CHAPTER 5 Flashcards
Within a millisecond or so, the
potential difference between the inside and outside, called
the
Diffusion potential
becomes great enough to block further net potassium diffusion to the exterior, despite the high
potassium ion concentration gradient.
Diffusion potential,
becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber,
-the potential difference required is about 94 millivolts, with negativity inside the fiber membrane.
Diffusion of the positively charged sodium
ions to the inside creates a membrane potential of opposite polarity to that in, with negativity outside
and positivity inside.
The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the
Nernst potential
The magnitude of this Nernst potential is determined by
the –
of the concentrations of that specific ion on the two sides of the membrane.
ratio
The greater this ratio, the
greater the tendency for the ion to diffuse in one direction, and therefore the greater the Nernst potential required
to prevent additional net diffusion.
Can be used to calculate
the Nernst potential for any univalent ion at normal body temperature of 98.6°F (37°C):
Nernst equation
When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors:
(1) the polarity of the electrical charge of each ion,
(2) the permeability of the membrane (P) to each ion, and
(3) the concentrations (C) of the respective ions on the inside (i) and
outside (o) of the membrane.
Formula, gives the calculated membrane potential on the inside of the membrane when two univalent positive
ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl−), are involved.
Goldman equation, or the Goldman-HodgkinKatz equation,
Is placed in the extracellular fluid,
and the potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter
“indifferent electrode,”
is a highly sophisticated electronic apparatus that is capable of measuring small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which has a lumen diameter usually less than 1 micrometer and a resistance more than a million ohms.
voltmeter
Then, as the recording electrode passes through the voltage change area at the
cell membrane (called the
Electrical dipole layer
Note that this is an electrogenic pump because more positive charges are pumped to the outside than to the inside(three Na+ ions to the outside for each two K+ ions to the inside), leaving a net deficit of positive ions on the inside; this causes a negative potential inside the cell membrane.
in the nerve membrane through which potassium can leak even in a resting cell.
“tandem pore domain,”
potassium channel, or
potassium (K+) “leak” channel,
Nerve signals are transmitted by
Action potentials
Which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane.
action potentials
This is the resting membrane potential before the action potential begins. The membrane is said to be “polarized” during this stage because of the −90
millivolts negative membrane potential that is present.
Resting Stage
At this time, the membrane suddenly becomes permeable to sodium ions, allowing tremendous numbers of positively charged sodium ions to diffuse to the interior of the axon.
Depolarization Stage
immediately neutralized
by the inflowing positively charged sodium ions, with the potential rising rapidly in the positive direction
depolarization
after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close and the
potassium channels open more than normal
Repolarization Stage.
Then, rapid diffusion of potassium ions to the exterior re-establishes the normal negative resting membrane potential
repolarization
The necessary actor in causing both depolarization and
repolarization of the nerve membrane during the action
potential is the
voltage-gated sodium channel
also plays an important role in
increasing the rapidity of repolarization of the membrane
voltage-gated potassium channel
This channel
has two gates—one near the outside of the channel called
activation gate
during this state, sodium ions can pour inward through
the channel, increasing the sodium permeability of the
membrane as much as 500- to 5000-fold
activated state
and another near the inside called the
inactivation gate
which is used to measure flow of ions through the
different channels.
voltage clamp
-For instance, the sodium channels can be blocked by a toxin called
-by applying it to the outside
of the cell membrane where the sodium activation gates
are located.
tetrodotoxin
Therefore, a sudden increase
in the membrane potential in a large nerve fiber from −90 millivolts up to about −65 millivolts usually causes the explosive development of an action potential. This level of −65 millivolts is said to be the
threshold for stimulation
In fact, the calcium ion
concentration needs to fall only 50 percent below normal before spontaneous discharge occurs in some peripheral
nerves, often causing muscle
muscle “tetany
tetraethylammonium ion
Blocks the potassium channels when it is applied to the interior of the nerve fiber.
This transmission of the depolarization process along a nerve or muscle fiber is
nerve or muscle impulse
Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, or it does not travel at all if conditions are not right. This is called the
all-or-nothing principle
when the spread of depolarization stops?
action potential reaches a
point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane.
Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This “greater than 1” requirement is called the
“safety factor” for propagation
This type of action potential occurs in
heart muscle fibers
The cause of the plateau is a combination of several factors. First, in heart muscle, two types of channels enter into the depolarization process:
(1) the usual
voltage-activated sodium channels, called fast channels, and
(2) voltage-activated calcium-sodium channels, which are slow to open and therefore are called
slow channels.
These rhythmical discharges cause:
These rhythmical
discharges cause
This is not enough negative voltage to keep the sodium and calcium channels totally closed. Therefore, the following sequence occurs:
(1) some sodium and
calcium ions flow inward;
(2) this increases the membrane
voltage in the positive direction, which further increases
membrane permeability;
(3) still more ions flow inward
(4) the permeability increases more, and so on, until an action potential is generated.
This continues for nearly a second after the preceding action potential is over, thus drawing the membrane potential nearer to the potassium Nernst potential. This is a state called
Hyperpolarization
But the increased potassium conductance (and the state of hyperpolarization) gradually disappears, as shown after
each action potential is completed in the figure, thereby allowing the membrane potential again to increase up to the
threshold
The large fibers are
myelinated
and the small ones are
unmyelinated
A typical myelinated fiber. The central
core of the fiber is the
axon
The axon is filled in its center with a
-which is a viscid intracellular fluid.
axoplasm
Surrounding the axon is a
myelin sheath
About once every 1 to 3 millimeters along the length of the myelin sheath is a
node of Ranvier
The myelin sheath is deposited around the axon by
Schwann cells
Then the Schwann cell rotates around the axon many times, laying down multiple layers of Schwann cell membrane containing the lipid subtance
sphingomyelin
Yet the action potentials
are conducted from node to node, as
this is called
saltatory conduction
Thus, the nerve impulse jumps along the fiber, which is the origin of the term
“saltatory.”
mechanical pressure to excite
nerve endings in the skin
chemical neurotransmitters to
transmit signals from one neuron to the next in the brain
Electrical current to transmit signals between
successive muscle cells in the heart and intestine.
These local potential changes are called
Acute local potentials
and when they fail to elicit an action potential, they are called
acute subthreshold potentials
Now the local potential has barely reached the level required to elicit an action potential, called the
threshold level
The period during which a second action potential cannot be elicited, even with a strong stimulus, is called the
Absolute refractory period
In contrast to the factors that increase nerve excitability, still others, called
membrane-stabilizing factors, can
decrease excitability
decreases membrane permeability to sodium ions and simultaneously reduces excitability
high extracellular fluid calcium ion concentration
Among the most important stabilizers are the many substances used clinically as local anesthetics, including p
procaine and tetracaine
The cathode ray tube itself is composed
basically of an
electron gun and a fluorescent screen
When excitability has been reduced so low that the ratio of action potential
strength to excitability threshold (called the
“safety factor”
