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Chapter 48

BIOL 1030 Chapter 48: Chapter 48 Nervous Systems

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University of Manitoba
Biological Sciences
BIOL 1030
Scott Kevin

Chapter 48 Nervous Systems Lecture Outline Overview: Command and Control Center • The human brain contains an estimated 1011 (100 billion) neurons. • Each neuron may communicate with thousands of other neurons in complex information-processing circuits. • Recently developed technologies can record brain activity from outside the skull. • One technique is functional magnetic resonance imaging (fMRI), which reconstructs a 3-D map of the subject’s brain activity. • The results of brain imaging and other research methods show that groups of neurons function in specialized circuits dedicated to different tasks. The ability of cells to respond to the environment has evolved over billions of years. • The ability to sense and react originated billions of years ago with prokaryotes that could detect changes in their environment and respond in ways that enhanced their survival and reproductive success. • Such cells could locate food sources by chemotaxis. • Later, modification of this simple process provided multicellular organisms with a mechanism for communication between cells of the body. • By the time of the Cambrian explosion, systems of neurons that allowed animals to sense and move rapidly had evolved in essentially modern form. Concept 48.1 Nervous systems consist of circuits of neurons and supporting cells Nervous systems show diverse patterns of organization. • All animals except sponges have some type of nervous system. • What distinguishes nervous systems of different animal groups is how the neurons are organized into circuits. • Cnidarians have radially symmetrical bodies organized around a gastrovascular cavity. • In hydras, neurons controlling the contraction and expansion of the gastrovascular cavity are arranged in diffuse nerve nets. • The nervous systems of more complex animals contain nerve nets, as well as nerves, which are bundles of fiberlike extensions of neurons. • With cephalization come more complex nervous systems. • Neurons are clustered in a brain near the anterior end in animals with elongated, bilaterally symmetrical bodies. • In simple cephalized animals such as the planarian, a small brain and longitudinal nerve cords form a simple central nervous system (CNS). • In more complex invertebrates, such as annelids and arthropods, behavior is regulated by more complicated brains and ventral nerve cords containing segmentally arranged clusters of neurons called ganglia. • Nerves that connect the CNS with the rest of the animal’s body make up the peripheral nervous system (PNS). • The nervous systems of molluscs correlate with lifestyle. • Clams and chitons have little or no cephalization and simple sense organs. • Squids and octopuses have the most sophisticated nervous systems of any invertebrates, rivaling those of some vertebrates. • The large brain and image-forming eyes of cephalopods support an active, predatory lifestyle. Nervous systems consist of circuits of neurons and supporting cells. • In general, there are three stages in the processing of information by nervous systems: sensory input, integration, and motor output. • Sensory neurons transmit information from sensors that detect external stimuli (light, heat, touch) and internal conditions (blood pressure, muscle tension). • Sensory input is conveyed to the CNS, where interneurons integrate the sensory input. • Motor output leaves the CNS via motor neurons, which communicate with effector cells (muscle or endocrine cells). • Effector cells carry out the body’s response to a stimulus. • The stages of sensory input, integration, and motor output are easy to study in the simple nerve circuits that produce reflexes, the body’s automatic responses to stimuli. Networks of neurons with intricate connections form nervous systems. • The neuron is the structural and functional unit of the nervous system. • The neuron’s nucleus is located in the cell body. • Arising from the cell body are two types of extensions: numerous dendrites and a single axon. • Dendrites are highly branched extensions that receive signals from other neurons. • An axon is a longer extension that transmits signals to neurons or effector cells. • The axon joins the cell body at the axon hillock, where signals that travel down the axon are generated. • Many axons are enclosed in a myelin sheath. • Near its end, axons divide into several branches, each of which ends in a synaptic terminal. • The site of communication between a synaptic terminal and another cell is called a synapse. • At most synapses, information is passed from the transmitting neuron (the presynaptic cell) to the receiving cell (the postsynaptic cell) by means of chemical messengers called neurotransmitters. • Glia are supporting cells that are essential for the structural integrity of the nervous system and for the normal functioning of neurons. • There are several types of glia in the brain and spinal cord. • Astrocytes are found within the CNS. • They provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters. • Some astrocytes respond to activity in neighboring neurons by facilitating information transfer at those neuron’s synapses. • By inducing the formation of tight junctions between capillary cells, astrocytes help form the blood-brain barrier, which restricts the passage of substances into the CNS. • In an embryo, radial glia form tracks along which newly formed neurons migrate from the neural tube. • Both radial glia and astrocytes can also act as stem cells, generating neurons and other glia. • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) are glia that form myelin sheaths around the axons of vertebrate neurons. • These sheaths provide electrical insulation of the axon. • In multiple sclerosis, myelin sheaths gradually deteriorate, resulting in a progressive loss of body function due to the disruption of nerve signal transmission. Concept 48.2 Ion pumps and ion channels maintain the resting potential of a neuron Every cell has a voltage, or membrane potential, across its plasma membrane. • All cells have an electrical potential difference (voltage) across their plasma membrane). • This voltage is called the membrane potential. • In neurons, the membrane potential is typically between ?60 and ?80 mV when the cell is not transmitting signals. • The membrane potential of a neuron that is not transmitting signals is called the resting potential. • In all neurons, the resting potential depends on the ionic gradients that exist across the plasma membrane. • In mammals, the extracellular fluid has a Na+ concentration of 150 millimolar (mM) and a K+ of 5 mM. • In the cytosol, Na+ concentration is 15 mM, and K+ concentration is 150 mM. • These gradients are maintained by the sodium-potassium pump. • The magnitude of the membrane voltage at equilibrium, called the equilibrium potential (Eion), is given by a formula called the Nernst equation. • For an ion with a net charge of +1, the Nernst equation is: • Eion = 62mV (log [ion]outside/[ion]inside) • The Nernst equation applies to any membrane that is permeable to a single type of ion. • In our model, the membrane is only permeable to K+, and the Nernst equation can be used to calculate EK, the equilibrium potential for K+. • With this K+ concentration gradient, K+ is at equilibrium when the inside of the membrane is 92 mV more negative than the outside. • Assume that the membrane is only permeable to Na+. • ENa, the equilibrium potential for Na+, is +62 mV, indicating that, with this Na+ concentration gradient, Na+ is at equilibrium when the inside of the membrane is 62 mV more positive than the outside. • How does a real mammalian neuron differ from these model neurons? • The plasma membrane of a real neuron at rest has many open potassium channels, but it also has a relatively small number of open sodium channels. • Consequently, the resting potential is around ?60 to ?80 mV, between EK and ENa. • Neither K+ nor Na + is at equilibrium, and there is a net flow of each ion (a current) across the membrane at rest. • The resting membrane potential remains steady, which means that the K+ and Na+ currents are equal and opposite. • The reason the resting potential is closer to EK than to ENa is that the membrane is more permeable to K+ than to Na+. • If something causes the membrane’s permeability to Na+ to increase, the membrane potential will move toward ENa and away from EK. • This is the basis of nearly all electrical signals in the nervous system. • The membrane potential can change from its resting value when the membrane’s permeability to particular ions changes. • Sodium and potassium play major roles, but there are also important roles for chloride and calcium ions. • The resting potential results from the diffusion of K+ and Na+ through ion channels that are always open. • These channels are ungated. • Neurons also have gated ion channels, which open or close in response to one of three types of stimuli. • Stretch-gated ion channels are found in cells that sense stretch, and open when the membrane is mechanically deformed. • Ligand-gated ion channels are found at synapses and open or close when a specific chemical, such as a neurotransmitter, binds to the channel. • Voltage-gated ion channels are found in axons (and in the dendrites and cell bodies of some neurons, as well as in some other types of cells) and open or close in response to a change in membrane potential. Concept 48.3 Action potentials are the signals conducted by axons • Gated ion channels are responsible for generating the signals of the nervous system. • If a cell has gated ion channels, its membrane potential may change in response to stimuli that open or close those channels. • Some stimuli trigger a hyperpolarization, an increase in the magnitude of the membrane potential. • Gated K+ channels open, K+ diffuses out of the cell, and the inside of the membrane becomes more negative. • Other stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential. • Gated Na+ channels open, Na+ diffuses into the cell, and the inside of the membrane becomes less negative. • These changes in membrane potential are called graded potentials because the magnitude of the change—either