BS2550 - Neuronal and Cellular Signaling Lecture 3 Action Potentials Electrical signals are produced by cells in response to a stimulus, this changes the resting membrane potential. Receptor potentials are due to the activation of sensory neurons by external stimuli. Action potentials are a special type of electrical signal that travels along the axon. This is a brief change in the membrane potential from negative to positive. Action potentials are said to be all or nothing - they occur fully or not at all. These potentials are generated across the membranes of neurons because of the differences in concentrations of specific ions and the membranes selective permeability to these ions. How Do Ions Move? The ion concentration gradients are established by proteins known as active transporters. These proteins move ions into and out of the cell by active transport ffagainst their concentration gradientfi. The selective permeability of membranes is due to ion channels that allow only certain kinds of ions to cross the membrane in the direction of their concentration gradients. These channels and protein transporters work against each other, in doing so they create the resting membrane potential and action potentials. An electrochemical equilibrium is created where there is an exact balance between the concentration gradient and opposing electrical gradient. Ionic Basis of Action Potentials An action potential is a rapid and transient change in the voltage across the membrane. It depends on ionic gradients and relative permeability of the membrane and is generated in excitable cells in response to a pre-synaptically released neurotransmitter ffall or nothing reactionfi and propagated over long distances without loss of amplitude. Every single cell has a potential difference across their membrane but are not all excitable cells. These are biological digital signals. Because the shape of each of the graphs are different, it shows that different ion channels are open at different times. The sodium hypothesis ffHodgkin and Katz, tt949fi states that sodium concentration outside of the cell is greater than inside of the cell meaning depolarization involves a rapid and highly specific increase in the permeability in the membrane to sodium ions. The resulting membrane potential approaches ENa. Potassium channels mediate the repolarization phase of an action potential, sodium is responsible for the depolarization of a membrane.
Channels can open and close -> they are gated. - Ligand-gating occurs when the channel opens or closes in response to the binding of a chemical ligand, e.g. channels at synapses that respond to extracellular transmitters and channels that respond to intracellular messengers. - Voltage-gating is when the channel opens or closes in response to changes in membrane potential such as channels involved in action potentials. - Mechanical gating occurs if a channel opens or closes in response to membrane deformation like channels in mechanoreceptors -> carotid sinus stretches receptors and hair cells. Voltage-gated channels are the ones involved the most in AP generation. Voltage-clamping controls the membrane potential keeping it constant so that the ionic currents can be measured. The Im electrode injects current