THE NEURON: A CELL FOR RAPID COMMUNICATION

Transcription

1 The only way in which cells communicate is chemically. Communication for a cell means having chemicals moving into and out of it. We will now see how the work of the two organ systems responsible for integration and control, the nervous system and the endocrine system, is based on the chemical communication between cells. THE NEURON: A CELL FOR RAPID COMMUNICATION How does a message travel through the nervous system? The cell theory tells us that the messages must travel along pathways composed of cells. The very specialized cells that make up these pathways are nerve cells, or neurons. The message itself is a nerve impulse. Nerve impulses travel through neurons very rapidly. Every neuron does three things: It receives, conducts, and sends impulses. The receiving end of a neuron is made up of a series of branching extensions called dendrites. Incoming impulses are gathered by the dendrites, which are attached to the cell body (cyton) of the neuron. The cell body contains the nerve cell s nucleus. Usually extending out of the cell body is a single long axon, which carries impulses away from the cell body. An axon typically makes up most of the length of a neuron. Neurons range from several millimeters in length to the 3-meter-long neurons that reach from a giraffe s legs to its spinal cord. The axon ends in a small series of terminal branches (end brushes), which send the impulse on its way. The most important part of a neuron involved in transmitting an impulse is the cell membrane. Through the rapid movement of ions across the cell membrane, an electrical voltage is created. In a neuron, the voltage changes that occur at one place trigger the same kind of changes at the next spot on the membrane. The movement of these cell membrane voltage changes along the length of the axon is the nerve impulse. CROSSING THE GAP: NEUROTRANSMITTERS If you accidentally touch a hot pot on a stove, you immediately pull your hand away. The nerve pathway that carries messages from your hand to your spinal cord and brain, and then back to your hand consists of many neurons. An impulse travels along a neuron as a wave of chemical and electrical changes in the cell membrane. Close examination shows that neurons do not touch each other. They are separated by a gap called a synapse. Chemicals known as neurotransmitters are released by the terminal branches of one neuron. The neurotransmitter is released as the impulse arrives at the terminal branches. These chemicals diffuse across the synapse to the dendrites of the next neuron. Once received by the dendrites of the next neuron, a neurotransmitter makes a new nerve impulse possible. In this way, the message continues along the entire nerve pathway, moving from one neuron to another. So far, at least 30 different neurotransmitters have been identified, including acetylcholine, the best known. Acetylcholine is the neurotransmitter involved in getting the muscles to contract. THE NERVOUS SYSTEM: AN INTERCELLULAR COMMUNICATION NETWORK Any event, change, or condition in the environment that causes an organism to react is a stimulus. The resulting reaction of the organism is a response. The nervous system of an organism makes both the detection of a stimulus and the response to a stimulus possible. A jellyfish has a nerve net. Its nerves are distributed equally in all directions throughout its body s cells. A jellyfish cannot send messages for a response to a particular location a jellyfish has no brain. The small planaria, a type of flatworm, shows an important change in its nervous system. In

