The Action Potential-An Electrical Signal

The nervous system is a wondrous thing. It is what makes your body function, gives you your senses, helps with that upcoming test, and more. The nervous system consists of the central nervous system, the brain and spinal cord, and the peripheral nervous system, everything else. We will focus on the peripheral nervous system for now.

Nerves (bundles of neurons) make up the peripheral nervous system. This system helps you receive external stimuli, such as when you touch something. These stimuli are received by neurons and transported to other neurons and so on until the signal becomes weak and eventually stops.

In order to understand these signals, we must first describe the neuron, a nerve cell. The neuron consists of four main parts. The cell body houses the nucleus and other organelles.  The dendrites carry signals to the cell body. The axon conducts signals toward and effector cell or another neuron. The myelin sheath, which only vertebrates have, makes a signal travel faster along the axon, because it has to jump around from the nodes of Ranvier, the only places on a neuron where a signal can be transmitted. The myelin sheath is a chain made up of Schwann cells. The synaptic knobs (in this diagram, the axon terminal) transmit signals from one neuron to another or to another neuron. A signal can only travel in one direction, from the dendrites to the synaptic knobs.

Neuron

The neuron is usually at resting potential, -70 mV (millivolts). The neuron's membrane keeps potassium and dissolved proteins (which have a negative charge) inside and sodium (positive charge) outside. Channels and pumps in the membrane also keep the charge stable at resting potential. Sodium-potassium pumps are a form of active transport. Sodium cells and forced out, and potassium cells are forced in. They move more sodium than potassium.

Finally, we can proceed to the action potential, also known as a nerve signal!

A neuron receives an external stimulus, and immediately the resting potential changes. The stimulus causes sodium channels to open, so the inside of the cell becomes more positively charged than before. If the threshold potential (-55

mV) is met, then an action potential is reached. More sodium channels open, and the inside of the cell becomes increasingly positive. This is called depolarization.

After the action potential is reached, sodium gates close, and potassium gates open. This causes the cell to become negative again. This is repolarization. There is a brief undershoot, because potassium channels close slowly. This is hyperpolarization. The cell is once again at resting potential.

The neuron will have short period where it is unable to receive another action potential. When an action potential occurs in one part of a neuron, it stimulates the next section of the axon and the next and the next. 

Propogation of an Action Potential Down an Axon

And now imagine. All of this has happened in just a few milliseconds!