Summary GCE Edexcel Biology (A) Salters Nuffield Topic 8

Nerve impulse

a. Resting potential (around -70mV)
- Maintained by sodium-potassium pump (3Na+ out, 2K+ in)
- Sodium channels are always closed
- Potassium channels are partially open, some K+ ions diffuse out (membrane is
partially permeable to potassium ions)
- High concentration of K+ inside axon, low contraction of Na+ outside axon
- A type of active transport (against the conc. gradient), requires ATP

b. Action potential

1. Depolarisation
- Once the threshold potential is exceeded, sodium channels open, sodium ions
diffuse into the axon
- Potassium channels remain closed
- Change from negative resting potential (-70mV) to positive action potential (around
+30/+40 mV)

2. Repolarisation
- Potassium channels open, potassium ions diffuse out of the axon
- Sodium channels remain closed
- Change from positive action potential (-30/-40mV) back to negative resting potential
(around -70 mV)

c. Hyperpolarisation
- Potassium channels remain open, more potassium ions diffuse out of the axon
- Potential difference falls below resting potential of (-70mV)
- This ensures that an action potential cannot be generated, and that the action
potential can only travel in one direction
- A second action potential can only be generated if stimuli is greater than the
threshold value (-55mV)

Myelination:
- Myelin sheath produced by Schwann cell
- Myelin sheath is an electrical insulator
- Action potential only occurs at nodes of Ranvier in a myelinated axon
- Allows nerve impulses to jump from ‘node to node’ resulting in saltatory conduction

Why does a loss of myelin sheath lead to muscle weakness?
- When myelin sheath is damaged, the transmission of impulses is slower
- Fewer nerve impulses reach muscles
- Fewer muscle fibres are stimulated to contract, leading to muscle weakness
- Less saltatory conduction/impulses cannot jump from node to node

, Sensory Neuron Vs Motor Neuron:

- Both have axon
- Both have a cell body containing nucleus
- Both have dendrites on one end and terminal branches on the other end
- Cell body in sensory is located in the middle of the axon, cell body in motor is on one
end




SYNAPSE:

Describe the sequence of events that lead to an excitatory postsynaptic potential during the
nerve impulse transmission. (4-5 marks)
1. Arrival of nerve impulse/action potential at presynaptic membrane stimulates Ca2+
channels open
2. Ca2+ diffuses into presynaptic membrane
3. This causes vesicles (containing neurotransmitters) to fuse with presynaptic
membrane
4. Neurotransmitter (or acetylcholine/Ach) is released via exocytosis
5. Neurotransmitters diffuse across synaptic cleft and binds to specific receptor on
postsynaptic membrane
6. This stimulates the opening of the sodium channels (on postsynaptic membrane)
7. Sodium ions enter postsynaptic membrane causing depolarisation
8. An action potential (nerve impulse) is set up once threshold potential is exceeded
*after transmission, the neurotransmitter will either be:
- taken up by diffusing through the reuptake protein/channel back into the
presynaptic membrane
- an enzyme will break down Ach after transmission process

How can cocaine help a person to have an increased sense of pleasure?
- Cocaine binds to the reuptake protein/channel
- Dopamine accumulates in the synaptic cleft
- Dopamine binds to receptors on postsynaptic membrane
- Sodium channels open, sodium ions diffuse in (the postsynaptic membrane)
- Membrane is continuously depolarised in the postsynaptic membrane, stimulates
prolonged feelings of pleasure