Organization of the Nervous System

Introduction to the Nervous System and Its General Functions

The nervous system acts as the master controlling and communicating system of the body. Every thought, action, and emotion reflects its activity. Its signaling device uses electrical impulses, which are rapid and specific, causing almost immediate responses. Working alongside the endocrine system, it maintains homeostasis. The nervous system performs three overlapping functions:

1. Sensory input: Millions of sensory receptors monitor changes occurring both inside and outside the body. These gathered changes are called stimuli.
2. Integration: The nervous system processes and interprets sensory input and decides what should be done at each moment.
3. Motor output: The nervous system activates effector organs—muscles and glands—to cause a response.

Structural Organization: Central versus Peripheral Nervous Systems

Anatomically, the nervous system is divided into two principal subdivisions: the Central Nervous System and the Peripheral Nervous System.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It occupies the dorsal body cavity and acts as the integrating and control center of the nervous system. It interprets sensory input and dictates motor output based on past experience, reflexes, and current conditions.
• Peripheral Nervous System (PNS): Consists of parts of the nervous system outside the CNS. It mainly consists of nerves that extend from the brain and spinal cord. Spinal nerves carry impulses to and from the spinal cord, while cranial nerves carry impulses to and from the brain. These nerves serve as communication lines linking all parts of the body to the CNS.

Anatomy and Functional Divisions of the Central Nervous System

The central nervous system is encased in bone—the skull protects the brain, and the vertebral column protects the spinal cord. Within this protected environment, different regions handle specialized tasks. The cerebrum forms the largest part of the brain and manages higher mental functions, conscious thought, and voluntary movement. The cerebellum coordinates balance, posture, and smooth skeletal muscle movements. The brainstem controls vital autonomic functions like breathing, heart rate, and blood pressure. Continuous with the brainstem, the spinal cord serves as a major reflex center and a two-way conduction pathway between the body and the brain. A common misconception is that the spinal cord only relays signals; it actually initiates independent reflex arcs without waiting for brain input.

Anatomy and Functional Divisions of the Peripheral Nervous System

The peripheral nervous system has two functional subdivisions based on the direction in which nerve fibers transmit impulses:

• Sensory (Afferent) Division: Keeps the CNS informed of events by conducting impulses from sensory receptors throughout the body toward the CNS. Somatic sensory fibers convey impulses from the skin, skeletal muscles, and joints. Visceral sensory fibers transmit impulses from visceral organs.
• Motor (Efferent) Division: Transmits impulses from the CNS out to effector organs, which are muscles and glands. These impulses activate muscles to contract and glands to secrete, effecting a motor response.

Somatic versus Autonomic Nervous Systems

The motor division of the peripheral nervous system further splits into two main systems that regulate different types of effectors:

Clinical observation reveals that somatic nerve damage leads to flaccid paralysis of skeletal muscles, whereas autonomic damage affects vegetative functions like digestion and blood pressure control.

Sympathetic and Parasympathetic Divisions of the Autonomic Nervous System

The autonomic nervous system is divided into two parts that usually have antagonistic effects on the same visceral organs:

• Sympathetic Division: Mobilizes the body during activity; it is the "fight-or-flight" system. During exercise, excitement, or emergency, it directs blood to skeletal muscles, dilates bronchioles, and increases heart rate.
• Parasympathetic Division: Promotes housekeeping functions during rest; it is the "rest-and-digest" system. It keeps body energy use as low as possible while directing activities like digestion, defecation, and diuresis.

While most visceral organs receive nerve fibers from both divisions, the balance between sympathetic and parasympathetic tone determines normal baseline organ function.

Microscopic Organization: Neurons and Neuroglial Cells

Nervous tissue is composed of two principal cell types: neurons and neuroglia.

• Neurons: Structural units of the nervous system. They are extreme specialists that conduct messages in the form of nerve impulses from one part of the body to another. Key features include extreme longevity, an inability to divide (amitotic), and a high metabolic rate requiring continuous oxygen and glucose.
• Neuroglial Cells (Glial cells): Smaller cells that surround and wrap delicate neurons. In the CNS, glial cells include astrocytes, microglia, ependymal cells, and oligodendrocytes. In the PNS, neuroglia include Schwann cells and satellite cells. Unlike neurons, glial cells can divide and generally support, insulate, and protect neurons.

Clinical Implications for Nursing Practice

Understanding nervous system organization directly guides nursing assessments and interventions. When assessing a patient with a suspected stroke or trauma, nurses test specific dermatomes and cranial nerves to localize the lesion to either the CNS or PNS. Autonomic dysreflexia in spinal cord injury patients demonstrates the dangerous interplay between sympathetic and parasympathetic divisions, where a noxious stimulus below the injury triggers massive, uninhibited sympathetic vasoconstriction. Recognizing whether a deficit involves voluntary skeletal muscle (somatic) or involuntary organ function (autonomic) helps nurses anticipate specific complications, such as aspiration risks, bowel or bladder incontinence, and hemodynamic instability.