A synapse is an anatomically specialized junction between two neurons, at which the electrical activity in a presynaptic neuron influences the electrical activity of a postsynaptic neuron. Anatomically, synapses include parts of the presynaptic and postsynaptic neurons and the extracellular space between them. According to the latest estimate, there are approximately 100 trillion! synapses in our CNS.
Activity at synapses can increase or decrease the likelihood that the postsynaptic neuron will fire action potentials by producing a brief, graded potential in the postsynaptic membrane. The membrane potential of a postsynaptic neuron is brought closer to threshold (i.e., depolarized) at an excitatory synapse, and it is either driven farther from threshold (i.e., hyperpolarized) or stabilized at its resting potential at an inhibitory synapse.
Hundreds or thousands of synapses from many different presynaptic cells can affect a single postsynaptic cell (convergence), and a single presynaptic cell can send branches to affect many other postsynaptic cells (divergence).
Action potential can travel both ways on a neuron, but they are transferred to adjacent neurons only though the synapses, from axon terminal to the postsynaptic membrane.
Action potential can travel both ways on a neuron, but they are transferred to adjacent neurons only though the synapses, from axon terminal to the postsynaptic membrane.
There are two types of synapses: electrical and chemical. At electrical synapses, the plasma membranes of the pre- and post- synaptic cells are joined by gap junctions,allowing the local current from arriving action potential to flow directly across the junction at places where communication in extremely rapid. At chemical synapses, the axon of the presynaptic neuron ends in a slight swelling, the axon terminal, which holds synaptic vesicles that contain neurotransmitters. The postsynaptic membrane adjacent to the axon terminal has a high density of intrinsic and extrinsic membrane proteins and receptors that make up the postsynaptic density. A 10 to 20 nm extracellular synaptic cleft seperates the pre- and the post- synaptic neurons.
(Widmaier EP, Raff H, Strang KT. Vander's Human Physiology: the mechanisms of body function. 12 ed.New York: McGraw-Hill International Edition; 2011)
(Widmaier EP, Raff H, Strang KT. Vander's Human Physiology: the mechanisms of body function. 12 ed.New York: McGraw-Hill International Edition; 2011)
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