10

I'm wondering:

there is the musculus extensor digitorum, that extends finger 2 to 5 up.

There is an extra muscle for the 2. and the 5. finger (m. extensor digiti minimi and m. extensor indicis).

But why can I than lift my middle finger (3. finger) up without lifting my ring finger (4. finger) up?

I mean, when the musculus extensor digitorum is stimulated (by the nervus radialis), shouldn't it than extend EVERY finger (2 to 5), not only one of them?

My guess was that the antagonist of the musculus extensor digitorum is pulling the other fingers back, but the musculi flexor digitorum superficialis and profundus are also just one big muscle, so this doesn't seem to be the right explanation.

Another idea was that I can activate only parts of the nervus radialis and with that only activate parts of the musculus extensor digitorum. But that doesn't seem to be very likely.

So, what is the reason behind that?

BlueRaja - Danny Pflughoeft
  • 233
  • 1
  • 6
Zwerg
  • 131
  • 1
  • 4

1 Answers1

11

Muscles aren't singular entities, they're made up of multiple muscle fibers. In the case of the extensor digitorum, the muscle fibers are attached to one of various tendons that manipulate the fingers, see e.g. this image:

Extensor_digitorum_muscle

Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain.

Multiple motoneurons innervate the muscle, and triggering different fibers moves different fingers through the attached tendons. Some sharing of tendons can make it more difficult to move individual fingers, but for most peoples' anatomy this is most difficult for the ring finger and not the others. Hu X, Suresh NL, Xue C, Rymer WZ. Extracting extensor digitorum communis activation patterns using high-density surface electromyography. Front Physiol. 2015 Oct 6;6:279. doi: 10.3389/fphys.2015.00279. PMID: 26500558; PMCID: PMC4593961. refers to both the motoneuron mapping and individual variability:

The extensor digitorum communis muscle plays an important role in hand dexterity during object manipulations. This multi-tendinous muscle is believed to be controlled through separate motoneuron pools, thereby forming different compartments that control individual digits. However, due to the complex anatomical variations across individuals and the flexibility of neural control strategies, the spatial activation patterns of the extensor digitorum communis compartments during individual finger extension have not been fully tracked under different task conditions.

