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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 Overview: Command and Control Center • The human brain contains about 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 is a technology that can reconstruct a threedimensional map of brain activity • Brain imaging and other methods reveal that groups of neurons function in specialized circuits dedicated to different tasks Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Information Processing • Nervous systems process information in three stages: sensory input, integration, and motor output Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-3 Sensory input Sensor Integration Motor output Effector 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-4 Quadriceps muscle Cell body of sensory neuron in dorsal root ganglion Gray matter White matter Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Interneuron Neuron Structure • Most of a neuron’s organelles are in the cell body • 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 • Many axons are covered with a myelin sheath Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-5 Dendrites Cell body Nucleus Axon hillock Axon Presynaptic cell Signal direction Synaptic Myelin sheath terminals Synapse Postsynaptic cell • Neurons have a wide variety of shapes that reflect input and output interactions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-6 Dendrites Axon Cell body Sensory neuron Interneurons Motor neuron Supporting Cells (Glia) • Glia are essential for structural integrity of the nervous system and for functioning of neurons • Types of glia: astrocytes, radial glia, oligodendrocytes, and Schwann cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In the CNS, astrocytes provide structural support for neurons and regulate extracellular concentrations of ions and neurotransmitters Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 50 µm LE 48-7 • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) form the myelin sheaths around axons of many vertebrate neurons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-8 Nodes of Ranvier Layers of myelin Axon Schwann cell Axon 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 cell’s inside is negative relative to the outside • Membrane potential of a cell can be measured Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-9 Microelectrode –70 mV Voltage recorder Reference electrode The Resting Potential • Resting potential is the membrane potential of a neuron that is not transmitting signals • Resting potential depends on ionic gradients across the plasma membrane Animation: Resting Potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concentration of Na+ is higher in the extracellular fluid than in the cytosol • The opposite is true for K+ • By modeling a neuron with an artificial membrane, we can better understand resting potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-10 CYTOSOL EXTRACELLULAR FLUID [Na+] 15 mM [Na+] 150 mM [K+] 150 mM [K+] 5 mM [Cl–] [Cl–] 120 mM 10 mM [A–] 100 mM Plasma membrane LE 48-11 Inner chamber –92 mV Outer chamber 150 mM KCl Inner chamber 15 mM NaCl 5 mM KCl +62 mV Outer chamber 150 mM NaCl Cl– K+ Potassium channel Cl– Na+ Sodium channel Artificial membrane Membrane selectively permeable to K+ 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 • Diffusion of K+ and Na+ leads to a separation of charges across the membrane, producing the resting potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gated Ion Channels • Gated ion channels open or close in response to one of three stimuli: – Stretch-gated ion channels open when the membrane is mechanically deformed – Ligand-gated ion channels open or close when a specific chemical binds to the channel – Voltage-gated ion channels respond to a change in membrane potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 magnitude of the membrane potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-12 Stimuli Stimuli +50 –50 Threshold Resting potential Hyperpolarizations 0 1 2 3 4 5 Time (msec) Graded potential hyperpolarizations +50 Membrane potential (mV) 0 Membrane potential (mV) Membrane potential (mV) +50 –100 Stronger depolarizing stimulus 0 –50 Threshold Resting potential –100 0 –50 Threshold Resting potential Depolarizations 0 1 2 3 4 5 Time (msec) Graded potential depolarizations Action potential –100 0 1 2 3 4 5 6 Time (msec) Action potential • Other stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Hyperpolarization and depolarization are called graded potentials • The magnitude of the change in membrane potential varies with the strength of the stimulus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Production of Action Potentials • Depolarizations are usually graded only up to a certain membrane voltage, called the threshold • A stimulus strong enough to produce depolarization that reaches the threshold triggers a response called an action potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane • It carries information along axons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Voltage-gated Na+ and K+ channels are involved in producing 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 • During the refractory period after an action potential, a second action potential cannot be initiated Animation: