The nervous system, a complex network of communication, relies on specialized cells called neurons to process and transmit information throughout the body. Each neuron acts as a communication unit, structured to manage the flow of electrical and chemical signals. Understanding how a neuron receives, processes, and sends these signals requires differentiating its two primary extensions: the dendrite and the axon. This distinction clarifies the fundamental mechanisms that govern all neural activity.
Dendrites: The Neuron’s Input Receptors
Dendrites represent the receptive, tree-like extensions that radiate outward from the neuron’s cell body. Their structure is highly branched, which maximizes the surface area available for communication with neighboring neurons. This extensive branching allows a single neuron to receive thousands of inputs simultaneously from various upstream cells.
Many dendrites feature tiny, mushroom-shaped protrusions along their surface known as dendritic spines. These spines are the primary sites where communication from other neurons is received, functioning as specialized contact points for synaptic connections. This structural complexity gathers incoming chemical messages, which are then converted into electrical signals.
The electrical signals generated in the dendrites are called graded potentials, meaning their strength is proportional to the intensity of the initial stimulus. Graded potentials weaken as they travel passively toward the cell body. The neuron’s cell body then sums up all these incoming graded potentials, both excitatory and inhibitory, to determine if the signal is strong enough to be transmitted further down the line.
Axons: The Neuron’s Output Highway
In contrast to the multiple, short dendrites, the axon is typically a single, long projection extending away from the cell body at a specialized region called the axon hillock. This hillock acts as the integration center, where the combined graded potentials are assessed to initiate an action potential if a specific voltage threshold is reached. Once initiated, the action potential is a rapid, self-propagating electrical impulse that travels down the axon without losing strength.
The axon’s primary function is to transmit this electrical signal over potentially long distances, sometimes stretching a meter or more from the spinal cord to a muscle in the foot. To ensure rapid and efficient signal transmission, many axons are insulated by a fatty layer called the myelin sheath. This sheath is formed by specialized glial cells, such as Schwann cells in the peripheral nervous system.
The myelin sheath is interrupted at regular intervals by small, uninsulated gaps known as the Nodes of Ranvier. These nodes are the only places along the myelinated axon where the electrical signal is actively regenerated. This mechanism, known as saltatory conduction, allows the electrical impulse to effectively jump from one node to the next, increasing the conduction velocity significantly compared to unmyelinated fibers. This allows signals to travel at speeds up to 120 meters per second.
Communication at the Synapse
Neural communication relies on the physical and functional connection between the axon of a sending neuron and the dendrite of a receiving neuron, a junction known as the synapse. The axon terminates in a structure called the presynaptic terminal, which is separated from the postsynaptic terminal on the dendrite by a microscopic gap called the synaptic cleft.
When an action potential arrives at the presynaptic terminal, it triggers the release of chemical messengers called neurotransmitters. These neurotransmitters are stored in small sacs called synaptic vesicles and are quickly released into the synaptic cleft.
They diffuse across the gap and bind to specific receptor proteins located on the membrane of the postsynaptic dendrite. The binding of the neurotransmitter to the dendrite’s receptors causes ion channels to open, which leads to the generation of a new electrical signal in the receiving neuron. This process converts the electrical signal from the axon into a chemical signal across the gap, and then back into an electrical signal in the dendrite. This chemical relay is the fundamental mechanism by which a neuron’s output (axon) successfully influences the input (dendrite) of the next cell in the circuit.