Compound Action Potentials
Date Performed: 15FEB2011
Date Due: 01MAR2011
Neurons are the cells that receive and transmit electrical signals (University of North Texas, 2010). The ability of the neuron to conduct these impulses is because of an electrochemical voltage across the plasma membrane of that neuron. An action potential is an all or nothing response to a stimulus along a single axon. A compound action potential is a graded response that results from the stimulation of more than one axon. Action potentials can be broken down into five different phases: resting potential, threshold, rising, falling, and recovery. The inside of a cell is negatively charged and the potential difference across the plasma membrane is between 50 and 90 mV. In a resting cell, the membrane is more permeable to potassium than sodium. When synaptic activity makes the cell less negative, the sodium channels open. If the cell voltage goes past a certain level, an action potential is produced. Action potentials, changes in the membrane potential that happen when a nerve cell membrane is stimulated (Ritchison), happen within milliseconds. After the voltage has reached the threshold potential, more voltage-gated sodium channels open and the voltage of the membrane reaches its most positive value. The voltage-gated sodium channels close soon after opening, and with that, the potassium channels open. Now potassium is rushing back into the cell, repolarizing the cell back to its resting level. Now, because there are many more potassium gates are open than there were when the cell was resting, the cell hyperpolarizes. Finally, the extra potassium gates that were open, close and the sodium channels are ready to open again. There is no such thing as a weak or a partial action potential because action potentials are all-or-none (Randall, French, & Burggren, 2002). Either the threshold is reached, causing an action potential, or it’s not (Ritchison). Right after an action potential, the membrane does not respond to any stimulation because the sodium gates aren’t activated, but the potassium gates are active. This period is called the absolute refractory period (University of North Texas, 2010). A little later, another action potential can be produced, but the stimulus must be greater than the threshold because during this time, not all of the sodium gates are activated. This is the relative refractory period. At the beginning of this period, a stimulus largely greater than the threshold is required to produce an action potential, but as time goes on the nerve cell membrane will need a smaller and smaller stimulus to cause an action potential. By the end of the relative refractory period, only a slightly above threshold stimulus will get the job done. Once initiated near the soma, action potentials grow and distribute down the axons (Sasaki, Matsuki, & Ikegaya). The movement of action potentials all the way down an axon is caused by positive charges in the cell leaking to an unstimulated region. Action potentials only take place in the nodes of myelinated cells because the insulation keeps the action potential currents from leaking out of the membrane.
Materials and Methods
Fine tipped glass rod
Recording Nerve Bath with lid
The frog was dissected in order to expose the sciatic nerve. The sciatic nerve was then removed and placed in a bath of Ringer’s solution. The nerve is then placed in a nerve bath that is connected to the Power Lab in order to receive stimuli. The computer recorded the data. There were three parts to this experiment: threshold voltage and maximal compound action potential amplitude, refractory period, and conduction velocity of the nerve.
The first part of this experiment was to dissect a frog and remove...