ニューロンの構造

A neuron has three primary structures: the soma (cell body) containing the nucleus, dendrites that receive signals from other neurons, and an axon that transmits signals to other cells. Dendrites are often highly branched, allowing neurons to receive tens of thousands of inputs. The axon extends from the cell body and can vary greatly in length, sometimes spanning from the spinal cord to the foot.

The myelin sheath is a fatty insulating layer wrapped around axons by supporting cells called glia. This insulation helps maintain the electrical signal as it travels along the axon. In myelinated axons, the action potential is regenerated at gaps called nodes of Ranvier, allowing the signal to propagate efficiently over long distances without degradation.

The action potential is first generated at the axon hillock, the junction between the soma and the axon. This is where the neuron integrates incoming signals from dendrites and determines whether to send an electrical signal forward. Once generated, the action potential travels down the axon to transmit information to other neurons or cells.

When an action potential reaches the axon terminal, synaptic vesicles fuse with the cell membrane and release neurotransmitter molecules into the synaptic cleft, the gap between cells at a synapse. Different neurotransmitters produce varying effects on the target cell, including excitatory and inhibitory effects of neurotransmitters that either increase or decrease the likelihood of an action potential in the postsynaptic cell.

Dendrites receive signals from other neurons at junctions called synapses, typically at specialized structures called spines that protrude from the dendrite surface. These spines contain receptors for neurotransmitters and other chemical signals. Signals received at synapses travel down the dendrite to the soma, where the neuron processes the information and decides whether to generate an action potential.

Neuronal morphology—the overall shape of neurons—varies dramatically and often relates to their specific function. Some neurons have few dendritic branches and a single axon, while others have highly convoluted dendritic arbors or axons spanning the organism's length. The number of synaptic connections on dendrites influences how a cell responds to signals, making dendritic morphology an important feature for defining neuron types.

Synapses are junctions where neurons connect and communicate with other cells. At the presynaptic terminal, neurotransmitters are released into the synaptic cleft. The postsynaptic cell receives these signals through receptors on its membrane. This chemical communication allows neurons to integrate multiple inputs and coordinate activity across neural circuits that process specific information.