The nervous system receives information about what is happening both inside and outside your body. It also directs the way in which your body responds to this information. In addition, the nervous system helps maintain homeostasis.
A stimulus is any change or signal in the environment that can make an organism react. After your nervous system analyzes a stimulus, it causes a response. A response is what your body does in reaction to a stimulus.
The cells that carry information through your nervous system are called neurons, or nerve cells. The message that a neuron carries is called a nerve impulse.
A neuron has a large cell body that contains the nucleus, threadlike extensions called dendrites, and an axon. The dendrites carry impulses toward the cell body. The axon carries impulses away from the cell body. Axons and dendrites are sometimes called nerve ﬁbers. A bundle of nerve ﬁbers is called a nerve.
Different kinds of neurons perform different functions. Three kinds of neurons are found in the body-sensory neurons, interneurons, and motor neurons. Together they make up the chain of nerve cells that carry an impulse through the nervous system.
A sensory neuron picks up stimuli from the internal or external environment and converts each stimulus into a nerve impulse. An interneuron is a neuron that carries nerve impulses from one neuron to another. A motor neuron sends an impulse to a muscle or gland, and the muscle or gland then reacts in response.
Nerve impulses begin in a dendrite, move toward the cell body, and then move down the axon. A nerve impulse travels along the neuron in the form of electrical and chemical signals.
The axon tip ends at a synapse. A synapse is the junction between each axon tip and the next structure. A small gap separates these two structures. For a nerve impulse to be carried along at a synapse, it must cross the gap between the axon and the next structure. The axon tips release chemicals that carry the impulse across the gap.
Your nervous system has two divisions that work together. The central nervous system consists of the brain and spinal cord. The peripheral nervous system consists of all the nerves located outside of the central nervous system.
The central nervous system is the control center of the body and includes the brain and the spinal cord. The brain, located in the skull, is the part of the central nervous system that controls most functions in the body. The spinal cord is the thick column of nerve tissue that links the brain to most of the nerves in the peripheral nervous system.
There are three main regions of the brain that receive and process information. These are the cerebrum, the cerebellum, and the brain stem. The cerebrum interprets input from the senses, controls movement, and carries out complex mental processes such as learning and remembering. It is the largest part of the brain. The cerebrum is divided into a left half and a right half. The left half controls the right side of the body, and the right half controls the left side of the body.
The cerebellum coordinates the actions of your muscles and helps you keep your balance. The brain stem controls your body's involuntary activities such as breathing and heartbeat.
The peripheral nervous system consists of a network of nerves that branch out from the central nervous system and connect it to the rest of the body. The peripheral nervous system is involved in various actions, both voluntary and involuntary. A total of 43 pairs of nerves make up the peripheral nervous system.
The nerves of the peripheral nervous system can be divided into the somatic and autonomic nervous systems. The nerves of the somatic nervous system control voluntary actions-activities you can choose to do or not do. Nerves of the autonomic nervous system control involuntary actions such as adjusting the diameter of blood vessels.
A reﬂex is an automatic response that occurs very rapidly and without conscious control. Reﬂexes help to protect the body. In some reﬂex actions, skeletal muscles contract with the involvement of the spinal cord only-not the brain.
Concussions and spinal cord injuries are two ways in which the nervous system can be damaged. A concussion is a bruiselike injury of the brain. A concussion happens when soft tissue of the brain collides against the skull. Spinal cord injuries happen when the spinal cord is cut or crushed. Spinal cord injuries can result in paralysis, which is the loss of movement in some part of the body
Your eyes respond to the stimulus of light. They convert that stimulus into impulses that your brain interprets, enabling you to see. When rays of light ﬁrst strike the eye, they pass through the cornea, the clear tissue that covers the front of the eye. Behind the cornea is the pupil, the opening through which light enters the eye. Around the pupil is the iris, a circular structure that regulates the amount of light entering the eye. Light that passes through the pupil strikes the lens. The lens is a ﬂexible structure that focuses light. Light ﬁnally strikes the retina, the layer of receptor cells that lines the back of the eye. Light hitting the retina causes nerve impulses to begin. The optic nerve carries nerve impulses from the retina to the cerebrum.
