Brain bits and X-nuclei MRI: What I did at work today

A collaboration I’m involved in has just been published [1]. It’s about a special kind of MRI focused on the chemical element sodium, using meta-analysis to combine estimates form different studies in different brain regions. So what’s the fuss about?

Sodium: Why are we interested?

Sodium (²³Na) is an essential mineral that has a number of important biological functions. For neurons in particular, it is important to maintain a difference in the concentration of sodium inside the cell compared to outside.

This concentration ‘gradient’ powers transport of ions, neurotransmitters and nutrients across the cell membrane:

Some transport pathways used by cells in the nervous system. From [2]

It helps regulates the forces on the cell:

From [3]

And the distribution sodium is one of the preconditions for generating and maintaining action potentials: the electrical impulses that are a major mode of communication in the nervous system.

From [4]

Maintaining the ‘electrochemical’ gradient of sodium across the membrane of neurons takes a lot of energy. The brain in general is a very hungry organ: despite representing about 2% of body weight, it accounts for up 20% of energy consumed. Stated differently, that’s a factor of ten more than we would predict on the basis of weight alone [5]. It’s thought that the largest portion of that energy is going towards maintaining the ion gradients, including those involving sodium. This means measuring sodium in the brain could potentially be an indicator of metabolic health – when everything is functioning as it should. Alternatively, it might be a useful indicator of pathology when the normal sodium distributions are disturbed.