A large, multicellular organism, which consists of many different kinds of systems, must have some way of integrating various s so that it can survive. The various systems must be coordinated to maintain a reasonably constant internal environment. Recall from chapter 18 that this condition of maintaining a constant internal environment is called homeostasis. To allow for homeostasis there must be constant monitoring and modification of the way specific parts of the organism . If the organism does not rspond appropriately, it will die. There are many kinds of sense organs located within organs and on the surfaces that respond to specific kinds of stimuli. A stimulus is any change in the environment that organism can detect. Some stimuli, like light or sound, are typically external to the organism; others,like the pain generated by an infection, are internal. The reaction of the organism to a stimulus is known as a response.
The nervous and endocrine systems are the major systems of the body that integrate stimuli and generate appropriate responses necessary to maintain homeostasis. The nervous system consists of a net work of cells with fibrous extensions that carry information alongy very specific pathways from one part of the body to another. The endocrine system consists of a number of glands that communicate with one another with other tissues through chemicals distributed throughout the organism. Glands are organs that manufacture specific molecules that are either secreted into surrounding tissue, where they are picked up by the circulatory system, or are secreted through ducts into the cavity of an organ or to the body surface. Endocrine glands have no ducts and secret their products into the circulatory system. The molecules produced by endocrine glands are called the hormones. A hormone is a specific molecule released by the one organ that is transported to another organ where it triggers a change in the other organ's activity. Other glands, such as the digestive glands and sweat glands, empty their products through ducts. These kinds of glands are called exocrine glands.
Although the s of the nervous and endocrine systems can overlay and be interrelated, these two systems have quite different methods of action. The nervous system s very much like a computer. A message is sent along established pathways from a specific initiating point to a specific end point, and the transmission is very rapid. The endocrine system s ina manner analogous to a radio broadcast system. Radio stations send their signals in all directions, but only those radio receivers that are tuned to the correct frequency can receive the message. Messageer molecules are typically distributed throughout the body by the circulatory system, but only those cells that have the proper receptor sites can receive and respond to the molecules.
the structure of the nervous system
The basic unit of the nervous system is a specialized cell called neuron, or nerve cell. A typical neuron consists of a central body called the soma, or cell body, which contains the nucleus and several long, protoplasmic extension called nerve fibers. There are two kinds of fiber; axons, which carry information away from the cell body, and dendrites, which carry information toward the cell body. Most nerve cells have one axon and several dendrites.
Neurons are arranged into two major systems. The central nervous system, which consists of the brain and spinal cord, is surrounded by the skull and the vertebrae of the spinal column. It receives input from sense organs, interprets information, and generates responses. The peripheral nervous system is located outside the skull and spinal column and consists of bundles of long axons and dendrites called nerves. There are two different sets of neurons in teh peripheral nervous system. Motor neurons carry messages from the central nervous system to the muscles and glands, and sensory neurons carry input from sense organs to the central nervous system. Motor neurons typically have one long axon that runs from the spinal cord to a muscle or gland; sensory neurons have long dendrites that carry input from the sense organs to the central nervous system.
The nature of the nerve impulse
Because most nerve cells have long fibrous extensions, it is possible for imformation to be passed along the nerve cell from one end to the other. The message that travels along a neuron is known as a nerve impulse. A nerve impulse is not like an electric current but involve a specific sequency of chemical events involving activities at the cell membrane.
Because all cell membranes are differentially permeable, it is difficult for some ions to pass through the membrane and the combination of ions inside the membrane is different from that on the outside. Cell membranes also contain proteins that actively transport specific ions from one side of the membrane to the other. Active transport involves the cell's use of adenosione triphosphate to move mateirals from one side of the cell membrane to the other. Because ATP is required this is an ability that cells lose when they die. One of the ions that is actively transported from cells is the sodium in. At the same time sodium ions are being transported out of cells, potassium ions are being transported into the normal resting cells. However, there are more sodium ions transported out than potassium ions transported in.
Because a normal resting cell has more positively charged Na+ ions on the outside of the cell than on the inside, a small but measurable voltage exists across the membrane of the cell. (voltage is a measure of the electricial charge difference that exists between two points or objects). The voltage difference between the inside and outside of a cell membrane is about 70 millivolts. the two sides of the cell membrane are , therefore, polarized in the same sense that a battery is polarized, with a positive and negative pole. A resting neuron has its positive pole on the outside of the cell membrane and its negative pole on the inside of the membrane.
