131555
Description
Mind Map by harry_bygraves, updated more than 1 year ago
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Created by harry_bygraves
over 11 years ago
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Nerve impulses;
resting potential
- A resting neurone is so called because it does not convey a nerve impulse. A resting neurone expends much energy in
maintaining a potential difference across its membrane. The potential difference is called the resting potential, and it is defined
as the potential difference that is maintained across them membrane of an axon when a neurone is not conducting an impulse
- The inside of the membrane is negative relative to the outside and the potential difference measures about -70mV
- Differential membrance permeability. During the resting potential, the inside of the neurone
is negative relative to the outside because of an unequal distribution of charged ions. this is
due mainly to the difference in permeabilty of the membrane to sodium and potassium ions.
Sodium ions (Na+) are present in higher concentrations outside the cell than inside. By
contrast, the inside of the cell has a higher concentration of potassium ions (K+)
- The unequal distribution of ions results from a combination of active transport and diffusion of
sodium and potassium ions across the cell memabrane. a sodium potassium pump acitvely
transports sodium ions out of the neurone and potassium in. For every three sodium ions pumped
out, only two potassium ions are pumped inwards. On its own, this is owuld result in only a slight
potential difference across the membrane. However, this difference is amplified by the membrane
being about 50 times more permeable to potassium ions than to sodium ions.
- Potassium ions are able to diffues freely back out of the cell, down their concentration gradient,
but the sodium ions diffuse back into the axoplasm (cytoplasm of the neurone only very slowly
- This results in the total number of positively charged ions on the outside of the
membrane being greater than the total number inside, and creates a negative electrical
charge inside compared to the outside. Without active transport, an equilbrium would
eventually be reached and there would be no potential difference across the membrane
- Electrochemical gradients. The diffusion of ions accross the membrane of a neurone is due
to a combination of electrical and chemical gradients. Sodium and Potassium ions are
postiviely charged and therefore tend to move down an electrical gradient towards a
negatively charged region. The ions will also diffuse down a chemical gradient from a region
where they are at high concentrations to a region where they are at low concentration.
- Movements of sodium and potassium ions. The rate
of diffusion of sodium and potassium ions down a
chemical gradient depends on channel proteins that
are specific to each ion. Some channel proteins allow
sodium ions to diffuse through the membrane into the
axoplasm; others allow potassium to diffuse out.
These proteins are volted gated to control their,
permeability, that is the opening and closing of gated
channel proteins in controlled by the changes in the
potential difference accross the cell membrane.
When the gates are open, the ions can pass through
the membrane; when the gates are closed, they
cannot. Membrane permability to an ion depends on
the proportion of gates open or closed. In a 'resting'
axon, relatively more potassium gates gates are open
that sodium gates. This explains why the membrane
is so much more permeable to potassium ions that to
sodium ions, and why more potassium ions move out
of the axoplasm than sodium ions move in.
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