Anatomy of ieurons and Glia<\/h2>\n

\n
• Nervous system contains 2 kinds of cells:\n
  \n
  1. ieurons<\/strong>: receive info and transmit it to other cells<\/li>\n
  2. Glia<\/strong>: many functions<\/li>\n<\/ol>\n<\/li>\n
  3. Brain is composed of individual cells; small gap that separates tips of one neuron\u2019s fibers from the surface of next neuron (~ 100 billion neurons) The Structure of an Animal Cell<\/strong><\/li>\n
  4. Neurons have a lot in common with the rest of the body cells.<\/li>\n
  5. Membrane (Plasma membrane)<\/strong>: surface of a cell; separates inside and outside of cell; 2 layers of fat molecules (phospholipids). Most molecules can\u2019t cross membrane, some specific ion channels exist for important molecules.<\/li>\n
  6. All animal cells have a nucleus<\/strong> \u2013 except for mammalian red blood cells<\/li>\n
  7. Mitochondrion<\/strong>: performs metabolic activities, provides energy for cell, requires fuel and oxygen to function<\/li>\n
  8. Ribosomes<\/strong>: cell synthesizes new protein molecules; provides building material for cell and facilitate chemical reactions<\/li>\n
  9. Endoplasmic reticulum<\/strong>: network of thin filaments that transport newly synthesized proteins to other locations The Structure of a ieuron<\/strong><\/li>\n
  10. Shape of a neuron can vary; they have long branching extensions \uf0b7 Larger neurons have 4 components:\n
      \n
      1. Dendrites<\/li>\n
      2. Cell body<\/li>\n
      3. Axon<\/li>\n
      4. Presynaptic terminals<\/li>\n<\/ol>\n<\/li>\n
      5. Tiny neurons lack axons, and some well-defined dendrites<\/li>\n<\/ul>\n

         Motor neuron: <\/strong>soma in spinal cord, receives excitation from other neurons through the dendrites and conducts impulses along axon to muscle<\/p>\n

         \n
         • Sensory neuron<\/strong>: specialized at 1 end to be highly sensitive to certain stimulation<\/li>\n
         • Dendrites<\/strong>: branching fibers, get more narrow at end; surface lined up with specialized synaptic receptors that receive info from other neurons. Some contain dendritic spines<\/strong>, outgrowths that increase SA for synapses<\/li>\n
         • Cell body<\/strong>: contains nucleus, ribosomes, mitochondria. Covered with synapses on its surface in many neurons<\/li>\n
         • Axon<\/strong>: fibre of constant diameter, sends info, convey impulse toward other neurons<\/li>\n
         • Myelin sheath<\/strong>: insulates axons, interruptions called iodes of Ranvier<\/strong>.<\/li>\n
         • Presynaptic terminal<\/strong>: Axon has many branches swelling at the tip; where axon releases chemicals that cross through the junction \uf0b7 A neuron can have many dendrites, but only one axon.<\/li>\n
         • Afferent axon<\/strong>: bring info into a structure<\/li>\n
         • Efferent axon<\/strong>: carries info away from structure<\/li>\n
         • Every sensory<\/u> neuron is afferent<\/u> to rest of NS and every motor<\/u> neuron is an efferent<\/u> from NS. A neuron is efferent from one and afferent to another structure.<\/li>\n
         • Interneuron\/intrinsic neuron<\/strong>: if a cell\u2019s dendrites and axon are entirely contained within a single structure Variations Among ieurons<\/strong><\/li>\n
         • Vary in size, shape and function; shape determines connections with other neurons and determines its function\/contributions to the nervous system. More branching = connect with more targets Glia (ieuroglia)<\/strong><\/li>\n
         • Don\u2019t transmit over long distances, smaller and higher in number than neurons \uf0b7 Several types of glia:\n
           \n
           1. Astrocytes<\/strong>: (star-shaped) wrap around presynaptic terminal of group of functionally related axons. Take up ions released by axons, release them back to axons \uf0e0 helps synchronize activity of axons \uf0e0 send messages in waves. Remove waste material created when neurons die, control blood flow to each brain area; during periods of heightened activity in some brain areas \uf0e0 dilate blood vessels so more nutrients travel to that area<\/li>\n<\/ol>\n<\/li>\n<\/ul>\n