2 this animal, nerves are grouped into two parallel nerve cords that run along the length of a planarian s body. These nerve cords meet in a cluster of nerves in the head. In fact, a planarian has a brain. The nerve cords and brain can be called a central nervous system (CNS). Between the two nerve cords, an entire series of nerves makes up the peripheral nervous system (PNS). Specialized structures that receive this information are called sensory receptors. THE VERTEBRATE NERVOUS SYSTEM The nervous system of vertebrates includes a central and a peripheral nervous system. The central nervous system (CNS) consists of the brain and spinal cord. The spinal cord runs from the base of the brain to the lower portion of the back. In most vertebrates, the spinal cord is surrounded by hollow bony vertebrae that make up the backbone. The peripheral nervous system (PNS) consists of neurons with axons that travel out of the CNS to all parts of the body. Those neurons that carry signals out of the CNS are motor neurons. The signals carried by motor neurons are delivered to effectors, such as muscles or glands. They, in turn, put into effect the instructions carried in the message. The sensory neurons carry signals into the CNS from receptors such as those in the ears and eyes. The brain, made up of billions of interconnected neurons, has a mass of only about 1.5 kilograms. However, it uses 25 percent of the body s oxygen supply. The brain can be divided into three major parts: the cerebrum; the cerebellum; and the brain stem, or medulla oblongata. The cerebrum is the most highly developed part of the human brain. The left side, or hemisphere, of the brain controls functions of the right side of the body, and vice versa. Scientists have mapped the outer surface of the cerebrum, the cerebral cortex, for sensory and motor functions. Sensory information from each part of the body comes to a specific location on the cerebrum. The largest portion of the sensory cortex receives information from the face and the hands. Similarly, the motor cortex consists of several regions, each of which controls movements for a specific part of the body. The cerebrum is the area of the brain that is responsible for thought processes and for the creativity we associate with humans. The cerebellum, below and toward the rear of the cerebrum, also processes a vast amount of sensory information and coordinates body movements. In the brain, information is processed by interconnected neurons. The greater the complexity of processing needed, the greater the number of neurons that are interconnected. It is therefore logical, although amazing, that one neuron in the cerebellum may receive impulses from up to 80,000 other neurons! The part of the brain that evolved most recently is the cerebrum. The part that is the oldest and most similar to the brains of vertebrates such as fish and amphibians is the brain stem. The lowest part of the brain stem, or medulla oblongata, controls and coordinates involuntary activities such as breathing, heart rate, swallowing, peristalsis, and blood pressure. When you pull your hand back from a hot object it is a reflex. You did not have to learn it; you did not even have to think about it. For example, the automatic knee-jerk response tested by a physician begins with a receptor, which detects when the knee is tapped. An impulse travels from the knee to the spinal cord along a sensory neuron. Immediately an impulse returns along a motor neuron to the leg muscle, the effector. The leg moves, kicking upward. In the meantime, information travels to the brain, telling it the knee has been tapped. Awareness occurs after the reflex response has already happened.

3 The PNS includes the autonomic system, which controls the involuntary activities of the body. This system is made up of the sympathetic nervous system and the parasympathetic nervous system. Instructions from each of these two systemss, so important for homeostasis, is maintained by the relationship of instructions from these two systems. For example, impulses from the sympathetic nervous system speed up the heart, add sugar to the blood, and increase the level of oxygen in the blood. These changes prepare the body to deal with difficult, dangerous, or stressful situations. The body is ready for action. The parasympathetic system is in charge of more ordinary functions, such as slowing the heart, releasing saliva to chew food and emptying the bladder of urine. DISEASES THAT AFFECT THE NERVOUS SYSTEM People with cerebral palsy experience brain damage just before or after birth. This brain damage does not get worse over time. Persons with cerebral palsy are taught to be as independent as possible while living with the effects of this disease. Multiple sclerosis occurs when myelin, the fatty substance that covers axons in the brain and spinal cord, is destroyed gradually. A wide variety of symptoms including shaking of the hands, blurred vision, and slurred speech occur in people with multiple sclerosis. Symptoms may appear and disappear for many years. Alzheimer s disease is a progressive, degenerative disease. Eventually, memory loss and the inability to think, speak, or care for oneself occur. This disease is usually fatal. The exact cause of Alzheimer s disease is currently unknown. Parkinson s disease also involves the brain; however, its cause is known. A group of neurons in the brain use dopamine as their neurotransmitter. Loss of function in these neurons produces the typical shaking motion, poor balance, lack of coordination, and stiffening of the muscles that occur with this disease. THE ENDOCRINE SYSTEM: ANOTHER COMMUNICATION NETWORK The mineral calcium is necessary for our body. Four small glands in the neck can detect a drop in blood calcium levels and release a hormone that acts to solve the problem. This hormone causes a small amount of calcium to be removed from the bones and less calcium to be excreted in the urine. However, these steps could put too much calcium in the blood. So another gland releases a hormone that reverses the effects of the first hormone, thus stabilizing the amount of calcium. Two sets of glands with two opposing hormones work to maintain the careful balance of calcium that life requires. This system of glands and hormones, including the glands that regulate calcium levels, comprises the endocrine system. The key function of this important system is maintaining homeostasis. Endocrine glands produce hormones, chemical messengers that are released into the blood and carried throughout the body by the circulatory system. At some place or places in the body often far away from the endocrine gland that made it the hormone arrives at its special target cells. It is at its target cells that the hormone puts into effect whatever changes it has been designed to produce. How do hormones do their work? Hormones bind to specific receptor proteins found either in the cell membranes or to receptor proteins in the cytoplasm. Impulses sent by the nervous system usually produce rapid responses. Frequently, these responses are produced by the actions of muscles. Hormones generally produce slower, more longlasting changes that often involve metabolic activity within the target cells.