hyperpolarization or depolarization—varies with the strength of the stimulus. • A larger stimulus causes a larger change in membrane permeability and, thus, a larger change in membrane potential. • In most neurons, depolarizations are graded only up to a certain membrane voltage, called the threshold. • A stimulus strong enough to produce a depolarization that reaches the threshold triggers a different type of response, called an action potential. • An action potential is an all-or-none phenomenon. • Once triggered, it has a magnitude that is independent of the strength of the triggering stimulus. • Action potentials of neurons are very brief—only 1–2 milliseconds in duration. • This allows a neuron to produce them at high frequency. • Both voltage-gated Na+ channels and voltage-gated K+ channels are involved in the production of an action potential. • Both types of channels are opened by depolarizing the membrane, but they respond independently and sequentially: Na+ channels open before K+ channels. • Each voltage-gated Na+ channel has two gates, an activation gate and an inactivation gate, and both must be open for Na+ to diffuse through the channel. • At the resting potential, the activation gate is closed and the inactivation gate is open on most Na+ channels. • Depolarization of the membrane rapidly opens the activation gate and slowly closes the inactivation gate. • Each voltage-gated K+ channel has just one gate, an activation gate. • At the resting potential, the activation gate on most K+ channels is closed. • Depolarization of the membrane slowly opens the K+ channel’s activation gate. • How do these channel properties contribute to the production of an action potential? • When a stimulus depolarizes the membrane, the activation gates on some Na+ channels open, allowing more Na+ to diffuse into the cell. • The Na+ influx causes further depolarization, which opens the activation gates on still more Na+ channels, and so on. • Once the threshold is crossed, this positive-feedback cycle rapidly brings the membrane potential close to ENa during the rising phase. • However, two events prevent the membrane potential from actually reaching ENa. • The inactivation gates on most Na+ channels close, halting Na+ influx. • The activation gates on most K+ channels open, causing a rapid efflux of K+. • Both events quickly bring the membrane potential back toward EK during the falling phase. • In fact, in the final phase of an action potential, called the undershoot, the membrane’s permeability to K+ is higher than at rest, so the membrane potential is closer to EK than it is at the resting potential. • The K+ channels’ activation gates eventually close, and the membrane potential returns to the resting potential. • The Na+ channels’ inactivation gates remain closed during the falling phase and the early part of the undershoot. • As a result, if a second depolarizing stimulus occurs during this refractory period, it will be unable to trigger an action potential. Nerve impulses propagate themselves along an axon. • The action potential is repeatedly regenerated along the length of the axon. • An action potential achieved at one region of the membrane is sufficient to depolarize a neighboring region above the threshold level, thus triggering a new action potential. • Immediately behind the traveling zone of depolarization due to Na+ influx is a zone of repolarization due to K+ efflux. • In the repolarized zone, the activation gates of Na+ channels are still closed. • Consequently, the inward current that depolarizes the axon membrane ahead of the action potential cannot produce another action potential behind it. • Once an action potential starts, it normally moves in only one direction—toward the synaptic terminals. • Several factors affect the speed at which action potentials are conducted along an axon. • One factor is the diameter of the axon: the larger the axon’s diameter, the faster the conduction. • In the myelinated neurons of vertebrates, voltage-gated Na+ and K+ channels are concentrated at gaps in the myelin sheath called nodes of Ranvier. • Only these unmyelinated regions of the axon depolarize. • Thus, the impulse moves faster than in unmyelinated neurons. • This mechanism is called saltatory conduction. Concept 48.4 Neurons communicate with other cells at synapses • When an action potential reaches the terminal of the axon, it generally stops there. • However, information is transmitted from a neuron to another cell at the synapse. • Some synapses, called electrical synapses, contain gap junctions that do allow electrical current to flow directly from cell to cell. • Action potentials travel directly from the presynaptic to the postsynaptic cell. • In both vertebrates and invertebrates, electrical synapses synchronize the activity of neurons responsible for rapid, stereotypical behaviors. • The vast majority of synapses are chemical synapses, which involve the release of chemical neurotransmitter by the presynaptic neuron. • The presynaptic neuron synthesizes the neurotransmitter and packages it in synaptic vesicles, which are stored in the neuron’s synaptic terminals. • When an action potential reaches a terminal, it