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Nervous SystemFriday, November 16, 201212:40 PMDivisions of the Nervous System:1.) CNS-has the spinal cord and the brain2.) PNS-has the nerves and sense organsGeneral Characteristics:-of a nerve cell:*dendrites-> receptive region*axon->conducting region a neuron can have only one axon: axon is the only region that can generate and transmit (conduct) action potentials which travel away from the cell body to the axon terminal-axon terminals contain vesicles which have neurotransmitters*neurotransmitters are released when action potentials arrive at the axon terminalsCLASSIFICATION OF NEURONS:1.)structural classification:*number of processes extending from the cell bodya.)multipolar neuronb.)bipolar neuronc.)pseudounipolar neuron2.)functional classification:*direction of impulse conductiona.)sensory (afferent neurons)b.)motor (efferent neurons)c.)association (interneurons)MULTIPOLAR NEURONS:-most abundant structural class-there are always one axon in perspective of the number of processes extending from the cell body-cell body with 1001 processes:*one axon*1000 dendritesBIPOLAR NEURONS:-has one axon and one dendritePSEUDOUNIPOLAR NEURON:-a short stalk extends from the cell body and bifurcates into 2 structures-one is myelinated (like axon= only axons can be myelinated)-have uniform diameter (like axons: dendrites are tapered)-one of the structures conducts impulses (impulses traveling action potential) away from the cell body like axons-the other structure conduct impulses towards the cell body like dendrite-based on this conundrum the 2 structures are referred to as peripheral process and central process*the peripheral process will conduct impulses towards cell body and central will take impulses awayFUNCTIONAL CLASSIFICATION:-defines a simple reflex arca.)sensory (afferent) neurons->axons generate and conduct (transmit) action potentials toward the CNS and the sensory neurons synapse the neurons in the CNS calledb.)association neurons (interneurons)-> integrate/ interpret the information and and provide command throughc.)the motor (efferent) neurons for action to be taken-in humans-> most of the neurons are association neurons and since the most abundant structural class is the multipolar neurons, association neurons are of the structural class, multipolar neurons*multipolar neurons are mostly of the functional class association neurons)AXONS:-do not exist in isolation: they associate with other axons to form bundles called:*tracts in the CNS*nerves in the PNSSTRUCTURAL ORGANIZATION OF TRACT OR NERVE:-considered organs Nervous tissue (CT & blood vessels) is composed of 2 types of cells: Neurons & supporting cells Tract - bundle of axons in CNSNerve - bundle of axons in PNSNucleus - cluster of neuron cell bodies in CNSGanglion - cluster of neuron cell bodies in PNS Structure of Nerve:Endoneurium - wraps an axonPerineurium (CT) - wraps a fascicleEpineurium (CT) - wraps a bundle of perineurium-covered fascicles (forms nerve/tract) Supporting Cells in CNS:1. Astrocytes - Blood-Brain barrier2. Microglia - macrophages3. Ependymal cells - ciliated columnar cells4. Oligodendrocytes - myelinated axons Supporting Cells in PNS:1. Schwann Cells - myelinated axons2. Satellite Cells Myelination (only axons can be myelinated) -->-Axons in the PNS are myelinated by Schwann Cells --> each Schwann cell wraps around the axon &concentric rings of plasma membrane of schwann cell --> MYELIN SHEATH (whitish fatty protein)The rest of the schwann cell (cytoplasm & nucleus) - form the NEURILEMMASchwann cells myelination of the axon is not complete, gaps in-between myelin sheath called NODES OF RANVIER (axolemma is exposed), myelin sheath located outside of axolemmaIn the tract/nerve, the endoneurium is outside the myelin sheathOne schwann cell can myelinate a segment of only one axon-Axons in the CNS by the Oligodendrocytes --> oligodendrocytes myelinated axons use extensions to wrap around segments of axonsOligodendrocytes can myelinate segments of several axonsCytoplasm & nucleus of oligodendrocytes are located a distance from the site of myelination, neurolemma is absentFunction of Myelin sheath/myelination:-Protects the axolemma of the axon-Electrical Insulation : Prevents interference of electrical activity from neighboring axons in the tract/nerve, acts as insulator (bad conductors of electrical current), electrical activity action potential can only be generated at the Nodes of RanvierConduction of action potentials down a myelinated axon appears to jump from node to node towards the axon terminals, this type of conduction is SALTATORY conduction...Faster than… Continuous conductors which occurs in unmyelinated axon where entire surface of axolemma must be depolarized -Increase in rate of impulse transmission *Severed Axons in PNS CAN REGENERATE*Severed Axons in CNS CANNOT REGENERATE Phases of Action Potential:Generated/transmitted only by axonsResting Membrane Potential1. Depolarization Phase - due to SODIUM INFLUX, Na+ goes down concentration gradient by moving into the axon, since Na+ is a cation, the RMP becomes less negative, the RMP potential is destroyed and as the potential gets to a critical potential called THRESHOLD potential, explosive entry of Na+ that drives the potential to 0 --> +30mV, increase in the potential between threshold potential and +30mV is called the UPSTROKE (or spike), The spike for action potential is the same for a weaker stimulus as the a stronger stimulus --> +30mV2. Repolarization Phase - due to closing of Na+ channels, which halt sodium influx & due to opening of K+ channels, which cause K+ EFFLUX. At +30mV, K+ channels open and K+ goes down concentration gradient --> K+ (potassium) leaves the axon for the external environment, termed K+ Efflux. Hence, potential reverses towards the RPM3. Hyperpolarization Phase - when the potential reaches the RMP, since K+ channels remain open, more K+ leaves the axon than is required to establish RMP --> potential becomes more negative than the RMP --> this more neg. potential is referred to as UNDERSHOOT (or hyperpolarization). Na+/K+ pumps actively transport 3Na+ out of the axon,


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UMD BSCI 201 - Nervous System

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