Nervous System

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Nervous System

The nervous system is the communication network for the body. It is the most highly organized system. The main function of the nervous system is to coordinate all of the body’s activities. When changes occur inside or outside the body, this system allows it to recognize them and respond as needed.


The basic element of the nervous system is the neuron, or nerve cell. Its job is to transmit a message from one cell to the next. To do this, the nerve cell contains special fibers that extend from the cytoplasm of the cell body. •Dendrites conduct impulses toward the cell. A nerve cell may have several dendrites. •Axons conduct impulses away from the cell. Each nerve cell has only one axon. Many axons are covered by a fatty tissue called the myelin sheath. This sheath protects the axons and speeds up the impulse as it travels. The axon of one neuron lies close to many dendrites of other neurons. The space between a dendrite and an axon is known as a synapse. Special chemicals called neurotransmitters help impulses "jump" the synapse to pass the message from cell to cell. Neurons form bundles called nerves, and the impulses can follow many different routes throughout the body.

The basic element of the nervous system is the nerve cell, also known as the _______.


Which of these fibers conduct impulses away from a nerve cell?

A single axon

Which of these fibers conduct impulses toward a nerve cell?

Many dendrites

What is the fatty tissue that covers axons and speeds up impulses as they travel from cell to cell?

Myelin sheath

How does an impulse travel from one neuron to another?

Chemical neurotransmitters help the impulse "jump" across the space between the cells.

Types of Neurons

The three types of neurons are afferent, efferent, and associative neurons. •Afferent neurons are also called sensory neurons. They carry messages from all parts of the body to the brain and spinal cord. •Efferent neurons are also called motor neurons. They carry messages from the brain and spinal cord to the muscles and glands. •Associative neurons are also called interneurons. They carry messages from afferent neurons to efferent neurons.

What type of neuron is also called motor because it carries messages from the brain and spinal cord to the muscles and glands to put them into action?


What type of neuron is also called an interneuron because it carries messages between the other two types of neurons?


Divisions of the Nervous System

The two main divisions of the nervous system are the central nervous system and the peripheral nervous system. •The central nervous system consists of the brain and spinal cord. •The peripheral nervous system consists of nerves that reach all parts of the body. It has a special division called the autonomic nervous system, which controls the involuntary, or automatic, activities of the vital organs.

Which division of the nervous system consists of the brain and spinal cord?

Central nervous system

Central Nervous System: Brain

The brain interprets, organizes, and stores information. It also controls and directs body functions. The main sections of the brain include the cerebrum, diencephalon, cerebellum, and brain stem. •The cerebrum is the largest and highest section of the brain. It is separated into four lobes: frontal, parietal, temporal, and occipital. It is concerned with reasoning, the senses, speech, and voluntary body movement. •The diencephalon includes two parts. ◦The thalamus directs sensory impulses to the cerebrum. ◦The hypothalamus controls the autonomic nervous system, body temperature, appetite, water balance, sleep, and blood vessel constriction and dilation. The hypothalamus also plays a role in emotions such as anger, fear, pleasure, pain, and affection. •The cerebellum is responsible for coordination of muscle movements, balance and posture, and muscle tone. The last main section of the brain is the brain stem. •The brain stem includes three parts. ◦The midbrain conducts impulses between the brain parts and for certain eye and auditory reflexes. ◦The pons directs messages to other parts of the brain and for chewing, tasting, and saliva production. It also helps with respiration. ◦The medulla oblongata connects with the spinal cord and regulates heartbeat, respiration, swallowing, coughing, and blood pressure.

Central Nervous System: Spinal Cord

The spinal cord starts at the base of the brain stem and extends to the area around the first lumbar vertebrae in the lower back. It is encased in the vertebral column. The spinal cord controls many reflex actions, and it acts as a pathway for messages to and from the brain and the nerves that go to the muscles and glands.

Peripheral Nervous System

The peripheral nervous system includes 12 pairs of cranial nerves and their branches and 31 pairs of spinal nerves and their branches. •Some of the cranial nerves process input from special senses, such as sight, hearing, taste, and smell. Others receive general sensations, such as touch, pressure, pain, and temperature, and send out impulses to control muscles. •The spinal nerves carry messages to and from the spinal cord. Each nerve goes directly to a certain part of the body or forms a network with other spinal nerves to reach a larger segment of the body.

Autonomic Nervous System

The autonomic nervous system is a special part of the peripheral nervous system. It controls the involuntary, or automatic, activities of the body. The two divisions of the autonomic nervous system are the sympathetic and parasympathetic systems. These two systems usually work together to maintain a balanced state for the body. In times of emergency, the sympathetic system prepares the body for action with the "fight or flight" response. It increases heart rate, respiration, and blood pressure and slows the rate of digestion. After the stress is over, the parasympathetic system reverses these actions to counteract the effects and return the body to normal working conditions.

Myasthenia Gravis

Myasthenia gravis is a disease where the proper nerve pulses are not sent to the muscles. As a result, progressive muscle weakness and paralysis occur. If the respiratory system muscles are affected, the disease can be fatal. The exact cause is not known, but it is thought that it may be related to an autoimmune process. There is no cure. Treatment involves medication and lifestyle changes to cope with the disease.

Cerebrovascular Accident

A cerebrovascular accident (CVA) is also known as a stroke, or "brain attack." It occurs when there is a loss of blood flow, and in turn oxygen, to the brain. It may be caused by a blood vessel bursting or being blocked by a blood clot. Symptoms vary depending on the area and the amount of brain tissue damaged. Common symptoms include weakness or paralysis on one side of the body, difficulty swallowing, visual or speech impairment, mental confusion, and loss of consciousness. Stroke is a leading cause of death in the United States. Controlling risk factors, such as smoking, heart disease, and diabetes, helps to prevent them. Treatment within the first three hours of a stroke, such as using drugs to break up a clot and restore blood flow, can help to prevent brain damage. Treatment for any damage involves therapy to help people recover from or adapt to the losses of mental or physical function

Multiple Sclerosis

Multiple sclerosis (MS) is a disease of the central nervous system that results in progressive loss of muscle control. The myelin sheath is slowly destroyed. The exact cause of this disease is not known, but it is thought that it may be related to a virus or an autoimmune process. Early symptoms include double vision, tingling and numbness, weakness, and fatigue. As the disease worsens, symptoms include tremors, speech impairment, and paralysis. There is no cure. Treatment involves medication and physical therapy to help control the symptoms and maintain functional ability as long as possible.

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