Bryan Krause
  • 14,047
  • 4
  • 34
  • 54
  • 1
    Never encountered that spelling "motoneuron". Is that a US thing or just less frequently in pop culture I wonder. – Jiminy Cricket. Feb 05 '24 at 14:55
  • 3
    @JiminyCricket. I'm familiar with them as "motor neurons", two words, or "motoneurons", one word; "motor neurons" are various neurons in the nervous system involved in motor control. It's common to separate them into "upper" and "lower" motor neurons. "Motoneuron" in my experience is used almost exclusively for the neurons specifically innervating muscle. – Bryan Krause Feb 05 '24 at 15:04
  • From a further look it seems like my experience may not generalize, and people do use "motoneuron" throughout the efferent pathway (though I haven't attempted to quantify this). – Bryan Krause Feb 05 '24 at 16:33
  • “for most peoples' anatomy this is most difficult for the ring and pinky fingers” — Really? I always thought it was the middle and ring fingers. There’s the old experiment where you put your hand on a table in a loose fist (intermediate/distal phalanges bent under the hand), extend one finger out onto the table and try to lift that finger without moving any of the others. I thought it was pretty universal that the result of that exercise is something along the lines of: index – easy; pinky – relatively easy; middle – slight lift possible; ring – impossible. – Janus Bahs Jacquet Feb 06 '24 at 09:46
  • 1
    @JanusBahsJacquet I believe it's hardest for most to move the ring finger independently because it also moves the pinky; for some it may also or instead move the middle. But usually the middle is easy to move by itself, I think... Maybe I need a more robust source on that. I can move all of mine independently in both directions which I assume is from learning to type at a young age. – Bryan Krause Feb 06 '24 at 13:38
  • I can move all mine independently in all directions as well when my hand is just freely held out in the air, but I cannot do what the girl in this YouTube video does at all, and I’ve never seen anyone else succeed either. (That is, the second part of the video, lifting the ring finger after bending in the middle finger – with all fingers stretched out, I can lift each one with no difficulty.) – Janus Bahs Jacquet Feb 06 '24 at 15:20
  • 1
    @JanusBahsJacquet I edited to just say "ring" finger; it seems from a quick Google scan that for most people the issue is with moving their ring finger; there are differences in whether the problem is that the pinky moves too or the middle moves too, but it seems like most can move the middle and pinky independently, and actually this fits very well with the diagram in my answer if you follow the tendons, since there isn't any connection to the ring finger that isn't at least partially shared. – Bryan Krause Feb 06 '24 at 15:26
  • @JanusBahsJacquet I'm fairly certain this is something you can learn (perhaps only when young.) One summer when I was very bored as a teenager, I somehow trained my 'index toe' to move with my big toe on my right foot. Before I could only move my big toe separately from the rest and that remains the case on my left foot. If I hold the big toe down, I can move the 'index toe' independently of the others. – JimmyJames Feb 06 '24 at 16:20
  • @JanusBahsJacquet You'll notice that her ring finger doesn't raise as much as the others. I tried and was able to raise it about half an inch, not as much as she can. So she's more flexible than most, but hardly a contortionist. – Barmar Feb 06 '24 at 17:12
  • @Barmar It's not really flexibility. I actually have somewhat hypermobile joints, and if I use my other hand to lift the ring finger, I can easily lift it up to about 45° (so it’s roughly perpendicular to the proximal phalanx of the index finger) – it’s only when I have to use the muscles/tendons in just that one hand that there’s no movement. But if Jimmy’s comment is anything to go by, perhaps it’s something that you can learn to do with practice (when you’re young, at least). – Janus Bahs Jacquet Feb 06 '24 at 17:49
  • @JanusBahsJacquet - It's both. The female in the video has hypermobile joints (not just somewhat); look at her pinky. If she fully extended her fingers, they would extend dorsally much more than the average person. But her anatomy is not average, just as her joints aren't. it's pretty impressive what she can do. And I, too, out of boredom as an adolescent trained my fingers and toes to do odd things. – anongoodnurse Feb 06 '24 at 18:07
  • @anongoodnurse This is getting off-topic, but how does one tell? Just from that video? Because apart from the ring-finger-with-bent-middle-finger part, I’d say I can move my fingers as much as she does, so maybe my finger joints are also more than just somewhat hypermobile (e.g., I can still, despite advancing years, ‘do the splits’ with most pairs of adjacent fingers, except middle/ring; and I can bend the knuckles enough that my fingers lie flat against the palm of my hand). – Janus Bahs Jacquet Feb 06 '24 at 18:17
  • 1
    @JanusBahsJacquet - I'd say your joints are somewhat hypermobile, maybe at the extreme of within normal limits. I've seen people hyperextend their fingers so that they almost touch the backs of their hands, which makes me very uncomfortable, because I know what that would do to normal joints, and it isn't pretty (think torture devices). Most people who can do that have an underlying connective tissue disorder that allows that elasticity. It has its pros and cons. In terms of teaching your fingers to do things... – anongoodnurse Feb 06 '24 at 23:13
  • 1
    ...if you have no neural ability to do, you can't, but if the "proper" nerves exist, you can do weird things with practice. I cannot wiggle my ears; I lack the muscle/innervation to do that. I can do the clover-leaf folding of my tongue, but had to practice in the mirror to learn which muscles to use. But about 80% of people don't have the ability (neurological/muscular level?); no amount of practice will make up for that inability. Did I answer your question? – anongoodnurse Feb 06 '24 at 23:20
  • @anongoodnurse Thank you – I didn’t realise actual neural ability differed like that. (I’ve seen people bend their fingers all the way back too – icks me out as well. I can only do it forwards; backwards they go to about 70–80° or so from the back of the hand, but no further.) – Janus Bahs Jacquet Feb 06 '24 at 23:26
  • 1
    @JanusBahsJacquet - Yes, those are very minor variations; does a nerve branch this way or that, or not at all? The same minor differences exist in skeletal structure and other organ systems, too (like I have an extra bone in my foot that was fractured when I broke my leg that the trauma orthopedist missed.) As long as the variants don't cause harm, they can stick around in the population. – anongoodnurse Feb 06 '24 at 23:48