synaptic transmission

what is a synapse?

junction between 2 neurones/ neurone and effector

what is the synaptic cleft?

tiny gap between cells at a synapse

what is the pre-synaptic membrane/ synaptic knob?

fat glob shape in pre-synaptic neurone, has the synaptic vesicles with neurotransmitters

what is the function of the synapse?

• action potential arrives at synaptic knob, leads to opening of Ca2+ channels, Ca2+ diffuses into knob

• vesicles w neurotransmitter move toward & fuse w presynaptic membrane,

describe the process occurring in cholinergic synapses. (5)

1. Depolarisation of presynaptic membrane;

   • Accept action potential for depolarisation.

2. Calcium channels open and calcium ions enter (synaptic knob);

   • Accept Ca2+.

3. (Calcium ions cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter;

   • Accept abbreviations for acetylcholine as term is in the question.

4. Acetylcholine/neurotransmitter diffuses across (synaptic cleft);

   • Accept abbreviations for acetylcholine as term is in the question.

5. (Acetylcholine attaches) to receptors on the postsynaptic membrane;

6. Sodium ions enter (postsynaptic neurone) leading to depolarisation;

   • Accept Na+.

   • Accept 'action potential' or 'generator potential' for depolarisation.

describe what excitatory neurotransmitters do

When Na+ ions move into the post synaptic neurone so it becomes more positive inside, making it more likely that an action potential is generated in the post-synaptic neurone

describe what inhibitory neurotransmitters do

• When the post synaptic neurone becomes more negative

• Due to either K+ ions moving out or Clions moving in

• This means that more Na+ ions would be required in order for an action potential to be reached

• So it is less likely that an action potential is generated in the post-synaptic neurone

describe the general process of summation

Spatial summation is when there is more than one pre-synaptic neuron synapsed to the same post-synaptic neuron.

what are the 2 types of summation?

Spatial and temporal

what is spatial summation?

Spatial summation is when there is more than one pre-synaptic neuron synapsed to the same post-synaptic neuron.

Describe how spatial summation makes it more possible for an action potential to occur in the post-synaptic neurone. (assume that it is an excitatory synapse)

• neurotransmitter from both pre-synaptic
  neurones bind to the receptors (on the ligand gated channels) on the post synaptic neurone

• so sufficient sodium ion ligand gated channels open

• so sufficient sodium ions move into the post-synaptic neuron by facilitated diffusion

• so threshold be exceeded and an action potential will be generated

what is temporal summation?

Temporal summation is when the pre-synaptic neuron has a high frequency of action potentials.

Describe how temporal summation makes it more possible for an action potential to occur in the post-synaptic neurone. (assume that it is an excitatory synapse)

• A large amount of neurotransmitter is realised by the pre-synaptic neurone in a short space of time

• so sufficient sodium ion ligand gated channels open

• so sufficient sodium ions move into the post-synaptic neuron by facilitated diffusion

• so threshold be exceeded and an action potential will be generated

what are neuromuscular junctions and what happens in them?

1. Depolarization causes Ca2+ voltage gated channels in the pre-synaptic knob open

2. Ca?+ move in and cause vesicles to move towards the presynaptic membrane and fuse with it

3. Acetylcholine is released into the neuromuscular junction

4. Acetylcholine binds to receptors on the sarcolemma, so the ion channels open

5. Nat move in by facilitated diffusion into the sarcolemma

6. Depolarization spreads along the sarcolemma and down the t-tubules

7. This causes the Ca2+ voltage gated channels on the sarcoplasmic reticulum to open

8. So Ca ions are released into the sarcoplasm from the sarcoplasmic reticulum allowing muscle contraction to take place

Describe how a synapse ensures unidirectionality of impulses

Only the pre-synaptic neurone contains synaptic vesicles with neurotransmitter

Only the post-synaptic neurone has receptors which the neurotransmitters can bind to

Write a flow-chart to describe what happens at a cholinergic synapse

1. Action potential reaches the pre-synaptic neurone

2. Calcium ion voltage gated channels open so Cat ions move in by facilitated diffusion

3. The Cat ions cause the synaptic vesicles to move towards the pre-synaptic membrane and fuse with it

4. The neurotransmitter (acetylcholine) is released (by exocytosis

5. Acetylcholine diffuses across the synaptic cleft

6. Acetylcholine binds to the receptors on (the ligand gated Nat channels) on the post synaptic membrane

7. Nat channels open

8. Nat ions move into the post synaptic neurone by facilitated diffusion (down their electrochemical gradient) causing it to depolarise

9. If threshold is exceeded Nat voltage gated channels open, so more Nat ions move in by facilitated diffusion and a new action potential is generated in the post synaptic neurone

10. Acetylcholinesterase hydrolyses the acetylcholine to form ethanoic acid and choline

11. The ethanoic acid and choline are reabsorbed into the pre-synaptic neurone

12. ATP is used to regenerate acetylcholine using the ethanoic acid and and choline