A synapse is a junction between two cells. A synaptic cleft is the gap between parts of two cells at a synapse. Synapses can occur between two neurons, a receptor cell and a neuron, or a neuron and an effector.
1. Synaptic transmission a. Impulses usually travel from dendrite or cell body, then along the axon to a synapse.
b. Axons have synaptic knobs at their distal ends that secrete neurotransmitters.
c. The neurotransmitter is released when a nerve impulse reaches the end of an axon, and the neurotransmitter diffuses across the synaptic cleft.
d. A neurotransmitter reaching the dendrite or cell body on the distal side of the cleft triggers a nerve impulse.
2. Synaptic potentials a. Some neurotransmitters can depolarize postsynaptic membranes, triggering an action potential. This is an excitatory postsynaptic potential (EPSP).
b. Others hyperpolarize the membranes, inhibiting action potentials. This is an inhibitory postsynaptic potential (IPSP).
c. EPSPs and IPSPs are summed in a trigger zone of the neuron.
3. Neurotransmitters a. The nervous system produces at least thirty types of neurotransmitters.
b. Calcium ions diffuse into synaptic knobs in response to action potentials, releasing neurotransmitters.
c. Neurotransmitters are quickly decomposed or removed from synaptic clefts.
4. Neuropeptides a. Neuropeptides are chains of amino acids.
b. Some neuropeptides are neurotransmitters or neuromodulators.
c. They include enkephalins, endorphins, and substance P.
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