The cell body, or soma, houses the nucleus, and the axon sends the output signal, but dendrites serve as the neuron’s primary sensory apparatus. These specialized cytoplasmic extensions function as receiving antennae, collecting electrochemical stimulation from thousands of other neurons. Their role is to capture, process, and combine these inputs to determine the cell’s overall activity.
The Physical Structure of Dendrites
Dendrites emerge from the cell body and branch out extensively, forming a complex, tree-like structure known as the dendritic arbor. The complexity and shape of this arbor vary significantly between neuron types, fundamentally dictating the neuron’s capacity to receive and process incoming signals.
The dendrites are covered in tiny, mushroom-shaped protrusions called dendritic spines, which are the main physical sites where communication from other neurons occurs. Each spine typically receives input from a single synapse, a specialized junction where signals are transmitted. These small, bulbous structures act as storage sites, collecting the postsynaptic potentials that arrive before relaying them to the main dendritic shaft.
The number of spines is considerable, often ranging from 20 to 50 per 10 micrometers of dendrite length. For example, a single Purkinje cell can connect with tens of thousands of other cells. The size and shape of the spines are dynamic elements whose morphology changes in response to activity, which has implications for long-term neural function.
Processing Electrical Signals
The primary function of the dendrite is to transform chemical signals received at the synapse into electrical signals and then combine them, a process termed synaptic integration. When a signal arrives, a chemical neurotransmitter is released into the synaptic gap, which binds to receptors on the dendritic spine or shaft. This binding causes ion channels to open, resulting in a localized change in the electrical potential of the dendrite’s membrane.
The incoming signals are categorized as either excitatory or inhibitory. Excitatory signals cause a depolarization, meaning they make the internal voltage of the cell slightly more positive, pushing the neuron closer to its firing threshold. Conversely, inhibitory signals cause a hyperpolarization, making the cell’s voltage more negative and actively suppressing the generation of an output signal.
Dendrites act as sophisticated calculators, summing all these positive and negative voltage fluctuations across their entire arbor. This summation occurs in two primary ways: spatially and temporally. Spatial summation involves the simultaneous addition of potentials arriving at different locations on the dendrite at the same moment. Temporal summation is the process by which signals arriving in rapid succession at the same synapse or nearby locations are added together over a short period of time.
The physical location of a synapse greatly influences its impact on the final output. Signals weaken, or attenuate, as they propagate electrically from the synapse toward the soma and the axon hillock. Synapses located closer to the cell body have a stronger influence on the neuron’s firing decision compared to those located on the dendritic tips. If the integrated sum of all these fluctuating potentials reaches a certain threshold at the axon hillock, the neuron generates an action potential, which is the cell’s main output signal.
Dendritic Plasticity and Cognitive Function
Dendrites are dynamic components capable of structural and functional modification, a concept known as dendritic plasticity. This adaptability allows the brain to change in response to new information and experiences, which is required for processes like learning and memory. The physical structure of the dendritic spines is continuously remodeled, changing their size, shape, and density.
Two major cellular mechanisms that underpin this plasticity are Long-Term Potentiation (LTP) and Long-Term Depression (LTD). LTP is a lasting increase in the strength of synaptic transmission, often triggered by high-frequency activity, and is associated with the growth of dendritic spines and an enlargement of the postsynaptic density. LTD, by contrast, is a persistent decrease in synaptic strength, which can result in the shrinking or even the retraction of dendritic spines.
These changes in spine morphology and density are believed to be the physical mechanisms by which the brain stores information. Learning experiences can lead to the formation of new spines or the strengthening of existing ones. Conversely, irregularities in dendritic structure and spine pathology are found in neuropsychiatric conditions. Disruptions in plasticity, such as impaired LTP, are implicated in disorders like depression and may contribute to the atrophy of neurons seen in Alzheimer’s disease.