Mitochondrion · Lobeworks/17

A mitochondrion is an organelle, a compartment inside the cell wrapped in two membranes, that burns the products of glucose with oxygen to make most of the cell's ATP, and neurons, which spend energy without pause, depend on them more than almost any other cell.

Mitochondrion. A mitochondrion is an organelle, a compartment inside the cell wrapped in two membranes, that burns the products of glucose with oxygen to make most of the cell's ATP, and neurons, which spend energy without pause, depend on them more than almost any other cell.

The outer membrane encloses an inner one folded into ridges, and on that inner membrane the respiratory chain pumps protons out and lets them flow back through ATP synthase, which turns the flow into ATP: oxidative phosphorylation, about fifteen times more ATP per glucose than glycolysis alone. Mitochondria descend from bacteria that entered an ancestral cell some two billion years ago, and they still carry a small genome of their own, a ring of 16,569 base pairs with 37 genes, 13 of them for parts of the respiratory chain; the rest of their proteins come from the nucleus.

Neurons send them where the work is. Mitochondria travel along the axon and gather at synaptic terminals and nodes, because the pump and the recycling of vesicles at a busy synapse need ATP right there.

They also hold calcium. They take up calcium during intense activity and shape its signals; overloaded, as in excitotoxicity, they fail and release signals that kill the cell.

Their faults show in the brain first. Mutations of mitochondrial DNA cause energy diseases whose signs are largely neurological (seizures, stroke-like episodes, loss of vision), because the brain has the least margin.

Every spike is paid for later in a mitochondrion.

Where a neuron works hardest, at its terminals, it parks its power stations.

Questions: Where in the neuron is most of its ATP made, and why there? In the inner membrane of its mitochondria, by oxidative phosphorylation, and the neuron moves those mitochondria to where the work is: the synaptic terminals and the nodes of the axon, where the pumps restore the gradients and the vesicles are recycled. ATP diffuses poorly over the length of an axon, so the power stations travel instead.