The Nervous System 10 27 2010 Two Divisions of the Nervous System Central Nervous System CNS consists of the brain and the spinal cord located in the dorsal body cavity surrounded by meninges Peripheral Nervous System PNS consists of the all neural structures outside of the CNS including the cranial nerves spinal nerves and sensory receptors Composition of Nervous Tissue The Nervous System is composed mainly of Nervous Tissue connective tissue and blood vessels are also present Nervous tissue is composed of two types of cells Neurons and Supporting Cells Neurons nerve cells are conducting cells Supporting cells are non conducting cells Structure of a Neuron Three regions of a neuron Cell body 2 Types of Processes o Cell body soma perikaryon Contains the nucleus and all other cytoplasmic organelles except centrioles hence neurons are generally amitotic Contains well developed rough ER called Nissl Body or Chromatophilic substance Contains intermediate filaments called neurofibrils Biosynthetic region a neuron Tapering processes that act as the receptive regions of Receive and convey electrical signals toward the cell o Dendrites a neuron body o Axon A single process extending from the cell body each neuron has only one axon Generates and transmits action potentials conducting region of a neuron Branches at the end to form terminal branches which end in bulbous ends called axon terminals synaptic knobs boutons Classification of Neurons 2 Types o Three Structural Classification of Neurons Multipolar neuron has at least three processes one axon and at least two dendrites most abundant neuron Bipolar neuron has two processes one axon and one in the human body dendrites Unipolar neuron has one short process from the cell body and it bifurcates into a central process and a peripheral process o Three Functional Classification of Neurons Motor or efferent neuron transmits impulses away from the CNS to effector organs glands organs Sensory or afferent neuron transmits impulses from sensory receptors toward the CNS Association neurons or interneuron located in the CNS between the sensory neurons and the motor neurons Most of the neurons 99 in the body are associated Definitions neurons Tract a bundle of axons in the CNS Nerve a bundle of axons in the PNS Nucleus a cluster of neuron cell bodies in the CNS Ganglion a cluster of neuron cell bodies in the PNS Structure of a Nerve Tract The plasma membrane of an axon is called an axolemma Each axon is wrapped in a delicate connective tissue membrane A bundle of endoneurium covered axons is called a fascicle Each fascicle is covered by the coarse connective tissue membrane called endoneurium called the perineurium A bundle of perineurium covered fascicles form the nerve or a tract which is covered in a tough connective tissue membrane called the eipneurium Six Types of Supporting Cells Supporting cells neuroglia Four supporting cells are located in the CNS o Astrocytes Most abundant Numerous extensions that wrap around neurouns Involved in forming the blood brain barrier a selective barrier that regulates the chemicals environment of the brain Regulate brain function o Microglia Since the specific immune system does not have access to the CNS the microglia act as macrophages to engulf destroy pathogens and cell debris o Ependymal cells Ciliated columnar cells that line the ventricles cavities in the brain that contain cerebrospinal fluid CSF Currents created by beating of cilia circulate the CSF o Oligodendrocytes Their extensions myelinate axons of neurons in the CNS Two supporting cells are located in the PNS o Schwann cells neurolemmocytes Myelinate axons of neurons in the PNS o Satellite cells Surround cell bodies of neurons and control their chemical environment Myelination of Axons Myelination of axons in the PNS by Schwann cells o Each Schwann cell wraps around a segment of an axon external to the axolemma o Schwann cell squeezes around the segment of axon wrapping concentric rings of its plasma membrane called myelin sheath around the axon o The cytoplasm and the nucleus of the Schwann cell squeezed outside the myelin sheath is called the neurilemma o The spaces between adjacent myelin sheaths are called nodes of ranvier Myelination of axons in the CNS by oligodendrocytes o The axons in the CNS are myelinated by extensions from the oligodendrocytes hence neurilemma is absent Severed Axons in the PNS Can Regenerate But Severed Axons in the CNS Cannot Severed axons in the PNS can regenerate because o When 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 regeneration o The neurilemma of the Schwann cell forms a regeneration tube that guides regeneration of the severed axon Severed axons in the CNS fails to regenerate because o The microglia poorly clean up area of damage debridement is not complete o No neurilemma guide growth of severed axon o Presence of growth inhibiting proteins in the CNS inhibit regeneration of a severed axon Neurophysiology Generation of Action Potential Resting membrane potential RMP is 70mV Depolarization phase entry of sodium ions sodium influx makes membrane potential less and less negative o Threshold potential action potential develops an all or none phenomenon o Upshoot or spike due to an explosive entry of sodium ions a positive membrane potential reached Repolarization phase sodium channels close and potassium channels open and potassium ions rush out potassium efflux and reversal of membrane potential toward a negative membrane potential Hyperpolarization phase more potassium ions leave the cell driving the membrane potential below the RMP Characteristics of Action Potentials All or none phenomenon an action potential will be generated if depolarization reaches a threshold potential Self 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 impulse Since 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 stimulus Two Refractory Periods During an Action Potential Absolute Refractory Period the depolarization phase of the action potential when sodium
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