Question

. Define Temporal and Spatial summation and explain how each can propagate the occurrence of an action potential in the axon of the neuron?(25 points)

Answer 1

Spatial summation occurs when multiple presynaptic neurones together release enough neurotransmitter (e.g. acetylcholine) to exceed the threshold of the postsynaptic neurone. For example, neurone A and neurone B may individually release insufficient neurotransmitter but when these quantities are combined, threshold may be exceeded and an action potential generated. According to Ricco’s law, within the eye, the intensity and area are inversely variable due to the combination of signals from rods to bipolars which turn into ganglion cells.

-Temporal summation occurs when one presynaptic neurone releases neurotransmitter many times over a period of time. The total amount of neurotransmitter released may exceed the threshold value of the postsynaptic neurone. The higher the frequency of the action potential the more quickly the threshold may be exceeded. There is always another rise in the action potential just before the previous potential ends. The previous and second potential points will summate thus generating a larger potential. When this occurs, the potential can reach its threshold to begin another action potential.With regards to vision, temporal summation is involved. The Bunsen-Roscoe law is the inverse proportion of intensity and time. According to the law, the longer the stimulus, the better chance it can attain the number of quanta needed for vision.