For the love of brain (well, the hippocampus) II

So I’ve already described how much I love being able to look at live, functional neurons in real time. But there are other things I enjoy seeing everyday too, in particular my favourite part of the brain; the hippocampus.

The rodent hippocampus in stained, coronal sections

The rodent hippocampus in stained, sequential coronal sections

At the start of every patch clamp experiment, when I look down the microscope at a brain slice, I need to locate a specific structure before I can focus on finding individual neurons. This structure is called the hippocampus (so called because a cross section of it looks like a seahorse) and every brain has a pair of them (one in each hemisphere). The hippocampus is very special because it has a well-defined and distinctive structure, which makes it instantly recognisable to anyone who has studied neuroanatomy.

Freshly cut transverse brain slices. I store them in a plastic tea strainer, submerged in  carboxygentated aCSF. The arrow points to where the hippocampus is loacted

Freshly cut transverse brain slices. I store them in a plastic tea strainer, submerged in carboxygentated aCSF. The arrow points to where the hippocampus is located

 

Acute, transverse hippocampus as seen during my patch experiments - notice the patch electrode emerging from the CA1 region in the right-side image

Acute, transverse hippocampus as seen during my patch experiments – notice the patch electrode emerging from the top of the CA1 region in the image on the right

As I’ve said before, the brain is not a homogenous blob, but contains many intricate and beautiful structures. Many of these structures look pretty much the same across the majority of species, no matter what size or shape the brain may be, and the hippocampus is no exception.

Drawing by Camillo Golgi of a hippocampus stained using silver nitrate

Drawing by Camillo Golgi of a hippocampus stained using silver nitrate

Why do I love the hippocampus? Well, for one thing I love how ‘organised’ the anatomy is (I won’t go into detail here, but I do find the Wikipedia page extremely useful). The hippocampus is a network and I love how you can stimulate a particular pathway and get other neurons to fire in response (ie. stimulate the axons of the CA3 pyramidal neurons and the CA1 pyramidal neurons will produce a response). I also love the fact that it’s involved in lots of different aspects of learning and memory.

So, going back to the point about being able to stimulate pathways in the hippocampus, I also do a lot of experiments using multi-electrode arrays (MEAs). Being able to stimulate particular pathways and measuring evoked responses can tell you a lot about the synapse involved and the neurons on either side. For my experiments, I stimulate the Schaffer collaterals (axons of CA3 pyramidal neurons) and record the responses of the CA1 pyramidal neurons. Instead of observing signals from individual neurons, I look at the combined response of a whole population of neurons, which all fire in unison when stimulated by an extracellular electrode.

I take a photograph (before and after!) of every hippocampus I use for my MEA experiments. It allows me to select which electrodes to stimulate so that I can record evoked field potentials

I take a photograph of every hippocampus I use for my MEA experiments so that I can select which electrodes to stimulate in order to produce evoked responses. This is not a stained section – it’s the actual colour of a ‘living’ hippocampus!

I’d love to go into more detail about MEA experiments, but I’ll save it for another post 🙂