Buildup of neurotransmitter released by a single presynaptic end-bulb firing two or more times in rapid succession results in:
A) convergence B) divergence C) parallel-after discharge D) spatial summation E) temporal summation
The correct answer and explanation is:
The correct answer is E) Temporal summation.
Explanation:
Temporal summation refers to the process where a single presynaptic neuron releases neurotransmitters in rapid succession. When the neuron fires multiple action potentials within a short time frame, the neurotransmitter released during each pulse can add up (or “summate”) at the postsynaptic membrane. If these successive impulses are spaced closely enough, their effects can overlap and cause a greater change in the postsynaptic potential.
This phenomenon is key in determining whether or not the postsynaptic neuron reaches the threshold required to initiate an action potential. The rapid, repeated release of neurotransmitters can lead to the cumulative depolarization of the postsynaptic neuron, making it more likely to fire an action potential.
In temporal summation, the “timing” of the presynaptic action potentials is critical. The postsynaptic neuron doesn’t get a single large influx of neurotransmitter all at once. Instead, it experiences multiple smaller inputs in quick succession, which accumulate over time. This form of summation contrasts with spatial summation (option D), where multiple presynaptic neurons fire at the same time, adding their effects together. In temporal summation, the same presynaptic neuron is responsible for the accumulation.
An example of temporal summation occurs when a sensory neuron repeatedly fires signals, creating a stronger response from the motor neuron, allowing an individual to feel a consistent stimulus, like pressure. If the firing rate of the sensory neuron is fast enough, the motor neuron will receive enough neurotransmitter to reach the threshold and trigger a response.