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Chapter 28 The Nervous system PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko 1 Nervous systems receive sensory input, interpret it, and send out appropriate commands Nervous systems are the most intricately organized data processing systems Brain contains--100 billion neurons, Nerve cells that transmit signals from one location in the body to another. A neuron consists of a cell body, containing the nucleus and other cell organelles, and long, thin extensions that convey signals. Each neuron may communicate with thousands of others, forming networks that enable us to learn remember, perceive our surroundings, and move. Nervous systems have two main anatomical divisions. 1. central nervous system (CNS), consists of the brain and, in vertebrates, the spinal cord. 2. peripheral nervous system (PNS),is made up mostly of nerves that carry signals into and out of the CNS. A nerve is a communication line consisting of a bundle of neurons tightly wrapped in connective tissue. In addition to nerves, the PNS also has ganglia (singular, ganglion), clusters of neuron cell bodies. © 2012 Pearson Education, Inc. A nervous system has three interconnected functions 1. Sensory input is the conduction of signals from sensory receptors, such as lightdetecting cells of the eye, to the CNS. 2. Integration is the analysis and interpretation of the sensory signals and the formulation of appropriate responses. 3. Motor output is the conduction of signals from the integration centers to effector cells, such as muscle cells or gland cells, which perform the body’s responses. The integration of sensory input and motor output is not usually rigid and linear, but involves the continuous background activity symbolized by the circular arrow © 2012 Pearson Education, Inc. Three functional types of neurons correspond to a nervous system’s three main functions: 1. Sensory neurons convey signals from sensory receptors into the CNS. 2. Interneurons are located entirely within the CNS. They integrate data and then relay appropriate signals to other interneurons or to motor neurons. 3. Motor neurons convey signals from the CNS to effector cells. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 2 Neurons are the functional units of nervous systems Most of a neuron’s organelles, including its nucleus, are located in the in the cell body. Arising from the cell body are two types of extensions: numerous dendrites and a single axon. 1. Dendrites (from the Greek dendron, tree) are highly branched extensions that receive signals from other neurons and convey this information toward the cell body. Dendrites are often short. 2. The axon is typically a much longer extension that transmits signals to other cells, which may be other neurons or effector cells. Some axons, such as the ones that reach from your spinal cord to muscle ells in your feet, can be over a meter long. 3. The axon ends in a cluster of branches. A typical axon has hundreds or thousands of these branches, each with a synaptic terminal at the very end. 4. The junction between a synaptic terminal and another cell is called a synapse. © 2012 Pearson Education, Inc. To function normally, neurons of all vertebrates and most invertebrates require supporting cells called glia. Depending on the type, glia may nourish neurons, insulate the axons of neurons, or help maintain homeostasis of the extracellular fluid surrounding neurons. In the mammalian brain, glia outnumber neurons by as many as 50 to 1. The glial cell is called a Schwann cell, which is found in the PNS. (Analogous cells are found in the CNS.) In many vertebrates, axons that convey signals rapidly are enclosed along most of their length by a thick insulating material, analogous to the plastic insulation that covers electrical wires. This insulating material, called the myelin sheath, resembles a chain of oblong beads. Each bead is actually a Schwann cell, and the myelin sheath is essentially a chain of Schwann cells, each wrapped many times around the axon. © 2012 Pearson Education, Inc. The gaps between Schwann cells are called nodes of Ranvier,and they are the only points along the axon that require nerve signals to be regenerated, which is a time-consuming process. The myelin sheath insulates the axon, preserving the signal and allowing it to propagate quickly. Thus, a nerve signal travels along a myelinated axon will be much faster In the human nervous system, signals can travel along a myelinated axon about 150 m/sec (over 330 miles per hour), which means that a command from your brain can make your fingers move in just a few milliseconds. Without myelin sheaths, the signals would be over 10 times slower. The debilitating autoimmune disease multiple sclerosis (MS) demonstrates the importance of myelin. MS leads to a gradual destruction of myelin sheaths by the individual’s own immune system. The result is a progressive loss of signal conduction, muscle control, and brain function. © 2012 Pearson Education, Inc. 28.11 Vertebrate nervous systems are highly centralized Vertebrate nervous systems are diverse in structure and level of sophistication. The nervous system of dolphins and humans are much more complex structurally than those of frogs or fishes --- more powerful integrators. All vertebrate nervous systems have fundamental similarities-- distinct central and peripheral elements and are highly