4 HORMONES: IN ANIMALS AND PLANTS In some insect and amphibian species, the young forms do not resemble the adults, for example; young moths and butterflies or tadpoles and frogs. The change from egg to larva to pupa into an adult is called complete metamorphosis. It is no surprise that the dramatic metamorphosis of a frog or of insects is controlled by the actions of hormones. TYPES OF HORMONES AND HOW THEY ACT Today, almost 200 hormones have been investigated. Hormones can be placed into two main groups; Steroid hormones formed from cholesterol. The sex hormones in vertebrates are steroid hormones. The other group of hormones is protein hormones. They are either protein molecules or modified amino acids, the subunits of proteins. Insulin, the hormone that controls the level of sugar in the blood, is a protein hormone. A characteristic of hormones is that only small amounts are needed to produce the required effect. A group of target cells for a hormone is sensitive to its particular hormone. Negative feedback works to control the amounts of many hormones that are released. THE HUMAN ENDOCRINE SYSTEM The close link between the nervous and endocrine systems exists. The hypothalamus is a part of the brain. The hypothalamus receives information about conditions in the body as blood passes through it. It also receives information from nerve impulses that are carried to it by neurons. In turn, the hypothalamus uses the information it receives to control hormones that are released from its neighbor in the brain, the pituitary gland. The pituitary gland, only about the size of a pea, is sometimes called the master gland because it controls the activities of many other glands of the endocrine system. The pituitary gland consists of a forward, or anterior, part and a rear, or posterior, part. The adrenal gland attached to the top of each kidney consists of two glands, the adrenal cortex and the adrenal medulla. The most important hormone from the adrenal cortex, cortisol, is released only after that gland is instructed to do so by adrenocorticotropic hormone (ACTH) made in the pituitary. This process of a pituitary hormone triggering another gland to release its hormone occurs throughout the body. Other major glands of the endocrine system are the thyroid gland in the neck, the four small parathyroids connected to the thyroid, the pancreas, and the ovaries and testes. DISEASES OF THE ENDOCRINE SYSTEM Goiter is a disease of the thyroid gland. Too little iodine in the diet causes a goiter to form. The chemical element iodine is needed for the manufacture of the hormone thyroxin. Too little thyroxin slows down a person s metabolism, the rate at which chemical reactions occur in the body s cells. Iodine is now added to table salt so that people get enough in their diet to prevent goiter. Grave s disease, in which the thyroid releases too much hormone, makes a person s metabolism overactive. Dwarfism, in which a person s body is much smaller than is normal, can result from too little growth hormone being produced by the anterior pituitary gland. Diabetes is a disease that affects about one in 20 Americans. Diabetes occurs for a variety of reasons. However, in all people with diabetes, something goes wrong with the metabolism of carbohydrates. Carbohydrate metabolism involves the hormone insulin, which is released from the pancreas. Anything that goes wrong with the production or functioning of insulin affects the levels of sugar in the blood and urine. Proper treatment of this disease can greatly reduce the damage that may occur to the eyes, kidneys, heart, arms, and legs.

5 VOCABULARY Define each of these terms in your own words. acetylcholine, adrenal gland, axon, cell body, central nervous system, dendrites, endocrine system, hypothalamus, interneuron, motor neurons, nerve impulse, neurons, neurotransmitters, peripheral nervous system, pituitary gland, protein hormones, response, sensory neurons, sensory receptors, steroid hormones, stimulus, synapse 1. The system of the body that rapidly receives, relays, and responds to internal and external stimuli is 2. Hormones control a. the amount of sugar in the blood b. the development of a tadpole into a frog c. human growth d. all of these. 3. Another term that means nerve cell is 4. If you walk past a bakery, and the delicious scent of cookies causes your mouth to water, the stimulus is a. walking b. the bakery c. the smell d. your mouth watering. 5. The endocrine disease caused by insufficient insulin is 6. An animal s fright, fight, flight responses are controlled by its a. sympathetic nervous system b. parasympathetic nervous system c. somatic nervous system d. autonomic nervous system. 7. What structures are shown in the diagram? Identify the parts labeled A, B, and C. 8. What happens in the part of the diagram labeled E? In the parts labeled D and F? 9. Differentiate and explain the relationships between these terms: central nervous system and peripheral nervous system; motor neurons and sensory neurons; axon and dendrite. 10. Compare and contrast the functioning of the nervous and endocrine systems. 11. Do you think it is appropriate that the pituitary is called the master gland? Justify your answer.

