Key Concepts:
Vocabulary: neurons, cerebrum, cerebellum, brain stem, epilepsy, cerebral palsy
The Nervous System
How the Nervous System Works
Main Idea: The nervous system coordinates all of the activities in the body.
Your nervous system is a complex network that allows communication between the brain and parts of the body. It stores information and coordinates all activities, from breath- ing or digesting food to sensing pain and feeling fear. The brain, spinal cord, and nerves work together, transmitting messages between organs, tissues, and cells.
The nervous system has two main divisions. The central nervous system (CNS) consists of the brain and spinal cord.The peripheral nervous system (PNS) gathers information from inside and outside your body. It includes nerves that extend from the brain, spinal cord, and sensory receptors, such as those in the skin that sense pressure, temperature, or pain. The CNS receives messages from the nerves in the PNS, interprets them, and sends out a response.
Understanding Neurons
Main Idea: Neurons transmit messages from the brain and spinal cord to the rest of the body.
Neurons, or nerve cells, transmit messages to and from the spinal cord and brain. The three types of neurons-sensory neurons, motor neurons, and interneurons-are classified by function. Sensory neurons carry messages from receptors in the body to the CNS. Motor neurons carry messages from the CNS back to muscles or glands in response to an impulse. Interneurons communicate with and connect other neurons. Figure 14.9 illustrates the nerve impulse. A neuron consists of three main parts:
The Central Nervous System
Main Idea: The central nervous system is made up of the brain and spinal cord.
The two organs that make up the CNS-the brain and spinal cord-send and receive impulses to and from nerves in the body. Figure 14.10 shows how nerves extend to various parts of the body.
The spinal cord is a long column of nerve tissue about the thickness of your index finger. The tissue ofthe spinal cordis surrounded by several layers of connective tissue called the spinal meninges. The meninges, along with the vertebrae-the bones of the spine-help protect the spinal cord. The spinal cord is also bathed in cerebrospinal fluid that absorbs shock and nourishes the nerve tissue.
An adult human brain weighs up to 3 pounds and rests in the protective cavity formed by the bones of the skull. Like the spinal cord, the brain is protected from injury by layers of cranial meninges and cerebrospinal fluid. The brain depends on oxygen to survive. It can last for only four to five minutes without oxygen before suffering irreversible damage.
Sections of the Brain
The brain, shown in Figure 14.11, coordinates and controls the activities of the nervous system. Your brain helps you to receive and process messages; to think, remem- ber, reason, and feel emotions; and to coordinate muscle move- ments. The brain has three main divisions: the cerebrum, the cerebellum, and the brain stem.
The Cerebrum
The cerebrum (seh-REE-brum) is the largest and most complex part of the brain. Billionsofneuronsinthe cerebrum are the center of conscious thought, learning, and memory. The cerebrum’s right and left sides, or hemispheres, communicate with each other to coordinate movement. The right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body. The left hemisphere is the center of language, reasoning, and critical thinking skills. The right hemisphere is the center for processing music and art and comprehending spatial relationships. Each hemisphere has four lobes:
The frontal lobe controls voluntary movements and has a role in the use oflanguage. The prefrontal areas are thought to be involved with intellect and personality.
The parietal lobe is involved with sensory information, including feelings of heat, cold, pain, touch, and body position in space.
The occipital lobe controls the sense of sight.
The Cerebellum
The cerebellum(ser-eh-BEL-um)is the second largest part of the brain. It coordinates the movement of skeletal muscles. This area of the brain also continually receives messages from sensory neurons in the inner ear and muscles. It uses this information to maintain the body’s posture and balance. Being able to carry out a complex series of muscle movements, such as serving a volleyball or playing the violin, is made possible by the cerebellum.
The Brain Stem
The brain stem is a 3-inch-long stalk ofnerve cells and fibers that connects the spinal cord to the rest of the brain. Incoming sensory impulses and outgoing motor impulses pass through the brain stem. It has five parts:
The medulla oblongata regulates heartbeat, respiratory rate, and reflexes such as coughing and sneezing.