When a cell is stimulated at a specific point on the cell membrane, the cell membrane changes it permeability and let sodium ions pass through it from the outside to the inside. The membrane is thus depolarized; it loses its difference in charge as sodium ions diffuse into cell from the outside. sodium ions diffuse into the cell because, initially, they are in greater concentration outside the cell than inside. When the membrane becomes more permeable, they are able to diffuse into the cell, toward the area of lower concentration. The depolarization of one piont on the cell membrane causes the adjacent portion of the cell membrane to change its permeability as well, and it also depolarizes. Thus a wave of depolarization passes along the length of the neuron from one end to the other . The depolarization and passage of an impulse along any portion of the neuron is a momentary event. As soon as a section of the membrane has been depolarized, potassium ions diffuse out of the cell. This re-establishes the original polarized state and the membrane is said to be repolarized. subsequently, the continuous active transport of sodium ions out of the cell and potassium ions into the cell restores the original concentration of ions on both sides of the cell membrane. When the nerve impuls reaches the end of the axon, it stimulates the release of a molecule that stimulates depolarization of the next neuron in the chain.
Activities at the synapse
Between the fibers of adjacent neurons in a chain is a space called the synapse. Many chemical events occur in the synapse that are important in the of the nervous system. When a neuron is stimulated, an impulse passes along its length from one end to the other. When the impulse reaches a synapse, a molecule called a neurotransmitter is released into the synapse from the axon. it diffuses across the synapse and binds to specific receptor sites on the dendrite of the next neuron. When enough neurotransmitter molecules have been bound to the second neuron, an impulse is initiated in it as well. Several kinds of neurotransmitters are produced by specific neurons. These include dopamine, epinephrine, acetylcholine, and several other molecules. The first neurotransmitter indentified was acetycholine. Acetylcholine molecules are manufactured in the soma and migrate down the axon where they are stored until needed.
As long as a neurotransmitter is bound to its receptor it continues to stimulate the nerve cell. Thus if acetylcholine continues to occupy receptors, the neuron continues to be stimulated again and again. An enzyme called acetylcholinesterase destroys acetylcholine and prevents this from happening. (The breakdown products of the acetylcholine can be used to remanufacture new acetylcholine molecules.) The destruction of acetylcholine allows the second neuron in the chain to return to normal. Thus it will be ready to accept another burst of acetylcholine from the first neuron a short time later. Neurons must also constantly manufacture new acetylcholine molecules or they will exhaust their supply and be unable to conduct an impulse across the synapse.
Certain drugs, such as curare and strychnine, interfere with activities at the synapse. Curare blocks the synapse and causes paralysis, whereas strychnine causes nerve cells to be continually stimulated. Many of the modern insecticides are also nerve poisons and are therefore quite hazardous.
Because of the way the synapse works, impulse can go in only one direction: only axons secrete acetylcholine, and only dendrites have receptors. This explains why there are sensory and motor neurons to carry messages to and from the central nervous system.
The nervous system is organized in a fashion similar to a computer. information from various input devices is delivered to the central processing unit by way of wires. The information is interpreted in the central processing unit. eventually messages can be sent by way of cables to drive external machinery (muscle and glands). This concept allows us to understand how the s of various portion of the nervous system have been identified. It is possible to electrically stimulate specific portions of the nervous system or to damage certain parts of the nervous system in experimental animals and determine the s of different parts of the brain and other parts of the nervous system. For example, because peripheral nerves carry bundles of both sensory and motor fibers, damage to a nerve may result in both a lack of feeling because sensory messages cannot get through and an inability to move because the motor nerves are damaged.
The organization of the central nervous system
Major s of specific portions of the brain have been identified. Certain parts of the brain are involved in controlling fundamental s such as breathing and heart rate. Others are involved in generating emotions, whereas others decode sensory input, or coordinate motor activity. The human brain also has considerable capacity to store information and creat new responses to environmental stimuli.
The brain consists of several regions each of which has specific s. The s of the brain can be roughly divided into three major levels: automatic activities, basic decision making and emotions, and thinking and reasoning. If we begin with the spinal cord and work our way forward we will proceed from the more fundamental, automatic activities of the brain to the more complex thinking portions of the brain. The spinal cord is a collection of nerve fibers surrounded by the vertebrae that convey information to and from the brain. At the base of the brain where the spinal cord enters the skull is a portion of the brain known as the medulla oblongata. This region of the brain controls fundamental activities such as blood pressure, breathing, and heart rate. Most of the fibers of the spinal cord cross from one side of the body to the other in the medulla oblongata. this is why the left side of the brain affects the right side of the body.
the cerabellum is a large bulge at the base of the brain that is connected to the medulla oblongata. the primary of the cerebellum is coordination of muscle activity. It receives information from sense organs such as the portions of the ear that involve balance, the eyes, and pressure sensors in muscles and tendons. This information is used to make adjustments to the strength and order of contraction of muscle necessary to move in a coordinated fashion.
