- A synapse is a region where communication occurs between two neurons or between a neuron and an effector cell (muscle cell or glandular cell).
- The term presynaptic neuron (pre- before) refers to a nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal.
- A postsynaptic cell is the cell that receives a signal. It may be a nerve cell called a postsynaptic neuron (post- after) that carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse.
Figure 11- Synapse
Most synapses between neurons are axodendritic (from axon to dendrite), while others are axosomatic (from axon to cell body) or axoaxonic (from axon to axon).
8.1 Types of Synapse
- In addition, synapses may be electrical or chemical and they differ both structurally and functionally.
- Electrical Synapses
- At an electrical synapse, action potentials (impulses) conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions.
- Electrical synapses have two main advantages:
- Faster communication: Because action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses.
- At an electrical synapse, the action potential passes directly from the presynaptic cell to the postsynaptic cell.
- Synchronization: Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. In other words, a large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions.
- Chemical Synapses
- Although the plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close, they do not touch.
- They are separated by the synaptic cleft.
- At most chemical synapses, only one-way information transfer can occur—from a presynaptic neuron to a postsynaptic neuron or an effector, such as a muscle fiber or a gland cell. As a result, action potentials move in one direction.
Figure 12- Signal Transmission at a Chemical Synapse
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Synapse & Its Types