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Chapter 48 Nervous Systems PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organization of Nervous Systems • The simplest animals with nervous systems, the cnidarians – Have neurons arranged in nerve nets Nerve net Figure 48.2a (a) Hydra (cnidarian) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In relatively simple cephalized animals, such as flatworms – A central nervous system (CNS) is evident Eyespot Brain Nerve cord Transverse nerve Figure 48.2c (c) Planarian (flatworm) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In vertebrates – The central nervous system consists of a brain and dorsal spinal cord – The PNS connects to the CNS Brain Spinal cord (dorsal nerve cord) Figure 48.2h Sensory ganglion (h) Salamander (chordate) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Information Processing • 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) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Central nervous system (CNS) • The three stages of information processing – Are illustrated in the knee-jerk reflex (monosynaptic) 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 Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Figure 48.4 1 The reflex is initiated by tapping the tendon connected to the quadriceps (extensor) muscle. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Dendrites Cell body Nucleus Synapse Signal Axon direction Axon hillock Presynaptic cell Postsynaptic cell Myelin sheath Figure 48.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Synaptic terminals Supporting Cells (Glia) • Glia are supporting cells – That are essential for the structural integrity of the nervous system and for the normal functioning of neurons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In the CNS, astrocytes Figure 48.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 50 µm – Provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) – Are glia that form the myelin sheaths around the axons of many vertebrate neurons Node of Ranvier Layers of myelin Axon Schwann cell Axon Myelin sheath Nodes of Ranvier Schwann cell Nucleus of Schwann cell Figure 48.8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0.1 µm • Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron • Across its plasma membrane, every cell has a voltage – Called a membrane potential • The inside of a cell is negative – Relative to the outside • The resting potential – Is the membrane potential of a neuron that is not transmitting signals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The membrane potential of a cell can be measured APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of neurons and other cells. TECHNIQUE A microelectrode is made from a glass capillary tube filled with an electrically conductive 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The concentration of Na+ is higher in the extracellular fluid than in the cytosol – While the opposite is true for K+ Outer chamber –92 mV +62 mV + – 150 mM KCL 5 mM KCL + Cl– Artificial membrane – + – Figure 48.11a, b (a) Membrane selectively permeable to K+ Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Outer chamber – 150 mM NaCl 15 mM NaCl Cl– K+ Potassium channel Inner chamber + Inner chamber + – Sodium + channel – Na+ (b) Membrane selectively permeable to Na+ • A neuron that is not transmitting signals – Contains many open K+ channels and fewer open Na+ channels in its plasma membrane • The diffusion of K+ and Na+ through these channels – Leads to a separation of charges across the membrane, producing the resting potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some stimuli trigger a hyperpolarization – An increase in the magnitude of the membrane Stimuli potential – Harder to fire impulse – Further from threshold 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Other stimuli trigger a depolarization – A reduction in the magnitude of the membrane Stimuli potential 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Hyperpolarization and depolarization – Are both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A stimulus strong enough to produce a depolarization that reaches the threshold – Triggers a different type of response, called an Stronger depolarizing stimulus action potential – all-or-none depolarization Membrane potential (mV) +50 Action potential 0 –50 Threshold Resting potential –100 Figure 48.12c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0 1 2 3 4 5 6 Time (msec) (c) Action potential triggered by a depolarization that reaches the threshold. • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • As the action potential subsides – K+ channels open, and K+ flows out of the cell • A refractory period (undershoot) follows the action potential (brief hyperpolarization) – During which a second action potential cannot be initiated Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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. + – – + – + + – Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 • The vast majority of synapses however – Are chemical synapses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In a chemical synapse, a presynaptic neuron – Releases chemical neurotransmitters, which are stored in the synaptic terminal Postsynaptic neuron 5 µm Synaptic terminal of presynaptic neurons Figure 48.16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • When an action potential reaches a terminal – Ca++ enters promoting vesicle fusion and NT release – The final result is the release of neurotransmitters into the synaptic cleft (gap) Postsynaptic cell Presynaptic cell Synaptic vesicles containing Presynaptic neurotransmitter membrane Voltage-gated Ca2+ channel 1 Ca2+ 4 2 Synaptic cleft Figure 48.17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3 Ligand-gated ion channels Postsynaptic membrane 5 Na+ K+ Neurotransmitter Postsynaptic membrane Ligandgated ion channel 6 Direct Synaptic Transmission • The process of direct synaptic transmission – Involves the binding of neurotransmitters to ligand-gated ion channels • Neurotransmitter binding – Causes the ion channels to open, generating a postsynaptic potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • After its release, the neurotransmitter – Diffuses out of the synaptic cleft – May be taken up by surrounding cells and degraded by enzymes • Unlike action potentials – Postsynaptic potentials are graded and do not regenerate themselves Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Postsynaptic potentials fall into two categories – Excitatory postsynaptic potentials (EPSPs) – Inhibitory postsynaptic potentials (IPSPs) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Since most neurons have many synapses on their dendrites and cell body – A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron Terminal branch of presynaptic neuron Membrane potential (mV) Postsynaptic neuron Figure 48.18a E1 Threshold of axon of postsynaptic neuron 0 Resting potential –70 E1 E1 (a) Subthreshold, no summation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • If two EPSPs are produced in rapid succession – An effect called temporal summation can occur E1 Axon hillock Action potential E1 E1 (b) Temporal summation Figure 48.18b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In spatial summation – EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together E E2 1 Action potential E1 + E2 (c) Spatial summation Figure 48.18c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Through summation – An IPSP can counter the effect of an EPSP E1 I E1 Figure 48.18d I (d) Spatial summation of EPSP and IPSP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings E1 + I Indirect Synaptic Transmission • 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, producing a slowly developing but long-lasting effect Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Neurotransmitters • The same neurotransmitter – Can produce different effects in different types of cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Major neurotransmitters Table 48.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Acetylcholine • Acetylcholine – Is one of the most common neurotransmitters in both vertebrates and invertebrates – Can be inhibitory or excitatory (skeletal muscle) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biogenic Amines • Biogenic amines – Include epinephrine, norepinephrine, dopamine, and serotonin – Are active in the CNS and PNS Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 48.5: The vertebrate nervous system is regionally specialized • In all vertebrates, the nervous system – Shows a high degree of cephalization and distinct CNS and PNS components Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Cranial nerves Ganglia outside CNS Spinal nerves Figure 48.19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Peripheral Nervous System • The PNS transmits information to and from the CNS – And plays a large role in regulating a vertebrate’s movement and internal environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The PNS can be divided into two functional components – The somatic nervous system and the autonomic nervous system Peripheral nervous system Somatic nervous system Autonomic nervous system Sympathetic division Figure 48.21 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Parasympathetic division Enteric division • The somatic nervous system – Carries signals to skeletal muscles • The autonomic nervous system – Regulates the internal environment, in an involuntary manner – Is divided into the sympathetic, parasympathetic, and enteric divisions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The sympathetic and parasympathetic divisions – Have antagonistic effects on target organs Parasympathetic division Sympathetic division Action on target organs: Location of preganglionic neurons: brainstem and sacral segments of spinal cord Neurotransmitter released by preganglionic neurons: acetylcholine Action on target organs: Dilates pupil of eye Constricts pupil of eye Inhibits salivary gland secretion Stimulates salivary gland secretion Constricts bronchi in lungs Sympathetic ganglia Cervical Accelerates heart Slows heart Location of postganglionic neurons: in ganglia close to or within target organs Stimulates activity of stomach and intestines Stimulates gallbladder Thoracic Inhibits activity of pancreas Stimulates glucose release from liver; inhibits gallbladder Promotes emptying of bladder Figure 48.22 Location of postganglionic neurons: some in ganglia close to target organs; others in a chain of ganglia near spinal cord Lumbar Stimulates adrenal medulla Promotes erection of genitalia Neurotransmitter released by preganglionic neurons: acetylcholine Inhibits activity of stomach and intestines Stimulates activity of pancreas Neurotransmitter released by postganglionic neurons: acetylcholine Relaxes bronchi in lungs Location of preganglionic neurons: thoracic and lumbar segments of spinal cord Inhibits emptying of bladder Synapse Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sacral Promotes ejaculation and vaginal contractions Neurotransmitter released