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Chapter 48 • The Nervous System Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Functional magnetic resonance imaging – 3-D map of brain activity Figure 48.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Brain imaging reveals groups of neurons function in specialized circuits dedicated to different tasks Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • All animals except sponges have some type of nervous system Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organization of Nervous Systems • Simplest animals nerve nets Nerve net Figure 48.2a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (a) Hydra (cnidarian) • Sea stars nerve net connected by radial nerves to a central nerve ring Radial nerve Nerve ring Figure 48.2b (b) Sea star (echinoderm) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Simple cephalized animals, e.g. flatworms – central nervous system (CNS) Eyespot Brain Nerve cord Transverse nerve Figure 48.2c (c) Planarian (flatworm) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Annelids and arthropods – Segmentally arranged clusters of neurons called ganglia • Ganglia connect to the CNS – and make up a peripheral nervous system (PNS) Brain Ventral nerve cord Segmental ganglion Figure 48.2d, e Brain Ventral nerve cord Segmental ganglia (d) Leech (annelid) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (e) Insect (arthropod) • Molluscs • Sessile molluscs simple systems • Complex molluscs sophisticated Anterior nerve ring Ganglia Brain Longitudinal nerve cords Figure 48.2f, g (f) Chiton (mollusc) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ganglia (g) Squid (mollusc) • Vertebrates – CNS brain and dorsal spinal cord – The PNS connects to the CNS Brain Spinal cord (dorsal Sensory ganglion nerve cord) Figure 48.2h Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (h) Salamander (chordate) Information Processing • 3 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) • Sensory information is sent to the CNS – Where interneurons integrate the information • Motor output leaves the CNS via motor neurons – Communicate with effector cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 3 stages 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 6 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 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 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 • Dendrites – Receive signals from other neurons • Axon – Transmits signals to other cells at synapses – May be covered with myelin sheath Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dendrites Axon Cell body Figure 48.6a–c (a) Sensory neuron (b) Interneurons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (c) Motor neuron Supporting Cells (Glia) • Structural integrity & normal functioning of neurons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In the CNS, astrocytes 50 µm – structural support for neurons Figure 48.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) – glia that form the myelin sheaths around the axons Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Axon Nodes of Myelin sheath Ranvier Nucleus of Schwann cell Figure 48.8 0.1 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Ion pumps and channels maintain the resting potential of a neuron • Every cell has a voltage – Called a membrane potential • The inside of a cell is negative (-) – Relative to the outside Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Measurement 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 Resting Potential • Membrane potential of a neuron that is not transmitting signals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Depends on ionic gradients EXTRACELLULAR FLUID CYTOSOL [Na+] 15 mM [K+] 150 mM [Cl–] 10 mM [A–] 100 mM – + [Na+] 150 mM – + [K+] 5 mM – + – + – + [Cl–] 120 mM Plasma membrane Figure 48.10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Conc. of Na+ is higher in the extracellular fluid than in the cytosol • K+ higher in cytosol Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gated Ion Channels • Open or close in response to a change in the membrane potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action potentials • Signals conducted by axon Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • hyperpolarization – 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • depolarization – Reduction in the magnitude of the membrane Stimuli potential Membrane potential (mV) +50 0 –50 Threshold RestingDepolarizations 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 Production of Action Potentials • A stimulus strong enough to produce a depolarization that reaches the threshold action potential Stronger depolarizing stimulus Membrane potential (mV) +50 Action potential 0 –50 Threshold Resting potential –100 0 1 2 3 4 5 6 Time (msec) Figure 48.12c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (c) Action potential triggered by a depolarization that reaches the threshold. • Action potential – Brief all-or-none depolarization of a neuron’s plasma membrane – Signal that carries information along axons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings generation of an action potential Na+ – – – – + + + + Na+ + + – – Na+ + + + + K+ 3. Rising phase of 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 3 Rising phase of the action potential the membrane positive with respect to the outside. 4 +50 + + + + K+ – A stimulus opens the + activation gates on some Na channels. Na+ influx through those channels depolarizes the membrane. If the depolarization reaches the Depolarization threshold, it triggers an action potential. – – 2. Depolarization 2 – – – – 0 Membrane potential (mV) + + 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. Action potential Na+ + + 3 2 –50 4 –100 5 1 1 Resting potential 5. Undershoot Time Threshold Na+ Extracellular fluid Na+ Potassium channel Cytosol Na+ + + – – – – + + K + – – – Sodium channel + + Activation gates Plasma + + membrane + + + + + + igure 48.13 4. Falling + + phase of Acton – – Potential +K Na+ 1. Resting 1 state Na+ K+ Inactivation gate 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 • Travels 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 an electrical current depolarizes the neighboring region of the axon membrane Axon Action potential – – + + + + + + + + +Na + – – – – – – – – – – – – – + + + + + + + – K+ + – – + + – – K+ + Action potential – + – + Na+ + – + – K+ Figure 48.14 + + – – – – + + + – – + – – + + K+ + – – + – + – – + – – + + + + – Action potential – + Na++ + – – – + – + + – + + – – Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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. is Conduction Speed • Increases with the diameter of an axon • Myelinated axons also faster 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 • Neurons communicate with other cells at synapses • Electrical synapse – current flows directly from one cell to another via a gap junction • Most synapses are chemical synapses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Presynaptic neuron releases 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 action potential reaches a terminal release of neurotransmitters into the synaptic cleft Postsynaptic cell Presynaptic cell Synaptic vesicles containing neurotransmitter 5 Presynaptic membrane Ligandgated ion channel Ca2+ channel Ca2+ 4 2 3 Synaptic cleft Figure 48.17 Neurotransmitter Postsynaptic membrane Voltage-gated 1 Na+ K+ Ligand-gated ion channels Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Postsynaptic membrane 6 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 • One of the most common neurotransmitters • Can be inhibitory or excitatory Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biogenic Amines • Include epinephrine, norepinephrine, dopamine, and serotonin Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gases • e.g. nitric oxide and carbon monoxide – local regulators in the PNS Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Specialization and regionalization • High degree of cephalization and distinct CNS and PNS components in verts. 