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Lecture 2, Kee

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Michelle French

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Lecture Two Central Nervous System: – CNS: brain and spinal cord – protected by back-bone and skull – necessary for maintenance of homeostasis – contains 10^11 neurons – contains 10^14 synapses (connections between neurons – processing happens here) – responsible for everything we perceive, do, feel, and think Peripheral Nervous System: – everything else that lies outside the CNS – functions to connect the CNS to the organs (including limbs) – divided into the somatic and the autonomic nervous system – doesn't have protection Input: – somatic(muscle), special(hearing, vision, sound, taste), and visceral(blood pressure, how organs are doing) senses (afferent) input to brain and spinal cord – makes the brain process the information Output: – output goes through either somatic or autonomic nervous system – somatic → skeletal muscles. + voluntary movements, stuff that you can control – Autonomic + control internal environment (ie: cardiac, smooth muscles. Glands) + you don't know when to use the muscle but the brain does II. Cells of the Nervous System: – Neurons + excitable cells – can change membrane potentials (can fire an action potential, a signal) + have connections up to 10,000 other neurons + make up a small percentage of cells in nervous system + most cells aren't neurons... not excitable. Can't transfer information + can conduct electricity – passive signals – Glial cells: + support cells (glue – putting everything together) + makes up most of the cells in CNS – 90% Components of a Neuron: – Soma: + contains nucleus and most organelles + cell body + made up of dendrites – Dendrites: + reception of incoming information – Axon: + takes the information + transmits electrical impulses called action potentials + transmits information to another axon – Axon hillock: + where axon originates and action potentials initiated – Axon terminal: + releases neurotransmitter + signal generated at axon hillock comes to here + leaves via synapse Glial Cells: – Astrocytes + helps with transmitter removal + provides energy + provides guidance for developing neurons + looks like stars – Ependymal cells – Microglia + involved in phagocytosis to protect the CNS from foreign matter – Oligodendrocytes – Schwann cells Myelin Forming Cells: – Oligodendrocytes: + found in CNS + One oligodendrocyte - forms several myelin sheaths - myelinates sections of several axons at a time - wraps whole bunch of axons individually + myelination is like insulation... - made up mainly of fat - looks white under microscope – Schwann cell: + found in PNS + One Schwann cell - forms one myelin sheath - myelinates one section of an axon – both Oligodendrocytes and Schwann cells forms a insulating wrap around the cells so that the message could be transmitted more faithfully – myelin sheath Myelin Sheath: – oligodendrocytes when wrapping one section at a time, similar idea to Schwann cells, it leaves a little gap before wrapping around the outer section – Node of Ranvier (important for action potentials of the axons) – wraps around the axon – if have lots of insulations wrapped around, the conduction velocity would be faster cause there would be a lot less of current leaking out of the axon + 80-100m/sec – cannot react faster than that because that is the limit which the signal travels + for an unmyelinated axon, a contrast, it's low as 2m/sec Case Study: Q: What would happen if we lost the Myelin Sheath? A: Multiple Sclerosis + neuro-system disease that affects the brain and spinal cord and the myelin sheaths of these patients are damaged – slowing down or blocking of messages between brain and body. - visual disturbance, muscle weakness, numbing sensation + treatment: prevent leakage of electrical signals... block potassium channels Ion Channels: Neurons: – Leak channels + always open + open to anything leaking out/in – Ligand-gated channels + open/close in response to ligand binding + not constantly open.. – Voltage-gated channels + open/close in response to change in membrane potential Voltage-gated Channels: – Sodium and potassium channels: + sensitive to voltage difference that occurs in the membrane – react by either opening or closing + present throughout the neuron, but more in axon (especially axon hillock) – transmission and signaling happens + voltage gate of sodium channels important because makes the cell/neuron excitable in order to fire an action potential + action potentials – Calcium channels + axon terminal + release of neurotransmitter III. Establishment of the Resting Membrane Potential – Determining the equilibrium potentials for potassium and sodium ions – All cells have a resting membrane potential + negatively charged → ie: Neuron is -70mV – Resting membrane potential of neurons – Exists because more negative charges inside cell and more positive charges outside cell Equilibrium Potentials: – Determining the equilibrium potentials for potassium and sodium ions – Two factors critical in determining resting membrane potential + ion concentration gradients – difference exists inside the membrane, cell, and outside. + membrane semi-permeability to these ions – at sometimes, open to let in potassium/sodium or mostly closed off. + ion channels – diffuse in and out following the concentration gradient (high to low) – Inside the cell is negative, outside is positive → membrane potential + In neurons, it's -70mV - more negative charges are sitting in the inside of the membrane, more positive charges on the outside Na+/K+ Pump: – 20% of resting membrane potential directly due to Na/K-ATPase – Create concentration difference – concentration gradient + electrogenic: 2 Na+ out, 3 K+ in. + lots of sodium on the outside, very little sodium on the inside → opposite for potassium + all cells have the Na+/K+ pump + 1/3 of what you eat... the energy goes to fuel the pump + Net +1 out – pumping one positive sodium ion out from the pump → inside of the cell will be more negative – 80% of resting membrane potential indirectly due to Na/K-ATPase + produces concentration gradients + Na+: high outside, low inside + K+: low outside, high inside Potassium Equilibrium Potential: – K+ chemical driving force is out of cell – K+ diffuses out of cell – As K+ diffuses out of cell, inside of cell becomes more negative – Electrical driving force acts to “pull” K+ back into the cell – Cell eventually reaches equilibrium – Chemical and electrical driving forces are opposite in direction & equal in magnitude + EK = -94 mV – An ion is at equilibrium when there is no net force for it to move across the membrane + chemical force = negative electrical force + or electrochemical force = 0 – Lots of potassium on the inside of cell → there is a chemical force that drives potassium from the inside to the outside. + taking positive charges away, making the membrane negative + as K+ comes out of cell and gathers an a area which will become positively charged, a cation... eventually repulse K+ from leaving the cell via electrical force. + eventually, cell will come to a place where one K+ ion will be pushed out at the same time via chemical force as one K+ ion will be pushed in via electrical force + an equilibrium will be reached if chemical and electrical forces are in opposite directions and equal in magnitude (force
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