by postganglionic neurons: norepinephrine • The sympathetic division – Correlates with the “fight-or-flight” response • The parasympathetic division – Promotes a return to self-maintenance functions • The enteric division – Controls the activity of the digestive tract, pancreas, and gallbladder Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Embryonic Development of the Brain • In all vertebrates – The brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrain Embryonic brain regions Forebrain Midbrain Hindbrain Midbrain Hindbrain Forebrain Figure 48.23a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (a) Embryo at one month The Brainstem • The brainstem consists of three parts – The medulla oblongata, the pons, and the midbrain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The medulla oblongata – Contains centers that control several visceral functions • The pons – Also participates in visceral functions • The midbrain – Contains centers for the receipt and integration of several types of sensory information Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Arousal and Sleep • A diffuse network of neurons called the reticular formation – Is present in the core of the brainstem Eye Reticular formation Figure 48.24 Input from touch, pain, and temperature receptors Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Input from ears The Cerebellum • The cerebellum – Is important for coordination and error checking during motor, perceptual, and cognitive functions • The cerebellum – Is also involved in learning and remembering motor skills Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The hypothalamus regulates – Homeostasis – Basic survival behaviors such as feeding, fighting, fleeing, and reproducing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Circadian Rhythms •The hypothalamus also regulates circadian rhythms – Such as the sleep/wake cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The cerebrum has right and left cerebral hemispheres – That each consist of cerebral cortex overlying white matter and basal nuclei Left cerebral hemisphere Right cerebral hemisphere Corpus callosum Neocortex Figure 48.26 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Basal nuclei •The thalamus – Is the main input center for sensory information going to the cerebrum and the main output center for motor information leaving the cerebrum Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In humans, the largest and most complex part of the brain – Is the cerebral cortex, where sensory information is analyzed, motor commands are issued, and language is generated Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 48.6: The cerebral cortex controls voluntary movement and cognitive functions • Each side of the cerebral cortex has four lobes – Frontal, parietal, temporal, and occipital Frontal lobe Parietal lobe Speech Frontal association area Taste Speech Smell Somatosensory association area Reading Hearing Auditory association area Visual association area Vision Figure 48.27 Temporal lobe Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Occipital lobe Lateralization of Cortical Function • During brain development, in a process called lateralization – Competing functions segregate and displace each other in the cortex of the left and right cerebral hemispheres Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The left hemisphere – Becomes more adept at language, math, logical operations, and the processing of serial sequences • The right hemisphere – Is stronger at pattern recognition, nonverbal thinking, and emotional processing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Language and Speech • Studies of brain activity – Have mapped specific areas of the brain responsible for language and speech Max Hearing words Seeing words Min Figure 48.29 Speaking words Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Generating words •Broca’s area and Wernicke’s area – Are essential for the generation and understanding of language Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Memory and Learning • The frontal lobes – Are a site of short-term memory – Interact with the hippocampus and amygdala to consolidate long-term memory Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cellular Mechanisms of Learning • Experiments on invertebrates – Have revealed the cellular basis of some types of learning (a) Touching the siphon triggers a reflex that causes the gill to withdraw. If the tail is shocked just before the siphon is touched, the withdrawal reflex is stronger. This strengthening of the reflex is a simple form of learning called sensitization. Siphon Mantle Gill Tail Head Figure 48.31a, b (b) Sensitization involves interneurons that make synapses on the synaptic terminals of the siphon sensory neurons. When the tail is shocked, the interneurons release serotonin, which activates a signal transduction pathway that closes K+ channels in the synaptic terminals of the siphon sensory neurons. As a result, action potentials in the siphon sensory neurons produce a prolonged depolarization of the terminals. That allows more Ca2+ to diffuse into the terminals, which causes the terminals to release more of their excitatory neurotransmitter onto the gill motor neurons. In response, the motor neurons generate action potentials at a higher frequency, producing a more forceful gill withdrawal. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gill withdrawal pathway Touching the siphon Siphon sensory neuron Gill motor neuron Sensitization pathway Shocking the tail Interneuron Tail sensory neuron Gill