Structure and Function of the Human Nervous System

Introduction to the Nervous System

The nervous system is the most complex and highly ordered of the various systems that make up the human body. It is the system concerned with the association and integration of a variety of bodily processes, as well as the responses and adjustments of the organism to its surroundings. The nervous system as a whole comprises the Central Nervous System (CNS), which is made up of the brain and spinal cord, and the Peripheral Nervous System (PNS), whose nerve fibers connect all parts of the body with the central nervous system.

The Peripheral Nervous System is further subdivided into two branches: the Somatic Nervous System and the Autonomic Nervous System. All these nerves are exterior to the Central Nervous System. The Somatic Nervous System controls musculoskeletal movement and transports sensory messages from the body to the CNS. The Autonomic Nervous System has two divisions — the Sympathetic and the Parasympathetic — which control the spontaneous processes of the body, including the viscera, sense organs, glands, and blood vessels. In evolutionary terms, it is older than the CNS, and its anatomical circuitry is largely dispersed, creating a generalized response quite different from the highly specific pathways and responses of the CNS. This broadly dispersed structure allows it to mediate overall changes in physiological state. It is also part of the limbic system, which has been referred to as the mammalian or emotional brain (Schoenberg, Marsh, & Lerner, 2011).

Central Nervous System

The Central Nervous System is made up of the brain and spinal cord. The primary form of communication in the CNS is the neuron. The brain and spinal cord are vital to life and functioning, so a number of protective barriers surround them. These include the skull and spine, along with membrane tissues known as meninges. Additionally, both structures are suspended in a protective liquid known as cerebrospinal fluid (Nervous System, n.d.).

While neurons are the building blocks of the body's communication system, it is the network of neurons that allows signals to move between the brain and body. These organized networks, made up of approximately one trillion neurons, constitute what is known as the nervous system. The human nervous system is made up of two parts: the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, which is composed of nerves and nerve networks throughout the body (Nervous System, n.d.).

The CNS is responsible for processing every sensation and thought that a person experiences. Sensory information gathered by receptors throughout the body is passed on to the central nervous system. The CNS also sends messages out to the rest of the body in order to manage movement, actions, and reactions to the immediate environment (Nervous System, n.d.).

Peripheral Nervous System

The Peripheral Nervous System (PNS) is the part of the nervous system that contains all the nerves lying outside of the central nervous system. The main role of the PNS is to connect the CNS to the organs, limbs, and skin. These nerves extend from the central nervous system to the furthest areas of the body. The nerves that make up the peripheral nervous system are in fact the axons, or bundles of axons, from neuron cells. In some cases these nerves are very small, but some nerve bundles are large enough to be seen with the naked eye. The PNS is not protected by bone or the blood-brain barrier, leaving it exposed to toxins and mechanical injury. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system (Flaherty, 2011).

Autonomic Nervous System: Sympathetic and Parasympathetic Divisions

Autonomic Nervous System

The autonomic system is the part of the peripheral nervous system responsible for regulating involuntary body functions such as blood flow, heartbeat, digestion, and breathing. This system is further divided into two parts: the sympathetic system, which regulates the fight-or-flight response, and the parasympathetic system, which helps maintain normal body functions and conserve physical resources (El-Sheikh, Hinnant, & Erath, 2011).

Sympathetic Nervous System

The sympathetic nervous system initiates what is commonly known as the fight-or-flight response. Like other parts of the nervous system, the sympathetic nervous system operates through a series of interconnected neurons. Sympathetic neurons are generally considered part of the peripheral nervous system, although many also lie within the central nervous system (Sherman, Bunyan, Creswell, & Jaremka, 2009).

The sympathetic nervous system is responsible for up- and down-regulating many homeostatic mechanisms in living organisms. Fibers from the SNS innervate tissues in nearly every organ system, providing at least some regulatory function to processes as varied as pupil diameter, gut motility, and urinary production. It is perhaps best known for mediating the neuronal and hormonal stress reaction commonly known as the fight-or-flight response. This reaction is also referred to as the sympatho-adrenal reaction of the body, as the preganglionic sympathetic fibers that terminate in the adrenal medulla secrete acetylcholine, which triggers the large release of adrenaline and, to a lesser extent, norepinephrine. As a result, this reaction acts primarily on the cardiovascular system, mediated directly through impulses conveyed through the sympathetic nervous system and partly through catecholamines secreted from the adrenal medulla (Sherman, Bunyan, Creswell, & Jaremka, 2009).

