Muscle spindles and tendon organs

Structure and functions of muscle spindles

Muscle spindles are intrinsic muscle receptors that respond to muscle length and the rate of change of that length. The higher the rate of lengthening of the muscle belly, the higher the frequency of impulses from the receptor.

Anatomical features:

• The structure includes a connective tissue capsule enclosing specialized intrafusal muscle fibers.
• The receptor is attached by one end to the muscle fascia and by the other to the tendon.
• Spindles are located strictly parallel to the main working (extrafusal) fibers.
• The number of these receptors in a muscle directly depends on its function: the more precise and delicate the movements performed by the muscle, the more spindles it contains.

Innervation and the $\gamma$-system

The connection between the muscle spindle and the central nervous system is provided by sensory (afferent) and motor (efferent) pathways.

The afferent limb is represented by two types of stretch receptors:

1. Group Ia afferents — have a high impulse conduction velocity (up to 120 m/s).
2. Group II afferents — conduct excitation more slowly (up to 70 m/s).

The efferent limb is formed by the processes of $\gamma$-motoneurons, which approach the intrafusal fibers. This motor $\gamma$-system is divided into two branches:

• Static $\gamma$-efferents: terminate on intrafusal fibers with a nuclear bag. They are activated during static loads, leading to an increase in the static responses of both afferent groups (Ia and II).
• Dynamic $\gamma$-efferents: approach fibers with a nuclear chain. They are excited during dynamic loads, resulting in a secondary amplification of the dynamic responses of group Ia afferents.

Myotatic reflex and tone regulation

Muscle tone is regulated through the interaction of muscle spindles and spinal cord $\alpha$-motoneurons via the myotatic reflex mechanism.

How the reflex works:

1. Stimulus: passive muscle stretch occurs.
2. Signal transmission: excitation from $\gamma$-afferent fibers is directed through the dorsal roots into the spinal cord.
3. Central processing: the signal is relayed directly to $\alpha$-motoneurons of the ventral horns of the spinal cord within the same segment.
4. Effect: the excitability of $\alpha$-motoneurons increases, leading to enhanced tone and muscle contraction.

A feedback mechanism also exists. When a muscle contracts, the sensory endings inside the spindles cease to be stretched and are not excited. The stream of stimulating impulses to $\alpha$-motoneurons stops, their excitability drops, and muscle tone naturally decreases.

Golgi tendon organs

Unlike spindles, Golgi receptors are localized in muscle tendons and are responsible for preventing excessive tension in muscle tissue. They are about 1 mm long and up to 100 µm in diameter. They are relatively few in number within tendons.

The main structural feature is that they are connected in series with extrafusal muscle fibers. Innervation is provided by fast group Ib afferent fibers (speeds up to 120 m/s).

These receptors respond to both stretch and contraction, but the maximum response occurs specifically during active muscle contraction.

Inverse myotatic reflex (inhibitory mechanism):

• The signal from activated Golgi receptors enters the spinal cord.
• With the participation of inhibitory Renshaw cells, the activity of $\alpha$-motoneurons of the contracting muscle itself (the agonist) and its synergists is suppressed.
• Simultaneously, some of the impulses are directed to the motoneurons of antagonistic muscles, causing their excitation and activation.

Joint receptors

Joint receptors also make a significant contribution to maintaining proprioception and muscle tone.

The stimulus for their activation is the stretching of joint capsules. Information is coded in such a way that the level of receptor activation strictly corresponds to the angle of limb rotation in a specific joint. Their sensitivity threshold triggers at displacements ranging from 2–3° to 30°.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Morphological differences between nuclear bag and nuclear chain intrafusal fibers
• Interaction of $\alpha$- and $\gamma$-motoneurons (coactivation) during voluntary movements
• Analysis of electromyography (EMG) and mechanogram during muscle receptor activity
• Detailed mechanisms of proprioceptive information encoding by joint receptors

Other topics in the "Motor Control" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.