BSC 215 Lecture Notes - Lecture 26: Membrane Potential, Resting Potential, Action Potential
11 Jun 2018
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Transmission of Nerve Impulses
The transmission of a nerve impulse along a neuron from one end to the other occurs as a result
of electrical changes across the membrane of the neuron. The membrane of an unstimulated
neuron is polarized—that is, there is a difference in electrical charge between the outside and
inside of the membrane. The inside is negative with respect to the outside.
Polarization is established by maintaining an excess of sodium ions (Na +) on
the outside and an excess of potassium ions (K +) on the inside. A certain
amount of Na + and K + is always leaking across the membrane through
leakage channels, but Na +/K + pumps in the membrane actively restore the
ions to the appropriate side.
The main contribution to the resting membrane potential (a polarized nerve) is
the difference in permeability of the resting membrane to potassium ions
versus sodium ions. The resting membrane is much more permeable to
potassium ions than to sodium ions resulting in slightly more net potassium
ion diffusion (from the inside of the neuron to the outside) than sodium ion
diffusion (from the outside of the neuron to the inside) causing the slight
difference in polarity right along the membrane of the axon.
Other ions, such as large, negatively charged proteins and nucleic acids,
reside within the cell. It is these large, negatively charged ions that contribute
to the overall negative charge on the inside of the cell membrane as
compared to the outside.
In addition to crossing the membrane through leakage channels, ions may
cross through gated channels. Gated channels open in response to
neurotransmitters, changes in membrane potential, or other stimuli.
The following events characterize the transmission of a nerve impulse
Resting potential. The resting potential describes the
unstimulated, polarized state of a neuron (at about –70
millivolts).
• Graded potential. A graded potential is a change in the
resting potential of the plasma membrane in the response to
a stimulus. A graded potential occurs when the stimulus
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Document Summary
The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of electrical changes across the membrane of the neuron. The membrane of an unstimulated neuron is polarized that is, there is a difference in electrical charge between the outside and inside of the membrane. The inside is negative with respect to the outside. Polarization is established by maintaining an excess of sodium ions (na +) on the outside and an excess of potassium ions (k +) on the inside. A certain amount of na + and k + is always leaking across the membrane through leakage channels, but na +/k + pumps in the membrane actively restore the ions to the appropriate side. The main contribution to the resting membrane potential (a polarized nerve) is the difference in permeability of the resting membrane to potassium ions versus sodium ions.
