A neuron is a cell of the nervous system responsible for transmitting information. One of its main components is the dendrite, a specialized extension that protrudes from the neuron’s cell body, known as the soma. Often compared to tree branches for their intricate appearance, these structures are the primary recipients of signals from other nerve cells. This design allows a single neuron to connect with thousands of others, forming complex networks.
The Structure of a Dendrite
A dendrite’s defining characteristic is its branching pattern, called a dendritic arbor. This arborization can vary dramatically between neuron types; some have simple branches, while others, like Purkinje cells, form dense trees. This diversity allows neurons to sample information from different sources. Dendrites are cytoplasmic extensions, filled with the same fluid as the cell body and containing organelles like mitochondria.
A feature of many dendrites is the presence of tiny protrusions called dendritic spines. These structures stud the surface of the branches and are the primary points of contact for incoming signals. The number and shape of these spines are not fixed, and their density can be immense, with an estimated ten trillion in the human cerebral cortex. The dendritic arbor and its spines greatly increase the surface area for receiving information.
This architecture is directly related to a neuron’s role within a neural circuit. Neurons with extensive dendritic arbors can receive signals from tens of thousands of other cells. The branches taper as they extend from the cell body, unlike the axon, which maintains a constant diameter. This arrangement is how dendrites collect and channel information toward the neuron’s soma.
How Dendrites Receive Information
The transfer of information from one neuron to another occurs at a synapse, the small gap between an axon and a dendrite. For many neurons, these synaptic connections are made directly onto dendritic spines. Each spine acts as a postsynaptic contact site, equipped to receive a specific chemical message.
Communication begins when an action potential travels down the axon of the sending neuron, prompting the release of neurotransmitters into the synaptic cleft. These molecules cross the gap and bind to receptors on the receiving dendrite or dendritic spine. This binding event converts the chemical signal into an electrical one.
When neurotransmitters attach to receptors, they cause ion channels on the dendritic membrane to open. This allows charged particles (ions) to flow into the dendrite, altering the electrical charge across its membrane. This localized change in voltage is called a postsynaptic potential (PSP).
Postsynaptic potentials can be either excitatory, making the neuron more likely to fire, or inhibitory, making it less likely. An excitatory PSP involves the influx of positive ions like sodium, causing a depolarization where the cell’s interior becomes less negative. An inhibitory PSP results from the influx of negative ions like chloride, making the cell’s charge more negative.
Integrating Signals for Neuron Communication
A neuron often receives thousands of postsynaptic potentials simultaneously from its dendritic tree. These electrical currents propagate from the dendritic branches toward the neuron’s cell body, or soma. The dendrites act as channels, funneling this stream of incoming information to a central location.
The soma combines these individual signals in a process known as summation. It adds together all the excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials from across the dendritic arbor. The location of the synapse influences its impact; an inhibitory signal on the path to the soma can cancel out an excitatory current. This integration is a complex calculation influenced by the timing and location of each input.
This summation determines the neuron’s output. The combined electrical charge converges at the axon hillock, a region where the soma connects to the axon. If the total signal depolarizes this region to the threshold of excitation, the neuron fires an action potential. An action potential is an all-or-nothing electrical impulse that travels down the axon to communicate with other cells.
Dendritic Changes and Brain Adaptability
Dendrite structure is not fixed; it is dynamic and changes in response to experience, a phenomenon known as neuroplasticity. The formation of new memories and learning new skills are associated with physical alterations in dendritic architecture. These changes can involve growing new branches or the formation, stabilization, or elimination of dendritic spines.
For instance, practicing a new skill can lead to the formation of new dendritic spines, creating more robust connections between the involved neurons. This makes communication along that pathway more efficient. Conversely, the brain uses synaptic pruning to eliminate less active dendritic spines and their synapses. This process refines neural circuits and improves signaling efficiency.
These adaptive processes are part of normal brain development, with rapid spine growth and pruning occurring during childhood and adolescence. Disruptions in these processes are linked to various neurological and developmental conditions. For example, reduced spine density is associated with schizophrenia, while altered spine development is observed in autism spectrum disorders.