OCR A2 Biology F214: Nerves

Sensory receptors

• Are...cells in which a change in the internal or external environment produces a nerve impulse (action potential).
• All receptors are transducers - the change energy from one form to another.

Sensory neurons

• Are...neurons that transmit nerve impulses from a sensory receptor to the central nervous system (CNS). 
• The cell body is at the side of the dendron.
• The dendrites are longer than the axon.
• Has synaptic knobs.

Motor neurons

• Are...neurons that transmit nerve impulses from the CNS to an effector (e.g. muscle or gland).
• The cell body is at the end of the axon.
• The axon is longer than the dendrites.
• Has motor end plates.

Resting potential

• Is...the potential difference (or voltage) across the neuron cell membrane while the neuron is at rest - not conducting a nerve impulse (= 70mV).

1. Sodium-potassium pumps in the plasma membrane actively transport 3Na+ OUT OF and 2K+ INTO the axon.

2. The membrane is relatively impermeable to the Na+ ions so they build up on the outside of the axon.

3. The membrane is permeable to K+ ions which diffuse from inside of the membrane, out of the membrane.

4. This results in the outside of the axon being positively charged compared to inside the axon.

Action potentials

• Are...depolarisation of the cell membrane so that the inside is more positive compared to the outside (= +40mV).

1. Stimulus is applied which exceeds the threshold value, opening Na+ channels.

2. Voltage gated Na+ channels open and Na+ ions diffuse INTO the axon, down a gradient.

3. The membrane depolarises.

4. The inside of the axon becomes more positive compared to the outside.

5. The voltage gated Na+ channels close.

6. The voltage gated K+ channels open.

7. K+ ions diffuse out of the axon, and the potential difference becomes negative.

8. Hyperpolarisation occurs - the membrane cannot depolarise.

9. Resting potential is restored.

Threshold potential

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