Topic 8- Grey Matter

Question 1

What is the structure of the myelin sheath?

Answer 1

The myelin sheath is made up of Schwann cells, types of glial cells. They form spirals around the axon and have a nucleus like other cells.

Question 2

How do the structures of motor and sensory neurones differ?

Answer 2

Motor neurones have a cell body at one end, whereas sensory neurones have the cell body in the middle of the axon

Question 3

What structures are found at the ends of all neurones?

Answer 3

Dendrons

Question 4

What is the advantage of myelinated neurones?

Answer 4

Myelin sheath acts as electrical insulator, preventing ions crossing the axon membrane. This facilitates saltatory conduction, where an action potential can jump to the nodes of ranvier between Schwann cells.

Question 5

How and when do pupils constrict?

Answer 5

Pupils constrict in very light conditions by contraction of the circular muscles of the iris

Question 6

How and why do pupils dilate?

Answer 6

Pupils dilate to allow as much light into the eye as possible in dark conditions. This happens when the radial muscles of the iris contract

Question 7

What is found between two neurones?

Answer 7

A synaptic cleft

Question 8

What is the advantage of having synapses?

Answer 8

Synapses allow flexibility of response and integration of information from different neurones, allowing a coordinated response

Question 9

What happens in the presynaptic membrane when an action potential arrives?

Answer 9

The membrane depolarising causes calcium ion channels to open and allow ca2+ ions into the cell, which cause exocytosis of vesicles that contain neurotransmitter

Question 10

How does the neurotransmitter travel across the synaptic cleft?

Answer 10

Diffusion

Question 11

What happens on the postsynaptic membrane as the neurotransmitter arrives?

Answer 11

The neurotransmitter binds with specific receptors on the postsynaptic membrane and cation channels open, allowing Na+ ions into the cell. This causes depolarisation of the membrane, and if the hires hold is reached an action potential is created

Question 12

What happens to neurotransmitters after they bind to receptors?

Answer 12

Some are broken down by enzymes and the products are re absorbed by the presynaptic membrane to be resynthesised using ATP. Some diffuse away from the cleft or are taken up by other cells in the nervous system

Question 13

Name some human neurotransmitters

Answer 13

Noradrenaline
Dopamine
Acetylcholine

Question 14

Which enzyme breaks up acetylcholine at the postsynaptic membrane?

Answer 14

Acetylcholinesterase

Question 15

What is the resting potential of an axon membrane?

Answer 15

-70mV

Question 16

How is the resting potential of axon membranes reached/maintained?

Answer 16

Na+ pumped out and K+ pumped in in ratio 3:2. Potassium diffuses out of cell down concentration gradient but sodium can’t cross membrane. This causes a less positive environment in the cell, setting up potential difference of -70mV. Electrochemical gradient causes some K+ to enter the cell, at -70mV concentration and electrochemical gradients balance out.

Question 17

What is the first step in an action potential and how does this happen?

Answer 17

Depolarisation.
Na+ channels open and ions flood into cell, reversing potential difference to about +40mV. Depolarisation causes more Na+ channels to open, called positive feedback

Question 18

What happens after depolarisation and how?

Answer 18

Repolarisation.
Na+ channels close after a short time and K+ channels open. K+ attracted by low concentration and negativity outside of cell and floods out, returning potential difference to -70mV

Question 19

Which types of human neurones are myelinated?

Answer 19

Motor neurones and sensory neurones are myelinated.

Relay neurones are not myelinated

Question 20

What is the name for the time between hyper polarisation and the return to the resting potential?

Answer 20

The refractory period

Question 21

What affects the speed of a nerve impulse?

Answer 21

Axon size (bigger=faster)
Amount of myelin (more=faster)
Temperature (hotter=faster)

Question 22

How does an action potential move to the next node?

Answer 22

The positive ions in the axon during depolarisation diffuse towards negative area of axon at next node. This causes slight depolarisation. If the resting potential gets high enough it triggers Na+ channels to open, and depolarisation begins at that node.

Question 23

Why do nerve impulses not go backwards?

Answer 23

When the positive ions diffuse back to the area by the previous node, it is undergoing hyper polarisation and is too negative to be increased to the point where Na+ channels open.

Question 24

How can drugs effect nerve impulses?

Answer 24

They can cause or inhibit action potentials crossing synapses. They can open or close receptors and ion channels. They can competitively or allosteric ally inhibit enzymes.

Question 25

What are the two types of drugs?

Answer 25

Antagonist drugs: inhibitory

Agonist drugs: opens channels