The human nervous system orchestrates every thought, movement, and sensation through a complex network of specialized cells called neurons. These cells are the fundamental units for transmitting information throughout the body, forming the basis of our ability to interact with the world. This article explores how these signals travel, detailing the mechanisms by which information moves both within a single neuron and then bridges the gap to communicate with the next.
Understanding the Neuron
A neuron’s structure is adapted for transmitting signals, comprising several distinct parts. Dendrites, resembling tree-like branches, receive signals from other neurons. The cell body, or soma, acts as the neuron’s control center, integrating all incoming information. The axon, a slender projection, transmits electrical signals away towards other neurons or target cells. Axon terminals, at the end of the axon, pass signals to the subsequent neuron.
The Electrical Signal Within a Neuron
A neuron at rest maintains an electrical difference across its membrane, known as the resting potential, typically around -70 millivolts (mV). This state is characterized by a higher concentration of positively charged sodium ions outside the cell and more potassium ions inside. When a neuron receives sufficient stimulation, this resting state changes. If the electrical input reaches the threshold potential, often around -55 mV, it triggers an electrical impulse called an action potential.
The action potential begins with a rapid depolarization phase where voltage-gated sodium channels open, causing a swift influx of positively charged sodium ions into the neuron. This influx makes the inside of the cell temporarily more positive, reaching a peak around +40 mV. The repolarization phase follows as sodium channels inactivate, and voltage-gated potassium channels open, allowing potassium ions to flow out of the cell. This outward movement restores the negative charge inside the neuron, bringing the membrane potential back towards its resting state.
This electrical wave propagates along the axon in a one-way direction. A myelin sheath, a fatty insulating layer around many axons, significantly increases transmission speed. Myelin allows the electrical signal to “jump” between unmyelinated gaps called Nodes of Ranvier, a process known as saltatory conduction. This is much faster than continuous conduction along unmyelinated axons, ensuring rapid communication across long distances.
Bridging the Gap Between Neurons
Once the action potential reaches the end of the presynaptic neuron’s axon, it must traverse a specialized junction called a synapse to reach the next neuron. The synapse consists of three main components: the presynaptic neuron’s axon terminal, the synaptic cleft, and the postsynaptic neuron’s membrane. The synaptic cleft is a tiny fluid-filled space separating the two neurons, preventing direct electrical transmission.
Upon action potential arrival at the presynaptic axon terminal, voltage-gated calcium channels open, allowing calcium ions to enter. This influx triggers synaptic vesicles, small sacs containing neurotransmitters, to fuse with the presynaptic membrane. Neurotransmitters are then released into the synaptic cleft.
Neurotransmitters bind to specific receptor proteins on the postsynaptic neuron’s membrane. This binding causes ion channels on the postsynaptic membrane to open. Neurotransmitters are quickly removed from the synaptic cleft through processes like reuptake or enzymatic degradation, preparing the synapse for the next signal.
How the Signal Continues
Neurotransmitter binding to postsynaptic neuron receptors can have two primary effects: excitation or inhibition. Excitatory neurotransmitters cause depolarization of the postsynaptic membrane, known as an excitatory postsynaptic potential (EPSP). This makes the neuron’s internal charge more positive and more likely to generate an action potential. Conversely, inhibitory neurotransmitters cause hyperpolarization or stabilization, leading to an inhibitory postsynaptic potential (IPSP). This makes the neuron’s internal charge more negative and less likely to fire an action potential.
The postsynaptic neuron receives many excitatory and inhibitory signals. These signals are integrated at a specialized region, the axon hillock, at the cell body’s base. The neuron sums all incoming potentials. If the combined effect of these graded potentials reaches the threshold potential at the axon hillock, the postsynaptic neuron generates its own action potential, continuing the signal to the next neuron.