              \u00a0<\/em><\/p>\n

              \n
              1. Microglia<\/strong>: small cells; remove waste materials, viruses, fungi, other microorganisms; function like immune system<\/li>\n
              2. Oligodendrocytes<\/strong>: in brain and spinal cord<\/li>\n
              3. Schwann cells<\/strong>: periphery of body; build myelin sheaths that surrounds and insulate vertebrate axons<\/li>\n
              4. Radial glia<\/strong>: guide migration of neurons, axons, and dendrites during embryonic development. Most will differentiate into neurons or other glia The Blood-Brain Barrier <\/u><\/strong><\/li>\n<\/ol>\n
                 \n
                 • Brain needs nutrients from the blood, most chemicals can\u2019t cross the blood to brain<\/li>\n<\/ul>\n

                   Why we need a blood-brain barrier<\/h3>\n

                   \n
                   • When a virus invades a cell, cell will extrude virus particles through membrane so that the immune system can find them.<\/li>\n
                   • When immune system cells identify a virus, they kill it and cell that contains it<\/li>\n
                   • To minimize risk of brain damage \uf0e0 body builds a wall along sides of brain\u2019s blood vessels \uf0e0 keeps out viruses, bacteria and harmful chemicals<\/li>\n
                   • Some viruses that invade brain can lead to death, some killed by glia, but a virus that enters the NS will stay with you for life How the Blood-Brain Barrier Works<\/strong><\/li>\n
                   • Depends on endothelial cells that form walls of capillaries<\/li>\n
                   • Outside brain, cells separated by small gaps; in brain, they are joined close so nothing passes. Barrier keeps out useful and harmful chemicals \uf0b7 For brain to function \uf0e0 body needs mechanism to get chemicals across \uf0b7 Mechanisms are:\n
                     \n
                     1. Small uncharged molecules cross freelyo Special protein channels in wall of endothelial cells (water)<\/li>\n
                     2. Molecules that dissolve in fats of membrane cross passively<\/li>\n
                     3. Active transport<\/strong>: protein-mediated process, expends energy to pump chemicals from the blood to the brain<\/li>\n<\/ol>\n<\/li>\n
                     4. Essential to health; people with Alzheimer\u2019s or similar conditions, endothelial cells lining brain\u2019s blood vessels shrink and harmful chemicals enter brain iourishment in Vertebrate ieurons <\/u><\/strong><\/li>\n
                     5. Glucose \uf0e0 vertebrate neurons depend entirely on it<\/li>\n
                     6. Metabolic pathway requires O2; brain uses 20% of O2 consumed<\/li>\n<\/ul>\n

                        Glucose is only nutrient that crosses the blood-brain barrier after infancy; except for ketones<\/p>\n

                        \n
                        • Glucose deficiency is rare (only inability to use); to use glucose \uf0e0 body needs vitamin B1 (Thiamine)<\/li>\n
                        • Korsakoff\u2019s syndrome \uf0e0 death of neurons by prolonged thiamine deficiency<\/li>\n<\/ul>\n

                          (alcoholism) \uf0e0 severe memory impairments \u00a0<\/u><\/strong><\/p>\n

                           <\/p>\n

                          THE iERVE IMPULSE <\/u><\/strong><\/p>\n

                          \n
                          • Axon doesn\u2019t conduct electrical impulse, it regenerates impulse at each point; impulse travels without weakening; axons transmit info at moderate speeds; properties of impulse conduction in axon adapted to exact needs of info transfer in NS<\/li>\n
                          • In vision, brain needs to know whether one stimulus began slightly before\/after another one unlike touch sense<\/li>\n<\/ul>\n

