The Action Potential

Neurotransmitters are chemical messengers that travel across the gap (synapse) with the message to the dendrite of a neighboring neuron. Some neurotransmitters excite and others inhibit. An impulse travels one way from the dendrites long the axon and away from the soma. This process is both electrical and chemical. The fluid inside and outside the cell contains chemically charged particles called ions. The language used by neurons to communicate involves simple "yes-no," "on-off"" electrochemical impulses. Neurons will fire an impulse when they are stimulated. When neurotransmitters attach to receptor sites on a neuron's dendrite, they can increase (hyperpolarization) or decrease (depolarization) membrane potential. This is known as action potential. During depolarization, membrane potential decreases from –70 milivolts. This value is the resting potential of the neuronal membrane. If depolarization reaches –50 milivolts, an action potential is triggered. During the action potential the membrane potential reaches +40 millivolts. Thresholds are points of no return. There is no halfway. Triggering an action potential is like squeezing the trigger of a gun. If enough pressure is applied, the gun will fire. If too little pressure/depolarization is applied, neither the gun nor the neuron will fire; but it is all or nothing. While the neuron is returning to the resting state, it becomes super negatively charged. During this refractory period, the neuron cannot generate another action potential. Thus, the "all or none" law is the principle stating that a neuron fires at full strength or not at all. Our behavior is, in part, the consequence of millions of cells talking to each other via these chemical and electrical processes.

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