Nervous system interneuron

Figure 4.2 Gastric add production. Two cell types in the mucosa of the corpus of stomach are principally responsible for secretion of acid. Histamine secreted from nearby enterochromaffin-like (ECL) cells stimulates parietal cells to secrete acid. A variety of substances can stimulate the ECL cell to secrete histamine, including PCAP, pituitary adenyl cyclase-activab ng peptide (released from enteric nervous system interneurones in the gastric mucosa), and gastrin, both stimulating ECL cells via adenyl cyclase to raise intracellular levels of cAMP that lead to histamine secretion.

GABA is the most prominent inhibitory neurotransmitter in the mammalian nervous system and acts via GABA receptors. Activation of GABAb receptors by GABA released from local spinal interneurons (Fig. 1) negatively modulates nociceptive transmission in the spinal cord. Agonists at GABAb receptors... 

An interneuron together with a sensory afferent and motor efferent form a polysynaptic reflex (Figure 2.2) this comprises the initial stage of information input (sensory afferent), the processing/computing an appropriate response (interneurons) and the execution of a behavioural response (motor efferent). The simplest reflexes in the nervous system are monosynaptic reflexes, such as the familiar tendon (knee) jerk, these do not involve an interneuron. The sensory afferent activated by the mechano-receptor (the tap of the patellar hammer) forms a synapse with the motor efferent in the spinal cord, which then causes the skeletal muscle to contract and the crossed leg to jerk forward. With a synaptic delay of 1 millisecond (ms), the time between input and output increases with the number of synapses introduced into the circuit. As an... 

NGF also has actions within the CNS, although it is not particularly abundant in the CNS. Its synthesis appears to be largely restricted to the hippocampus and neocortex, and even in these regions it is present at relatively low concentrations relative to the other neurotrophins. The most prominent population of NGF-responsive neurons expressing TrkA are the basal forebrain cholinergic neurons. The principal projections of these neurons are to the hippocampus and cortex, which conforms with the concept that NGF acts as a target-derived trophic factor in the CNS, just as it does in the peripheral nervous system (PNS). NGF also acts on a subpopulation of cholinergic neurons within the striatum. These interneurons express the NGF receptor, TrkA, and respond to NGF. However, they do not appear to rely entirely on NGF for their survival, and the specific actions of NGF on this neuronal population have not been clearly defined. NGF may also have autocrine actions in the CNS, as some neuronal populations have been identified that express both TrkA and NGF. 

Diazepam Facilitates GABAergic transmission in central nervous system (see Chapter 22) Increases interneuron inhibition of primary motor afferents in spinal cord central sedation Chronic spasm due to cerebral palsy, stroke, spinal cord injury acute spasm due to muscle injury Hepatic metabolism duration, 12-24 h Toxicities See Chapter 22... 

The central nervous system (CNS) - the brain and spinal cord - is involved in the reception and interpretation of peripheral afferent nociceptive impulses. Reflexes mediated by spinal interneurons and the gating functions of the dorsal horn of the spinal cord are particularly crucial. However, our knowledge of brain mechanisms is still limited. 

Benkirane S, Arbilla S, Langer SZ (1987) A functional response to D] dopamine receptor stimulation in the central nervous system inhibition of the release of [3H]-serotonin from the rat substantia nigra. Naunyn-Schmiedeberg s Arch Pharmacol 335 502-7 Bennett BD, Wilson CJ (1999) Spontaneous activity of neostriatal cholinergic interneurons in vitro. J Neurosci 19 5586-96... 

A nervous system is essential for the passage of information through the body by means of electrical signals. As shown in Figure 7.1, in insects reflex is initiated when a sensory receptor detects a stimulus. A sensory neuron conveys the electrical impulse to the effector organs via an interneuron involving two synapses. 

These neurons were of different t3rpes interneurons, motor neurons, and neurosecretory cells. A small, stereotyped population of PLI neurons was found in the Drosophila larval central nervous system (CNS). In the periphery, proctolin-immunoreactive neuromuscular endings were identified on both visceral and skeletal muscle fibers. On the hindgut, the neuropeptide is associated with endings on intrinsic circular muscle fibers. In this study, the presence of proctolin was verified in the CNS, hindgut, and segmental body wall by tissue extraction followed by reverse-phase HPLC and quantitative bioassay. Evidence for a proctolin-like substance has also been found in the adult Colorado potato beetle... 

In the peripheral nervous system, ACh is found as the neurotransmitter in the autonomic ganglia, the parasympathetic postganglionic synapse, and the neuromuscular endplate. Cholinergic neurons in the central nervous system are either wide ranging projection neurons or short ranging interneurons in (1) the basal forebrain (septum, diagonal band, nucleus basalis of Meynert) projects to the entire cortex, the hippocampus,... 

The fundamental cellular component of the nervous system, the neuron, has long, branching, cyllndrlc processes (dendrites and axon) that extend from the cell body. Dendrites are modified for signal reception and transduction and form extensive networks that permit Interneuronal communication, coordination, and integration of nervous-system function. The axon typically is a long extension of Che neuron specialized for transmission of electric signals and, at Its distal end, for chemical communication of Information to other neurons or to muscle at sites termed synapses and neuromuscular junctions, respectively. 

