8.6—Synapses, neurotransmitters and pupil response

An action potential opens calcium channels in the presynaptic knob. Calcium triggers vesicles to release neurotransmitter, which diffuses across the cleft and binds receptors on the postsynaptic membrane, opening ion channels.

Transmission is one-way because release and receptors are on opposite sides. Summation of repeated or converging inputs can bring the postsynaptic membrane to threshold.

At the pupil pathway, receptor impulses reach the CNS and motor neurones activate antagonistic iris muscles; synapses ensure the signal is routed rather than reflected backward.

Neurotransmitter does not electrically jump the cleft, and binding is not enough unless the postsynaptic effect reaches threshold.