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Two Divisions of the Nervous SystemCentral Nervous System (CNS)—consists of the brain and the spinal cord; located in the dorsal body cavity surrounded by meningesPeripheral Nervous System (PNS)—consists of the all neural structures outside of the CNS including the cranial nerves, spinal nerves, and sensory receptorsComposition of Nervous TissueThe Nervous System is composed mainly of Nervous Tissue; connective tissue, and blood vessels are also presentNervous tissue is composed of two types of cells: Neurons and Supporting CellsNeurons=nerve cells are conducting cellsSupporting cells are non-conducting cellsStructure of a NeuronThree regions of a neuron: Cell body + 2 Types of ProcessesCell body=soma=perikaryonContains the nucleus and all other cytoplasmic organelles except centrioles hence, neurons are generally amitoticContains well-developed rough ER called Nissl Body or Chromatophilic substanceContains intermediate filaments called neurofibrilsBiosynthetic region a neuronDendritesTapering processes that act as the receptive regions of a neuronReceive and convey electrical signals toward the cell bodyAxonA single process extending from the cell body—each neuron has only one axonGenerates and transmits action potentials=conducting region of a neuronBranches at the end to form terminal branches which end in bulbous ends called axon terminals=synaptic knobs=boutonsClassification of Neurons2 Types:Three Structural Classification of Neurons:Multipolar neuron has at least three processes—one axon and at least two dendrites; most abundant neuron in the human bodyBipolar neuron has two processes—one axon and one dendritesUnipolar neuron has one short process from the cell body and it bifurcates into a central process and a peripheral processThree Functional Classification of Neurons:Motor or efferent neuron transmits impulses away from the CNS to effector organs=glands, organsSensory or afferent neuron transmits impulses from sensory receptors toward the CNSAssociation neurons or interneuron located in the CNS between the sensory neurons and the motor neuronsMost of the neurons (99%) in the body are associated neuronsDefinitionsTract—a bundle of axons in the CNSNerve—a bundle of axons in the PNSNucleus—a cluster of neuron cell bodies in the CNSGanglion—a cluster of neuron cell bodies in the PNSStructure of a Nerve (Tract)The plasma membrane of an axon is called an axolemmaEach axon is wrapped in a delicate connective tissue membrane called endoneuriumA bundle of endoneurium-covered axons is called a fascicleEach fascicle is covered by the coarse connective tissue membrane called the perineuriumA bundle of perineurium-covered fascicles form the nerve or a tract which is covered in a tough connective tissue membrane called the eipneuriumSix Types of Supporting CellsSupporting cells=neurogliaFour supporting cells are located in the CNSAstrocytesMost abundantNumerous extensions that wrap around neurounsInvolved in forming the blood-brain barrier, a selective barrier that regulates the chemicals environment of the brainRegulate brain functionMicrogliaSince the specific immune system does not have access to the CNS; the microglia act as macrophages to engulf/destroy pathogens and cell debrisEpendymal cellsCiliated columnar cells that line the ventricles—cavities in the brain that contain cerebrospinal fluid (CSF)Currents created by beating of cilia circulate the CSFOligodendrocytesTheir extensions myelinate axons of neurons in the CNSTwo supporting cells are located in the PNSSchwann cells=neurolemmocytesMyelinate axons of neurons in the PNSSatellite cellsSurround cell bodies of neurons and control their chemical environmentMyelination of AxonsMyelination of axons in the PNS by Schwann cells:Each Schwann cell wraps around a segment of an axon (external to the axolemma)Schwann cell squeezes around the segment of axon wrapping concentric rings of its plasma membrane called myelin sheath around the axonThe cytoplasm and the nucleus of the Schwann cell squeezed outside the myelin sheath is called the neurilemmaThe spaces between adjacent myelin sheaths are called nodes of ranvierMyelination of axons in the CNS by oligodendrocytes:The axons in the CNS are myelinated by extensions from the oligodendrocytes hence, neurilemma is absentSevered Axons in the PNS Can Regenerate, But Severed Axons in the CNS CannotSevered axons in the PNS can regenerate becauseWhen the axon is severed in the PNS, cells of the immune system clean up the damaged area of cell debris, a process known as debridement, which sets the stage for regenerationThe neurilemma of the Schwann cell forms a regeneration tube that guides regeneration of the severed axonSevered axons in the CNS fails to regenerate becauseThe microglia poorly clean up area of damage—debridement is not completeNo neurilemma guide growth of severed axonPresence of growth-inhibiting proteins in the CNS inhibit regeneration of a severed axonNeurophysiology—Generation of Action PotentialResting membrane potential (RMP) is -70mVDepolarization phase—entry of sodium ions=sodium influx, makes membrane potential less and less negativeThreshold potential—action potential develops=an all-or-none phenomenonUpshoot or spike due to an explosive entry of sodium ions=a positive membrane potential reachedRepolarization phase—sodium channels close and potassium channels open and potassium ions rush out=potassium efflux, and reversal of membrane potential toward a negative membrane potentialHyperpolarization phase—more potassium ions leave the cell driving the membrane potential below the RMPCharacteristics of Action PotentialsAll-or-none phenomenon—an action potential will be generated if depolarization reaches a threshold potentialSelf propagating—once generated by the axon, it is propagated down the axon to the axonal terminals; a propagated or transmitted action potential is called an impulseSince all action potentials appear the same have the same shape and amplitude irrespective of stimulus strength; thus, the difference between a stronger stimulus that causes the generation of an action potential is that the stronger stimulus causes the impulse to be generated at a higher frequency than the weaker stimulusTwo Refractory Periods During an Action PotentialAbsolute Refractory Period—the depolarization phase of the action potential when sodium channels are opened, another action potential cannot be generatedRelative Refractory


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UMD BSCI 201 - The Nervous System

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