Summary
Highlights
Neurons communicate through electrical impulses called action potentials, which are like a simple 'ping' that varies in frequency but not in strength or speed. This single signal communicates all thoughts, actions, and emotions. The brain interprets these signals based on frequency, location, and magnitude.
The body can be thought of as a sack of batteries, maintaining electrical neutrality overall but with localized positive and negative charges separated by membranes. These separations create potential energy, similar to a battery. Voltage in cells is called membrane potential, and current refers to the flow of ions across cell membranes. Resistance hinders this flow, with insulators having high resistance and conductors having low resistance.
A resting neuron has a negative charge inside (-70 millivolts) compared to the outside, a state known as resting membrane potential or polarization. This is maintained by the sodium-potassium pump, which moves three sodium ions out for every two potassium ions pumped in, creating an electrochemical gradient.
Ion channels in the neuron membrane allow ions to pass through, evening out the electrochemical gradient. These channels can be voltage-gated (opening at specific membrane potentials), ligand-gated (opening with specific neurotransmitters or hormones), or mechanically-gated (opening due to physical stretching). The movement of ions through these channels is crucial for all electrical events in neurons.
Small, localized changes in membrane potential caused by a few open ion channels are called graded potentials. For long-distance signaling, a larger change is needed to trigger an action potential. This requires depolarizing the neuron to a threshold of about -55mV.
Once the threshold of -55mV is reached, voltage-gated sodium channels open, causing a rapid influx of positive sodium ions. This massive depolarization makes the cell's interior positive, reaching about +40mV. This is an 'all-or-nothing' phenomenon; if the stimulus is too weak, no action potential occurs.
After depolarization, voltage-gated potassium channels open, allowing potassium ions to flow out, leading to repolarization. The membrane briefly hyperpolarizes, dropping to about -75mV, before the channels close and sodium-potassium pumps restore the resting potential. During the refractory period, the axon cannot respond to new stimuli, preventing signals from traveling backward.
While the strength of an action potential is constant, its frequency changes. Weak stimuli cause less frequent action potentials, while intense stimuli increase the frequency. Action potentials also vary in speed (conduction velocity), being faster in reflexes and slower in glands and guts. Myelin sheaths on axons significantly increase transmission speed through saltatory conduction, where the impulse leaps between Nodes of Ranvier.