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31.1 The Neuron
Functions of the Nervous System The nervous system collects information about the body’s internal
and external environment, processes that information, and responds to it.
The peripheral nervous system consists of nerves and supporting cells. It collects information about
the body’s internal and external environment.
The central nervous system consists of the brain and spinal cord. It processes information and creates a
response that is delivered through the peripheral nervous system.
Neurons Nervous system impulses are transmitted by cells called neuro
neurons.
ns. The three types of neurons are
sensory, motor, and interneurons. All neurons have certain features:
The cell body contains the nucleus and much of the cytoplasm.
Dendrites receive impulses from other neurons and carry impulses to the cell body.
The axon is the long fiber that carries impulses away from the cell body. In some neurons, the axon is
surrounded by an insulating membrane called the myelin sheath.
The Nerve Impulse Nerve impulses are similar to the flow of an electric current through a wire.
wir
Neurons have a charge, or electric potential, across their membranes. When resting, the inside of a
neuron has a negative charge compared to the outside. This difference is called the resting potential.
When a neuron is stimulated, the inside of its m
membrane
embrane temporarily becomes more positive than the
outside. This reversal of charges is called an action potential,, or nerve impulse. The nerve impulse
moves along the axon.
Not all stimuli are capable of starting an impulse. The minimum level of a stimul
stimulus
us that is required to
start an impulse in a neuron is called its threshold.
At the end of the axon, impulses can be transmitted to the next neuron. The point at which a neuron
transfers an impulse to another cell is called a synapse. When an impulse arrives
ves at the synapse,
neurotransmitters,, chemicals that transmit an impulse across a synapse to another cell, are released
from the axon.
31.2 The Central Nervous System
The Brain and Spinal Cord The central nervous system consists of the brain and spinal cord. Some
kinds of information, including some reflexes, are processed directly in the spinal cord. A reflex is a quick,
automatic response to a stimulus.
The largest region of the human brain is the cerebrum, which controls learning,
g, judgment, and
voluntary actions of muscles.
• The cerebrum is divided into right and left hemispheres. Each deals primarily with the opposite side
of the body.
• The outer layer of the cerebrum is the cerebral cortex. It processes information from the sense
organs and controls body movements.
The limbic system controls functions such as emotion, behavior, and memory.
The thalamus receives messages from sensory receptors throughout the body and sends the information
to the proper region of the cerebrum for processing.
The hypothalamus controls the recognition and analysis of hunger, thirst, fatigue, anger, and body
temperature. It helps coordinate the nervous and endocrine systems.
The cerebellum is the second largest region of the brain. It receives infor
information
mation about muscle and joint
position and coordinates the actions of these muscles.
The brain stem connects the brain and spinal cord. It regulates the flow of information between the
brain and the rest of the body.
Addiction and the Brain Almost all ad
addictive substances affect brain synapses.
Many drugs cause an increase in the release of the neurotransmitter dopamine. The brain reacts to high
dopamine levels by reducing the number of receptors.
With fewer dopamine receptors available, larger amounts of drugs are required to produce a high. This
can result in an addiction.
31.3 The Peripheral Nervous System
The Sensory Division The peripheral nervous system consists of all the nerves and associated cells
that are not part of the brain or spinal cord. It is made up of the sensory division and the motor
division.
The sensory division transmits impulses from sense organs to the central nervous system.
Sensory receptors are cells that transmit information about changes in the internal and external
environment. Chemoreceptors respond to chemicals. Photoreceptors respond to light. Mechanoreceptors
respond to touch,
uch, pressure, vibrations, and stretch. Thermoreceptors respond to temperature change.
Pain receptors respond to tissue injury.
The Motor Division The motor division, which is divided into the somatic and autonomic nervous
systems, transmits impulses from the central nervous system to muscles and glands.
The somatic nervous system regulates processes under voluntary control.
Actions of the somatic nervous system called reflexes occur automatically. The impulses controlling
these actions travel on a pathway
ay called a reflex arc. An impulse travels through a sensory neuron, to
the spinal cord, and then back through a motor neuron.
The autonomic nervous system regulates activities that are involuntary. It consists of two parts, the
sympathetic nervous system and the parasympathetic nervous system.
• In general, the sympathetic nervous system prepares the body for intense activity. It prepares the body
to “fight or flee” in response to stress.
• The parasympathetic nervous system causes the “rest and digest” respo
response.
31.4 The Senses
Lesson Objectives
Discuss the sense of touch and identify the various types of sensory receptors in the skin.
Explain the relationship between smell and taste.
Identify the parts of the ears that make hearing and balance possible.
Describe the major parts of the eye and explain how the eye enables us to see.
Touch and Related Senses Different sensory receptors in the body respond tto
o touch, temperature, and
pain.
Skin contains at least seven types
pes of sensory rreceptors that respond to touch.
Thermoreceptors respond to heat and cold. They are foun
found
d in the skin and hypothalamus.
Pain receptors are found throughout the body. The
They respond to physical injuries.
Smell and Taste Sensations of smell aand
nd taste are the result of impulses sent to the brain by
chemoreceptors. Sense organs that detect taste are called taste buds.. Sensory cells in taste buds respond to
salty, bitter, sweet, sour, and savory foods.
Hearing and Balance Mechanoreceptors found in parts of the ear transmit impulses to the brain. The
brain translates the impulses into sound and information about balanc
balance.
Vibrations cause pressure waves in the fluid
fluid-filled cochlea of the inner ear. Tiny hair cells in the
cochlea are pushed back andd forth by the pressure waves. The hair cells send nerve impulses to the
brain, which interprets them as sound.
The ears also help maintain balance. The semicircular canals and two fluid-filled
filled sacs behind them
monitor the position of the
he body in relation to gravity.
Vision Vision occurs when photoreceptors in the eyes transmit impulses to the brain, which translates these
impulses into images.
The cornea is a tough, transparent layer of cells. The cornea helps focus the light, which passes through
a chamber filled with
ith fluid called aqueous humor.
The iris is the colored portion of the eye. In the middle of the iris is a small opening called the pupil,
through
ugh which light enters the eye.
The lens is located behind the iris. The shape of the lens is chan
changed
ged by tiny muscles to adjust the eye’s
focus.
The lens focuses light on the retina
retina,, the inner layer of the eye. Photoreceptors in the retina convert light
energy into nerve impulses that are carried to the brain via the optic nerve. There are
a two kinds of
photoreceptors:
• Rods are very sensitive to light but do not distinguish colors.
• Cones are less sensitive to light,
ght, but do distinguish colors.