Understanding the Brain: The Neurobiology of Everyday Life

On chewing neural circuits or the privilege of chewing without biting

your tongue at the same time! (Most of the time).

By Miguel Camarena

A little bit about me

• I’m an electronics engineer working at a semiconductor company with a passion for neuroscience.

• I’m also interested in biology and I’m a supporter and newbie collaborator to Openworm.org, which is an open-source scientific project whose goal is to simulate a full biological model of the worm Caenorhadbitis elegans.

Project Requirements

1. Identify the parts of the nervous system that are active in your example: CPG, motor neuron (MN), synapse, genioglossus MN, masseter muscle MN, styloglossum MN, jaw closing MN (JCMN), digastric MN, protractor MN, premotor neurons pMN, retrograde virus transport in neurons.

2. Describe the functions of the nervous system that are apparent and/or impaired in your example: muscular junctions, feeding behavior, jaw closing and opening behavior.

3. Explain in detail the ways in which this course has allowed you to better analyze the events and phenomena around you. Understanding of basic components of motion, understand function of CPGs, understanding of MNs, retrograde virus infection a la polio, motor hierarchy. In the slides’ notes more details will be expounded.

Introduction

• We take so much for granted in our lives, specially the most apparently simple motor actions.

• It is amazing how the brain has evolved to facilitate actions that seems so useful in every day life.

• Chewing involves coordination of jaw and tongue motion circuits, otherwise we would bite ourselves during every meal.

• In these slides we will review a fascinating approach that was invented to illuminate the architecture of… chewing circuits!

Simultaneously performing two motor tasks is hard!

• Tasks such as circularly rubbing your tummy whilst tapping your head are hard without practice. This is a classice example as seen in previous classes. (P Masson., 2011).

• Other instances occur as we age. For example, it appears that talking while walking becomes increasingly difficult (Lundin-Olsson et al., 1997).

• It is even more difficult to simultaneously execute two movements using the same group of muscles, such as talking and chewing. With noxious consequences… such as choking.

Muscles involved in chewing that we need to keep a track of

• We can divide chewing into two phases. The jaw opening phase (JO) and the jaw closing phase (JC). We’ll see pictures of these muscles in the next slide.– During the JO phase, the digastric muscles (which

opens the jaw) and the genioglossus muscle (which protracts the tongue) are active.

– Whereas in the JC phase, the masseter muscles(which closes the jaw) and the styloglossus (retractsthe tongue) are both on the go during the jaw closing (JC) phase.

A brief foray into human oro-facial anatomy

Masseter

Digastric

Genioglossum

Styloglossum

Muscles for JC phase

Muscles for JO phase

Temporal sequence of the chewing behavior

• What the spiking diagram below illustrates is that MN chewing behavior occurs in sequence: when the jaw is closed by the masseter muscles, the tongue protractor muscle is inhibited to avoid biting one’s tongue.

How do we find the right circuits?

• To being understanding this behavior we need to determine the architecture or connectivity of the neurons and muscles involved.

• An fluorescence-based imaging technique will be exposed that sheds light into the architecture of the chewing behavior.

• This technique also allows to discard competing hypothesis of circuit connections as we reveal the actual components of the chewing circuits.

• This protocol was developed by Fang et al.

*Mapping of the oro-facial motor neuron circuits.

Morquette P , and Kolta A eLife Sciences 2014;3:e03235

*Firstly we need to label the neurons involved.

*Then we determine which circuit architecture is reflected in reality. It could be either B or C (or none or something else).

Fluorescent labelling of neurons

We use the virus with fluorescent red and green tags as a molecular tracer to detect which neural paths are involved in the studied behaviors.

The mice’s neural tissue were sliced thinly to allow illumination of those neurons towards which the virus migrated.

Dedicatedcircuits hypothesis

One model proposes that there are two dedicated circuits for each process JC and JO.

One circuits shunts the other one in sequence. This implies parallel sets of IMNs and CPGs.

Extensive imaging discarded this oro-facial circuit hypothesis.

The authors propose to use a single multipurpose circuit.

The complete dynamical workings of the circuits haven’t been established but, hey, we need first to be sure what are the parts involved.

Eventually this first part is done. They have provided an overview of the neural architecture.

Multipurposecircuits hypothesis

Conclusions

• The course helped me understand the paper and how the components used for the experimental assay are logically used.

• It helped me understand the neuronal parts of the chewing behavioral components involved in the circuit.

• It was also lots of fun!

Thanks!

• Thanks for this great course Peggy!

• Thanks TAs and all the MOOC students who made this a great MOOC.

Bibliography

• Eric Kandel et al, Principles of Neural Science, 2013, McGraw-Hill.

• Dale Purves et al, Neuroscience, 2011, Sinauer.• Peggy Mason, Medical Neurobiology, 2011, Oxford

University Press.• Fan Wang, Monosynaptic premotor circuit tracing reveals

neural substrates for oro-motor coordinations, 2014, eLife.• Philippe Morquette et al, Orofacial coordination: How do

we walk and chew gum at the same time?, 2014, eLife– http://elifesciences.org/content/3/e03235