Muscle spindle

Muscle spindles are sensory organs in the muscles that detect the strain state of skeletal muscle. They belong to the group of proprioceptors and PD sensors ( proportional and derivative property).

Muscle spindles also protect muscle from over stretching. When the rapid stretching of the muscle they trigger the so-called stretch reflex, causing the muscle to contract again. The doctor checks the correct function of the stretch reflex, for example, with patellar (knee -jerk ). With a light tap with a hammer below the kneecap to the thigh muscle is stretched briefly. The contraction of the muscle by the stretch reflex will only take place when the impact is already over. This contraction makes the lower leg forward fast.

Construction

Muscle spindles consist of five to ten, in humans one to three millimeters long, striated muscle fibers, which are surrounded by a connective tissue sheath. Between the muscle fibers of the leg extensor ( quadriceps femoris) in the thigh are five hundred to a thousand muscle spindles embedded, which are up to ten millimeters long. The more muscle spindles are present in a muscle, the fine associated with this movement can be adjusted.

Proprioceptive reflex

Lying receptor and effector in the same organ, the stimuli can be answered very quickly. The reflex is then referred to as proprioceptive reflex and the monosynaptic excitation transfer runs. One example is the patellar tendon.

Stretch reflex

The non- contractile middle of the muscle spindle afferent fibers from sensory nerve fibers, wound the Ia fibers. If the muscle is stretched, it is also the muscle spindle and thus the middle part, the so-called nuclear bag region, stretched, which in the Ia fibers, a signal ( action potential ) is generated. The signal is passed through the spinal nerve in the dorsal horn of the gray matter of the spinal cord and is transmitted via a synapse in the ventral horn monosynaptic on α - motoneurons, which cause the contraction of skeletal muscle fibers in the stretched muscle. The α - motor neuron diverges branches, one branch to the Renshaw cell is that to which they previously caused by its inhibitory effect innervating α - motoneuron that the contraction of each muscle is only briefly. By this negative feedback, a certain length of the muscle can be maintained constant in spite of disturbances.

The less muscle fibers are innervated by an α - motoneuron, the more finely tuned may be the movement: In case of eye and finger muscles a motoneuron supplies approximately 100 muscle fibers in other muscles up to 2000 (see Motorized Unit).

The line speed of the α - motor neurons is 80 to 120 ms - 1, the γ - motoneurons 40 ms -1.

Control of muscle length

About the so-called γ - spindle loop can control the muscle length. The muscle fibers of the muscle spindles ( intrafusal muscle fibers) are on the contractile ends, connected with motor nerve fibers, the γ - motoneurons. If this is enabled, the ends of the muscle spindle fibers contract. This, however, their center is stretched, the Ia fibers generate an action potential, which in turn led into the dorsal horn of the spinal cord and transferred to α - motoneurons. This triggers a contraction of skeletal muscle fibers, causing the muscle spindle and thus also the central part of the muscle spindle fibers is released. This continues until the Ia fibers perceive no more stretch.

Spindle break and compensation of the spindle break

The spindle break occurs during static muscular work. There are only the α - motoneurons arbitrarily activated later the γ - motoneurons ( α - γ - coactivation ). This leads to the following phenomena:

• The extrafusal fibers ( muscle fibers outside the muscle spindles) are shortened ( the pulse passes through the voluntary motor cortex, that is, over the pyramidal tract )
• The central part of the muscle spindle relaxed thereby.
• Since there is no more tension on the fibers is also no impulse conduction is more (spindle break). The receptor is not active at that moment.
• By activating the γ - motor neurons, the voltage of the muscle spindle is restored. The spindle break is canceled, and information about the muscle length can be sent again.

Sensitivity adjustment

The sensitivity adjustment is unintentional on the γ - motor neurons of the efferent system in collaboration with the afferent part. The spindle is the only receptor in the body that is efferent supplied, all others are only afferent supplies. A similar adaptation to a stimulus is found, however, even in the hair cells of the acoustic system.

Loop

The muscle spindles are elements of a complex control and regulation system ( to the basics, see System), which has the following meanings:

• Protection against over-stretching of the muscles by the stretch reflex
• Setting and maintaining a constant muscle tension ( tonus )
• Thereby maintaining a certain joint and body position
• Fine metering of movements by switching on and off of muscle fibers ( similar to a servo mechanism )

The motor centers of the brain as a guide member serve as a setpoint encoder for the length of muscles. The setpoint value is transmitted as the activity of the γ - control element muscle fibers to the spindle. In the muscle spindle, the actual value, the length of the muscle fibers, and therefore the length of the firmly attached to the muscle muscle spindle compared with the target value. If the value is lower, it means that the center of the muscle spindle fiber is stretched. This section of fiber is used as measuring element, its elongation is encoded as activity of the Ia nerve fibers and transmitted as a control value of the α - motor neurons to the muscle fibers. The contraction acts as a manipulated variable as long as the muscle spindle is reduced as far as to the center of the fiber is not stretched. As a disturbance affects each stretch the muscles, whether changes in position of the body, blow on the tendon or contraction of the antagonist.

α - and γ - motor neurons are connected to motor centers of the brain, so that muscle contractions can be controlled arbitrarily and involuntarily. For complex movements, such as walking, the brain changes the setpoints for different muscle groups according to the motion program.