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Peripheral Nervous System

Introduces the sensory and motor divisions of the peripheral nervous system.

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Peripheral Nervous System

How does your brain connect to the rest of your body?

You know you have nerves in your fingers and toes because you can feel them. But how does your brain know what's going on in these nerves? You have a network of nerves running from your brain and spinal cord to your fingers and toes and the rest of your body. This network is known as the peripheral nervous system.

The Peripheral Nervous System

There are other nerves in your body that are not found in the brain or spinal cord. The peripheral nervous system (PNS) (Figure below) contains all the nerves in the body that are found outside of the central nervous system. They include nerves of the hands, arms, feet, legs, and trunk. They also include nerves of the scalp, neck, and face. Nerves that send and receive messages to the internal organs are also part of the peripheral nervous system.

The blue lines in this drawing represent nerves of the peripheral nervous system. Every peripheral nerve is connected directly or indirectly to the spinal cord.

The peripheral nervous system is divided into two parts, the sensory division and the motor division. How these divisions of the peripheral nervous system are related to the rest of the nervous system is shown below (Figure below). Refer to the figure as you read more about the peripheral nervous system in the text that follows.

The central nervous system interprets messages from sense organs and internal organs, and the motor division sends messages to internal organs, glands, and muscles.

The Sensory Division

The sensory division carries messages from sense organs and internal organs to the central nervous system. Human beings have several senses. They include sight, hearing, balance, touch, taste, and smell. We have special sense organs for each of these senses. What is the sense organ for sight? For hearing?

Sensory neurons in each sense organ receive stimuli, or messages from the environment that cause a response in the body. For example, sensory neurons in the eyes send messages to the brain about light. Sensory neurons in the skin send messages to the brain about touch. Our sense organs recognize sensations, but they don’t tell us what we are sensing. For example, when you breathe in chemicals given off by baking cookies, your nose does not tell you that you are smelling cookies. That’s your brain’s job. The sense organs send messages about sights, smells, and other stimuli to the brain (Figure below). The brain then reads the messages and tells you what they mean. A certain area of the brain receives and interprets information from each sense organ. For example, information from the nose is received and interpreted by the temporal lobe of the cerebrum.

Which senses would be stimulated by these raspberries?

The Motor Division

The motor division of the peripheral system carries messages from the central nervous system to internal organs and muscles. The motor division is also divided into two parts (Figure above), the somatic nervous system and the autonomic nervous system.

The somatic nervous system carries messages that control body movements. It is responsible for activities that are under your control, such as waving your hand or kicking a ball. The girl pictured below (Figure below) is using her somatic nervous system to control the muscles needed to play the violin. Her brain sends messages to motor neurons that move her hands so she can play. Without the messages from her brain, she would not be able to move her hands and play the violin.

This girl’s central nervous system is controlling the movements of her hands and arms as she plays the violin. Her brain is sending commands to her somatic nervous system, which controls the muscles of her hands and arms.

The autonomic nervous system carries nerve impulses to internal organs. It controls activities that are not under your control, such as sweating and digesting food. The autonomic nervous system has two parts:

  1. The sympathetic division controls internal organs and glands during emergencies. It prepares the body for fight or flight (Figure below). For example, it increases the heart rate and the flow of blood to the legs, so you can run away from danger.
  2. The parasympathetic division controls internal organs and glands during the rest of the time. It controls processes like digestion, heartbeat, and breathing when there is not an emergency.

The woman pictured here is just pretending to be frightened, but assuming that she really was scared, think of which division of the autonomic nervous system would prepare her body for an emergency.

Have you ever become frightened and felt your heart start pounding? How does this happen? The answer is your autonomic nervous system. The sympathetic division prepared you to deal with a possible emergency by increasing your heart rate. The fact that this happened in the blink of an eye shows how amazing the nervous system is.

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