The principal functional units of the brain and the nervous system are neurons.
The neuronis a living cell with a cell wall and a nucleus. Unlike other cells of the body, neurons specialize in transmitting messages. Of particular importance are two structures called the dendriteand the axon. A neuron often has more than one dendrite; dendrites are reminiscent of a root system. They act like antennas, picking up information and sending it in the direction of the cell body. The axon extends from the cell body like a long filament. Although there can be more than one dendrite, there is always just one axon. The axon consistently sends informa- tion away from the cell body, often to an adjacent neuron.
At the end of the axon there is anend foot,a slightly bulging structure that releases neurotransmitters. A neurotransmitter is a chemical messenger that allows a neuron to communicate with either other neurons or muscle fibers. The neurotransmitter travels across a physical gap called thesynaptic cleft.The func- tional connection between, for example, two neurons is called thesynapse.Note that there is a formal distinction between a synaptic cleft and a synapse. The first refers to a physical characteristic; the second refers to a functional characteristic. It is common to hear people refer to the gap itself as the synapse, and this is incorrect.
In order for a neurotransmitter to work, it must find a receptor site on an adjacent cell. The receptor site has a physical shape that matches that of the neu- ron. A useful analogy is a key and a lock. The neurotransmitter is like the key; the receptor site is like the lock.
(a) An axon consistently sends information . (b) A neurotransmitter is also referred to as a .
(c) In order for a neurotransmitter to work, what must it find on an adjacent cell?
Answers: (a) away from the cell body; (b) chemical messenger; (c) A receptor site.
Neurotransmitters have received a lot of attention in recent years. Their activ- ity often forms the basis of biological theories of mental disorders. For example, low levels of the neurotransmitter serotoninare associated with depression. Spe- cific drugs called selective serotonin reuptake inhibitors (SSRIs)increase the amount of useable serotonin at the synapse. (Prozac is such a drug.)
For a second example, excessive activity of the neurotransmitter dopamineis associated with schizophrenia, a mental disorder characterized by delusions. There is evidence to suggest that chronic schizophrenic patients have too many receptor sites for dopamine, and this causes excessive dopamine activity. It is this activity
Dendrites Cell wall Axon End foot
Synaptic cleft Nucleus
Two adjacent neurons with selected structures.
that induces delusions. Some of the principal antipsychotic drugs block receptor sites for dopamine. (See the section on drug therapy in chapter 15.)
(a) Identify a neurotransmitter associated with depression.
(b) Identify a neurotransmitter associated with schizophrenia.
Answers: (a) Serotonin; (b) Dopamine.
Three basic kinds of neurons are (1) sensory, (2) association, and (3) motor.
Sensory neuronsmake it possible for us to be in contact with the outside world.
They are sensitive to light, sound, chemicals that induce taste sensations, and so forth. The rods and cones in the retina of your eye are sensory neurons.
Association neuronscommunicate with each other. Most of the neurons in your brain are association neurons. They allow you to think, remember, and per- ceive. It is the rich complexity of association neurons that makes self- consciousness possible.
Motor neurons communicate with muscle fibers, and these too are cells of the body. Complex contractions and relaxations of muscle fibers make it possible for us to talk, walk, and otherwise act.
(a) Sensory neurons make it possible for us to be in contact with . (b) Association neurons communicate with .
(c) Motor neurons communicate with .
Answers: (a) the outside world; (b) each other; (c) muscle fibers.
The brain contains about 3 billion neurons. The rest of the nervous system contains approximately the same amount. The population of the planet Earth is rapidly approaching 6 billion people. It is impressive to think that the quantity of neurons you possess is roughly equivalent to the human population of our world.
When a neuron releases neurotransmitters it is like a gun; when it “fires,” it sends forth a spray of neurotransmitters. The formal term for “fire” is depolar- ize.When a neuron depolarizes, it alternates from (1) a resting negative electrical potential to (2) a positive electrical potential, and (3) returns to a negative electri- cal potential again. This completes a cycle. In some cases a neuron can complete 2,000 to 3,000 cycles in a single second.
A nerveis a bundle of axons. Think of the axons as thin rope fibers and the nerve as the thick rope itself. Nerves are identified as afferent or efferent. Affer- ent nervesmove toward an objective. Efferent nervesmove away from a source.
The optic nerve is an afferent nerve; it carries visual messages toward the brain.
On the other hand, when you pick up a pen you are using efferent nerves. Mes- sages are being carried away from the brain.
(a) When a neuron releases neurotransmitters it is either said to “fire” or to . (b) Nerves that move away from a source are called .
Answers: (a) depolarize; (b) efferent.