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Chapter 35
# Title
Nervous System
Summary
35–1 Human Body Systems
Cells of multicellular organisms are specialized for certain
functions. The levels of organization in a multicellular organism
include cells, tissues, organs, and organ systems.
• A cell is the basic unit of structure and function in living things.
Specialized cells are suited to perform a particular function.
• Groups of similar cells work together to form tissues.
Epithelial tissue covers body surfaces. Connective tissue
supports the body and connects its parts. Nervous tissue
carries messages throughout the body. Muscle tissue
allows movement.
• Groups of tissues that work together to perform complex
functions are called organs.
• Organs form organ systems.
Organ systems work together to keep conditions in the body
stable. This process is called homeostasis. Homeostasis is the
process by which organisms keep internal conditions relatively
constant, despite changes in external environments. Homeostasis
may involve feedback inhibition, or negative feedback. For example, the nervous system senses when the body cools and signals
the cells to give off more heat.
35–2 The Nervous System
The nervous system controls and coordinates body functions.
It responds to internal and external stimuli. Messages move
through the nervous system as electrical signals. The signals are
called impulses.
Neurons are nerve cells. Each neuron has a cell body, dendrites,
and an axon. The cell body is the largest part of a typical neuron.
It holds the nucleus. Short branches, called dendrites, carry
impulses toward the cell body. A long fiber, called the axon,
carries impulses away from the cell body. A myelin sheath
covers and insulates parts of some axons. A synapse is at the
end of an axon. One neuron transfers an impulse to another at
the synapse. Chemicals called neurotransmitters send impulses
across the synapse.
There are three types of neurons: sensory neurons, motor
neurons, and interneurons. Sensory neurons carry impulses to
the spinal cord. Motor neurons carry impulses from the spinal
cord. Interneurons link sensory and motor neurons.
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A resting neuron is one that is not carrying an impulse. When
a neuron is resting, the inside of the cell has a net negative charge.
The outside of the cell has a net positive charge. This charge difference across the cell membrane is called the resting potential of the
neuron. When a resting neuron is stimulated by another neuron
or by the environment, an impulse starts. Positive ions flow into
the neuron making the inside of the cell temporarily more positive
than the outside of the cell. This reversal of charges across the
membrane is the impulse, or action potential. Once started, the
action potential travels quickly down the neuron’s axon. At the
end of the axon, impulses can be transmitted to the next cell.
35–3 Divisions of the Nervous System
The nervous system has two major divisions: the central nervous
system and the peripheral nervous system.
1) The central nervous system is made up of the brain and spinal
cord. It is the control center of the body. It sends messages,
processes information, and analyzes information. The brain
and spinal cord are protected by bone and three layers of
connective tissue. Fluid between the layers cushions and
protects nerve tissue.
The brain has several regions. The cerebrum controls voluntary
actions. The cerebellum coordinates involuntary actions. The
brain stem controls basic body functions. The thalamus receives
impulses from the senses and sends them to the cerebrum.The
hypothalamus connects the nervous and endocrine systems.
The spinal cord connects the brain with the rest of the body.
Some reflexes are processed in the spinal cord. A reflex is a quick,
automatic response to a stimulus. Sneezing is an example. The
path of an impulse during a reflex is called the reflex arc.
2) The peripheral nervous system has two divisions.
• The sensory division sends impulses from sensory neurons
to the central nervous system.
• The motor division sends impulses from the central
nervous system to muscles and glands. The motor
division is further divided into somatic and autonomic
nervous systems.
The somatic nervous system controls voluntary actions. The
autonomic nervous system regulates activities that are automatic.
These nerves control functions that are not under conscious control.
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35–4 The Senses
Sensory receptors are neurons that react to stimuli in the environment. These receptors send impulses to the central nervous
system. There are five types of sensory receptors. Pain receptors
respond to pain. Thermoreceptors respond to temperature.
Mechanoreceptors respond to pressure. Chemoreceptors respond
to chemicals. Photoreceptors respond to light.
The sensory organ responsible for vision is the eye. Light enters
the eye through the pupil. The pupil is a small opening at the front
of the eye. Light then moves through the lens, which focuses the
light on the retina. The retina contains photoreceptors called rods
and cones. Rods are sensitive to dim light. Cones are sensitive
to colors.
Ears respond to sound stimuli. Sound vibrations enter the ear
and create pressure waves in the fluid-filled cochlea. Sensory
receptors in the cochlea send impulses to the brain. Ears also sense
balance. Semicircular canals in the ear contain fluid, hair cells, and
tiny grains. Movements make the grains bend the hair cells, which
send an impulse to the brain.
Chemoreceptors in the nose react to chemicals in the air and
send impulses to the brain. Taste buds are sense organs that
detect taste. Most taste buds are located on a person’s tongue.
The skin is the largest sense organ. It has receptors that respond
to temperature, touch, and pain.
35–5 Drugs and the Nervous System
A drug is any substance, other than food, that changes the structure or function of the body. Several types of drugs affect the
nervous system.
• Stimulants increase heart rate, blood pressure, and
breathing rate.
• Depressants decrease heart rate and breathing rate, lower
blood pressure, relax muscles, and relieve tension.
• Opiates act like natural brain chemicals called endorphins.
These brain chemicals normally help overcome pain. Opiates
are highly addictive.
• Marijuana can cause memory and concentration problems.
• Alcohol is a depressant that slows down the rate at which
the central nervous system functions.
Drug abuse can be defined as the intentional misuse of any
drug for nonmedical purposes. Addiction is an uncontrollable
dependence on a drug.
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