Hello irun,
Given that you have listed your question under Physiology -
Muscle/Neuro, I can restate your question as asking about the
mechanism for motor neurons to evoke action potential. In other
words, how do neurons get excited. North Arizona University gives an
explanation of outline form.
"II. Message Transmission – Neuron excitability
Action potential
1. Ligand-gated Na channels open
2. Na+ rushes into cell down gradient
3. Threshold potential is reached (more positive than resting)
4. Voltage gated Na channels open – positive feedback loop
5. Cell potential becomes positive (depolarized)
6. Voltage gated K channels open
7. Rapid exodus of K+ repolarizes neuron
7[sic]. Absolute refractory period – while Na gates are open
9. Relative refractory period – while K gates are open"
You can visit their web site page for more information on events
preceding and following the evoking of action potential in a motor
neuron.
Northern Arizona University, "Nervous System"
http://jan.ucc.nau.edu/~pe/exs336web/336pnerves.htm
A more detailed explanation is offered by Iowa State University.
"The nerve impulse travels down the axon cell membrane and occurs as
movement of ions across the cell membrane. Inside of a neuron has a
deficiency in positive ions so that it is negative with respect to the
outside. It is high in K+ and low in Na+. Just the opposite
immediately outside the axon. K+ and Cl- ions can move freely across
the membrane but the axon cell membrane is impermeable to Na+. The
membrane thus has a small charge imbalance that can be measured as a
voltage. During the resting stage this charge imbalance polarizes the
membrane. During an action potential or an impulse channels in the
cell membrane open to allow Na+ to come across the membrane. This
occurs through specific Na+ channels. When this happens the cell
membrane depolarizes. After a few milliseconds the Na+ channels close.
Na+ is pumped back out of the cell (by an ATPase pump) and the resting
potential is restored, repolarizing the membrane. This ion fluctuation
is what creates the nerve impulse. The action potential moves down the
surface of a neuron to the synapse. At the synapse the action
potential causes the release of a neurotransmitter across the synaptic
cleft to stimulate or inhibit the postsynaptic cell. Release of a
neurotransmitter is due to the influx of Ca++ through Ca++ channels."
The link to the Iowa State University web page "...a brief look at
neurons...":
http://www.ent.iastate.edu/dept/courses/ent555/555notes/nerves.html
Bryn Mar College gives the explanation as follows:
"GENERATION OF NERVE SIGNAL occurs by release of neurotransmitters at
SYNAPSES:
Synapses form between neurons (axon-dendrite), and between nerve and
muscle:
Neurotransmitters (NTs) released from pre-synaptic terminals activate
neurotransmitter-gated ion channels on post-synaptic membrane ->
activate voltage-gated ion channels
NT types released differ by functional nerve type and location:
Acetylcholine, adrenaline (PNS & CNS); many more used only in CNS:
many molecules used as neurotransmitters by nerves are also used as
hormones by endocrine cells (see final section of outline)
NTs rapidly inactivated or taken back up into nerve terminal
CELL MEMBRANE DEPOLARIZES: NT-gated ion channels activate
voltage-gated ion channels on post-synaptic membrane
DEPOLARIZATION IS TURNED INTO AN ACTION POTENTIAL if threshold
stimulus level is achieved:
Depolarization spreads without diminishing along axon"
The link to the Bryn Mar page from Biology 204 follows. The relevant
section is titled "4. PHYSIOLOGY OF NERVE SIGNAL GENERATION AND
TRANSMISSION:"
http://www.brynmawr.edu/biology/204_02/HlectWk_4.html
I've always found it quite magical, the way that a simple biochemical
process results in an entire body executing complex motion. For
addtional reading on the development of neurons and AchRs
(Acetylcholine receptors) you can read the following article from
Columbia University, with special attention to the section titled
"postsynaptic, part 3". "every nerve-evoked potential from the motor
neuron inhibits the synthesis of new AchRs. Action potentials spread
along the length of a given muscle fiber resulting in a massive rise
in [Ca++]i ª suppression of AchR synthesis at all muscle nuclei,
except those at junctional areas of the sarcolemma where the presence
of secreted neureglin counteracts the nerve-evoked down-regulation
with a stronger erbB receptor-induced up-regulation of AchR
synthesis."
Columbia University web site, "Neural Science 38" (you may have to cut
and paste the link):
http://216.239.51.100/search?q=cache:v4zBhB3k990C:cpmcnet.columbia.edu/dept/ps/2004/Academic/first_year/neuro/ns39.doc+neuron+synapse+motor+evoked+potential&hl=en&ie=UTF-8
Search strategy used:
neuron synapse motor action potential Ca++
Regards,
historybuff |
