The nerve message

The chemical that crosses the synaptic cleft and causes the transmission of the nerve message in an adjacent neuron or the stimulation of an effector cell (muscle or gland). 

Section (4) refers to the restrictions imposed on cation movement by membranes. The relative ease of movement of potassium through a nerve membrane at rest generates a potassium potential. Imposition of a perturbation upon the membrane changes its properties so that it is more permeable to sodium, and the activated membrane shows a sodium potential of reversed sign to the potassium potential. Thereupon a self-propagating spike of depolarization which is rapidly followed by recovery to the rest state fiows along the nerve cell and is the nerve message. 

The nervous system consists of two main units the central nervous system (CNS), which includes the brain and the spinal cord and the peripheral nervous system (PNS), which includes the body s system of nerves that control the muscles (motor function), the senses (the sensory nerves), and which are involved in other critical control functions. The individual units of the nervous system are the nerve cells, called neurons. Nenrons are a nniqne type of cell becanse they have the capacity to transmit electrical messages aronnd the body. Messages pass from one nenron to the next in a strnctnre called a synapse. Electric impnlses moving along a branch of the nenron called the axon reach the synapse (a space between nenrons) and canse the release of certain chemicals called neurotransmitters, one of which, acetylcholine, we described earlier in the chapter. These chemicals migrate to a nnit of the next nenron called the dendrites, where their presence canses the bnild-np of an electrical impnlse in the second nenron. 

In simple terms, messages travel along neurons (nerve cells) in the form of an electrical current that moves from one end of the neuron to its opposite end. The electric current is produced by a flow of sodium ions (Na ") and potassium ions (K ) across the nerve membrane, as shown in the diagram on page 11. When the electrical current reaches the end of the neuron, it causes the release of a chemical known as a neurotransmitter. Some examples of neurotransmitters are acetylcholine, serotonin, dopamine, GABA (gamma-aminobutyric acid), and norepinephrine. 

The neurochemical basis for these effects has also heen studied in some detail. Researchers have learned that MDMA (and its phenyl-ethylamine cousins) interferes with the normal function of at least two neurotransmitters in the brain, serotonin and dopamine. Under normal circumstances, nerve messages are transmitted through the CNS when an axon on one neuron releases a neurotransmitter, such as serotonin or dopamine, which travels across the synapse between two neurons and is taken up at a receptor site in the second neuron. 

The chemistry of such an experience is now quite well understood. Capsaicin interacts with a specific protein, a receptor, on the surface of nerve cells and triggers an influx of calcium into the cell. This then liberates a string of amino acids, known as substance P, from nerve endings. These send the pain message to the brain. And that pain can be intense. 

Classically, it has been held that this neurotransmitter-receptor complex initiates a process that reconverts the chemical message back into an electrical impulse in the second nerve. This is certainly true for rapid-onset neurotransmitters and can explain the initial actions of some slow-onset neurotransmitters as well. However, it is now known that the postsynaptic neuron has a vast repertoire of responses beyond just whether it changes its membrane polarization to make it more or less likely to fire. Indeed, many important biochemical processes are triggered in the postsynaptic neuron by neurotransmitters occupying their receptors. Some of these begin within milliseconds, whereas others can take days to develop (Figs. 1 — 11 to 1 — 13). 

The message of calcium is conveyed from the troponin to which it binds, via tropomyosin to the actin filament193-198).As soon as the nerve impulse ceases, the calcium becomes quickly removed and returned to the storage sites situated in the membranes of the sarcoplasmic reticulum. When Ca2+ concentration in the sarcoplasm reaches 1 x 10-7 molar, the fiber is relaxed. 

One problem with long-term inhalant abuse is that it can break down myelin in the body, according to a NIDA report. Myelin is a fatty tissue that surrounds many of the body s nerve cells called neurons. The nerve cells in the brain and spinal cord are like a command central for the body. They transmit messages that control just about everything the body does. If the myelin breaks down, the nerve cells may not be able to transmit messages. 

NEUROTRANSMITTER A chemical produced by one nerve cell that stimulates another nerve cell in the process of sending messages along the nerves. 

The mind-bending effects of alcohol begin soon after it appears in the blood. Within minutes it enters the brain, numbing the nerve cells and slowing messages... 

Modern measurements distinguish two groups of velocities. Nerves in most mammals have a myelin sheath interrupted by sections in which the nerve axon is in direct contact with the entire cellular fluid ( nodes ). The electrical impulses in the nodes travel at < 0.01 ms-1 while those in the myelinated sections travel at — 100 ms"1. Evidently Nernst measured a net velocity that would include the effect of interruption of the message by the axons. 

A neurotransmitter that is stored in vesicles at the nerve endings and secreted when a message needs to be passed from one nerve to another alongside it. 

Recently, studies have been carried out concerning the role of the Li+ ion in the human brain, and lithium carbonate has been used extensively in the treatment of manic-depressive patients. The Li+ ion apparently affects the levels of neurotransmitters, molecules that assist the transmission of messages along the nerve networks. Incorrect concentrations of these molecules can lead to depression or mania. (See Section 12.16 for a brief discussion of lithium s role in biological systems.)... 

GMT is primarily a disease of the nerves whereby the myelin or insulating sheath of myelin on the nerves does not stay intact and the messages from the brain to the muscles through the nerves are not carried properly. It differs from muscular dystrophy in that people who have CMT are bom with normal muscles. The muscles atrophy because the CMT affected nerves that serve them cannot properly send the message from the brain for them to move. Therefore, muscles can atrophy even though they are being used. People with muscular dystrophy have a problem with their muscles from the beginning. CMT is a muscular atrophy not a muscular dystrophy. 

Both calcium-45 and calcium-47 can be used to study how calcium is used in the body. A doctor may think that a person s body is not using calcium properly in making bones or regulating nerve messages. The doctor can use calcium-45 or calcium-47 to find out more about this problem. The radioactive isotope is injected into the person s bloodstream. Then its path can be followed by the radiation it gives off The doctor can then tell if the calcium is being used normally in the body. 

Sodium is also involved in sending nerve messages to and from cells. These impulses control the way muscles move. Again, an excess or lack of sodium can result in abnormal nerve and muscle behavior. Sodium is also needed to control the digestion of foods in the stomach and intestines. 

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