centralized. The brain and spinal cord make up the CNS, while the PNS comprises the rest of the nervous system © 2012 Pearson Education, Inc. The spinal cord, a jellylike bundle of nerve fibers that runs lengthwise inside the spine---conveys information to and from the brain and integrates simple responses to certain stimuli © 2012 Pearson Education, Inc. The master control center of the nervous system, the brain, includes 1. homeostatic centers that keep the body functioning smoothly; 2. sensory centers that integrate data from the sense organs; and (in humans, at least) 3. centers of emotion, 4. intellect 5. Sends motor commands to muscles blood-brain barrier A vast network of blood vessels services the CNS. Brain capillaries are more selective than those elsewhere in the body They allow essential nutrients and oxygen to pass freely into the brain, but keep out many chemicals, such as metabolic wastes This selective mechanism, called the blood-brain barrier, maintains a stable chemical environment for the brain. © 2012 Pearson Education, Inc. Fluid-filled spaces in the brain are called ventricles and are continuous with the narrow central canal of the spinal cord These cavities are filled with cerebrospinal fluid, which is formed within the brain by the filtration of blood. Circulating slowly through the central canal and ventricles (and then draining back into veins), the cerebrospinal fluid cushions the CNS and assists in supplying nutrients and hormones and removing wastes. © 2012 Pearson Education, Inc. Meninges Protecting the brain and spinal cord are layers of connective tissue, called meninges. If the cerebrospinal fluid becomes infected by bacteria or viruses, the meninges may become inflamed, a condition called meningitis. In mammals, cerebrospinal fluid circulates between layers of the meninges, providing an additional protective cushion for the CNS. CNS has white matter and gray matter. White matter is composed mainly of axons (with their whitish myelin sheaths); gray matter consists mainly of nerve cell bodies and dendrites. The ganglia and nerves of the vertebrate PNS are a vast communication network. Cranial nerves originate in the brain and usually end in structures of the head and upper body (eyes, nose, and ears, for instance). Spinal nerves originate in the spinal cord and extend to parts of the body below the head. All spinal nerves and most cranial nerves contain sensory and motor neurons. © 2012 Pearson Education, Inc. 28.12 The peripheral nervous system of vertebrates is a functional hierarchy The PNS can be divided into two functional components: 1. The Motor Nervous System The motor system carries signals to and from skeletal muscles, mainly in response to external stimulli. The control of skeletal muscles can be voluntary, as when you raise your hand to ask a question, or involuntary, as in a knee-jerk reflex controlled by the spinal cord. 2. The Autonomic Nervous System. The autonomic nervous system regulates the internal environment by controlling smooth and cardiac muscles and the organs and glands of the digestive, cardiovascular, excretory, and endocrine systems. This control is generally involuntary. The autonomic nervous system is composed of three divisions: parasympathetic, sympathetic, and enteric. © 2012 Pearson Education, Inc. 1. The neurons of parasympathetic division primes the body for activities that gain and conserve energy for the body (“rest and digest”) These include stimulating the digestive organs, such as the salivary glands, stomach, and pancreas; decreasing the heart rate; and increasing glycogen production. 2. Neurons of the sympathetic division tend to have the opposite effect, preparing the body for intense, energy-consuming activities, such as fighting, fleeing, or competing in a strenuous game (the “fight-or-flight” response). The digestive organs are inhibited, the bronchi dilate so that more air can pass, The heart rate increases, the liver releases the energy compound glucose into the blood, and the adrenal glands secrete the hormones epinephrine and norepinephrine. © 2012 Pearson Education, Inc. Fight-or-flight and relaxation are opposite extremes, but body usually operates at intermediate levels, Most of sympathetic signals. The an organ’s level. organs receiving both and parasympathetic opposing signals adjust activity to a suitable In regulating some body functions, the two divisions complement rather than antagonize each other. For example, in regulating reproduction, erection is promoted by the parasympathetic division while ejaculation is promoted by the sympathetic division. Sympathetic and parasympathetic neurons emerge from different regions of the CNS Neurons of the parasympathetic system emerge from the brain and lower part of the spinal cord. Most parasympathetic neurons produce their effects by releasing the neurotransmitter acetylcholine at synapses within target organs. Neurons of the sympathetic system emerge from the middle regions of the spinal cord. Most sympathetic neurons release the neurotransmitter norepinephrine at target organs. It is convenient to divide the PNS into motor and autonomic components, it is important to realize that these two divisions cooperate to maintain homeostasis. In response to a drop in body temperature, for example, the brain signals the autonomic nervous system to constrict surface blood vessels, which reduces heat loss. At the same time, the brain