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Chapter 48 Nervous Systems Figure 48.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organization of Nervous Systems The simplest animals with nervous systems, the cnidarians (neurons arranged in nerve nets) Sea stars have a nerve net in each arm (Connected by radial nerves to a central nerve ring) Radial nerve Nerve ring Nerve net Figure 48.2a (a) Hydra (cnidarian) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 48.2b (b) Sea star (echinoderm) In cephalized animals, such as flatworms (A CNS is evident) In vertebrates: The central nervous system consists of a brain and dorsal spinal cord (PNS connects to the CNS) Eyespot Brain Brain Nerve cord Transverse nerve Spinal cord (dorsal nerve cord) Figure 48.2h Figure 48.2c (c) Planarian (flatworm) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) – Are glia (supporting cells) 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 • 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 • In all neurons, the resting potential – Depends on the ionic gradients that exist across the plasma membrane – The concentration of Na+ is higher in the extracellular fluid than in the cytosol – While the opposite is true for K+ EXTRACELLULAR FLUID CYTOSOL [Na+] 15 mM – + [Na+] 150 mM [K+] 150 mM – + [K+] 5 mM – + 10 mM – [Cl–] + 120 mM [A–] 100 mM – + [Cl–] Plasma membrane Figure 48.10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Impulse begins when a neuron is stimulated by another neuron or by the environment • Electrical impulse moves in one direction: Dendrites → Cell Body → Axon • Synapse: gap between 2 neurons • Neurotransmitters send the signal to the following neuron Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action Potential • 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Depolarization: reduction in magnitude of membrane potential (inside becomes less negative) • Hyperpolarization: increase in the magnitude of the membrane potential (inside becomes more negative) • When a stimulus depolarizes the membrane – • As the action potential subsides – • Na+ channels open, allowing Na+ to diffuse into the cell 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 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 • 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 VIDEO Action potential – – + ++ Na + + – – K+ + + – – – – + + K+ Figure 48.14 + – – + + – – + + + + + + + – – + – – + – – + – – + – – + – – + + – – + + – – + Action potential – + Na – + + + + – – K+ + + – – – – + + 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 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 Schwann cell Depolarized region (node of Ranvier) Myelin sheath –– – Cell body Figure 48.15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings + ++ + ++ ––– –– – + + + ++ Axon –– – • 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 VIDEO Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 3 Synaptic cleft Figure 48.17 Ligand-gated ion channels Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Postsynaptic membrane 6 Major neurotransmitters • Can produce different effects in different types of cells Table 48.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 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 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 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 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 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 The Cerebellum • The cerebellum – Is important for coordination and error checking during motor, perceptual, and cognitive functions, learning and remembering motor skills Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Diencephalon • The embryonic diencephalon develops into three adult brain regions – The epithalamus, thalamus, and hypothalamus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The epithalamus – Includes the pineal gland and the choroid plexus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 • The hypothalamus regulates – Homeostasis – Basic survival behaviors such as feeding, fighting, fleeing, and reproducing – sleep/wake cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Cerebrum • The cerebrum – Develops from the embryonic telencephalon 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 • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 • 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 • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Schizophrenia • About 1% of the world’s population – Suffers from schizophrenia • Schizophrenia is characterized by – Hallucinations, delusions, blunted emotions, and many other symptoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alzheimer’s Disease • Alzheimer’s disease (AD) – Is a mental deterioration characterized by confusion, memory loss, and other symptoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 – No cure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings