Postsynaptic nerve

Rgure 22-2. Neurotransmission in the central nervous system. Neurotransmitter molecules (eg, norepinephrine), released by the presynaptic nerve, cross the synapse and bind with receptors in the cell membrane of the postsynaptic nerve, resulting in the transmission of the nerve impulse. 

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. 

Stimulating Postsynaptic Receptors. Neurotransmitters carry their signals by diffusing across the synapse from the axon terminals to receptors on the neighboring, so-called postsynaptic, nerve cell. Some medications stimulate postsynaptic receptors and thereby act by mimicking the action of the neurotransmitter itself. 

A similar number of amino acids in other proteins are involved in binding specific molecules, e.g. an antigen to the antibody receptor protein on B lymphocytes, a neurotransmitter to the receptor in a postsynaptic nerve, a hormone or growth factor to a receptor on the surface of a cell. 

The postsynaptic nerve ending, which is usually the tip of an axonal dendrite, has its own set of proteins, which varies to some extent with the nature of the neurotransmitter. In excitatory cells the plasma membrane of the postsynaptic neuron is thickened to — 30—40 ran to form the "postsynaptic density," a disc-like structure of clustered receptors of two types, which extends 30 ran into the cytosol.593 594 Only single receptor channels are indicated in Fig. 30-20, but many receptors are present in the clusters594 595 as are other specialized proteins. One of these, designated... 

Cholinesterases, e.g., acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholi-nesterase (BChE, EC 3.1.1.8), are serine hydrolases that break down the neurotransmitter acetylcholine and other choline esters [5]. In the neurotransmission processes at the neuromuscular junction, the cationic neurotransmitter acetylcholine (ACh) is released from the presynaptic nerve, diffuses across the synapse and binds to the ACh receptor in the postsynaptic nerve (Fig. 1). Acetylcholinesterase is located between the synaptic nerves and functions as the terminator of impulse transmissions by hydrolysis of acetylcholine to acetic acid and choline as shown in Scheme 4. The process is very efficient, and the hydrolysis rate is close to diffusion controlled [6, 7]. 

Figure 2 Sites of actions of antidepressants at the synapse (presynaptic nerve terminal, postsynaptic nerve cell, and synaptic cleft between the two are shown). MAO is present in mitochondria, and the shaded circles represent synaptic vesicles that contain neurotransmitter amines.

Acetylcholine moves to an enzyme called acetylcholinesterase which is situated on the postsynaptic nerve and which catalyses the hydrolysis of acetylcholine to produce choline and ethanoic acid. 

On the postsynaptic side, there are specific receptors located in the membrane on to which the transmitter binds, in a similar way to the type of hormone-receptor interaction proposed in the previous chapter. (It is also possible that cAMP is involved in the postsynaptic response to some transmitters.) The transmitter-receptor results in a change in the postsynaptic membrane structure. If the receptor is an excitatory one, this may result in an influx of Ca++ ions large enough for the postsynaptic membrane to become depolarized. If a sufficient number of synapses transmit excitatory messages to the postsynaptic nerve at around the same time, the result will be a general depolarization, and the second nerve wil 14 fire or the muscle contract. 

On the other hand, there are other transmitters which, working in the same way, produce a hyperpolarization of the postsynaptic nerve (i.e. make it more, rather than less negative) - thus making it harder to fire. These are inhibitory transmitters and are of equal importance to the excitatory ones (nerve cells need to be able to say No as well as Yes ). 

The molecular mechanism by which such chemoreceptor potentials arise is not yet well understood. However, it is thought that the receptor potentials may arise by processes similar to those (chemosensory membrane) occurring in the postsynaptic nerve or muscle cell. 

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