Written by Amanda Ng Ren Hui
Edited by Oh Sher Li
Over the course of recess week this semester (Semester 2, AY2016/17), two fascinating workshops on neuroimaging and neuroanatomy were held. The neuroimaging workshop introduced several common imaging techniques with a focus on magnetic resonance imaging (MRI), while the neuroanatomy workshop served as a brief look into parts of the nervous system and their general functions.
Several highlights during the workshop included seeing the MRI machine in action (thank you Stevia and Caroline), and seeing how the parts of the neural system actually look like from silent mentors and preserved brains and spinal cords. (Not exactly for the faint of heart.)
Neuroimaging: Taking Photos of the Brain in Action
What is MRI or, more appropriately, how does MRI work? To kick-start things off, Assistant Professor Christopher Lee Asplund gave us an overview of the principles behind MRI.
For readers familiar with nuclear magnetic resonance (NMR), MRI works by the same principle except that the data processing is slightly different.
The nucleus of atoms such as C13 has two magnetic spin states, due to an odd number of protons and/or neutrons. (These atoms, however, are not necessarily radioactive, which depends on the ratio of protons to neutrons instead.) As nuclei comprise of protons and neutrons, their overall charge is positive. The ability of nuclei to spin, results in a magnetic moment, so this is where “nuclear magnetic” in NMR comes from. You can imagine all these nuclei as tiny magnets or compass arrows.
In NMR and MRI machines, a strong magnetic field is applied, resulting in these nuclei or tiny magnets aligning with the magnetic field. Pulses of different frequencies are emitted in the machine. Some of these pulses correspond to the “right” energy level and can be absorbed by the nuclei. This absorption of energy knocks the nuclei out of alignment. This is similar to how you need to apply a certain amount of energy to a compass arrow to push it out of alignment from the Earth’s magnetic field. For the same nuclei, the “right” energy levels differ due to environmental factors (bonded to different atoms that stabilise or destabilise the magnetic spin state).
As the nuclei de-excites and emits the energy it has absorbed (i.e. if you stopped supplying energy to keep the compass arrow out of alignment), it will try to realign itself to the magnetic field, but this realignment doesn’t happen perfectly. The nuclei will oscillate, before coming to rest in alignment with the magnetic field. This emission is detected by the NMR and MRI machines.
Frequency of pulse absorbed = frequency of pulse emitted
The pulse emitted, thus, gives us information about the environment of the nuclei.
During de-excitation, photons are emitted. Photons of different frequency are emitted depending on how the nuclei de-excites. This creates a complex spectrum detected by the NMR and MRI machine. To distinguish what frequencies are emitted, this spectrum needs to be broken down into its constituent frequencies (via Fourier transformation).
While this is done for both NMR and MRI, these two machines have different objectives. NMR provides details on the structure of a molecule. MRI, on the other hand, deals with organs. Organs contain millions of molecules, making it an extremely complex NMR. However, we do not need to concern ourselves with the structure of the molecules in the organ, but rather, the characteristic molecules present or emission pattern detected. These are colour-coded by the processing software and converted into black and white photos of the organ (Figure 1).
Figure 1 – Transverse section of a person’s head using an MRI machine (taken from Graham Templeton)
Positron Emission Tomography: An Alternative Method
Positron emission tomography (PET) is another common neuroimaging method. It is used to track the level of metabolism in different parts of the body. A tracer is administered to a patient undergoing a PET scan. This tracer is a metabolite labelled with a radioactive substance like fluorodeoxyglucose (FDG). FDG is a glucose analogue. The radiolabel (e.g. F18) undergoes beta decay, releasing pairs of positrons in the process. These positrons emit gamma rays that are detected by the PET scan and are used to reconstruct the location the positron came from. The more tracer is present, the stronger the signal. This gives a good indicator of metabolic rates and demands of the tissue as well. It can be used to detect tumours, which tend to consume large quantities of glucose, meaning that tracers like FDG tend to localise there.
Wait… This method uses radiolabelled substances, right? Isn’t this harmful to the human body? It’s important to take note that there are different types of radioactive decay processes. Beta decay is one such example and is found in everyday objects like bananas (as they contain radioactive K40). It’s largely harmless.
Neuroanatomy: Regions of the Brain
Just like most topics under anatomy, the focus is on mapping out regions of organs involved in the system, and the general roles each region plays. It is more “it is how it is” rather than a derivation from physical science principles like neuroimaging techniques.
To summarise, the nervous system is made up of the central nervous system (CNS), which comprise the brain and spinal cord, and the peripheral nervous system (PNS). The CNS is less resistant to damage than the PNS, resulting in the need for many layers of protection (skull, cerebral column, meninges, etc.).
The main portions of the brain are highlighted in Figure 2 – frontal lobe, parietal lobe, temporal lobe and the occipital lobe. The brain is also often covered in this protective layer of meninges (this appears like a translucent film over the brain). It is quite a strong layer, a lot like a thin film of tough plastic.
The frontal and parietal lobes are separated by the central sulcus. Sulcus refers to the crevices in the brain that form as a result of folding. The central sulcus is a noticeably continuous crevice around the centre of the brain (notice the line running between the lobes in Figure 2). Reading from left to right, before the central sulcus, is the motor cortex that dictates movement. Right beside it and the central sulcus is the somatosensory cortex. It processes information from our five senses before.
The central sulcus ends abruptly with a horizontal line called the lateral sulcus. This sulcus is very deep compared to other sulci and is thus also known as the fissure. Below the fissure is the temporal lobe.
The occipital lobe is more difficult to notice from Figure 2, but if the brain is pulled at to look at the interior, the parieto-occipital sulcus can be observed.
Figure 2 – Parts of the brain (taken from Alicia).
We also looked at the spinal cord briefly. It shields nerves, which are sensitive to damage, and cannot self-repair easily. It is interesting to note that depending on where the nerves are severed, particularly in the neck, it may result in instant death or quadriplegia (paralysis from the neck down). The higher the location of damage, the more likely for instant death. This raises an interesting question of the distribution of nerves exiting the CNS into the PNS.
Nerves responsible for involuntary actions like respiration and the heart beating seem to exit the CNS earlier than those responsible for voluntary actions like running. If the cerebral column was damaged near the top, the nerves for involuntary actions would be damaged and cause instantaneous death.
The sensitivity and distribution of nerves is a fascinating topic to think about and how it came to be developed this way. It remains to be a question drawing no common consensus. The neuroanatomy workshop was insightful in giving an introduction to the parts of the nervous system. It worked well in combination with the neuroimaging workshop, where we got to understand how scientists identify the parts of the brain responsible for various functions amongst other things.
Snapshots from the Workshops
One of our members got to experience an MRI scan first-hand!
The participants of the neuroanatomy workshop
Graham Templeton. (2013). World’s most powerful MRI can lift a tank like Magneto, or see deep into your brain – ExtremeTech. Retrieved March 6, 2017, from https://www.extremetech.com/extreme/169526-worlds-most-powerful-mri-can-lift-a-tank-like-magneto-or-see-deep-into-your-brain
Alicia. (n.d.). 4 Main Parts of the Brain and Their Functions Explained! – EnkiVillage. Retrieved March 6, 2017, from http://www.enki-village.com/parts-of-the-brain-and-their-functions.html