What occurs during the process of summation as it applies to the excitory effect of neurons?
An assignment for Biology... Couldn't find the answer in my textbook.
Most neurons have many thousands of inputs from other neurons, sensory inputs, etc. Some of the inputs are positive, and some are negative. These inputs are said to "sum." If there are sufficient positive (excitatory) inputs to cause a depolarization to threshold, then an action potential (AP) will be fired.
Summation that can make a neuron more likely to fire an AP can be spatial or temporal. Let's take the example of sensing the touch of something on the skin of your arm. If a larger area is touched, then more neurons, with sensory endings in that area of your arm will fire. If those multiple sensory inputs converge on the same neuron, each of them sending excitatory signals, will make the neuron they all innervate more likely to fire an AP.
Similarly, temporal summation can occur. If a stimulus is larger than usual, it causes the sensory nerves to send excitatory messages much more quickly than usual. Each additional excitatory message causes more depolarization in the neuron they innervate, again making it more likely to fire an AP.