Neurons and Excitable Tissue

Study Notes

Nerves and muscles are called excitable tissue because they respond to chemical, mechanical, and electrical stimuli.
Muscles demonstrate by contraction, while nerves by integration and transmission.

Production of signals depend on two basic features of the plasma membrane of excitable cells: resting membrane potential and ion channels.

Ion channels open and close due to the presence of gates.
There are four kinds of ion channels:
Leakage channels: open and close randomly
Voltage-gated channels: open in response to a change in membrane potential (voltage)
Ligand-gated channels: open and close in response to chemical stimulus, such as Ach
Mechanical gated channels: open or close in response to mechanical stimulus

Definition: The difference in voltage across the cell membrane when a neuron or muscle cell is not producing an action potential.
A typical value is: -70 mV (-50 to -90)
A cell that exhibits a membrane potential is said to be polarized.
RMP is negative inside the cell relative to the outside because:
The resting membrane is 10-100 times more permeable to K+ than to Na+
Potassium inside the cell is 140meq/L and outside is 4meq/L
K+ tends to leak out of the cell down its concentration gradient, carrying +ve charge with it

Diffusion: movement of molecules from a region of higher concentration to a region of lower concentration
Electrical gradient: +ve ions move to the –ve area and –ve ions move to +ve area
Active transport: transport ions against their concentration gradient (e.g., Na+-K+ pump)

Definition: A sudden reversal of membrane polarity by a stimulus
Importance: AP occurs in living organisms to produce physiological effects such as transmission of impulses along nerve fibers and release of neurosecretions or chemical transmitters in synapses
Action potential takes place as a result of the triggered opening and subsequent closing of 2 specific types of channels: voltage-gated Na+ channels and voltage-gated K+ channels

Most important channels during AP
Have two gates: activation gate and inactivation gate
Activation gate opens when membrane potential becomes less negative than during resting state
Inactivation gate closes when membrane potential becomes less negative, leading to Na+ influx

Open during RMP, but are closed
Same stimulus that opens voltage-gated Na+ channels also opens voltage-gated K+ channels
Due to slow opening of these channels, they open just as Na+ channels are beginning to close
Simultaneous decrease in Na+ influx and increase in K+ outflux cause repolarization