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.
In order for skeletal muscle cells (muscle fibers) to contract, it is imperative they have communication with a motor neuron. A motor neuron is a nerve cell whose cell body is located in the spinal cord or brain. The axon of the nerve cell meanders toward the muscle it supplies. As it reaches the muscle, it branches: each branch going to a muscle fiber. The motor neuron, its axon branches, and all the muscle fibers it supplies is known as a motor unit and each muscle consists of numerous motor units (See Figure 1).