Calcium-Independent Neurotransmitter Release
The brain is not a muscle. And we should not expect communication between neurons to mimic the well studied events that occur during transmission at a neuromuscular junction. Although many neurons use Ca+2-dependent exocytosis to release transmitters, some retinal neurons now appear to use a different mechanism. A motor neuron rapidly mobilizes the release of sufficient transmitter to ensure muscle contraction. The pre-synaptic signal is an action potential, a 100 mV depolarization that lasts several milliseconds. In contrast, photoreceptors, and other neurons in the outer retina, signal the absorption of light with a slow, graded change in voltage. A dark-adapted rod photoreceptor maintains a potential of approximately −35 mV; the absorption of a photon produces a 30 μV hyperpolarization that lasts nearly one second (Schwartz, 1976). The mechanisms of synaptic transmission that operate in motor neurons and in photoreceptors may be optimized to ensure the reliable communication of their very different pre-synaptic signals.
KeywordsSynaptic Transmission Bipolar Cell Horizontal Cell Retinal Neuron Outer Retina
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