#biology

A neuron and a muscle cell do very different things. One participates in electrical signaling; the other contracts to produce movement. They have different shapes, different responsibilities, and different collections of proteins that make their specialized functions possible.

Yet, in an ordinary mammalian body, these two cells contain essentially the same DNA. Almost all the genetic instructions needed to build either kind of cell are present in both.

How can the same genetic information produce such different results?

Part of the answer is gene regulation. Cells do not use every gene in their genomes equally. Instead, they express some genes at high levels, keep others relatively quiet, and adjust their activity according to their identity and circumstances. When biologists say a gene is expressed, they mean that its information is being used to produce RNA, which in many cases provides the instructions for making a protein.

Cells have several interacting mechanisms for regulating this process. One involves something rather remarkable: making small chemical changes to DNA itself, without changing the sequence of genetic instructions.

This process is called DNA methylation.

DNA methylation is one of the molecular mechanisms studied under epigenetics, a field concerned with how cells regulate and sometimes maintain patterns of gene activity without altering the underlying DNA sequence. Not every epigenetic modification is permanent or inherited, but some can contribute to regulatory states that persist as cells divide.

Descriptions of the nervous system can sound contradictory.

Neurons are often said to carry electrical signals. But we also hear constantly about neurotransmitters like dopamine, serotonin, glutamate, and GABA: chemicals that neurons use to communicate with one another.

So which is it?

The useful answer is: both.

A typical neural signal repeatedly changes form. Within a neuron, information can travel as an electrical change in the cell membrane. At most synapses, the junctions between neurons, that electrical signal is converted into a chemical one. The receiving neuron then converts the chemical message back into an electrical change of its own.

The nervous system is therefore not purely electrical or purely chemical. It is an enormous network built around translating between the two.

Within a Neuron and Between Neurons

A useful first approximation is:

Within a neuron, signaling is electrical. Between neurons, signaling is usually chemical.

That is not a perfect rule. Some neurons communicate through electrical synapses, and the electrical activity inside a neuron is itself inseparable from chemistry. But as a mental model, it gets surprisingly far.

Imagine one neuron receiving signals from many others. Neurotransmitter molecules bind to receptors on its surface and alter what electrically charged particles, called ions, can move through the neuron's membrane.

Those changes affect the electrical voltage across the membrane.