The pons is connected to the anterior end of the medulla oblongata. It also connects to the cerebellum and to higher levels of the brain. It is involved in controlling many sensory and motor s of the sense organs of the head and face.
If we continue forward the pons is connected to a portion of the brain that froms two bulblike structures that altimately connect to the cerebrum. Although these portions of the brain still control many automatic activities, there are many activities that involve much more integration of information and some level of " decision making" occurs in this region. The primary regions are the thalamus and the hypothalamus. The thalamus relays information between the cerebrum and lower portions of the brain. It also provides some level of awareness in that it it determines pleasant and unpleasant stimuli and is involved in sleep and arousal. The hypothalamus is also involved in sleep and arousal and is important in emotions auch as anger, fear, pleasure, hunger, sexual response, and pain. Several other more automatic s are regulated in this region, such as body temperature, blood pressure, and water balance. The hypothalamus also is connected to the pituitary gland and influences the manufacture and release of its hormones. Figure 20.6 shows the relationship of the various more primitive parts of the brain.
The cerebrum is the largest portion of the brain in humans. The two hemispheres of the cerebrum cover all other portions of the brain except the cerebellum. the cerebrum is the thinking part of the brain. The surface of the cerebrum has been extensively mapped so that we know the location of many s. Abilities such as memory, language, control of movement, interpretation of sensory input, and thought are associated with specific areas of the cerebrum. figure 20.7 shows a diagram of the cerebrum and the locations of specific s.
The of the brain is not determined by structure alone. Many parts of the brain have specialized neurons that produce specific neurotransmitter molecules used only to stimulate specific sensitive cells that have the proper receptor sites. As we learn more about the ing of the brain, we are finding more kinds of specialized neurotransmitter molecules. Their discovery allows for the treatment of many types of mental and emotional diseases. Manipulating these neurotransmitter molecules can help correct inappropriate ing of the brain. However, one should not assure that we understand the brain. We are still at an early stage in our research to comprehend this organ that sets us apart from other animals.
Endocrine system
As mentioned previously, the endocrine system is basically a broadcasting system in which glands secrete messenger molecules, called hormones, that are distributed throughout the body by the circulatory system. However, each kind of hormone affects only certain cells. The specific cells that a particular hormone affects are often called target cells. The hormones target certain cells because the cells have specific receptor molecules on their surfaces to which specific hormones attach. The cells that receive the messages typically respond in one of three ways: 1. some cells release products that have been previously manufactured, 2. other cells are stimulated to synthesize molecules or to begin bolic activities, and 3. some are stimulated to divide and grow.
These different kinds of responses mean that some endocrine responses are relatively rapid, whereas others are very slow. For example, the release of the hormones epinephrine and norepinephrine from the adrenal medulla, located near the kidney, causes a rapid change in the behavior of an organism. The heart rate increases, blood pressure rises, blood is shunted to muscles, and the breathing rate increases. You have certainly experienced this reaction many times in your lifetime, such as when you nearly had an automobile accident or slipped and nearly fall.
Another hormone, called antidiuretic hormone, acts more slowly. it is released from the posterior pituitary gland at the base of the brain and regulates the rate at which the body loses water through the kidneys. It does this by encouraging the reabsorption of water from their collecting ducts. the effects of this hormone can be noticed in a matter of minutes to hours. Insulin is another hormone whose effects are quite rapid. Insulin is produced by the pancreas, located near the stomach, and stimulates cells --particularly muscle, liver, and fat cells--to take up glucose from the blood. After a high carbohydrate meal, the level of glucose in the blood begins to rise, stimulating the pancreas to release insulin. The increased insulin causes glucose levels to fall as the sugar is taken up by cells. People with diabetes have insufficient or improperly acting insulin or lack the receptors to respond to the insulin, and therefore have difficulty regulating glucose levels in their blood.
