1.2 Organization of the Human Nervous System
A Brief History of Neuropsychology
After Chapter 1.2, you will be able to:
· Correctly associate regions of the nervous system with the CNS or PNS
· Distinguish between afferent and efferent neurons
· Describe the functions of the somatic and autonomic nervous systems, as well as the sympathetic and parasympathetic nervous systems:
The “High-Yield” badge on this section indicates that the content is frequently tested on the MCAT.
The human nervous system is a complex web of over 100 billion cells that communicate, coordinate, and regulate signals for the rest of the body. Mental and physical action occurs when the body can react to external stimuli using the nervous system. In this section, we will look at the nervous system and its basic organization.
Note: Much of the information contained in this section is also discussed in Chapter 4 of MCAT Biology Review.
CENTRAL AND PERIPHERAL NERVOUS SYSTEMS
There are three kinds of nerve cells in the nervous system: sensory neurons, motor neurons, and interneurons. Sensory neurons (also known as afferent neurons) transmit sensory information from receptors to the spinal cord and brain. Motor neurons (also known as efferent neurons) transmit motor information from the brain and spinal cord to muscles and glands. Interneurons are found between other neurons and are the most numerous of the three types of neurons. Interneurons are located predominantly in the brain and spinal cord and are often linked to reflexive behavior. Neural circuits called reflex arcs control this type of reflexive behavior. For example, consider what occurs when someone steps on a nail. Receptors in the foot detect pain and the pain signal is transmitted by sensory neurons up to the spinal cord. At that point, the sensory neurons connect with interneurons, which then relay pain impulses up to the brain. Rather However, rather than waiting for the brain to send out a signal, interneurons in the spinal cord send signals to the muscles of both legs directly, causing the individual to reflexively withdraw the foot in pain while simultaneously reflexively transferring weight to the other foot. The original sensory information still makes its way up to the brain; however, by that time, the muscles have already responded to the pain, thanks to the cooperation of these several reflex arcs.
Let’s turn to the overall structure of the human nervous system, which is diagrammed in Figure 1.1. The nervous system can be broadly divided into two primary components: the central and peripheral nervous systems. The central nervous system (CNS) is composed of the brain and spinal cord. The peripheral nervous system (PNS), in contrast, is made up of nerve tissue and fibers outside the brain and spinal cord. Note that the peripheral nervous system includes all 31 pairs of nerves emanating from the spinal cord, which are called spinal nerves, and 12 pairs of nerves emanating directly from the brain, called cranial nerves. The olfactory and optic nerves (cranial nerves I and II) are structurally outgrowths of the central nervous system, but are still considered components of the peripheral nervous system. The PNS thus connects the CNS to the rest of the body.
Figure 1.1. Major Divisions of the Nervous System
The peripheral nervous system is further subdivided into the somatic and autonomic nervous systems. The somatic nervous system consists of sensory and motor neurons distributed throughout the skin, joints, and muscles. Sensory neurons transmit information toward the CNS through afferent fibers. Motor impulses, in contrast, travel from the CNS back to the body along efferent fibers.
Afferent neurons ascend in the cord toward the brain; efferent neurons exit the cord on their way to the rest of the body.
The autonomic nervous system (ANS) generally regulates heartbeat, respiration, digestion, and glandular secretions. In other words, the ANS manages the involuntary muscles associated with many internal organs and glands. The ANS also helps regulate body temperature by activating sweating or piloerection, depending on whether the body is too hot or too cold. The main thing to understand about all of these functions is that they are automatic, or independent of conscious control. Note the similarity between the words autonomic and automatic. This association makes it easy to remember that the autonomic nervous system manages automatic functions such as heartbeat, respiration, digestion, and temperature control.
THE AUTONOMIC NERVOUS SYSTEM
The ANS has two subdivisions: the sympathetic nervous system and the parasympathetic nervous system. These two branches often act in opposition to one another, meaning they are antagonistic. For example, the sympathetic nervous system acts to accelerate heart rate and inhibit digestion, while the parasympathetic nervous system decelerates heart rate and increases digestion.
The main role of the parasympathetic nervous system is to conserve energy. It is associated with resting and sleeping states, and acts to reduce heart rate and constrict the bronchi. The parasympathetic nervous system is also responsible for managing digestion by increasing peristalsis and exocrine secretions. Acetylcholine is the neurotransmitter responsible for parasympathetic responses in the body. The functions of the parasympathetic nervous system are summarized in Figure 1.2.
Figure 1.2. Functions of the Parasympathetic Nervous System
In contrast, the sympathetic nervous system is activated by stress. This can include everything from a mild stressor, such as keeping up with schoolwork, to emergencies that mean the difference between life and death. The sympathetic nervous system is closely associated with rage and fear reactions, also known as “fight-or-flight” reactions.
When activated, the sympathetic nervous system:
· Increases heart rate
· Redistributes blood to muscles of locomotion
· Increases blood glucose concentration
· Relaxes the bronchi
· Decreases digestion and peristalsis
· Dilates the eyes to maximize light intake
· Releases epinephrine into the bloodstream
Sympathetic and parasympathetic nervous systems:
· Sympathetic: “fight-or-flight”
· Parasympathetic: “rest-and-digest”
The functions of the sympathetic nervous system are summarized in Figure 1.3.
Figure 1.3. Functions of the Sympathetic Nervous System
MCAT Concept Check 1.2:
Before you move on, assess your understanding of the material with these questions.
1. What parts of the nervous system are in the central nervous system (CNS)? Peripheral nervous system (PNS)?
2. What do afferent neurons do? Efferent neurons?
3. What functions are accomplished by the somatic nervous system? The autonomic nervous system?
4. What are the effects of the sympathetic nervous system? The parasympathetic nervous system?