A neuroeffector junction is a site where a motor neuron releases a neurotransmitter to affect a target—non-neuronal—cell. This junction functions like a synapse. However, unlike most neurons, somatic efferent motor neurons innervate skeletal muscle, and are always excitatory. Visceral efferent neurons innervate smooth muscle, cardiac muscle, and glands, and have the ability to be either excitatory or inhibitory in function. Neuroeffector junctions are known as neuromuscular junctions when the target cell is a muscle fiber. Non-synaptic transmission is characteristic of autonomic neuroeffector junctions. The structure of the autonomic neuromuscular junction consists of several essential features including that: the terminal portions of autonomic nerve fibers are varicose and mobile, transmitters being released 'en passage' from varying distances from the effector cells; while there is no structural post-junctional specialization on effector cells, receptors for neurotransmitters accumulate on cell membranes at close junctions. Muscle effectors are bundles rather than single smooth muscle cells that are connected by gap junctions which allow electrotonic spread of activity between cells. A multiplicity of transmitters are utilized by autonomic nerves, and co-transmission occurs often involving synergistic actions of the co-transmitters, although pre- and post-junctional neuromodulation of neurotransmitter release also take place. It is suggested that autonomic neural control of immune, epithelial and endothelial cells also involves non-synaptic transmission. These are tight junctions, but in the autonomic nervous system and enteric nervous system the connecting junctions become much "looser", allowing for easier diffusion. This looseness allows for a wider signal receiving whereas in tighter junctions, more neurotransmitters get metabolized or broken down. In skeletal muscles, the junctions are mostly of the same distance and size because they innervate such definite structures of muscle fibers. In the Autonomic Nervous System however, these neuromuscular junctions are much less well defined. Analysis of non-noradrenergic/non-cholinergic (NANC) transmission at single varicosities or swellings indicates that individual synapses possess different probabilities for the secretion of transmitter as well as different complements of autoreceptors and mixtures of post-junctional receptor subunits. There is then a local determination of the quantitative properties of single synapses. Nerve terminals are the terminal part of the axon filled with neurotransmitters and are the location from which neurotransmitters are released. Nerve terminals may take different forms in different tissues. Nerve terminals appear like a button in the CNS, end plates in striated muscle and varicosities in many tissues including the gut. Buttons, endplates or varicosities all function to store and release neurotransmitters. In many peripheral tissues, the varicose axon branches in its proximal course and carries a covering of Schwann sheath, which is interrupted and finally lost in its most terminal part. The unmyelinated, preterminal axons with very long varicose branches are present in small axon bundles and varicose terminal axons are present as single isolated axons. The small axon bundles run parallel to and between muscle bundles and the "en passage" varicose axons are the main sources of innervations to the gut smooth muscle bundles. Nonsynaptic post-junctional receptors are mostly G-protein coupled metabotropic receptors that produce a slower response. They include metabotropic receptors for the classical neurotransmitters, monoamines, norepinephrine, purines and peptide transmitters. Post-junctional receptors also include some ionotropic receptors such as nicotinic receptors in the central nervous system (CNS) as well as the autonomic nervous system (ANS). Nonsynaptic junctional transmission is the only mode of transmission involving the varicosities that show no synaptic contacts that includes almost all nerve terminals whose target is not a neuron. Most smooth muscles exhibit both fast and slow junction potentials typically mediated by different classes of metabotropic receptors with different kinetics. The close junctional neurotransmission is characterized by synapse like close contact between the pre-junctional release site and the post-junctional receptors. However, unlike the synapse, the junctional space is open to the extravascular space; the pre-junctional release site lacks the distinguishing features of the presynaptic active zone and release of the soluble transmitters; and the post junctional receptors include metabotropic receptors or slower acting ionotropic receptors. Almost all tissues that exhibit close junctional neurotransmission also show wide junctional neurotransmission. Thus, wide junctional transmission has been described in many smooth muscles such as vas deferens, urinary bladder, blood vessels, gut as well as the nervous systems including ENS, autonomic ganglia and the CNS. Control of gastrointestinal (GI) movements by enteric motoneurons is critical for orderly processing of food, absorption of nutrients and elimination of wastes. Neuroeffector junctions in the tunica muscularis might consist of synaptic-like connectivity with specialized cells, and contributions from multiple cell types in integrated post-junctional responses. Interstitial cells of Cajal (ICC) – non-muscular cells of mesenchymal origin—were proposed as potential mediators in motor neurotransmission. Neuromuscular junctions in GI smooth muscles may reflect innervation of, and post-junctional responses in, all three classes of post-junctional cells. Transduction of neurotransmitter signals by ICC cells and activation of ionic conductances would be conducted electronically via gap junctions to surrounding smooth muscle cells and influence the excitability of tissues.
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