depolarizes the terminal membrane, opening voltage-gated calcium channels in the membrane. • Calcium ions (Ca2+) then diffuse into the terminal, and the rise in Ca2+ concentration in the terminal causes some of the synaptic vesicles to fuse with the terminal membrane, releasing the neurotransmitter by exocytosis. • The neurotransmitter diffuses across the narrow gap, called the synaptic cleft, which separates the presynaptic neuron from the postsynaptic cell. • The effect of the neurotransmitter on the postsynaptic cell may be direct or indirect. • Information transfer at the synapse can be modified in response to environmental conditions. • Such modification may form the basis for learning or memory. Neural integration occurs at the cellular level. • At many chemical synapses, ligand-gated ion channels capable of binding to the neurotransmitter are clustered in the membrane of the postsynaptic cell, directly opposite the synaptic terminal. • Binding of the neurotransmitter to the receptor opens the channel and allows specific ions to diffuse across the postsynaptic membrane. • This mechanism of information transfer is called direct synaptic transmission. • The result is generally a postsynaptic potential, a change in the membrane potential of the postsynaptic cell. • Excitatory postsynaptic potentials (EPSPs) depolarize the postsynaptic neuron. • The binding of neurotransmitter to postsynaptic receptors opens gated channels that allow Na+ to diffuse into and K+ to diffuse out of the cell. • Inhibitory postsynaptic potential (IPSP) hyperpolarizes the postsynaptic neuron. • The binding of neurotransmitter to postsynaptic receptors open gated channels that allow K+ to diffuse out of the cell and/or Cl? to diffuse into the cell. • Various mechanisms end the effect of neurotransmitters on postsynaptic cells. • The neurotransmitter may simply diffuse out of the synaptic cleft. • The neurotransmitter may be taken up by the presynaptic neuron through active transport and repackaged into synaptic vesicles. • Glia actively take up the neurotransmitter at some synapses and metabolize it as fuel. • The neurotransmitter acetylcholine is degraded by acetylcholinesterase, an enzyme in the synaptic cleft. • Postsynaptic potentials are graded; their magnitude varies with a number of factors, including the amount of neurotransmitter released by the presynaptic neuron. • Postsynaptic potentials do not regenerate but diminish with distance from the synapse. • Most synapses on a neuron are located on its dendrites or cell body, whereas action potentials are generally initiated at the axon hillock. • Therefore, a single EPSP is usually too small to trigger an action potential in a postsynaptic neuron. • Graded potentials (EPSPs and IPSPs) are summed to either depolarize or hyperpolarize a postsynaptic neuron. • Two EPSPs produced in rapid succession at the same synapse can be added in an effect called temporal summation. • Two EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron can be added, in an effect called spatial summation. • Summation also applies to IPSPs. • This interplay between multiple excitatory and inhibitory inputs is the essence of integration in the nervous system. • The axon hillock is the neuron’s integrating center, where the membrane potential at any instant represents the summed effect of all EPSPs and IPSPs. • Whenever the membrane potential at the axon hillock reaches the threshold, an action potential is generated and travels along the axon to its synaptic terminals. • In indirect synaptic transmission, a neurotransmitter binds to a receptor that is not part of an ion channel. • This binding activates a signal transduction pathway involving a second messenger in the postsynaptic cell. • This form of transmission has a slower onset, but its effects have a longer duration. • cAMP acts as a secondary messenger in indirect synaptic transmission. • When the neurotransmitter norepinephrine binds to its receptor, the neurotransmitter-receptor complex activates a G-protein, which in turn activates adenylyl cyclase, the enzyme that converts ATP to cAMP. • cAMP activates protein kinase A, which phosphorylates specific channel proteins in the postsynaptic membrane, causing them to open or close. • Because of the amplifying effect of the signal transduction pathway, the binding of a neurotransmitter to a single receptor can open or close many channels. The same neurotransmitter can produce different effects on different types of cells. • Each of the known neurotransmitters binds to a specific group of receptors. • Some neurotransmitters have a dozen or more receptors, which can produce very different effects in postsynaptic cells. • Acetylcholine is one of the most common neurotransmitters in both invertebrates and vertebrates. • In the vertebrate CNS, it can be inhibitory or excitatory, depending on the type of receptor. • At the vertebrate neuromuscular junction, acetylcholine released by the motor neuron binds to receptors on ligand- gated channels in the muscle cell, producing an EPSP via direct synaptic transmission. • Nicotine binds to the same receptors. • Acetylcholine is inhibitory to cardiac muscle cell