also signals the motor nervous system to cause shivering, which increases heat production. © 2012 Pearson Education, Inc. Enteric division of the autonomic nervous system The enteric division of the autonomic nervous system consists of networks of neurons in the digestive tract, pancreas, and gallbladder. Within these organs, neurons of the enteric division control secretion as well as activity of the smooth muscles that produce peristalsis. Enteric division can function independently, it is normally regulated by the sympathetic and parasympathetic divisions. © 2012 Pearson Education, Inc. 28.13 The vertebrate brain develops from three anterior bulges of the neural tube One of the four distinguishing features of chordates is the embryonic development of the vertebrate nervous system from the dorsal hollow nerve cord During early embryonic development, three bilaterally symmetric bulges—the forebrain, midbrain, and hindbrain—appear at the anterior end of the neural tube © 2012 Pearson Education, Inc. During course of vertebrate evolution, the forebrain and hindbrain gradually became subdivided—both structurally and functionally— into regions that assume specific responsibilities. Another trend in brain evolution was the increasing integrative power of the forebrain. Evolution of the most complex vertebrate behavior paralleled the evolution of the cerebrum– the most sophisticated center of homeostatic control and integration. During the embryonic development of the human brain, the most profound changes occur in the region of the forebrain. Rapid, expansive growth of during the second and third months creates the cerebrum, which extends over and around much of the rest of the brain By the sixth month of development, foldings increase the surface area of the cerebrum. This extensively convoluted outer region is called the cerebral cortex. The cerebrum develops into two halves, called the left and right cerebral hemispheres. – The brains of humans and other primates are strongly oriented toward visual perceptions. Humans have the largest brain surface area, relative to body size, of all animals. 28.14 The structure of a living supercomputer: The human brain Composed of up to 100 billion intricately organized neurons, with a much larger number of supporting cells, the human brain is more powerful than the most sophisticated computer Hind brain Two sections of the hindbrain, the medulla oblongata and pons, and the midbrain make up a functional unit called the brainstem. Consisting of a stalk with cap-like swellings at the anterior end of the spinal cord, the brainstem is, evolutionarily one of the older parts of the vertebrate brain. The brainstem coordinates and filters the conduction of information from sensory and motor neurons to the higher brain regions. It also regulates sleep and arousal and helps coordinate body movements, such as walking. Another part of the hindbrain, the cerebellum, is a planning center for body movements. It also plays a role in learning, decision making, and remembering motor responses. The cerebellum receives sensory information about the position of joints and the length of muscles, as well as information from the auditory and visual systems. It also receives input concerning motor commands issued by the cerebrum.The cerebellum uses this information to coordinate movement and balance Hand eye coordination Fore brain The thalamus, the hypothalamus, and the cerebrum. The thalamus The thalamus contains most of the cell bodies of neurons that relay information to the cerebral cortex. The thalamus first sorts data into categories (all of the touch signals from a hand). It also suppresses some signals and enhances others. The thalamus then sends information on to the appropriate higher brain centers for further interpretation and integration. The hypothalamus Regulates body temperature, blood pressure, hunger, thirst, sex drive, and fight-or-flight responses, and it helps us experience emotions such as rage and pleasure. A“pleasure center” in the hypothalamus could also be called an addiction center, for it is strongly affected by certain addictive drugs, such as amphetamines and cocaine. These drugs increase the effects of norepinephrine and dopamine at synapses in the pleasure center, producing a short-term high, often followed by depression. Cocaine addiction may involve chemical changes in the pleasure center and elsewhere in the hypothalamus. A pair of hypothalmic structures called the suprachiasmatic nuclei function as an internal timekeeper, our biological clock. Receiving visual input from the eyes (light/dark cycles), the clock maintains our circadian rhythms—daily cycles of biological activity such as the sleep/wake cycle. The cerebrum It is the largest and most complex part of our brain, consists of right and left cerebral hemispheres. each responsible for the opposite side of the body. A thick band of nerve fibers called the corpus callosum facilitates communication between the hemispheres, enabling them to process information together. Under the corpus callosum, groups of neurons called the basal nuclei are important in motor coordination. If they are damaged, a person may be immobilized. Degeneration of the basal nuclei occurs in Parkinson’s disease 28.3 Nerve function depends on charge differences across neuron membranes Membrane Potential Like all cells, a resting neuron has potential energy, can be put to work sending signals from one part of the body to another. It exists as an electrical charge difference across the neuron’s plasma membrane: The inside of the cell is negatively charged relative to the outside as a result of unequal distribution of positively and negatively charged ions. The opposite charges tend to move toward each other, a membrane stores energy by holding opposite charges apart, like a battery. The strength (voltage) of a neuron’s stored energy can be measured with microelectrodes connected to a voltmeter. The voltage across the plasma membrane of a resting neuron is called the resting potential. A neuron’s resting potential is about –70 millivolts (mV) 28.3 Nerve function depends on charge differences across neuron membranes The resting potential exists because of differences in ionic composition of the fluids inside and outside the neuron The plasma membrane surrounding the neuron has protein channels and pumps that regulate the passage of inorganic ions A resting membrane has many open potassium (K) channels but only a few open sodium (Na) channels, allowing much more potassium than sodium to diffuse across the membrane--Na more concentrated outside the neuron than inside But K, which is more concentrated inside, can flow out through the many open K channels. As the positively charged potassium ions diffuse out, the inside of the neuron becomes less positive—that is, more negative—relative to outside Also helping maintain the resting potential are membrane proteins called sodium-potassium (Na-K) pumps. Using energy from ATP, these pumps actively transport Na out of the neuron and K in, thereby helping keep the concentration of Na low in the neuron and K high. 28.3 Nerve function depends on charge differences across neuron membranes The resting potential exists because of differences in ionic composition of the fluids inside and outside the neuron The plasma membrane surrounding the neuron has protein channels and pumps that regulate the passage of inorganic ions A resting membrane has many open potassium (K) channels but only a few open sodium (Na) channels, allowing much more potassium than sodium to diffuse across the membrane--Na more concentrated outside the neuron than inside But K, which is more concentrated inside, can flow out through the many open K channels. As the positively charged potassium ions diffuse out, the inside of the neuron becomes less positive—that is, more negative—relative to outside Also helping maintain the resting potential are membrane proteins called sodium-potassium (Na-K) pumps. Using energy from ATP, these pumps actively transport Na of the neuron and K in, thereby helping keep the concentration of Na low in the neuron and K high. 28.4 A nerve signal begins as a change in the membrane potential Stimulating a neuron’s plasma membrane can trigger the use of the membrane’s potential energy to generate a nerve signal. A stimulus is any factor that causes a nerve signal to be generated. Examples include light, sound, a tap on the knee, or a chemical signal from another neuron. The discovery of giant axons in squids (up to 1 mm in diameter) gave researchers their first chance to study how stimuli trigger signals in a living neuron. From microelectrode studies with squid neurons, British biologists A. L. Hodgkin and A. F. Huxley worked out the details of nerve signal transmission in the 1940s, earning a Nobel Prize for their findings. The graph in the middle of the figure traces the electrical changes that make up an action potential, a change in membrane voltage that transmits a nerve signal along an axon. The graph records electrical events over time (in milliseconds) at a particular place on the membrane where a stimulus is applied. The stimulus is applied. If it is strong enough, the voltage rises to what is called the threshold (–50 mV, in this case). The difference between the threshold and the resting potential is the minimum change in the membrane’s voltage that must occur to generate the action potential(+20 mV, in this case). Once the threshold is reached, the action potential is triggered. The membrane polarity reverses abruptly, with the interior of the cell becoming positive with respect to the outside. The membrane then rapidly repolarizes as the voltage drops back down undershoots the resting potential, Finally returns to it In a typical mammalian neuron, this entire process takes just a few milliseconds, meaning that a neuron can produce hundreds of nerve signals per second. 28.5 The action potential propagates itself along the axon An action potential is a localized electrical event—a rapid change from the resting potential at a specific place along the neuron. A nerve signal starts out as an action potential generated in the axon, typically where the axon meets the cell body. To function as a long-distance signal, this local event must be passed along the axon from the cell body to the synaptic terminals. It does so by regenerating itself along the axon 1. When this region of the axon (blue) has its Na channels open, Na rushes inward, and an action potential is generated--corresponds to the upswing of the curve (step 2) 2. Soon, the K channels in that same region open, allowing K to diffuse out of the axon; at this time, its Na channels are closed and inactivated at that point on the axon---the downswing of the action potential 3. A short time later, we would see no signs of an action potential