Structure and Function of Muscle Spindles
Muscle spindles are intrinsic muscle receptors that respond to muscle length and the rate of change in length. The higher the rate of elongation of the muscle belly, the higher the frequency of action potentials generated by the receptor.
Anatomical Features:
- The structure includes a connective tissue capsule enclosing specialized intrafusal muscle fibers.
- The receptor is anchored at one end to the muscle fascia and at the other to a tendon.
- Spindles are located strictly in parallel with the main working (extrafusal) fibers.
- The density of these receptors in a muscle correlates directly with its function: muscles responsible for precise, delicate movements contain a higher density of spindles.
Innervation and the $\gamma$-System
The connection between the muscle spindle and the central nervous system is mediated by sensory (afferent) and motor (efferent) pathways.
Afferent Component: Represented by two types of stretch receptors:
- Group Ia afferents — characterized by high conduction velocity (up to 120 m/s).
- Group II afferents — conduct signals more slowly (up to 70 m/s).
Efferent Component: Formed by the axons of $\gamma$-motor neurons, which innervate the intrafusal fibers. This motor $\gamma$-system is divided into two branches:
- Static $\gamma$-efferents: terminate on nuclear bag intrafusal fibers. They are activated during static loading, increasing the static response of both Ia and II afferents.
- Dynamic $\gamma$-efferents: innervate nuclear chain fibers. They are activated during dynamic movements, selectively enhancing 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 $\alpha$-motor neurons via the myotatic (stretch) reflex mechanism.
Mechanism of Action:
- Stimulus: Passive stretch of the muscle occurs.
- Signal Transmission: Excitation from the spindle afferents enters the spinal cord via dorsal roots.
- Central Processing: The signal synapses directly onto $\alpha$-motor neurons in the ventral horns of the same spinal segment.
- Effect: The excitability of the $\alpha$-motor neurons increases, leading to reflexive contraction and increased muscle tone.
Conversely, a negative feedback loop operates during contraction: when the muscle shortens, the sensory endings within the spindle are unloaded and stop firing. The stimulatory drive to the $\alpha$-motor neurons ceases, their excitability drops, and muscle tone decreases naturally.
Golgi Tendon Organs
Unlike muscle spindles, Golgi tendon organs are located in muscle tendons and protect muscle tissue from excessive tension. They measure about 1 mm in length and up to 100 µm in diameter, and are relatively sparse within tendons.
A key structural feature is that they are connected in series with extrafusal muscle fibers. They are innervated by fast Group Ib afferent fibers (conduction velocity up to 120 m/s).
These receptors respond to both stretch and contraction, but their maximal response occurs during active muscle contraction.
Inverse Myotatic Reflex (Inhibitory Mechanism):
- Signals from activated Golgi tendon organs travel to the spinal cord.
- Via inhibitory interneurons (such as Renshaw cells), the activity of $\alpha$-motor neurons supplying the contracting muscle (agonist) and its synergists is suppressed.
- Simultaneously, excitatory impulses are directed to the motor neurons of antagonistic muscles, causing their activation.
Joint Receptors
Joint receptors also make a significant contribution to proprioception and muscle tone.
They are activated by the mechanical distortion and stretching of joint capsules. Information is encoded such that the level of receptor activation directly reflects the specific joint angle. Their threshold of sensitivity ranges from angular displacements of 2–3° up to 30°.