A. HUMAN NERVOUS SYSTEM. The human nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord. The peripheral nervous system is made up of the nerves that lie outside the central nervous system. The peripheral nervous system carries impulses to and from the central nervous system.
1. The human nervous system is divided into the central nervous system and the peripheral nervous system.
2. The brain and the spinal cord make up the central nervous system.
3. The nerves that lie outside the central nervous system make up the peripheral nervous system.
B. NERVE CELLS. The basic unit of the nervous system is the nerve cell. Nerve cells are also called neurons. Nerve cells receive impulses and send them to various body parts. They make up the structure of the nerves, brain, spinal cord, and receptors. A nerve cell is made up of a cell body, dendrites, and an axon. The cell body, or cyton, contains the nucleus and cytoplasm. Cell bodies often are present in groups called ganglia. The ganglia coordinate incoming and outgoing impulses. Dendrites and axons extend from the cell body. Dendrites are branched parts of a neuron that receive impulses from other neurons or sense organs. The impulses are then sent to the cell body. The axon is a single, long fiber that carries impulses away from the cell body. A neuron has many dendrites but only one axon. Between the dendrites of one neuron and the axon of another is a space called a synapse. Impulses cross the synapse by means of chemical neurotransmitters.
cyton: The cell body, or cyton, contains the nucleus and cytoplasm.
axon: The axon is a single, long fiber that carries impulses away from the cell body.
dendrite: Dendrites are branched parts of a neuron that receive impulses from other neurons or sense organs.
synapse: A space called a synapse is located between the dendrites of one neuron
and the axon of another. Impulses cross the synapse by means of chemical
ganglia: Cell bodies often are present in groups called ganglia. The ganglia coordinate incoming and outgoing impulses.
C. NERVES. A nerve is a bundle of neurons or parts of neurons. They are held together by a tough protective membrane. Nerves are classified as sensory, motor, or mixed. Sensory neurons carry impulses to the central nervous system. Motor neurons carry impulses from the central nervous system to various body parts such as muscles or glands. Mixed nerves contain both sensory and motor fibers. A nerve is like a telephone cable line carrying messages to and from your home.
D. THE BRAIN. The brain is the major control center of the body. It is located in the head in an area called the cranial cavity. The bones of the skull surround and protect the brain. The cerebrum, cerebellum, and medulla are the three parts of the brain. The cerebrum is the center of thinking, memory, emotions, sensory impulse interpretation, and all voluntary activity. It is the largest part of the brain. You are using your cerebrum when you take a test. The cerebellum is responsible for coordination of muscles and is the center of balance. When you bounce a basketball, you are using your cerebellum. Alcohol reduces the ability of the cerebellum to coordinate muscle activity. This results in slurred speech and unbalanced walking. The medulla or brain stem is the center of respiration, heartbeat, and other involuntary activities. Your medulla is controlling all the activities of your body that you cannot control such as breathing and peristalsis.
E. THE SPINAL CORD. The spinal cord is the tubular mass of nerve cells that extends from the brain stem downward through the center of the spinal column. It is surrounded and protected by the vertebrae of the spinal column (backbone). The spinal cord relays impulses to and from the brain and is the center of some reflex actions.
Injury to the spinal cord can result in paralysis and even death. Paralysis is caused by severe damage to the spinal cord. It results in loss of feeling and muscle function in the lower parts of the body. Paralysis occurs because impulses to and from the lower body muscles cannot be transmitted to the brain.
F. RECEPTORS. Receptors are special structures that are sensitive to stimuli. Receptors receive messages. External receptors, called sense organs, include the eyes, ears, nose, tongue, and skin. Stimuli for these receptors are light, sound, chemicals, and temperature and pressure changes. Internal receptors are located in the internal organs. They allow the brain to detect hunger, thirst, muscle position, and carbon dioxide levels in the blood.
G. INVOLUNTARY BEHAVIOR. The total response of an organism to stimuli is known as behavior. The responses may be inborn or acquired (learned), voluntary or involuntary. Involuntary behavior occurs automatically without conscious control. Some involuntary actions are inborn while others are learned. Examples of involuntary actions that are inborn include: contraction of heart and diaphragm muscles, secretion of glands, and simple reflexes such as the knee jerk and iris movement. Examples of involuntary actions that are acquired by
H. THE REFLEX ARC. The reflex is an example of involuntary behavior. It is an inborn, involuntary response to a particular stimulus. In a reflex response, nerve impulses travel in a set pathway called a reflex arc (Figure 12-4). The path an impulse follows in a reflex arc is as follows:
Receptor--->.Sensory Neuron--->lnterneuron--->Motor Neuron--->Effector
For example, the receptors in a knee jerk reflex are the nerve endings in the knee. The receptors receive the message. The sensory neuron passes along the message to the spinal cord. The interneuron connects the sensory neuron with the motor neuron. The motor neuron sends the message to the muscle in the leg called the effector. An effector does something. In this case it jerks the knee. A reflex arc is a short cut for nerve impulses. The message does not have to take the long route to the brain where thinking occurs. A reflex action does not require thought; it is an automatic action. Blinking is another example of a reflex.
I. VOLUNTARY BEHAVIOR. Voluntary behavior requires thinking. The impulses start in the brain and are carried by nerve cells to muscles or glands. The muscles or glands respond in a certain way. Voluntary actions include measuring with a meter stick, picking up a book, and building a model airplane.
J. NERVOUS SYSTEM DISORDERS. Some disorders of the nervous systems are cerebral palsy, meningitis, and polio.
Nervous System Disorders
• Meningitis. Meningitis is an inflammation of the membranes that surround the brain and spinal cord. The disease is caused by bacteria. The symptoms include severe headache and stiffness ol the neck. Meningitis can be fatal. Antibiotics are used to treat meningitis.
• Cerebral Palsy. Cerebral palsy is a group of birth disorders characterized by disturbances of the motor functions. It is a crippling disorder and treatment is still experimental.
• Polio. Polio is a viral disease of the central nervous system that can cause paralysis. It can be prevented by immunization
K. EFFECTS OF DRUGS ON THE NERVOUS SYSTEM. Drugs can be harmful or helpful, depending on how they are used. The incorrect use of a drug is called drug abuse. A drug is defined as a medicine used to treat or cure disease or pain in the body, such as, aspirin and penicillin. A drug can also be a substance that has a strong effect on the mind or body and that can become habit-forming, such as, heroine and cocaine.
Drugs affect the central nervous system (CNS) in many different ways. Some drugs create a feeling of euphoria (well-being). Other drugs cause a decrease in inhibitions, or an inability to stop doing something that a person would not normally do. Using some drugs results in tolerance when they are taken continually for a few weeks. This means that increasingly larger amounts are needed to get the desired effect. There are some drugs that cause stimulation (excitement) and feelings of high energy. Physical dependence, or addiction, can result from using drugs. When the drug is withheld, the addict suffers physical illness and withdrawal symptoms. The effects of some common drugs on the nervous system are described