The response that result from the growth of cells may take weeks or years to occur. For example, growth stimulating hormone is produced by the anterior pitutitary gland over a period of years and results in typical human growth. After sexual maturity, the amount of this hormone generally drops to very low levels, and body growth stops. Sexual development is also largely the result of the growth of specific tissues and organs. The male sex hormone testosterone, produced by the testes, causes the growth of male sex organs and a change to the adult body form. The female counterpart, estrogen, results in the development of female sex organs and body form. In all of these cases, it is the release of hormones over long periods, continually stimulating the growth of sensitive tissues, that results in a normal developmental pattern. The absence or inhibition of any of these hormones early in life changes the normal growth process.
Glands within the endocrine system typically interact with one another and control production of hormones. One commone control mechanism is called negative-feedback control. In negative-feedback control the increased amount of one hormone interferes with the production of a different hormone in the chain of events. The production of thyroxine and triiodothyronine by the thyroid gland exemflifies this kind of control. The production of these two hormone is stimulated by increased production of a hormone from the anterior pituitary called thyroid-stimualting-hormone. The control lies in the quantity of the hormone produced. When the anterior pituitary produces high levels of thyroid stimulating hormone, the thyroid is stimulated to grow and secrete more thyroxine and triiodothyronine. But when increased amounts of thyroxine and triiodothyronine are produced, these hormones ave a negative effect on the pituitary so that it decreases its production of thyroid-stimulating hormone, leading to reduced production of thyroxine and triiodothyronine. If the amount of the thyroid hormones falls too low, the pituitary is no longer inhibited and releases additional thyroid-stimulating hormone. As a result of the interaction of these hormones, their concentrations are maintaind within certain limits.
It is possible for the nervous and endocrine systems to interact. The pituitary gland is located at the base of the brain and is divided into two parts. The posterior pituitary is directly connected to the brain and develops from nerve tissue. The other part, the anterior pituitary, is produced from the lining of the roof of the mouth in early fetal development. Certain pituitary hormones are produced in the brain and transported down axons to the posterior pituitary where they are stored before being released. The anterior pituitary also receives a continous input of messenger molecules from the brain, but these are delivered by way of a special set of blood vessels that pick up hormones produced by the hypothalamus of the brain and deliver them to the anterior pituitary.
The pituitary gland produces a variety of hormones that are responsible for causing other endocrine glands, such as the thyoid, ovaries, and testes, and adrenals, to secrete their hormones. Pituitary hormones also influence milk production, skin pigmentation, body growth, mineral regulation, and blood glucose levels.
Because the pituitary is constantly receiving information from the hypothalamus of the brain, many kinds of sensory stimuli to the body can affect the ing of the endocrine system. One example is the way in which the nervous system and endocrine system interact to influence the menstrual cycle. At least three different hormones are involved in the cycle of changes that affect the ovary and the lining of the uterus. It is well documented that stress caused by tension or worry can interfere with the normal cycle of hormones and delay or stop menstrual cycles. In addition, young women living in groups, such as in college dormitories, often find that their menstrual cycles become synchronized. Athough the exact mechanism involved in this phenomenon is unknown, it is suspected that imput from the nervous system causes this synchronization. (ordors and sympathetic feelings have been suggested as causes.)
In many animals, the changing length of the day causes hormonal changes related to reproduction. In the spring, birds respond to lengthening days and begin to produce hormones that gear up their reproductive systems for the summer breeding season. The pineal body, a portion of the brain, serves as the receivers of light stimuli and changes the amounts of hormones secreted by the pituitary, resulting in changes in the levels of reproductive hormones. These hormonal changes modify the behavior of birds. courtship, mating, and nest-building behaviors increase in intensity. Therefore, it appears that a change in hormones level is affecting the behavior of the animal; the endocrine system is influenced the nervous system.
It has been known for centuries that changes in the levels of sex hormone cause changes in the behavior of animals. Castration (remove of the testes) of male domesticated animals, such as cattle, horses, and pigs, is sometimes done in part to reduce their aggressive behavior and make them easier to control. In humans, the use of anabolic steroids to increase muscle mass is known to cause behavioral changes and "moodiness".
Although we still tend to think of the nervous and endocrine systems as being separate and different, it is becoming clear that they are interconnected. As we learn more about the molecules produced in the brain, it is becoming clear that the brain produces many molecules that act as hormones. some of these molecules affect adjacent parts of the brain, others affect the pituitary, and still others may have effects on more distant organs. In any case, these are two systems cooperate to bring about appropriate response to environmental challenges.The nervous system is specialized for receiving and sending short-term messages, whereas activities that require long-term, growth-related actions are handled by endorine system.