The pons helps regulate breathing and controls the muscles of the eyes and face.
The midbrain controls eyeball movement, pupil size, and the reflexive response of turning your head.
The thalamus relays incoming sensory impulses from the eyes, the ears, and from pressure receptors in the skin.
The hypothalamus regulates body temperature, appetite, sleep, and controls secretions from the pituitary gland, affecting metabolism, sexual development, and emotions.
The Peripheral Nervous System
Main Idea: The peripheral nervous system is made up of the nerves that are not in the brain and spinal cord.
The peripheral nervous system (PNS) carries messages between the CNS and part of the body, signaling internal and external changes. The PNS is made up of the autonomic nervous system and the somatic nervous system.
The Autonomic Nervous System
The autonomic nervous system contcs such involuntary functions as digestion and heart rate. It consists of a network ofnerves divided into two smaller networks, the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system kicks in when you are startled, sending messages that cause your heart rate to increase. Blood vessels in your muscles dilate, allowing greater blood flow. This is the “fight-or-flight” response that prepares you to react in a dangerous situation. Figure 14.12 illustrates this reflex, the body’s spontaneous response to a stimulus, as when a doctor tests your knee-jerk reflex by tapping the ligament below your knee during a physical exam.
The parasympathetic nervous system opposes the action of the sympathetic nervous system by slowing body functions. During periods of rest, it slows heartbeat, relaxes blood vessels, and lowers blood pressure to conserve energy. The parasympathetic nervous system stimulates production of saliva and stomach secretions to promote the digestion of food.
The Somatic Nervous System
The somatic nervous system involves voluntary responses that are under your control. Sensory neurons relay messages from the eyes, ears, nose, tongue, and skin to the CNS. Motor neurons carry impulses from the CNS to skeletal muscles.
Caring for Your Nervous System
Main Idea: Making healthful choices can protect your nervous system from injury.
Eating a well-balanced diet, exercising regularly, getting enough sleep, and wearing protective devices will protect your nervous system. Always wear a safety belt when in a motor vehicle. Wear a helmet and other protective gear while riding a bicycle, motorcycle, or other open vehicle, or when enjoying a contact sport. Before diving, check the depth ofthe water. Never dive head first into shallow water or into water where you cannot see the bottom. Finally, drugs and alcohol can permanently damage nerve cells, so avoid using them.
Problems of the Nervous System
Injury to the nervous system affects the immediate tissues, and may lead to other problems, including the following:
• Headaches. Headaches can be caused by muscle tension, eyestrain, exposure to fumes, a sinus infection, dehydration, or food allergies. Migraines are recurrent headaches that may be accompanied by sensitivity to light.
• Head injuries. Each year, 435,000 American children and teens sustain brain injuries. Types of head injuries include concussion, a temporary loss of consciousness, contusion, a bruising of the brain tissues that causes swelling, and coma, caused by major trauma.
• Spinal injuries. Spinal cord injuries require medical care. Swelling of the spinal cord or the tissue around it can result in temporary loss of nerve function. Permanent nerve damage will result without treatment. If the spinal cord has been severed, paralysis results.
• Meningitis. Meningitis is an inflammation of the spinal and cranial meninges caused by bacterial or viral infection. Meningitis is very serious and can result in death. Symptoms include fever, head- ache, light and sound sensitivity, and neck stiffness.
Some nervous system diseases are degenerative, which means they occur over tiJne as cells break down. Multiple sclerosis, Parkinson’s disease and Alzheimer’s are examples of degenerative diseases. Other disorders result from injury or brain damage. Epilepsy is a disorder of the nervous system that is characterized by recurrent seizures-sudden episodes of uncontrolled electrical activity in the brain. Causes include brain damage at birth, infections, head injury, or exposure to toxins. Medications can help control seizures. Cerebral palsy refers to a group of neurological disorders that are the result of damage to the brain before, during, or just after birth or in early childhood. Physical therapy and medication help patients cope.
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