Stanford University is closed until January 6th. Logistical translation: the heat in the Stanford School of Medicine is off until January 6th.
Practical translation: I’ve been occupying the spare bedroom of my parents’ house for over a week, and I’m running out of entertaining neuroscience facts to spout.
A critical situation, as my primary purpose as the token Doctoral Candidate is the spouting of random neuroscience facts. Or holding forth about my research.
Does this sound at all familiar? Has your store of neuroscience tidbits been exhausted by the constant presence of relatives? Are you wondering how you will entertain folks on during the inevitable portion of the New Years Party when awkward pauses cry out for the utterance of a Neuroscience Fact™, newly learned in honor of the new year?
Here are 10 facts for you, in no particular order, carefully selected from Amazing Numbers in Biology, by Ranier Findt (at Amazon, also available for download at Stanford's Lane Library (Stanford affiliates only)).
1. The number of vertebrae in a spine
As readers will undoubtedly know, vertebrae are the bone segments that surround and protect the spinal cords of all ‘vertebrate’ animals. One might reasonably expect that given differences in the length of individual species, the number of vertebrae included within the spines of different species might vary. Nevertheless, I found that for some species the degree of variation, in comparison to other species, was … unexpected.
Humans have 34 vertebrae.
Turtles also have 34 vertebrae, which might lead us to conclude that most animals have similar numbers of vertebrae.
But they don’t.
Sharks are rocking an approximate 400 vertebrae. Impressive, perhaps implying a range of spinal motion I don’t commonly associate with sharks. On the other hand:
But lest you think sharks have the largest number of vertebrae in the list, they don’t. That honor belongs to a not-unsurprising animal, the boa, which boasts a maximum of 435 vertebrae.
2. The number of cervical vertebrae
In fact, vertebrae can be separated into categories (cervical, thoracic, lumbar); roughly, cervical vertebrae are those that located in the neck. Humans have 7 cervical vertebrae. In comparison, crocodiles have 2, while flamingos have 19, leading me to conclude that the more cervical vertebrae, the bendy-er the neck.
Which lead me to Google the number of cervical vertebrae in a giraffe neck (as thoughts of bendy necks lead invariably to the mighty giraffe). Contrary to thought, the giraffe has only 7 cervical vertebrae, like humans. Those vertebrae however, are longer than other mammals of its size, and are connected to each other via ball and socket joints, a highly unusual state of affairs that allows giraffes to tilt their heads vertically.
3. Number of strands of hair on the head of a human, by hair color
These figures quite delighted me, not because of their numerical values, but because of the thought that someone, at some point, thought to count them in the first place.
- Blonde: 150,000 strands
- Brown: 110,000 strands
- Black: 100,000 strands
- Red: 90,000 strands
(Note: those interested in the hair counts on various sections of the body should refer to table 4.2.3, located on page 213.)
4. (A+T): (G+C) ratios
As the reader will know, DNA contains four nucleotide base molecules: adenine, thymine, guanine and cytosine, abbreviated A, T, G and C, respectively. The bases are paired, A & T, G & C. Because of chemistry, 3 hydrogen bonds bind the G & C bases together, as opposed to only 2 between the A & T bases. The extra hydrogen bond makes a G+C base pair stronger; DNA stability depends on the percent of G+C pairs in the DNA sequence. More G+C pairs = more hydrogen bonds = greater stability.
Humans have a ratio of (A+T) : (G+C) bonds weighted towards the less stable pairing: 1.4-1.52. (So around 3 A+T pairs for every 2 G+C pairs).
Now, most other animals have a very similar ratio. But crabs have a whopping ratio of 17.5 (35 A+T pairs for every 2 G+C pairs). What is the consequence of this ratio shift, and the associated degrease in DNA stability? Unclear. I suggest a rousing discussion at your New Years event of choice.
5. Brain weights of several eminent humans.
Now, I’m not sure how this data was collected. Or why. So I present it without further comment.
- Lord Byron (36 yo): 2.22 kg
- Dante (56 yo): 1.42 kg
- Bismarck (83 yo): 1.807 kg
- Kant (80 yo): 1.65 kg
- Cromwell (59 yo): 2 kg
Note: The age at collection indicates this to be Oliver not Thomas Cromwell’s brain weight. I hold this bit of data under some suspicion, as it is unclear when it would have been collected. Presumably before his initial burial, which adds a certain dimension to the knowledge that Cromwell’s severed (and presumably now brainless) head was displayed on a pole outside of Westminster for several years after his exhumation and posthumous execution…
6. Number of taste buds
First, a baseline: humans have approximately 9,000 taste buds.
