Nervous System, Special Senses and Neurologic Disorders

Organization and Cellular Structure of the Nervous System

The human nervous system is an intricate communication network divided into two primary structural components: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord, serving as the master control center for processing information and directing bodily responses. In contrast, the peripheral nervous system includes the cranial nerves, spinal nerves, and associated nerve branches and ganglia located outside the brain and spinal cord. In the outpatient clinical setting, the medical assistant frequently rooms patients presenting with conditions affecting both divisions, collecting vital signs and documenting patient symptoms under the supervision of the healthcare provider.

At the microscopic level, the functional building blocks of nervous tissue are specialized cells called neurons. Each neuron is composed of a cell body, dendrites that receive impulses from neighboring cells or environmental stimuli, and an axon that transmits impulses away from the cell body toward other cells. To ensure rapid and efficient communication, many axons are wrapped in a myelin sheath, which is a specialized fatty layer covering axons that increases the speed of nerve impulse transmission. Communication between neurons, or between a neuron and an effector cell, occurs across a junction known as a synapse, where chemical messengers called neurotransmitters are released to propagate the signal.

The peripheral nervous system also encompasses the autonomic nervous system, which regulates involuntary bodily functions without conscious effort. The autonomic nervous system is divided into two functionally opposing divisions: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system activates what is commonly known as the fight-or-flight response, preparing the body for stressors by increasing heart rate, dilating bronchioles to enhance airflow, and dilating pupils. Conversely, the parasympathetic nervous system promotes the rest-and-digest response, conserving energy by slowing the heart rate, stimulating digestion, and constricting pupils. For example, when a patient experiences acute anxiety during a clinical visit, sympathetic activation causes an elevated pulse and dilated pupils, whereas sitting quietly in a calm room allows parasympathetic activity to return vital signs toward baseline.