20.2 sensory input
The activities of the nervous and endocrine system are often response to some kind of input received from the sense organs. Sense organs of various types are located throughout the body. Many of them are located on the surface, where environmental changes can be easily detected. Hearing, sight, and touch are good examples of such senses. Other sense organs are located within the body and indicate to the organism how its various parts are changing. For example, pain and pressure are often used to monitor internal conditions. The sense organs detect changes, but the brain is responsible for perception--the recognition that a stimulus has been received. Sensory abilities involve many different kinds of mechanisms, including chemical recognition, the detection of energy changes, and the monitoring of physical forces.
Chemical detection
All cells have receptors on their surfaces that can bind selectively to molecules they encounter. This binding process can cause changes in the cells in several ways. In some cells it causes depolarization. When this happens, the binding of molecules to the cell can stimulate neurons and cause messages to be sent to the central nervous system, informing it of some change in the surroundings. In other cases, a molecule binding to the cell surface may cause certain genes to be expressed, and the cell responds by changing the molecules it produces. This is typical of the way the endocrine system receives and delivers messages.
Most cells have specific binding sites for particular molecules. others, such as the taste buds on the tongue, appear to respond to classes of molecules. Traditionally we have distinguished four kinds of tastes: sweet, sour, salt, and bitter. However, recently, a fifth kind of taste, umami (meaty), has been identified that responds to the amino acid, glutamate, which is present in many kinds of foods and is added as a flavor enhancer (monosodium glutamate) to many kinds of foods.
The taste buds that give us the sour sensation respond to the presence of hydrogen ions. (acid foods taste sour). the hydrogen ions stimulate the cells in two ways: they enter the cell directly or they alter the normal movement of sodium and potassium ions across the cell membrane. In either case, the cell depolarizes and stimulates a nerve cell. sodium chloride stimulates the taste buds that give us the sensation of a salty taste by directly entering the cell, which causes the cell to polarize.
However, the sensations of sweetness, bitterness, and umami occur when molecules bind to specific surface receptors on the cell. Sweetness can be stimulated by many kinds of organic molecules, including sugars and artificial sweeters, and also by inorganic lead compounds. When a molecule binds to a sweetness receptor, a molecule is split and its splitting stimulates an enzyme that leads to the depolarization of the cell. The sweet taste of lead salts in old paints partly explains why children sometimes eat paint chips. Because the lead interferes with normal brain development, this behavior can have disastrous results. Many other kinds of compounds of diverse structures give the bitter sensation. The cells that respond to bitter sensations have a variety of receptor molecules on their surface. When a substance binds to one of the receptors, the cell depolarizes. In the case of umami, It is the glutamate molecule that binds to receptors on the cells of the taste buds.
Each of these tastes has a significance from a evolutionary piont of view. Carbohydrate are a major food source and many carbohydrates tastes sweet, therefore, this sense would be useful in identifying foods that have high food . Similarly, proteins and salts are necessary in the diet. Therefore, being able to indentify these itemss in potential foods would be extremely valuable. This is particularly ture for salt, which must often be obtained from meniral sources. On the other hand, bitter and sour materials are often harmful. Many plants produce toxic materials that are bitter tasting and acids are often the result of bacterial decomposition (spoiling) of foods. being able to identify bitter and sour would allow organisms to avoid foods that would be harmful.
It is also important to understand that much of what we often refer to as taste involves such things as temperature, texture, and smell. Cold coffee has a different taste than hot coffee even though they are chemically the same. Lumpy, cooked cereal and smooth cereal have different tastes. If you are unable to smell food, it doesn't taste as it should, which is why you sometimes lose your appetite when you have a stuffy nose. We still have much to learn about how the tongue detects chemicals and the role other associated senses play in modifying taste.
The other major chemcial sense, the sense of smell, is much more versatile; it can detect thousands of different molecules at very low concentrations. The cells that makeup the olfactory epithelium, the cells that line the nasal cavtiy and respond to smells, apparently bind molecules to receptors on their surfaces. Exactly how this can account for the large number of recognizably different odors is unknown, but the receptor cells are extremely sensitive. In some cases a single molecule of a substance is sufficient to cause a receptor cell to send a message to the brain, where the sensation of odor is perceived. These sensory cells also fatigue rapidly. you have probably noticed that when you first walk into a room, specific odors are readily detected, but after a few minutes you are unable to detect them. Most perfumes and aftershaves are undetectable after 15 minutes of continuous stimulation.
