Why is an action potential unidirectional

Figure Lodish 5th edition. At time 0, an action potential purple is at the 2-mm position on the axon. The membrane depolarization spreads passively in both directions along the axon Figure Each region of the membrane is refractory inactive for a few milliseconds after an action potential has passed.

Thus, the depolarization at the 2-mm site at time 0 triggers action potentials downstream only; at 1 ms an action potential is passing the 3-mm position, and at 2 ms, an action potential is passing the 4-mm position.

Figure , Lodish 4th Edition. Ion channels in neuronal plasma membranes. Each type of channel protein has a specific function in the electrical activity of neurons. One type c has a site for binding a specific extracellular neurotransmitter blue circle. The other type d is coupled to a neurotransmitter receptor via a G protein; it responds to intracellular signals red circle induced by binding of neurotransmitter to a separate receptor protein not shown.

Figure , Lodish 4th edition. Proposed transmembrane structures of four types of gated ion-channel proteins. Helix 4 maroon acts as a voltage sensor, and the nonhelical P segments between helices 5 and 6 line the ion pore. Since none of the transmembrane alpha helices act as a voltage sensor, these channels are not voltage-gated.

Rather, binding of cAMP or cGMP to a cytosolic segment triggers opening of these channels, which are abundant in the sensing cells of the visual and olfactory systems see Figure About 64 percent of the residues are similar or identical in sequence in both channels.

Each of the four homologous domains is thought to contain six transmembrane alpha helices similar in structure to those in the Shaker channel monomers in a. Figure , Lodish 5th edition. Increasing the size of the axon retains more of the sodium ions that form the internal depolarisation wave inside the axon. However, if we had to have axons the size of the squid giant axon in our brains, doorways would have to be substantially widened to accommodate our heads!!!

We could only have a few muscles located at any great distance from our brains - so we'd all be extremely short with very large heads The answer is to insulate the axonal membrane to prevent the dissipation of the internal depolarisation in small axons - myelin. Without the myelin sheath, we cannot function. This is demonstrated by the devastating effects of Multiple Sclerosis, a demyelinating disease that affects bundles of axons in the brain, spinal cord and optic nerve, leading to lack of co-ordination and muscle control as well as difficulties with speech and vision.

For further information on this disease, visit the MS Society 's web site. Brain Basics The fundamentals of neuroscience. Introduction What are neurons? How do neurons work? How do neurons conduct electrical impulses? Neurons conduct electrical impulses by using the Action Potential. On the right is the target that receives the signals. The arrow at fs is the input from stimulation at frequency of fs Hz.

If the stimulator frequency were less than Hz, there is a possibility that a naturally occurring AP would not be annihilated by a stimulator produced AP. Action Potentials AP propagation initiated by electrical stimulation with a nerve electrode can be stopped by applying an arresting current.

This suggests that the distance between the initiation site and the arrest site should be small. A nerve electrode with multiple contact surfaces for injecting currents can be placed around a nerve trunk to initiate APs and arrest propagation in one direction. Department of Biomedical Engineering.