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The Nervous SystemI. Cellular Communication A. Gap Junctions1. Communication between adjacent cells 2. Not efficient for communication between distant cells B. Neurons 1. Rapid transmission of information over long distances 2. Organized into intricate networks called nervous systems II. Neurons And Neuroglia A. Structure of a Neuron 1. Varied appearance, similar functional architecture 2. Cell body is an enlarged region containing nucleus 3. Dendrites extend from cell body a) One or more branched cytoplasmic extensions b) Cell receives input from many sources simultaneously 4. Surface of cell body integrates information arriving at dendrites 5. If membrane excitation is enough it triggers impulse sent along axon a) Electrical current travels outward from cell body b) Generally only one for each cell body, may be quite long c) Single axon in a giraffe may be three meters long B. Neuron Support System 1. Structural and functional support supplied by neuroglia cells a) Comprise half of the volume of vertebrate nervous systems b) Include Schwann cells and oligodendrocytes (1) Envelop axon at particular intervals, form myelin sheath (2) Insulating material of layers of neuroglial membrane (3) Interrupted at nodes of Ranvier 2. Nerve: bundles of myelinated and nonmyelinated fibers a) Myelinated axons have myelin sheath b) Unmyelinated axons lack myelin sheath III. The Nerve Impulse A. The Membrane Potential 1. Potential difference in electrical charge exists between two poles a) Example battery terminals b) One pole positive, other negative 2. Cells have potential difference across plasma membrane a) Side of membrane exposed to cytoplasm is negative b) Side of membrane exposed to extracellular fluid is positive c) Cellular voltage called membrane potential (1) Resting membrane potential: cell not producing impulses (2) Generally about 70 millivolts, written -70 mV 3. Origin of membrane potential a) Ionic compound in water dissociates into ions, forms electrolyte solution (1) Positive ions are cations (2) Negative ions are anions (3) Example: table salt dissociates into Na+ and Cl- ions b) Cellular electrolyte solution different from extracellular solution (1) Cytoplasm contains negatively charged proteins and organic phosphates (2) K+ is 30 times more concentrated in cytoplasm than extracellularly (3) Na+ and Cl- are 10-20 times higher in extracellular fluid c) Concentration gradients form between inside and outside of cell (1) Net diffusion of Na+ and Cl- into cell (2) Net diffusion of K+, proteins and organic phosphates out of cell 4. Cell membrane permeability a) Impermeable to proteins and organic phosphates, trapped in cytoplasm b) Somewhat permeable to Na+, K+ and Cl- c) Permeability due to ion channels, (1) Membrane-spanning transport proteins with water-filled pores (2) When open ions can diffuse through (3) Channels are specific for only one kind of ion 5. Permeability in resting neuron a) Channels with open pores are selective for K+ b) Cell is more permeable to K+ than any other ion c) If only K+ could pass, it would reach an equilibrium state (1) No more net movement, diffusion out equals diffusion in (2) Membrane potential called equilibrium potential (3) Value is about -90 mV for most vertebrates d) True resting potential different since some open channels are Na+ not K+ (1) Na+ diffuses in through these channels (2) Equilibrium potential for Na+ is +60mV (3) At rest neither Na+ or K+ is in equilibrium (4) K+ continually diffuses out, Na+ continually diffuses in (5) Sodium-potassium pump also contributes to membrane potential 6. Cell membrane potential can change in response to stimulation a) Depolarization: shift in positive direction (-70 mV to -50 mV) b) Hyperpolarization: shift to more negative (-70mV to -85 mV) B. Generation of Action Potentials 1. Nerve and muscle cells have unique Na+ and K+ channels a) Have gates, protein portion opens or closes pore with change in membrane potential b) Channel called voltage-gates ion channel c) Gates for Na+ and K+ are closed at resting membrane potential d) Open in response to membrane depolarization, allow ion to flow through 2. If initial depolarization is strong enough action potential is generated a) Depolarization must reach cell's threshold level b) Typically -55 mV, 15mV more positive than resting potential c) No action potential produced if depolarization below this level d) Depolarization at this level opens both Na+ and K+ channels, Na+ first (1) Rapid diffusion of Na+ into cell shifts membrane potential toward equilibrium potential for Na+ (+60mV) (2) Membrane reverses polarity as Na+ rushes in (3) On an oscilloscope this shows a rising phase of a spike (4) Membrane potential does not reach +60 mV because Na+ channels close e) At same time K+ channels finally open (1) K+ diffuses out of