- Describe the difference between the central and peripheral nervous systems and the somatic and autonomic nervous systems
- Describe the difference between the sympathetic and parasympathetic divisions of the autonomic nervous system
- Describe the endocrine system and how it affects behavior
the peripheral nervous system
We learned about the central nervous system (the brain and spinal cord); now we turn our attention to the rest of the body systems, starting with the peripheral nervous system (PNS). The peripheral nervous system is made up of thick bundles of axons, called nerves, carrying messages back and forth between the CNS and the muscles, organs, and senses in the periphery of the body. The PNS has two major subdivisions: the somatic nervous system and the autonomic nervous system.
the somatic nervous system
The somatic nervous system is associated with activities traditionally thought of as conscious or voluntary. It is involved in the relay of sensory and motor information to and from the CNS; therefore, it consists of motor neurons and sensory neurons. Motor neurons, carrying instructions from the CNS to the muscles, are efferent fibers (efferent means “moving away from”—away from the brain, in this instance). Sensory neurons, carrying sensory information to the CNS, are afferent fibers (afferent means “moving toward”). A helpful way to remember this is that efferent = exit and afferent = arrive. Each nerve is basically a bundle of neurons forming a two-way superhighway, containing thousands of axons, both efferent and afferent.
the autonomic nervous system
The autonomic nervous system controls our internal organs and glands and is generally considered to be outside the realm of voluntary control. It can be further subdivided into divisions. The two systems, the sympathetic and parasympathetic nervous systems, have complementary functions, operating in tandem to maintain the body’s homeostasis. Homeostasis is a state of equilibrium, or balance, in which biological conditions (such as body temperature) are maintained at optimal levels.
Fight-or-Flight
the sympathetic nervous system
The sympathetic nervous system is involved in preparing the body for stress-related activities. It is activated when we are faced with stressful or high-arousal situations.
- The activity of this system was adaptive for our ancestors, increasing their chances of survival. Imagine, for example, that one of our early ancestors, out hunting small game, suddenly disturbs a large bear with her cubs. At that moment, the body would undergo a series of changes—a direct function of sympathetic activation—preparing a person to face the threat. Their pupils dilate, their heart rate and blood pressure increase, their bladder relaxes, their endocrine system kicks in—their liver releases glucose, and adrenaline surges into their bloodstream.
the parasympathetic nervous system
The parasympathetic nervous system is associated with returning the body to routine, day-to-day operations and a relaxed state.
- Our hunter’s heart rate and blood pressure return to normal, their pupils constrict, they regain control of their bladder, and the liver begins to store glucose in the form of glycogen for future use. These restorative processes are associated with activation of the parasympathetic nervous system.
The “fight or flight” response is a physiological response that prepares the body to either confront a perceived threat or escape from it. For example, if a person sees a snake, whether the individual decides to fight the snake or run away from it, either action requires energy; in short, the sympathetic nervous system says “go, go, go.” If the person decides that it’s not a real threat, the parasympathetic nervous system curtails undue energy mobilization into muscles and glands and modulates the response by saying “stop, stop, stop.” This push–pull tandem system regulates fight-or-flight responses in all of us.
While it is clear that a fight-or-flight a response would be critical for survival for our ancestors, who lived in a world full of real physical threats, many of the high-arousal situations we face in the modern world are more psychological in nature. For example, think about how you feel when you have to stand up and give a presentation in front of a roomful of people, or right before taking a big test. You are in no real physical danger in those situations, and yet you have evolved to respond to any perceived threat with the fight or flight response. This kind of response is not nearly as adaptive in the modern world; in fact, we suffer negative health consequences when faced constantly with psychological threats that we can neither fight nor flee.