Perhaps the simplest and most fundamental movements are reflexes. These are relatively fixed, automatic muscle responses to particular stimuli, such as the slight extension of the leg when a physician taps the knee with a small rubber hammer.
All reflexes involve the activation of small sensory receptors in the skin, the joints, or even in the muscles themselves. For example, the reflexive knee movement is produced by a slight stretch of the knee extensor muscles when the physician taps the muscle tendon at the knee. This slight muscle stretch is “sensed” by receptors in the muscle called muscle spindles. Innervated by sensory fibers, the spindles send information to the spinal cord and brain about the length and speed of the shortening or lengthening of a muscle. This information is used to control both voluntary and involuntary movements. A sudden muscle stretch sends a barrage of impulses into the spinal cord along the muscle spindle sensory fibers. In turn, these fibers activate motor neurons in the stretched muscle, causing a contraction called the stretch reflex. The same sensory stimulus causes inactivation, or inhibition, of the motor neurons of the antagonist muscles through connecting neurons, called inhibitory interneurons, within the spinal cord. Thus, even the simplest of reflexes involves a coordination of activity across motor neurons that control agonist and antagonist muscles.
The brain can control not only the actions of motor neurons and muscles but even the nature of the feedback received as movements occur. For example, the sensitivity of the muscle spindle organs is monitored by the brain through a separate set of gamma motor neurons that control the specialized muscle fibers and allow the brain to fine-tune the system for different movement tasks. Other specialized sense organs in muscle tendons — the Golgi tendon organs — detect the force applied by a contracting muscle, allowing the brain to sense and control the muscular force exerted during movement. These complex feedback systems are coordinated and organized to respond differently for tasks that require precise control of position, such as holding a full teacup, than they do for those requiring rapid, strong movement, such as throwing a ball.
Another useful reflex is the flexion withdrawal that occurs when the bare foot encounters a sharp object. The leg is immediately lifted from the source of potential injury (flexion), but the opposite leg responds with increased extension so that we can maintain our balance. The latter event is called the crossed extension reflex. These responses occur very rapidly and without your attention because they are built into systems of neurons that are located within the spinal cord itself