The major biological determinant of human behavior is the nervous system, which is made up of two different kinds of cells: glial cells and neurons. Both types are very important, but the neurons tend to get most of the attention, because they are the cells that actually transmit information. Glial cells, or simply glia (the name means glue) exist to provide support for the neurons, by holding them in place, providing nourishment, destroying old and damaged neurons, and disposing of waste materials. Glia also provide myelin, an important insulating material for proper nervous transmission. Neurons are the cells that actually transmit information, making possible everything from higher-order thought processes to simple reﬂexes and movements. The body contains approximately one trillion neurons, with a large portion of them making up the brain.
Neurons come in a variety of shapes and sizes, from a microscopic scale to several feet long—there are single neurons that extend from the spinal column to the foot, for example. Despite these disparities, most neurons share a basic conﬁguration. This conﬁguration may be roughly divided into three parts: cell body, dendrites, and axon. The cell body is essentially a life-support center for the rest of the cell, and like most cell bodies it includes a nucleus, which is where the cell DNA is located. The dendrites, which may number in the hundreds on a single neuron, are ﬁngerlike projections (the name means ﬁngers) with the task of receiving information from other neurons. The axon is the extended arm of the neuron, ending in more branching projections (these are called terminal buttons) through which information is passed along to other neurons. On many neurons, especially the longer ones, the axon is wrapped in a sheet of a waxy, fatty substance called myelin, which serves to enable much faster neural transmission than is possible in unmyelinated neurons. Many elementary psychology textbooks compare the myelin sheath to the rubber insulation on the outside of a wire, but this analogy is fairly inaccurate, because transmission of electrical impulses along neurons is quite different from how wires work.
The process of neural transmission is more accurately called an electrochemical process than a purely electrical one, and its complexity makes the speed at which it occurs especially impressive. Like in all cells of the body, a neuron’s mass is made up largely of water, and the cell is surrounded by ﬂuid as well. Suspended in that ﬂuid are ions (atoms with a positive or a negative charge) of sodium (Na) and potassium (K), among other elements. These are the electrolytes that coaches worry will be lost through excessive sweating, and a major reason why dehydration is dangerous. Without the proper concentrations of these particles (too many or too few is bad), the cells that run things in the body cannot function properly.
Ordinarily, in its resting state, the inside of the cell has an excess of negatively charged ions, while the area surrounding it has an excess of positively charged ions. This balance, called a resting potential, is made possible by the cell membrane, which is selectively permeable, meaning it has “gates” through which positive ions cannot pass but negative ions can.
When a signal is received by the dendrites from another neuron or neurons telling the neuron to ﬁre (transmit a message to the next neuron), the membrane ﬂips open those gates at the ﬁrst bit of the axon, allowing the positive ions to ﬂood in. They are strongly attracted by the negative ions on the inside, rather like opposite poles on magnets. This ﬂooding depolarizes that part of the axon, which causes the next section of membrane to do the same thing, and then the next, and so on, and thus an electrical impulse (known as an action potential) travels the length of the axon.
After a section of the membrane has depolarized, it pumps the positive ions back out and gets ready to do it all again. While the membrane does this, it is in the refractory period, and incapable of ﬁring again until the resetting process is complete. Complex though this process sounds, some neurons can perform this set of functions 100 times in a single second.
The process is actually considerably more complex, considering the fact that each neuron can have dendritic connections to hundreds or even thousands of other neurons, all of which may be sending it signals at the same time. A neuron can transmit only two kinds of signal, excitatory and inhibitory. An excitatory signal encourages the cell to ﬁre, whereas an inhibitory signal tells it not to. The cell body takes all the different signals into account and determines whether excitatory signals outnumber inhibitory signals to a large enough degree (the threshold). Think of it as a small political gathering: if the yeas exceed the nays, the resolution passes and an action potential is sent along. Each neuron is only capable of ﬁring in a particular way, however; and ﬁring is an all-or-nothing response. A neuron either ﬁres or it doesn’t, and a greater degree of excitation will not produce a more intense response.
