Nervous System: Master Controlling and Communicating System of the Body
The nervous system is the master controlling and communicating system of the body. It serves as the chief coordinating agency. Conditions both within and outside of the body are constantly changing. Through the instructions and directions of the nervous system to the various organs, a person’s internal harmony and balance between him and his environment is maintained. The nervous system is compared to a telephone communication system wherein the brain and the spinal cord act as switching centers and the nerve trunks act as cables for transmitting messages to and from these centers. The nervous system has three overlapping functions: to monitor changes (stimuli) occurring inside and outside the body and to gather information (sensory input); to process and interpret the sensory input in order to make decisions (integration); and to activate the muscles or glands (motor input).
Structural Classification of the Nervous System
Central Nervous System
The Central Nervous System is consists of the brain, the spinal cord and the nerves that extend from the spinal cord. It could be compared to an electrical communication station. The brain is like a switchboard, receiving and answering messages continuously. The spinal cord is the main cable through which incoming and outgoing message flow. And the nerves are the wires that run from the main cable to the remote corners of the body, leaving no area without a signaling device. The incoming messages are carried on a network made up of the sensory nerves. The answers are delivered by way of a companion network composed of the motor nerves.
The brain is a mass of nerve tissue occupying the cranial cavity and is covered by membranes, fluid, and the bones of the skull. It is a little over three pounds. It is the largest and most complex mass of nervous tissue in the body. It controls all our thinking and all our nerve activities.
Almost all the sensory and motor nerves that activate the muscles extend from the spinal cord. The cord is a long cable-like structure made up of hundreds of nerve fibers, which is approximately 17 inches (42 cm) long. It is suspended in the bony canal formed by the long string of vertebral, bashed by a clear fluid called the spinal fluid and enclosed within the vertebral column. The cord runs up the spine and through the opening of the bottom of the cranium where it expands and becomes the brain. It extends from the foramen magnum of the skull to the first and second lumber vertebra. It provides a two-way conduction pathway to and from the brain. It is also a major reflex center where the spinal reflexes are completed.
Nervous SystemClick thumbnail to view full-size
Parts of the Neurons
Neurons are composed of the following:
- Cell body which contains the nucleus and is the metabolic center of the cell; and the
- Nerve processes or fibersextending from the cell body composed of:
- Dendrites - the nerve processes that conduct impulses towards the cell body. Each of them functions as receptor, where a stimulus is received and the sensory impulse begins.
- Axons - the nerve processes that conduct impulses away from the cell body. When these impulses reach the axonal terminals they stimulate the release of chemicals called neurotransmitters into the extra cellular space. The axonal terminals are separated from the next neuron by a tiny gap, the synaptic cleft. This is what we call synapse which is the region of communication between neurons. Some axons in the central and peripheral nervous systems are covered with a fatty insulating material called myelin. Axons covered with myelin are called white fibers found in the white matter of the brain and spinal cord as well as in the nerve trunks in all parts of the body. The fibers and cell bodies of the gray matter are not covered with myelin. A thin outer sheath called neurilemma covers the myelinated axons of the peripheral nervous system. Neuralgia supports and protects neurons in the central nervous system. A nerve impulse is an electrochemical event that causes change in neuron plasma membrane permeability allowing sodium ions to enter the cell.
Neurons are also called nerve cells. They are highly specialized to transmit nerve impulses from one part of the body to another. All living tissue is composed of tiny, microscopic bits of matter or cells. The cells that make up the nervous system are called neurons. All cells react to stimulation, but none in the unusual way that neurons do. Neurons are capable of transmitting an impulse to its neighboring cell. The billions of neurons packed together into the nerve paths deliver a message to the brain in the following way. The impulse travels rapidly in one direction; each neuron disturbing the next one along the route of a sensory nerve until the section of the concerned organ or body part is informed. The response is flashed back in identical manner through a motor nerve to the muscles it activates. The first neuron in a sensory nerve that is stimulated starts a chain reaction that ends with the contraction of a muscle through a motor nerve.