Chapter 15: Support, Movement, and Responses - Science

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The Divisions of the Nervous System Figure 16 shows how organs of the nervous system are grouped into two major divisions—the central nervous system (CNS) and the peripheral (puh RIH fuh rul) nervous system (PNS). The central nervous system includes the brain and spinal cord. The brain is the control center for all activities in the body. It is made of billions of neurons. The spinal cord is made up of bundles of neurons. An adult’s spinal cord is about the width of a thumb and about 43 cm long. Sensory neurons send impulses to the brain or spinal cord. The Peripheral Nervous System All the nerves outside the CNS that connect the brain and spinal cord to other body parts are part of the peripheral nervous system. The PNS includes 12 pairs of nerves from your brain called cranial nerves, and 31 pairs of nerves from your spinal cord called spinal nerves. Spinal nerves are made up of bundles of sensory and motor neurons bound together by connective tissue. They carry impulses from all parts of the body to the brain and from the brain to all parts of your body. A single spinal nerve can have impulses going to and from the brain at the same time. Some nerves contain only sensory neurons, and some contain only motor neurons, but most nerves contain both types of neurons. Somatic and Autonomic Systems The peripheral nervous system has two major divisions. The somatic system controls voluntary actions. It is made up of the cranial and spinal nerves that go from the central nervous system to your skeletal muscles. The autonomic system controls involuntary actions—those not under conscious control—such as your heart rate, breathing, digestion, and glandular functions. Spinal nerves (PNS) Brain (CNS) Topic: Nervous System Visit green.msscience.com for Web links to information about the nervous system. Activity Make a brochure outlining recent medical advances. Spinal cord (CNS) Figure 16 The brain and spinal cord (yellow) form the central nervous system (CNS). All other nerves (green) are part of the peripheral nervous system (PNS). SECTION 4 The Nervous System 451
Neuron Chemical Acetylcholine (uh see tul KOH leen) is a chemical produced by neurons that carries an impulse across a synapse to the next neuron. After the impulse is started, acetylcholine breaks down rapidly. In your Science Journal, make an inference about why the rapid breakdown of acetylcholine is important. 452 CHAPTER 15 Support, Movement, and Responses KS Studios Figure 17 Your response in a reflex is controlled in your spinal cord, not your brain. Direction of impulse Sensory neuron Muscle contracts Motor neuron Interneuron Receptor in skin Safety and the Nervous System Every mental process and physical action of the body involves structures of the central and peripheral nervous systems. Therefore, any injury to them can be serious. A severe blow to the head can bruise the brain and cause temporary or permanent loss of mental and physical abilities. For example, an injury to the back of the brain could result in the loss of vision. Although the spinal cord is surrounded by the vertebrae of your spine, spinal cord injuries do occur. They can be just as dangerous as a brain injury. Injury to the spine can bring about damage to nerve pathways and result in paralysis (puh RAH luh suhs), which is the loss of muscle movement. Major causes of head and spinal injuries include automobile, motorcycle, and bicycle accidents, as well as sports injuries. Just like wearing seat belts in automobiles, it is important to wear the appropriate safety gear while playing sports and riding on bicycles and skateboards. Reflexes You experience a reflex if you accidentally touch something sharp, something extremely hot or cold, or when you cough or vomit. A reflex is an involuntary, automatic response to a stimulus. You can’t control reflexes because they occur before you know what has happened. A reflex involves a simple nerve pathway called a reflex arc, as illustrated in Figure 17. A reflex allows the body to respond without having to think about what action to take. Reflex responses are controlled in your spinal cord, not in your brain. Your brain acts after the reflex to help you figure out what to do to make the pain stop. Why are reflexes important? Spinal cord
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Page 19: Nucleus Dendrite Cell body Figure 1
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Page 25 and 26: Nerve fibers Tongue Figure 21 Taste
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Chapter 15: Support, Movement, and Responses - Science

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