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Nervous System
Nervous System - Function
The basic functional units of the nervous system that transmit messages
are cells called neurons. Signals travel through a neuron as electrical
impulses. Neurons release chemical substances, known as
neurotransmitters, to transmit information to other neurons, to
muscles, or to glands. The chemical messages of the nervous system are
transmitted over short distances, and their effects are short-lived. The
nervous system allows for control and coordination of skeletal muscular
movements that may be consciously predetermined, or may happen
automatically, such as reflexes. Other parts of the nervous system
control and coordinate subconscious body activities, including heart
rate, gland secretions and smooth muscle movement in the digestive
system. Some activities, such as breathing, can be controlled both
subconsciously and consciously. The nervous system typically works
quickly. It also allows us to integrate and store information, such as
when you are learning.
The nervous system transmits signals to different parts of the body to
coordinate function. Electrochemical signals are processed in the brain
and sent down the spinal cord, which runs the length of the back. From
the spinal cord, peripheral nerves send signals out to the extremities.
Return signals come in through sensory nerves and either return to the
spinal cord for processing or back to the brain. The spinal cord
processes reflexes and repeated patterns.
Nervous System - Organs and Structures
The nervous system is often divided into two functional parts:
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The central nervous system, which processes incoming
information and initiates a response.
The peripheral nervous system, which brings sensory
information to, or carries motor output from, the central nervous
system to initiate a reaction.
Central Nervous System
The major structures within the central nervous system are:
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Brain
Spinal cord
The brain has several lobes, each of which carries out specific functions
and processes information associated with specific parts of the body.
The spinal cord is located within the vertebral column and processes
some reflexes but primarily transmits information to and from the brain
along neurons. Specialized membranes called meninges cover the
brain and the spinal cord to protect them. Additionally, a special fluid,
called cerebrospinal fluid, chemically and mechanically protects the
brain and spinal cord.
Peripheral Nervous System
The major structures within the peripheral nervous system are:
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Cranial nerves
Spinal nerves
The peripheral nervous system is composed of nerves outside the brain
and spinal cord. Nerves are bundles of extensions from neurons that
extend through the body in the peripheral nervous system. These nerves
are categorized into the following functional groups:
sensory nerves, which carry sensory input to the brain or spinal
cord from the environment.
• motor nerves, which carry motor impulses from the brain or
spinal cord to muscles or glands.
• mixed nerves, which have a combination of sensory and motor
neurons in one nerve.
The peripheral nervous system can be subdivided into two subdivisions:
the somatic and autonomic divisions. The somatic nervous
system includes sensory neurons that send sensory information from
sensory receptors of the skeletal muscle, skin and special senses
(including smell, taste, sight, hearing and equilibrium) to the central
nervous system and motor neurons that control skeletal muscle.
The autonomic nervous system monitors and regulates changes in
the body's internal environment. These changes are not under voluntary
control. Body processes controlled by the autonomic nervous system
include the contractions of the stomach and other digestive organs, the
heart rate, and contractions of blood vessels to control blood pressure
and flow though the body.
The autonomic nervous system is further divided into the sympathetic
and parasympathetic divisions. The sympathetic nervous
system controls functions that speed up the heart and increase energy
usage during emergencies or times of stress. On the other hand,
the parasympathetic nervous system controls functions that have
the opposite effect—they reduce heart rate and decrease overall energy
usage when the body is returning to normal after an emergency or
during normal functioning.
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Nervous System - Anatomy and Direction
The brain is protected inside the skull. The spinal cord runs from the
brain down through the bones of the spinal column. From the brain and
spinal cord, nerves run throughout the body, including to the limbs.