Journeying into the sheep brain
In this installment of laboratory videos, the goal is to compare the sheep and human brains. What we’ll see is the extreme similarity in the basic plan of the sheep and human brains. This is part of our mammalian inheritance, the common evolutionarily derived template that mammals share. In this first video, we look at sheep and human brains side by side and identify the major brain regions in each.
Now let’s go inside the sheep brain. The first important point is to see that, as in the human, the telencephalon arches over the diencephalon and brainstem in the sheep. And while the human telencephalic cap covers even more of the rest of the brain than does the sheep cap, the sheep telencephalon is no small potatoes. In fact, the telencephalon is impressive even in smooth-brained (= lissencephalic or lacking sulci and gyri) mammals such as the rat that have relatively small cerebral cortices.
By looking at sheep brains, I discovered something surprising; namely that the sheep superior colliculus appears larger than the human superior colliculus. I mentioned this in NeuroMOOC class and several students began to discuss this issue. These clever NeuroMOOCers came up with two intriguing ideas regarding how a large colliculus might render a behavioral or ecological advantage to sheep. First, the sheep with its laterally placed eyes has a wider visual field than does the human with its forward-facing eyes. The sheep can see close to 300° whereas humans have a visual field that is roughly half of that. The size of the visual field is important because while the superior colliculus receives auditory and somatosensory input, visual input dominates.
The second idea that NeuroMOOCers came up with is that the superior colliculus of the sheep but not of the human supports pinnal reflexes, meaning reflexes of the external ear or pinna. Remember that the superior colliculus is command headquarters for orienting movements. We may turn our gaze to orient to a sound or sight but many animals turn their pinnae (singular form is pinna; the lobe part of the external ear) to orient toward a stimulus. Cats exhibit this reflex spectacularly clearly. Sheep, as it turns out, also have pinnal reflexes, which fits with the large superior colliculus in this species. Humans can’t move their ear lobes; we don’t have the requisite muscle attachments; this fits with our smaller superior colliculus.
Now we look in at the brainstem with particular attention to 1) the connection of the cerebellum to the rest of the brain through the cerebellar peduncles; 2) the fourth ventricle, cerebral aqueduct and third ventricle; and 3) the vulnerability of CSF flow to aqueduct obstruction by a pineal tumor.
The vermis is the central portion of the cerebellum and is flanked on either side by lateral lobes or hemispheres. In humans, the cerebellar hemispheres are so large that they completely cover over the vermis but in sheep, the hemispheres are proportionally much smaller than the vermis. Consequently the vermis is easy to view in the sheep. And the cerebellar vermis of the sheep has a striking feature: it’s an asymmetric structure. Turns out, it is not just the sheep that has an asymmetric vermis. The asymmetry is a general feature of the mammalian cerebellum, including that of the human. However, in years of looking at human brains, I never noticed the asymmetry, probably because the human vermis is so hidden beneath the cerebellar hemisphere [sounds familiar, right? cortex expanding to take up more room and covering over neighbouring brain areas in the hindbrain as in the telencephalon]. It pays to look at the brain of different animals! Learning through comparative neuroanatomy!
In this next video, we look from the outside at CSF flow within the ventricular system from the lateral ventricles in the telencephalic hemispheres to the subarachnoid space that surrounds the brain and spinal cord.
Now let’s go inside the brains of both sheep and human to trace the ventricular system. In particular, the lateral ventricle can be followed as it completes a UChicago C with horns in the frontal, occipital and temporal lobe poles.
And that’s it for this installment. The next installment will be all new. Stay tuned.