Once that action potential reaches the terminal buttons at the other end of the axon, things get a bit more complicated. Although a cell’s terminal buttons and dendrites can make connections to thousands of other neurons, no actual physical contact occurs between neurons. They are separated by an open, ﬂuid-ﬁlled gap known as the synapse, or synaptic cleft. When the action potential reaches the terminal button, it triggers the release of messenger chemicals, called neurotransmitters, into the synapse. They cross the gap and attach to receptor sites on the dendrites of the next neuron, triggering slight changes (excitatory or inhibitory) in the cell membrane. Any neurotransmitter molecules that do not bind to receptor sites are reabsorbed by the terminal buttons that released them, in a process called reuptake. All of this takes about 1/10,000 of a second to occur. Many drugs, including the antidepressants known as SSRIs (selective serotonin reuptake inhibitors), work by preventing reuptake, thus making more neurotransmitter molecules available to the neurons in the brain.
There are dozens of different neurotransmitters, and particular pathways in the brain may only make use of one or two of them, thus giving certain neurotransmitters very particular effects on behavior, emotions, and cognitions:
- Dopamine—is involved in movement, learning, attention, and emotion. Insufﬁcient dopamine is a major factor in Parkinson’s disease, in which the primary symptom is difﬁculty coordinating movement, while excess activity at dopamine receptors is often seen in schizophrenia.
- Serotonin—is involved in regulating mood, hunger, sleep/waking cycles, and arousal. Prozac and several other popular antidepressants act primarily by maintaining high serotonin levels. LSD and other psychedelic drugs also act on serotonin levels, as do cocaine and other stimulants.
- Acetylcholine—is used by neurons involved in muscle action as well as learning and memory. In persons with Alzheimer’s disease, levels of acetylcholine are unusually low.
- Gamma-aminobutyric acid (GABA)—serves primarily in inhibitory pathways and is involved in sleep disorders and eating disorders.
- Norephinephrine—is involved in control of alertness and arousal.
- Endorphins—these neurotransmitters are chemically similar to morphine and other opiates (the name is short for endogenous morphine). They are released in response to pain and vigorous exercise—hence the widely known “runner’s high.”
Most psychoactive drugs act by either ﬁtting the receptor sites for neurotransmitters, thus mimicking their actions (drugs known as agonists); or by blocking the receptor sites, thus preventing the neurotransmitter from doing its job (antagonists).
The nervous system, where all those neurons do their work, can actually be thought of as several systems that work together. Traditionally, the nervous system has been divided into two main divisions, the central nervous system (CNS) and the peripheral nervous system. The central nervous system is made up of the brain and spinal cord, and the peripheral nervous system is made up of all the rest. The peripheral nervous system is further broken down into the somatic and the autonomic nervous systems. Another helpful way to think of these two systems is as the voluntary and involuntary systems. The somatic nervous system consists of all those neurons that are involved in bringing sensory information to the brain, and is also responsible for the production of voluntary movements and actions, whereas the autonomic nervous system is made up of neurons involved in producing involuntary reactions.
One example of an autonomic nervous system response with which everyone should be familiar is the ﬁght-or-ﬂight response, the set of involuntary physiological reactions that is often activated by stress. Assume, for example, that a leisurely, pleasant hike in the woods is suddenly interrupted by coming face-to-face with a hungry Bengal tiger. Without any need for conscious thought to motivate it, the body goes into action, getting ready to either run away very fast or to give that tiger a walloping he won’t soon forget (one of these is a more advisable plan of action than the other, considering the hiker’s unarmed state). Symptoms are likely to include the following: faster, shallower breathing (to get more oxygen to your cells more rapidly), elevated pulse and blood pressure (same reason), rush of blood to the face (evolutionarily speaking, this is probably to intimidate the tiger, but it isn’t likely to be effective), profuse perspiration (to cool off the ﬁghting-or-ﬂeeing body), dilated pupils (so the hiker can see his fate more clearly), and, under great stress, release of bladder and/or bowel control (probably for purposes of dropping ballast and lightening the load—this one made sense biologically before clothing).
The autonomic nervous system is also subdivided into two parts: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic system is the one that produces the ﬁght-or-ﬂight response, but that state (fast heart rate, high blood pressure, etc.) isn’t a healthy one to remain in for very long, so a separate system (the parasympathetic) exists simply to slow things back down to normal once the danger is past. In most (less dangerous than an encounter with a tiger) anxiety-producing circumstances, a lesser version of the response may occur (sweaty palms when about to meet someone important, discomfort in the stomach upon hearing a scary sound late at night), but the autonomic nervous system is still responsible.
Sylwester, R. A Celebration of Neurons: An Educator’s Guide to the Human Brain. Association for Supervision and Curriculum Development, 1995.