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 • Brain integrative power • Spinal cord integrates simple responses to certain kinds of stimuli and conveys information to and from the brain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Spinal cord and 4 ventricles of the brain – Are hollow, (derived from the dorsal embryonic nerve cord) Gray matter White matter Ventricles Figure 48.20 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Peripheral Nervous System • Transmits information to and from the CNS – regulates movement and internal environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PNS Peripheral nervous system Somatic Autonomic nervous system nervous system Sympathetic division Figure 48.21 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Parasympathetic division Enteric division • Somatic nervous system – signals to skeletal muscles • Autonomic nervous system – Regulates internal environment, involuntary – Divided into the sympathetic, parasympathetic, and enteric divisions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sympathetic and Parasympathetic divisions • Antagonistic 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 Sacral Synapse Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Promotes ejaculation and vaginal contractions Neurotransmitter released by postganglionic neurons: norepinephrine • Sympathetic division – “fight-or-flight” response • Parasympathetic division – return to self-maintenance functions • Enteric division – Activity of the digestive tract Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Embryonic Development of the Brain – 3 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • By 5th week of development – 5 brain regions have formed Embryonic brain regions Telencephalon Diencephalon Mesencephalon Metencephalon Myelencephalon Mesencephalon Metencephalon Diencephalon Myelencephalon Spinal cord Telencephalon Figure 48.23b (b) Embryo at five weeks Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • As development continues – most profound change occurs in forebrain, cerebrum Brain structures present in adult Cerebrum (cerebral hemispheres; includes cerebral cortex, white matter, basal nuclei) Diencnephalo (thalamus, hypothalamus, epithalamus) Midbrain (part of brainstem) Pons (part of brainstem), cerebellum Medulla oblongata (part of brainstem) Diencephalon: Cerebral hemisphere Hypothalamus Thalamus Pineal gland (part of epithalamus) Brainstem: Midbrain Pons Pituitary gland Medulla oblongata Spinal cord Cerebellum Central canal Figure 48.23c (c) Adult Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Brainstem • Medulla oblongata, the pons, and the midbrain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The medulla oblongata – visceral functions • The pons – visceral functions • The midbrain – integration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Arousal and Sleep • reticular formation Eye Input from ears Reticular formation Figure 48.24 Input from touch, pain, and temperature receptors Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Cerebellum • Coordination of motor, perceptual, and cognitive functions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diencephalon • Epithalamus, thalamus, and hypothalamus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Thalamus – sensory input and information output Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Hypothalamus – Homeostasis – Basic survival behaviors such as feeding, fighting, fleeing, and reproducing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hypothalamus & Circadian Rhythms • Such as the sleep/wake cycle • Biological clock Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Biological clocks require external cues for 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 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4 8 12 The Cerebrum • Develops from the embryonic telencephalon Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Right and left cerebral hemispheres Left cerebral hemisphere Right cerebral hemisphere Corpus callosum Neocortex Figure 48.26 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Basal nuclei • In humans, the largest and most complex part of the brain – Is the cerebral cortex, sensory information is analyzed, motor commands issued, and language generated Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Corpus callosum – Communication between the right and left cerebral cortices Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Cerebral cortex controls voluntary movement and cognitive functions • 4 lobes for ea. side – 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 Frontal lobe Parietal lobe Genitalia Toes Lips Jaw Tongue Tongue Pharynx Primary motor cortex Figure 48.28 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abdominal organs Primary somatosensory cortex • The left hemisphere – language, math, logical operations, and the processing of serial sequences • The right hemisphere – pattern recognition, nonverbal thinking, and emotional processing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Portions of the frontal lobe, (Broca’s area and Wernicke’s area) language Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Emotions • Limbic system Thalamus Hypothalamus Prefrontal cortex Olfactory bulb Amygdala Figure 48.30 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hippocampus • limbic system includes 3 parts – The amygdala, hippocampus, and olfactory bulb • mediate primary emotions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Memory and Learning • The frontal lobes – short-term memory – Interact with the hippocampus and amygdala to consolidate long-term memory Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Neural Stem Cells • cells that can differentiate into mature neurons Figure 48.34 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diseases and Disorders of the Nervous System Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Schizophrenia • Hallucinations, delusions, blunted emotions, and many other symptoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Depression • Bipolar disorder and major depression Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Bipolar disorder – Manic (high-mood) and depressive (low-mood) phases • Major depression – Persistent low mood Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alzheimer’s Disease • deterioration characterized by confusion, memory loss, and other symptoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • AD is caused by the formation of – Tangles and senile plaques in the brain Senile plaque Neurofibrillary tangle Figure 48.35 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 20 m Parkinson’s Disease • Motor disorder characterized by difficulty in initiating movements, slowness of movement, and rigidity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings