What role do motor neurons play in basic bodily functions?

As you read this, muscles are contracting and relaxing regularly to move air in and out of your lungs. When you walk to the kitchen for a cup of coffee, muscles in your legs contract and relax in rhythmic sequence to move you forward. For animals to function, motor behaviours like breathing and walking must be reliably controlled by the nervous system. Muscles need to contract in the same order, for roughly the same duration, each time a breath or step is taken. There is a pattern of activity that must be maintained. But the system also has to be flexible enough to respond to changes in the environment, such as obstacles in your path that you have to step around. There are many open questions about how the nervous system controls rhythmic movements, permitting reliability and flexibility. What determines the timing of the motor pattern? Under what conditions can the timing be altered, and how? To what extent can these behaviours be recovered after injury?

How does the nervous system produce the rhythmic sequence of movements required for walking?
Image credit: Eadward Muybridge 1887, via Boston Public Library

Rhythmic motor pattern generation

Example of a motor network, in which motor neurons are not part of the CPG. Image credit: E. McKiernan.

Rhythmic motor behaviours are controlled by networks of neurons which communicate electrically and chemically¹. Although the exact organization of these networks varies, many can be divided into four principal groups of cells. One group includes a subset of neurons in the central nervous system called a central pattern generating (CPG) network, which produces the core rhythm. In some systems, motor neurons, which send signals directly to muscle fibres, participate in generating the rhythm and are part of the first group. However, in other systems, motor neurons do not belong to the CPG network and are considered a second group receiving input from the first. Muscle fibres constitute a third group of cells comprising whole muscles that contract or relax to produce movements. A fourth group, sensory neurons, responds to the movement of muscles and sends feedback to the central nervous system about the output that was produced, or whether there are environmental perturbations, like obstacles.

Where is Everyone? The Fermi Paradox, Astrobiology and Exoplanets

Since the middle of the last century, against the backdrop of greatly expanding space technology and understanding, scientists have wondered about our place in the vast universe and whether we are alone or not. When it comes down to it, why would we be? There is no reason, be it physical or chemical, life couldn't exist elsewhere. At first glance it seems that we live on a relatively normal planet, our parent star is of a fairly common variety and our corner of the galaxy isn't all that extraordinary. Water and other 'building block' organic compounds, thought crucial for life in any imaginable form, are relatively abundant throughout the galaxy.

There are at least 100 billion (that's a 1 followed by eleven zeroes) stars in the Milky Way galaxy; many we now know come complete with a family of planets in their orbit. On top of that, several of these newly-discovered 'exoplanets' (that's the name for a planet outside our solar system) are not that different from the Earth in mass or orbital distance from their parent stars. In fact, a recent study calculated that a staggering 17 billion Earth-like planets are likely to exist in the Milky Way alone! Surely, more than one of those worlds would have life of some kind or the other clinging to its surface? And if there was life, even if it was almost vanishingly rare, could another species with a similar level of intelligence to humans exist on another one of those billions of planets out there in the reaches of space?

 

Artist's impression of Gliese 667Cc, a possible Earth-like exoplanet 22 light years distant, in the constellation Scorpius. 

Credit: ESO/L. Calçada

Mass Extinction

Why do we study past biodiversity, and is it applicable to the current 'biodiversity crisis'?

It is no great secret that the Earth is on the brink of a large-scale extinction. The extinction of both species and populations is becoming a major concern in the scientific and policy domains for social, environmental and economic reasons. Although genuinely documented recent species or population extinctions are relatively limited (when compared to the Permian-Triassic mass extinction, in which an estimated 96% of species went kaput, around 252 million years ago), it is extremely likely that any figure is a vast under-estimate of actual extinction rates, as we simply do not have any rigorous estimate for how many species there are currently on this planet.

Marine biodiversity holds some of the most enigmatic
and beautiful forms we currently know. Image Credit: Richard Ling (Wikimedia commons)

Species don't just die out - new species are coming into being all the time, through evolutionary processes. If the rate at which species are becoming extinct is at least 75% higher than the rate at which new species are appearing (speciation), traditionally over a period of 2 million years or so, then palaeontologists call this a "mass extinction". Seeing as this is quite irrelevant to current processes happening in just tens to thousands of years, some would prefer to adapt this definition to something more relevant to modern biodiversity issues.

The IUCN (International Union for Conservation of Nature) have undertaken the most recent and thorough extinction status analysis, but this is limited to only 2.7% of the nearly two million named extant species (species we know to be alive today). This low figure calls into question any claims that we are entering a mass extinction, as the analysis simply doesn't cover enough species for us to be sure it gives a comprehensive picture of what is happening. 

Nevertheless, there are mounting concerns that environmental, ecological, and climatic factors are being perturbed in a way that will be substantially detrimental to global biodiversity, issuing the onset of a new, and 6th, 'biodiversity crisis' or mass extinction.