lecture 3

• Neurons are the structural and functional units of the nervous system.
• Saltatory conduction is faster than conduction in an unmyelinated nerve fiber.
• Nerve cells respond to electrical, chemical, or mechanical stimuli.
• A neuron is an electrical cell of the nervous system.
• Action Potential is a digital one-way electrical pulse from axon hillock to axon terminals.
• Each ion that can flow through channels reaches equilibrium between two forces.
• Equilibrium Potential is the voltage across cell membrane if only one ion could diffuse.
• Equilibrium potential for each ion is determined by Nernst Equation.
• E K+ = +58 mV log 10 ([K+] outside/[K+] inside) for room temperature, squid axon.
• E K+ = 58 mV log 10 1/20 = - 75 mV.
• E Na + = 58 mV log 10 10/1 = + 58 mV.
• E Cl - = - 58 mV log 10 15 = - 68 mV.
• E Ca ++ = +58 mV log 10 10,000 = +220 mV.
• K+ make potentials; Na+ make + potentials.
• Membrane Potential is the difference in charge across membranes.
• Resting Membrane Potential (RMP) is the membrane voltage of a cell not producing impulses.
• Neurons have a resting potential of − 70 mV.
• At around +30 mV, voltage-gated K+ channels open, and K+ rushes out of the cell following the electrochemical gradient, this makes the cell repolarize back toward the potassium equilibrium potential.
• The strength of the stimulus will NOT affect action potential duration.
• The absolute refractory period occurs during the action potential.
• A stronger stimulus may also activate more neurons in a nerve, this is called recruitment.
• A stronger stimulus WILL make action potentials occur more frequently.
• Sodium rushes in due to the electrochemical gradient.
• The strength of the stimulus will NOT affect the size of the action potential; it will always reach +30 mV.
• Voltage-Gated Na+Channels open if the membrane potential depolarizes to −55 mV, this is called the threshold.
• Na+ channels are inactive (not just closed) during the relative refractory period.
• Hyperpolarization is inhibitory.
• RMP of most cells is – 65 to – 85 mV.
• In myelinated axons, action potentials are produced only at the nodes of Ranvier, this is called saltatory conduction.
• Action potentials can only increase in frequency to a certain point.
• In unmyelinated axons, action potentials are produced fractions of a micrometer apart, this is called continuous conduction.
• These channels are deactivated at +30 mV.
• A refractory period occurs after an action potential when the neuron cannot become excited again.
• Once threshold has been reached, an action potential will happen.
• Membrane potential climbs toward sodium equilibrium potential.
• RMP depends on concentrations of ions inside and out and on permeability of each ion.
• Affected most by K+ because it is most permeable.
• Some Na+ diffuses in so RMP is less negative than E K+.
• At rest, there is a high concentration of K+ inside the cell and Na+ outside the cell.
• All potentials result from ions moving across membranes.