Neuronal communication

Chemical neurotransmission is the way in which neurons communicate by releasing chemical substances that are received by the receptors in the next neuron (or the target) and excite or inhibit it. About 50% or more of dtug mechanisms are based on modification of chemical neurotransmission. 

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. 

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. 

The individual unit of the nervous system is the neuron, a specialized cell that both receives and transmits information. The nervous system contains more than 100 billion neurons and is a major user of metabolic energy in the human body. It is also a region particularly susceptible to injury from toxic chemicals, lack of oxygen, and other assaults. Depending on the nervous region in which they reside, neurons may have different anatomical features and may use different chemical transmitters. Neurons communicate with each other and with their end organs by these chemical signals, which are released from the nerve terminal and interact with specific receptors on adjacent neurons or cells. 

Synaptic transmission is the process whereby neurons communicate with each other and with the target organs whose physiology they are influencing synaptic transmission permits the action potential to jump from one neuron to the next. It is imperative... 

Drugs that alter sleep produce their effects on the brain by altering the actions of neurotransmitters and consequently how neurons communicate with each other. However, different drugs can alter the actions of neurotransmitters in different ways. Stimulants such as amphetamine cause neurons to release excess amounts of neurotransmitters like dopamine and serotonin. Other drugs, such as the prescription sleeping pills Halcion or Ambien or antihistamines, can interact directly with the neurons receptors to either enhance or block the effects of the neurotransmitters. In later chapters, we will discuss how drugs that help you sleep or stay awake alter the chemistry of the brain. 

Gap junctions in synapses. Not all neurons communicate via chemical synapses. Gap junctions, which are found in both neurons, astrocytes, and other cells, serve as electrical synapses. Thus, heart cells are all electrically coupled together by gap junctions.606-608 Gap junctions are formed with the aid of hexameric connexons, which are present in each of the opposed membranes and are aligned one with the other (Fig. 1-15F,G).607 609 610 There may be thousands of connexons in a single gap junction, which resemble ion channels in appearance but contain pores 1.5 nm in diameter. They are formed from 26- to 43- kDa... 

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 i—i. Top The anatomy of a few brain regions that will be discussed later. Bottom How individual neurons communicate with each other. See text for details. 

CB1 receptors are widely prevalent in the brain and are particularly concentrated in areas known as the basal ganglia, hippocampus, cerebellum, and cerebral cortex. By binding or blocking actions of the cannabinoid receptors, THC interrupts normal neuronal communications and so creates the effects that are associated with marijuana usage. Studies now show that the behavioral and physical effects associated with THC and marijuana strongly correlate with the amounts of cannabinoid receptors in these areas of the brain. 

Winder DG, Conn PJ (1996) Roles of metabotropic glutamate receptors in glial function and glial-neuronal communication. J Neurosci Res 46 131-7... 

Norenberg, M.D., Neary, J.T., Bender, A.S., Dombro, R.S. (1992). Hepatic encephalopathy A disorder in glial-neuronal communication. Progress Brain Res. 94,261-269. 

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). 

The elevation of presynaptic Ca2+ causes synaptic vesicles to fuse with the plasma membrane and release neurotransmitter into the synaptic cleft. Neurotransmitters then diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane. This can result in the opening or closing of ion channels, thus changing the probability that the postsynaptic neuron will fire an action potential, or the triggering of biochemical cascades within the postsynaptic neuron (collectively termed signal transduction). In this manner, neuronal communication occurs as information is transmitted from the presynaptic membrane to the postsynaptic membrane, and thus from one neuron to another. 

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