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Nervous Systems Chapt 48 (pp 1011-1025) 4/21/06 IB-202-14-06 Complex Brain-Human! • Overview: Command and Control Center • The human brain contains an estimated 100 billion nerve cells, or neurons • Each neuron may communicate with thousands of other neurons • Functional magnetic resonance imaging – Is a technology that can reconstruct a threedimensional map of brain activity Colored areas of brain active during language processing. The results of brain imaging and other research methods reveal that groups of neurons function in specialized circuits dedicated to different tasks Figure 48.1 Simple Nervous Systems • Concept 48.1: Nervous systems consist of circuits of neurons and supporting cells • All animals except sponges have some type of nervous system • What distinguishes the nervous systems of different animal groups is how the neurons are organized into circuits • Most invertebrate nervous systems are simple Organization of Nervous Systems • The simplest animals with nervous systems, the cnidarians have neurons arranged in nerve nets When prey touch a tentacle, the hydra can contract its tentacle to its mouth and engulf the prey item. Figure 48.2a Nerve net (a) Hydra (cnidarian) Star fish • Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring. No Photosensitive Organs Radial nerve Each radial nerve would have smaller nerves sending signals to the water vascular system as well as muscles. Nerve ring Figure 48.2b (b) Sea star (echinoderm) Appearance of cephalization and centralization of nervous system • In relatively simple cephalized animals, such as flatworms a central nervous system (CNS) is evident Eyespot 1st appearance of eye spots at head end! Allow it to turn away from light! Brain Nerve cord Transverse nerve Two ventral nerve cords (interconnected so communicate with each other)! Figure 48.2c (c) Planarian (flatworm) Segmented invertebrates • Annelids and arthropods – Have segmentally arranged clusters of neurons called ganglia • These ganglia connect to the CNS and make up a peripheral nervous system (PNS) Brain Brain Ventral nerve cord Segmental ganglion Figure 48.2d, e Ventral nerve cord Segmental ganglia (d) Leech (annelid) (e) Insect (arthropod) Molluscs • Nervous systems in molluscs – Correlate with the animals’ lifestyles • Sessile molluscs (clams sitting in the mud) have simple systems while more complex molluscs have more sophisticated systems like the squid and octopus both which have eyes and are capable of complex behavior, including learning. Anterior nerve ring Well developed brain and eyes in squid! Ganglia Brain Longitudinal nerve cords Figure 48.2f, g (f) Chiton (mollusc) Ganglia (g) Squid (mollusc) Can “grasp” items with tentacles and manipulate them! Vertebrates have a brain encased in a skull for protection. • In vertebrates – The central nervous system consists of a brain and dorsal spinal cord – The periferal nerves system connects to the CNS Brain Spinal cord (dorsal nerve cord) Figure 48.2h Dorsal sensory ganglion (h) Salamander (chordate) What does the nervous system do? Gathers information about its surroundings, processes it and acts on it with some sort of output. • Nervous systems process information in three stages--sensory input, integration, and motor output Sensory input Integration Sensor Motor output Effector Figure 48.3 Peripheral nervous system (PNS) Central nervous system (CNS) Knee jerk as an example of info processing outside of the brain • The three stages of information processing 2 Sensors detect a sudden stretch in the quadriceps. 3 Sensory neurons convey the information to the spinal cord. Cell body of sensory neuron in dorsal root ganglion 4 The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward. Gray matter 5 Sensory neurons from the quadriceps also communicate with interneurons in the spinal cord. Quadriceps muscle White matter Stretching of quadriceps when leaning forward! Information is in the form 1 is of electrical initiatedTheby reflex tapping the tendon connected signals. to the quadriceps Figure 48.4 (extensor) muscle. Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Interneuron 6 The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps. Neuron Structure • Most of a neuron’s organelles are located in the cell body. Axons conduct impulse away from cell body! Dendrites Cell body Nucleus Synapse Signal Axon direction Axon hillock Presynaptic cell Postsynaptic cell Myelin sheath Figure 48.5 Synaptic terminals • Most neurons have dendrites – Highly branched extensions that receive signals from other neurons • The axon is typically a much longer extension – That transmits signals to other cells at synapses – That may be covered with a myelin sheath • Neurons have a wide variety of shapes – That reflect their input and output interactions Dendrites Axon Cell body Figure 48.6a–c (a) Sensory neuron (b) Interneurons (c) Motor neuron • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) are supporting cells that form the myelin sheaths around the axons of many vertebrate neurons Node of Ranvier Layers of myelin Axon Schwann cell Axon Figure 48.8 Myelin sheath Nodes of Ranvier Schwann cell Nucleus of Schwann cell 0.1 µm Basis for generation of anelectrical signal is the alteration of the resting membrane potential of excitable cells! 1. Every cell has a voltage, or membrane potential, across its plasma membrane • A membrane potential is a localized electrical gradient across membrane. The basis for the gradient is the disproportionate distribution of charged ions. – Anions are more concentrated within a cell. – Cations are more concentrated in the extracellular fluid. – A greater number of negative charges within the cell Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings The resting membrane potential of a cell can be measured APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of neurons and other cells. A microelectrode is made from a glass capillary tube filled with an electrically conductive TECHNIQUE salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell. Microelectrode –70 mV Voltage recorder Figure 48.9 Reference electrode • Concept 48.2: Ion pumps and ion channels maintain the resting potential of all cells including neurons • For a neuron the resting potential is the membrane potential of a cell that is not transmitting signals • Cells that can transmit signals are called excitable cells (nerves and muscles) • How a Cell Maintains a Membrane Potential. – Cations. • K+ the principal intracellular cation. • Na+ is the principal extracellular cation. – Anions. • Proteins, amino acids, sulfate, and phosphate are the principal intracellular anions. • Cl– is principal extracellular anion. