Question

A moth species responds to sounds by way of simple reflex arcs connecting each ear to flight control muscles on the opposite side of the body. These arcs include: 1) one sensory neuron connected to the tympanic membrane, 2) an association neuron, and 3) a motor neuron that innervates the flight control muscles. A very low-volume playback of a bat call fails to elicit a response.  A louder playback causes the moth to fly in the opposite direction. 

Explain how temporal summation can explain this difference in response.  Define temporal summation in your answer.  

Answer 1

In moths, noctuid ears contain two auditory receptor neurons with differing sensitivity: A1 and A2. A1 neural cell is more sensitive than A2 by approximately 20 dB. Sound evoked excitatory post-synaptic potentials (EPSPs) receive input from A1 neural cell either directly or indirectly. Two neurons namely 501 and 504 recieve monosynaptic signal from the A1 receptor. However, the decay time differ among these two neurons with 501's decay time constant is three times of that of 504. EPSPs produced by a single A1 are brief lasting within 12 ms and hence are small to reach the threshold of 501. If signal arrives at a rate exceeding 250 Hz leads to temporal summation of EPSPs which causes spikes by 501. Such high signal by A1 is achieved only in cases like bat-like ultrasounds and leads to temporal summation in 501 and hence recognition of threat by bat. In this case temporal summation acts as a noise filter.

The threshold of post synaptic potential is achieved by summation of signal by a single neuron is known as temporal summation.