Action Potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-13_5 Na+ Na+ Na+ Na+ K+ K+ Rising phase of the action potential Falling phase of the action potential Na+ Na+ Membrane potential (mV) +50 0 –50 K+ Action potential –100 Threshold Resting potential Time Depolarization Na+ Na+ Extracellular fluid Na+ Potassium channel Activation gates K+ Plasma membrane Cytosol Resting state Undershoot Sodium channel K+ Inactivation gate 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-14c Axon Action potential Na+ An action potential is generated as Na+ flows inward across the membrane at one location. K+ Action potential Na+ K+ 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. K+ Action potential Na+ K+ 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 axon’s diameter • In vertebrates, axons are myelinated, also causing an action potential’s speed to increase • Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-15 Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon Concept 48.4: Neurons communicate with other cells at synapses • In an electrical synapse, 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 stored in the synaptic terminal Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-16 Synaptic terminals of presynaptic neurons 5 µm Postsynaptic neuron • When an action potential reaches a terminal, the final result is release of neurotransmitters into the synaptic cleft Animation: Synapse Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-17 Presynaptic cell Postsynaptic cell Synaptic vesicles containing neurotransmitter Na+ K+ Presynaptic membrane Neurotransmitter Postsynaptic membrane Ligandgated ion channel Voltage-gated Ca2+ channel Postsynaptic membrane Ca2+ Synaptic cleft Ligand-gated ion channels Direct Synaptic Transmission • Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels • Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential 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 • After release, the neurotransmitter diffuses out of the synaptic cleft • It may be taken up by surrounding cells and degraded by enzymes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Summation of Postsynaptic Potentials • Unlike action potentials, postsynaptic potentials are graded and do not regenerate • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-18 Terminal branch of presynaptic neuron Postsynaptic E1 neuron E1 E2 E1 E1 I Membrane potential (mV) Axon hillock 0 Action potential Action potential Threshold of axon of postsynaptic neuron Resting potential –70 E1 E1 Subthreshold, no summation E1 E1 Temporal summation E1 + E2 Spatial summation E1 I E1 + I Spatial summation of EPSP and IPSP • If two EPSPs are produced in rapid succession, an effect called temporal summation occurs 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Through summation, an IPSP can counter the effect of an EPSP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • Effects of indirect synaptic transmission have a slower onset but last longer 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Acetylcholine • Acetylcholine is a common neurotransmitter in vertebrates and invertebrates • It can be inhibitory or excitatory Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biogenic Amines • Biogenic amines include epinephrine, norepinephrine, dopamine, and serotonin • They are active in the CNS and PNS Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Amino Acids and Peptides • Four amino acids are known to function as neurotransmitters in the CNS • Several neuropeptides, relatively short chains of amino acids, also function as neurotransmitters Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gases • Gases such as nitric oxide and carbon monoxide are local regulators in the PNS Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 48.5: The vertebrate nervous system is regionally specialized • Vertebrate nervous systems show a high degree of cephalization and distinct CNS and PNS components Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-19 Central nervous system (CNS) Peripheral nervous system (PNS) Brain Spinal cord Cranial nerves Ganglia outside CNS Spinal nerves • 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 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 because they are derived from the dorsal embryonic nerve cord • Axons within the CNS are often found in bundles that have a whitish appearance • This white matter is distinguishable from gray matter, which consists mainly of dendrites, unmyelinated axons, and neuron cell bodies Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-20 Gray matter White matter Ventricles The Peripheral Nervous System • The PNS transmits information to and from the CNS and regulates movement and internal environment • Cranial nerves originate in the brain and terminate mostly in organs of the head and upper body • Spinal nerves originate in the spinal cord and extend to parts of the body below the head • The PNS has two functional components: the somatic and autonomic nervous systems Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-21 Peripheral nervous system Somatic nervous system Autonomic nervous system Sympathetic division 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The autonomic nervous system has sympathetic, parasympathetic, and enteric divisions • The