3.6.2.2 Synaptic transmission

What is a synapse

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

How do impulses cross from one neurone to the next

chemical neurotransmitter

What does acetyl choline bind to?

cholinergic receptors

Why does acetylcholine bind to cholinergic receptors?

complimentary shapes

Why are synapses important?

they give the nervous system control over the transmission of impulses

Give 3 things that can happen at a synapse?

• impulses can be stopped

• impulses passed on to more than one neurone

• directed along certain neuronal pathways

Are the neurones at a synapse in direct contact?

no

What is the gap between neurones called?

synaptic cleft

What is the size of a synaptic cleft?

20-30nm

What does a neurotransmitter do on the adjacent neurone?

initiates of inhibits action potentials

Describe the steps of synaptic transmission at a cholinergic joint

1. action potential arrives at presynaptic knob

2. calcium ion channels open and calcium ions diffuse into the presynaptic knob

3. these are later pumped out by active transport

4. synaptic vesicles containing acetylcholine move toward the presynaptic membrane (requiring ATP)

5. vesicle fuses with the presynaptic membrane and acetylcholine is released from synaptic vesicles into the synaptic cleft by exoytosis

6. acetyl choline diffuses across synaptic cleft to post synaptic membrane

7. acetyl choline binds to receptors on post synaptic membrane

8. sodium ion channels on post synaptic membrane ipen and sodium ions enter, depolarising the membrane

9. acetylcholinesterase breaks down acetyl choline and it diffuses back into synaptic know where it is resynthesised (requiring ATP)

What does an action potential cause at a presynaptic membrane?

calcium ion channels open

As Ca2+ channels open, what happens to Ca2+ ions?

they diffuse into the pre-synaptic knob

What happens to Ca2+ ions after synaptic transmission?

they are actuvely transported out of the pre-synaptic knob

How are Ca2+ ions pumped out of the synaptic knob?

active transport

What do synaptic vesicles contain?

acetylcholine

What does the movement of Ca2+ ions into the synaptic knob stimulate for?

vesicles containg acetylcholine fuse with presynaptic membrane

What does the movement of synaptic vesicles require?

ATP

How is acetyl choline released into the synaptic cleft?

fusing with pre-synaptic membrane

Where does acetylcholine diffuse across and to?

across synaptic cleft to post synaptic membrane

What does acetyl choline to at the post-synaptic membrane?

binds to preotein receptors

What does acetyl choline bind to at the post-synaptic membrane?

receptor

What is the result of acetylcholine binding to receptors on the post synaptic membrane?

sodium ion channels open

What happens when sodium ions enter the post-synaptic neurone?

depolarisation

What happens to acetyl choline after a synaptic transmission?

it is broken down and dissuses into pre-synaptic knob where it is resynthesised

What breaks down acetylcholine?

acetylcholinesterase

What happens after acetyl choline is broken down?

it diffuses into pre-synaptic knob and is resynthesised

what does the resynthesis of ACh require?

ATP

What are excitatory synapses?

synapses where the binding of a neurotransmitter opens sodium channels and initates an action potential

What is the effect of binding of the neurotransmitter at inhibitory synapses?

the resting potential of the post synaptic memvrae becomes more negative (hyperpolarised)

What happens to the resting potential of the post synaptic membrane at an inhibitory synapse?

it is more negative

What is the effect of hyperpolarisation?

it is less likely that threshold potential will be reached

Give an example of an inhibitory neurotransmitter

GABA

What is the effect of GABA?

chloride ions enter the post-synaptic neurone

Give 2 roles of synapses

1. allows the nerve impulse to be transmitted between neurones and ensures that transmission of action potentials is unidirectional

2. summation

Why is action potential transmission unidirectional?

vesicles are only in the presynaptic knob and receptors only exist on the post-synaptic membrane

Describe summation

several action potentials are needed to proiduce enough neurotransmitter to reach threshold potential in the post-synaptic membrabe

Give 2 types of summation

• temporal

• spatial

What is spatial summation?

several action potentials from different neurones arrive at the post synaptic membrane at the same time, each releasing small quantities of neurotransmitter which combines to produce an action potential

Draw a graph for spatial summation

Describe temporal summation

severla action potentials arrive very closely one after the other, with each releasing a small quantity of neurotransmitter which combines to initiate an action potential in the post synaptic neurone

How does temporal summation cause an action potential

several impulses release a set volume of neyro transmitter which combines to initiate an action potential

What are the benefits of temporal summation?

prevents overstimulation and fatigue of the neurone