18.5
Neuron Structure
Overview
Neurons are the main type of cell in the nervous system that generate and transmit electrochemical signals. They primarily communicate with e…
Neurons are the cells of the nervous system that transmit electrical signals. They come in a variety of shapes and sizes but generally have some common features. All neurons have a cell body, also called the soma, that contains the nucleus. Most neurons also have dendrites and an axon that extends from the cell body.
Dendrites are often highly branching and they receive signals from other neurons at junctions called synapses. The axon, on the other hand, transmits signals to neurons and 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.
Axons are often wrapped in a fatty myelin sheath, made by support cells called glia, which insulates them, helping to maintain the electrical signal as it is transmitted along. The nodes of ranvier, gaps in the myelin sheath, are regions where the action potential is repeatedly regenerated down the axon.
At the end of the axon is the terminal, containing synaptic vesicles filled with neurotransmitter molecules. When an action potential reaches the terminal, neurotransmitter is released into the synaptic cleft, a region of space between cells at a synapse. Depending on the type of channel, the neurotransmitter can help transmit the signal to the next cell.
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Q1: What are the main structural components of a neuron?
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.
Q2: How do dendrites and axons differ in function?
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.
Q3: What role does the myelin sheath play in neuronal signaling?
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.
Q4: What happens at the axon terminal when an action potential arrives?
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.
Q5: How does neuronal morphology relate to function?
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.
Q6: What is the significance of the axon hillock in neuronal signaling?
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.
Q7: How do postsynaptic terminals on dendrites receive signals?
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.