at this (far-left) spot because the axon membrane here has restored itself and returned to its resting potential. In step 1 of the figure, the blue arrows pointing sideways within the axon indicate local spreading of the electrical changes caused by the inflowing Na associated with the first action potential. These changes are large enough to reach threshold in the neighboring regions triggering the opening of Na channels. As a result, a second action potential is generated, as indicated by the blue region in step 2. In the same way, a third action potential is generated in step 3, and each action potential generates another all the way down the axon. The net result is the movement of a nerve impulse from the cell body to the synaptic terminals. As the blue arrows indicate, local electrical changes do spread in both directions in the axon. However, these changes cannot open Na channels and generate an action potential when the Na channels are inactivated Thus, an action potential cannot be generated in the regions where K is leaving the axon (green in the figure) and Na channels are still inactivated. Consequently, the inward flow of Na that depolarizes the axon membrane ahead of the action potential cannot produce another action potential behind it. Once an action potential starts where the cell body and axon meet, it moves along the axon in only one direction: toward the synaptic terminals. How, do action potentials relay different intensities of information (such as a loud sound versus a soft sound) to your central nervous system? It is the frequency of action potentials that changes with the intensity of stimuli. For example, in the neurons connecting the ear to the brain, loud sounds generate more action potentials per second than quiet sounds. 28.6 Neurons communicate at synapses If an action potential travels in one direction along an axon, what happens when the signal arrives at the end of the neuron? To continue conveying information, the signal must be passed to another cell. This occurs at a synapse, or relay point, between a synaptic terminal of a sending neuron and a receiving cell. The receiving cell can be another neuron or an effecter cell such as a muscle cell or endocrine cells Synapses come in two varieties: Electrical synapses --- In an electrical synapse, electrical current flows directly from a neuron to a receiving cell. The receiving cell is stimulated quickly and at the same frequency of action potentials as the sending neuron. Lobsters and many fishes can flip their tails with lightning speed because the neurons that carry signals for these movements communicate by fast electrical synapses. In the human body, electrical synapses are found in the heart and digestive tract, where nerve signals maintain steady, rhythmic muscle contractions. Chemical synapses-- when an action potential reaches a chemical synapse, it stops there at a narrow gap, called the synaptic cleft, separating a synaptic terminal of the sending (presynaptic) cell from the receiving (postsynaptic) cell. The cleft is very narrow—only about 50 nm, about 1/1,000th the width of a human hair— but it prevents the action potential from spreading directly to the receiving cell. The action potential (an electrical signal) is first converted to a chemical signal consisting of molecules of neurotransmitter. The chemical signal may then generate an action potential in the receiving cell. Molecules of the neurotransmitter are in membrane-enclosed sacs called synaptic vesicles in the sending neuron’s synaptic terminals. An action potential arrives at the synaptic terminal. The action potential causes some synaptic vesicles to fuse with the plasma membrane of the sending cell The fused vesicles release their neurotransmitter molecules by exocytosis into the synaptic cleft, and the neurotransmitter rapidly diffuses across the cleft. The released neurotransmitter binds to complementary receptors on ion channel proteins in the receiving cell’s plasma membrane. The neurotransmitter is broken down by an enzyme or is transported back into the signaling cell, and the ion channels close. 28.7 Chemical synapses enable complex information to be processed • A neuron may interact with many others-----a neuron may receive information via neurotransmitters from hundreds of other neurons connecting at thousands of synaptic terminals (red and green in the drawing). The inputs can be highly varied because each sending neuron may secrete a different quantity or kind of neurotransmitter. The binding of a neurotransmitter to a receptor may open ion channels in the receiving cell’s plasma membrane or trigger a signal transduction pathway that does so. A. The effect of the neurotransmitter depends on the kind of membrane channel it opens. Neurotransmitters that open Na channels, for instance, may trigger action potentials in the receiving cell. Such effects are referred to as excitatory (green in the drawing). B. Many neurotransmitters open membrane channels for ions that decrease the tendency to develop action potentials in the receiving cell—such as channels that admit Cl–or release K. These effects are called inhibitory (red). The effects of both excitatory and inhibitory signals can vary in magnitude. In general, the more neurotransmitter molecules that bind to receptors on the receiving cell and the closer the synapse is to the base of the receiving cell’s axon, the stronger the effect. The receiving neuron’s plasma membrane may receive signals—both excitatory and inhibitory—from many different sending neurons. If the excitatory signals are collectively strong enough to raise the membrane potential to threshold, an action potential will be generated in the receiving cell. 28.8 A variety of small molecules function as neurotransmitters The propagation (transfer) of nerve signals across chemical synapses depends on neurotransmitters. A variety of small molecules serve as neurotransmitters. Many neurotransmitters are small, nitrogen-containing organic molecules. 