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human phys lec 4 - 1...

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There are two classes of membrane potential h th t h ll b ti changes that occur when cells become active 1. Graded potentials . These can be small, or large, or anywhere in between. They can be brief or sustained. They do not travel very far from site of origin. 2. Action potentials . These are all or none. They are large changes in membrane potential that have a generally fixed waveform . Action potentials occur in neurons, muscle cells, and a few varieties of hormone secretory cells (adrenal chromaffin cells pancreatic beta cells) secretory cells (adrenal chromaffin cells, pancreatic beta cells). Action potentials are triggered when the membrane potential crosses a threshold . (Like an explosive that goes off once a certain temperature is reached). Cells capable of discharging action potentials are referred to as excitable cells .
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Examples of neuronal and cardiac action potentials Neuron Cardiac cell (ventricular myocyte)
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A train of action potentials (a.k.a. spikes) in a neuron (mV) Note that they are all pretty much the same waveform. Note large amplitude (from 60 mV to +10 mV). They are all or none.
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A sequence of action potentials in a cardiac cell 50 mV 400 ms Note that they all have the same waveform (although h l h l i i l ) they are slower than neuronal action potentials). Note large amplitude. They are all or none.
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It is possible to record action potentials using an extracellular electrode placed close to a cell This is usual done in vivo. The electrodes can be made of glass or metal. The electrode detects small extracellular electric fields caused by an action potential in a cell , so the signals tend to be 1 mV or less (1 2% of the amplitude of an action potential). Size of signal is determined by size of the cell and how close the electrode is to the cell. Extracellular electrodes as a rule can only detect action potentials , not graded potentials.
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Example of a graded potential in a sensory neuron Graded sensory potential Amplitude of graded receptor potential (mV) Stimulation intensity Pressure applied to skin This is a small depolarization (0.4 1.5 mV) caused by a mechanical stimulation of a touch sensitive cell in the skin. The depolarization gets larger as the intensity of the stimulation is increased. They are not all the same amplitude. They are not all or none.
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Three main types of graded potentials in nerve and muscle. 1. Synaptic potentials 2. Receptor potentials (found in sensory cells) 3. Pacemaker potentials (usually oscillatory).
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