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.
1 Because alpha motor neurons are the only way to stimulate extrafusal muscle fibers, the loss of these neurons causes a decrease in muscle tone. 2  Hyporeflexia. 3  The myotatic (stretch) reflex is weak or absent with lower motor neuron disorders, because the alpha motor neurons that cause muscle contraction are damaged. Fibrillation. 2  With further degeneration of the alpha motor neuron, only remnants of the axons near the muscle fibers remain. 3  These individual axon fibers can also generate spontaneous action potentials; however, these action potentials will only cause individual muscle fibers to contract.