🧑⚕️9.11 SU MedSchool: Exploring Skeletal Muscle Contradiction Through Stimulus Intensity and Frequency
It is a privilege to serve as a teaching assistant in the Fall functional physiology lab at West China University (Sichuan University) due to my previous experience with experiments and medical practice. Today's lab focuses on the relationship between stimulus intensity, stimulus frequency, and skeletal muscle contraction.
Purpose and Principle:
The purpose of this experiment is to explore the principles and methods of studying the relationship between stimulus intensity, stimulus frequency, and skeletal muscle contraction. This includes successfully preparing a viable frog sciatic nerve-gastrocnemius specimen, recording and determining the relationship between stimulus intensity and muscle contraction amplitude, and the relationship between stimulus frequency and muscle contraction patterns. We will integrate the knowledge learned to explain the underlying mechanisms.
Both nerve and muscle tissues are excitable tissues. The excitation of motor neurons triggers muscle contraction. However, the excitability of different tissues and cells varies. A functional unit, known as a motor unit, is composed of a single motor neuron fiber and the skeletal muscle fibers it innervates. The sciatic nerve-gastrocnemius specimen is composed of many intertwined motor units.
If the stimulus intensity is too low, no muscle contraction will occur. When the intensity reaches a certain threshold, only a few highly excitable nerve fibers are activated, causing slight contractions of the skeletal muscle fibers they innervate. This threshold intensity is called the threshold stimulus. As stimulus intensity increases, more motor units will be activated, resulting in a progressive increase in muscle contraction amplitude or tension. This type of stimulus is called supraliminal stimulus. However, when the intensity reaches a certain critical value, all motor units in the specimen are activated, and the muscle contraction amplitude or tension reaches its maximum. Beyond this point, further increases in stimulus intensity will not result in an increase in muscle contraction amplitude. The optimal stimulus intensity is the minimum stimulus intensity required to induce the maximum response, also known as the adequate stimulus.
The total duration of a single skeletal muscle contraction includes the latent period, contraction period, and relaxation period. If a series of continuous stimuli are applied at a frequency such that the interval between two stimuli is shorter than the total contraction time but longer than the latent and contraction periods, a compound contraction will occur. If the interval between stimuli is shorter than the total contraction time but longer than the latent period, new contractions will start before the muscle fully relaxes, leading to incomplete tetanic contraction. The contraction amplitude will be higher than that of a single contraction. If the interval between stimuli is shorter than the latent and contraction periods, the muscle will begin a new contraction while the previous one is still in the contraction phase, leading to complete tetanic contraction, with an amplitude higher than that of incomplete tetanic contraction.
Experimental Subject:
Frogs (or toads).
Experimental Equipment and Chemicals:
Frog surgical instruments (frog board, anesthetic board, scissors, surgical forceps, probes, glass needles, frog pins, fine thread, droppers), tension transducer, biological signal acquisition and processing system, stimulating electrodes, Ringer's solution, Petri dishes (250 mL), culture dishes, and other related materials.
Lab Questions for Students:
How can threshold stimuli, subthreshold stimuli, suprathreshold stimuli, and optimal stimuli be distinguished? Why does muscle contraction amplitude increase with increasing stimulus intensity up to the optimal stimulus, but then remain constant beyond the optimal stimulus?
What factors constitute the latent period of a single contraction? What differences are observed in specimens with and without nerves?
How can the threshold stimulus and optimal stimulus for the specimen be accurately identified? How can the critical frequency for incomplete and complete tetanic contractions be accurately determined?