1. Acetylcholine, is important in the brain and at synapses between motor neurons and muscle cells. Depending on the kind of receptors on receiving cells, acetylcholine may be excitatory or inhibitory. Acetylcholine makes our skeletal muscles contract but slows the rate of contraction of cardiac muscles. Botulinum toxin (sold as Botox), made by the bacteria that cause botulism food poisoning, inhibits the release of acetylcholine. Botox injections disable the synapses that control certain facial muscles, eliminating wrinkles around the eyes or mouth. 2. Four other neurotransmitters—aspartate, glutamate, glycine, and GABA (gamma aminobutyric acid)—are amino acids. All are important in the central nervous system. Aspartate and glutamate act primarily at excitatory synapses, while glycine and GABA act at inhibitory synapses. 3. Biogenic amines are neurotransmitters derived from amino acids. It includes epinephrine, norepinephrine, serotonin, and dopamine. Biogenic amines are important neurotransmitters in the central nervous system. Serotonin and dopamine affect sleep, mood, attention, and learning. Imbalances of biogenic amines are associated with various disorders. For example, the degenerative illness Parkinson’s disease is associated with a lack of dopamine in the brain. Reduced levels of norepinephrine and serotonin seem to be linked with some types of depression. Some psychoactive drugs, including LSD and mescaline, apparently produce their hallucinatory effects by binding to serotonin and dopamine receptors in the brain. 4. Many neuropeptides, relatively short chains of amino acids, also serve as neurotransmitters. The endorphins are peptides that function as both neurotransmitters and hormones, decreasing our perception of pain during times of physical or emotional stress. Endorphins may be released in response to a wide variety of stimuli, including traumatic injury, muscle fatigue, and even eating very spicy foods. 5. Neurons also use some dissolved gases, notably nitric oxide (NO), as chemical signals. During sexual arousal in human males, certain neurons release NO into blood vessels in the erectile tissue of the penis, and the NO triggers an erection. Neurons produce NO molecules on demand, rather than storing them in synaptic vesicles. The dissolved gas diffuses into neighboring cells, produces a change, and is broken down—all within a few seconds. Chapter 30 How Animals Move PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko 30.4 Bones are complex living organs Bones are actually complex organs consisting of several kinds of moist, living tissues. The bone itself contains living cells that secrete a surrounding material, or matrix. Bone matrix consists of flexible fibers of the protein collagen with crystals of a mineral made of calcium and phosphate bonded to them. The collagen keeps the bone flexible and nonbrittle, while the hard mineral matrix resists compression. A sheet of fibrous connective tissue, covers most of the outside surface, helps to form new bone in the event of a fracture. A thin sheet of cartilage forms a cushion-like surface for movable joints, protecting the ends of bones as they glide against one another. The shaft of this long bone is made of compact bone, or dense structure. 30.4 Bones are complex living organs Bones are actually complex organs consisting of several kinds of moist, living tissues. The bone itself contains living cells that secrete a surrounding material, or matrix. Bone matrix consists of flexible fibers of the protein collagen with crystals of a mineral made of calcium and phosphate bonded to them. The collagen keeps the bone flexible and nonbrittle, while the hard mineral matrix resists compression. A sheet of fibrous connective tissue, covers most of the outside surface, helps to form new bone in the event of a fracture. A thin sheet of cartilage forms a cushion-like surface for movable joints, protecting the ends of bones as they glide against one another. The shaft of this long bone is made of compact bone, or dense structure. 30.4 Bones are complex living organs Bones are actually complex organs consisting of several kinds of moist, living tissues. The bone itself contains living cells that secrete a surrounding material, or matrix. Bone matrix consists of flexible fibers of the protein collagen with crystals of a mineral made of calcium and phosphate bonded to them. The collagen keeps the bone flexible and nonbrittle, while the hard mineral matrix resists compression. A sheet of fibrous connective tissue, covers most of the outside surface, helps to form new bone in the event of a fracture. A thin sheet of cartilage forms a cushion-like surface for movable joints, protecting the ends of bones as they glide against one another. The shaft of this long bone is made of compact bone, or dense structure. 