Diffen âº Science âº Biology âº Microbiology. Fraternal twins are âdizygotic,â meaning that they developed from two different eggs fertilized by two different sperm cells, while identical twins are âmonozygoticâ i.e., they developed from a single fertilized egg that split.
A T-tubule (or transverse tubule) is a deep invagination of the sarcolemma, which is the plasma membrane of skeletal muscle and cardiac muscle cells.These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell.ransverse tubules are invaginations of the sarcolemma. Though the invaginations are normally perpendicular to the length of the fiber, many T-tubules can lie axial to the long-axis of the fiber, both wrapping around and running alongside myofilament bundles.
One motor neuron (nerve cell) may stimulate a few muscle cells or hundreds of them, depending on the. particular muscle and the work it does. One neuron and all the skeletal muscle cells it stimulates are a. motor unit. When a long threadlike extension of the neuron, called the nerve fiber or axon, reaches the. muscle, it branches into a number of axon terminals, each of which forms junctions with the. sarcolemma of a different muscle cell. These junctions are called neuromuscular (literally, ânerve-. muscleâ) junctions.