IntegrationPET3322 Exam 2 Study GuideChapter 11 – Fundamentals of the Nervous System and Nervous Tissue- Nervous systemo Sensory function: to sense changes in internal or external environment through sensory receptorso Integrative function: to analyze the sensory info, store some aspects, and make decisions regarding appropriate behaviors; association or interneurons serve thisfunctiono Motor function: response to stimuli by initiating action- Central Nervous Systemo Consists of brain and spinal cord- Peripheral Nervous Systemo Consists of cranial and spinal nerves that contain both sensory and motor fiberso Connects CNS to muscles, glands, and all sensory receptorso Sensory (afferent) Somatic fibers: impulses from skin, skeletal muscles, and joints to the brain Visceral fibers: impulses from visceral organs to the braino 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!- Neuronso Functional unit of nervous systemo Have capacity to produce action potentials Electrical excitabilityo Cell body Single nucleus with prominent nucleolusSensory input (ex: seeinga glass of water) =AFFERENTMotor output (picking upglass) = EFFERENTo Cell processes = dendrites & axonso Dendrites and soma: receptive or input regions of the neuron- Structural Diversity and Classification of Neuronso Functional classifications: Sensory/afferent Motor/efferent Interneurons- Myelinated and unmyelinated axonso 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 transmissiono Unmyelinated axons Slower nerve impulses than Myelinated- Conduction velocities of axonso Conduction velocities vary widely among neuronso 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 Mattero White matter: primarily myelinatedo Gray matter: neuronal cell bodies, dendrites, unmyelinated axons, axon terminals- Neurons Communicate with Other Cellso Neurons are electrically excitable due to the voltage difference across their membraneo Communicate with 2 types of electric signals Action potentials that can travel long distanceso In living cells, a flow of ions occurs through ion channels in the cell membrane- Ion Channelso Ligand-gated channels: open and close in response to a chemical stimulus Results in neuron excitabilityo Voltage-gated channels: respond to a direct change in the membrane potential; internal voltage becomes less negativeo Mechanically gated channel: responds to mechanical vibration or pressure- Resting Membrane Potentialo Negative ions along inside of cell membrane and positive ions along outside Potential energy difference at rest is -70 mVo 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 Signalo 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 impulseo 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 processo 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 mVo Hyperpolarization Occurs when the inside of the membrane becomes more negative than the resting potential- Factors That Affect Speed of Propagationo Amount of myelinationo Axon diameter- The Synapse is a Functional Junction Between Neuronso 2 tyoes of synapses Electrical Chemical- One-way info transfer from a presynaptic neuron to a postsynaptic neurono Axodendritic: from axon to dendriteo Axosomatic: from axon to cell body- Chemical Synapseso Action potential reaches end bulb and voltage-gated Ca2+ channelso Ca2+ flows inward triggering release of neurotransmittero Neurotransmitter crosses synaptic cleft and binds to ligand-gated receptors The more neurotransmitter released the greater the change in potential of the postsynaptic cello One-way info transfer- Termination of Neurotransmitter Effectso NT bound to a postsynaptic neuron: 1. Produces a continuous postsynaptic effect2. Must be removed from its receptoro 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 Potentialso The effect of a NT can be either excitory or inhibitoryo A depolarizing/excitory postsynaptic potential is called an EPSP Results from the opening of ligand-gated Na+ channelso 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
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