with
the inside value indicating the difference
Causes of
Resting Potential
(-65mvolts
in neurons)
Both
active and passive channels are involved in maintaining the
potential of the neuron membrane
1. active transport of Na &
K....
(2 Na+ out/3 K+
in)
leads to high Na+ outside
& high K+ inside
2. passive Na+ and K+ channels
(voltage gated)
differential
permeability of Na (slower in)
& K (faster
out)
i.e. Na+ channels are closed,
K+ channels are "leaky"
3. lots of protein anions (-) inside
which cannot diffuse through the
membrane
4. normal diffusion of Cl- into the
cell
all
of which make inside of cell negative (-)
Examine
the image to see relative concentrations of all ions.
This is
called the RESTING POTENTIAL of the cell. It's value is -70
mV
The cell
is said to be POLARIZED
EXCITABILITY
- the ability of nerve cells to respond to stimuli and convert
themn into nerve impulses
NERVE
IMPULSE - a combination of self-propagating electrical and
chemical events which create an electrical current which travels
across the membrane
STIMULUS
causes
membrane's permeability to Na+ ions to increase (i.e. passive Na+
channels open) at the point of stimulation
Na+
flood into the neuron (only at this point)
the
electrical potential of the membrane begins to change because more
+ ions are entering
at 0
mV the membrane is depolarized (charge inside and outside are the
same)
Na+ continue to rush
in, creating a more + charge inside, until the potential
difference reaches +30 mV (i.e. outside is now more negative than
the inside) (this is the "spike" on the graph)
the
value of +30 mV does not change..........."all or none
principle"...........depolarization occurs or it
doesn't..........potential difference achieved is always the same
(due to voltage gated channels)
at
this point it can be said that an ACTION POTENTIAL has been
initiated
this
is because this point will have a high enough charge to stimulate
the polarized point in the membrane which is adjacent to it
(causing this to happen there)......this is self perpetuating and
will travel down the neuron
............back at the original point of
stimulus............
REPOLARIZATION
occurs ........the Na+ channels close so that Na+ stops diffusing
in..........K+ channels open creating an outward diffusion of K+
ions..........causing the external surface of the membrane to
become more positive.........and the internal to be more
negative
the
quantity of Na+ and K+ ions inside and outside the membrane is
returned to original state
repolarization returns the cell to resting potential
HYPERPOLARIZATION
..after
repolarization is achieved the K+ channels close but not before
too many K+ have diffused out, so that the potential difference
becomes greater than -70mV
the active Na+ and K+ pump is still working at bringing things
back to normal.........so resting potential is achieved again
the time for these events is about 2 -
3 msec...........occurs from point to point in a wave down the
neural membrane
SOME OTHER
FEATURES
THRESHOLD
STIMULUS..........
a stimulus must be strong enough to initiate an impulse........a
SUBTHRESHOLD stimulus will cause Na+ channels to open briefly, but
the depolarization may only reach about -65 mV before they close
again.........this is not enough to stimulate the next point on
the membrane (no action potential.....no impulse)
SUMMATION
- a second (or a series) of subsequent subthreshold stimuli
(quickly applied) can be cumulative and lead to an
impulse
ALL
OR NONE PRINCIPLE
.......as stated before the value of the action potential is
always +30mV
REFRACTORY
PERIOD.........sodium
channels cannot be activated again for a short period after the
passing of a nerve impulse......thus a short period of time must
pass before another impulse can be generated (larger diameter
fibres recover faster and so can transmit up to 2500 impulses per
second, while smaller diameter fibres have a longer refractory
period and so may only transmit as few as 250 impulses per
second)
SALTATORIAL
CONDUCTION........spaces
between Schwann cells on myelinated axons are called Nodes of
Ranvier......the sheath contains lipoproteins which does not
conduct and electric current.....so membrane depolarization can
not occur where it is surrounded by the
sheath.......depolarization occurs in the space (node) and will
jump to the next node.......creates a much faster
impulse........requires less energy because less action required
by Na+/K+ pump
SPEED
OF NERVE IMPULSE
- impulses can travel up to 130 m/second in large (diameter)
fibers.......ie. all motor nerves leading to skeletal muscle and
large sensory neurons associated with touch, pressure, heat, cold
and position of joints........other fibres which stimulate smooth
muscle and glands have a smaller diameter and transmit about 10
m/sec.......while some other smaller fibres are as slowas 0.5
m/sec
IMPULSES TRAVEL ONE WAY ONLY -
although an action potential can stimulate points to either side,
the synapse (junction between two neurons) can only carry the
impulse in one direction
cell must be
alive since the active Na+/K+ pumps require energy