Synapses and EPSP/IPSP

Question 1

Explain transmission between synapses.

Answer 1

1. A nerve impulse reaches at a presynaptic bulb causing calcium channels to open so the membrane is more permeable to CALCIUM IONS.
2. Calcium ions diffuse into the bulb causing vesicles containing acetylcholine to fuse with the presynaptic membrane. The vesicles then release the neurotransmitter by exocytosis into the SYNAPTIC CLEFT.
3. Aceytl diffuses to the postsynaptic membrane’s RECEPTOR SITES causing Na channel proteins in the postsynaptic membrane to open. More permeable to Na ions which diffuse into the postsynaptic neurone.
4. This sets up an EPSP

Question 2

How is an EPSP formed?

Answer 2

Forms because sodium entry depolarises the membrane causing a change in potential difference.
If EPSP is large enough to reach a threshold value then an action potential is set up and the impulse is conducted down the neurone.

Question 3

How is an IPSP formed?

Answer 3

• When a neurotransmitter binds to receptors on the postsynaptic membrane, different ion channels (K and Cl) open allowing NEGATIVE ions to diffuse in.
• This makes the inside more negative than the resting potential (hyperpolarisation) making it less likely that an action potential will form.

Question 4

What happens to neurotransmitters after they are used?

Answer 4

They are broken down into its components by enzymes stopping transmission and the continued stimulation and firing of impulses.
They enter the presynaptic bulb and are used to reform a neurotransmitter which requires ATP.

Question 5

What is temporal and spatial summation?

Answer 5

SPATIAL:
- A number of presynaptic neurones converge onto one postsynaptic neurone, and require the release of neurotransmitters from more than one bulb to reach a threshold EPSP and fire an impulse.

TEMPORAL:
- A single presynaptic bulb fires a neurotransmitter multiple times to one postsynaptic membrane (the effect of many action potentials to release sufficient neurotransmitters).