TYPES OF SYNAPSES
A synapse is a site where an arriving action potential, through excitation-secretion coupling involving Ca2+ influx, triggers the release of one or more neurotransmitters into the synaptic cleft (typically 20 µm across).
from its resting state. These postsynaptic potentials are called graded potentials. Most synapses carrying information toward a target neuron terminate as axodendritic or axosomatic synapses. Specialized synapses, such as reciprocal synapses or complex arrays of synaptic interactions, provide specific regulatory control over the excitability of their target neurons. Dendrodendritic synapses aid in the coordinated firing of groups of related neurons such as the phrenic nucleus neurons that cause contraction of the diaphragm.
The configurations of the synapses of key neuronal populations in particular regions of the brain and of target cells in the periphery determine the relative influence of that input. At the neuromuscular junction, a sufficient amount of acetylcholine is usually released by an action potential in the motor axon to guarantee that the muscle end plate potential reaches threshold and initiates an action potential. In contrast, the neuronal inputs into reticular formation neurons and many other types of neurons require either temporal or spatial summation to allow the target neuron to reach threshold; this orchestration involves coordinated multisynaptic regulation. In some key neurons such as lower motor neurons (LMNs), input from brain stem upper motor neurons (UMNs) is directed mainly through spinal cord interneurons and requires extensive summation to activate the LMNs; in contrast, direct monosynaptic corticospinal UMNs input onto some LMNs, such as those regulating fine finger movements, terminate close to the axon hillock/initial segment; and can directly initiate an action potential in the LMNs. Some complex arrays of synapses among several neuronal elements, such as those seen in structures such as the cerebellum and retina, permit modulation of key neurons by both serial and parallel arrays of connections, providing lateral modulation of neighboring neuronal excitability.