C13 The synapse

Question 1

What are neurones separated by

Answer 1

Synapses

Question 2

what is the role of a synapse

Answer 2

to send the nervous impulse between neurones in one direction only

Question 3

what are the two classes of synapse

Answer 3

Electrical synapse

Chemical synapse

Question 4

what is the main thing to remember about electrical synapses

Answer 4

it is 3nm gap which means that an electrical impulse is transmitted directly from one neurone to the next

Question 5

what are the main things to remember about a chemical synapse

Answer 5

it is a 20nm gap

too big for the nervous impulse to jump

most junction between neurones are chemical synapses

Branches of axons lie close to dendrites of other neurone but do not touch

Question 6

what is the impulse transmitted by

Answer 6

neurotransmitter

which is a chemical that diffuses across the synaptic cleft from the presynaptic membrane of one neurone to the postsynaptic membrane of an adjacent neurone where a new impulse is initiated

Question 7

What is a neurotransmitter

Answer 7

a chemical secreted in response to an action potential

which carries a chemical signal across a synapse

from one neurone to the next where a new action potential is generated

Question 8

What are the steps involved in the synaptic transmission sequence

Answer 8

1/ the arrival of an impulse at the synaptic end bulb alters its membrane permeability, opening voltage-dependent calcium channels, so calcium ions diffuse into the end bulb down their concentration gradient

2/ the influx of calcium ions causes the synaptic vesicles to move towards and fuse with the pre-synaptic membrane. this releases the neurotransmitter ACETYLCHOLINE by exocytosis into the synaptic cleft

3/ The neurotransmitter diffuses across the synaptic cleft and binds to a receptor, an intrinsic protein spanning the postsynaptic membrane. The proteins sub-units have two receptor sites and the two acetylcholine molecules show co-operative binding when they attach

4/ when acetylcholine molecules bind with both of these sites, the receptor protein changes shape, opening a channel and sodium ions diffuse in, down their concentration gradient.

5/The postsynaptic neurone is consequently depolarised and if the membrane is depolarised enough a threshold potential difference is reached and an action potential is initiated

6/ but if insufficient acetylcholine is bound the postsynaptic membrane will not be depolarised enough to exceed the threshold and so an action potential wont be generated

Question 9

What would happen if the acetylcholine were to remain in the synaptic cleft?

Answer 9

It would constantly initiate new impulses in the postsynaptic membrane and impulses would not be distinct

Question 10

How can this be prevented

Answer 10

Direct uptake of acetylcholine in the presynaptic neurone so none remains in the synaptic cleft to bind to the postsynaptic receptor

Active transport of calcium ions out of the synaptic bulb end so no more exocytosis of acetylcholine occurs

Hydrolysis of acetylcholine

Question 11

Study point

Answer 11

Calcium ions enter through the pre synaptic membrane to release acetylcholine

sodium ions enter through the post synaptic membrane to initiate a new action potential

Question 12

what happens after the release

Answer 12

acetylcholine is quickly destroyed by the enzyme acetlycholinesterase in the synaptic clefts

Question 13

Properties of a synapse

Answer 13

Transmit information between neurones

Pass impulses in one direction generating precision in the nervous system

act as junctions

Protect the response system from overstimulation bc the impulse is always the same size whatever the size of the stimulus

Filter out low-level stimuli:

An action potential only initiated when the depolarization was enough to reach a threshold value (-55mv)

Question 14

What is a drug

Answer 14

a molecule that has a physiological effect on the body when ingested
inhaled, absorbed or injected.

Question 15

What are the 4 molecules

Answer 15

acetylcholine
GABA
Monoamines
Neuropeptides

Question 16

how do different drugs affect the action of different neurotransmitters?

Answer 16

Sedatives e.g. alcohol inhibits the nervous system creating fewer action potentials in postsynaptic neurones

Stimulants e.g. amphetamines stimulate the nervous system by allowing more action potentials in postsynaptic neurones.

Question 17

Key things to remember about drugs

Answer 17

They may have the same shape and bind to the post synaptic neurone in the same way

which can increase the frequency of action potential

Drugs can prevent the breakdown of neurotransmitters

If addicted the body may become habituated to said drug and may mean that the body needs the drug

Question 18

psychoactive drugs what are they

Answer 18

primarily on the CNS by affecting different neurotransmitters or their receptors which effects the firing of neurones this alters brain function and consequently perception mood consciousness and behaviour