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An example of normal spontaneous depolarization is the constant contractions of cardiac muscle, causing the heart to beat. Usually, intentional movement of the involved muscle causes fasciculations to cease immediately, but they may return once the muscle is at rest again. Tics must also be distinguished from fasciculations.
Depiction of smooth muscle contraction. Muscle contraction is the activation of tension-generating sites within muscle cells. [1] [2] In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. [1]
Once arousal has peaked (1:35), the penis returns to its flaccid state, the scrotum loosens, and the testicles descend. Please note that the arousal/orgasm process can take anywhere from a few seconds to several hours, and this video is intended to demonstrate the typical process, not the typical duration (see an alternate example here).
A myoclonic seizure (myo "muscle", clonic "jerk") is a sudden involuntary contraction of muscle groups. The muscle jerks consist of symmetric, mostly generalized jerks, localized in the arms and in the shoulders and also simultaneously with a head nod; both the arms may fling out together and simultaneously a head nod may occur.
A secondary cause of muscle atrophy is that denervated muscles are no longer supplied with trophic factors from the α-MNs that innervate them. Alpha motor neuron lesions also result in abnormal EMG potentials (e.g., fibrillation potentials) and fasciculations, the latter being spontaneous, involuntary muscle contractions.
Myoclonic dystonia or Myoclonus dystonia syndrome is a rare movement disorder that induces spontaneous muscle contraction causing abnormal posture. The prevalence of myoclonus dystonia has not been reported, however, this disorder falls under the umbrella of movement disorders which affect thousands worldwide. [1]
Stretch of the muscle membrane opens a stretch-activated ion channel. The cells then become depolarized and this results in a Ca 2+ signal and triggers muscle contraction. No action potential is necessary here; the level of entered calcium affects the level of contraction proportionally and causes tonic contraction.
The origin of a muscle is the bone, typically proximal, which has greater mass and is more stable during a contraction than a muscle's insertion. [14] For example, with the latissimus dorsi muscle, the origin site is the torso, and the insertion is the arm. When this muscle contracts, normally the arm moves due to having less mass than the torso.