Neuron communication between

FIGURE 2.2 The anatomy of the neuron. Communication between two neurons occurs at the synapse. The presynaptic neuron produces and releases the neurotransmitter into the synaptic cleft. Four mechanisms (1 ) are important to understand the function of most neurotransmitter systems. The release of neurotransmitter can be modulated via presynaptic receptors (1). The amount of neurotransmitter in the synaptic cleft can be decreased by reuptake into the presynaptic neuron (2) or via enzymatic degradation. Neurotransmitter effects at the target neuron are relayed via fast-acting ion channel—coupled receptors (3) or via slower-acting G protein—coupled receptors (4). Down-stream effects of postsynaptic receptors include the phosphorylation (P) of nuclear proteins. 

Each neuron is a distinct anatomic unit, and no structural continuity exists between most neurons. Communication between nerve cells— and between nerve cells and effector organs—occurs through the release of specific chemical signals, called neurotransmitters, from the nerve terminals. This release depends on processes that are triggered by Ca++ uptake and regulated by phosphorylation of synaptic proteins. The neurotransmitters rapidly diffuse across the synaptic cleft or gap (synapse) between nerve endings and combine with specific receptors on the postsynaptic (target) cell (see pp. 37 and 57). 

Montana V, Ni Y, Sunjara V, Hua X, Parpura V (2004) Vesicular glutamate transporter-dependent glutamate release from astrocytes. J Neurosci 24 2633-2642 Murphy TH, Blatter LA, Wier WG, Baraban JM (1993) Rapid communication between neurons and astrocytes in primary cortical cultures. J Neurosci 13 2672-2679 Murphy PM, Baggiolini M, Charo IF, Hebert CA, Horuk R, Matsushima K, Miller LH, Oppenheim JJ, Power CA (2000) International union of pharmacology. XXll. Nomenclature for chemokine receptors. Pharmacol Rev 52 145-176... 

Fasti L, Volterra A, Pozzan T, Carmignoto G (1997) Intracellular calcium oscillations in astrocytes a highly plastic, bidirectional form of communication between neurons and astrocytes in situ. J Neurosci 17 7817-7830... 

Figure 3.1 Schematic representation of communication between pontine cholinergic REM-ON, NA-ergic REM-OFF and GABA-ergic neurons in the brainstem and influence of brainstem wake- and sleep-inducing areas on REM-ON and REM-OFF neurons for the regulation of REM sleep. Refer to the text for details. The abbreviations are same as in the text (+) indicates excitation and (—) indicates inhibition.

Communication between neurons involves neurotransmitters. Up until the beginning of the last century, synaptic transmission was regarded as probably electrical. It was suggested that the close apposition of two neurons allowed the current to jump the synaptic cleft, rather like an electrical spark between two closely positioned wires. There is indeed evidence for electrical synapses in animal species where the synaptic cleft is particularly narrow (2 nm, or nanometres), as well as in the myocardium where the close coupling of cells allows electrical current to flow from one cell to the next,... 

Interruption of the communication between the motor neuron cell body and axon by transection, crush or avulsion induces motor neuron injury 734... 

The communication between neurons occurs at either gap junctions (electrical synapses) or chemical synapses with release of neurotransmitters from a presynaptic neuron and their detection by a postsynaptic nerve cell (Fig. 17.1). Neurotransmitters not used in the synaptic cleft are removed promptly by either uptake into adjacent cells, reuptake in the presynaptic neuron, or are degraded by enzymatic systems. 

Synapses are the local sites of communication between neurones. 

Communication between one neurone and another or between a neurone and an effector tissue such as muscle is mediated via neurotransmitters. Although the message, for example the sensation of pain, may be passed a long distance, neurotransmitters... 

A series of networks were considered in this study. In all cases the networks were built of a number of basic systems (biochemical neurons), and communication between the individual neurons was achieved by chemical species passing from one neuron to another, where they participate in processes taking place therein. This type of communication is operative in natural neuronic systems, where information from one neuron to the other is passed as neurotransmitter molecules crossing the synapses connecting the participating neurons. 

Over 100 years ago, a debate was raging between the two most famous neuroscientists in the world concerning the nature of the nervous system. Golgi believed that all neurons were connected in a nerve net or syncytium whereas Ramon y Cajal believed that neurons were separated from each other by tiny spaces called synapses. Cajal proved to be correct, and it was later learned that neurons communicate across the synapse by releasing chemical substances known as neurotransmitters or by releasing electrical charges. Because chemical neurotransmission is much more common than electrical transmission, especially in the brain, and it is chemical neurotransmission that is modulated by psychiatric medicines, our discussion will focus on the chemotransmitter process. In simplest terms, the process of chemical neurotransmission occurs in three steps neurotransmitter production, neurotransmitter release, and neurotransmitter action on specific receptors. 