sympathetic and parasympathetic divisions have antagonistic effects on target organs Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The sympathetic division correlates with the “fightor-flight” response • The parasympathetic division promotes a return to self-maintenance functions • The enteric division controls activity of the digestive tract, pancreas, and gallbladder Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-22 Sympathetic division Parasympathetic 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 Action on target organs: Dilates pupil of eye Constricts pupil of eye Inhibits salivary gland secretion Stimulates salivary gland secretion Constricts bronchi in lungs Cervical Sympathetic ganglia Accelerates heart Slows heart Stimulates activity of stomach and intestines Inhibits activity of stomach and intestines Thoracic Inhibits activity of pancreas Stimulates activity of pancreas Stimulates gallbladder Stimulates glucose release from liver; inhibits gallbladder 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 Lumbar Neurotransmitter released by postganglionic neurons: Promotes emptying acetylcholine of bladder Promotes erection of genitalia Relaxes bronchi in lungs Location of preganglionic neurons: thoracic and lumbar segments of spinal cord Stimulates adrenal medulla Inhibits emptying of bladder Sacral Synapse Promotes ejaculation and vaginal contractions Neurotransmitter released by postganglionic neurons: norepinephrine Embryonic Development of the Brain • All vertebrate brains develop from three embryonic regions: forebrain, midbrain, and hindbrain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-23 Brain structures present in adult Embryonic brain regions Telencephalon Cerebrum (cerebral hemispheres; includes cerebral cortex, white matter, basal nuclei) Diencephalon Diencephalon (thalamus, hypothalamus, epithalamus) Forebrain Midbrain Mesencephalon Midbrain (part of brainstem) Metencephalon Pons (part of brainstem), cerebellum Myelencephalon Medulla oblongata (part of brainstem) Hindbrain Mesencephalon Metencephalon Cerebral hemisphere Diencephalon: Hypothalamus Thalamus Midbrain Hindbrain Diencephalon Myelencephalon Pineal gland (part of epithalamus) Brainstem: Midbrain Pons Spinal cord Forebrain Telencephalon Pituitary gland Medulla oblongata Spinal cord Cerebellum Central canal Embryo at one month Embryo at five weeks Adult • By the fifth week of human embryonic development, five brain regions have formed from the three embryonic regions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • As a human brain develops further, the most profound change occurs in the forebrain, which gives rise to the cerebrum Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Brainstem • The brainstem has 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 receipt and integration of sensory information Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Arousal and Sleep • The core of the brainstem has a diffuse network of neurons called the reticular formation • Part of this formation, called the reticular activating system (RAS), regulates sleep and arousal Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-24 Eye Input from ears Reticular formation Input from touch, pain, and temperature receptors The Cerebellum • The cerebellum is important for coordination and error checking during motor, perceptual, and cognitive functions • It is also involved in learning and remembering motor skills Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Diencephalon • The diencephalon develops into three regions: the epithalamus, thalamus, and hypothalamus • The epithalamus includes the pineal gland and choroid plexus • The thalamus is the main input center for sensory information to the cerebrum and the main output center for motor information leaving the cerebrum • The hypothalamus regulates homeostasis and basic survival behaviors such as feeding, fighting, fleeing, and reproducing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Circadian Rhythms • The hypothalamus also regulates circadian rhythms such as the sleep/wake cycle • Animals usually have a biological clock, a pair of suprachiasmatic nuclei (SCN) in the hypothalamus • Biological clocks usually require external cues to remain synchronized with environmental cycles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-25 12 hr light–12 hr dark cycle Light Dark Constant darkness Light Dark 1 Days of experiment 5 10 15 20 12 16 24 4 20 Time of day (hr) 8 12 12 16 24 4 20 Time of day (hr) 8 12 The Cerebrum • The cerebrum develops from the embryonic telencephalon Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The cerebrum has right and left hemispheres • Each cerebral hemisphere consists of a cerebral cortex overlying white matter and basal nuclei • A thick band of axons called the corpus callosum provides communication between the right and left cerebral cortices Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The basal nuclei are important centers for planning and learning movement sequences • In mammals, the cerebral cortex has a convoluted surface called the neocortex Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-26 Left cerebral hemisphere Right cerebral hemisphere Corpus callosum Basal nuclei Neocortex • In humans, the cerebral cortex is the largest and most complex