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Transcript
SBI 4U January 17 th , 2012 Luigi Galvani: muscles of dead frog twitch (partially correct) Continued research began to reveal differences between electrical and neural transmission Nerve impulses more slow than electric current Nerve impulses do not undergo loss of strength during transmission Synapse: site of connection between neurons/neuron and effector Pre-synaptic and post-synaptic Chemical vs. Electrical synapse Neurotransmitter released by terminal at synapse Synaptic cleft: separation of plasma membranes by a 25 nm wide gap Presynaptic cell makes direct contact with the postsynaptic cell, allowing current to flow via gap junctions between the cells Unbroken transmission Rapid transmission and synchronous activity Outside of cell – positive Inside of cell – negative Charge separation -> voltage, or electrical potential difference Membrane potential Plasma membrane selectively permeable select ions move through ion channels Some ion channels closed during resting state, others open upon the binding of a specific substance Neurons and muscle cells are exciteable Na+/K+ ion active transport pump, uses energy from ATP to pump 3 Na+ out of the cell for every 2 K+ pumped in -more Na+ outside and more K+ inside -inside of cell more negative due to anions. Outside more positive -70 mV – resting potential. Unstimulated neuron Polarized Action potential: the voltage difference across a nerve cell membrane when the nerve is excited Causes the interior to be less negative Incoming positive ions raise membrane potential to less negative depolarization! If depolarization continues, membrane potential reaches the threshold potential (~ -50 to -55 mv) Na+ channels open Na+ channels remain open flows across concentration gradient action potential fires! Action potential rises high, inside of plasma membrane positive Action potential reaches peak, reaching +30 mV or more Na+ channels close and K+ channels open K+ channels open membrane potential falls during the process of repolarization Voltage-gated K+ channels begin to close slowly Membrane repolarizes Undershoot of membrane potential Final phase Membrane potential stabilizes at the resting value Ready for a new action potential! Many stimuli can cause some degree of depolarization All-or-nothing principle Propagation of action potential does not need further triggering events At the peak of an action potential, membrane enters a resting period, the refractory period (few milliseconds) During refractory period, threshold required for action potential much higher than normal Lasts until membrane has stabilized Intensity not dependent upon the magnitude of stimulus