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Light sensitive cells in the retina detect light intensity and range of wavelengths (colour)
Olefactory cells in the nasal cavity detect the presence of volatile chemicals.
Tastebuds detect the presence of soluble chemicals.
Pressure receptors in the skin detect pressure on the skin.
Sound receptors in the cochlea detect vibrations in the air.
Muscle spindles detect the length of muscle fibres.
These are all transducers and convert the stimulus into a nerve impulse.

A motor neurone has a cell body at the end of the neurone, in the central nervous system (CNS), many short dendrites that carry the impulses to the cell body, a long axon carrying the impulse away from the cell body, that ends at a motor end plate.
A sensory neurone as a cell body in the centre of the neurone, in the peripheral nervous system (PNS), a dendron carrying impulses from a receptor to the cell body, with dendrites at the end of the dendron, a shorter axon carrying impulses from the cell body to the central nervous system. 

The neurones resting potential is maintained at -60mV because sodium-potassium pumps are actively transporting three Na+ ions out for every two K+ ions into the neurone. The axon contains inorganic anions, which the membrane is impermeable to. There is a slight loss of K+ ions through the permeable membrane.

1. The membrane is at resting state, -60mV inside the cell compared to the outside. The membrane is polarised.
2. A stimulus causes some Na+ ion channels to open and some Na+ ions diffuse into the neurone.
3. The neurone depolarises, reaching the threshold potential of -50mV.
4. Voltage-gated sodium ion channels open, and there is a rapid influx of Na+ ions. As more Na+ ions enter, the neurone becomes positively charged inside compared with the ouside.
5. The potential difference across the membrane reaches +40mV.
6. Voltage-gated sodium ion channels close and the potassium ion channels open.
7. K+ ions diffuse out of the cell, reducing the potential difference down until it becomes negatively charged with respect to the outside of the cell once more - repolarisation.
8. The potential difference overshoots slightly, making the cell hyperpolarised.
9. The original resting potential is restored, -60mV.

A neurone will either conduct an action potential or not; this is known as the all or nothing law. A stimulus at a higher intensity does not cause a…


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