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Chapter 48 Nervous Systems Mr. Karns AP biology for Biology, Seventh Edition Neil Campbell and Jane Reece • 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 Figure 48.1 • The results of brain imaging and other research methods – Reveal that groups of neurons function in specialized circuits dedicated to different tasks • 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 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) • Sea stars have a nerve net in each arm – Connected by radial nerves to a central nerve ring Radial nerve Nerve ring Figure 48.2b (b) Sea star (echinoderm) • 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) • 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) • Nervous systems in molluscs – Correlate with the animals’ lifestyles • Sessile molluscs have simple systems – While more complex molluscs have more sophisticated systems Anterior nerve ring Ganglia Brain Longitudinal nerve cords Figure 48.2f, g (f) Chiton (mollusc) Ganglia (g) Squid (mollusc) • 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) 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) Central nervous system (CNS) • Sensory neurons transmit information from sensors – That detect external stimuli and internal conditions • Sensory information is sent to the CNS – Where interneurons integrate the information • Motor output leaves the CNS via motor neurons – Which communicate with effector cells • The three stages of information processing – Are illustrated in the knee-jerk reflex 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. 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 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 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 • In the CNS, astrocytes Figure 48.7 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 Figure 48.8 Myelin sheath Nodes of Ranvier Schwann cell Nucleus of Schwann cell 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 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 The Resting Potential • The resting potential – Is the membrane potential of a neuron that is not transmitting signals • In all neurons, the resting potential – Depends on the ionic gradients that exist across the plasma membrane EXTRACELLULAR FLUID CYTOSOL [Na+] 15 mM – + [Na+] 150 mM [K+] 150 mM – + [K+] 5 mM – + [Cl–] 10 mM – [Cl–] + 120 mM [A–] 100 mM – + Plasma membrane Figure 48.10 • The concentration of Na+ is higher in the extracellular fluid than in the cytosol – While the opposite is true for K+ • By modeling a mammalian neuron with an artificial membrane – We can gain a better understanding of the resting potential of a neuron –92 mV Outer chamber – 150 mM KCL +62 mV + 5 mM KCL + Cl– Artificial membrane – + – Figure 48.11a, b (a) Membrane selectively permeable to K+ 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 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 • Concept 48.3: Action potentials are the signals conducted by axons • 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 Stimuli potential 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 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. • Hyperpolarization and depolarization – Are both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus Production of Action Potentials • 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 Stronger depolarizing stimulus action potential 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 • 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 • 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 • 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 + ++ + ++ ––– Figure 48.15 –– – + + + ++ –– – Axon • 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 – Are chemical synapses • 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 • When an action potential reaches a terminal – The final result is the release of neurotransmitters into the synaptic cleft Postsynaptic cell Presynaptic cell 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 Figure 48.17 3 Ligand-gated ion channels Postsynaptic membrane 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 • Postsynaptic potentials fall into two categories – Excitatory postsynaptic potentials (EPSPs) – Inhibitory postsynaptic potentials (IPSPs) – Write these down- learn the acronym • After its release, the neurotransmitter – Diffuses out of the synaptic cleft – May be taken up by surrounding cells and degraded by enzymes Summation of Postsynaptic Potentials • Unlike action potentials – Postsynaptic potentials are graded and do not regenerate themselves • 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 • If two EPSPs are produced in rapid succession – An effect called temporal summation occurs E1 Axon hillock Action potential E1 E1 (b) Temporal summation Figure 48.18b • 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 • Through summation – An IPSP can counter the effect of an EPSP, so an action potential is not reached E1 I E1 Figure 48.18d I E1 + I (d) Spatial summation of EPSP and IPSP 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 Neurotransmitters • The same neurotransmitter – Can produce different effects in different types of cells • Major neurotransmitters Table 48.1 Acetylcholine • Acetylcholine – Is one of the most common neurotransmitters in both vertebrates and invertebrates – Can be inhibitory or excitatory Biogenic Amines • Biogenic amines – Include epinephrine, norepinephrine, dopamine, and serotonin – Are active in the CNS and PNS Amino Acids and Peptides • Various amino acids and peptides – Are active in the brain Gases • Gases such as nitric oxide and carbon monoxide – Are local regulators in the PNS • 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 • The brain provides the 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 central canal of the spinal cord and the four ventricles