Brain Dissections with Peggy Mason

On and off for years and years, I taught neuroanatomy, solo or with my friend and colleague Cliff Ragsdale, to medical, graduate and undergraduate students at The University of Chicago. The last time that Cliff and I did this was Winter 2020, with the final week of class – exams – shut down by the pandemic. Consequently, UChicago undergraduates who began their neuroscience classes in the pandemic hit academic year of 2020-21 have not been taught neuroanatomy. The NEURO club, a student-run organization dedicated to “Neuroscience, Education, University Research, & Outreach” then approached me to lead them in brain dissections.

To give an overview of neuroanatomy, I happily recorded an hour of dissections designed to provide a neuroanatomical framework. To start with, it is critical to understand the parts of the brain and for this we need to go back to development. By about day 28 of gestation (in humans), the neural tube consists of four parts, a spinal cord and three vesicles or swellings with Latin names where the head will develop. They are from caudal, towards the tail, to rostral, towards the snout:

  • spinal cord
  • rhombencephalon
  • mesencephalon
  • prosencephalon

The spinal cord at day 28 is destined to develop into the spinal cord. The Latin named vesicles will become, again from caudal to rostral, the hindbrain, midbrain and forebrain. The prosencephalon splits once more to form two daughter vesicles: the diencephalon and the telencephalon. There are no English terms for these that are currently in use. At one point, the terms interbrain (di’) and endbrain (tel’) were used but sadly, no more. So, I will refer to the structures that develop from these two vesicles by their vesicle names.

We are going to leave aside the spinal cord and restrict our focus to the brain. What are the structures that develop from each vesicle?

  • rhombencephalon (hindbrain) -> medulla, pons, cerebellum
  • mesencephalon (midbrain) -> midbrain
  • prosencephalon (forebrain)
    • diencephalon -> thalamus or dorsal thalamus, hypothalamus (and epithalamus if you want to be really complete)
    • telencephalon -> cerebral cortex including neocortex (also called pallium), striatum and pallidum (the core structures of the basal ganglia), amygdala; and if you want to be complete, the claustrum

Now a word about the telencephalon. The hungry beast of the brain lurks within the telencephalon and in the dorsal telencephalon to be specific. There are a crowd of subcortical structures such as the striatum and amygdala with normal appetites for neural real estate, appetites not appreciably different from that of the hindbrain or midbrain.

Neocortex is different. This structure which forms the rind of the telencephalon is only found in mammals and its appetite is voracious. The neocortex expands every which way – up, forward, toward the midline, laterally and most prominently, the neocortex expands caudally. It does what I call a “comb-over” atop the rest of the brain. The upshot is that as you look down on a mammalian brain, you see telencephalon (cortex) and you do not see diencephalon or midbrain at all. Depending on the mammal and the view, you may see a touch of hindbrain – cerebellum to be specific.

So the basic messages are:

The four parts of the brain (hindbrain, midbrain, diencephalon, telencephalon) are lined up all nice in box car fashion until you get to the derivatives of the dorsal telencephalon: the cortex.

Cortex expands every which way and most prominently does a comb-over atop the rest of the brain. This fold-up of the brain is how the brain fits in the unforgiving bony vault of the cranium (the space in the skull where the brain sits).

Between the expanded cortex and the underlying hindbrain, midbrain, and diencephalon are spaces which are not part of the brain. They are outside of the brain albeit inside the cranium. One such space, the velum interpositum, is present between the diencephalon and telencephalon.

Although I did not discuss it here, note that the cerebellum also has a cortex and also does a mini-comb-over. As superficial stuctures that depend on surface area, cortices are hungry beasts.

There are additional points made in this hour-long video (too long, I know). I introduce the retina and optic nerve, internal capsule, corpus callosum, pineal gland, superior colliculus, substantia nigra where the dopamine cells that die in Parkinson’s disease are located, lateral ventricle, third ventricle, cerebral aqueduct, fourth ventricle, dura and a few more structures. If you want more of this, leave a detailed comment about what you are interested in and I will try to oblige.

7 Comments »

  1. I’m interested in how brains change when key physical structures like the eyes (blindness) or the ears (deaf) are not being used. I read that the brain real estate changes because these areas of the brain are not being used. What does that actually look like inside the brain?

    Liked by 1 person

  2. That is a great question Cynthia. In general, macro changes are not apparent or not easily so. But there are changes that can be discerned using either staining or fMRI methods. I will give you two examples.

    There is a highly instructive story about a woman who is congenitally blind and has a stroke in her visual cortex. You would at first glance think, well no problem. But it turns out that her visual cortex is being used after all – for “reading.” Of course her version of reading is Braille reading. After the stroke, she can no longer read Braille. Yet she can still use her fingers to detect bumps and report how many there are and how closely spaced and so on. What this means is that she uses her visual cortex to sense Braille and thereby read while using her somatosensory cortex to sense texture as we all do. Her visual cortex deprived of input from the eyes has been repurposed to receive input from the somatosensory cortex and derive linguistic meaning from that input. Here is a link to the paper: https://pubmed.ncbi.nlm.nih.gov/10674462/

    The second way that changes following sensory loss can be detected is through changes in connectivity. What I mean by this is that for example, inputs from the visual part of thalamus (the lateral geniculate nucleus) may end up targeting hearing or somatosensory cortical areas in individuals born without sight.

    Thanks for the question,
    Peggy

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  3. Thank you for this and for your excellent and inspiring coursera course.

    In a situation of a stroke does the brain have capacity to actually repair damage or does it utilize compensatory mechanisms, within say the same region?

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    • Hi Wigi,
      Some of both. Some repair and some compensation. I would also say that the great thing about the cerebrum is that if a person survives the acute phase after a stroke, lots of recovery is possible. Much less is possible as you march caudally in the neuraxis. Not much possibility of amelioration after spinal cord damage but forebrain has a lot of recovery-potential.
      Thanks for the question,
      Peggy

      Like

  4. thank you and Hallo again!

    What exactly are thoughts?

    In an individual born stone deaf and blind how does thought process evolve?

    would I be able to understand the “language of their thoughts”?

    What happens to Broca’s area and Wernicke’s area in such a case?

    In the tactile language, developed by and for such individuals , are there observable changes in the somatosensory cortex of hearing and seeing people who engage in this form of communication?

    In such cases would there be atrophy of say the Visual and Auditory systems or do they remain pristine?

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  5. thank you and Hallo again.

    (more questions from a lay person)

    1 In persons born Blind and Deaf are Broca’s area and Wernicke’s area dormant?

    2 Do they become active when Tactile Language may be learned?

    3 Do the visual and auditory system structures atrophy or do they remain pristine?

    4 Do hearing and seeing people who learn to communicate inTactile Language develop greater activity in the Somatosensory Cortex?

    5 What is known of the “language” of thought process and creativity in such individuals?

    Like

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