What causes body movement in humans?
What causes our body movement like how we move our legs, toes, and arms? I know at the most basic causes, the bones and joints are attached to muscle, and an electric impulse sent by the brain causes us to want to move. But what causes the electric impulse in the brain in the first place(or in other words, what causes us to make us want to move?). I move without even thinking about moving, because it happens naturally, but there seems to be no trigger that causes me to want to move my body parts. Is there a cause to what makes us want to move?
When we touch something hot(like a pan of boiling water, it makes are hand want to move. This is easy to figure out because we already know the stimulus - the hot boiling pan of water.
But what if there is no stimulus? What makes us want to move? I don't see a stimulus thats why Im asking this quesiton.
This is a very interesting question.You are right to point out that the movement caused by you burning yourself is different from the movement caused by you deciding to move - whether subconsciously or consciously.
I am presuming that your question is not about the molecular mechanics of how muscles contract, instead you are more interested in what forms the original signal to the muscle.
I will start with the boiling water analogy to describe reflexive action - since this is easier to explain and forms a nice backdrop. In your hands, (and all over your skin, for that matter), you have neurons that express a protein called TRPV1 which causes a rise in electrical potential at elevated temperature (wikipedia has a good page on this). This rise in potential causes a dorsal root ganglion cell to fire an action potential. This impulse travels up your arm and into the back of the spinal cord. Here, the brain is very cleverly wired up in development. The arrival of the pain-sensing impulse directly excites motor neurons that causes the contraction of your hand away from the stimulus. Hence motor action is unconscious and involuntary.
But here is where the fun begins. The conscious part of your motor nervous system (aka somatomotor) can be roughly split in two. "Lower motor neurons" are like the ones described above that go from the spinal cord and travel to your muscles. When a painful signal arrives to them from a heat or pressure sensor, they will fire and cause movement. But they are regulated, usually negatively, by "upper motor neurons". These neurons are incredibly long and travel all the way from your cortex (the bumpy part of the brain), down to the spinal cord. Imagine that, a single cell over a metre long - it still blows my mind. When these neurons fire, they cause their lower motor neuron to fire. If a brain surgeon were to probe around the part of your cortex called the primary motor region, then they could make you move any of the muscles in your body that you have voluntary control over.
But how do these motor areas in the cortex get activated? Here's where the story gets more interesting! So far, what I have described is like a panel of buttons in the head, which when pushed cause motion of that part of the body. Well, who or what does the pushing?! Around the primary motor cortex lies less defined secondary motor cortex areas. If a neurosurgeon pokes around this area, then they get more complex, coordinated movements. Exciting the primary motor cortex causes a single muscle to twitch, whereas exciting the secondary motor cortex might cause your leg muscles to work together to lift your leg and ankle at the same time ready to take a step. We learn these complex motions as a baby and store 'lift leg to move forward' in a part of our cortex. When activated, it will send the correct sequence of impulses to the primary motor cortex and we will perform a 'complex movement'. A whole region of the brain, called the cerebellum, is largely devoted to perfecting muscle motions to achieve certain tasks like hitting a bullseye. It is, in my opinion, the most beautiful tissue structure of the entire body - I recommend looking it up!.
Now here we get much more crazy! So how do we decide to make a complex motion? How do we decide which motion is most suitable for our goal? This part involves a subconscious-involuntary bit of processing. We have this amazing circuit loop in our body called the basal ganglia. One of its functions is to be an 'idea filter'. If I decide to stand up, many different ideas of exactly how I will do that will enter this loop, and involuntarily, it will magically select the optimal way to stand up and excite the appropriate regions of the secondary motor cortex to create the complex movements of the primary motor cortex. Disorders in this system include Parkinsons, and Huntingdons disease. In Parkinsons, the filter says 'no' to everything, and thus patients are rigid and find it difficult to initiate movements. Makes sense right - no request get through to the secondary motor cortex! In Huntingdons, the opposite is the case. The neurons in the "No" pathway start dying, so the brain says "yes" to any and every subconscious idea that your brain has - the result is called 'chorea' which means dancing. These patients constantly swing their arms/legs around and can't be still.
Now we reach the final, and hardest to define part of the story. What decides we want to sit up from our chair to start the basal ganglia going to select the best motion in the secondary motor cortex to activate the correct parts of our primary motor cortex, which activate the specific muscles? This is what we would call 'executive function'. It is the highest level of our conscious mind. It makes decisions and acts on them in 'rational ways'. When things go wrong here, as many drug abusers find, we behave irrationally and are psychotic. This part of the brain is located at the front of our head, and is called the 'prefrontal cortex'. A very famous patient, named Phineas Gage from the 19th century got an iron pole stuck through his skull, and it destroyed part of his frontal cortex. His executive function changed - high level behavioural things. He went from being a religious, polite and kind man, to constantly swearing, and highly violent. This part of the brain is the domain of human behaviour and intelligence. Predictably, it is the most enlarged part with reference to other animals. How does this part of the brain work? How does it make us 'conscious'. How can we predict what we will do next? These are the frontiers of modern neuroscience. Join us, and help us understand the most incredible machine in the entire universe.