2-Minute Neuroscience: Corpus Callosum

In this video, I discuss the corpus callosum. The corpus callosum is the largest collection of white matter in the brain. It connects the left and right cerebral hemispheres and acts as an important pathway for communication between the two hemispheres. In this video, I describe the function of the corpus callosum along with the deficits that appear after damage to the structure.

2-Minute Neuroscience: Long-Term Depression (LTD)

Long-term depression, or LTD, is a process by which synaptic connections between neurons are weakened. Although the functions of LTD are not completely understood, it may be important to memory formation---perhaps by resetting previous synaptic changes to allow for new memories to be formed via long-term potentiation (LTP). In this video, I discuss the best understood mechanism underlying LTD, which involves AMPA and NMDA glutamate receptors.

Know your brain: Telencephalon

Where is the telencephalon?

The telencephalon highlighted red. The telencephalon not only includes the cerebral cortex (visible here) but also a large number of subcortical structures, pathways, etc.

The telencephalon is also known as the cerebrum, and it consists of the largest part of the brain (it makes up about 85% of the total weight of the brain). It contains the cerebral hemispheres, and thus includes the cerebral cortex and a number of other structures lying below it (subcortical structures), along with a variety of important fiber bundles like the corpus callosum. The inferior boundaries of the telencephalon are found at the diencephalon (e.g. thalamus and hypothalamus) and the brainstem. Posteriorly, it is bordered by the cerebellum.

What is the telencephalon and what does it do?

The telencephalon begins to emerge in embryonic development at about 5 weeks. At this time, the nervous system consists of tube-shaped piece of tissue called the neural tube. The neural tube begins to develop swellings (called vesicles) that will later develop into important structures in the nervous system. The swelling that forms at the farthest end of the neural tube is called the telencephalon (telencephalon is Greek for "far brain"). 

As development continues, the growth of the telencephalon far outpaces the growth of the other structures of the nervous system. The telencephalon begins to expand into two symmetrical structures that sit alongside one another at the very end of the neural tube; these will become the cerebral hemispheres. Initially, the surface of each cerebral hemisphere is smooth, but over the course of neural development it becomes more convoluted until it takes on the appearance of an adult brain with its many sulci and gyri. Thus, the cerebral cortex is part of the telencephalon---as are all of the divisions of the cerebral cortex like the prefrontal cortex, motor cortex, somatosensory cortex, occipital cortex, and so on. 

In addition to the cortex and its recognizable features, there are a large number of subcortical structures that are considered part of the telencephalon. These include the hippocampus, amygdala, and a majority of the regions included in the basal ganglia, among others. Also a multitude of major pathways traverse the telencephalon, such as the corpus callosum---a large bundle of fibers that connects the two cerebral hemispheres---and the internal capsule---another prominent collection of neurons that carries almost all information to and from the cerebral cortex.

The telencephalon is too large an area of the brain to try to link it with a function or short list of functions. It plays a role in most of our brain activity and thus is more analagous to an entire division of the nervous system than to a particular delimited brain structure.

References:

Haines DE. Fundamental Neuroscience for Basic and Clinical Applications. 4th ed. Philadelphia, PA: Elsevier; 2013.

Vanderah TW, Gould DJ. Nolte's The Human Brain. 7th ed. Philadelphia, PA: Elsevier; 2016.

History of neuroscience: Julien Jean Cesar Legallois

The idea that different parts of the nervous system are specialized for specific functions has been a pervasive concept in brain science since ancient times, perhaps best exemplified by the belief---dating back to the 4th century CE---that the four cavities of the brain known as the ventricles each were responsible for a different function, e.g. perception in the two lateral ventricles, cognition in the third ventricle, and memory in the fourth ventricle. By the early 1800s, however, there was still no definitive experimental evidence linking a particular function to a circumscribed area of the brain.

Image showing the medulla oblongata, the region of the brainstem that Legallois found was essential to respiration.

Image showing the medulla oblongata, the region of the brainstem that Legallois found was essential to respiration.

This changed with Julien Jean Cesar Legallois, a young French physician who was driven to identify the parts of the brain and body that were essential for maintaining life. The thinking at the time was that the heart and brain were both integral to life, but there was some debate about where the life-sustaining centers in the brain were located. Some, for example, considered the cerebellum to be the organ that controlled vital functions like heartbeat and respiration. Research conducted in the second half of the 18th century by the French physician Antione Charles de Lorry, however, had suggested that the area of the brain most critical to life was found in the upper spinal cord. Legallois would take Lorry's research a step further by conducting a series of gruesome experiments with rabbits that would help him to specifically pinpoint the center of vital functions in the brain.

Before detailing these experiments, it's important to mention that Legallois' studies were done at a time when the ethical treatment of animals in research---and indeed ethics in research at all---were not given much thought. Legallois was a vivisectionist, meaning that he performed surgery on living animals in his experiments. Legallois' work would not be likely to be approved by a university or research institution today, and indeed when you read Legallois' own impassive descriptions of his grisly experiments they sound like something a budding serial killer might have dreamed up before he moved on to human victims. But this was a different time, when thoughts about animal welfare were not as well formulated as they are now---and Legallois was far from the only vivisectionist of his day. Indeed, a great deal of our current neuroscience knowledge was developed using experimental methods we would consider unjustifiably cruel today. 

Legallois' method of exploring the centers of vital functions in the brain primarily involved the decapitation of rabbits. Legallois observed that after a decapitation made at certain levels of the brainstem, the headless body of a rabbit could still continue to breathe and "survive" for some time (up to five and a half hours according to Legallois). Decapitation further down the brainstem, however, would cause respiration to cease immediately. This observation was in agreement with Lorry's. Legallois then set out to isolate the particular part of the brainstem where these respiratory functions were located.

To do this, Legallois opened the skull of a young rabbit (while the rabbit was still alive), and began to remove portions of the brain---slice by slice. He found that he could remove all of the cerebrum and cerebellum and much of the brainstem, and respiration would continue. But, when he reached a particular location in the medulla oblongata---at the point of origin for the vagus nerve---respiration stopped. Thus, Legallois surmised that respiration did not depend on the whole brain but on one circumscribed area of the medulla. He concluded that the "primary seat of life" was in the medulla, not the cerebellum or cerebrum.

Legallois published the details of his seminal experiment in 1812. We now consider the medulla to be a critical area for the control of respiration as well as the regulation of heart rate, and the region is often considered to be a center of vital functions in the nervous system. Indeed, Legallois was influential in establishing the hypothesis that the brain is involved in the regulation of heart rate as well (prior hypotheses had emphasized the ability of the heart to act alone---without the influence of the brain). While Legallois was not the first to hypothesize that vital functions are localized to the medulla (he was preceded by Lorry), he was the first to provide clear experimental evidence linking the medulla to such functions, and he greatly refined Lorry's estimation of where the vital centers were located. In the process, Legallois gave us our first clear evidence that linked a function to a localized area of the brain.

Cheung T. 2013. Limits of Life and Death: Legallois's Decapitation Experiments. Journal of the History of Biology. 46: 283-313.

Finger, S. 1994. Origins of Neuroscience. New York, NY: Oxford University Press.

For more about the medulla oblongata's role in vital functions, read this article: Know your brain - Medulla oblongata