The nervous and endocrine systems regulate the bodies functions, allowing it to maintain homeostasis, a state of balance. The nervous system is the control centre and can make fast changes while the endocrine system produces and secretes hormones that act more slowly. We are more concerned with the fast acting nervous system.
Nerves transmit electrochemical responses around the body and they do it real fast. The ways that these nerves are connected (here we go again) plays an important role in their function, especially in relation to movement. The connection between nerves in the brain, the spine and the limbs facilitates the transmission of data that controls how we determine our position in space and how we move to change that position. These specific pathways are in the main not predetermined but are developed throughout life. As we learn to catch a ball as a child we are creating neurological patterns that allow us to respond to our environment and move accordingly. We see the ball, we move, we catch it but we have to practice these things. In the practice we are establishing connections in our nervous system that trigger the appropriate sensory receptors and motor units (more about those soon). In the same way when learning to play a guitar we are establishing new connections in the nervous system that allow awkward stiff movements to evolve into automatic fluid ones.
muscle memory
Sensory organs perceive changes in our environment, or more accurately our relationship with our environment, and generate an electro-chemical impulse that is passed on to a sensory nerve and eventually to the brain for processing. Some specific types of impulses are intercepted and processed in the spine but they are by far the exception, we'll just say the brain to keep the language as simply as possible.
The brain then processes this input and spits out a signal along a motor nerve pathway that eventually reaches either a muscle or a gland. An action follows that changes our relationship with our environment, the sensory data regulates that change, we adjust motor activity where necessary and eventually reach again a point of equilibrium again.
The vast majority of sensory input is visual so we're used to getting our information this way. The convenient grid shape of the strings on the fretboard and the tendency to play in shapes and boxes within this grid make the guitar a very visual instrument. This tends to make us lazy, too many guitarists play to shapes rather than sounds, never bother to develop really good aural skills and get stuck playing the same old licks all the time because our fingers have memorised these shapes. This is not to mention of course the disastrous impact on our posture that is inevitable when we're looking down at the guitar all the time.
So, as sight-impaired people have learned to do, we need to develop other ways of getting information to our brain. Skeletal muscle is innervated not only by motor nerves but by sensory nerves as well. Proprioceptors gather information about muscle tension and send it north for processing. From this information the brain can determine the relative amounts of contraction and relaxation in various muscle groups and by extension exactly, more or less, where we are in space. When you close your eyes with arms outstretched and bring your finger in to touch your nose you're testing your proprioception.
Of course the only way to develop these skills, to establish the necessary neural pathways, is through practice. There are many technical exercises available for guitar players, those that have been developed for the TuneUp program look very specifically at developing proprioception and muscle memory. With each of them is a more detailed description of how they do it.
The nervous system is crucial to our understanding of movement and biomechanics. As the quick-acting regulator it is the nervous system that instigates movement and regulates muscle tone. When you stretch a muscle you're not actually stretching the muscle tissue, you're adjusting the way that the nervous system controls muscle length. Over time this allows the actual fibres to lengthen by a process called creep.
Simple nerve endings in the myofascia called receptors give the brain information with which it can regulate muscle tone and determine your position in space.
Golgi Tendon Organs are located at the junction of the tendon and the main body of the muscle, excess tension here results in a signal to the brain to reduce the signal to the muscle to contract. The active stretching exercises and some of the massage techniques in the remedial program work because they activate the Golgi Tendon organs.
Muscle spindles, or stretch receptors, work the other way. If a muscle is being overstretched it is in danger of tearing and signals from muscle spindles tell the brain to increase tone to maintain the integrity of the muscle. If these receptors are over active they'll be telling the muscles to contract.The sort of physical relaxation that you experience from a good massage and from the exercises in the relaxation program probably works on reducing the activity of these muscle spindles.
Proprioceptors guage the tension in the myofascia. With the information from many different propriceptors you can determine the position in space of parts of your body. You can play without looking at your hands because you've done it enough times that your brain knows that the pattern it's getting from all the proprioceptors represents a certain position.