The nervous system is unique in its ability to accept and interpret various stimuli and develop a response to that stimuli, whether action based or emotional. In addition to seemingly voluntary outputs, it works to involuntarily maintain homeostasis throughout the body. The highest concentration of neurons, the basic unit of our nervous system, is within the brain and numbers in the tens of billions.1 When regarding the nervous system as a whole, the total number of neurons has been hypothesized to exceed 100 billion. Though knowledge of the architecture of our nervous system is quite vast, novel research is constantly pushing the boundaries of our understanding and providing new insights into this incredibly intricate system. Through the context of this chapter we will touch upon aspects of the central and peripheral nervous system, including its anatomy and physiology. In addition to this we will outline ways to interpret the health of a patient's nervous system through physical examination and diagnostic tools. This text is meant to serve as a primer of information to create a foundation. At times details may be purposefully foregone as they will be discussed in later chapters with detail.
DIVISIONS OF THE NERVOUS SYSTEM
The nervous system as a whole is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and the spinal cord, and the peripheral nervous system consists of the individual nerves with nerve fibers that either exit the spinal cord or feed into it from the periphery. The movement of information toward the CNS and ascending to the brain is considered afferent signaling, whereas information descending down the spinal cord and exiting the CNS structures to end organs, muscles, and glands is considered efferent signaling. The PNS is then further divided into the somatic and autonomic nervous systems. The somatic nervous system consists of peripheral nerves responsible for motor control of body movements as well as sensory information. The autonomic nervous system is responsible for unconscious bodily functions and homeostasis through nerve innervations to viscera, smooth muscle, and glandular structures. The autonomic nervous system is even further divided into the sympathetic and parasympathetic nervous system, each with specific functions (Figs. 3–1 and 3–2).
Overview of the nervous system.
General organization of the central and peripheral nervous system. (Reproduced with permission from Nerve Tissue & the Nervous System. In: Mescher AL, eds. Junqueira's Basic Histology, 14e New York, NY: McGraw-Hill; 2016.)
NERVE ANATOMY AND PHYSIOLOGY
Critical to our understanding of the nervous system is the anatomy of a nerve cell or neuron. A neuron consists of a cell body with receptive extensions known as dendrites, ...