# Action potentials

1. Jan 28, 2012

### jsmith613

1. The problem statement, all variables and given/known data
Not sure if this is a homework question but figured this is the best place to put it.

When I touch something, how is the stimulus turned into an action potential? how does the body convert the energy of the stimulus into energy that can be used to modify an electrochemical gradient?

2. Relevant equations

3. The attempt at a solution
I know between neurons it is via a neutrotransmitter but how does it start?

2. Jan 28, 2012

### Ygggdrasil

The energy from an action potential does not come from the stimulus; rather, neurons continually expend energy to keep themselves primed to fire an action potential. Various ion pumps in the neuron use the energy from ATP hydrolysis to pump ions across the cell membrane such that there is an excess of positive charges outside of the neuron. A stimulus, such as choice, simply opens an ion channel that allows the positive ions to flow back into the cell. The flow of ions into the cell decreases the voltage across the cell membrane, which triggers nearby voltage-gated ion channels that help to propagate the action potential through the neuron.

A good analogy here is that of a toilet. When you push the lever on the toilet, the energy you use to push the lever is not what shoots the water into the toilet bowl. Rather, the water sits in a reservoir above the bowl and it uses its gravitational potential energy powers its propagation down the drain. The work is done after the flush is over as the pumps in the toilet re-fill the reservoir after the flush.

In a similar way, the positively-charged ions outside of the neurons are like the water in the reservoir. They have ample electrochemical potential energy to push them into the cell, and all they are waiting for is for someone to open a valve for them to enter into the cell. Also like a toilet, the neuron expends the most energy right after the action potential has fired, as its ion pumps seek to re-fill the reservoir of positively charged ions outside the neuron in order to prepare the neuron for the next action potential.

3. Jan 29, 2012

### jsmith613

But I still don't get why a stimulus would do this?

4. Jan 29, 2012

### nobahar

5. Jan 29, 2012

### Ygggdrasil

Hmm, don't know why I wrote choice there. I meant to write, "A stimulus, such as touch..."

As nobahar said, in touch and hearing, the stimulus (touch or sound waves) mechanically deforms the sensory cell. This deformation of the cell changes the shape of the mechano-sensitive ion channel, which opens the pore of the ion channel, allowing ions to flow into the cell.

Mechanosensation is probably the easiest to understand because the stimulus directly opens the ion channel. In other cases, such as vision and olfaction, the stimulus indirectly opens ion channels. Vision, how photoreceptor cells sense light and change their firing in response, is the process where where the molecular details have been worked out in the greatest detail. For a summary see: https://en.wikipedia.org/wiki/Photoreceptor_cell#Phototransduction

This process is somewhat strange as vision works in reverse from many other senses: the photoreceptors fire action potentials in the absence of a stimulus and decrease their firing in the presence of the stimulus.