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Ungated ion channels allow ions to diffuse across the plasma membrane. – These channels are always open. • This diffusion does not achieve an equilibrium since sodium-potassium pump transports these ions against their concentration gradients. If poison the pump they will. Fig. 48.7 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Size of arrow represents the rate of diffusion. Faster for K+ than Na+ Excitable cells have the ability to generate large changes in their membrane potentials because they have gated ion channels. – Gated ion channels open or close in response to stimuli. (These are separate and different from the ion channels in the former slide) • The subsequent movement of ions across the membrane leads to a change in the membrane potential. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Gated Ion Channels • Gated ion channels open or close – In response to membrane stretch or the binding of a specific ligand – In response to a change in the membrane potential Production of Action Potentials • In most neurons, depolarizations – Are graded only up to a certain membrane voltage, called the threshold • Some stimuli trigger a hyperpolarization – An increase in the magnitude of the membrane potential Stimuli Membrane potential (mV) +50 0 –50 Threshold Resting potential Hyperpolarizations –100 0 1 2 3 4 5 Time (msec) (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus produces Figure 48.12a a larger hyperpolarization. • Other stimuli trigger a depolarization – A reduction in the magnitude of the membrane potential Stimuli Membrane potential (mV) +50 0 –50 Threshold Resting Depolarizations potential –100 0 1 2 3 4 5 Time (msec) (b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+. The larger stimulus produces a Figure 48.12b larger depolarization. • Hyperpolarization and depolarization – Are both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus • A stimulus strong enough to produce a depolarization that reaches the threshold of -55mV triggers a different type of response, called an action potential Stronger depolarizing stimulus Membrane potential (mV) +50 Action potential 0 –50 Threshold Resting potential –100 Figure 48.12c 0 1 2 3 4 5 6 Time (msec) (c) Action potential triggered by a depolarization that reaches the threshold. • An action potential – Is a brief all-or-none depolarization of a neuron’s plasma membrane – Is the type of signal that carries information along axons • Both voltage-gated Na+ channels and voltagegated K+ channels – Are involved in the production of an action potential • When a stimulus depolarizes the membrane – Na+ channels open, allowing Na+ to diffuse into the cell changing the potential to a positive value • As the action potential subsides – K+ channels open, and K+ flows out of the cell • A refractory period follows the action potential – During which a second action potential cannot be initiated • An action potential can travel long distances – By regenerating itself along the axon • The generation of an action potential Na+ Na+ – – – – – – – – + + + + + + + + K+ Rising phase of the action potential Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respect to the outside. Na+ + + + + – – – – +50 + + – – K+ – – –50 Na+ + + + + + + + + + + – – – – – – – – 3 2 4 Threshold 5 1 1 Resting potential Na+ Potassium channel + + Activation gates + + + + – – – – + + + + – – – – + + K+ – – – – – – – – Cytosol – – Sodium channel 1 Na+ + + Plasma membrane Figure 48.13 Falling phase of the action potential The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on most K+ channels open, permitting K+ efflux which again makes the inside of the cell negative. Time Depolarization A stimulus opens the activation gates on some Na+ channels. Na+ influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. Extracellular fluid + + Action potential 0 –100 2 + + 4 Na+ + + + + K+ Membrane potential (mV) 3 Na+ Na+ – – K+ – – Inactivation gate Resting state The activation gates on the Na+ and K+ channels are closed, and the membrane’s resting potential is maintained. 5 Undershoot Both gates of the Na+ channels are closed, but the activation gates on some K+ channels are still open. As these gates close on most K+ channels, and the inactivation gates open on Na+ channels, the membrane returns to its resting state. Conduction of Action Potentials • An action potential can travel long distances – By regenerating itself along the axon • At the site where the action potential is generated, usually the axon hillock – An electrical current depolarizes the neighboring region of the axon membrane Axon Action potential – – + ++ Na + + – – K+ + + – – – – + + K+ Figure 48.14 + – – + + – – + + + + + + + – – + – – + – – + – – + – – + – – + + – – + + – – + Action potential – + Na – + + + + – – K+ + + – – – – + + K+ + – – + Action potential – – + ++ Na + + – – – + + – 1 An action potential is generated as Na+ flows inward across the membrane at one location. 2 The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 3 The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon. + – – + – + + – Conduction Speed • The speed of an action potential – Increases with the diameter of an axon • In vertebrates, axons are myelinated – Also causing the speed of an action potential to increase Myelinated axons conduct impulses faster than non-myelinated • Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction Schwann cell Depolarized region (node of Ranvier) Myelin sheath –– – Cell body + ++ + ++ ––– –– – + + Axon + ++ –– – Figure 48.15 • Concept 48.4: Neurons communicate with other cells at synapses • In an electrical synapse – Electrical current flows directly from one cell to another via a gap junction (tail flick escape response in lobster uses electrical connection because it must be as fast as possible). • The vast majority of synapses – Are chemical synapses • In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters, which are stored in the synaptic terminal Postsynaptic neuron body 5 µm Synaptic terminal of presynaptic neurons Figure 48.16 • When an action potential reaches a terminal – The final result is the release of neurotransmitters into the synaptic cleft Postsynaptic cell Presynaptic cell Action potential results in influx of calcium! Calcium causes vesicles to fuse with presynaptic membrane releasing neurotransmitter Figure 48.17 Synaptic vesicles containing neurotransmitter 5 Presynaptic membrane Na+ K+ Neurotransmitter Postsynaptic membrane Ligandgated ion channel Voltage-gated Ca2+ channel 1 Ca2+ 4 2 Synaptic cleft 3 Ligand-gated ion channels Postsynaptic membrane 6