Structures of the Nervous System

The nervous system is a complex, highly organized network of billions of neurons and even more neuroglia. The structures that make up the nervous system include the brain. Cranial nerves and their branches, the spinal cord, spinal nerves and their branches, ganglia, enteric plexuses, and sensory receptors . The skull encloses the brain, which contains about 100 billion (10H) neurons. Twelve pairs (right and left) of cranial nerves, numbered I through XII, emerge from the base of the brain. A nerve is a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lays out- side the brain and spinal cord. Each nerve follows a defined path and serves a specific region of the body. For example, the right cranial nerve I carries signals for the sense of smell from the right side of the nose to the brain.
The spinal cord connects to the brain through the foramen magnum of the skull and is encircled by the hones of the vertebral column. It contains about 100 million neurons. Thirty-one pairs of spinal nerves emerge from the spinal cord, each serving a specific region on the right or left side of the body. Ganglia are small masses of nervous tissue, containing primarily cell bodies of neurons, that are located outside the brain and spinal cord. Ganglia are closely associated with cranial and spinal nerves. In the walls of organs of the gastrointestinal tract are extensive networks of neurons called enteric plexuses that help regulate the digestive system. Sensory receptors are either the dendrites of sensory neurons (described shortly) or separate, specialized cells that monitor changes in the internal or external environment, such as photoreceptors in the retina of the eye.

Functions of the Nervous System

The nervous system carries out a complex array of tasks, such as sensing various smells producing speech, remembering, providing signals that control body movements, and regulating the operation of internal organs. These diverse activities can be grouped into three basic functions: sensory, integrative, and motor.

Sensory function: Sensory receptors direct internal stimuli, such as an increase in blood acidity and external stimuli, such as a raindrop landing on your arm. The neurons that carry sensory information from cranial and spinal nerves into the brain and spinal cord or from a lower to higher level in the spinal cord and brain are sensory or afferent neurons.

Integrative function: The nervous system integrates (processes) sensory information by analyzing and storing some of it and by making decisions for appropriates responses. Many of the neurons that participate in integration are interneurons, whose axons extend only for a short distance and contact near by neurons in the brain, spinal cord or a ganglion. Interneurones comprise the vast majority of neurons in the body.

Motor function:  The nervous system's motor function involves responding to integration decisions. The neurons that serve this function are motor or efferent neurons. Motor neurons carry information from the brain toward the spinal cord or out of the brain and spinal cord into cranial or spinal nerves. The cells and organs connected by motor neurons in cranial and spinal nerves are termed effectors. Muscle fiber and glandular cells are examples of effectors.