The Nervous System
Your nervous system is living tissue composed if cells. The cells in the nervous system fall into two major categories: glia and neurons. Glia are cells found throughout the nervous system that provide structural support and insulation for neurons. Glia hold the nervous system together and help maintain the chemical environment of the neurons. Neurons are individual cells in the nervous system that receive, integrate, and transmit information. They are the basic links that permit communication within the nervous system. The vast majority of them communicate only with other neurons. However, a small
minorities receive signals from outside the nervous system (from sensory organs) or carry messages from the nervous system to the muscles that move the body. A highly simplified drawing of a few “typical” neurons is shown in Figure 1.
Figure 1 Structure of the neuron.
The soma, or cell body, contains the cell nucleus and much of the chemical machinery common to most cells. The rest of the neuron is devoted exclusively to handling information. The neuron at the left in Figure 3.1 has a number of branches called dendritic trees. Each individual branch is a dendrite. Dendrites are the parts of neuron that are specialized to receive information. Most neurons receive information from many other cells—Sometimes thousands of others—and so have extensive dendritic trees.
From the many dendrites, information flows into the cell body and then travels away from the soma along the axon. The axon is a long, thin fiber that transmits signals away from the soma to other neurons or to muscles or glands.
In humans, many axons are wrapped in cells with a high concentration of a white. fatty substance called myelin. The nivelin sheath is insulating material. derived form glial cells, that encases some axons. The myelin sheath speeds up the transmission of signals that move along axons. If an axon’s
myelin sheath deteriorates, its signals may not be transmitted effectively. The loss of muscle control seen with the disease multiple sclerosis is due to a degeneration of myelin sheaths.
The axon ends in a cluster of terminal buttons, which are small knobs that secrete chemicals called neurotransmitters. These chemicals serve as messengers that may activate neighboring neurons. The points at which neurons interconnected are called sinapses. A synapse is a junction where information is transmitted from one neuron to another.
The Neural Impulse
The Resting Potential and The Action Potential
Neural impulse is a complex electro-chemical reaction. Both inside and outside the neuron are fluids containing electrically charged atoms and molecules called ions. Positively charged sodium and potassium ions and negatively charged chloride ions flow back and forth across the cell membrane, but they do not cross at the same rate. The difference in flow rates leads to a slightly higher concentration of negatively charged ions inside the cell. The resulting voltage means that the neuron at rest is a tiny battery, a store of potential energy. The resting potential of a neuron is its stable, negative charge when the cell is inactive.
As long as the voltage of a neuron remains constant, the cell is quiet, and no messages are being sent. When he neuron is stimulated, channels in its cell membrane open, briefly allowing positively charged sodium ions to rush in. For an instant, the neuron’s charge is less negative, or even positive, creating an action potential. An action potential is a very brief shift in a neuron’s electrical charge that travels along an axon.
The All or None Law
The neural impulse is an all-or-none proposition, like firing a gun. You can’t half-fire a gun. The same is true of the neuron’s firing of action potentials. Either the neuron fires or it doesn’t, and its action potentials are all the same size. That is, weaker stimuli do not produce smaller action potentials.
Various neurons transmit neural impulses at different speeds. For example, thicker axons transmit neural impulses more rapidly than thinner ones do. Although neural impulses do not travel as fast as electricity along a wire, they are very fast. moving at up to 100 meters per second.
The Synapse: Where Neurons Meet
In the nervous system, the neural impulse functions as a signal. For that signal to have any meaning for the system as a whole, it must be transmitted from the neuron to other cells. This transmission takes place at special junctions called synapses, which depend on chemical messengers.
Figure 2 The synapse.
A “typical” synapse is shown in Figure 2. The first thing that you should notice is that the two neurons don’t actually touch. They are separated by the synaptic cleft, a microscopic gap between the terminal button of one neuron and the cell membrane of another neuron. Signals have to jump this gap to permit neurons to communicate. In this situation, the neuron that sends a signal across the gap is called the pre-synaptic neuron, and the neuron that receives the signal is called the postsynaptic neuron. How do messages travel across the gaps between neurons? The arrival of an action potential at an axon’s terminal buttons triggers the release of neurotransmitters—chemicals that transmit information from one neuron to another. Within the buttons, most of these chemicals are stored in small sacs, called synaptic vesicles.
Neurotransmitters and Behavior
As we have seen, the nervous system relies on chemical couriers to communicate information between neurons. These neurotransmitters are fundamental to behavior, playing a key role in everything from muscle movement to moods and mental health. Let’s briefly review some of the most interesting findings about how neurotransmitters regulate behavior.
The discovery that cells communicate by releasing chemicals was first made in connection with the transmitter acetylcholine (ACh). ACh has been found throughout the nervous system. It is the only transmitter between motor neurons and voluntary muscles. Every move you make—walking, talking, breathingdepends on ACh released to your muscles by motor neurons. ACh also appears to contribute to attention, arousal, and perhaps memory.
The activity of ACh (and other neurotransmitters) may be influenced by other chemicals in the brain. Although synaptic receptor sites are sensitive to specific neurotransmitters. sometimes they can be fooled by other chemical substances. For example, if you smoke tobacco, some of your ACh synapses will be stimulated by the nicotine that arrives in your brain. At these synapses, the nicotine acts like ACh itself. In technical language, nicotine is an ACh agonist. An agonist is a chemical that mimics the action of a neurotransmitter.
Not all chemicals that fool synaptic receptors are agonists. In effect, they temporarily block the action of the neural transmitter by occupying its receptor sites, rendering them unusable. Thus, they act as antagonists. An antagonist is a chemical that opposes the action of a neurotransmitter. For example, the drug curare is an ACh antagonist. It blocks action at the same Ach synapses that are fooled by nicotine. As a result, muscles are unable to move. Some South American natives use a form of curare on arrows. If they wound an animal, the curare blocks the synapses from nerve to muscle. paralyzing the animal.
The monoamines include three neurotransmitters: dopamine. norepinephrine, and serotonin. Neurons using these transmitters regulate many aspects of everyday behavior. Dopamine (DA), for example, is used by neurons that control voluntary movements. The degeneration of such neurons apparently causes Parkinsonism, a disease marked by tremors, muscular rigidity. and reduced control over voluntary movements.
Although other neurotransmitters are also involved, serotoninreleasing neurons appear to play a prominent role in the regulation of sleep and wakefulness and eating behavior. There also is considerable evidence that neural circuits using serotonin modulate aggressive behavior in animals and some preliminary evidence relating serotonin activity to aggression and impulsive behavior in humans.
Abnormal levels of monoamines in the brain have been related to the development of certain psychological disorders. For example, people who suffer from depression appear to have lowered levels of activation at norepinephrine (NE) and serotonin synapses.
In a similar fashion, abnormalities in activity at dopamine synapses have been implicated in the development of schizophrenia. This severe mental illness is marked by irrational thought, hallucinations, poor contact with reality, and deterioration of routine adaptive behavior.
Endorphins and their receptors are widely distributed in the human body and that they clearly contribute to the modulation of pain, as well as a variety of other phenomena. The term endorphins refers to the entire family of internally produced chemicals that resemble opiates in structure an effects.
The discovery of endorphins has led to new theories and findings on the neurochemical bases of pain and pleasure. In addition to their painkilling effect, opiate drugs such as morphine and heroin produce highly pleasurable feelings of euphoria. This euphoric effect explains why heroin is so widely abused. Researchers suspect that the body’s natural endorphins may also be capable of producing feelings of pleasure.