contraction. • Biogenic amines are neurotransmitters derived from amino acids. v • One group, known as catecholamines, consists of neurotransmitters produced from the amino acid tyrosine. • This group includes epinephrine and norepinephrine and a closely related compound called dopamine. • Another biogenic amine, serotonin, is synthesized from the amino acid tryptophan. • The biogenic amines are usually involved in indirect synaptic transmission, most commonly in the CNS. • Dopamine and serotonin affect sleep, mood, attention, and learning. • Imbalances in these neurotransmitters are associated with several disorders. • Parkinson’s disease is associated with a lack of dopamine in the brain. • LSD and mescaline produce hallucinations by binding to brain receptors for serotonin and dopamine. • Depression is treated with drugs that increase the brain concentrations of biogenic amines such as norepinephrine and serotonin. • Prozac inhibits the uptake of serotonin after its release, increasing its effect. • Four amino acids function as neurotransmitters in the CNS: gamma aminobutyric acid (GABA), glycine, glutamate, and aspartate. • GABA is the neurotransmitter at most inhibitory synapses in the brain, where it produces IPSPs. • Several neuropeptides, relatively short chains of amino acids, serve as neurotransmitters. • Most neurons release one or more neuropeptides as well as a nonpeptide neurotransmitter. • Neuropeptides usually operate via signal transduction pathways. • The neuropeptide substance P is a key excitatory neurotransmitter that mediates our perception of pain. • Other neuropeptides, endorphins, act as natural analgesics. • Opiates such as morphine and heroin bind to receptors on brain neurons by mimicking endorphins, which are produced in the brain under times of physical or emotional stress. • Some neurons of the vertebrate PNS and CNS release dissolved gases, especially nitric oxide and carbon monoxide, which act as local regulators. • During male sexual arousal, certain neurons release NO into the erectile tissue of the penis. • In response, smooth muscle cells in the blood vessel walls of the erectile tissue relax, allowing the blood vessels to dilate and fill the spongy erectile tissue with blood, producing an erection. • Viagra inhibits an enzyme that slows the muscle-releasing effects of NO. • Carbon monoxide is synthesized by the enzyme heme oxygenase. • In the brain, CO regulates the release of hypothalamic hormones. • In the PNS, it acts as an inhibitory neurotransmitter that hyperpolarizes intestinal smooth muscle cells. • NO and CO are synthesized by cells as needed. • They diffuse into neighboring target cells, produce an effect, and are broken down, all within a few seconds. Concept 48.5 The vertebrate nervous system is regionally specialized Vertebrate nervous systems have central and peripheral components. • In all vertebrates, the nervous system shows a high degree of cephalization and has distinct CNS and PNS components. • The brain provides integrative power that underlies the complex behavior of vertebrates. • The spinal cord integrates simple responses to certain kinds of stimuli and conveys information to and from the brain. • The vertebrate CNS is derived from the dorsal embryonic nerve cord, which is hollow. • In the adult, this feature persists as the narrow central canal of the spinal cord and the four ventricles of the brain. • Both the canal and the ventricles are filled with cerebrospinal fluid, which is formed in the brain by filtration of the blood. • Cerebrospinal fluid circulates through the central canal and ventricles and then drains into the veins, assisting in the supply of nutrients and hormone and the removal of wastes. • In mammals, the fluid cushions the brain and spinal cord by circulating between two of the meninges, layers of connective tissue that surround the CNS. • White matter of the CNS is composed of bundles of myelinated axons. • Gray matter consists of unmyelinated axons, nuclei, and dendrites. The divisions of the peripheral nervous system interact in maintaining homeostasis. • The PNS transmits information to and from the CNS and plays an important role in regulating the movement and internal environment of a vertebrate. • The vertebrate PNS consists of left-right pairs of cranial and spinal nerves and their associated ganglia. • Paired cranial nerves originate in the brain and innervate the head and upper body. • Paired spinal nerves originate in the spinal cord and innervate the entire body. • The PNS can be divided into two functional components: the somatic nervous system and the autonomic nervous system. • The somatic nervous system carries signals to and from skeletal muscle, mainly in response to external stimuli. • It is subject to conscious control, but much skeletal muscle activity is actually controlled by reflexes mediated by the spinal cord or the brainstem. • The autonomic nervous system regulates the internal environment by controlling smooth and cardiac muscles and the organs of the digestive, cardiovascular, excretory, and endocrine systems. • Three divisions make up the autonomic nervous syst
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