The basic structure of a neuron
The neuron, the basic cell of the nervous system, is made up of a cell body, dendrites, and an axon. The cell body houses the nucleus and organelles; dendrites are branch-like extensions that receive signals from other neurons; the axon is a long fiber that carries the signal generated in the cell body over distance to the next neuron, muscle, or gland, handing it off at the axon terminal.
Resting membrane potential (-70mV)
When there is no stimulus, the inside of a neuron's cell membrane sits at a voltage roughly -70mV lower than the outside. This voltage difference comes from the concentration gradient of sodium (Na+) and potassium (K+) ions across the membrane, which the sodium-potassium pump continuously maintains using ATP energy. This baseline voltage, held while there's no stimulus, is called the resting membrane potential.
How an action potential (nerve impulse) arises
When a stimulus crosses a threshold, sodium channels open and the membrane potential briefly reverses β this is the action potential. Sodium ions rushing into the cell cause depolarization, flipping the potential positive, followed quickly by potassium ions flowing out, repolarizing it back to negative. Once triggered, this electrical signal doesn't weaken β it travels the full length of the axon at constant strength.
The myelin sheath and saltatory conduction
Thanks to the myelin sheath wrapping the axon, the signal jumps only between the gaps called nodes of Ranvier, traveling much faster. New action potentials only form at these unmyelinated nodes, while the signal leaps across the covered stretches in between. This saltatory conduction makes myelinated nerves transmit signals far faster than unmyelinated ones.
The synapse and the synaptic cleft
Where one neuron meets another is called a synapse, and it includes a very narrow gap β the synaptic cleft β roughly 20-40 nanometers wide. In most synapses, the two neurons' cell membranes don't touch directly; they're separated by this tiny gap. Since the electrical signal (action potential) can't leap across this gap on its own, a chemical method is needed to carry the signal across.
The role of neurotransmitters
When an action potential reaches the axon terminal, vesicles containing neurotransmitters burst and release their contents into the synaptic cleft. The released neurotransmitters (dopamine, serotonin, acetylcholine, and others) cross the gap and bind to receptors on the next neuron, and this binding generates a new electrical signal in that neuron β at the synapse, the electrical signal essentially converts to a chemical signal and then back to an electrical one.
Excitatory and inhibitory signals
Depending on the type of neurotransmitter, the next neuron can be excited or, conversely, inhibited. Excitatory signals make the next neuron more likely to fire an action potential, while inhibitory signals make it less likely. A single neuron receives both types of signals simultaneously from many other neurons and decides whether to fire based on the combined sum.
The reflex arc: a response that bypasses the brain
Like instantly pulling your hand away from something hot, some responses are handled directly by the spinal cord without going all the way to the brain. The pathway where a sensory neuron's signal passes through an interneuron in the spinal cord straight to a motor neuron is called a reflex arc. Because it skips the round trip up to the brain and back, it allows a much faster reaction to dangerous stimuli.