Basic physiological properties of muscle tissue
Absolutely all types of muscle tissue possess three fundamental physiological properties that ensure their normal functioning and reaction to external or internal stimuli. These properties include:
- Excitability — the ability of tissue to enter a state of physiological activity in response to a stimulus.
- Conductivity — the ability to propagate the generated action potential (excitation) along the cell membrane.
- Contractility — a specific property of muscle tissue consisting in the ability of the muscle to shorten and increase its tension (tone) when exposed to threshold and suprathreshold stimuli.
Functions of skeletal muscles
Skeletal muscles perform a number of vital tasks in the body that are not limited solely to movement. There are four main functions:
- Motor function. Responsible for the direct movement of the entire body, as well as its individual parts in three-dimensional space.
- Static (postural) function. Ensures the maintenance of body balance in various spatial positions and during movement. This function is implemented by continuously maintaining basic muscle tone.
- Heat production. Muscles play a crucial role in the body's thermoregulation. Heat generation increases significantly both during targeted muscle contraction and involuntary muscle shivering in cold conditions.
- Pumping function. Rhythmic contractions of skeletal muscles exert mechanical pressure on the blood and lymph vessels passing through them, actively facilitating lymph flow in the body.
The "all-or-none" law and fiber recruitment
In normal physiology, there is the "all-or-none" law, but its applicability to muscle tissue depends on whether we are considering an individual cell or the organ as a whole.
An individual muscle fiber strictly obeys this law: when the threshold stimulus strength is reached, it responds with the maximum possible contraction, and does not contract at all under a subthreshold stimulus.
Unlike a single fiber, an intact skeletal muscle does not obey the "all-or-none" law. This is due to its heterogeneous structure and innervation principles:
- The muscle contains a vast number of fibers, and each of them has its own individual threshold of excitation.
- As the strength of the applied stimulus increases, recruitment occurs — an increasing number of muscle fibers with higher thresholds are involved in the process of excitation and subsequent contraction.
- A direct consequence of this mechanism is that the amplitude of the overall muscle contraction increases smoothly in proportion to the stimulus strength.
Key parameters of the muscle response
When evaluating the functional state and characteristics of a skeletal muscle's response to stimulation, physiologists analyze a number of basic parameters. These include:
- Duration — the time during which the contraction and relaxation cycle lasts.
- Frequency — the number of stimuli arriving at the muscle per unit of time.
- Amplitude — the degree of muscle shortening, which, as noted above, depends on the number of recruited fibers.
- Force — the tension developed by the muscle during contraction.
Briefly: answers to topical questions
What types of muscle contractions are distinguished depending on the stimulus frequency?
Depending on the frequency of stimulation, the following types of muscle contractions are distinguished:
- Single contractions — occur with infrequent impulses (low frequency), when each cycle ends with complete relaxation.
- Tetanic contractions (tetanus) — fused contractions that occur when the stimulation frequency increases. These include:
- Incomplete (fused/serrated) tetanus — occurs when the subsequent impulse arrives during the relaxation phase of the previous contraction.
- Complete (smooth) tetanus — occurs at a high frequency when impulses arrive during the shortening phase, making the contraction fused and sustained.
With an excessive increase in frequency, there may be a lack of response to each impulse (rhythm transformation) or pessimum.
What types of muscle fibers make up skeletal muscles?
Skeletal muscles contain different types of muscle fibers:
- Phasic — generate a full-fledged propagating action potential; among them, fast (white, glycolytic) and slow (red, oxidative) fibers are distinguished.
- Tonic — incapable of generating a full-fledged action potential.
- According to histochemical classification, there are red type I fibers, white type II fibers, and intermediate fibers.
- There are also intrafusal fibers located inside the muscle spindle, and extrafusal fibers — the fibers of the given muscle that are not part of the muscle spindle.
- Within motor units: slow S-units contain type I fibers; fast fatigable FF-units contain type IIB fibers; fast fatigue-resistant FR-units contain type IIA fibers.
What are the main modes of muscle contraction?
There are three main modes of muscle contraction based on changes in muscle length and tone:
- Isometric contraction — tension (tone) increases, but muscle length does not change.
- Isotonic contraction — the muscle shortens (length changes), but tone (tension) remains constant.
- Auxotonic (mixed) contraction — both muscle length and tension change. Under natural conditions, mixed contractions are the most common.
What is the mechanism of muscle contraction according to the sliding filament theory?
The mechanism of muscle contraction is based on the mutual sliding of actin and myosin filaments without changing their own length. The process involves a cross-bridge cycle using ATP hydrolysis energy:
- Upon excitation, the concentration of calcium ions increases, which bind to troponin. Troponin changes its conformation, exposing sites on actin.
- The cross-bridge (myosin head) attaches to actin.
- Inorganic phosphate is released, and the myosin head performs a power stroke, moving the actin filament toward the middle of the sarcomere.
- Binding of a new ATP molecule leads to the detachment of the cross-bridge from actin.
- The energy of ATP hydrolysis transitions myosin into a high-energy conformation for a new cycle.
To memorize the main functions of skeletal muscles, use the mnemonic: Movement Creates Heat Pump (Motor, Static, Heat production, Pumping).
Class and exam questions
Why does a whole muscle not obey the "all-or-none" law?
Because an intact muscle consists of many muscle fibers, each of which has its individual excitation threshold. When stimulus strength increases, a larger number of fibers are excited, leading to a smooth increase in contraction amplitude.
What is meant by contractility in physiology?
Contractility is the ability of muscle tissue to change its length (shorten) and increase tone when exposed to a threshold or suprathreshold stimulus.
How do muscles participate in lymph flow?
Skeletal muscles perform a pumping function. During contraction, muscles thicken and compress lymphatic vessels, pushing lymph forward.
What else to study on this topic
- Mechanisms of muscle fiber recruitment with increasing stimulus intensity
- Influence of stimulus parameters (duration, frequency, amplitude, strength) on the nature of contraction
- The role of muscle tone in maintaining static (postural) function
- Differences between threshold and suprathreshold stimulation for muscle tissue
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