Re: Action Potential

[E.S.Prakash]

Ethan Leong: 

A stimulus that is just strong (intense) enough to bring a neuron's membrane potential (from RMP) to the 'firing level' is called a "threshold stimulus". 

You are right; when the membrane potential has come to firing level, the rate of depolarization increases sharply (usually) that we call it a "spike" (upstroke of the action potential etc). 

When a neuron's membrane potential comes to the firing level (a typical value is - 55 mV), this voltage is sensed by voltage-gated Na channels at the site of the neuron membrane that has been excited to - 55 mV. Opening of voltage-gated Na channels causes inrush of enough Na ions enough to transiently reverse the membrane potential at that site to a positive value. This depolarizes the adjacent segment completely by opening voltage-gated Na channels, and this wave of depolarization is propagated through the neuron membrane (action potential) until the end of the neuron (axon terminal in the case of a motor neuron) is reached. 

See this animation 

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

and see if you agree with my description above. 

When a neuron has been depolarized from RMP to firing level, virtually nothing can stop an action potential from occurring - i.e., action potential occurs. When a neuron has not been depolarized to the threshold or firing level, an action potential does not occur, and because of the continued operation of Na-K ATPase and open K channels, the membrane's potential tends to go back toward the "RMP". The two facts above are summarized as the All or None Law (with regard to action potentials in neurons).

What these animation misses to show is that the type of ion channels that are activated when we apply a stimulus (it could be touch - a mechanical stimulus; adrenaline - a chemical stimulus; or electrical current), the ion channels that are initially opened by the stimulus are not the "voltage gated Na channels"

There are several types of ion channels in addition to voltage-gated Na channels. 

Mechanically gated ion channels - activated by touch and other mechanical stimuli (in the case of sensory neurons)

Ligand gated ion channels - activated by neurotransmitters released by presynaptic neurons; 

The opening of one or more of the above channels is responsible for the depolarization from RMP to the threshold or firing level. 

Voltage-gated Na channels, however, are opened only when the membrane potential comes to "firing level" - so ion flux through these channels is responsible for the full fledged depolarizing phase of the action potential.

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On Sat, Dec 4, 2010 at 4:47 PM, Ethan Leong <char_...@hotmail.com> wrote:

Dear sir,
 
Regarding the topic of action potential, i have some doubt that i hope sir can help me on it.
 
Firing level is a point where depolarization process become very fast, so if a stimulus a given and it depolarized the neuron to -55mV which is the point of firing level, the stimulate stop and will the AP being continue to be generated or it stop and consider as an subthreshold stimulus.