                            The Resting Potential of the ieuron<\/h2>\n

                            \n
                            • Neuronal messages develop from disturbances of the resting potential<\/li>\n
                            • All parts of membrane covered by membrane composed of 2 layers of phospholipid molecule; among phospholipids are protein molecules, so chemicals can pass.<\/li>\n
                            • Structure\uf0e0 flexibility and firmness, controls flow of chemicals<\/li>\n
                            • Electrical gradient (polarization)<\/strong>: difference in electrical charge between inside and outside of cell; neuron inside membrane has \u2013ve charge compared to outside<\/li>\n
                            • Resting potential<\/strong>: difference in voltage in a resting neuron<\/li>\n
                            • Measure resting potential by inserting microelectrode into the cell body; a reference electrode outside the cell completes the circuit; typical level -70mV Forces Acting on Sodium and Potassium Ions<\/strong><\/li>\n
                            • If charged ions could cross membrane freely \uf0e0 would depolarize<\/li>\n
                            • Selectively permeable<\/strong>: membrane; chemicals pass freely \uf0e0 O2, CO2, H2O through channels; large or electrically charged ions don\u2019t cross; Na\/K\/Cl cross through membrane channels<\/li>\n
                            • When membrane is at rest, Na channels are closed, K almost closed<\/li>\n
                            • ia-K pump:<\/strong> transports 3 Na ions out of cell, drawing in 2 K ions in cell<\/li>\n
                            • Active transport that requires energy. Na more concentrated outside, K inside<\/li>\n
                            • Effective due to selective permeability of membrane<\/li>\n
                            • Selective permeability prevents Na to come back in; some K leak out<\/li>\n<\/ul>\n

                              (carrying +ve charge) \uf0e0 increases electrical gradient<\/p>\n

                              \n
                              • When neuron at rest 2 forces push Na into the cell\n
                                \n
                                1. Electrical gradient: Na+ wants to enter -ve cell; Na +ve charged and inside \u2013ve charged<\/li>\n
                                2. Concentration gradient (difference in distribution of ions across a membrane); Na higher outside cell \uf0e0 wants to come in, but Na channels closed when cell is at rest, so no Na+ enters the cell<\/li>\n<\/ol>\n<\/li>\n
                                3. K+ electrically wants to move in, but K more concentrated inside cell \uf0e0 gradient drives it out; if K channels open, K would leave in small quantities. Na\/K pump pulls more K into cell<\/li>\n
                                4. Negative anions inside the cell are responsible for membrane\u2019s polarization (Cl-) Why a Resting Potential?<\/strong><\/li>\n
                                5. Prepares neuron to respond rapidly<\/li>\n
                                6. Exciting neuron opens channels that allow Na to enter cell fast (as membrane maintained concentration gradients for Na already) The Action Potential <\/u><\/strong><\/li>\n
                                7. Action potential<\/strong>: messages sent by axons<\/li>\n
                                8. When a membrane is at rest, there is a \u2013ve potential inside cell<\/li>\n
                                9. Hyperpolarization<\/strong>: further increase of \u2013ve charge; increased polarization; stimulation ends \uf0e0 charge returns to original resting level<\/li>\n
                                10. Depolarization<\/strong>: neuron reduces polarization toward 0<\/li>\n
                                11. Threshold of excitation<\/strong>: produces massive depolarization of membrane; opens Na+ channels, Na+ floods into cell; rapid depolarization \uf0e0 reversal = action potential that peaks at +30mV<\/li>\n
                                12. Sub-threshold stimulation<\/strong>: small response proportional to the current<\/li>\n<\/ul>\n

                                    Molecular Basis of the Action Potential<\/h3>\n

                                    \n
                                    1. Start \uf0e0 Na+ mostly outside, K mostly inside<\/li>\n
                                    2. When membrane depolarized \uf0e0 Na\/K channels open 3. At peak of AP \uf0e0 Na channels close<\/li>\n<\/ol>\n
                                       \n
                                       • Membrane has many channels that can open or close<\/li>\n
                                       • Voltage-gated channels<\/strong>: regulates Na\/K; permeability depends on voltage difference across the membrane,<\/li>\n<\/ul>\n