Many internal sense organs also respond to specific molecules. for example, the brain and aorta contain cells that respond to concentrations of hydrogen ions, carbon dioxide, and oxygen in the blood. Remember, too, that the endocrine system relies on the detection of specific messenger molecules to trigger its activities.
Light detection
The eyes primarily respond to changes in the flow of light energy. The structure of the eye is designed to focus light on a light-sensitive layer of the back of the eye known as the retina of the eye. The cells called rods respond to a broad range of wavelengths of light and are responsible for black-and-white vision. Because rods are very sensitive to light, they are particularly useful in dim light. Rods are located over most of the retina surface except for the area of most acute vision known as the fovea centralis. The other receptor cells, called cones, are found throughout the retina but are particularly concentrated in the fovea centralis. Cones are not as sensitive to light. This combination of receptors gives us the ability to detect color when light levels are high, but we rely on black-and-white vision at night. There are three types of cones: one type responds best to red light, another responds best to green light, and the third responds best to blue light. Stimulation of various combinations of these three kinds of cones allows us to detect different shades of color.
Rods and the three different kinds of cones each contain a pigment that decomposes when struck by light of the proper wavelength and sufficient strength. The pigment found in rods is called rhodopsin. This change in the structure of rhodopsin causes the rod to polarize. Cone cells have a similar mechanism of action, and each of the three kinds of cones has a different pigment. Because rods and cones synapse with neurons, they stimulate a neuron when depolarized and cause a message to be sent to the brain. Thus the pattern of color and light intensity recorded on the retina is detected by rods and cones and converted into a series of nerve impulses that are received and interpreted by the brain.
Sound detection
The ears respond to changes in sound waves. Sound is produced by the vibration of molecules. Consequently, the ears are detecting changes in the quantity of energy and the quality of the sound wave. Sound has several characteristics . Loudness, or volume, is a measure of the intensity of sound energy that arrives at the ear. Very loud sounds will literally wibrate your body, and can cause hearing lose if they are too intense. Pitch is a quality of sound that is determined by the frequency of the sound vibrations. High-pitched sounds have short wavelengths; low-pitched sounds have long wavelengths.
Figure 20.14 shows the anatomy of the ear. The sound that arrives at the ear is first funneled by the external ear to the tympanum, also known as the eardrum. The cone-shaped nature of the external ear focuses sound on the tympanum and causes it to vibrate at the same frequency as the sound waves reaching it. Attached to the tympanum are three tiny bones known as the malleus (hammer), incus (anvil), and stapes (stirrup). The malleus is attached to the tympanum, the incus is attached to the malleus and stapes, and the stapes is attached to a small, membrane-covered opening called the oval window in a snail-shaped structure known as the cochlea. The vibration of the tympanum causes the tiny bones to vibrate, and they in turn cause a corresponding vibration in the membrane of the oval
The cochlea of the ear is the structure that detects sound and consists of a snail-shaped set of fluid-filled tubes. When the oval window vibrates, the fluid in the cochlea begins to move, causing a membrane in the cochlea, called the basilar membrane, to vibrate. High-pitched , short-wavelength sounds cause the basilar membrane to vibrate at the base of the cochlea near the oval Low-pitched, long-wavelength sounds vibrate the basilar membrane far from the oval Loud sounds cause the basilar membrane to vibrate more vigoriously than do faint sounds. Cells on this membrane depolarize when they are stimulated by its vibrations. Because they synapse with neurons, messages can be sent to the brain.
Because sounds of different wavelengths stimulate different portions of the cochlea, the brain is able to determine the pitch of a sound. Most sounds consists of a mixture of pitches that are heard. Louder sounds stimulate the membrane more forcefully, causing the sensory cells in the cochlea to send more nerve impulses per second. Thus the brain is able to perceive the loudness of various sounds as well as the pitch.
Associated with the cochlea are two fluid-filled chambers and a set of fluid-filled tubes called the semicircular canals. These structures are not involved in hearing but are involved in maintaining balance and posture. In the wall of these chambers and canals are cells similar to those found on the basilar membrane. These cells are stimulated by movements of the head and by the position of the head with respect to the force of gravity. The constantly changing position of the head results in sensory input that is important in maintaining balance.
Touch
What we normally call the sense of touch consists of vaviety of different kinds of input. Some receptors respond to pressure, others to temperature, and others, which we call pain receptors, usually respond to cell damage. When these receptors are appropriately stimulated, they send a message to the brain. Because receptors are stimulated in particular parts of the body, the brain is able to localize the sensation. However, not all parts of the body are equally supplied with these receptors. The tips of the fingers, lips, and external genitals have the highest density of these nerve endings, whereas the back, legs, and arms have far fewer receptors.
