Synapses

Question 1

What are synapses?

Answer 1

The gaps between the end of the axon on one neuron and the dendrite of another
Action potential is transmitted as neurotransmitters that diffuse across synapse

Question 2

Structure of a synapse

Answer 2

Synaptic cleft- small gap that separates neurones (20-30nm wide)
Presynaptic neurone releases neurotransmitter + axon of this neurone ends with a synaptic knob containing lots of mitochondria, endoplasmic reticulum + vesicles containing neurotransmitters
Postsynaptic neurone has receptors which receive the neurotransmitters

Question 3

How are synapses unidirectional?

Answer 3

Can only pass info in one direction- from presynaptic neurone to postsynaptic neurone
Only presynaptic neurone contains neurotransmitter + only postsynaptic neurone contains receptors

Question 4

What is summation?

Answer 4

The rapid build-up of neurotransmitters in the synapse to help generate an action potential by spatial summation or temporal summation
-This is needed as a small amount of neurotransmitter may be released which may not be enough to reach threshold + trigger an action potential
Spatial summation: Many different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of postsynaptic neurone + together trigger a new action potential
Temporal summation: one presynaptic neurone releases neurotransmitter repeatedly over a short period of time allowing it to build up so action potential will be triggered

Question 5

Transmission across a cholinergic synapse

Answer 5

-Action potential arrives at presynaptic knob causing calcium ion protein channels to open and Ca2+ to enter synaptic knob by facilitated diffusion
-Influx of Ca2+ into presynaptic neurone causes vesicles containing acetylcholine to fuse with presynaptic membrane, releasing acetylcholine into synaptic cleft through exocytosis
-Acetylcholine diffuses across synaptic cleft quickly as diffusion pathway is short. Acetylcholine binds to receptor sites on sodium ion protein channels in postsynaptic membrane
- Sodium ion channels open + an influx of sodium ions depolarises postsynaptic membrane

Question 6

Describe inhibitory synapses

Answer 6

• At inhibitory synapses the neurotransmitter hyperpolarises the membrane by binding to chloride ion channels on postsynaptic neurone, causing them to open (chloride ions move in) + potassium channels to open (potassium ions move out)
• Inside of postsynaptic membrane is more negative so less likely a new impulse will be created