The most fundamental and perhaps the simplest movements are known as the reflexes. These are relatively fixed, automatic muscle responses to particular stimuli, the most well-known example of a reflex of course being the slight extension of the leg when a physician taps your knee with a rubber hammer.

Sensory Activation

All reflexes involve the activation of small sensory receptors in the skin, the joints or even in the muscles themselves. For example, the reflex mentioned above is produced by a slight extension of the knee extensor muscles when the muscle tendon at the knee is tapped by the rubber hammer. This slight muscle stretch is sensed by receptors in the muscles known as muscle spindles. These spindles, when innervated by sensory fibers, send information to the brain and the spinal cord about the length and speed of the shortening or lengthening of a muscle. This information is subsequently used in reflex control at the joint at which this specific muscle acts.

Motor Activation

This sudden muscle stretch sends a barrage of impulses into the spinal cord along the muscle sensory fibers. This, in turn, activates the motor neurons in the stretched muscle, causing a contraction called the stretch reflex. The same sensory impulses cause an inactivation or inhibition of the antagonists of the motor neurons. This task is performed by connecting neurons, called inhibitory neurons, within the spinal cord. So, even the simplest reflexes involve a complex coordination of activity across those motor neurons that control agonist and antagonist muscles.

Brain Control

An interesting fact is that the brain not only controls the actions of motor neurons and muscles, but also the nature of the feedback it receives from the sensory receptors in the muscles as movements occur. For example, the sensitivity of the aforementioned muscle spindles is controlled by the brain through a separate set of gamma motor neurons. These neurons control the specialized muscle fibers and allow the brain to fine-tune the system for different movement tasks.

Golgi Tendon Organs

In addition to this excellent sensing and control of muscle length by muscle spindles, other specialized sense organs in the muscle tendons, called golgi tendon organs, detect the force applied by a contracting muscle. Through this knowledge, the brain is allowed to also sense and control the muscular force exerted during the movement when it occurs.

The interaction of these complex systems allow precise control of position and force of the muscles involved.