Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Figure 48.2a Nerve net (a) Hydra (cnidarian) Figure 48.2b Nerve ring Radial nerve (b) Sea star (echinoderm) 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)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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) Figure 48.2h Brain Spinal cord (dorsal nerve cord) Sensory ganglion (h) Salamander (chordate) Figure 48.2c Eyespot Brain Nerve cord Transverse nerve (c) Planarian (flatworm)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Information Processing Nervous systems process information in three stages – Sensory input, integration, and motor output Figure 48.3 Sensor Effector Motor output Integration Sensory input Peripheral nervous system (PNS) Central nervous system (CNS)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Figure 48.5 Dendrites Cell body Nucleus Axon hillock Axon Signal direction Synapse Myelin sheath Synaptic terminals Presynaptic cell Postsynaptic cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Myelin sheath Nodes of Ranvier Schwann cell Schwann cell Nucleus of Schwann cell Axon Layers of myelin Node of Ranvier 0.1 µm Axon Figure 48.8

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 + CYTOSOL EXTRACELLULAR FLUID [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 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 action potential Figure 48.12c –50 –100 Time (msec) Threshold Resting potential Membrane potential (mV) Stronger depolarizing stimulus Action potential (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 – 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The generation of an action potential – – – – – + – + – + – + – + – + – + – + – + – + – + – + – + – + – + – + Na + K+K+ K+K+ K+K+ K+K+ K+K+ 5 1 Resting state 2 Depolarization 3 Rising phase of the action potential 4 Falling phase of the action potential Undershoot Sodium channel Action potential Resting potential Time Plasma membrane Extracellular fluid Activation gates Potassium channel Inactivation gate Threshold Membrane potential (mV) –50 –100 Threshold Cytosol Figure 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. 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. 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. The activation gates on the Na + and K + channels are closed, and the membrane’s resting potential is maintained. 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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure – + – – + – – + – – + – – + – – –– – + – + – – –– – –– – – –– – – –– –– – – – – – –– – – – – – –– – – Na + Action potential K+K+ K+K+ K+K+ Axon An action potential is generated as Na + flows inward across the membrane at one location. 1 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. K+K+ 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 VIDEO

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 Cell body Schwann cell Myelin sheath Axon Depolarized region (node of Ranvier) – – – – – – Figure 48.15

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Figure Presynaptic cell Postsynaptic cell Synaptic vesicles containing neurotransmitter Presynaptic membrane Postsynaptic membrane Voltage-gated Ca 2+ channel Synaptic cleft Ligand-gated ion channels Na + K+K+ Ligand- gated ion channel Postsynaptic membrane Neuro- transmitter 1Ca

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Figure Central nervous system (CNS) Peripheral nervous system (PNS) Brain Spinal cord Cranial nerves Ganglia outside CNS Spinal nerves

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 Parasympathetic division Enteric division Figure 48.21

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 divisionSympathetic division Action on target organs: Location of preganglionic neurons: brainstem and sacral segments of spinal cord Neurotransmitter released by preganglionic neurons: acetylcholine Location of postganglionic neurons: in ganglia close to or within target organs Neurotransmitter released by postganglionic neurons: acetylcholine Constricts pupil of eye Stimulates salivary gland secretion Constricts bronchi in lungs Slows heart Stimulates activity of stomach and intestines Stimulates activity of pancreas Stimulates gallbladder Promotes emptying of bladder Promotes erection of genitalia Cervical Thoracic Lumbar Synapse Sympathetic ganglia Dilates pupil of eye Inhibits salivary gland secretion Relaxes bronchi in lungs Accelerates heart Inhibits activity of stomach and intestines Inhibits activity of pancreas Stimulates glucose release from liver; inhibits gallbladder Stimulates adrenal medulla Inhibits emptying of bladder Promotes ejaculation and vaginal contractions Sacral Location of preganglionic neurons: thoracic and lumbar segments of spinal cord Neurotransmitter released by preganglionic neurons: acetylcholine Location of postganglionic neurons: some in ganglia close to target organs; others in a chain of ganglia near spinal cord Neurotransmitter released by postganglionic neurons: norepinephrine Figure 48.22

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Corpus callosum Neocortex Right cerebral hemisphere Basal nuclei Figure 48.26

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 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 Temporal lobeOccipital lobe Parietal lobe Frontal association area Speech Smell Hearing Auditory association area Vision Visual association area Somatosensory association area Reading Speech Taste Somatosensory cortex Motor cortex Figure 48.27

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 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