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The Nervous System Functions of the Nervous System The nervous system is the communication and control center of our body. Our nervous system controls all of our body’s actions and functions. It originates and coordinates physical reactions to the environment and controls involuntary muscles and organs such as the heart and lungs. It maintains a balanced state within the body (homeostasis). Structure of the Nervous System Two main divisions of the nervous system 1) Central Nervous System (CNS) 2) Peripheral Nervous System (PNS) The PNS gathers information from inside and outside the body. The CNS receives and analyzes this information and initiates responses. PNS then picks up and carries the response signals. The information is transmitted throughout our body by means of electrical charges called impulses. (up to 248 mph) The messengers and receivers of these transmissions are neurons (nerve cells) Functions of Neurons Classified according to their functions. Three main types of neurons: sensory neurons, motor neurons, and interneurons. Sensory Neurons – carry signals from sense receptors (eyes, ears, skin) into the CNS. Motor Neurons – carry signals from the CNS to muscles or glands. Interneurons – form all the electrical connections within the CNS itself. How all of this works: Nerve cells (neurons), unlike other cells, cannot replace themselves. If the cell body is damaged or degenerates, the cell dies! The three basic parts of a neuron: cell body, dendrites, axons (refer to hand-out) Cell body – consists of a nucleus and cytoplasm enclosed by the cell membrane. The nucleus receives and sends nerve impulses. Regulates amount and type of proteins made. Dendrites – are branching projections of the cell body. They receive and carry impulses toward the cell body. Axons – threadlike extension that carries impulses away from the cell body. Branches at its end forming terminals, signals are sent to target cells (other neurons, muscle cells, or glands) Length can vary from ~ mm to > 1 meter Most are coated (sheath) with myelin (fatty material). Myelin insulates the nerve fiber, speeds transmission of impulses. Central Nervous System Consists of two main parts: The brain and spinal cord Every bodily function involves the CNS. Messages from outlying receptors and sensors arrive at the CNS, where they are interpreted; the CNS then sends out reaction impulses. The Spinal Cord o Cylinder of nerve tissue about 43 cm long (17-18 inches) and as thick as index finger. o Downward extension of brain. o Runs down canal in spine, protected by vertebrae o Also protected by cerebrospinal fluid (shock absorber), three layers of connective tissue called spinal meninges. o Shaped like a flattened cylinder, slightly winder than a pencil for most of its length The center region of the cord is white matter made up of numerous sheathed nerve fibers that run lengthwise through the spinal cord. The interior is gray matter. The gray matter contains the cell bodies of neurons and supporting cells. Some are interneurons. Others are motor neurons whose axons pass out of the spinal cord in bundles and extend to muscles and glands. Axons of sensory neurons enter the gray matter and connect with motor neurons and interneurons. Thirty-one pairs of spinal nerves branch out from the vertebrae to the right and left sides of the body. The nerves are large cables of sensory and motor fibers. Each nerve pair controls a particular area of the body and is identified by the number of the vertebrae over which it leaves the spinal cord. Cervical nerves (8 pairs) for neck, shoulders, and arms Thoracic nerves (12 pairs) for ribs, back, and abdomen Lumbar nerves (5 pairs) for lower back and thighs Sacral nerves (5 pairs) for buttocks, genitals, legs, and feet Coccygeal (1 pair) for hips, tailbone, buttocks Nerve signals can only travel one way along a nerve, so some nerves carry signals down the spine (the descending pathway) and some carry signals up (the ascending pathway). Spinal cord reaches full length at about 4 to 5 years of age. The Brain Your brain is the largest most complex part of the nervous system. Brain helps you receive and process messages To think, remember, and reason Coordinates your muscle movement Involved in your emotions and everything you sense The brain weighs about three pounds and contains almost 100 billion neurons. At birth, the brain weighs about one pound. The brain is fully grown by your sixth birthday. It makes up only about two percent of total body weight, but use more than 20 percent of the oxygen you inhale. Without oxygen the brain can only last four to five minutes before suffering serious and irreversible damage. The brain has three main divisions: the cerebrum, the cerebellum, and the brain stem. The Cerebrum This is the largest, most complex part of the brain. This is the site of the most conscious and intelligent activities. Conscious thought occurs in the outer layer (cortex). Cerebrum is divided into two halves called cerebral hemispheres. Hemispheres are connected to the rest of the CNS in a cross-over fashion. That means that the right hemisphere controls the muscular activity of and receives sensory input from the left half of the body. The left hemisphere does the same for the right half of the body. In humans: the left hemisphere is mainly responsible for language, logic, arithmetic calculation, analysis, and critical thinking. The right hemisphere is concerned with imagination and visual thinking—spatial relationships, form artistic activities, and emotional responses. Each hemisphere has four lobes. Each lobe is named after the bone in the skull that protects it. The frontal lobe controls voluntary movements and also has a role in the use of language. The prefrontal areas of the lobe are believed to be involved with intellect and personality. The parietal lobe is involved with a wide variety of sensory information— heat, cold, pain, touch, and body position in space. The occipital lobe contains the sense of vision. The temporal lobe contains the senses of hearing and smell as well as memory, thought, and judgment. The Cerebellum Cerebellum is the second largest part of the brain. It’s a rounded structure located beneath the occipital lobes of the cerebrum. The cerebellum is divided into two hemispheres. The cerebellum: Concerned mainly with maintaining posture and balance and coordinating skeletal muscle movement. Receives impulses from balance organs of the inner ear and muscles. After receiving information, cerebellum refines orders being sent to muscles from motor cortex in cerebrum, ensures smooth coordinated movements The Brain Stem The brain stem is a 3-inch long (7.6 cm) stalk of nerve cells and fibers that connects the spinal cord to the rest of the brain. It acts partly as a pathway for messages traveling between other parts of the brain and the spinal cord. It also connects with 10 and the 12 pairs of cranial nerves. Controls our basic functions: breathing heartbeat, and eye reflexes. Activities of the brainstem are not under conscious control The brain stem consists of three main parts: medulla oblongata pons midbrain Medulla oblongata— Lowest part of the brain stem (top end of the spinal cord) Regulates/controls heartbeat, breathing blood pressure, and digestion Is also control center for swallowing, vomiting, sneezing, and coughing Receives and relays taste sensations from the tongue and is involved in speech and tongue movements Pons Located just above the medulla Is the pathway for nerve impulses passing to and from the cerebrum Connects the cerebrum with the cerebellum Joins the hemispheres of the cerebellum Relays sensory information from the ear, face, teeth, signals that move the jaw, and adjust our facial expressions. Midbrain Shortest part of the brain stem Located just above the pons Contains two pairs of cranial nerves that control eye movements and size and reactions of the pupils. Closely associated to the brain stem are the thalamus and hypothalamus. Thalamus Located above the midbrain, two lobes that are egg-shaped (walnutsized). Important relay center for incoming sensory impulses. Clusters of nerve cells, receive information from different sense organs such as eyes and ears. Through the spine, the thalamus also receives information from touch and pressure receptors in the skin. Hypothalamus Located behind the eyes and under the thalamus Have nerve centers that control various body processes and keep the conditions balanced. Nerve cells regulate body temperature, stimulate appetite for food and drink, and regulate sleep. Also controls secretions ( hormones ) from the pituitary gland that control many processes such as metabolism and sexual development and emotional responses. Peripheral Nervous System Peripheral = away from center The peripheral nervous system (PNS) consists of nerves that fan out from the CNS to the muscles, skin, internal organs, and glands. PNS carries messages between the CNS and the rest of the body.