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Summary Notes on Nerve Impulses - AQA A Level Biology
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Nerve impulses – AQA A Level Biology Summary NotesPart of cell Function
Cell body Contains all usual cell organelles including a nucleus and large
amount of RER
Associated with production of neurotransmitters and proteins
Dendrons Extensions of the cell body which smaller branched fibres
called dendrites that carry nerve impulses towards the cell
body
Axon A single long fibre that carries nerve impulses away from the
body
Axon hillock Location where action potential is generated
Schawnn cell Surrounds the axon protecting it and providing electrical
insulation
Carry out phagocytosis and play part in nerve regeneration
Schwann cells wrap themselves around many times, so that
layers of membrane build up around it
Myelin sheath Forms a covering to the axon and is made up of membranes
of the Schawnn cells
These membranes are rich in a lipid known as myelin
Neurones with myelin sheath are called myelinated neurones
Node of Ranvier Constrictions between adjacent Schawnn cells
No myelin where they occur
Constrictions/gaps are 2-3 um long and occur every 1-3 mm in
humans
, Resting potential
- When potential difference across membrane of neurone is - 70 mV
- Established by moving sodium and potassium ions across the membrane
- Can be measured by inserting micro-electrodes into axon
The axon membrane contains 3 different types of protein involved in the RP:
1. Sodium ion channels: closed, so sodium cannot move through them
2. Potassium ion channels: mostly closed, but a few remain open so a small amount of
potassium can move out of axon by facilitated diffusion down its concentration gradient (a
bit leaky)
3. Sodium-potassium ion pumps: actively transport 3 sodium ions out of the axon in exchange
for 2 potassium ions going in
= net effect of this is that more ions accumulate outside of the axon, giving the inside of the
membrane a negative charge (-70mV) with respect to the outside
Action potential
Changes in membrane permeability lead to depolarisation and the generation of an action
potential
If a stimulus is detected by a receptor, the membrane will depolarise
So sodium ion channels open, allowing sodium to move in, down their electrochemical gradient
The membranes polarity decreases
If the stimulus is sufficiently strong, and enough sodium ions enter, the depolarisation will reach
“threshold” (-55mV)
So all the voltage gated sodium challenge open, causing a large influx of sodium ions, so the
membrane becomes positive inside
= depolarisation
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