Muscle

What happens when skeletal muscle motor units lose innervation?

A: They atrophy, replaced by connective tissue. Innervation also determines the type of muscle fiber.

Some differences between cardiac muscle and skeletal muscle?

A: Triad of SR-T-tubule-SR in skeletal, diad in cardiac. In skeletal the triad is at the AI junction. In cardiac it is at the Z line, so only one per sarcomere. In cardiac muscle calcium is actively transported into the cell by L-type channels to maintain contraction; this is important in calcium-induced calcium release. In skeletal muscle all calcium derives from the SR in a regenerative process. Skeletal muscle is innervated by somatic motor. Cardiac is spontaneous, with gap junctions, regulated by ANS. Only skeletal muscle has the ability to hypertrophy (hyperplasia as well). Once dead, cardiac muscle does not regenerate (or very little). Skeletal muscle is formed by fusion of myoblasts so it's a long, electrically communicative cylinder. On the other hand, cardiac myocytes are electrically coupled by intercalated discs (fascia adherens (doesn't go all the way around cell so not zonula adherens), desmosomes, gap junctions), but maintain their individuality. Skeletal: nuclei thin, peripherally located. Cardiac: centrally located more round nuclei, with pale staining perinuclear area with mitochondria and glycogen.

Smooth muscle has dense bodies, visceral innervation, can regenerate by mitosis, no striations.