Muscle spindles are sensory organs in the muscles that record the state of stretch of the skeletal muscles . They belong to the group of proprioceptors and are PD sensors (proportional and differential property).
Muscle spindles also protect muscles from overstretching. When the muscle is suddenly stretched, they trigger the so-called stretch reflex, which causes the muscle to contract again. The doctor checks the correct function of the stretch reflex, for example with the patellar tendon reflex ( hamstring reflex ). The thigh muscle is briefly stretched by tapping gently with a hammer below the kneecap. However, the muscle does not contract through the stretch reflex until the stroke is over. This contraction causes the lower leg to snap forward.
Muscle spindles consist of five to ten, in humans one to three millimeters long, striated muscle fibers that are surrounded by a sheath of connective tissue . Between the muscle fibers of the stirrup ( quadriceps femoris ) in the thigh are five hundred to a thousand muscle spindles embedded that are up to ten millimeters long. The more muscle spindles there are in a muscle, the more finely the movements associated with it can be coordinated.
If the receptor and effector are in the same organ, the stimuli can be answered very quickly. The reflex is then called the self-reflex and the transmission of excitation is monosynaptic. One example is the patellar tendon reflex.
The non-contractile middle of the muscle spindle fibers is wrapped in afferent sensory nerve fibers , the Ia fibers . If the muscle is stretched, the muscle spindle and thus the middle part, the so-called core sac region, is also stretched, which generates a signal ( action potential ) in the Ia fibers . The signal is passed on via the spinal nerve to the dorsal horn of the gray matter of the spinal cord and is transmitted monosynaptically via a synapse in the anterior horn to α- motor neurons , which cause the skeletal muscle fibers to contract in the stretched muscle. The α-motor neuron diverges, branches out, with a branch going to the Renshaw cell , which, through its inhibitory influence on the α-motor neuron, which previously innervated it, causes the respective muscle to contract only briefly. Due to this negative feedback , a certain muscle length can be constantly maintained despite disturbances.
The fewer muscle fibers that are innervated by an α-motor neuron, the more finely tuned the movement can be: In eye and finger muscles, one motor neuron supplies around 100 muscle fibers, in other muscles up to 2000 (see motor unit ).
The conduction velocity of the α motor neurons is 80 to 120 ms −1 , that of the γ motor neurons 40 ms −1 .
Control of muscle length
The muscle length can be controlled via the so-called γ-spindle loop. The muscle fibers of the muscle spindles ( intrafusal muscle fibers ) are connected at the contractile ends with motor nerve fibers , the γ motor neurons . When these are activated, the ends of the muscle spindle fibers contract. As a result, however, their middle is stretched, the Ia fibers generate an action potential , which in turn is conducted into the anterior horn of the spinal cord and transferred to α motor neurons . These trigger a contraction of the skeletal muscle fibers, which relaxes the muscle spindle and thus also the middle part of the muscle spindle fibers. This continues until the Ia fibers no longer perceive any stretching.
Spindle pause and compensation of the spindle pause
The spindle break occurs during static muscle work. First the α- motor neurons are activated at will, then later the γ-motor neurons (α-γ co-activation). This leads to the following phenomena:
- The extrafusal fibers (muscle fibers outside the muscle spindles) shorten (the impulse runs through the voluntary motor cortex , that is, over the pyramidal tract )
- The middle part of the muscle spindle slackens.
- Since there is no longer any tension on the fibers, there is no longer any impulse transmission (spindle pause). The receptor is inactive at this moment.
- By activating the γ motor neurons, the tension of the muscle spindle is restored. The spindle pause is canceled and information about the muscle length can be sent again.
The sensitivity regulation takes place unconsciously via the γ-motor neurons of the efferent system in cooperation with the afferent part. The spindle is the only receptor in the body that is supplied efferent , all others are only supplied afferent . A similar adaptation to a stimulus can also be found in the hair cells of the acoustic system.
The muscle spindles are elements of a complex control and regulation system (for the basics see system ), which has the following meanings:
- Protection against overstretching the muscles through the stretch reflex
- Setting and maintaining constant muscle tension ( tone )
- thereby maintaining a certain joint and body position
- Fine adjustment of movements by switching muscle fibers on and off (comparable to a servo mechanism )
The motor centers of the brain as a guide link serve as setpoint generators for the length of muscles. The target value is defined as activity of the γ-fibers to the control element passed muscle spindle. In the muscle spindle, the actual value , the length of the muscle fibers and thus the length of the muscle spindle firmly connected to the muscle fibers, is compared with the target value. If the actual value is smaller, it means that the middle of the muscle spindle fibers is stretched. This fiber section serves as a measuring element ; its stretching is coded as the activity of the Ia nerve fibers and transmitted as a control value to the muscle fibers via the α motor neurons. Their contraction acts as a manipulated variable until the muscle spindle is shortened to such an extent that the fiber center is no longer stretched. Any stretching of the muscles acts as a disturbance , whether it is a change in the position of the body, a blow to the tendon or a contraction of the antagonist .
α and γ motor neurons are connected to motor centers in the brain so that muscle contractions can be controlled voluntarily and involuntarily. In the case of complex movement sequences, such as walking, the brain changes the target values for different muscle groups according to the exercise program.
- Rainer Klinke, Hans-Christian Pape, Stefan Silbernagl (eds.): Textbook of Physiology. 5th edition. Thieme, Stuttgart 2005, ISBN 3-13-796003-7 , pp. 743/744.
- Th. H. Schiebler (Ed.): Anatomie. 9th edition. Springer-Verlag, 2005, ISBN 3-540-21966-8 .