13.4 Synapses

Question 1

Define:
Synapse
Synaptic cleft
Presynaptic neurone
Synaptic knob
Postsynaptic neurone
Neurotransmitters

Answer 1

• a synapse is junction between a neurone and another neurone, or between a neurone and an effector cell
• the tiny gap between the neurone is known as the synaptic cleft
• the presynaptic neurone is the one before the synapse
• the presynaptic neurone has a swelling called a synaptic knob. This contains synaptic vesicles filled with chemicals known as neurotransmitters
• neurotransmitters are chemicals used to pass formation from one neurone to the next

Question 2

What causes neurotransmitters to be released?

Answer 2

When an action potential reaches the end of a neurone, it causes neurotransmitters to be released into the synaptic cleft.
They diffuse across to the postsynaptic membrane (one after the synapse) and bind to specific receptors.
When neurotransmitters bind to receptors they trigger an action potential or response in the next neurone or effector

Neurotransmitters are removed from the cleft so that the response does not keep happening e.g. they’re taken back to the pre synaptic neurone or they’re broken down by enzymes

Question 3

Describe and explain how a nerve impulse is transmitted across as cholinergic synapse

Answer 3

1) An action potential arrives at the synaptic knob of the presynaptic neurone.
The action potential stimulates voltage-gated calcium ion channels to open.
Calcium ions diffuse into the synaptic knob (they’re pumped out after by active transport)

2) The influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane. They then fuse with the presynaptic membrane.
The vesicles release ACh (acetylcholine) into the synaptic cleft via exocytosis.

3) ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane. This causes sodium ion channels in the post-synaptic neurone to open.
The influx of sodium ions into the post synaptic neurone causes depolarisation. Action potential is generated if the threshold is reached.
ACh is removed from the synaptic cleft so response doesnt keep happening.
It’s broken down by AChE and the products are reabsorbed by the presynaptic neurone and used to makes more ACh

Question 4

Describe disruption of synaptic transmission

Answer 4

• can be affected by drugs and toxins

Question 5

What is synaptic convergence and synaptic divergence?

Answer 5

Synaptic divergence: when one neurone connects to many neurones, so that information can be spread to different parts of the body
Synaptic convergence: when many neurones connect to one neurone, so that information can be amplified (made stronger)

Question 6

What is summation?
What are the 2 types?

Answer 6

If a stimulus i weak, only a small amount of neurotransmitter will be released from a neurone into the synaptic cleft. This might not be enough to excite the posts synaptic membrane to the threshold level and stimulate an action potential.

Summation is where the effect of neurotransmitters released from many neurones (or one neurone that is stimulated a lot in a short period of time) is added together. It means synapses accurately process info.

2 types:
Spatial summation
Temporal summation

Question 7

Describe spatial summation

Answer 7

• when two or more presynaptic neurones converge and release their neurotransmitters at the same time onto the same post synaptic neurone.
  The small amount of neurotransmitter released from each of these neurones can be enough if their effect is added together to reach the threshold in the post synaptic neurone and trigger an action potential.

Stimuli might arrive from different sources, but spatial summation allows signals from multiple stimuli to be coordinated into a single response

Question 8

Describe temporal summation

Answer 8

• when two or more nerve impulse arrive in quick succession from the same presynaptic neurone
• this makes an action potential more likely because more neurotransmitter is released into the synaptic cleft

Question 9

How is unidirectional transmission achieved?

Answer 9

• nerve impulses can only travel in one direction as only the postsynaptic membrane has the specific receptors that the neurotransmitters can bind to
  And neurotransmitters are only released from the presynaptic neurone