The lenses in eyeglasses can help correct vision problems that result when the lens of the eye does not focus light properly. People with nearsightedness can see nearby objects clearly. They have trouble seeing distant objects. People with farsightedness can see distant objects clearly. They have trouble seeing nearby objects.
Your ears are the sense organs that respond to the stimulus of sound. The ears convert the sound to nerve impulses that your brain interprets. Sound is produced by material that is vibrating. The vibrations create waves. The ear is structured to receive sound waves.
The ear consists of three sections-the outer ear, middle ear, and inner ear. The outer ear helps gather sound waves. Sound waves move through the ear canal to the eardrum, which separates the outer ear from the middle ear. The eardrum is a membrane that vibrates when sound waves strike it. Three tiny bones in the middle ear-the hammer, anvil, and stirrup-transmit the vibrations from the eardrum to the cochlea in the inner ear. The cochlea is a snail-shaped tube lined with receptors that respond to sound. These receptors create nerve impulses that are carried through the auditory nerve to the brain. Your inner ear contains structures that help control your sense of balance. Above the cochlea are the semicircular canals, which are the structures that are responsible for your sense of balance.
The senses of smell and taste work closely together. Both depend on chemicals in food and in the air. The chemicals trigger responses in receptors in the nose and mouth.
The sense of touch is found in the skin. Your skin contains different kinds of touch receptors that respond to different kinds of stimuli. Some of these receptors respond to light touch, heavy pressure, pain, and temperature change.
To a scientist, a drug is any chemical taken into the body that causes changes in a person's body or behavior. Drug abuse is the deliberate misuse of drugs for purposes other than medical ones. Both legal and illegal drugs can be abused. Most commonly abused drugs, such as marijuana, alcohol, and cocaine, are especially dangerous because they act on the brain and other parts of the nervous system. In addition, long-term drug abuse can lead to addiction and other health and social problems. Most abused drugs can alter, or change, a person's mood and feelings. Because of this effect, these drugs are often called mood-altering drugs.
If a person takes a drug regularly, the body may develop a tolerance to the drug. Tolerance is a state in which a drug user needs larger and larger amounts of the drug to get the same effect. Tolerance can cause people to take an overdose and become unconscious or even die.
Repeated use of many commonly abused drugs can result in addiction. In addiction, the body becomes physically dependent on the drug. When the drug addict misses a few doses of the drug, the body reacts to the lack of the drug. The person is experiencing withdrawal, a period of adjustment that occurs when a person stops taking a drug. People can also become emotionally dependent on the feelings and moods produced by a drug.
The use of drugs can also affect a person's health indirectly. HIV, the pathogen that causes AIDS, can be transmitted when drug users share needles. In addition, a person caught using or selling an illegal drug may have to pay a ﬁne or go to jail.
Commonly abused drugs include depressants, stimulants, inhalants, hallucinogens, anabolic steroids, and alcohol. Many drugs affect the central nervous system, while others affect the overall chemical balance of the body. Depressants are drugs that slow down the activity of the central nervous system. Stimulants speed up body processes. Inhalants are inhaled, or breathed in. They can produce mood-altering effects. Hallucinogens can make people see or hear things that do not really exist. Some athletes try to improve their performance by taking anabolic steroids, synthetic chemicals that are similar to hormones produced in the body. They can cause mood changes and permanent physical damage.
Alcohol is a powerful depressant. It is quickly absorbed by the digestive system. The amount of alcohol in the blood is usually expressed as blood alcohol concentration, or BAC. Heavy drinking, especially over a long period, can result in signiﬁcant health problems. The abuse of alcohol can cause the destruction of cells in the brain and liver, and it can also lead to addiction and emotional dependence. Abuse of alcohol can result in alcoholism, which is a disease in which a person is both physically addicted to and emotionally dependent on alcohol.
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