For example, we can will ourselves breathe/blink faster, or choose to hold our breath. But at the same time, we can’t will a faster or slower heart rate or digestion when it might be advantageous to do so. What is the difference in the muscles involved or brain regions associated with these automated functions?
In: Biology
those functions have been outsourced to the autonomic nervous system because evolution has found this to be efficient to our survival.
from a contemporary human perspective we outsource simple and repetitive jobs to robots so we can focus our more expensive labor on problems that are more unique. so it should likely be this way with nature as well. the job of monitoring your blood pressure, body temperature and plasma salt and acidity levels etc, are essentially boring processes that can be dealt with without conscious intervention. this frees up brain power to deal with problems that are not repetitive such as, “how do I catch that rabbit for dinner?” as opposed to “our temperature dropped 0.1 degrees. turn up the metabolism!”
this is not to say that we cannot intervene in the mundane tasks of our bodies. there are recorded instances of people being able to control things like body temperature , blood pressure , even the dilation of their pupils without the typical external stimuli needed to effect such changes. but this requires extraodinary skill and free time. and what good does that serve to the species apart from entertainment?
years in the future when only driverless vehicles are allowed on the bigger roads people might ask “how come we can drive in some places but not others?” and I think the answer will be the same.
tldr
you can’t control some functions because your body has outsources them to automated systems millions of years ago. to try and intervene manually would both threaten your survival (you can’t be manually better than a robot which has been optimized over generations to do this job) as well as take precious brain power away from things that your species excels at (unique problem solving)
The heart and the intestines have a type of muscle called cardiac and smooth muscle respectively. These muscles are not innervated by nerves that can be voluntarily controlled by the brain.
Breathing and blinking fuctions are controlled by skeletal muscles (like your arms or legs) which are innervated by nerves that can be voluntarily controlled.
Extra info for the curious: blood vessels also have smooth muscle cells that control the width of your vessels to adjust your blood pressure. These are all innervated by the sympathic nerves (unvoluntary control).
You have probably heard of the fight or flight response. They use the sympathic innervations to STOP your gut fonctions (because they are useless in a life or death situation) and INCREASE heart rate and blood pressure.
Edit: clarification and typo
Edit2: yes my bad, the heart is actually made of cardiac muscle like people are mentionning, had a brain fart last night. But the sympathic innervation remains the same.
Disclaimer: not a doctor or biologist, just someone who reads a lot.
Lots of these posts are talking about why some things are more important than others, which is why they are reflexes and not actively processed (using the heart as an example). Another point about the heart which isn’t being mentioned, is that your heart develops to pump blood before you have a fully formed brain stem. So regardless of the relative importance between these body parts in a fully grown human – we couldn’t have evolved if the heart couldn’t beat on its own
For stuff like breath and blinking, those are controlled by muscles that receive signals from our brain to contract and relax. This is why we can override those actions.
For heart rate, it is controlled by pacemaker cells that are independent of the brain. Another example is when the doctor taps on your knee and your leg kicks, you can’t stop it. The signal never actually reaches your brain, just to your spinal chord and back.
Edit: clarification
Edit: you can indirectly control your heart rate by influencing it with other factors (movement and breathing). But you cannot only change your bpm through sheer willpower.
Basically, because of evolution.
Some body functions (like breathing) can be left automated most of the time, but have a few instances where it’s more useful to control them (for example, when you’re running for a long time and need to breath manually to be more efficient).
Other body functions, like your heart rate, or the various hormones that automatically regulate your body are best left doing their own things. Either their work is passive and controlling them would be useless (like your heart or your digestion), or their purpose is to regulate your body and send you warning signals when needed (that’s what hormones do). Controlling those things would result in more bad than good, so we can’t.
This may be explained but I’ll add
The body have two defences so to speak. What you can consciously do and what you cant.
You can consciously tell your body to breathe or walk or blink. But the body can also handle these without your input, else we would spend every second thinking like this
“Step, step, breathe in, step, blink, step, breathe out,” you would never get anything done. It would be huge strain on the brain.
Also, the second defense is at a local level. By this, as someone has explained the heart has it’s own nerve system and pacemaker, that doesn’t need a brain to control. Which is why when someone “dies” the heart can actually be restarted, and sometimes bring them back, or sometimes the person is brain dead but the heart can live.
Which is why a heart transplant can be done. The organ can control it self.
Another example is the spine. If you stub your toe , what happens is the toe senses danger, that travels up the foot, up the leg and into the spine. Now instead of going to the brain only, the spine will tell the leg to pull away quickly, in doing so it also knows to throw your weight to the other leg to balance. All that is done without a brain!
There was a study done, nasty to see. Where they cut the brain stem of a cat, and put it onto a treadmill. The “dead” cat was able to walk and step over a hurdle, all without any input from the brain.
You may have heard of the headless chicken? True story. No brain, but it lived based purely on body reaction alone.
Aren’t there monks that can slow their heart rate?
These answers are good, but tend to be in terms of mechanics. How things work instead of why.
The likely reason that breathing is automated but we can take control of it is that there have been advantages to having breathing automated and significant advantages to being able consciously hold or otherwise control our breath so we evolved to be about to do it. We control our breathing to make vocal sounds, speak, and hold our breath under water (although the latter could probably be delegated to automated response).
By the way, if you are lying in bed and want your heart to beat faster, take a sudden deep breath and hold it for a few seconds while noticing your pulse.
The diaphragm muscle moves down causing a vacuum in your chest.
That vacuum causes air to enter your lungs faster, but it ALSO sucks more blood back into your heart through the veins in your chest (increases “venous return”) (so now the walls of your heart stretch farther apart like a super inflated balloon.
The extra blood and stretch in your heart tricks the heart into needing to beat faster (“Starlings law”), so … surprise… you can voluntarily sort of control your heart rate that way if you really want to.
(Read about physiology; it’s awesome!)
You can slow it down by thinking about ice bath (James Bond movie style).
There are other ways of making your heart beat faster while lying in bed, but that’s beyond the scope of this ELI5 post. 🙂
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