Multipolar neuron
A multipolar neuron is the most common type of neuron in the vertebrate central nervous system, distinguished by a single long axon extending from the cell body and multiple shorter dendrites that branch out to receive incoming signals.[1][2] The cell body, or soma, houses the nucleus and essential organelles, while the dendrites function primarily as receptive surfaces for synaptic inputs, and the axon conducts electrical impulses away from the soma to transmit information to other cells.[1][3] These neurons are predominantly located in the brain and spinal cord, where they comprise motor neurons that innervate skeletal muscles and interneurons that integrate signals between sensory and motor pathways.[2][1] Notable subtypes include pyramidal cells in the cerebral cortex, Purkinje cells in the cerebellum, and stellate cells, each adapted for specific roles in neural computation and coordination.[3] Through their multipolar architecture, these neurons enable the integration of diverse inputs, facilitating complex functions such as voluntary movement, sensory processing, and cognitive activities.[1][2]Structure and Morphology
Cell Body Features
The cell body, or soma, of a multipolar neuron serves as the central nucleated structure, typically measuring 4–100 micrometers in diameter. This size range accommodates the neuron's metabolic hub, housing essential components such as the nucleus, which contains genetic material and a prominent nucleolus for ribosomal RNA synthesis, along with Nissl bodies—clusters of rough endoplasmic reticulum dedicated to protein production. Additionally, the soma includes abundant mitochondria for energy generation and a Golgi apparatus for processing and packaging cellular products.[4][5][6] The substantial volume of the multipolar neuron's soma supports its elevated metabolic requirements, enabling the synthesis of proteins and lipids necessary for maintaining cellular integrity and function. This enlarged structure provides the biophysical and biochemical capacity to sustain extensive dendritic arborization, ensuring efficient integration of synaptic inputs across a broad receptive field. Neurons with larger somas, such as many multipolar types, exhibit enhanced cellular machinery to handle the energy costs associated with complex branching patterns.[7][4] Characteristic of multipolar neuron somas is the presence of an eccentrically positioned nucleus and prominent nucleoli, adaptations linked to heightened transcriptional activity that drives ongoing gene expression in these post-mitotic cells. These features underscore the soma's role in supporting the neuron's overall synthetic demands, with the nucleus often displaced toward the periphery to optimize cytoplasmic space for organelles. From this central soma, multiple dendrites and a single axon extend to facilitate signal reception and transmission.[8][6][9]Dendritic and Axonal Processes
Multipolar neurons are characterized by a single long axon that serves as the primary output pathway, along with two or more shorter dendrites that function as input receptors, setting them apart from unipolar neurons, which possess a single process that bifurcates into both axonal and dendritic branches, and bipolar neurons, which feature exactly one axon and one dendrite.[1][2][10] The dendritic processes of multipolar neurons form elaborate, highly branched trees that taper progressively from the soma, enabling extensive receptive fields for synaptic inputs. These structures often exhibit secondary and tertiary branching patterns, with individual dendrites extending up to several millimeters in length in certain subtypes, such as cortical pyramidal cells, thereby vastly increasing the total surface area—sometimes by orders of magnitude—for receiving signals from presynaptic neurons. Dendritic trees are densely covered in spines, small protrusions typically 0.5 to 2 micrometers in length that serve as sites for excitatory synapses, enhancing the neuron's capacity for signal reception without compromising spatial organization.[8][11][12] In contrast, the axon of a multipolar neuron originates from the axon hillock—a conical region of the soma—and extends as a singular, often cylindrical process capable of propagating action potentials over considerable distances, ranging from millimeters to over a meter in some motor neurons. Axons may be myelinated, featuring insulating layers of Schwann cells or oligodendrocytes that facilitate saltatory conduction for faster signal transmission, or unmyelinated, relying on continuous propagation along the membrane; this variation depends on the neuron's location and function. The initial segment of the axon, immediately adjacent to the hillock and typically unmyelinated, plays a critical role in spike initiation due to its high density of voltage-gated sodium channels, ensuring reliable generation and propagation of electrical impulses.[13][4][14] The establishment of neuronal polarity, which differentiates the axon from dendrites in multipolar neurons, involves the selective localization of microtubule-associated proteins (MAPs), such as MAP2, which predominantly stabilizes microtubules in dendrites to support their branched morphology, versus tau proteins, which enrich axons to promote their elongation and stability. This segregation begins early in development, with tau directing the specification and growth of the axonal process while MAP2 confines to somatodendritic compartments, a process mediated by differential mRNA transport and local translation that ensures structural and functional asymmetry.[15][16][17]Classification
Comparison to Other Neuron Types
Multipolar neurons are distinguished from other neuron types primarily by their morphology, featuring a single axon and multiple dendrites extending from the cell body, which enables complex signal integration. In contrast, unipolar neurons, which are rare in vertebrates and more common in invertebrates, possess a single process that bifurcates into peripheral and central branches, typically serving sensory functions in simpler nervous systems. Bipolar neurons have one axon and one dendrite extending in opposite directions, facilitating straightforward relay of sensory information, while pseudounipolar neurons exhibit a T-shaped structure where a single process splits shortly after emerging from the soma, optimizing rapid conduction in sensory pathways.[1] Evolutionarily, multipolar neurons predominate in vertebrates, supporting advanced neural processing and cortical complexity, whereas simpler unipolar and bipolar forms are more prevalent in invertebrates for basic sensory and motor tasks.[18] In humans, multipolar neurons constitute over 99% of all neurons, underscoring their role in enabling the intricate connectivity of the central nervous system.[19]| Neuron Type | Number of Processes | Typical Locations | Primary Functions |
|---|---|---|---|
| Multipolar | One axon, multiple dendrites | Central nervous system (e.g., cortex, spinal cord) | Signal integration, motor control, interneuronal communication[1] |
| Unipolar | Single process bifurcating into two | Invertebrate sensory systems | Basic sensory detection[1] |
| Bipolar | One axon, one dendrite | Retina, olfactory epithelium | Sensory relay (e.g., vision, smell)[1] |
| Pseudounipolar | Single process splitting into two branches | Dorsal root ganglia, peripheral sensory nerves | Rapid sensory conduction (e.g., touch, pain)[1] |