7/21/2009 There are two major divisions of the nervous system. The Nervous System • The central nervous system is your body’s command center. The anatomy an d p hys io logy o f • It includes the brain and animals illustrate the complementary spinal cord. nature of structure and function. Peripheral Nervous System Neurons and nerve impulses • Your nervous system is made of hundreds • The peripheral nervous of billions of specialized cells called system consists of nerves that connect all areas of neurons. the body to the central • A neuron has three parts: the cell body, a nervous system. long stalk called the axon, and finger-like • You can think of the projections called dendrites. peripheral nervous system as the “information highway” of your nervous system. Neurons send signals called nerve impulses throughout your body. • A nerve impulse is wave of electrical How your body responds to a and chemical stimulus activity transmitted The withdrawal reflex between neurons. 1 7/21/2009 Imagine you’re relaxing on the couch, watching your favorite television show. • Someone sneaks up behind you and touches the back of your neck with a wet, frosty ice cube. Before you even have a chance to think “who did that?” your body springs into action. • The ice cube triggers an automatic response called a withdrawal reflex that happens without a conscious decision on your part. A withdrawal reflex happens because nerve impulses are sent through the nerves in your body. Sensory and Motor Nerves • When an ice cube touches the back of your neck, sensory nerves in your skin send nerve impulses through wire-like nerve fibers to your spinal cord. • In the spinal cord, the nerve impulse is transferred to motor nerves. Motor nerves control muscle contractions. • Impulses from your motor nerves cause the muscles in your neck and back to contract, jerking your body away from the ice cube. How a nerve impulse works • All of this happens in a split second! A withdrawal reflex starts when sensory nerves in your skin receive a stimulus from outside the body. 2 7/21/2009 • That stimulus starts a nerve impulse along the cell • Other channels open and let positively membrane. charged particles out of the cell. • When a neuron is at rest, the inside of the cell • As they leave, the inside of the cell membrane is electrically negative compared with membrane once again becomes negatively- the outside. charged compared with the outside. • The stimulus causes the cell membrane to open • channels that let ppygpositively charged particles into The nerve impulse travels down the axon the cell. like dom inoes fa lling. • The inside of the cell becomes positively charged • When the impulse reaches the end of the compared with the outside. axon, chemicals are released and picked up by a neighboring neuron, causing the nerve impulse to continue. Each second, your body fires off about five trillion nerve impulses. • Your emotions, decisions, and physical actions all happen through nerve impulses traveling through neurons in your brain, spinal cord and nerves. • A single neuron can have up to ten thousand dendrites connecting to other neurons. • It is estimated that just one cubic millimeter of brain tissue contains a billion connections between cells! The brain • What is the brain? The brain is the processing and control center of your nervous system. The brain and spinal cord are made of tissues called gray and white matter. 3 7/21/2009 The brain has three parts • Gray matter is mostly made up of the cell bodies of neurons. • The three parts of the • White matter is mostly made up of the brain are the axons coming from those cell bodies. cerebrum, the • In gg,gyeneral, grey matter makes up pp the parts cerebellum, and the of the brain responsible for information medulla. processing. • White matter is responsible for transmitting nerve impulses. The largest part of your brain is the The cerebrum is divided into two halves dome-shaped cerebrum. called hemispheres. • The right hemisphere controls • The cerebrum controls the left side of the body and the voluntary movements and left hemisphere controls the the senses (touch, taste, right side of your body! smell, vision, hearing). • But both sides are involved in • It also allows you to think, most activities. talk, solve problems, and • These parts are all connected imagine. but each part has its own function The medulla is the part of the brain that The cerebellum controls your spinal cord. • The cerebellum provides • It also controls your feedback on the position of involuntary breathing, heart the body in space. rate, blood pressure, and some • It receives sensory information other involuntary activities. anddd sends nerve impulses to different skeletal muscles to keep • It receives sensory input from the heart and you balanced. blood vessels and sends nerve impulses back • The cerebellum is located to those organs to control their function. underneath the back of your • The medulla is located underneath the cerebrum. cerebrum and in front of the cerebellum. 4.