                                         At rest, Na channels are closed, K channels are almost closed<\/p>\n

                                         \n
                                         • Opening of K channels does little difference because electrical and concentration gradient almost balanced; Na channels make a big difference, as both gradients allow Na to enter<\/li>\n
                                         • When depolarization reaches threshold, Na channels open wide \uf0e0 Na enters cell rapidly until there is a reversed polarization \uf0e0 Na channels shut<\/li>\n
                                         • When many Na ions cross membrane, inside of the cell is slightly +ve \uf0e0 K driven out of the cell \uf0e0 temporary hyperpolarization; membrane returns to resting potential; inside has more Na and less K; Na\/K pump restores the correct gradient (excessive Na build up toxic to cell)<\/li>\n
                                         • Local anaesthetic attach to Na channels, prevent Na from entering cell; stop APs The All-or-ione Law<\/strong><\/li>\n
                                         • AP starts in an axon \uf0e0 propagates without loss along an axon<\/li>\n
                                         • Once started, \u201cback-propagate\u201d to cell body and dendrites \uf0e0 don\u2019t conduct AP\u2019s in same way as axons, but passively register electrical event<\/li>\n
                                         • When voltage across axon membrane reaches threshold, voltage-gated Na channels open, Na depolarizes membrane \uf0e0<\/li>\n
                                         • All AP are equal in intensity and velocity, all-or-none law<\/strong>: amplitude and velocity of AP independent of intensity of triggering stimulus.<\/li>\n
                                         • AP may vary between neurons<\/li>\n
                                         • More frequent APs (NOT more intense AP) \uf0e0 greater intensity of stimulus The Refractory Period<\/strong><\/li>\n
                                         • While potential returning from peak towards rest \uf0e0 still above threshold<\/li>\n
                                         • Refractory Period<\/strong>: Right after AP, resists production of new AP (Na channels are closed, K flowing outside the cell at a fast rate)<\/li>\n
                                         • Absolute refractory period<\/strong>: membrane can\u2019t produce AP, regardless of stimulation<\/li>\n
                                         • Relative refractory period<\/strong>: stronger than usual stimulus \uf0e0 initiates AP<\/li>\n<\/ul>\n

                                           \u00a0Propagation of the Action Potential<\/h2>\n

                                           \n
                                           • In motor neuron, AP starts at axon hillock<\/strong> \uf0e0 where Na enters axon. This spot is temporarily +ve compared to other areas on axon \uf0e0 +ve ions flow to nearby regions, slightly depolarizing next area \uf0e0 area reaches threshold, opens Na+ channels \uf0e0 AP regenerates at that point<\/li>\n
                                           • AP near center of axon doesn\u2019t trigger another AP in areas it passed already because those areas are in refractory period<\/li>\n<\/ul>\n

                                             \u00a0<\/em><\/p>\n

                                             Area of axon reaches threshold \uf0e0 Na + K channels open \uf0e0 K channels little affect at 1^{st<\/sup> \uf0e0 Na channels opening causes Na to rush into cell \uf0e0 +ve charges flow down axon, open nearby Na channels \uf0e0 at peak of AP, Na channels close (depolarization) \uf0e0 K channels open \uf0e0 K ions flow out (regular potential) \uf0e0 K channels close<\/p>\n}

                                             \u00a0The Myelin Sheath and Saltatory Conduction<\/h2>\n

                                             \n
                                             • Increasing diameter of an axon \uf0e0 faster conduction\/velocity<\/li>\n
                                             • Myelin<\/strong>: insulating material composed of fat and protein<\/li>\n
                                             • Myelinated axons: <\/strong>those covered with myelin sheath; interrupted periodically with Nodes of Ranvier<\/li>\n
                                             • After AP occurs at node, Na+ enters axon, push ions to next node \uf0e0 regenerate AP<\/li>\n
                                             • Saltatory conduction<\/strong>: jumping of AP from node to node; rapid conduction of impulses, conserves energy Local ieurons <\/u><\/strong><\/li>\n
                                             • Axons produce AP<\/li>\n
                                             • Neurons without axons exchange info with only close neighbors \uf0e0 local neurons<\/strong><\/li>\n
                                             • Don\u2019t follow all-or-none law because no axons; after receiving info from other neurons, it has a graded potential<\/strong>: membrane potential that varies in magnitude in proportion to intensity of stimulus<\/li>\n
                                             • Change in membrane potential conducted to adjacent areas of cell, weaken with time; areas of cell contact other neurons, then excite\/inhibit other neurons through synapses<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"

                                               Anatomy of ieurons and Glia Nervous system contains 2 kinds of cells: ieurons: receive info and transmit it to other cells Glia: many functions Brain is composed of individual… Continue Reading The cells of the nervous system<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[114],"tags":[],"_links":{"self":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4497"}],"collection":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/comments?post=4497"}],"version-history":[{"count":0,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4497\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/media?parent=4497"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/categories?post=4497"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/tags?post=4497"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}