problem bc it prevents the axon from reinnervating structure if scar tissue is blocking it
4) directs neural growth during development
5) controls the interstitial environment e.g. by recycling excess NTs not recycled at the synapse

Question 13

Describe microglia

Answer 13

• They are small, slender cells with numerous cytoplasmic processes which have fine branches
• They form from the same stem cells as macrophages and monocytes and undertake a similar function, just in the brain
• Usually they make up 5% of glial cells but their numbers rapidly increase if infection is present

Question 14

Describe ependymal cells

Answer 14

• Line the chambers and passageways of the CNS which are filled with cerebrospinal fluid (CSF)
• Unlike other epithelial cells, they have branching processes which make contact with other glial cells and interstitial cells
• They have a function in monitoring the composition of CSF and ensuring the whole network has an integrated function through connection to the other glial cells

Question 15

Describe oligodendrocytes

Answer 15

• They have processes which come into contact with axons either wrapping them together or myelinating them to improve the conducting velocity of the axons
• The cell body is in the centre on multiple axons and it is never in isolation - other oligodendrocyte processes form neighbouring myelin
• Many axons are completely ensheathed in myelin which wraps itself around a section of the axon forming a multi-layered structure

Question 16

What are the two supporting cells in the PNS?

Answer 16

1) Schwann cells

2) Satellite cells

Question 17

How are myelinated axons supported by Schwann cells?

Answer 17

• The Schwann cells myelinate a section on axon up to 1mm in length, making myelinated axons usually 1-25 microns in diameter
• 1:1 relationship between Schwann cell and the axon
• Schwann cell wraps itself around the axon, producing myelin

Question 18

How are unmyelinated axons supported by Schwann cells?

Answer 18

• The Schwann cell wraps around multiple axons with no gaps (∴ no nodes of Ranvier) with the nucleus of the Schwann cell in the centre of multiple axons
• The Schwann cell envelops the axons rather than wrapping itself continuously around them
• They are usually <2.5 microns in diameter