Resting Potential
Fall 2002
Neurons are designed to carry messages from one
place to another.
Today, we will learn how the message flows
within a single neuron. Next week we will learn how messages travel between
neurons.
Everything neurons do follow the laws of physics.
That is, it is logical, not mystical.
The message that travels within a neuron is electrical.
Electricity (i.e., the flow of current)
requires 2 things:
1) Charged particles--neurons use ions
(charged particle)
2) Separation of charge--created by neural
membrane
The inside of the cell is called the intracellular
space
The outside of the cell is called the extracellular
space
The cell membrane is made of two layers of fat (called
lipid
bilayer) (Figure 3.3). Particles can not pass through this lipid bilayer.
However, proteins in the membrane form channels for particles (Figure 3.6
& 3.7). These channels can open and close which allow particles to
enter and leave the cell.
Neurons make use of several ions to create electric
currents.
Sodium has a positive charge (Na+)
Chloride has a negative charge (Cl-)
Potassium has a positive charge (K +)
Proteins in cell have a negative charge
(A-)
Three forces act on ions:
1) concentration gradient (Figure 3.8)
2) electrical force (Figure 3.9)
3) Na+/K+ pump--special proteins in the
membrane that move ions
Given that the membrane is permeable only to certain
ions (e.g., K+), these forces create what is called a resting potential.
Potential means a separation of charge. In this
case the separation is across the membrane.
Resting means that no current is flowing across
the membrane.
These simple rules create the resting potential:
1) Concentration gradient moves ions from high
to low concentration
2) Electrical force moves ions with same charge
away from each other and ions with opposite charge towards each other.
3) Na+/K+ pump moves Na+ to the extracellular
space. It can't get back in because the membrane is impermeable to Na+.
4) The membrane is permeable to K+ (i.e., it
flows either way)
5) Large negatively charged proteins (A-) are
stuck inside the neuron
Na+ and K+ are driven intracellularly to A- by
the electrical force.
The concentration gradient is going to push Na+
intracellularly and K+ extracellularly.
The Na+/K+ pump is going to move Na+ extracellularly.
Because of this, the extracellular space has
lots of Na+ and ends up relatively positive compared to the intracellular
space where there are lots of A-. In other words, the resting potential
has the inside of the neuron 70mV more negative than the extracellular
space (V stands for voltage). Voltage is a measure of potential (the difference
in electric charge between two places).
Where would you expect most Cl- ions to be located
during the resting potential? (see Figure 3.9; think about the electrical
force)
Scientists can measure a neuron's potential with
an intracellular electrode (Figure 3.11).
When a neuron becames more polarized (more negative;
e.g., -75 mV) it is said to be hyperpolarized
When a neuron becomes less polarized it is said
to be depolarized
Changes in resting potential occur when ions move
in or out of the neuron.
Example 1: Moving K+ out of the neuron will
cause a hyperpolarization (more negative inside).
Example 2: Moving Na+ into the neuron will cause
a depolarization (less negative inside).
Concentration gradient and electrical force are both
trying to push Na+ in. The membrane is impermeable to Na+ so it can't get
in. However, if Na+ channels in the membrane open, then Na+ will rush in.
This is called an Action Potential (other names include impulse,
fire, spike). Action potentials are how electrical messages are transmitted
within a neuron. (see next lecture for more information about action potentials).
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