Transcript Lecture 4
Transmission 1. innervation - cell body as integrator 2. action potentials (impulses) - axon hillock 3. myelin sheath 4. Voltage-gated ion channels - large concentration in hillock - found along the axon Neuron signaling 1. afferent vs. efferent 2. interneurons 3. circuits 4. synapses - presynaptic terminal - postsynaptic terminal - neurotransmitters - ligand-gated ions channels on postsynaptic membrane Nervous System A. Organization of neurons 1. circuits for stimulus-response 2. exchange of information Figure 7.1 Central Nervous System 1. brain 2. nerve cord - ganglia associated 3. axons project to and from the body (CNS PNS) 4. cell bodies in CNS except some in ganglia Support cells (neuroglia) - more abundant than neurons - more mitotic capability Membrane Potentials A. Measured across the cell membrane 1. use internal and external electrodes - reference electrode and recording microelectrode 2. measure potential difference between ICF and ECF (voltage) 3. determines Vm (membrane potential) - intracellular potential relative to extracellular potential - extracellular potential considered zero B. Resting potential Vrest 1. steady state negative potential of ICF - usually between -20 and -100 mV 2. reflects an electrical gradient (energy) Electrical Properties of Membranes A. Conductance (g) 1. conferred by ion channels 2. is inversely related to resistance 3. Ohm’s law: ∆Vm = ∆I x R ∆Vm = change in voltage across the membrane ∆I = current across the membrane (in amps) R = electrical resistance of the membrane (in Ohms) Electrical Properties of Membranes B. Capacitance (ability to store an electric charge) 1. conferred by membrane itself bilayer is an insulating layer separating charges 2. capacitative current - ability of ions to interact across the membrane without crossing the bilayer - charges collect on either side of the membrane - energy of the charges “stored” by the capacitor Electrochemical Potentials A. Factors responsible 1. ion concentration gradients on either side of the membrane - maintained by active transport Electrochemical Potentials A. Factors responsible 2. selectively permeable ion channels B. Gradients not just chemical, but electrical too 1. electromotive force can counterbalance diffusion gradient 2. electrochemical equilibrium C. Establishes an equilibrium potential for a particular ion based on Donnan equilibrium Assume Cl- cannot cross the membrane Nernst equation (pp. 69-71) 1. What membrane potential would exist at the true equilibrium for a particular ion? - What is the voltage that would balance diffusion gradients with the force that would prevent net ion movement? 2. This theoretical equilibrium potential can be calculated (for a particular ion). Eion = RT ln [X]outside zF [X]inside Goldman Equation 1. quantitative representation of Vm when membrane is permeable to more than one ion species 2. involves permeability constants (P) +] +] -] RT P [K + P [Na + P [Cl K out Na out Cl in ENa,K,Cl = ___ln _____________________________ +] + P +] + P [Cl-] P [K [Na K in Na in Cl out F pp 72-73 Resting Potential A. Vrest 1. represents potential difference at non-excited state -30 to -100mV depending on cell type 2. not all ion species may have an ion channel 3. there is an unequal distribution of ions due to active pumping mechanisms - contributes to Donnan equilibrium - creates chemical diffusion gradient that contributes to the equilibrium potential Resting Potential B. Ion channels necessary for carrying charge across the membrane 1. the the concentration gradient, the greater its contribution to the membrane potential 2. K+ is the key to Vrest (due to increased permeability) - opening K+ channels will greatly alter Vrest Resting Potential C. Role of active transport ENa is + 63 mV in frog muscle Vm is -90 to -100mV in frog muscle