The Nerve Impulse Flashcards
polarization
a difference in electrical charge between the inside and outside of the cell
resting potential.
electrical potential inside the membrane is slightly nega- tive with respect to the outside, mainly because of negatively charged proteins inside the cell
difference in voltage is called the resting potential
selective permeability
some chemicals pass through it more freely than others do
Oxygen, carbon dioxide, urea, and water cross freely through channels that are always open
sodium, potassium, calcium, and chloride, cross through membrane channels (or gates) that are sometimes open and sometimes closed
sodium–potassium pump
a protein complex, repeatedly transports three sodium ions out of the cell while drawing two potassium ions into it.
sodium ions are more than 10 times more concentrated outside the membrane than inside, and potassium ions are more concentrated in- side than outside
forces acting on membrane at rest
electrical gradient
concentration gradient
electrical gradient
Sodium is positively charged and the inside of the cell is negatively charged. Opposite electrical charges attract, so the electrical gradient tends to pull sodium into the cell
concentration gradient
difference in distribution of ions across the membrane.
Sodium is more concentrated outside than inside, so just by the laws of prob- ability, sodium is more likely to enter the cell than to leave it.
forces acting on potassium
positively charged and the inside of the cell is negatively charged, so the electrical gradient tends to pull potassium in
potassium is more concentrated inside the cell than outside, so the concentration gradient tends to drive it out
forces acting on sodium
electrical and concentration tend to pull sodium in
maintaining negative charge at rest
Negatively charged pro- teins inside the cell sustain the membrane’s polarization
Chloride ions, being negatively charged, are mainly outside the cell. When the membrane is at rest, the concentration gradient and electrical gradient balance, so opening the chloride channels would produce little effect. However, chloride does have a net flow when the membrane’s polarization changes.
action potentials
Messages sent by axons
hyperpolarization
when the inside of the cell increases its negative charge;
ie, increased polarization
depolarization
reduce its polarization toward zero
threshold of excitation
stimulation beyond this produces massive depolarization of the membrane
all-or-none law
the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it, provided that the stimulus reaches the threshold
Molecular Basis of the Action Potential
- At the start, sodium ions are mostly outside the neuron, and potassium ions are mostly inside.
- When the membrane is depolarized, sodium and potas- sium channels in the membrane open.
- At the peak of the action potential, the sodium channels close.
voltage-gated channels
axon channels regulating sodium and potas- sium
their permeability depends on the voltage difference across the membrane
At the resting potential, the sodium channels are fully closed and the potassium channels are almost closed, allowing only a little flow of potassium
As the membrane becomes depolarized, both the sodium and the potassium channels begin to open, allowing freer flow.
peak of action potential
at the peak of the action potential, the sodium gates snap shut
Local anesthetic
attach to the sodium channels of the membrane, preventing sodium ions from entering
propagation of the action potential
the transmission of an action potential down an axon
back propagation
action potential “back-propagates” from the axon into the cell body and dendrites
hey pas- sively register the electrical event that started in the nearby axon
the dendrite be- comes more susceptible to the structural changes responsible for learning
myelin
insulating material composed of fats and proteins
myelinated axons
those covered with a myelin sheath
found only in vertebrates
interrupted periodically by short sections of axon called nodes of Ranvier, each one about 1 micrometer wide
After an action potential occurs at a node, sodium ions enter the axon and diffuse, push- ing a chain of positive charge along the axon to the next node, where they regenerate the action potential
saltatory conduction
jumping of action potentials from node to node
This flow of charge moves considerably faster than the regeneration of an action potential at each point along the axon.
multiple sclerosis
immune system attacks myelin sheaths. An axon that never had a myelin sheath conducts im- pulses slowly but steadily, but an axon that has lost its myelin is not the same, because it lacks sodium channels where the myelin used to be, meaning the action potential dies out
refractory period
period during which it resists the production of further action potentials
absolute and relative
absolute refractory period
membrane cannot produce another action potential, regardless of the stimulation
relative refractory period
a stronger- than-usual stimulus is necessary to initiate an action potential
time of refractory periods
absolute refractory period is about 1 millisecond (ms), and the relative refractory period is another 2 to 4 ms.
local neurons
Neurons without an axon that exchange information with only their closest neighbors
graded potential
a membrane potential that varies in magnitude in proportion to the intensity of the stimulus.
ie, it does not function by the all or none principle
change in membrane poten- tial is conducted to adjacent areas of the cell, in all directions, gradually decaying as it travels.