cell restoring resting membrane potential (2) Repolarization appears as falling phase on oscilloscope (3) Repolarization may slightly undershoot and be more negative for brief period f) Entire sequence of events over in a few milliseconds 3. Action potentials have three useful characteristics a) Produced only when depolarizing stimulus reaches cell's threshold b) When produced follow an all-or-none law c) Are always separate events, can not add together or interfere with each other (1) Due to refractory period that exists after action potential is generated (2) New action potential cannot form for a brief period of time 4. Production of action potential results entirely from passive diffusion of ions a) At end cytoplasm has a little more Na+ and a little less K+ b) Active transport returns cell to normal C. Propagation of Action Potentials 1. Action potential in membrane causes adjacent-gated channels to open 2. Each action potential serves as depolarizing stimulus for another action potential down the membrane 3. Propagates action potential along axon or muscle cell 4. Action potential remains same size along entire length, no strength lost 5. Velocity of propagation dependent on fiber diameter and myelination a) Greater conduction velocity in larger fibers b) Myelination dramatically increases velocity via saltatory conduction (1) Myelin sheath interrupted at nodes of Ranvier (2) Action potential jumps from node to node IV. Transferring Information From Neurons To Other Cells A. Synapses 1. Axon/dendrite junctions, sites on muscles or gland cells 2. Synaptic cleft: narrow intercellular gap between axon and target cell a) Presynaptic membrane: side where axon transmits action potential b) Postsynaptic membrane: opposite side of cleft 3. Signals cross synaptic cleft chemically a) Axonal ending on presynaptic side contain synaptic vesicles b) Vesicles contain chemicals called neurotransmitters c) With impulse, membrane depolarized, neurotransmitters released into cleft d) Chemicals bind to receptors on postsynaptic side e) Cause chemically gated ion channels to open f) Not opened by membrane depolarization like voltage-gated channels of axons 4. Advantages of chemical junctions a) Different chemicals in different junctions permit varied responses b) More than sixty chemicals are transmitters or modify activity of transmitters B. The Vertebrate Neuromuscular Junction 1. Synapse between axon and muscle cell 2. Neurotransmitter is acetylcholine (ACh) a) Acetylcholine molecules bind to receptors in muscle fiber membrane b) Open chemically gated channels that allow both Na+ and K+ to flow c) More Na+ enter than K+ leave, depolarize muscle fiber membrane d) Opens voltage-gated channels e) Action potential produced and conducted by muscle fiber f) Depolarization excites muscle fiber: excitatory postsynaptic potential (EPSP) g) Neuromuscular junction is an excitatory synapse h) Stimulates entry of calcium ions, triggers muscle contraction 3. ACh must be destroyed for next transmission to occur or muscle remains contracted a) Acetylcholinesterase: enzyme that destroys acetylcholine b) Super fast-acting cleaves one ACh every 40 microseconds 4. Organic phosphates inhibit acetylcholinesterase a) Nerve gases tabun and sarin, agricultural insecticide parathion b) Produce continual neuromuscular stimulation, cause paralysis of respiratory muscles C. Synapses Between Neurons 1. Utilize ACh or other chemical transmitters, possess other possible outcomes 2. Excitatory synapse a) Neurotransmitter opens both Na+ and K+ channels b) Depolarization small compared to postsynaptic neuron's threshold c) Makes postsynaptic neuron more likely to produce action potential and EPSP 3. Inhibitory synapse a) Makes postsynaptic neuron less likely to generate action potential b) Binding frequently causes hyperpolarization c) Membrane potential change called an inhibitory postsynaptic potential (IPSP) d) Neurotransmitter may be gamma-aminobuteric acid (GABA) (1) Opens a Cl- channel (2) Inward diffusion of Cl- produces hyperpolarization 4. Nerve cell can possess both excitatory and inhibitory synapses a) EPSPs and IPSPs from both synapses reach cell body b) Small EPSPs add together, IPSPs subtract from effects of EPSPs c) Process called synaptic integration V. Organization Of The Vertebrate Nervous System A. Composition of the Vertebrate Central Nervous Systems (CNS) 1. Brain and spinal cord a) Do most information processing b) Consist primarily of interneurons and neuroglia 2. White vs. gray matter a) White matter: myelinated axons b) Gray matter: unmyelinated cell bodies and dendrites c) In spinal cord gray matter surrounded by tracts of white matter 3. Ascending tracts carry sensory information to brain 4. Descending tracts carry impulses from brain a) Go to motor neurons and interneurons in ventral part of spinal cord gray matter b) Control