NERVOUS SYSTEMCONCEPTUAL LIFE SCIENCENERVOUS SYSTEMCENTRAL NERVOUS SYSTEMThe central nervous system consists of the brain and the spinal cord. The brain has two sides called hemispheres. It is like having a left and right brain. The cerebral cortex is in charge of learning. The learning pathways are found in the cerebral cortex. Itperforms thinking and reasoning. The brain has specialized areas for different functions such as vision and speech.Corpus callosumThe corpus callosum connects the left and right brains. Something learned in one hemisphere is sent to the other. The exception to this is speech.The right hemisphere controls the left side of the body. It learns with the left handand the left eye. The left hemisphere controls the right side of the body. It learns with the right hand and eye. This is why, in the case where an individual has a stroke that paralyzes the right side of the body, the doctors look on the left side of the brain for the blood clot.Cerebral dominanceThe “talking hemisphere” is the dominant one. The speech center is in the left brain about 90% of the time. This is independent of left- and right-handedness, which is controlled by different genes. The non-talking hemisphere contains the three-dimensional perception center.CerebellumThe cerebellum integrates and coordinates muscle action. It helps maintain balance and posture. It is located in the back of the brain. Body position is determined by nerve transmissions from the semicircular canals in the inner ear.Medulla oblongataThe medulla oblongata controls the autonomic nervous system. It contains centersto regulate body temperature, heartbeat, breathing, and other basic functions.13-113-2Brain stemThe brain stem (pons) provides integration of auditory, visual and cerebral mechanisms. This is important for such activities as writing, speech and playing musical instruments.ThalamusThe thalamus processes sensory information from the body and all of the sense organs except the sense of smell. The processed information is then sent to the cerebrum (cerebral cortex). HypothalamusThe hypothalamus regulates activities associated with basic needs and emotions. It also controls secretion of hormones by the pituitary. This provides a link between the nervous system and the endocrine system.PERIPHERAL NERVOUS SYSTEMThe neuronThe neuron is the cell of the nervous system. There is a cell body (cyton), an axon, and terminal branches. A sheath of membranes surrounds the axon. The nerve cell conducts electrical impulses. In a motor neuron, the terminal branches end in a muscle ora gland.Nerve fiberA nerve fiber is made up of the axons of many nerve cells. Axons are often very long. The cytons for the nerves of the legs are located in the spinal cord and the axons travel the entire length of the legs.Propagation of impulsesThe nerve cell has a positive charge on the outside and a negative charge on the inside. The nerve impulse causes a temporary change in the charge of the cell membrane resulting from the inflow of sodium ions. After the impulse has passed, the membrane restores its original charge during the “refractory” period.The space between two nerve cells is called a synapse. Nerve cells do not touch each other so they cannot transmit nerve impulses electrically between cells. To send a nerve impulse between cells, the nerve that is sending the impulse produces13-3acetylcholine, which is a neurotransmitter that causes the recipient nerve to fire and continue propagating the impulse. When the process is completed, the acetylcholine is destroyed by cholinesterase, a special enzyme, to prevent the same nerve impulse from being transmitted repeatedly.Sensory neuronsThe sensory neurons carry impulses from the sense organs to the central nervous system. When a sensory receptor is stimulated, it sends impulses to the brain or spinal cord using a sensory neuron.Motor neuronsMotor neurons carry impulses from the central nervous system to organs, muscles and glands. A shortcut from a sensory neuron to a motor neuron is known as a reflex arc.Reflex motionIn a reflex arc, a sensory neuron transmits an impulse to an associative neuron (interneuron) in the spinal cord. From there, the impulse is transmitted to a motor neuron. The result is a response by the organ that was stimulated by the motor neuron.THE AUTONOMIC NERVOUS SYSTEMThe autonomic nervous system (ANS) has special nerves for internal organs. Thesympathetic division of the ANS uses noradrenalin as the neurotransmitter. This stimulates the activity of the organs. Acetylcholine is the neurotransmitter for the parasympathetic division of the ANS. It causes the internal organs to slow down. See Table XIII-1.Table XIII-1. Neurotransmitters of the autonomic nervous system.Division Neurotransmitter EffectSympathetic Noradrenalin Speeds up the organsParasympathetic Acetylcholine Slows down the organsThe result is a balance between the divisions of the ANS. The response of the organs willdepend upon which neurotransmitter is received.Ganglion cellsGanglion cells are the special nerve cells of the autonomic nervous system. The cell bodies of ganglion cells are located outside of the central nervous system. The organs affected by the autonomic nervous system will respond according to which neurotransmitter they receive from the ganglion