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Chapter 48+ (sections of 49/50) Neurons, Synapses, and Signaling w/in the Nerv. Syst. Concept 48.1: Neuron organization and structure reflect function in information transfer • Nervous systems process information in 3 stages: sensory input, integration, and motor output – Sensors detect external stimuli and internal conditions and transmit information along sensory neurons – Sensory information is sent to the brain or ganglia, where interneurons integrate the information – Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity • Many animals have a complex nervous system which consists of: – central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord Sensory input – A peripheral nervous system (PNS), which brings information into and out of the CNS Motor output Peripheral nervous system (PNS) Central nervous system (CNS) Neuron Structure and Function Fig. 48-4 Dendrites Stimulus Nucleus Cell body Axon hillock Presynaptic cell Axon Information is transmitted from a presynaptic cell (neuron) to a postsynaptic cell (neuron, muscle, or gland cell) Synapse Synaptic terminals Postsynaptic cell Neurotransmitter Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron • Membrane potential : difference in electrical charge across plasma membrane • The resting potential is the membrane potential of a neuron not sending signals Microelectrode –70 mV Voltage recorder Reference electrode Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Formation of the Resting Potential Fig. 48-6 The basis of the membrane potential Key Na+ K+ OUTSIDE CELL OUTSIDE [K+] CELL 5 mM INSIDE [K+] CELL 140 mM [Na+] [Cl–] 150 mM 120 mM [Na+] 15 mM [Cl–] 10 mM [A–] 100 mM INSIDE CELL (a) (b) 1. Na+/K+ pump 2. K+ leak channels 3. Anions Sodiumpotassium pump Potassium channel Sodium channel Resting potential: membrane potential at rest; polarized – Na+ outside, K+ inside cell (Salty Banana) – Na+/K+ Pump use ATP to maintain these gradients across the plasma membrane – Voltage-gated Na+ channel = CLOSED – Voltage-gated K+ channel = OPEN Nerve impulse: stimulus causes a change in membrane potential – Action potential: neuron membrane depolarizes – All-or-nothing response Na+ channels open Na+ enters cell K+ channels open K+ leaves cell Concept 48.3: Action potentials (nerve impulses) are the signals conducted by axons • Neurons contain gated ion channels that open or close in response to stimuli • You’ll want to review this pic after you understand the action potential Polarized Production of Action Potentials • Voltage-gated Na+ and K+ channels respond to a change in membrane potential • When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell • The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open • A strong stimulus results in a massive change in membrane voltage called an action potential Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane • Action potentials are signals that carry information along axons Strong depolarizing stimulus +50 Action potential Membrane potential (mV) • An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold 0 –50 Threshold Resting potential –100 0 (c) Action potential 1 2 3 4 5 Time (msec) 6 Generation of Action Potentials: A Closer Look • A neuron can produce hundreds of action potentials per second • The frequency of action potentials can reflect the strength of a stimulus • An action potential can be broken down into a series of stages Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 48-10-1 Key Na+ K+ Membrane potential (mV) +50 Action potential –50 Plasma membrane Cytosol Inactivation loop 1 Resting state 2 –100 Sodium channel 4 Threshold 1 5 Resting potential Depolarization Extracellular fluid 3 0 Time Potassium channel 1 Fig. 48-10-2 Key Na+ K+ Membrane potential (mV) +50 Action potential –50 2 Plasma membrane Cytosol Inactivation loop 1 Resting state 2 –100 Sodium channel 4 Threshold 1 5 Resting potential Depolarization Extracellular fluid 3 0 Time Potassium channel 1 Fig. 48-10-3 Key Na+ K+ 3 Rising phase of the action potential Membrane potential (mV) +50 Action potential –50 2 Plasma membrane Cytosol Inactivation loop 1 Resting state 2 –100 Sodium channel 4 Threshold 1 5 Resting potential Depolarization Extracellular fluid 3 0 Time Potassium channel 1 Fig. 48-10-4 Key Na+ K+ 3 4 Rising phase of the action potential Membrane potential (mV) +50 Action potential –50 2 Plasma membrane Cytosol Inactivation loop 1 Resting state 2 –100 Sodium channel 4 Threshold 1 5 Resting potential Depolarization Extracellular fluid 3 0 Time Potassium channel 1 Falling phase of the action potential Fig. 48-10-5 Key Na+ K+ 3 4 Rising phase of the action potential Falling phase of the action potential Membrane potential (mV) +50 Action potential –50 2 2 4 Threshold 1 1 5 Resting potential Depolarization Extracellular fluid 3 0 –100 Sodium channel Time Potassium channel Plasma membrane Cytosol Inactivation loop 5 1 Resting state Undershoot • During the refractory period after an action potential, a second action potential cannot be initiated • The refractory period is a result of a temporary inactivation of the Na+ channels 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, an electrical current depolarizes the neighboring region of the axon membrane • Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards • Action potentials travel in only one direction: toward the synaptic terminals (axon terminals) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Conduction of an action potential Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Conduction Speed • The speed of an action potential increases with the axon’s diameter • In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase • Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 48-12a Node of Ranvier Layers of myelin Axon Schwann cell Nodes of Myelin sheath Ranvier Axon Fig. 48-12b Schwann cell Nucleus of Schwann cell Myelinated axon (cross section) 0.1 µm • Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltagegated Na+ channels are found • Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon Concept 48.4: Neurons communicate with other cells at synapses • At electrical synapses, the electrical current flows from one neuron to another • At chemical synapses, a chemical neurotransmitter carries information across the gap junction • Most synapses are chemical synapses Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 48-15 5 Synaptic vesicles containing neurotransmitter Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 Synaptic cleft Presynaptic membrane 3 Ligand-gated ion channels 6 K+ Na+ Neurotransmitters • Chemicals released from vesicles by exocytosis into synaptic cleft • Diffuse across synapse • Bind to receptors on neurons, muscle cells, or gland cells • Broken down by enzymes or taken back up into surrounding cells • Types of neurotransmitters: – Excitatory: speed up impulses by causing depolarization of postsynaptic membrane – Inhibitory: slow impulses by causing hyperpolarization of postsynaptic membrane Summation of Postsynaptic Potentials • Unlike action potentials, postsynaptic potentials are graded – A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron • Most neurons have many synapses on dendrites and cell body Terminal branch of presynaptic neuron E2 E1 E2 Membrane potential (mV) Postsynaptic neuron E1 E1 E1 E2 E2 I I Axon hillock I I 0 Action potential Threshold of axon of postsynaptic neuron Action potential Resting potential –70 E1 E1 (a) Subthreshold, no summation E1 E1 (b) Temporal summation E1 + E2 (c) Spatial summation E1 I E1 + I (d) Spatial summation of EPSP and IPSP Table 48-1 Examples of Neurotransmitters • Acetylcholine (ACh): stimulates muscles, memory formation, learning – Sarin gas; Botox • Epinephrine: Adrenaline; fight-or-flight • Norepinephrine: fight-or-flight • Dopamine: reward, pleasure (“high”) – Loss of dopamine Parkinson’s Disease – Excessive amounts Schizophrenia – LSD binds to dopamine receptors hallucinations Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Examples of Neurotransmitters • Serotonin: well-being; happiness – LSD binds to serotonin receptors hallucinations – Low levels of serotonin depression – Prozac enhances effects of serotonin by inhibiting reuptake • GABA: inhibitory neurotransmitter – Affected by alcohol – Valium uses these receptors • Endorphins: decrease pain perception – Morphine/heroine bind to same receptors Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Regional Specialization of Vertebrate Brain • • • Forebrain cerebrum Midbrain brainstem Hindbrain cerebellum Human Brain Structure Function Cerebrum • Information processing (learning, emotion, memory, perception, voluntary movement) • Right & Left cerebral hemispheres • Corpus callosum: connect hemispheres Brainstem *Oldest evolutionary part* •Basic, autonomic survival behaviors •Medulla oblongata –breathing, heart & blood vessel activity, digestion, swallowing, vomiting •Transfer info between PNS & CNS Cerebellum • Coordinate movement & balance • Motor skill learning Fig. 49-15 Frontal lobe Parietal lobe Speech Frontal association area Somatosensory association area Taste Reading Speech Hearing Smell Auditory association area Visual association area Vision Temporal lobe Occipital lobe You should now be able to: 1. Distinguish among : sensory neurons, interneurons, motor neurons; membrane potential, resting potential; ungated and gated ion channels; electrical synapse, chemical synapse; EPSP, IPSP; temporal, spatial summation 2. Explain the role of the Na+/K+ pump in maintaining the resting potential 3. Describe the stages of an action potential; explain the role of voltage-gated ion channels in each stage of this process 4. Explain why the action potential cannot travel back toward the cell body 5. Describe the factors that affect the rate of neuronal signal transmission; explain saltatory conduction 6. Describe the events that lead to the release of neurotransmitters into the synaptic cleft 7. Explain the statement: “Unlike action potentials, which are all-or-none events, postsynaptic potentials are graded” 8. Describe five categories of neurotransmitters; given information, be able to apply info to use/blockage of specific signals via simulation or blockage of neurotransmitter 9. Describe how the brain integrates information. 10. Describe the different functions of the different regions of the brain. Fig. 48-UN1 Action potential Membrane potential (mV) +50 Falling phase 0 Rising phase Threshold (–55) –50 –100 Resting potential –70 Depolarization Time (msec) Undershoot