The Gamma Motor System
The muscle spindles are very sensitive to changes in muscle length; they will increase their rate of firing when the muscle is lengthened by a very small amount. The interesting thing is that this detection mechanism is adjustable. Remember that the ends of the intrafusal muscle fibers can be contracted by activity of the associated efferent axons of the gamma motor neurons; their rate of firing determines the degree of contraction. When the muscle spindles are relaxed, they are relatively insensitive to stretch. However, when the gamma motor neurons are active, they become shorter and hence become much more sensitive to changes in muscle length. This property of adjustable sensitivity simplifies the role of the brain in controlling movement. The more control that can occur in the spinal cord, the fewer messages must be sent to and from the brain.