This I compare with raised eyebrow to the incredible 100,000 taste buds found in the mouth of a catshark. A quick Internet search reveals that sharks use their sense of taste to test potential food for palatability. Given my lack of experience in consuming the primary diet of catsharks, crustaceans, I will restrict my commentary to supposing some evolutionary pressure that would drive increased taste acuity, presumably to allow accurate differentiation between crustaceans and crustacean-looking non-edible objects (such as rocks).
On the opposite end of the “cares about tasting what it’s eating” spectrum is the chicken, which possesses a meager 24 taste buds. As an avid observer of chicken behavior, I can attest to the fact that a chicken’s main category of food is “stuff that it can see”.
7. Photoreceptors in the eye
Again, as readers will know, photoreceptors are the components of the eye responsible for translating light into the language of the brain, electrical activity. As a general rule of thumb, more photoreceptors means higher sensitivity to light, and, often, more acute vision (although this depends somewhat on the circuitry of the retina, more on that later).
Humans possess 160,000 photoreceptors per square mm in our eyes; as vision is our primary sensory modality we must assume this is a fairly respectable number.
However, the tawny owl possesses 680,000 photoreceptors/mm^2, with the brown rat clocking in at 1.4 million!
8. Number of photoreceptors per retinal ganglion cell (RGC)
Here is a ruthlessly simplified explanation of retinal circuitry: photoreceptors turn light into electrical activity, retinal ganglion cells collect that electrical activity and relay it up to the brain proper. In the retina of some animals, the number of photoreceptors that connect to any one RGC will vary, depending on location within the retina. As a rule of thumb, the fewer photoreceptors per RGC, the more precise the spatial visual information encoded by that RGC.
At the periphery of the human retina (the part that encodes our “peripheral” vision), there are 130 photoreceptors per RGC.
In the center of our retina (the fovea), our vision is highly acute, a physiological reality supported by an astonishingly precise wiring of 1 photoreceptor per RGC.
Such high acuity vision as is found in the human fovea must be the norm for the sparrow, as the sparrow retina also contains a wiring pattern of 1 photoreceptor per RGC dendrite.
“Ah!” I hear you cry, “but is that number only in the sparrow fovea? Perhaps the sparrow’s peripheral vision is as crap as ours!”
Although Amazing Numbers in Biology does not specify sparrow foveal and peripheral numbers, a 1952 article by James D Lockie does. And unlike humans, the peripheral portions of the sparrow’s retina do not exhibit a precipitous increase in photoreceptor convergence onto retinal ganglion cells. Indeed, outside the fovea, the sparrow maintains a photoreceptor: RGC ratio of 1.7, suggesting a rather alarmingly acute visual field (or at least a limited reliance on all the tricks of foveated vision we human employ so blithley. Saccades, I’m looking at you.)
But least I leave you embarrassed at our retinal configuration, consider this: Tigers have 2,500 photoreceptors per RGC.
9. Flicker fusion rate
Flicker fusion rate is defined as the frequency required for two images to appear as one continuous image. Below the rate, the two images are perceived as just that, two images, presented sequentially. Above the rate, and our brain is tricked into seeing those two images as a single, perhaps moving, image. The concept of flicker fusion rate is what allows moving pictures to… well… work.
For humans, our flicker fusion rate is 21 frames per second. Modern motion pictures are therefore presented at 24 frames per second, except, of course for the Hobbit movies, because, Pete Jackson.
Not even the Jackson-preferred 48 fps would enable a pigeon to appreciate the sight of Orlando Bloom’s acrobatic shenanigans (anyone else think the Hobbit trilogy just turned into a Legolas origin story? Not that I’m complaining.) Pigeons have a flicker fusion rate of 148 frames per second, meaning that just about the only moving pictures they can enjoy are certain BBC Sports broadcasts.
10. Age-related changes in amount of peripheral vision.
At this point, I imagine most folks know the neuroscience trivia related to our ability to hear high-pitched tones? How as we grow older, we can no longer perceive very high-pitched sounds. This lead to scattered reports of teenagers setting their ring tones to a sound their middle-aged teachers simply could not hear.
Since everyone knows this, I’ll instead conclude with another example of age-related decreases in our ability to perceive the world around us. As we grow older, the total angle of the world that we can see decreases. Imagine the world as a 360 degree field of space. When we are young, in our 20s, our two eyes can capture light from approximately 176 degrees of that field.By the time we are in our 70s, that area has shrunk to 155 degrees.
Happy New Year. Here’s to one year less of peripheral vision.
**The author would like to thank her non-neuroscientist sister for initial determination of whether the above facts were “even remotely interesting”. However, the author maintains that the Freezing Point Depression and Osmolarity of the Blood of Selected Animals (Table 1.7.12) is clearly fascinating.