part of the brain • Here 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 • Each lobe contains primary sensory areas and association areas Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-27 Frontal lobe Parietal lobe Speech Frontal association area Taste Somatosensory association area Reading Speech Hearing Smell Temporal lobe Auditory association area Visual association area Vision Occipital lobe Information Processing in the Cerebral Cortex • Specific types of sensory input enter the primary sensory areas • Adjacent areas process features in the sensory input and integrate information from different sensory areas • In the somatosensory and motor cortices, neurons are distributed according to the body part that generates sensory input or receives motor input Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-28 Frontal lobe Parietal lobe Toes Genitalia Lips Jaw Tongue Pharynx Primary motor cortex Abdominal organs Primary somatosensory cortex Lateralization of Cortical Function • During brain development, competing functions segregate and displace each other in the cortex of the left and right cerebral hemispheres • This process results in lateralization of functions • The left hemisphere is more adept at language, math, logic, and 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 areas responsible for language and speech • Portions of the frontal lobe, 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 LE 48-29 Max Hearing words Seeing words Min Speaking words Generating words Emotions • The limbic system is a ring of structures around the brainstem • It 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 survival-related functions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-30 Thalamus Hypothalamus Prefrontal cortex Olfactory bulb Amygdala Hippocampus • Structures of the limbic system form in early development and provide a foundation for emotional memory, associating emotions with particular events or experiences Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Memory and Learning • The frontal lobes are a site of short-term memory • They 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-31a Siphon Mantle Gill Tail Head LE 48-31b Gill withdrawal pathway Touching the siphon Siphon sensory neuron Gill motor neuron Gill Sensitization pathway Shocking the tail Interneuron Tail sensory neuron • In the vertebrate brain, a form of learning called long-term potentiation (LTP) involves an increase in the strength of synaptic transmission Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-32 PRESYNAPTIC NEURON NO NMDA receptor Glutamate AMPA receptor NO P Ca2+ Signal transduction pathways POSTSYNAPTIC NEURON Consciousness • Modern brain-imaging techniques suggest that consciousness is an emergent property of the brain based on activity in many areas of the cortex Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 48.7: CNS injuries and diseases are the focus of much research • Unlike the PNS, the mammalian CNS cannot repair itself when damaged by disease • Current research on nerve cell development and stem cells may one day make it possible to repair or replace damaged neurons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 signal source Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-33a Midline of spinal cord Developing axon of interneuron Growth cone Netrin-1 receptor Netrin-1 Floor plate Growth toward the floor plate Cell adhesion molecules Growth across the midline Slit Slit receptor No turning back Growth of an interneuron axon toward and across the midline of the spinal cord (diagrammed here in cross section) LE 48-33b Developing axon of motor neuron Netrin-1 receptor Slit receptor Slit Netrin-1 Growth of a motor neuron axon away from the midline of the spinal cord • Genes and basic events 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Neural Stem Cells • The adult human brain contains stem cells that can differentiate into mature neurons • Induction of stem cell differentiation and transplantation of cultured stem cells are potential methods for replacing neurons lost to trauma or disease Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 10 µm LE 48-34 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 longterm health care 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 other symptoms • Available treatments focus on brain pathways that use dopamine as a neurotransmitter 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 (highmood) and depressive (low-mood) phases • In major depression, patients have a persistent low mood • Treatments for these types of depression include drugs such as Prozac and lithium 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 • AD is caused by the formation of neurofibrillary tangles and senile plaques in the brain • A successful treatment in humans may hinge on early detection of senile plaques Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 48-35 Senile plaque Neurofibrillary tangle 20 µm Parkinson’s Disease • Parkinson’s disease is a motor disorder caused by death of dopamine-secreting neurons in the substantia nigra • It is characterized by difficulty in initiating movements, slowness of movement, and rigidity • There is no cure, although various approaches are used to manage symptoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings