 | only cells with excitable membranes (neurons and muscle cells)
can generate action potentials (APs) |
| simply put, an AP is a brief reversal of membrane potential -- from
-70mV to +30mV -- over a few milliseconds |
| unlike graded potentials, APs, do not decrease in strength over distance |
| threshold stimulus is needed to generate the AP (also called
the nerve impulse) |
| only an axon is capable of generating an AP
 | a local graded potential that occurs in a dendrite and
spreads to the axon may result in an AP |
| the transition from graded potential to AP takes
place typically at the axon hillock
|
| the special voltage-regulated gated channels that
allow an AP to be generated are found only on the axon |
|
| the process of AP generation involves three consecutive, overlapping
changes in membrane permeability, which are all due to opening and closing
of active ion gates :
| resting state : active channels closed --
all voltage-gated Na+ and K+ channels are closed. Some K+ is leaking out;
even less Na+ is leaking in. |
| depolarizing phase : increase in Na+ permeability
and reversal of the membrane potential -- when the axonal membrane is
depolarized by the graded potentials spreading from the soma and/or dendrites,
voltage-dependent Na+ channels open, and Na+ rushes into the cell. This causes
even further depolarization of the area, and the intracellular area becomes
even less negative.
 | threshold is between
-50mV and -55mV, at which point the depolarization becomes self-perpetuating
(a depolarization of 15-20mV) |
| Na+ influx is due to positive feedback |
| once all Na+ channels are open,
Na+ permeability is ~1000 times greater than at resting stage |
|
| depolarizing phase : decrease in sodium
permeability -- as the membrane potential becomes positive, the ionic
gradient begins to resist further influx of positive charges, and the Na+
channels begin to close after a few milliseconds of depolarization |
| repolarizing phase : increase in potassium
permeability -- once Na+ entry declines, the voltage-regulated K+ channels
open, allowing K+ to rush out of the cell, following the electrochemical
gradient (more negative and less K+ outside the cell). As a result, the cell's
interior becomes more and more negative, with respect to the extracellular
fluid. |
| undershoot : potassium permeability continues
-- the K+ channels are slow to respond to the depolarization signal,
so more K+ leaks out than is necessary to restore resting potential, allowing
for a brief period of hyperpolarization. |
|
| Propagation of the AP
| the impulse will move away from the point of origin
due to the closed sodium gates of the area where an AP was just generated. |
|
| All-or-none principle
| there is a point (threshold) where the
AP will be propagated. |
| below that point, nothing happens at all. |
| above that point, an impulse occurs.
| an impulse, not a "strong"
impulse or a "weak" impulse, but an impulse. |
|
| either the impulse happens or it doesn't. That
is the "all-or-none" principle. |
| given that, the CNS determines a "strong" or "weak"
stimulus by the frequency of impulses, not the strength
| a strong stimulus will result in
a greater frequency of impulses |
|
|
| Absolute and relative refractory periods
| the absolute refractory period occurs when an
area is currently generating an AP and the sodium gates are open. At this
point, no matter how strong the stimulus is, the neuron is absolutely incapable
of responding to it. |
| directly after the absolute refractory period
is the relative refractory period, when the sodium gates are closed, the
potassium gates are open and repolarization is occuring. During the relative
refractory period, there is a possibility for another impulse, but the stimulus
must be extremely stronger; the threshold has been elevated. |
|
| a nerve fiber may conduct an impulse up to 100m/s
| rate of conduction depends on axonal diameter
-- wider axons conduct more quickly (they have less resistance) |
| rate of conduction also depends on the presence
of a myelin sheath -- myelinated fibers (white fibers) conduct impulses more
quickly than unmyelinated (gray) fibers.
| myelin acts as an insulator to
prevent charge leakage from the axon |
| the nodes of Ranvier are the only
places where current can pass through a myelinated axonal membrane |
| all voltage-regulated Na+ channels
are located at the nodes, so the local depolarizing current does not dissipate
through the membrane, but is maintained and moves to the next node
| this is called saltatory
conduction
|
|
| Multiple Sclerosis (MS) is a demyelinating
disorder of an autoimmune nature.
| symptoms : visual disturbances
(blindness), muscle weakness and clumsiness, incontinence |
| myelin sheaths in the CNS are
gradually destroyed and replaced with hardened sheaths called
scleroses
|
| loss of myelin slows and eventually
stops nerve conduction in the affected fibers |
| axons are not damaged, and generate
more and more Na+ channels to try to compensate, which may account for periods
of "remission" |
|
|
| Other things that affect nerve impulses :
| alcohol, sedatives, anesthetics
: block nerve impulses by reducing membrane permeability to Na+. |
| cold and continuous pressure interrupt
blood circulation to the neuronal processes, which will also impair nerve
conduction |
|
|
