Just seeing if you're paying attention!

GE 345: Week 4

Electrical Potentials

Electrical potentials exists across almost all cell membranes in the body. Nerve and mucle cell are excitable -- capable of generating their own electrochemical impulses, which can transmit signals along their membranes.

Ion Diffusion and Membrane Potentials

Ion concentration differences across a semi-permeable membrane can cause the creation of a negative membrane potential in its resting state. When activated, the membrane polarizes and repolarizes through ion diffusion.

Resting Potential: "polarized" -- large negative membrane potential. Na⁺-K⁺ pump actively transports 3 Na⁺ outside the membrane for every 2 K⁺ it pumps inside the membrane. At the same time, K⁺-Na⁺ leak channels allow K⁺ back into the membrane, while allowing Na⁺ back in. Resting potential is about the same in nerve and muscle fibers.

Depolarization: Rapid rise of potential in positive direction triggers voltage-gated Na⁺ channels to open, allowing Na⁺ to diffuse back inside the membrane. Conduction velocity of the AP is faster in nerves than in muscle.

Repolarization: Na⁺ channels deactivate, while voltage-gated K⁺ channels open, allowing large amounts of K⁺ to diffuse to the outside of the membrane, after which these channels also close back up. Duration of the AP is about the same in muscle and nerve fibers.

Other Ions

Impermeant anions (negative ions) are inside the membrane.

A Ca²⁺ pump, similar to that for Na⁺, as well as voltage-gated Ca²⁺ channels, also exist. This system, which is slower to activate, is numerous in cardiac and smooth muscle, and almost completely replaces the Na⁺ system in in some smooth muscles.

Causes of Excitation

3 factors control excitation of nerve and muscle:

Mechanical membrane disturbance, such as pressure on the sensory nerve endings.
Chemical stimuli such as neurotransmitters.
Electricity through the membrane, such as in cardiac muscle.