ニューロンの構造

All neurons have a cell body, or soma, containing the nucleus. Most neurons also have dendrites, which receive signals from other neurons, and an axon, which transmits signals to other cells. The axon hillock, where the cell body meets the axon, generates the action potential, the primary form of electrical signaling in the nervous system.

Dendrites are highly branching structures that receive signals from other neurons at junctions called synapses. Axons, by contrast, transmit signals away from the cell body to other neurons and cells. This specialized division allows neurons to integrate incoming information and relay it forward through the nervous system.

The myelin sheath is a fatty insulating layer wrapped around axons by support cells called glia. It maintains the electrical signal as it travels along the axon. The nodes of Ranvier, gaps in the myelin sheath, are regions where the action potential is repeatedly regenerated, allowing efficient signal transmission over long distances.

When an action potential reaches the axon terminal, synaptic vesicles filled with neurotransmitter molecules release their contents into the synaptic cleft, the space between cells at a synapse. Depending on the neurotransmitter type and receptor, the signal can be transmitted to the next cell, facilitating communication across the synapse and neurotransmitters.

Neuronal shape varies dramatically and often reflects 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 morphology an important feature for defining neuron types.

The axon hillock is the junction between the soma and the axon where the action potential is first generated. This region is critical because it determines whether incoming signals will trigger an electrical response that propagates down the axon. The axon hillock essentially acts as the decision point for signal transmission in the neuron.

Postsynaptic terminals, frequently located on small bumps called spines protruding from dendrites, contain receptors for neurotransmitters and other chemical signals. Dendrites are often highly branched, allowing some neurons to receive tens of thousands of inputs. Signals received at synapses travel down the dendrite to the soma for processing.