6 1. What is a neuron? What is a nerve impulse? 2. Draw a diagram of a neuron and label its dendrites, cyton, axon, myelin sheath, and terminal branches. 3. Describe a dendrite and what it does. 4. What is the function of the axon? 5. What is a synapse? 6. In a paragraph, completely explain how neurotransmitters released by a neuron can trigger a nerve impulse in an adjacent neuron. 7. What is the difference between a stimulus and a response? 8. What makes up the endocrine system? 9. What is the hypothalamus? Where is it found? How does it control our hormones? 10. The pituitary gland is a small gland at the base of the brain. Why is it sometimes nicknamed the "master gland? 11. Where is the adrenal gland located? 12. Besides the pituitary gland and the adrenal gland, list 5 other glands in the human endocrine system 13. What is a cause of goiter? How does this effect the body metabolic rate? 14. What is a common cause of diabetes? 15. Explain what is meant by negative feedback and list an example of this. 16. Where is thyroxin produced and what is its function? 17. What is the function of insulin in our body 18. What is the function of glucagon in our body? 19. How does an Endocrine Gland differ from an Exocrine Gland?

7 Endocrine System Across 1. Hormones travel from endocrine glands through bloodstream to organs.[6] 6. Adrenaline is called the "fight or hormone".[6] 9. Female reproductive organ.[5] 10. Hormone that converts stored glycogen back into blood sugar.[8] 11. Male reproductive organ that makes testosterone.[6] 12. Disease of poor blood sugar regulation.[8] Down 2. Glands that are situated on top of kidneys.[7] 3. Gland in neck that regulates metabolism.[7] 4. Master gland in the brain.[9] 5. The Islets of Langerhans that make insulin are situated in the.[8] 7. Chemical produced by an endocrine gland.[7] 8. Gland that produces hormones.[9] 10. HGH stands for Human Hormone.[6]

8 Label the diagram of the brain to show the cerebrum, cerebellum, and brain stem. Write the name of the part labeled above that matches each description in the table. Includes the medulla and pons Controls conscious activities and movement Important for keeping your balance If damaged, heart rate might be affected If damaged, memory might be affected Ensures that movements are coordinated Complete the table by filling in the missing information in each case. Structure Function carry impulses toward the brain and spinal cord dendrites motor neurons transmit impulses within the brain and spinal cord carry impulses away from neuron cell bodies

9 Nervous System and Senses Across Down 1. The chemical substance transmits nerve 2. A knee jerk is an example of a arc.[6] messages between nerve cells.[11] 3. Scientific name for nerve cell.[6] 7. A neuron is a cell.[5] 4. Type of nerve that sends messages to 8. The nose is the sense organ for.[5] muscles.[5] 9. Long part of a nerve cell.[4] 5. Sense organ for hearing and balance.[3] 12. Sense organ for sight.[3] 6. Midbrain.[10] 13. Liquid that protects brain and spinal cord is 10. Hindbrain is the medulla.[9] cerebrospinal.[5] 11. Gap between nerve cells.[7] 14. Nervous system that comprises brain and spinal 14. Forebrain.[8] cord.[7] 17. Sense detected by taste buds on tongue.[5] 15. Bone that protects brain.[5] 16. Bones that protect spinal cord.[9] 18. Type of nerve that sends messages from sense organs to brain.[7]

10 The diagram below is of a nerve cell or neuron. Add the following labels to the diagram. Axon; Myelin sheath; Cell body; Dendrites; Muscle fibers; Color in the diagram as suggested below. Axon - purple; Myelin sheath - yellow; Cell body - blue; Dendrites - green; Muscle fibers red; Now indicate the direction that the nerve impulse travels.