30.4 Bones are complex living organs Bones are actually complex organs consisting of several kinds of moist, living tissues. The bone itself contains living cells that secrete a surrounding material, or matrix. Bone matrix consists of flexible fibers of the protein collagen with crystals of a mineral made of calcium and phosphate bonded to them. The collagen keeps the bone flexible and nonbrittle, while the hard mineral matrix resists compression. A sheet of fibrous connective tissue, covers most of the outside surface, helps to form new bone in the event of a fracture. A thin sheet of cartilage forms a cushion-like surface for movable joints, protecting the ends of bones as they glide against one another. The shaft of this long bone is made of compact bone, or dense structure. The compact bone surrounds a central cavity, contains yellow bone marrow, which is mostly stored fat brought into the bone by the blood. The ends, or heads, of the bone have an outer layer of compact bone and an inner layer of spongy bone. The cavities contain red bone marrow--specialized tissue that produces blood cells Like all living tissues, bone cells carry out metabolism. Blood vessels that extend through channels in the bone transport nutrients and regulatory hormones to its cells and remove waste materials. Nerves running parallel to the blood vessels help regulate the traffic of materials between the bone and the blood. 30.6 Joints permit different types of movement Much of the versatility of the vertebrate skeleton comes from its joints. Bands of strong fibrous connective tissue called ligaments hold together the bones of movable joints. 1. Ball-and-socket joints, such as are found where the humerus joins the pectoral girdle, enable us to rotate our arms and legs and move them in several planes. A ball-and-socket joint also joins the femur to the pelvic girdle. 2. Hinge joints permit movement in a single plane, Our elbows and knees are hinge joints. Hinge joints are especially vulnerable to injury in sports like volleyball, basketball, and tennis that demand quickturns, which can twist the joint sideways. 3. A pivot joint enables us to rotate the forearm at the elbow. A pivot joint between the first and second cervical vertebrae allows movement of the head from side to side. 30.7 The skeleton and muscles interact in movement Muscles are connected to bones by tendons. For example, the upper ends of the biceps and triceps muscles are anchored (attached) to bones in the shoulder. The lower ends of these muscles are attached to bones in the forearm. The action of a muscle is always to contract, or shorten. A muscle pulls the bone to which it is attached—it can only move the bone in one direction. A different muscle is needed to reverse the action. Thus, back-and-forth movement of body parts involves antagonists, a pair of muscles that can pull the same bone in opposite directions. While one antagonist contracts, the other relaxes. Examples, the biceps and triceps muscles and the quadriceps and hamstring muscles All animals—very small ones like ants and giant ones like elephants—have antagonistic pairs of muscles that apply opposite forces to move parts of their skeleton. 30.8 Each muscle cell has its own contractile apparatus The skeletal muscle system is a beautiful illustration of the relationship between structure and function. Each muscle in the body is made up of a hierarchy of smaller and smaller parallel strands, from the muscle itself down to the contractile protein molecules that produce body movements. Figure 30.8 shows the levels of organization of skeletal muscle. A muscle consists of many bundles of muscle fibers—roughly 250,000 in a typical human biceps muscle—oriented parallel to each other. Each muscle fiber is a single long, cylindrical cell that has many nuclei. Most of its volume is occupied by hundreds or thousands of myofibrils, discrete bundles of proteins that include the contractile proteins actin and myosin. A sarcomere is the region between two dark, narrow lines, called Z lines, in the myofibril. Each myofibril consists of a long series of sarcomeres. Functionally, the sarcomere is the contractile apparatus in a myofibril— the muscle fiber’s fundamental unit of action. The pattern of horizontal stripes is the result of the alternating bands of thin filaments, composed primarily of actin molecules, and thick filaments, which are made up of myosin molecules. The Z lines consist of proteins that connect adjacent thin filaments. The light band surrounding each Z line contains only thin filaments. 30.9 A muscle contracts when thin filaments slide along thick filaments According to the sliding-filament model of muscle contraction, a sarcomere contracts (shortens) when its thin filaments slide along its thick filaments. In the contracting sarcomere, the Z lines and the thin filaments have moved closer together. In the fully contracted sacromere, the thin filaments overlap in the middle. Contraction shortens the sarcomere without changing the lengths of the thick and thin filaments, muscle can shorten about 35% of its resting length. 30.9 A muscle contracts when thin filaments slide along thick filaments According to the sliding-filament model of muscle contraction, a sarcomere contracts (shortens) when its thin filaments slide along its thick filaments. In the contracting sarcomere, the Z lines and the thin filaments have moved closer together. In the fully contracted sacromere, the thin filaments overlap in the middle. Contraction shortens the sarcomere without changing the lengths of the thick and thin filaments, muscle can shorten about 35% of its resting length. 