The nervous system allows organisms to sense, organize, and react to information in the environment. The basic unit of the nervous system is the neuron. Synapses form between the neurons, allowing them to communicate to other neurons or other systems in the body. The general flow of information is that the peripheral nervous system (PNS) takes in information through sensory neurons, then sends it to the central nervous system (CNS) to be processed. After processing, the CNS “tells” the PNS what to do—what muscles to flex, whether the lungs need more oxygen, which limbs need more blood, any number of biological processes—and the PNS makes it happen through muscle control. The neurons responsible for taking information to the CNS are known as afferent neurons, while the neurons that carry the responses from the CNS to the PNS are known as efferent neurons.

The central nervous system includes the spinal cord and the brain. The brain is the body’s main control center. The main function of the CNS is the integration and processing of sensory information. It synthesizes sensory input to compute an appropriate motor response, or output.

The peripheral nervous system includes a large system of nerves that are linked to the brain and spinal cord. It is comprised of sensory receptors, which process changes in internal and external stimuli and communicate that information to the CNS. The PNS can be further subdivided into the autonomic nervous system and the somatic nervous system.

The autonomic nervous system regulates involuntary actions such as internal-organ function and blood-vessel movement. It supplies nerves to (“innervates”) cardiac and smooth muscle tissue. The autonomic nervous system is made of two components, which work in opposition to one another: the sympathetic nervous system, responsible for the body’s “fight-or-flight” response to danger, and the parasympathetic nervous system, which calms the body back down.

The somatic nervous system controls voluntary movements such as those in the skin, bones, joints, and skeletal muscles.

Practice Questions

 

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Key Points

• The nervous system is the body’s main communication system; it gathers, synthesizes, and uses data from the environment.

• The most basic unit of the nervous system is the neuron, which serves as both a sensor and communicator of internal and external stimuli.

• The nervous system can be broken down into two major parts—the central nervous system and the peripheral nervous system.

• The central nervous system, the main data center of the body, includes the brain and spinal cord.

• The peripheral nervous system includes all of the neurons that sense and communicate data to the central nervous system.

• The peripheral nervous system can be further divided into the autonomic system, which regulates involuntary actions, and the somatic system, which controls voluntary actions.

Key Terms

central nervous system: in vertebrates, the part of the nervous system comprising the brain, brainstem, and spinal cord

peripheral nervous system: the part of the nervous system comprising a large system of nerves that are linked to the brain and spinal cord; this system is divided into the autonomic and somatic nervous systems

neuron: a cell of the nervous system, which conducts nerve impulses; consisting of an axon and several dendrites. Neurons are connected by synapses

spinal cord: a thick, whitish cord of nerve tissue that is a major part of the central nervous system. It extends from the brain stem through the spine, with nerves branching off to various parts of the body

somatic nervous system: the part of the peripheral nervous system that transmits signals from the central nervous system to skeletal muscle and from receptors of external stimuli to the central nervous system, thereby mediating sight, hearing, and touch

autonomic nervous system: the part of the nervous system that regulates the involuntary activity of the heart, intestines, and glands, including digestion, respiration, perspiration, metabolism, and blood-pressure modulation

sympathetic nervous system: the part of the autonomic nervous system that raises blood pressure and heart rate, constricts blood vessels, and dilates the pupils in situations of stress

parasympathetic nervous system: one of the divisions of the autonomic nervous system; located between the brain and the spinal cord; slows the heart and relaxes the muscles