Musculoskeletal Key

Muscle fibers that must remain active over a long time (type I or slow-twitch fibers) are rich in mitochondria, whose iron-containing cytochrome oxidase enzymes and abundant myoglobin content give the…

The voltage-dependent sodium channel, which mediates the muscle action potential, is also time dependent: prolonged depolarization inactivates the action potential. This can be reversed only by a repolarization of the…

The tension of the twitch can be measured under conditions in which the muscle is not allowed to shorten (isometric contraction). Because all the sarcomeres are activated, the single-twitch strength…

The membrane of the nerve terminal has a different assortment of ion channels: fewer sodium channels, several types of potassium channels, and, most important, voltage-dependent calcium channels. When an action…

In mammalian skeletal muscle, each muscle fiber is innervated by only one motor neuron. The combination of a single motor neuron and all the muscle fibers it innervates is called…

However, for the action potential to affect the sarcoplasmic reticulum deep within the muscle fiber, it must be propagated inward as well as along the surface. This is accomplished through…

The muscle fiber is surrounded by a connective tissue basement membrane that continues into the synaptic cleft, sending extensions into the junctional folds. The basement membrane contains a large amount…

The immediate trigger for muscular contraction is a sudden increase in the concentration of calcium ions in the cytoplasm of the muscle fiber, the sarcoplasm. To prevent the muscle from…

When the muscle fiber is electrically excited, calcium (Ca2+) ions released from the sarcoplasmic reticulum bind to the troponin C subunit of the troponin molecules on the actin filaments, with…

The contraction of a muscle fiber in response to a single nerve impulse is called a twitch. Under a given set of starting conditions, the force of a single fiber’s…