Parasympathetic Nervous System

The Parasympathetic system is the part of the Autonomic Nervous System responsible for the body's ability to recover and return to a balanced state known as homeostasis after enduring pain or stress. It is often called the "rest and renew" system. The parasympathetic system functions in opposition to the sympathetic nervous system. When the sympathetic system activates in response to a stressor, the parasympathetic reacts in turn to bring the body back to a state of equilibrium. The parasympathetic system is constantly active at a low level, but its activity increases when it is necessary to return the body to balance from a state of high sympathetic activity (Dulleck, Ristl, Schaffner, & Torgler, 2011).

The primary parasympathetic nerve is the vagus nerve, also known as cranial nerve X. When active, the parasympathetic system slows heart rate, dilates blood vessels, activates digestion, and stores energy. Unlike the sympathetic system, the parasympathetic response does not automatically perform all of these functions simultaneously, but rather selectively, as circumstances warrant (Dulleck, Ristl, Schaffner, & Torgler, 2011).

There appears to be a direct link between the parasympathetic nervous system and a person's mood. The PNS can go into overdrive, leading to depression, despair, withdrawal, and feelings of shame. A person's energy turns inward and they become more introspective. The parasympathetic and sympathetic nervous systems work together; at a point of balance, both may be operating simultaneously, with neither overriding the other. When the SNS is active, a person feels stressed or excited. Ultimately, a compensatory reaction from the PNS occurs, bringing the nervous system down. When the parasympathetic nervous system moves into overdrive, people may experience fear-provoked states such as the freeze response or shame. What helps to shift these states is the regulating function of a higher cortical center in the area of the brain called the prefrontal cortex, located between and just above the eyes (Dulleck, Ristl, Schaffner, & Torgler, 2011).

Conclusion

As the most multifaceted system in the body, the nervous system serves as the body's control center and communications network, operating through electrical and chemical signals. As a key homeostatic regulatory and coordinating system, it senses, interprets, and reacts to alterations in internal and external circumstances. The nervous system integrates vast amounts of information and generates appropriate responses by sending electrochemical impulses through nerves to effector organs such as muscles and glands. The brain and spinal cord constitute the central nervous system, while the connecting nerve processes to effectors and receptors form the peripheral nervous system. Special sense receptors provide for taste, smell, sight, hearing, and balance, and nerves carry all messages exchanged between the CNS and the rest of the body.

The complex composition of the nervous system often makes it difficult to determine what has gone wrong when problems arise. Disorders of the nervous system can affect people on a physical level, a psychological level, or both simultaneously. This is why a thorough understanding of the nervous system's structure and function is so important. Because of its very nature and intricate organization, it is sometimes difficult to determine precisely where a problem originates. Neuroscience — the study of the human nervous system — encompasses understanding human thought, emotion, and behavior, since all of these are interconnected with one another and with the ways the body communicates with itself. Neuroscientists use tools such as computers and specialized imaging techniques to examine molecules, nerve cells, networks, brain systems, and behavior. Through these studies, they learn how the nervous system develops and functions normally, and what goes wrong when neurological disorders occur.

References

Dulleck, U., Ristl, A., Schaffner, M., & Torgler, B. (2011). Heart rate variability, the autonomic nervous system, and neuroeconomic experiments. Journal of Neuroscience, Psychology, and Economics, 4(2), 117–124.

El-Sheikh, M., Hinnant, J., & Erath, S. (2011). Developmental trajectories of delinquency symptoms in childhood: The role of marital conflict and autonomic nervous system activity. Journal of Abnormal Psychology, 120(1), 16–32.

Flaherty, A. (2011). Brain illness and creativity: Mechanisms and treatment risks. Canadian Journal of Psychiatry / Revue Canadienne de Psychiatrie, 56(3), 132–143.

Ihlen, E. F., & Vereijken, B. (2010). Interaction-dominant dynamics in human cognition: Beyond 1/f fluctuation. Journal of Experimental Psychology: General, 139(3), 436–463.

Nervous System. (n.d.). Retrieved from http://stresshelp.tripod.com/id15.html

Psychophysiology Studies. (2010). Retrieved from http://www.adinstruments.com/solutions/research/Psychophysiology-Studies/

Schoenberg, M. R., Marsh, P. J., & Lerner, A. J. (2011). Neuroanatomy primer: Structure and function of the human nervous system. In M. R. Schoenberg & J. G. Scott (Eds.), The little black book of neuropsychology: A syndrome-based approach (pp. 59–126). New York, NY: Springer Science + Business Media.

Sherman, D. K., Bunyan, D. P., Creswell, J., & Jaremka, L. M. (2009). Psychological vulnerability and stress: The effects of self-affirmation on sympathetic nervous system responses to naturalistic stressors. Health Psychology, 28(5), 554–562.