Explain the ionic basis of membrane potentials
- Single Cell Physiology: Concepts and Methods (only need to read the first half)
Action potentials rely on the opening and closing of various ion channels throughout the cell membrane. Usually, these ion channels are voltage-gated sodium channels and voltage-gated potassium channels (with some exceptions, such as in the heart).
The "resting potential" of most cells (i.e. the membrane potential when cells are not being stimulated) is around -70mV. There are several reasons why the resting potential is negative: the Na+/K+ pump pumps more positive ions out of the cell than into the cell, and many proteins within the cell form anions (negative ions). The membrane potential can be increased or decreased by the opening or closing of channels in the cell membrane. However, small changes in membrane potential are incapable of sending a signal. In order to send an electrical signal, the membrane potential needs to be increased past a threshold, usually around -55mV.
Once the cell's membrane potential exceeds the threshold potential, voltage-gated sodium and potassium channels start to open. However, since sodium channels open more quickly than potassium channels, the sodium channels open first, allowing positively-charged sodium ions to rush into the cell, increasing the membrane potential so that it becomes positive. This is called an action potential. Eventually, potassium channels open, allowing positively-charged potassium ions to leave the cell, decreasing the membrane potential until it becomes even lower than the resting potential (this is called "hyperpolarisation"). The Na+/K+ pump on the cell membrane then works to restore the initial concentrations of sodium and potassium inside and outside of the cell, so that another action potential can occur when the cell is stimulated again.
Note that action potentials are all-or-nothing. An action potential will either not occur, or it will occur at the same amplitude every time. Cells can, however, have more frequent or less frequent action potentials.
Explain the process of sensory transduction. Describe how action potentials are propagated along membranes
The process of sensory transduction generally goes along the lines of this: something stimulates cell -> cell fires action potential -> COOL STUFF HAPPENS (e.g. neurotransmitters get released, muscle cell contracts, etc.).
In order to understand sensory transduction, it is important to understand how an action potential travels from one end of the cell to another. Simply put, when one area of the membrane is stimulated and becomes positive, nearby areas become more positive as well. In this way, an action potential can propagate from one membrane microdomain to another. In nerves, for instance, action potentials generally start at the axon hillock (where the axon, which is the long process of the neuron, joins the soma, which is the area where the organelles are) before firing down the axon. (Note that the soma of the neuron does not produce action potentials.)
You might be wondering how the action potential avoids going backwards. Well, wonder no more! Remember that, due to the slow opening and closing of potassium channels, the cell becomes hyperpolarised (more negative than usual) following an action potential. As such, that area of the membrane becomes harder to stimulate, so the signal will not travel to places where an action potential has just occurred.
Action potentials on their own can be slow to propagate: a rate of roughly 2m/s, which is pretty slow when you consider that signals from your brain need to travel down to your feet in order for you to move them. Thankfully, there's a solution, and that solution is called myelination. Myelin, which is secreted by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system, wraps around axons. There are spaces between myelinated sections, called Nodes of Ranvier. When a nerve is stimulated, the action potentials can "jump" between the Nodes of Ranvier in a process known as saltatory conduction. The wider the neuron, the more myelin is used, and the faster the conduction occurs!
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