Muscle spindles are sensory organs that belong to the proprioceptor group and detect the state of stretch and change in stretch of skeletal muscles and deliver the generated signals to fast afferent Ia nerve fibers. Muscle spindles also have efferent nerve connections that control their sensitivity. Via the gamma spindle loop, muscle spindles also serve to control muscle length and associated muscle contractions.
What is a muscle spindle?
Muscle spindles, in their capacity as sensors of the state of stretch of skeletal muscles, belong to the group of proprioceptors, with the help of which a positional picture of the position of individual limbs and the body is created in the corresponding brain centers. At the same time, the positional image and the muscle spindles are used for the control of conscious and unconscious movements – including the control of muscle reflexes. Muscle spindles have proportional and differential properties as sensors. This means that they detect both static stretch states of individual muscles and the dynamic rate of change of their stretch and transmit them via afferent Ia nerve fibers, which have the highest conduction velocity in the human body. The frequency distribution of muscle spindles in individual skeletal muscles provides a measure of fine or gross motor control capabilities of the muscle. For example, the quadriceps (Musculus quadriceps femoris), which is a leg extensor attached to the front of the thigh, has 500 to 1,000 muscle spindles. They are embedded between the muscle fibers of the skeletal muscles, parallel to the orientation of the muscle fibers, and reach a length of 1 to 3 millimeters.
Anatomy and structure
The core of the muscle spindles is formed by a bundle of five to ten striated intrafusal muscle fibers and are encased in a connective tissue sheath. Intrafusal muscle fibers are found exclusively in muscle spindles. Their distinctive feature is that they are contractile, i.e., active, at each of their ends, while their midsection is extensible and passively adapts to the state of stretch of the skeletal muscle. The passive middle part of the muscle spindles consists of core sac fibers and core chain fibers. When the muscle contracts, the muscle spindle also shortens. The core sac fibers bulge a little, causing the central part of the muscle spindle to thicken. To capture the dynamics of change, the core sac fibers are wrapped exclusively by fast-conducting afferent Ia nerve fibers, which respond to any change in thickness. The core chain fibers, which detect the more static stretch state of the muscle, are also connected to Ia nerve fibers, but are additionally connected to class II afferent fibers as secondary innervation. Class II fibers have lower sensitivity and conduct impulses more slowly than Ia fibers. The two contractile terminals of the intrafusal muscle fibers are connected to efferent gamma neurons, through which the sensitivity of the muscle spindles and the target of muscle contraction are controlled.
Function and tasks
Muscle spindles simultaneously perform multiple tasks and functions to coordinate gross and fine motor movements, establish and maintain static postures, and protect individual skeletal muscles from overstretching. Muscle spindles are thus part of a complex control and regulation system. Coordinated movement requires that specific muscles each assume a predetermined static stretch state or follow a predetermined dynamic change in stretch state. The motor centers of the brain can perform these tasks because the muscle spindles simultaneously perform the passive function of a sensor and the active role of a target for the muscle. Via the contractile terminals of the intrafusal muscle fibers, the muscle spindles can follow and adapt to the respective stretch state of the muscles or generate the set point for the muscle. The length of the muscle is changed by appropriate contraction commands in such a way that a 0-potential is created with respect to the muscle spindle. In this case, the muscle adapts to the muscle spindle and not vice versa. To fulfill their protective function against overstretching of the muscles, muscle spindles take over the control of involuntary stretch reflexes.As soon as the stretch state of a muscle exceeds a certain threshold value, which is detected by the muscle spindles, this triggers an involuntary contraction signal to the muscle concerned, which is also controlled by the muscle spindles. A typical example of such a contraction reflex is the patellar tendon reflex. A brief blow with the reflex hammer on the patellar tendon below the kneecap briefly signals overstretching of the quadriceps, which leads to the contraction reflex as the lower leg performs an involuntary twitch in the direction of leg extension.
Diseases
Independent morphologic diseases explicitly affecting muscle spindles are not known. This is probably due to the fact that muscle spindles are specialized muscle fibers that tend to follow diseases of the muscles in which they are embedded. First and foremost are muscular atrophies caused by underuse of the muscles. The corresponding muscle regresses as a result of the underuse and, in parallel, the muscle spindles also regress. Muscle atrophy is often caused by nervous diseases or by injury to the corresponding motor neurons, from which the muscle can no longer receive impulses. An example of neurogenically induced muscle atrophy is amyotrophic lateral sclerosis (ALS). This is a non-curable degenerative disease of the motor nervous system. Another rare disease is spinal muscular atrophy, which is caused by a gradually progressive loss of motor nerves in the anterior horn of the spinal cord. A number of diseases that result in changes in the motor endplates in the intrafusal muscle fibers of the muscle spindles are also due to neurological disorders and diseases. There is a cross-link between combating Alzheimer’s disease and the functioning of muscle spindles. A group of researchers in Berlin has found that the enzyme beta-secretase, which is blamed for the harmful protein deposits in Alzheimer’s, is apparently important for the functional efficiency of muscle spindles, so suppression of the enzyme in Alzheimer’s patients would also be expected to cause coordination disorders in movement.