Some receptors , such as pain and pressure receptors, are important in allowing us to monitor our internal activities. many pains generated by the internal organs are often perceived as if they were somewhere else. For example, the pain associated with heart attack is often perceived to be in the left arm. Pressure receptors in joints and muscles are important to providing information about the degree of stress being placed on a portion of the body. this is also important information to send back to the brain so that adjustments can be made in movements to maintain posture. If you have ever had your foot "go to sleep" because the nerve stopped ing, you have experienced what is like to lost this constant input of nerve messages from the pressure sensors that assist in guiding the movements you make. your movements become uncoordinated until the nerve returns to normal.
20.3 output coordination
The nerve system and endocrine system cause changes in several ways. Both systems can stimulate muscles to contract and glands to secrete. The endocrine system is also able to change the bolism of cells and regulate the growth of tissues. The nervous system acts upon two kinds of organs muscles and glands. the actions of muscles and glands are simple and direct: muscles contract and glands recrete.
Muscles
The ability to move is one of the fundamental characteristics of animals. Through the coordinated contraction of many muscles, the intricate, precise movements of a dancer, basketball plaer, or writer are accomplished. It is important to recognize that muscles can pull only by contracting; they are unable to push by lengthening. The work of any muscle is done during its contraction. Relaxation is the passive state of the muscle. There must always be some force available that will stretch a muscle after it has stopped contracting and relaxes. Therefore, the muscles that control the movements of the skeleton are present in antagonistic sets--foy every muscle's action there is another muscle that has the opposite action. For example, the biceps muscle causes the arm to flex as the muscle shortens. The contraction of its antagonist, the triceps muscle, causes the arm to extend (straighten) and at the same time stretchs the relaxed biceps muscle.
what we recognize as a muscle is composed of many muscle cells, which are in turn make up of myofibrils that are composed of two kinds of myofilaments. The mechanism by which muscle contracts is well understood and involves the movement of protein filaments past one another as ATP is utilized. ATP (adnosine triphosphate) is the primary molecule used by cells for their immediate energy needs. The filements in muscle cells are of two types, arranged in a particular pattern. Thin filements composed of the proteins actin, tropomyosin, and troponin alternate with thick filaments composed primarily of protein known as myosin.
The myosin molecules have a shape similar to a golf club. the head of the club-shaped molecule sticks out from the thich filament and can combine with the action of the thin filament. However, the troponin and tropomyosin protein associated with the action cover the actin in such a way that myosin cannot bind with it. when actin is uncovered, myosin can bind to it and contraction of a muscle will occur when ATP is utilized.
The process of muscle-cell contraction involves several steps. When a nerve impulse arrives at a muscle cell, its cells depolarize, calcium ions contained within membranes are released among the action and myosin filaments. The calsium ions combine with the tropnin molecules, causing the tropsin-tropomyosin complex to expose actin so that it can bind with myosin. While the actin and myosin molecules are attached, the head of the myosin molecule can flex as ATP is used and the actin molecule is pulled past the myosin molecule. Thus a tiny section of the muscle cell shortens. When one of our muscles contracts, thousands of such interactions take place within a tiny portion of a muscle cell, and many cells within a muscle all contract at the same time.
There are three major types of muscle: skeletal, smooth, and cardiac. these differ from another in several ways. Skeletal muscle is voluntary muscle; it is under the control of the nervous system. The brain and spinal cord sends a message to skeletal muscles, and they contract to move the legs, fingers, and other parts of the body. This does not mean that you must make a consious decision every time you want to move a muscle. Many of the movements we make are learned initially but become automatic as a result of practice. for example, walking, swimming, or riding a bicycle required a great amount of practice originally, But now you probably perform these movements without thinking about them. They are,however, still considered voluntary actions.
Skeletal muscles are constantly bombarded with nerve impulses that result in repeated contractions of different strength. Many neurons end in each muscle, and each one stimulates a specific set of muscle sells called a motor unit. Because each muscle consists of many motor units, it is possible to have a wide variety of intensities of contration within one muscle organ. This allows a single set of muscles to serve a wide variety of s. For example, the same muscles of the arms and shoulders that are used to play a piano can be used in other combinations to tightly grip and throw a baseball. If the nerves going to a muslce are destroyed, the muscle becomes paralyzed and begins to shrink. Regular nervous stimulation of skeletal muscle is necessary for muscle to maintain size and strength.any kind of prolonged inactivity leads to the degeneration of muscles known as astrophy. Muscle maintenance is one of the primary s of physical therapy and a benefit of regular exercise.