of the brain – Are hollow, since they are derived from the dorsal embryonic nerve cord Gray matter White matter Ventricles Figure 48.20 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 • The cranial nerves originate in the brain – And terminate mostly in organs of the head and upper body • The spinal nerves originate in the spinal cord – And extend to parts of the body below the head • 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 Parasympathetic division Enteric division • The somatic nervous system ( somatic = body) – 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 • 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 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 ( allows us to sleep) • The enteric division – Controls the activity of the digestive tract, pancreas, and gallbladder 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 (a) Embryo at one month • By the fifth week of human embryonic development – Five brain regions have formed from the three embryonic regions Embryonic brain regions Telencephalon Diencephalon Mesencephalon Metencephalon Myelencephalon Mesencephalon Metencephalon Diencephalon Myelencephalon Spinal cord Telencephalon Figure 48.23b (b) Embryo at five weeks • As a human brain develops further – The most profound change occurs in the forebrain, which gives rise to the cerebrum Brain structures present in adult Cerebrum (cerebral hemispheres; includes cerebral cortex, white matter, basal nuclei) Diencephalon (thalamus, hypothalamus, epithalamus) Midbrain (part of brainstem) Pons (part of brainstem), cerebellum Medulla oblongata (part of brainstem) Cerebral hemisphere Diencephalon: Hypothalamus Thalamus Pineal gland (part of epithalamus) Brainstem: Midbrain Pons Pituitary gland Spinal cord Cerebellum Central canal Figure 48.23c (c) Adult Medulla oblongata The Brainstem • The brainstem consists of three parts – The medulla oblongata, the pons, and the midbrain • 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 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 Input from ears • A part of the reticular formation, the reticular activating system (RAS) – Regulates sleep and arousal 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 The Diencephalon • The embryonic diencephalon develops into three adult brain regions – The epithalamus, thalamus, and hypothalamus • The epithalamus – Includes the pineal gland and the choroid plexus • 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 • The hypothalamus regulates – Homeostasis – Basic survival behaviors such as feeding, fighting, fleeing, and reproducing Circadian Rhythms • The hypothalamus also regulates circadian rhythms – Such as the sleep/wake cycle • Animals usually have a biological clock – Which is a pair of suprachiasmatic nuclei (SCN) found in the hypothalamus • Biological clocks usually require external cues – To remain synchronized with environmental cycles EXPERIMENT In the northern flying squirrel (Glaucomys sabrinus), activity normally begins with the onset of darkness and ends at dawn, which suggests that light is an important external cue for the squirrel. To test this idea, researchers monitored the activity of captive squirrels for 23 days under two sets of conditions: (a) a regular cycle of 12 hours of light and 12 hours of darkness and (b) constant darkness. The squirrels were given free access to an exercise wheel and a rest cage. A recorder automatically noted when the wheel was rotating and when it was still. (a) 12 hr light-12 hr dark cycle Light Dark Light Dark 1 Days of experiment RESULTS When the squirrels were exposed to a regular light/dark cycle, their wheel-turning activity (indicated by the dark bars) occurred at roughly the same time every day. However, when they were kept in constant darkness, their activity phase began about 21 minutes later each day. (b) Constant darkness 5 10 15 20 Figure 48.25 12 16 20 24 4 Time of day (hr) 8 12 12 16 20 24 4 Time of day (hr) CONCLUSION The northern flying squirrel’s internal clock can run in constant darkness, but it does so on its own cycle, which lasts about 24 hours and 21 minutes. External (light) cues keep the clock running on a 24-hour cycle. 8 12 The Cerebrum • The cerebrum – Develops from the embryonic telencephalon • 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 Basal nuclei • The basal nuclei – Are important centers for planning and learning movement sequences • In mammals – The cerebral cortex has a convoluted surface called the neocortex • 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 • A thick band of axons, the corpus callosum – Provides communication between the right and left cerebral cortices • 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 Occipital lobe • Each of the lobes – Contains primary sensory areas and association areas Information Processing in the Cerebral Cortex • Specific types of sensory input – Enter the primary sensory areas • Adjacent association areas – Process particular features in the sensory input and integrate information from different sensory areas • In the somatosensory cortex and motor cortex – Neurons are distributed according to the part of the body that generates sensory input or receives motor input Frontal lobe Parietal lobe Toes Lips Jaw Tongue Tongue Pharynx Primary motor cortex Figure 48.28 Genitalia Abdominal organs Primary somatosensory cortex 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 • 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 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 Generating words • Portions of the frontal lobe, Broca’s area and Wernicke’s area – Are essential for the generation and understanding of language Emotions • The limbic system – Is a ring of structures around the brainstem Thalamus Hypothalamus Prefrontal cortex Olfactory bulb Amygdala Figure 48.30 Hippocampus • This limbic system includes three parts of the cerebral cortex – The amygdala, hippocampus, and olfactory bulb • These structures interact with the neocortex to mediate primary emotions – And attach emotional “feelings” to survivalrelated functions • Structures of the limbic system form in early development – And provide a foundation for emotional memory, associating emotions with particular events or experiences Memory and Learning • The frontal lobes – Are a site of short-term memory – Interact with the hippocampus and amygdala to consolidate long-term memory • Many sensory and motor association areas of the cerebral cortex – Are involved in storing and retrieving words and images 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. Gill withdrawal pathway Touching the siphon Siphon sensory neuron Gill motor neuron Sensitization pathway Shocking the tail Interneuron Tail sensory neuron Gill • In the vertebrate brain, a form of learning called long-term potentiation (LTP) – Involves an increase in the strength of synaptic transmission 1 The presynaptic neuron releases glutamate. 2 Glutamate binds to AMPA receptors, opening the AMPAreceptor channel and depolarizing the postsynaptic membrane. PRESYNAPTIC NEURON 7 NO diffuses into the presynaptic neuron, causing it to release more glutamate. NO 6 Ca2+ stimulates the postsynaptic neuron to produce nitric oxide (NO). Glutamate AMPA receptor NO Figure 48.32 5 Ca2+ initiates the phosphorylation of AMPA receptors, making them more responsive. Ca2+ also causes more AMPA receptors to appear in the postsynaptic membrane. NMDA receptor 3 Glutamate also binds to NMDA receptors. If the postsynaptic membrane is simultaneously depolarized, the NMDA-receptor channel opens. P Ca2+ Signal transduction pathways POSTSYNAPTIC NEURON 4 Ca2+ diffuses into the postsynaptic neuron. Consciousness • Modern brain-imaging techniques – Suggest that consciousness may be an emergent property of the brain that is based on activity in many areas of the cortex • Concept 48.7: CNS injuries and diseases are the focus of much research • Unlike the PNS, the mammalian CNS – Cannot repair itself when damaged or assaulted by disease • Current research on nerve cell development and stem cells – May one day make it possible for physicians to repair or replace damaged neurons Nerve Cell Development • Signal molecules direct an axon’s growth – By binding to receptors on the plasma membrane of the growth cone • This receptor binding triggers a signal transduction pathway – Which may cause an axon to grow toward or away from the source of the signal Midline of spinal cord Developing axon of interneuron Developing axon of motor neuron Growth cone Netrin-1 receptor Netrin-1 receptor Slit receptor Netrin-1 Floor plate 1 Growth toward the floor plate. 2 Cells in the floor plate of the spinal cord release Netrin-1, which diffuses away from the floor plate and binds to receptors on the growth cone of a developing interneuron axon. Binding stimulates axon growth toward the floor plate. Figure 48.33a, b Slit Netrin-1 Cell adhesion molecules Growth across the mid-line. 3 Once the axon reaches the floor plate, cell adhesion molecules on the axon bind to complementary molecules on floor plate cells, directing the growth of the axon across the midline. Slit Slit receptor No turning back. Now the axon synthesizes receptors that bind to Slit, a repulsion protein released by floor plate cells. This prevents the axon from growing back across the midline. (a) Growth of an interneuron axon toward and across the midline of the spinal cord (diagrammed here in cross section) Netrin-1 and Slit, produced by cells of the floor plate, bind to receptors on the axons of motor neurons. In this case, both proteins act to repel the axon, directing the motor neuron to grow away from the spinal cord. (b) Growth of a motor neuron axon away from the midline of the spinal cord • The genes and basic events involved in axon guidance – Are similar in invertebrates and vertebrates • Knowledge of these events may be applied one day – To stimulate axonal regrowth following CNS damage Neural Stem Cells • The adult human brain – Contains stem cells that can differentiate into mature neurons Figure 48.34 • The induction of stem cell differentiation and the transplantation of cultured stem cells – Are potential methods for replacing neurons lost to trauma or disease Diseases and Disorders of the Nervous System • Mental illnesses and neurological disorders – Take an enormous toll on society, in both the patient’s loss of a productive life and the high cost of long-term health care Schizophrenia • About 1% of the world’s population – Suffers from schizophrenia – 99% of AP biology teachers. jk • Schizophrenia is characterized by – Hallucinations, delusions, blunted emotions, and many other symptoms • Available treatments have focused on – Brain pathways that use dopamine as a neurotransmitter Depression • Two broad forms of depressive illness are known – Bipolar disorder and major depression • Bipolar disorder is characterized by – Manic (high-mood) and depressive (low-mood) phases • In major depression – Patients have a persistent low mood • Treatments for these types of depression include – A variety of drugs such as Prozac and lithium Alzheimer’s Disease • Alzheimer’s disease (AD) – Is a mental deterioration characterized by confusion, memory loss, and other symptoms • AD is caused by the formation of – Neurofibrillary tangles and senile plaques in the brain 20 m Senile plaque Figure 48.35 Neurofibrillary tangle • A successful treatment for AD in humans – May hinge on early detection of senile plaques Parkinson’s Disease • Parkinson’s disease is a motor disorder – Caused by the death of dopamine-secreting neurons in the substantia nigra – Characterized by difficulty in initiating movements, slowness of movement, and rigidity • There is no cure for Parkinson’s disease – Although various approaches are used to manage the symptoms