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Animal Physiology Index
There are many useful sites with some good animations.
The Nerve Impulse
Cells and membrane potentials
All animal cells generate a small voltage across their membranes. This is because there is a large amount of small organic molecules in the cytoplasm. To balance this, animal cell pump Na+ out of the cells. This regulates osmosis but it leaves a large number of organic molecules that are overall negatively changed (anions) in the cytoplasm, giving the cell a potential difference (voltage) across its membrane.
The resting potential
Overall a slow leakage of K+ ions through K+ leakage channels and the poor permeability of the membrane to Na+ ions bring about the membrane potential of neurones so they cannot get in.
As the K+ leaks the inside of the resting cell becomes more negatively charged.
Neurones like other cells are more negatively charged inside than outside. This results in a membrane potential of about - 70 milliVolts. This is called the resting potential of the neurone.
This has an effect on the passive movement of K+ and Na+ across the neurone's plasma membrane.
Two forces determine the passive movement of ions across a cell membrane:
The concentration gradient - causing the ions to diffuse down their concentration gradient.
The electrical potential - causing ions to be attracted to the opposite charge to the one they carry.
The two important ions in a nerve cell (neurone or neuron) are K+ and Na+, which are both cations (positively charged ions).
NOTE: Na+ ions move more slowly across the membrane than K+ or Cl- ions. This is because although the Na+ ion is smaller than the K+ ion it has a larger coating of water molecules giving it a bigger diameter. This makes the plasma membrane 25 times more permeable to K+ than Na+.
In addition to this K+ ions leak out of K+ ion pores when the nerve cell is at rest.
So to maintain the high concentration of K+ inside the cell, it has to be actively pumped inwards a bit when the cells is at rest.
The result is that the resting potential of the neurone is almost at the equilibrium for K+ ions. They leak out a bit and need pumping in. The resting potential, however, is well below the equilibrium for the Na+. Na+ ions are actively pumped out and kept out.
Potassium ion movement in a resting cell can be summarised as follows
So the K+ ions want to move out a little, they need pumping back in.
Sodium ion movement in a resting can be summarised as follows
So the Na+ ions want to move in but the membrane is relatively impermeable towards them.
A coupled Na+- K+ pump also actively pumps Na+ ions out.
Proof that it is a coupled pump
Depolarisation of neurone membranes gets them excited
As the neurones membrane at rest is more negative inside than outside, it is said to be polarised.
Neurones belong to a group of cells, which include muscle cells and receptor cells (sensory cells), called excitable cells.
The cells are excited when their membranes become depolarised.
Depolarising membranes may be achieved by:
Nerve impulses are self-propagating like a trail of gunpowder. Localised currents in the ions occur just ahead of the impulse causing localised depolarisation.
NOTE: Nerve impulses are not like electrical signals travelling down a wire. Electrical signals in metal wire use a flux of electrons.
The action potential
The action potential is the state of the neurone membrane when a nerve impulse passes by.
A small change in the membrane voltage will depolarise the membrane enough to flip open Na+ channels. These are called voltage-gated Na+ channels.
As Na+ moves into the cell more and more Na+ channels open.
A small change in the membrane permeability to Na+ results in a big change in membrane potential. This is because the volume of the axon is minute compared to the volume of the extracellular fluid.
As Na+ moves in the cell will become more positive with respect to the outside.
The ion pumps resist the change in the membrane potential but it only has to rise by 15mV, from -70mV to -55mV, and the pumps cannot restore the equilibrium. Na+ floods in.
This means that nerve impulses all look the same, there are not big ones and little ones. This is the all-or-nothing law.
-55mV represents the threshold potential. Beyond this we get a full action potential.
The membrane potential rises to +35mV this is the peak of the action potential. The cells are almost at the equilibrium for Na+ ions.
After Na+ moves in passively until the Na+ channels start to close. At the same time K+ permeability increases as voltage-gated K+ channels open - they are a bit slower to respond to the depolarisation than the Na+ channels. The K+ ions move out. This makes the cell negative inside with respect to outside again. The membrane potential falls.
The membrane potential falls below the resting potential of -70mV. It is said to be hyperpolarised.
Gradually active pumping of the ions (K+ in and Na+ out) restores the resting potential.
During this period no impulses can pass along that part of the membrane. This is called the refractory period.
The trace of an action potential as it passes by in a neurone membrane
The maximum frequency recorded for nerve impulses is up to 2500 s-1.
Nevertheless the whole action potential passes by in less than 2ms.
Nerve impulses are very fast up to 120ms-1 in myelinated neurones.
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