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Physiological Processes of the Nervous System

Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Your nervous system is like your body's personal computer, constantly collecting, sorting and responding to all the information around you. Read on to learn more about how this happens.

What Is the Nervous System?

Do you remember the last time you touched boiling water or a hot skillet? You felt a sharp, distressing sensation called pain. That pain was your nervous system responding to a change in your environment and telling you, ''Stop touching that right now!'' Your body responded to the command before you had a chance to consciously think about it.

Your nervous system is like your body's personal computer, constantly collecting, sorting and responding to all the information around you. It is responsible for controlling and communicating with all of your body parts; it is the center of your mental activity, including thought and memory; and it monitors and responds to the environment, both inside your body and out.

The nervous system is the organ system consisting of the brain, spinal cord and sense organs (such as the nose, skin and ear), as well as the nerve fibers that connect these organs with the rest of the body. Its primary roles are sensing changes in the environment (like your hand on a hot pan), integrating or processing all of this incoming information (''This is damaging our skin!''), and responding in the form of a motor reaction (''Move your hand immediately!'').

The Nervous System Is Made of Nerve Cells

The nervous system and all of its organs, including the brain, spinal cord and nerves fibers, consist of specialized nerve cells called neurons. Neurons have the unique ability to communicate with each other through the transmission of electrical impulses. The basic structure of a neuron is composed of a cell body, an axon (wrapped in myelin) and dendrites.

Diagram of a neuron.
Diagram of neuron.

The cell body is the main part of the cell. Similar to other cells in your body, it contains a nucleus and carries out the basic functions of the cell. Dendrites are short, hair-like extensions of the cell body and serve to pick up impulses and carry them into the neuron. Finally, the axon is a single, long extension that carries the impulse away from the cell body, and towards the dendrites of the next neuron. Axons are wrapped in a white, fatty substance called myelin. The myelinated axons make up the so-called, ''white matter'' of the brain and spinal cord; the rest of neuron makes up the ''gray matter''.

The white matter in your brain is made of axons wrapped in myelin. The gray matter consists of the cell bodies, dendrites and axon terminals.
Cross-section of a human brain.

Functions of the Nervous System

The primary responsibilities of the nervous system can be sorted into three interrelated categories: the sensory functions, the integrative functions and the motor functions.

Sensory Functions

Using millions of sensory receptors in your body, your nervous system is constantly gathering sensory information by monitoring the changes, or stimuli, that occur both inside and outside of your body. These receptors detect external variations, such as temperature, touch, light and sound, as well as internal variations, like pH, concentrations of carbon dioxide, levels of salt and water, and the positions of your joints and muscles. All these various sensory stimuli are then translated into electrical impulses and sent to the brain for translation and interpretation.

Integrative Functions

The sensory signals all make their way to the brain and spinal cord. Here they are evaluated, and then either organized or discarded. Some might become physical sensations like pain, touch or hunger; some produce thoughts, and some are committed to memory. For example, if you sense that a car is going too fast behind you, this sensory information allows you to think ''maybe I should slow down and let them pass''. Or, say you view a really great sunset, integrative functions help you turn that visual stimuli into a memory. This continual process of evaluating and organizing the incoming sensory information is referred to as integration.

Motor Functions

Using sensory stimuli and integration information, the nervous system reacts with a motor response. For example, if you need to slow down your car so another car can pass, the brain or spinal cord sends signals to muscles telling them to contract. The brain may also tell glands to produce hormones. When viewing a great sunset, the brain might say ''release some serotonin so she really enjoys and remembers this incredible sunset.''.

How much control you have over these actions depends on where these reactions originate (the brain or the spinal cord) and where they end up (such as the muscles in your arm or the muscles in your stomach). The operation of your organs and the release of hormones are never under your conscious control. But the muscles in your arms and legs sometimes are and sometimes aren't. When your brain suggests that you slow down and let the speeding car pass, your foot muscles are under voluntary, conscious control.

Sometimes, however, the situation is urgent, like when you are touching a hot skillet. In that case, there is not enough time for the impulse to travel to the brain and back. Instead, the spinal cord intervenes, quickly sending out a signal that moves the muscles in your hand before you are consciously aware of it. This is known as a reflex.

In a reflex reaction, the electrical impulses from the stimulus (in this case a pin in the paw of an animal) never reach the brain. The integration and the motor response occur in the spinal cord.
Diagram of a reflex arc.

The Central and Peripheral Nervous Systems

Don't let the names fool you; you really only have one, highly-integrated nervous system. Its parts, however, are divided into categories based on their functions.

The central and peripheral nervous systems.
Diagram of the central and peripheral nervous systems.

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