muscles of body B. Peripheral Nervous System (PNS) 1. Includes all nerve pathways outside of brain and spinal cord 2. Somatic nervous system: motor pathway, stimulates skeletal muscle contractions 3. Autonomic nervous system: regulates activity of smooth muscles, cardiac muscle, glands 4. Nerves connect CNS with muscles and glands a) Also connect to ganglia b) Collections of neurons outside CNS VI. Evolution Of The Vertebrate Brain A. Early Chordate Brains Were Quite Primitive 1. Little more than swellings at end of nerve cord 2. Served as sensory centers, received messages from sensory receptors 3. Brains evolved independently in vertebrates, cephalopods, arthropods B. Basic Organization of the Vertebrate Brain 1. Hindbrain (rhombencephalon) a) Principal component of early brains b) Primary component of present-day fishes c) Composed of cerebellum, pons and medulla d) Extension of the spinal cord, coordinates motor reflexes e) Cerebellum is coordinating center (1) Size increased in more advanced vertebrates (2) Involved in evaluating limb position, muscle state, general body position 2. Midbrain a) Composed of optic lobes in fishes b) Receive and process visual information 3. Forebrain a) Compose olfactory lobes in fishes b) Receive and process olfactory (smel l) information C. The Dominant Forebrain 1. Further development in amphibians, more prominent in reptiles 2. Diencephalon: thalamus and hypothalamus a) Thalamus integrates sensory information b) Hypothalamus participates in basic drives and emotions, controls pituitary secretions 3. Telencephalon a) Located in front of forebrain b) Devoted to associative activity c) Called cerebrum in mammals D. Expansion of the Cerebrum 1. Ratio of brain mass to body mass a) Discontinuity between fish/reptiles and birds/mammals b) Mammalian brains particularly large in comparison to body mass 2. Cerebrum is the center for correlation, association and learning a) Receives sensory data from thalamus b) Issues motor commands to spinal cord via descending tracts c) Pyramidal tracts: major descending tracts (1) Control motor neurons in spinal cord (2) In turn stimulate skeletal muscles to contract VII. Anatomy And Function Of The Human Forebrain A. Cerebrum 1. Split into two cerebral hemispheres 2. Connected by nerve tract = corpus callosum 3. Hemispheres divided into lobes: frontal, parietal, occipital, temporal 4. Each hemisphere receives information from opposite side of body a) Controls motor activities on that side b) Damage due to stroke results in loss of sensation, paralysis on opposite side B. Cerebral Cortex 1. Outer layer of cerebral surface 2. Location of most neural activity 3. Surface highly convoluted to increase surface area 4. Three categories of activity: motor, sensory, associative 5. Primary motor cortex a) Lies along posterior border of frontal lobe b) Each point on surface associated with movement of a body part 6. Primary somatosensory cortex a) Lies along anterior edge of parietal lobe b) Each point receives input from sensory neuron on part of body 7. Auditory cortex a) Lies within temporal lobe b) Surfaces associated with different sound frequencies 8. Visual cortex a) Lies on occipital lobe b) Processes information from different positions on retina 9. Association cortex a) Portion not occupied by any of the other motor/sensory cortexes b) Site of higher mental activities 95% of surface in humans, 5% of surface in mouse C. Basal Ganglia 1. Buried deep within white matter of cerebrum, produce islands of gray matter 2. Receive sensory information and motor commands from cerebral cortex 3. Participate in control of muscle activity 4. Damage produces tremor associated with Parkinson's disease D. Thalamus 1. Primary site of sensory integration 2. Relays information from sensors to appropriate area of brain E. Hypothalamus 1. Integrates visceral activities: temperature, respiration, heartbeat 2. Associated with limbic system 3. Directs secretion of the pituitary gland, in turn regulates endocrine organs 4. Interconnected with cerebral cortex and brainstem control centers 5. Helps coordinate neural and hormonal responses to internal stimuli and emotions F. Limbic System 1. Composed of hippocampus and amygdala 2. Linked structures involved with emotional responses 3. Hippocampus also involved with memory function and recall VIII. Activites Of The Brain A. Arousal and Sleep 1. Reticular system is a diffuse collection of neurons in brainstem a) Reticular activating system controls consciousness and alertness b) All sensory pathways feed into this system c) When stimulated, increases level of activity in many parts of brain d) Neural pathways depressed by anesthetics and barbiturates 2. Controls both sleep and waking states a) Easier to sleep in darkened room due to fewer visual stimuli b) Activity reduced by serotonin c) Activity measured by an