Edit 2: I found here a good and simple ppt/pdf on AP propagation and muscle contraction. A graduated muscle response works as follows: if the fibers are stimulated while a previous contraction is still occurring, the second contraction is stronger. This response is called wave summation because the excitation-contraction coupling effects of successive motor neuron signaling are added or added together (Figure 10.4.4a). At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate more cross bridges while the muscle contracts again from the first stimulus. Summation leads to a stronger contraction of the motor unit. A single action potential of a motor neuron creates a single contraction in the muscle fibers innervated by the motor neuron. This isolated contraction is called contraction. A contraction can last between a few milliseconds and 100 milliseconds, depending on the type of muscle fiber. The voltage generated by a single contraction can be measured by a myogram, an instrument that measures the voltage generated over time (Figure 10.4.3). The invention of modern dry cells dates back to studies of frog muscle by the Italian anatomist Luigi Galvani (1737-98). He hung isolated frog legs on a copper hook and noticed that they contracted when hit by an iron scalpel. He attributed this to “animal electricity” in the legs. The physicist Alessandro Volta (1745-1827) further studied Galvani`s discovery.

He concluded that when two different metals (such as the copper hook and the iron scalpel) are separated by an electrolyte solution (tissue fluids of a frog), a chemical reaction occurs that creates an electric current. This current had stimulated the muscle in the legs of the Galvani frogs and caused the contractions. Based on this principle, Volta invented the first simple voltaic cell, the precursor of today`s dry cells. Muscle tone is achieved through a complex interaction between the nervous system and skeletal muscles, which leads to the activation of certain motor units simultaneously, most likely cyclically. In this way, the muscles never get tired completely, because some motor units are in a state of recovery, while others actively create tension. To move an object called a strain, the muscle fibers of a skeletal muscle must shorten. The force generated by a contracting muscle is called muscle tension. Muscle tension can also be generated when muscle contracts against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions (Figure 10.4.1). Figure 11.14 Relationship between stimulus intensity (tension) and muscle tension. Weak stimuli (1-2) do not stimulate nerve fibers and therefore do not create muscle contraction. When stimuli reach or exceed the threshold (3-7), they stimulate more and more nerve fibers and motor units, creating ever stronger contractions. This is the sum of several motor units (recruitment).

Once all the nerve fibers are stimulated (7-9), an additional increase in irritating power does not lead to a further increase in muscle tension. The size of a motor unit determines its function. A small motor unit, consisting of a motor neuron and only a few muscle fibers, allows a very fine motor control of a muscle. For example, extraocular eye muscles have thousands of muscle fibers, each with 5-10 fibers, provided by a single motor neuron; This allows for exquisite control of eye movements, allowing both eyes to quickly focus on an object. Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for gripping, texting, etc. The absence of the weak contractions that lead to muscle tone is called hypotension or atrophy and can result from damage to parts of the central nervous system (CNS) such as the cerebellum or the loss of innervations on skelet muscle, as in poliomyelitis. Hypotonic muscles have a flaccid appearance and have functional impairments, such as .B. weak reflexes. Conversely, excessive muscle tone is called hypertension, accompanied by hyperreflexia (excessive reflex reactions), often the result of damage to the upper motor neurons of the CNS. Hypertension can occur with muscle rigidity (as seen in Parkinson`s disease) or spasticity, a phasic change in muscle tone, in which a limb “folds” from passive stretching (as in some strokes).

In isotonic contractions, where the tension in the muscle remains relatively constant, a load is shifted as the length of the muscle changes. With concentric contraction, the muscle shortens to move a load. An example of this is the contraction of the biceps brachii muscle when a weight from the hand is brought upwards to the body. Eccentric contraction occurs when muscle tension decreases and a muscle lengthens. This type of contraction is observed when the same hand weight is slowly and controlled lowered by the biceps brachii. Most of the body`s actions are the result of a combination of isotonic and isometric contractions that work together to achieve a wide range of results. These muscular activities are under the control of the nervous system. A crucial aspect of controlling the nervous system of skeletal muscles is the role of motor units.

If necessary, the maximum number of motor units in a muscle can be recruited at the same time, creating the maximum contraction force for that muscle, but it may not take very long due to the energy required to maintain the contraction. To prevent complete muscle fatigue, not all motor units are usually active at the same time, but some motor units rest while others are active, allowing for longer muscle contractions. The nervous system therefore uses recruitment as a mechanism to effectively use skeletal muscle. The following factors contribute to the strength and maximum duration of a muscle contraction: Most muscles in the human body have a mixture of small and large motor units, which gives the nervous system wide control over the muscle. The smaller motor units of a muscle have motor neurons that are more excitable. The initial activation of these smaller motor units results in a relatively low tension generated in a muscle. When more power is needed, larger motor units are used to generate more voltage. This process, in which additional motor units are called to generate more tension, is called recruitment. This process allows a muscle like the biceps brachii to pick up a feather with minimal strength generation, rather than picking up a heavy weight that requires much greater strength generation. 2. What factors contribute to the amount of tension generated in a single muscle fiber? Three phases are known for muscle contraction. The first phase is the latency period, during which the action potential spreads along the sarcolemma and Ca++ ions are released from the sarcoplasmic reticulum.

This is the phase where excitation and contraction are coupled, but contraction has not yet taken place. The contraction phase occurs when the muscle creates increasing tension; Ca++ ions in sarcoplasm are bound to troponin, tropomyosin has moved away from actin binding sites, bridges have formed, and sarcomeres are actively shortening. The final phase is the relaxation phase, in which the tension decreases when the Ca++ ions are pumped from the sarcoplasm into the sarcoplasmic reticulum and the muscle fibers are returned to their resting state. Muscle contraction events that describe the concept of sliding filament are listed as follows. As already mentioned, the contraction of skeletal muscle fibers is triggered by the signaling of a motor neuron. Each muscle fiber is innervated by a single motor neuron, but a single motor neuron can innervate several muscle fibers. A motor unit is defined as a single motor neuron and all the muscle fibers it innervates (Figure 10.4.1a). Isometric contraction occurs when a muscle creates tension without changing the length of the muscle. Isometric contractions involve a shortening of the sarcoma and an increase in muscle tension, but do not displace the load because the force generated cannot overcome the resistance provided by the load. For example, if you try to lift a hand weight that is too heavy, there will be activation and shortening of the sarcoma to a certain point and ever-increasing muscle tension, but no change in the position of the hand weight. In everyday life, isometric contractions are active in maintaining posture and maintaining the stability of bones and joints. Once the elastic components of the series are tightened, the muscle begins to create external tension and move a resistant object or load.

This is called the contraction phase of contractions. In the gastrocnemic preparation of frog, the charge is the sensor of the recording device; In the body, it is usually a bone. .