how signals are propagated along a nerve?
http://health.howstuffworks.com/human-body/systems/nervous-system/nerve4.htm
synapses
Signals are propagated along a nerve through a process called action potential or nerve impulse. This involves the movement of ions across the cell membrane of the nerve cells, known as neurons. Here's an explanation of how signals are propagated along a nerve:
1. Resting State: Neurons have a resting membrane potential, which means there is a difference in electrical charge on either side of the cell membrane. The inside of the neuron is negatively charged compared to the outside.
2. Depolarization: When a stimulus is received by a neuron, it causes a change in the permeability of the cell membrane, allowing positively charged ions, such as sodium (Na+), to enter the neuron. This influx of positive ions reduces the electrical difference across the membrane, causing depolarization.
3. Action Potential: If the depolarization reaches a certain threshold, it triggers an action potential. This is a rapid and temporary reversal of the electrical charge across the cell membrane. The action potential is propagated along the neuron as a series of depolarization and repolarization events.
4. Repolarization: After depolarization, the cell membrane becomes permeable to another positively charged ion, potassium (K+). Potassium ions exit the neuron, restoring the electrical difference across the membrane and repolarizing it.
5. Hyperpolarization: In some cases, the membrane potential can become even more negative than the resting state, termed hyperpolarization, due to the prolonged outflow of potassium ions. This makes the neuron temporarily less responsive to additional stimuli.
6. Refractory Period: Following an action potential, a brief refractory period occurs during which the neuron is temporarily unable to generate another action potential. This ensures that the signal moves in one direction along the nerve.
7. Propagation: As the action potential reaches one section of the neuron, it depolarizes the adjacent section of the membrane, leading to a chain reaction. This ensures that the signal is propagated along the length of the nerve without losing its strength.
It's important to note that the process of signal propagation along a nerve is facilitated by the myelin sheath, a fatty layer that wraps around certain neurons. The myelin sheath acts as an insulator, allowing the action potential to "jump" from one node of Ranvier to another, significantly increasing the speed of signal transmission. This is known as saltatory conduction.