electroencephalogram (EEG) d) EEG signals associated with sleep (1) Sleep characterized by large alpha waves (2) Slow-wave sleep associated with reduced body activities (3) Dreaming occurs in REM phase of sleep B. Language 1. Dominant (left in most peopl e) hemisphere specializes in language 2. Wernicke`s area interprets language and formulates speech 3. Broca`s area generates motor output resulting in speech C. Spatial Recognition 1. Specialization of the non-dominant (righ t) hemisphere 2. Facial recognition site 3. Involved with spacial relationships and musical activity 4. Consolidates memories of non-verbal experiences D. Memory and Learning 1. Memory not located in particular identifiable portions of the brain 2. Removal of portions of temporal lobe result in impaired memory 3. First stage of memory is transient, short-term memory a) Can be removed from the brain by electrical shock b) Long-term memories are preserved 4. Long-term memory based on changes in synaptic efficacy 5. Damage to temporal lobes, hippocampus and amygdala a) Affects short-term memory and memory consolidation b) Affects ability to process short-term into long-term memory 6. Long-term potentiation (LTP) a) Synapses used intensively for short period have more effective transmission b) Presynaptic neuron may release increased amounts of neurotransmitter c) Postsynaptic neuron may become increasingly sensitive to neurotransmitter d) May be responsible for some aspects of memory storage IX. Peripheral Nervous System A. Controls Voluntary and Involuntary Body Functions 1. Voluntary functions controlled by somatic system 2. Involuntary functions controlled by autonomic system, endocrine glands B. The Somatic Nervous System 1. Reflex Control of Skeletal Muscles a) Somatic motor neurons stimulate skeletal muscles to contract (1) In response to conscious commands (2) As part of reflexes that do not require conscious control b) Example: knee jerk reflex (1) Example of a muscle stretch reflex (2) Sensory neuron makes excitatory synapse directly on somatic motor neuron a) When muscle stretched, sensory neurons excite motor neurons b) Action potentials in motor neurons cause muscle to contract (3) Monosynaptic reflex: only one synapse crossed by reflex arc c) Most reflexes involve a relay of information (1) Information from sensory neuron relayed through one or more interneurons (2) Goes to motor neuron (3) Motor neuron stimulates contraction of appropriate muscle (4) Function may be modulated by interneurons d) Dorsal root ganglia (1) Axons of sensory neurons enter dorsal surface of spinal cord in dorsal root (2) Cell bodies grouped in ganglia at each level in spinal cord e) Motor neuron axons exit from ventral surface f) Dorsal and ventral roots combine to form spinal nerves 2. Antagonistic Control of the Skeletal Muscles a) Movement controlled by opposing sets of muscles (1) Muscles shorten and pull insertion toward origin (2) Antagonistic actions produced when muscles attach to bones differently b) When one muscle contracts, other stretches c) Normally stretch in muscle produces reflex contraction d) When brain stimulates muscle to contract it also inhibits antagonistic muscle C. The Autonomic Nervous System 1. Composed of two elements that act antagonistically 2. Parasympathetic nervous system (PNS) a) Axons extend from brain and sacral spinal cord to visceral organs b) Synapse with neurons located in parasympathetic ganglia c) Axons from ganglia neurons innervate visceral organs 3. Sympathetic nervous system (SNS) a) Axons leave spinal cord at thoracic and abdominal levels b) Travel short distance to chain of ganglia just outside spinal cord c) Synapse with neurons whose axons innervate visceral organs 4. Control of the Autonomic Nervous System a) Two sets of neurons in PNS or SNS (1) Preganglionic neurons have cell bodies in CNS, axons extend to ganglia (2) Postsynaptic neurons cell bodies in ganglia, axons synapse with target organ b) Neurotransmitters (1) Acetylcholine always at synapse between pre and postganglionic neuron (2) Acetylcholine at second synapse with PNS (3) Norepinephrine at second synapse with SNS c) Preganglionic sympathetic axons stimulate adrenal medulla to secrete epinephrine d) With SNS activation (1) Visceral organs stimulated by norepinephrine from sympathetic neurons (2) Also stimulated by epinephrine from adrenal glands (3) These two chemicals initiate flight-or-flight reaction a) Heart beats faster, bronchioles of lung dilate b) Liver glycogen converted to glucose c) Digestive functions inhibited e) PNS promotes opposite response through release of acetylcholine (1) Heart slows, bronchioles constrict, digestive functions stimulated (2) Inhibited during fight-or-flight reaction (3) Activated following a meal f) Both systems work in antagonistic manner to achieve fine control Layout by J.T. Poirier © 2001 |