AU mental activity is achieved by two types of cells, neurones and glial cells, and approximately 50 different chemicals that provide communication between the neurones. The neurones are specialised cells that generate differences in electrical potential that are transmitted along their surface to carry information. Within the brain, integration and coordination is brought about by around 10" neurones, com-... 

In the CNS, glial cells aid in the communication between the densely packed neurons of the CNS. These cells also play a big part in forming the blood-brain barrier. The blood-brain barrier keeps some classes of chemicals from entering the brain, which can make it very difficult to treat diseases of the brain. However, some chemicals, such as caffeine, readily enter the brain, as do many other neuroactive compounds. Compounds essential for function are actively transported across this barrier. 

This book adds to numerous preceding texts on consciousness the relatively new concept that particular neurotransmitters may be central to the process. As outlined in the Preface, communication between neurons is essential for consciousness and such communication, on the timescale applicable to conscious perception, is principally mediated by chemical neurotransmission. As Susan Greenfield (2000) points out in The Private Life of the Brain , acetylcholine may enable a whole population of cells to become more important than individual units, a kind of neuroscientific Marxism If the concept of transmitter NCC is incorporated into future discussions of the neurobiology of consciousness, or adds a further dimension to the neuropharmacology of disorders of the brain which affect conscious awareness, this book will have more than served its purpose. 

Hastings If there is an extra-SCN oscillator, presumably it is influenced by the SCN. In the two conditions it is getting a different type of perturbation. If we lesion the SCN this oscillator has no afferent input from the SCN. In the VPAC2 mutant the neurons are still there and the SCN appears fine anatomically. And so some residual communication between SCN and its targets is possible even if it is not rhythmic. It is not the same as complete deafferentation. 

Synaptic communication between neurons does not only involve the classical neurotransmitter systems reviewed in this chapter. Two other classes of neurotransmitters are also known to affect brain function neuropeptides and atypical neurotransmitters, such as nitric oxide and carbon monoxide. These neurotransmitters are not stored in vesicles, are not released by exocytosis, and do not bind to postsynaptic receptors. We have chosen not to review these neurotransmitters here, since they do not yet have implications for the current practice of neuropsychopharmacology. They are, however, promising targets for the development of new treatment strategies, and the interested reader is referred to other articles for review (Hokfelt, 1991 Snyder and Ferris, 2000). 

Neurons are the specialized cells through which virtually all the processes that occur in the central nervous system take place, from the control of movement and autonomic functions like breathing to complicated tasks like producing conscious thoughts. Internally, neurons use a combination of chemical and electrical signals, while communication between neurons is done mainly through chemical messengers called neurotransmitters. 

The communication between neurons in the CNS occurs through chemical synapses in the majority of cases. (A few instances of electrical coupling between neurons have been documented, and such coupling may play a role in synchronizing neuronal discharge. However, it is unlikely that these electrical synapses are an important site of drug action.) The events involved in synaptic transmission can be summarized as follows. 

Neurons send electrical impulses from one part of the cell to another part of the same cell via their axons, but these electrical impulses do not jump directly to other neurons. Neurons communicate by one neuron hurling a chemical messenger, or neurotransmitter, at the receptors of a second neuron. This happens frequently, but not exclusively, at the sites of synaptic connections between them (Fig. 1 — 3). Communication between neurons is therefore chemical, not electrical. That is, an electrical impulse in the first neuron is converted to a chemical signal at the synapse between it and a second neuron, in a process known as chemical neurotransmission. This occurs predominantly in one direction, from the presynaptic axon terminal, to any of a variety of sites on a second postsynaptic neuron. However, it is increasingly apparent that the postsynaptic neuron can also talk back to the presynaptic neuron with chemical messengers of its own, perhaps such as the neurotransmitter nitric oxide. The frequency and extent of such cross-communication may determine how... 

FIGURE 4—11. Shown here is normal communication between two neurons, with the synapse between the red and the blue neuron magnified. Normal neurotransmission from the red to the blue neuron is being mediated here by neurotransmitter binding to postsynaptic receptors by the usual mechanism of synaptic neurotransmission. 

---