Skeletal muscles are able to contract quickly, but they cannot remain contracted for long periods. Even when we contract a muscle for a minute or so, the muscle is constantly shifting the individual motor units within it that are in a state of contraction. A single skeletal muscle cell cannot stay in a contracted state.
Smooth muscle make up the walls of muscular internal organs, such as the gut, blood vessels, and reproductive organs. They have the property of contracting as a response to being stretched. Because much of the digestive system is being stretched constantly, the responsive contractions contribute to the normal rhythmic movements associated with the digestive system. These are involuntary muscles; they can contract on their own without receiving direct messages from the nervous system. This can be demonstrated by removing portions of the gut or uterus from experimental animals. When these muscular organs are kept moist with special solution, they go through cycles of contraction without any possible stimulation from neurons. However, they do receive nervous stimulation, which can modify the rate and strength of their contraction. This kind of muscle also has the ability to stay contracted for long periods without fatigue. Many kinds of smooth muscle, such as the muscle of the uterus, also respond to the presence of hormones. Specially , the hormone oxytocin, which is released from the posterior pituitary, causes strong contractions of the uterus during labor and birth. similarly, several hormones produced by the duodenum influence certain muscles of the digestive system to either contract or relax.
Cardiac muscle is the muscle that makes up the heart. It has the ability to contract rapidly like skeletal muscle, but does not require nervous stimulation to do so. Nervous stimulation can , however, cause the heart to speed or slow its rate of contraction. Hormones, such as epinephrine and norepinephrine, also influence the heart by increasing its rate and strength of contraction. Cardiac muscle also has the characteristic of being unable to stay contracted. It will contract quickly but must have a short period of relaxation before it will be able to contract a second time. This makes sense in light of its continuous, rhythmic, pumping . Table 20.1 summarizes the differenes among skeletal, smooth, and cardiac muscles.
Glands
The glands of the body are of two different kinds. Those that secrete into the bloodstream are called endocrine glands. We have already talked about several of these: the pituitary, thyroid,ovary, and testis are examples. the exocrine glands are those that secrete to the surface of the body or into one of the tubular organs of the body, such as the gut or reproductive tract. Examples are the salivary glands, intestinal mucus glands, and sweat glands. Some of these glands, such as salivary glands and sweat glands, are under nervous control. When stimulated by the nervous system, they secrete their contents.
The Russian physiologist Ivan Petrovich Pavlov showed that salivary glands were under the control of the nervous system when he trained dogs to salivate in response to hearing a bell. You may recall from chapter 17 that, initially, the animals were presented with food at the same time the bell was rung. Eventually they would salivate when the bell was rung even if food was not present. This demonstrated that saliva release was under the control of the central nervous system.
Many other exocrine glands are under hormonal control. Many of the digestive enzymes of the stomach and intestine are secreted in response to local hormone produced in the gut. These are circulated through the blood to the digestive glands, which respond by secreting the appropriate digestive enzymes and other molecules.
Growth responses
the hormones produced by the endocrine system can have a variety of effects. As mentioned earlier, hormones can stimulate smooth muscle to contract and can influence the contraction of cardiac muscle as well. Many kinds of glands, both endocrine and exocrine, are caused to secrete as result of a hormonal stimulus. However, the endocrine system has one major effect that is not equaled by the nervous system: Hormones regulate growth. Several examples of the many kinds of long-term growth changes that are caused by the endocrine system were given earlier in the chapter. Growth-stimulating hormone is produced over a period of years to bring about the increase in size of most of the structures of the body. A low level of this hormone results in a person with small body size. It is important to recognize that the amount of growth-stimulating hormone present varies from time to time. It is present in fairly high amounts throughout childhood and results in steady growth. It also appears to be present at higher levels at certain times, resulting in growth spurts. Finally, as adulthood is reached, the level of this hormone falls, and growth stops.
Similarly, testosterone produced during adolescence influences the growth of bone and muscle to provide men with larger, more muscular bodies than those of women. In addition, there is growth of penis, growth of the larynx, and increased growth of hair on the face and body. The primary female hormone, estrogen, causes growth of reproductive organs and development of breast tissue. It is also ivolves, along with other hormones in the cyclic growth and sloughing of the wall of the uterus.
