Skeletal muscle contractions can be broadly separated into twitch, summation, and tetanic contractions. In a twitch contraction, a single action potential, whether from a motor neuron or from within the muscle, causes the muscle fiber to produce a single contraction and relaxation cycle called a twitch.uscle contractions can be described based on two variables: length and tension. A muscle contraction is described as isometric if the muscle tension changes but the muscle length remains the same. In contrast, a muscle contraction is isotonic if muscle length changes but the muscle tension remains the same.
Â© A Dictionary of Earth Sciences 1999, originally published by Oxford University Press 1999. Gutenberg discontinuity The seismic-velocity discontinuity between the Earth's mantle and core. The boundary is at a depth of about 2600 km and is thought to have surface irregularities of a few kilometres. Cite this article. Pick a style below, and copy the text for your bibliography.
The Gutenberg discontinuity separates the outer core and the mantle. Core: The Earth has a iron-nickel core that is about 2,100 miles in radius. The inner core may have a temperature up to about 13,000Â°F (7,200Â°C = 7,500 K), which is hotter than the surface of the Sun.he crust has a variable thickness, being 35-70 km thick in the continents and 5-10 km thick in the ocean basins. The crust is composed mainly of alumino-silicates. The topmost layer is the lithosphere, which is comprised of the crust and solid portion of the upper mantle.