Human Biology Book Ch. 4.1 Senses connect the human body to its environment.
To maintain homeostasis and to survive, your body must constantly monitor the environment in which you live. This
involves organs that interact so closely with the nervous system that they are often considered extensions of the
nervous system. These are your sense organs. They give you the ability to see, smell, touch, hear, and taste.
Each of the senses can detect a specific type of change in the environment. For example, if you have begun to cross
the street but suddenly hear a horn blowing, you may stop and step back onto the curb. Your sense of hearing
allowed your brain to perceive that a car was coming and thus helped you to protect yourself.
The sound of the horn is a ​stimulus.​ A stimulus is a change in your environment that you react to, such as a smell,
taste, sound, feeling, or sight. Your brain interprets any such change. If it did not, the information perceived by the
senses would be meaningless.
Sight
If you have ever tried to find your way in the dark, you know how important light is for seeing. Light is a stimulus.
You are able to detect it because your eyes, the sense organs of sight, capture light and help turn it into an image,
which is processed by the brain.
Light enters the eye through the lens, a structure made of transparent tissue. Muscles surrounding the lens change
its shape so the lens focuses light. Other muscles control the amount of light that enters the eye by altering the size
of the pupil, a dark circle in the center of the eye. To reduce the amount of light, the area around the pupil, called the
iris, contracts, making the pupil smaller, thus allowing less light to enter. When the iris relaxes, more light can enter
the eye.
At the back of the eye, the light strikes a layer called the retina. Among the many cells of the retina are two types
of receptors, called rods and cones. Rods detect changes in brightness, while cones are sensitive to color.
Hearing
Your eyes perceive light waves, but your ears perceive and interpret a different type of stimulus, sound waves. Sound
waves are produced by vibrations. A reed on a clarinet vibrates, and so do your vocal cords. So does a bell after it has
been hit by a mallet. The motion causes changes in the air that surrounds the bell. These changes can often be
processed by the ear as sound, although many vibrations are too low or high to be heard by humans.
Sound waves enter the ear and are funneled into the auditory canal, a tube-shaped structure that ends at the
eardrum. The eardrum vibrates when the sound waves strike it, and it transmits some of the vibrations to a tiny bone
called the stirrup. Pressure caused by vibrations from the stirrup causes fluid in the ear to move. The movement of
the fluid sends signals to the brain that are interpreted as sound.
Touch
The sense of touch depends on tiny sensory receptors in the skin. Without these you wouldn't be able to feel pressure,
temperature, or pain. Nerves in the outer layer of your skin, or epidermis, sense textures, like smooth glass or rough
concrete. Nerves deeper in the skin, in the dermis, sense pressure. Receptors also sense how hot or cold an object is
and can thus help protect you from burning yourself. The sense of touch is important in alerting your brain to danger.
Though you might wish that you couldn't feel pain, it serves a critical purpose. Without it, you could harm your body
without realizing it.
Smell
Whereas sight, touch, and hearing involve processing physical information from the environment, the senses of smell
and taste involve processing chemical information. Much as taste receptors sense chemicals in food, smelling
receptors sense chemicals in the air. High in the back of your nose, a patch of tissue grows hairlike fibers covered in
mucus. Scent molecules enter your nose, stick to the mucus, and then bind to receptors in the hairlike fibers. The
receptors send an impulse to your brain, and you smell the scent.
Taste
Your tongue is covered with small sensory structures called taste buds, which are also found in the throat and on the
roof of the mouth. Each taste bud includes about 100 sensory cells that are specialized to detect sweet, sour, bittter,
and salty. The thousands of tastes you experience are due to sense organs in your nose. When you have a cold, your
ability to taste decreases.
The central nervous system controls functions.
The ​central nervous system ​consists of the brain and spinal cord. The brain is located in and protected by the skull,
and the spinal cord is located in and protected by the spine. The central nervous system communicates with the rest
of the nervous system through electrical signals sent to and from neurons. Impulses travel very quickly, some as fast
as 90 meters (295 ft) per second. That's like running almost the entire length of a soccer field in one second!
Brain
The average adult brain contains nearly 100 billion nerve cells, called ​neurons.​ The brain directly controls voluntary
behavior, such as walking and thinking. It also allows the body to control most involuntary responses such as
heartbeat, blood pressure, fluid balance, and posture.
As you can see in the diagram, every area of the brain has a specific function, although many functions may involve
more than one area. For example, certain areas in the brain control the senses, while other areas help you stand up
straight. The lower part of the brain, called the brain stem, controls activities such as breathing and vomiting.
Spinal Cord
The spinal cord is about 44 centimeters (17 in.) long and weighs about 35–40 grams (1.25–1.4 oz). It is the main
pathway for information, connecting the brain and the nerves throughout your body. The spinal cord is protected and
supported by the vertebral column, which is made up of small bones called vertebrae. The spinal cord itself is a
double-layered tube with an outer layer of nerve fibers wrapped in tissue, an inner layer of nerve cell bodies, and a
central canal that runs the entire length of the cord. Connected to the spinal cord are 31 pairs of nerves, which send
sensory impulses into the spinal cord, which in turn sends them to the brain. In a similar way, spinal nerves send
impulses to muscles and glands.
The peripheral nervous system is a network of nerves.
Nerves, which are found throughout the body, are often referred to all together as the ​peripheral nervous system ​.
Both sensory and motor nerves are parts of the peripheral nervous system. Sensory nerves receive information from
the environment–such as heat or cold–and pass the information to the central nervous system. Motor nerves send
signals to your muscles that allow you to move. The peripheral nervous system includes both voluntary motor nerves
and involuntary responses.
Another type of motor nerves controls the involuntary responses of the body. In times of danger, there is no time to
think. The body must respond immediately. In less stressful situations, the body maintains activities like breathing
and digesting food. These functions go on without conscious thought. They are controlled by part of the peripheral
nervous system called the autonomic (​AW​-tuh-NAHM-ihk) nervous system.
The ​autonomic nervous system ​controls the movement of the heart, the smooth muscles in the stomach, the
intestines, and the glands. The autonomic nervous system has two distinct functions: to conserve and store energy
and to respond quickly to changes. You can think of the autonomic nervous system as having a division that performs
each of these two main functions.
Each division is controlled by different locations on the spinal cord, or within the brain and the brain stem. The
cerebellum, which is located at the rear of the brain, coordinates balance and related muscle activity. The brain stem,
which lies between the spinal cord and the rest of the brain, controls heartbeat, respiration, and the smooth muscles
in the blood vessels.
When you are under stress, one part of the autonomic nervous system causes what is called the “fight or flight
response.” Rapid changes in your body prepare you either to fight the danger or to take flight and run away from the
danger. The response of your nervous system is the same, whether the stress is a real danger, like falling off a
skateboard, or a perceived danger, like being worried or embarrassed.
The ​voluntary nervous system ​monitors movement and functions that can be controlled consciously. Every
movement you think about is voluntary. The voluntary nervous system controls the skeletal muscles of the arms, the
legs, and the rest of the body. It also controls the muscles that are responsible for speech and the senses.