PET3322 Exam 2 Study Guide Chapter 11 Fundamentals of the Nervous System and Nervous Tissue Nervous system sensory receptors o Sensory function to sense changes in internal or external environment through o Integrative function to analyze the sensory info store some aspects and make decisions regarding appropriate behaviors association or interneurons serve this function o Motor function response to stimuli by initiating action Sensory input ex seeing a glass of water AFFERENT Motor output picking up glass EFFERENT Integration Central Nervous System o Consists of brain and spinal cord Peripheral Nervous System o Consists of cranial and spinal nerves that contain both sensory and motor fibers o Connects CNS to muscles glands and all sensory receptors o Sensory afferent Somatic fibers impulses from skin skeletal muscles and joints to the brain Visceral fibers impulses from visceral organs to the brain o Motor efferent Impulses from CNS to effector organs Somatic nervous system impulse sent from CNS to skeletal muscle via motor fiber voluntary Autonomic NS impulses sent from CNS to smooth muscle cardiac muscle and glands sympathetic parasympathetic divisions OR enteric motor neurons via motor fibers involuntary Neurons o Functional unit of nervous system o Have capacity to produce action potentials Electrical excitability o Cell body Single nucleus with prominent nucleolus o Cell processes dendrites axons o Dendrites and soma receptive or input regions of the neuron Structural Diversity and Classification of Neurons o Functional classifications Sensory afferent Motor efferent Interneurons Myelinated and unmyelinated axons o Schwann cells myelinate wrap around axons in the PNS Whitish fatty protein lipoid segmented sheath around most long axons Increase the speed of nerve impulse transmission o Unmyelinated axons Slower nerve impulses than Myelinated Conduction velocities of axons o Conduction velocities vary widely among neurons o Rate of impulse propagation is determined by Axon diameter the larger the diameter the faster the impulse Presence of a myelin sheath Myelination dramatically increases impulse speed Distribution of Gray and White Matter o White matter primarily myelinated o Gray matter neuronal cell bodies dendrites unmyelinated axons axon terminals Neurons Communicate with Other Cells o Neurons are electrically excitable due to the voltage difference across their membrane o Communicate with 2 types of electric signals Action potentials that can travel long distances o In living cells a flow of ions occurs through ion channels in the cell membrane Ion Channels o Ligand gated channels open and close in response to a chemical stimulus Results in neuron excitability o Voltage gated channels respond to a direct change in the membrane potential internal voltage becomes less negative o Mechanically gated channel responds to mechanical vibration or pressure Resting Membrane Potential o Negative ions along inside of cell membrane and positive ions along outside Potential energy difference at rest is 70 mV o Resting potential exists because Concentration of ions different inside and outside Extracellular fluid rich in Na and Cl Cytosol full of K organic phosphate and amino acids Membrane permeability differs for Na and K 50 100 greater permeability for K Inward flow of Na can t keep up with outward flow of K Na K pump removes Na as fast as it leaks in Action Potential is an All or Nothing Electrical Signal o An action potential AP or nerve impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential depolarizing phase and then restore it to the resting state repolarizing phase During an action potential voltage gated Na and K channels open open in sequence If a stimulus reaches threshold the action potential is always the same A stronger stimulus will not cause a larger impulse o Depolarizing Phase Chemical or mechanical stimulus caused a graded potential to reach at least 55mV threshold Voltage gated Na channels open and Na rushes into cell In resting membrane inactivation gate of Na channel is open and activation gate is closed Na cannot get in When threshold 55 mV is reached both open and Na enters Inactivation gate closes again in few ten thousandths of second Only a total of 20 000 Na actually enter the cell but they change the membrane potential considerably up to 30 mV Positive feedback process o Repolarizing Phase When K channels finally do open the Na channels have already closed Na inflow stops K outflow returns membrane potential to 70 mV If enough K leaves the cell it will reach a 90 mV membrane potential and enter the after hyperpolarizing phase K channels close and the membrane potential returns to the resting potential of 70 mV o Hyperpolarization Occurs when the inside of the membrane becomes more negative than the resting potential Factors That Affect Speed of Propagation o Amount of myelination o Axon diameter The Synapse is a Functional Junction Between Neurons o 2 tyoes of synapses Electrical Chemical One way info transfer from a presynaptic neuron to a postsynaptic neuron o Axodendritic from axon to dendrite o Axosomatic from axon to cell body Chemical Synapses o Action potential reaches end bulb and voltage gated Ca2 channels o Ca2 flows inward triggering release of neurotransmitter o Neurotransmitter crosses synaptic cleft and binds to ligand gated receptors The more neurotransmitter released the greater the change in potential of the postsynaptic cell o One way info transfer Termination of Neurotransmitter Effects o NT bound to a postsynaptic neuron 1 Produces a continuous postsynaptic effect 2 Must be removed from its receptor o Removal of NTs occurs when they Are degraded by enzymes acetylcholinesterase Diffuse from synaptic cleft Move down concentration gradient Uptake by neurons NT transporters Excitory and Inhibitory Postsynaptic Potentials o The effect of a NT can be either excitory or inhibitory o A depolarizing excitory postsynaptic potential is called an EPSP Results from the opening of ligand gated Na channels o An inhibitory postsynaptic potential is called an IPSP Results from the opening of ligand gated Cl or K channels Causes the postsynaptic cell to become more negative hyperpolarized thus harder to depolarize or stimulate Postsynaptic cell is less likely to reach threshold Causes the membrane to become more permeable to K and Cl ions Chapter 12 The Central Nervous System Nerves Nerves
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