The Neuron: The Building Block of the Nervous System
The principal functional units of the brain and the nervous system are neurons. Neurons specialize in transmitting messages. Of particular importance, there are two structures called the dendrite and the axon. A neuron often has more than one dendrite; that act like antennas, picking up information and sending it in the direction of the cell body.
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 the synaptic cleft. 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.
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 neuron. A useful analogy is a key and a lock. The neurotransmitter is like the key; the receptor site is like the lock.
Three basic kinds of neurons are: Sensory neurons make 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 neurons communicate with each other. Most of the neurons in your brain are association neurons. They allow you to think, remember, and perceive. 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.
The Nervous System: The Body’s Communication Network
The nervous system is the body’s communication network. The nervous system has two main divisions. These are: (1) The central nervous system consists of the brain and the spinal cord, the brain is the subject of the next section. (2) The peripheral nervous system itself has two divisions. These are the autonomic nervous system and somatic nervous system.
The autonomic nervous system is involved in the regulation of such bodily processes as digestion, blood pressure, pulse, breathing, and internal temperature. This system has two additional divisions. These are the sympathetic division and the parasympathetic division. The sympathetic division is active primarily when there is an increase in autonomic activity. Excitement provides an example. When you are excited, your blood pressure goes up, your pulse increases, and the rate of respiration rises. These are all due to the activity of the sympathetic division. The parasympathetic division is active primarily when there is a decrease in autonomic activity. Relaxation provides an example. When you are relaxed, your blood pressure goes down, your pulse decreases, and your respiration rate falls.
The Brain: The CEO of the Organization
The brain is the chief executive officer (CEO) of the nervous system. It is traditional in studying the brain to describe its structures and their functions. Each structure is given a name and a location in the brain. Then the purpose of the structure is identified.
Located within the brain stem is a structure called the reticular activating system (RAS). When you wake up suddenly, the RAS has stimulated you to do so. When you fall asleep quickly, the RAS has also induced this behavior. On the other hand, when you are drowsy, the RAS is acting like a rheostat.
The medulla is located toward the front of the brain stem. One of its principal functions is to regulate the respiration rate. The pons, associated with the medulla, is a bulging structure also located toward the front of the brain stem; it resides above the medulla. It too is involved in the regulation of breathing. In addition, it plays a role in the regulation of sleep and attention.
Cerebellum is a small brain that serves to maintain a sense of balance and coordinate muscle movement is a function of cerebellum.
The hypothalamus has a number of functions. Of particular interest is the regulation of biological drives. The hypothalamus is not the id, because the id is a psychological concept and the hypothalamus is a physiological structure.
The pituitary gland—one of the endocrine glands—is located toward the front of the hypothalamus. It is usually called the “master gland” of the body. This is because it plays a role in regulating the action of the other endocrine glands. One of the hormones secreted by the pituitary gland is growth hormone (GH). GH has an effect on growth and stature. People who are abnormally tall have a pathology of the pituitary gland and are said to suffer from the disease of giantism.
Located just above the hypothalamus is the thalamus. One of the principal functions of the thalamus is to act as a relay center for the sense organs.
The cerebral cortex is the highest part of the brain. Cortex has a number of functions. It has dedicated areas for vision, hearing, taste, touch, and smell. It has motor areas allowing for voluntary movements. It has association areas allowing for learning, thinking, and memory.
The Two Hemispheres of the Brain: Does the Right Side Know What the Left Side Is Doing?
These two hemispheres are connected by a structure called the corpus callosum (“thick body”). The function of the corpus callosum is to provide a way for the two hemispheres to communicate with each other.
It is evident from studies of subjects with a severed corpus callosum that the right hemisphere mediates nonverbal patterning—the kind of mental functioning required in drawing, making up a melody, dancing, and creating visual images. The right hemisphere is sometimes called the “romantic” hemisphere. The left hemisphere mediates verbal and mathematical thinking—the kind of mental functioning required in writing, talking, scientific analysis, working an algebra problem, and so forth. The left hemisphere is sometimes called the “logical” hemisphere.
The right side of the brain does know what the left side is doing if the corpus callosum is intact—as it usually is.
The Endocrine System: Moods and Your Glands
You have a set of glands in your body that have a lot to do with your moods, your emotional states, and your behavior in general. These glands are called the endocrine system. The glands are called endocrine glands because they secrete their substances directly into the bloodstream without ducts (exocrine glands, such as salivary or digestive glands, secrete their substances “outside” of the bloodstream.)
The pineal gland is a tiny gland located nearly in the center of the brain. According to René Descartes, perhaps because it is small and centered, that the pineal gland is the place where the soul interacts with the body. Today’s research indicates that the pineal gland secretes a hormone called melatonin. This hormone plays a role in controlling the biological timetable for sexual maturation. Another of its functions is to regulate the sleep cycle.
The principal function of thyroxin, the hormone secreted by the thyroid gland, is to regulate metabolism.
There are two adrenal glands, and these are located on top of the kidneys. The adrenal glands produce such hormones as the corticosteroid hormones and epinephrine. The corticosteroid hormones regulate the way the body utilizes such substances as glucose, salt, and water during times of stress. Epinephrine induces an increased sense of arousal and excitement. It plays an important role in the fight-or-flight reaction, an involuntary process in which the body prepares itself to cope with threatening situations.
The pancreas gland is a large gland located under the stomach. One of the hormones secreted by the pancreas is insulin.
The gonads are the sexual glands. In females the gonads are called the ovaries. In males the gonads are called the testes. Estrogen is one of the principal hormones produced by the ovaries. Testosterone is one of the principal hormones produced by the testes.
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