30.9 A muscle contracts when thin filaments slide along thick filaments According to the sliding-filament model of muscle contraction, a sarcomere contracts (shortens) when its thin filaments slide along its thick filaments. In the contracting sarcomere, the Z lines and the thin filaments have moved closer together. In the fully contracted sacromere, the thin filaments overlap in the middle. Contraction shortens the sarcomere without changing the lengths of the thick and thin filaments, muscle can shorten about 35% of its resting length. Myosin acts as the engine of movement. Each myosin molecule has a long “tail” region and a globular “head” region. The tails of the myosin molecules in a thick filament lie parallel to each other, with their heads sticking out to the side. Each head has two binding sites. One of the bindingsites matches a binding site on the actin molecules (subunits) of the thin filament ATP binds at the other site, which is also capable of hydrolyzing the ATP to release its energy—the energy that powers muscle contraction. Each myosin head pivots back and forth in a limited arc as it changes shape from a lowenergy configuration to high energy configuration and back again. During these changes, the myosin head swings toward the thin filament, binds with an actin molecule, and drags the thin filament through the remainder of its arc. The myosin head then releases the actin molecule and returns to its starting position to repeat the same motion with a different actin molecule. key events of this process of sacromere contraction 1. The myosin head binds a molecule of ATP, at low-energy position. 1. Myosin hydrolyzes the ATP to ADP and phosphate (P), releasing energy that extends the myosin head toward the thin filament. The myosin head extends further, and its other binding site latches on to the binding site of an actin. The result is a connection between the two filaments—a cross-bridge. 4. ADP and Phosphate are released, and the myosin head pivots back to its low-energy configuration. This action, called the power stroke, pulls the thin filament toward the center of the sarcomere. 5. The cross-bridge remains intact until another ATP molecule binds to the myosin head, and the whole process repeats. On the next power stroke, the myosin head attaches to an actin molecule ahead of the previous one on the thin filament , This sequence—detach, extend, attach, pull, detach—occurs again and again in a contracting muscle. a typical thick filament has about 350 heads, each of which can bind and unbind to a thin filament about five times per second. The combined action of hundreds of myosin heads on each thick filament ratchets the thin filament toward the center of the sarcomere, as long as sufficient ATP is present, the process continues until the muscle is fully contracted or until the signal to contract stops. 30.10 Motor neurons stimulate muscle contraction What prevents muscles from contracting whenever ATP is present? Signals from the central nervous system, conveyed (transfer) by motor neurons are required to initiate and sustain muscle contraction. A motor neuron sends out an action potential, its synaptic terminals release the neurotransmitter acetylcholine, which diffuses across the synapse to the plasma membrane of the muscle fiber the plasma membrane of a muscle fiber is electrically excitable—it can propagate action potentials----- and the plasma membrane extends deep into the interior of the muscle fiber via (by) infoldings called transverse (T) tubules. The T tubules are in close contact with the endoplasmic reticulum The action potential causes channels in the ER to open, releasing calcium ions into the cytoplasmic fluid How does the endoplasmic reticulum help regulate muscle contraction? In a resting muscle fiber (not transferring signals), the regulatory proteins tropomyosin and troponin block the myosin binding sites on the actin molecules. The muscle fiber cannot contract while these sites are blocked. When Ca binds to troponin, the tropomyosin moves away from the myosin binding sites, allowing contraction When motor neurons stop sending action potentials to the muscle fibers, the ER pumps Ca back out of the cytoplasmic fluid, and binding sites on the actin molecules are again blocked, thus the sarcomeres stop contracting, and the muscle relaxes. Motor unit A large muscle such as the calf muscle is composed of roughly a million muscle fibers. About 500 motor neurons run to the calf muscle. Each motor neuron has axons that branch out to synapse with many muscle fibers distributed throughout the muscle. An action potential from a single motor neuron in the calf causes the simultaneous contraction of roughly 2,000 muscle fibers. A motor neuron and all the muscle fibers it controls is called a motor unit. The organization of individual neurons and muscle cells into motor units is the key to the action of whole muscles. The amount of force can be increased several times in biceps and triceps, when additional motor units are activated ---resulting in more signals for forceful contraction from CNS In muscles requiring precise control, such as those controlling eye movements, a motor neuron may control only a single muscle fiber.