The network formed by the billions of neurons in your body is responsible for all five senses, controls movement and consciousness. This article explores the various substructures of the neuron to prepare the reader for a lesson on connectomics.
The cell body is the large, round part of the neuron. It holds the nucleus and most of the other organelles in a cell. Because it contains most of the organelles, most of the proteins are synthesized in the cell body.
Dendrites are the extensions of the neuron which receive most of the signals from other cells. In general, dendrites do not extend very far away from the cell body, although some of the neurons in the brain have very long dendrites to receive signals from cells that are far away.
The shape that the dendrites make, or the cell’s dendritic arbor, is often unique to a certain type of neuron, and thus dendritic arbor shapes are often used in classification of neurons.
Neurons have a wide variety of dendritic arbors — although no two are exactly the same their shape and size tell us their type. This is similar to identifying leaves of a tree. Scientists are still discovering new neuron classifications the likes of which they’ve never seen before.
Check out the pictures below and see if you can use the neuron’s dendritic arbor to identify its classification (answers at the end of article).
(options: double bouquet cell; chandelier cell; pyramidal cell)
While dendrites are the primary means by which a cell receives a signal, the cell’s axon is how the neuron sends a signal to another neuron. Axons can be extremely long, some reaching up to 1 meter in full-grown humans. Rather than transmitting signals by a constant change of the voltage difference across the cell membrane, axons instead carry action potentials. Action potentials almost always have the same amplitude (voltage difference) for a given cell. Instead of transmitting the intensity of a signal through the amplitude, cells change the frequency at which action potentials are generated.
Synapses are the connections between neurons that allow the neurons, through chemical or electrical means, to communicate with other neurons.
Chemical synapses are more common than electrical synapses. When the action potential in the presynaptic cell reaches the location of the synapse, neurotransmitters are released into the small space between the presynaptic and postsynaptic terminals. The neurotransmitters then bind to neurotransmitter receptors on the postsynaptic cell, which either causes or prevents the postsynaptic cell from firing.
Synapses are either excitatory or inhibitory, meaning that they will either cause the postsynaptic cell to fire, or prevent it from firing. After the neurotransmitters have bound to the receptors, they must be removed from the space between the cells to allow for further use of the synapse. This usually occurs in one of three ways: the neurotransmitters may diffuse away, to be broken down at another location; enzymes may be released to break them down; or they may be reabsorbed into the presynaptic terminal for later use.
Electrical synapses are less common than chemical synapses, but they are much quicker. They are also bidirectional, and are always excitatory. Instead of sending a chemical signal from one cell to another, electrical synapses work by directly transmitting the electrical signal through a gap junction, which is a connection between the cytoplasm of the two cells.
Want to know more about the parts of the neuron? Check out the wiki!
Keep an eye on the blog for the next installment of Behind the Science.
(answers, from left to right: pyramidal cell; double bouquet cell; chandelier cell)