Neuroeffector junction
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.
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.
Nerve terminals appear like a button in the CNS, end plates in striated muscle and varicosities in many tissues including the gut.
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.
Most smooth muscles exhibit both fast and slow junction potentials typically mediated by different classes of metabotropic receptors with different kinetics.
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.
[5] Control of gastrointestinal (GI) movements by enteric motoneurons is critical for orderly processing of food, absorption of nutrients and elimination of wastes.
[6] Studies do not exclude the possibility of parallel excitatory neurotransmission to ICC-DMP (deep muscular plexus) and smooth muscle cells.
However, it was not until the advent of the electron microscope that we were able to provide us with a comprehensive view of the relationship between these varicose endings and smooth muscle.
There is no reason to assume a priori that responses to neurotransmitters released from neurons and exogenous transmitter substances are mediated by the same cells, receptors or post-junctional (transduction) signaling pathways.
Besides smooth muscle, autonomic neural control of immune, epithelial, and endothelial cells also involves nonsynaptic transmission.
The active zone of individual sympathetic varicosities, delineated by a high concentration of syntaxin, occupies an area on the pre-junctional membrane of about 0.2 μm2; this gives a junctional gap between the pre-junctional active zone and post-junctional membranes that varies between about 50 and 100 nm.
The post-junctional membrane beneath the varicosity can possess a patch about 1 μm2 of purinergic P2X1 receptors in high density, although this is not always the case.
A multiplicity of transmitters are utilized by autonomic nerves, and cotransmission occurs, often involving synergistic actions of the cotransmitters, although pre- and post-junctional neuromodulation of neurotransmitter release also take place.
Cotransmission without co-storage occurs in parasympathetic nerves, where terminals staining for the vesicular acetylcholine transporter can also contain nitric oxide synthase, suggesting that they release NO as a gaseous neurotransmitter.
Neuroeffector Ca2+ transients (NCT) have been used to detect the packeted release of the neurotransmitter ATP acting on post-junctional P2X receptors to cause the Ca2+ influx.
This could arise by autoinhibition (by the pre-junctional action of noradrenaline or purines) or due to a transient shortage of vesicles readily available for release.
[5] Over the past 20 years, many studies have given evidence that Interstitial cells of Cajal (ICC): (i) serve as pacemaker cells with unique ionic currents that generate electrical slow waves in GI muscles; (ii) provide a pathway for active slow wave propagation in GI organs; (iii) express receptors, transduction mechanisms and ionic conductances allowing them to mediate post-junctional responses to enteric motor neurotransmission; (iv) regulate smooth muscle excitability by contributing to resting potential and affecting syncytial conductance; and (v) manifest stretch-receptor functions regulating excitability and regulating slow wave frequency.
Calcium imaging studies in the colon have shown that ICC-MY is innervated by nitrergic and cholinergic nerve terminals, though the nature of the contacts has not been well defined.
ICC-IM play a critical role in the reception and transduction of cholinergic excitatory and nitrergic inhibitory neurotransmission.
Recent morphological evidence using anterograde tracing methods, has shown close apposition between vagal and spinal afferents and ICC-IM within the stomach wall (Fig.