Sensory neurons transduce physical stimuli, such as smell, light, or sound, into action potentials, which are then transmitted to the spinal cord or brain. Sensory neurons, which bring information into the central nervous system, are also referred to as afferent neurons. Motor neurons transmit nerve impulses away from the brain and spinal cord to muscles or glands and are also called efferent neurons. Intemeurons transmit nerve impulses between sensory neurons and the motor neurons. Interneurons are responsible for receiving, relaying, integrating, and sending nerve impulses. Intemeurons are found exclusively in the central nervous system and account for almost 99% of all the nerve cells in the body. 

The elimination of y-BHC symptoms by avermectin may be explained as follows The primary action site of y-BHC is likely to involve the picrotoxinin receptor (5). Therefore, one could assume that y-BHC, at least in part, acts in an identical manner to picrotoxinin which has already been shown to antagonize the action of avermectin Kass et al. (29) reported two types of avermectin action on the nervous system of the nematode, based on the inhibition by picrotoxinin. One is the avermectin-induced blockage of interneuron-excitatory motorneuron transmission, which can be reversed by picrotoxinin. The other is the avermectin-induced blockage of the transmission between inhibitory motoneurons and muscle, which cannot be reversed by picrotoxinin. 

Only in the last few centuries has the link between the brain and behaviour become clear, and only at the end of the nineteenth century was it demonstrated that the nervous system was made up of billions of separate nerve cells or neurons. We now know that during evolution complex networks of such neurons have developed in order to effect certain behaviours. Whilst the neurons of the central, peripheral and autonomic nervous systems vary enormously in form and function, they can be classed into three broad groups sensory neurons which convey information into the central nervous system effector neurons which carry information out of the central nervous system to muscles and other effector organs and interneurons within the central nervous system which link the sensory and effector neurons and also have links with one another. 

Postsynaptic responses to neurotransmitters are invariably initiated by the binding of the transmitter to a specific recognition site, or receptor. This finding is true both for interneuronal communications and the transmission of signals from neurons to effector cells. Perhaps the only known exception to this observation is the presumed communication in the central nervous system between electrotonic synapses, a topic beyond the scope of this chapter [see Weight (1971) and Schmitt et al. (1976) for further details]. 

As we have seen in Drosophila, many developmental signals act in a graded fashion, inducing different cell fates depending on their concentration. The same phenomenon exists in vertebrates, for example, in the development of the mammalian central nervous system from the neural tube, which forms early in embryogenesis. The neural tube is a simple rolled-up sheet of cells, initially one cell thick. Cells in the ventral part will form motor neurons lateral cells will form a variety of interneurons. The different cell types can be distinguished prior to morphological differentiation by the proteins that they produce. 

Because of its size and availability, Ascaris smm has been the parasitic nematode of choice for neuromuscular analysis. The Ascaris nervous system contains approximately 90 motoneurons and interneurons (42). If both the head and tail of the worm are removed, these 90 motoneurons and interneurons are sufficient to control locomotion. Ascaris moves by propagating a wave of motion along its body axis anteriorly or posteriorly (42). The muscle cells that are innervated are arranged in two longitudinal masses, the dorsal and the ventral. They are independently innervated by neurons from the dorsal or ventral nerve cord (42). This distinctive pattern of nerve-muscle interaction ensures that the propagated wave moves through the axis of the worm in the dorsal-ventral plane. 

The extreme sensitivity of the receptor neurons is combined with a most efficient processing of their responses by the central nervous system. Via the axons of the receptor neurons the nerve impulses are conducted to the antennal lobe, the first synaptic station of the central olfactory pathway in insects. The axons of the pheromone receptor neurons terminate on local interneurons and projection neurons (PN) of the macroglomerular complex (MGC) (Hildebrand 1996). The silk moth has... 

The generated spike activity of the-different sensory cells in response to stimuli is decoded by the central nervous system (CNS). To investigate the sensory code of the receptor cells of a sensillum or several sensilla, different approaches can be used. The most direct method is to record from central neurons. In contrast to the investigation of central neurons responding to olfactory stimulations (Chapter 2), such recordings have been performed only recently from interneurons of the thoracic ganglion of the blowfly in response to the stimulation of tarsal contact chemoreceptive sensilla (Rook et al., 1980). 

Rook, M. B., Wolk van der, F. M. and Starre van der, H. (1980) Functional-anatomical mapping of taste-associated interneurons in the central nervous system of the blowfly, Calliphora vicina (R.-D.). In Olfaction and Taste, Vol. VII (van der Starre, H., ed.) p. 281. Information Retrieval, London. 

In the peripheral and central nervous system, there are several inhibitory mechanisms such as A-beta neiu-ons and enkephalins. Repetitive stimulation of nerve fibers called A-beta mechanoreceptives, for example, initially excites and then inhibits dorsal horn spinothalamic tract (STT) neurons via interposed interneurons releasing y-aminobutyric acid (GABA) or enkephalin, which decreases the stimulation of C fibers. 

---