13.5 Synapses

Question 1

What is the structure of a synapse?

Answer 1

Question 2

What is the synaptic cleft?

Answer 2

The gap which separates the axon of one neurone from the dendrite of the next neurone. approx 20-30 nm

Question 3

Definition of a synapse

Answer 3

The junction between two neurones or between a neurone and an effector cell

Question 4

How are impulses transmitted across a synapse?

Answer 4

By neurotransmitters

Question 5

What is the synaptic knob?

Answer 5

The swollen end of the presynaptic neurone, contains many mitochondria and endoplasmic reticulum for the manufacture of neurotransmitters

Question 6

What are synaptic vesicles?

Answer 6

Vesicles containing neurotransmitters which fuse with the presynaptic membrane and release their contents into the synaptic cleft

Question 7

What are neurotransmitter receptors?

Answer 7

Receptor molecules which the neurotransmitter binds to in the postsynaptic membrane

Question 8

What are the two types of neurotransmitters?

Answer 8

Excitatory, Inhibitory

Question 9

What are synapses that use Acetylcholine called?

Answer 9

Cholinergic synapses

Question 10

What are excitatory neurotransmitters?

Answer 10

Neurotransmitters depolarise the postsynaptic neurone, making it fire an action potential if the threshold is reached

Question 11

What is an example of an excitatory neurotransmitter?

Answer 11

Acetylcholine

Question 12

What are inhibitory neurotransmitters?

Answer 12

They bind to receptors on the postsynaptic membrane, they cause hyperpolarisation preventing an action potential from being fired

Question 13

What is an example of an inhibitory neurotransmitter?

Answer 13

GABA

Question 14

How does GABA act as an inhibitory neurotransmitter?

Answer 14

It binds to receptors on chlorine ion channels on the postsynaptic membrane, when open Cl- ions flood into the post synaptic membrane causing hyperpolarisation, preventing the neurone from reaching threshold potential and ultimately preventing an action potential

Question 15

What are the steps for the transmission across a cholinergic synapse?

Answer 15

1. An action potential arrives at the synaptic knob of the presynaptic neurone, this stimulates calcium ion channels to open and calcium ions to diffuse into the synaptic knob by facilitated diffusion down their electrochemical gradient
2. The influx of calcium ions causes synaptic vesicles to move to the pre-synaptic membrane. They then guise with the presynaptic membrane and release their contents Acetylcholine into the synaptic cleft by exocytosis, this sets up a concentration gradient across the synaptic cleft
3. Acetylcholine diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane, causing sodium ion channels to open. The influx of sodium ions into the post synaptic membrane causes depolarisation, and an action potential is generated if the threshold potential is reached (-55 mV)
4. Acetylcholinesterase an hydrologic enzyme hydrolyses Acetylcholine into choline and ethanoic acid (acetyl) which diffuse back across the synaptic cleft into the presynaptic neurone. This also prevents a continuous action potential being generated and recycles the neurotransmitters
5. ATP is released from the mitochondria to recombine choline and ethanoic acid (acetyl) into acetylcholine. This is stored in vesicles for future use. Sodium ion channels also close in the absence of the acetylcholine on receptor sites

Question 16

What is the role of synapses?

Answer 16

-Ensure impulses are unidirectional, as neurotransmitter receptors are only present on the postsynaptic membrane
-Allow impulses from one neurone to be transmitted to a number of neurones at multiple synapses, results in a single stimulus creating a number of simultaneous responses (synaptic divergence)
-Allows a number of neurones to feed into the same synapse with a single postsynaptic neurone, results in stimuli from different receptors interacting to produce a single result (syntactic convergence)

Question 17

Definition of summation

Answer 17

The build up of neurotransmitter in a synapse to sufficient levels to trigger an action potential

Question 18

What is spatial summation?

Answer 18

A number of presynaptic neurones connect two one postsynaptic neurone. Each releases neurotransmitter which builds up to reach a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone

Question 19

What is temporal summation?

Answer 19

When a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period. This builds up in the synapse until the quantity is sufficient enough to trigger an action potential