Physiology of muscle contraction

Tendon receptors and protection against overstrain

Golgi tendon organs play a key role in the self-regulation of muscle tone. These are small structures (about 1 mm long and up to 100 µm in diameter) located within tendons and connected in series with extrafusal muscle fibers.

Their primary task is to prevent excessive tension and potential tissue rupture.

• Activation: The receptors respond to both stretch and contraction, but the maximal response occurs specifically during active muscle contraction.
• Inverse myotatic reflex: Upon strong excitation, the signal travels via fast afferent fibers (group Ib, conduction velocity up to 120 m/s) to the spinal cord. There, via inhibitory Renshaw cells, the activity of alpha motor neurons of the working muscle (agonist) and its synergists is suppressed. Simultaneously, some impulses excite motor neurons of antagonist muscles, causing them to activate.

Specific features of smooth muscle tissue

Smooth musculature of internal organs (stomach, gallbladder, urinary bladder, sphincters) differs cardinally from skeletal muscle. It is characterized by low excitability, slow signal conduction velocity, and extremely low labileness.

1. Single contraction: Develops slowly and lasts a long time, yet is capable of achieving significant force. This is critically important for the reliable sphincter closure function.
2. Automaticity: Smooth muscles are capable of spontaneous excitation without external stimuli. Even upon complete denervation (severing connections with the CNS), they retain their tone and ability to contract.
3. Conduction of excitation: The signal is transmitted from cell to cell via specialized gap junctions — nexuses. Because of this, smooth muscle does not obey the law of isolated conduction. It responds to a stimulus as a single whole, obeying the "all-or-none" law (unlike skeletal musculature).
4. Chemical sensitivity: Smooth muscles possess heightened sensitivity to chemical substances. While skeletal fibers contract only under the action of nerve impulses, smooth muscles respond to chemical factors, neural influences, and possess intrinsic automaticity.

Plasticity and response to stretch

A crucial property of hollow organs is plastic tone (plasticity). This is the ability of the tissue to maintain a new length imparted to it by slow stretching, without a significant change in internal tension.

• If an organ is stretched slowly, plasticity manifests: it fills up without increasing internal pressure (for example, during the gradual accumulation of fluid in the urinary bladder).
• If stretching occurs rapidly, it acts as an adequate stimulus. A sharp contraction of the muscular walls occurs, aimed at emptying the organ.

For comparison: skeletal musculature possesses elasticity — after stretching, it always tends to return to its initial length.

Basics of skeletal muscle contraction

The work of skeletal musculature is based on a strict structural and functional hierarchy: from the muscle fiber and myofibril to the basic contractile unit — the sarcomere.

• Initiation of the contractile act is impossible without the participation of calcium ions (Ca2+).
• Depending on loading conditions, isotonic, isometric, and auxotonic contraction regimes of the muscle are distinguished.
• The amplitude of the muscle response directly depends on the strength of the stimulus and the initial length of the sarcomere.
• At high stimulation frequencies, single responses fuse, forming a prolonged, powerful contraction — tetanus.
• The functional control unit is the motor unit. The number of muscle fibers in it strictly depends on the specific muscle's function (fewer for precise movements, more for coarse ones).

Class and exam questions

What else to study on this topic

• The mechanism of muscle contraction and its stages at the myofilament level
• The role of calcium ions in initiating the contractile act
• Types of tetanus and mechanisms of tetanic contractions
• Chemical and thermal processes during muscle work
• The concept of absolute and total muscle strength, basics of dynamometry

Other topics in the "Motor Control" section

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