The Nervous System

As the most complex system, the nervous system serves as the body control center and communications electrical-chemical wiring network. As a key homeostatic regulatory and coordinating system, it detects, interprets, and responds to changes in internal and external conditions. The nervous system integrates countless bits of information and generates appropriate reactions by sending electrochemical impulses through nerves to effector organs such as muscles and glands. The brain and spinal cord are the central nervous system (CNS); the connecting nerve processes to effectors and receptors serve as the peripheral nervous system (PNS). Special sense receptors provide for taste, smell, sight, hearing, and balance. Nerves carry all messages exchanged between the CNS and the rest of the body.

The neuron transmits electric signals like an electric wire. The perikaryon (cell body) is the neuron central part. Dendrites, short branches, extend from the neuron. These input channels receive information from other neurons or sensory cells (cells that receive information from the environment). A long branch, the axon, extends from the neuron as its output channel. The neuron sends messages along the axon to other neurons or directly to muscles or glands.

Neurons must be linked to each other in order to transmit signals. The connection between two neurons is a synapse. When a nerve impulse (electrical signal) travels across a neuron to the synapse, it causes the release of neurotransmitters. These chemicals carry the nerve signal across the synapse to another neuron.

Nerve impulses are propagated (transmitted) along the entire length of an axon in a process called continuous conduction. To transmit nerve impulses faster, some axons are partially coated with myelin sheaths. These sheaths are composed of cell membranes from Schwann cells, a type of supporting cell outside the CNS. Nodes of Ranvier (short intervals of exposed axon) occur between myelin sheaths. Impulses moving along myelinated axons jump from node to node. This method of nerve impulse transmission is saltatory conduction.

The brain has billions of neurons that receive, analyze, and store information about internal and external conditions. It is also the source of conscious and unconscious thoughts, moods, and emotions. Four major brain divisions govern its main functions: the cerebrum, the diencephalon, the cerebellum, and the brain stem.

The cerebrum is the large rounded area that divides into left and right hemispheres (halves) at a fissure (deep groove). The hemispheres communicate with each other through the corpus callosum (bundle of fibers between the hemispheres). Surprisingly, each hemisphere controls muscles and glands on the opposite side of the body. Comprising 85 percent of total brain weight, the cerebrum controls language, conscious thought, hearing, somatosensory functions (sense of touch), memory, personality development, and vision.

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