The Nervous System1) 2 divisionsa) Central Nervous System: consists of the brain and spinal cord; located in the dorsal body cavity surrounded by meningesi) Also tracts and nucleib) Peripheral Nervous System: consists of all neural structures outside of CNS including the cranial nerves, spinal nerves and sensory receptorsi) Nerves and ganglia2) Composition of Nervous Tissuea) Nervous System composed mainly of nervous tissuei) CT and blood vessels present toob) Composed of neurons and supporting cellsi) Neurons = nerve cells are conducting cellsii) Supporting cells are non-conducting cells(1) Supporting cells are also called neuroglia(2) 6 types(a) In CNS:(i) Astrocytes1. Most abundant2. Numerous extensions that wrap around neurons3. Involved in forming blood-brain barrier, selective barrier that regulate the chemical environment of the brain4. Regulate brain function5. Lipid-based substances are not prevented by the barrier6. Form continuous tight junction to prevent substances from entering in b/w astrocytesa. Substances can transverse the barrier only by diffusing through PM of astrocytes(ii) Microglia1. Since specific immune system does not have access to CNS, these act as macrophages to destroy pathogens/cell debris2. Not as effective as macrophages (ultimate phagocytes) in body (outside the CNS)(iii) Ependymal cells1. Ciliated columnar cells that line ventricles (cavities in brain that contain cerebrospinal fluid or CSF)2. Cilia beat to create a current that circulates CSF in ventricles and in central canal(iv) Oligodendrocytes1. Have cellular extensions that wap around segments of axons in the CNS to myelinate axons of neuronsa. Myelinated axons appear white because myelin sheath is a white, fatty covering(b) In PNS:(i) Schwann cells (neurolemmocytes)1. Myelinate axons in PNS2. Analogous to oligodendrocytes (ii) Satellite cells(iii) Surround cell bodies of neurons and control chemical environment3) Structure of neurona) 3 regions of neuron: cell body + 2 types of processesi) Cell body = soma or perikaryon (1) Contains nucleus and all other cytoplasmic organelles except centriols(a) Generally amitotic(2) Well-developed rough ER called nissi body/chromatophilic substance(3) Intermediate filaments called neurofilaments(4) Biosynthetic region of neuronii) Dendrites(1) Tapering processes that act as receptive regions of a neuron(2) Receive and convey electrical signals toward cell bodyiii) Axon(1) Single process extending from cell body (each neuron can only have 1 axon)(2) Generates and transmits action potentials away from soma(3) Conducting region of neuron(4) Branches at end into telodendria which end in axon termals4) Myelinationa) Process by which concentric rings of Schwann cells or cellular extensions of oligodentrocytes wrapped around a segment of an axonb) Differences b/w myelination in PNS (by Schwann cells) and in CNS (oligodendrocytes)i) Neurilemma is present around the myelinated axons in PNS; neurilemma is absent around myelinated axons in CNSc) Similarityi) Myelinated axons in CNS and in PNS have nodes of ranvier 5) Action Potentiala) Electrical signals produced by the axolemma of an axonb) All or none phenomenonc) So far as the axolemma can be depolarized to the threshold potential, an AP will be producedd) Generated AP travels down the axon to the axon terminals with the intensity unchanged (remains the same as it travels down the axon)i) A traveling AP is referred to as a nerve impulseii) Electrical signals produced by dendrites are referred to as graded potentials(1) Intensity of graded potentials wanes (decreases) over distance(2) Hence, AP are used in long distance communications in the nervous systemiii) All Aps have the same tracingthey all reach about 35 mV irrespective ofthe strength of the stimulus(1) Any stimulus that can depolarize the axolemma to the threshold potential will cause the production of AP6) How do you tell the difference between a weak stimulus that caused production of an AP and a stronger stimulus that caused the production of AP?a) The amplitude of the types of AP will look the sameb) Stronger stimulus will generate Aps at a higher frequency than the weak stimulus7) Action Potentiala) RMPi) Cytoplasmic face of the axolemma is negative compared to external face of the axolemma due to the partial nature of the axolemma(1) Allows potassium ions to move out of the axon but not sodium ions to move in(a) Permeability of K+ is 75 times that of Na+ in a resting cell where the resting membrane potential is establishedii) When the neuron is stimulated, Na+ channels in the axolemma open up (depolarization phase of the action potential)(1) 2 subphases(a) Threshold potential(i) Critical potential caused by Na+ entry into the axon (Na+ influx)(ii) Less negative than the RMP (-90 mV)(iii) Once the region of the axolemma is depolarized to the threshold potential, an AP spike occurs(iv) When the threshold potential is reached due to positive feedback mechanism there is an explosive entry of Na+ as more and more Na+ channels open increase of Na+ influx membrane potential becomes less negative ~35 mV1. Na+ channels are sensitive to positive potential, hence Na+ channels closea. Na+ influx halts (repolarization phase)i. First Na+ channels close, then K+ channels openii. When K+ channels open, K+ leaves axon to exterior (K+ efflux)(b) Spike or the upshoot of the AP8) Repolarization phases of AP2 sequential eventsa) As K+ efflux occurs, the membrane potential of the axolemma becomes less positivei) 0 mV-90 mV because K+ channels are sluggish to close and more K+ leaves the axon than is required to establish the RMP -100 mV(1) This phases of the AP is referred to as the hyperpolarization phase (undershoot)(2) Na+/K+ pump (active transport) works to re-establish the RMP9) Periods of Action Potential a) Absolute Refractory Periodi) Have Na+ influx occurring and the Na+ channels are therefore open HENCE, another action potential cannot be initiated during the absolute refractory periodb) Relative Refractory Periodi) Occurs during the repolarization phase of the AP (action potential) when the Na+ channels are closed; K+ channels are opened and K+ efflux is occurring ii) An exceptionally strong stimulus can cause the closed Na+ channels in therelative refraction period to open to cause Na+ influx to initiate another AP (without the previous AP repolarizing to the RMP by overriding repolarization to begin another AP) HENCE, a stronger
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