Smooth Muscle Flashcards
Summarize the process of smooth muscle contraction.
âšš [Flowchart]
âšš Depolarization of the cell membrane opens the voltage-gated calcium channels hence an influx of calcium ions into the cell. [Besides neural stimulation, stretch, local factors e.g. O2 and CO2 concentrations (in the case of vascular smooth muscle) and hormones which affect ligand-gated calcium channels may also induce action potentials that cause smooth muscle to contract.]
âšš With an increase in cytoplasmic calcium concentration, the calcium-induced calcium release mechanism takes effect, thus further increase in calcium ion concentration.
âšš Unlike in skeletal muscle, smooth muscle lacks troponins, but instead has a protein called calmodulin to which calcium binds, forming a complex known as the calcium-calmodulin complex.
âšš A protein known as calponin bound to actin and tropomyosin inhibits myosin ATPase activity. [ATPases are a group of enzymes that catalyze the hydrolysis of a phosphate bond in adenosine triphosphate (ATP) to form adenosine diphosphate (ADP).]
âšš The calcium-calmodulin complex binds to calponin and activates a protein kinase that phosphorylates the calponin and its inhibitory function is removed.
âšš The calcium-calmodulin complex also activates an enzyme on the regulatory light chain of myosin called the myosin light chain kinase.
âšš Myosin light chain kinase phosphorylates myosin hence activating it.
âšš Activated myosin can then attach to actin so that cross-bridge cycling can occur.
Summarize the process of smooth muscle relaxation.
âšš Calcium ATPase pumps on the sarcoplasmic reticulum and the plasma membrane transport calcium back to the sarcoplasmic reticulum or back into the extracellular fluid respectively.
âšš There are also pumps known as sodium-calcium exchangers that transport calcium out of the cell in exchange for sodium.
âšš Through any of these methods, the calcium levels in the cell drop.
âšš Calcium is released from calmodulin, but that is not enough for the muscle cell to relax.
⚚ The myosin is still phosphorylated and for it to become inactive again, it needs to be dephosphorlyated. This happens through an enzyme called myosin light chain phosphatase. It removes the phosphate from myosin’s regulatory light chain, hence inactivating it.
âšš The cross-bridge cycling stops and the muscle relaxes.
âšš It is important to note however, that the muscle may not necessarily relax upon dephosphorylation of the myosin light chain. The myosin cross-bridges may remain attached to actin for some time after calcium concentration falls. This is known as the latch bridge mechanism. This produces sustained contraction with little expenditure of energy, which is especially important in vascular smooth muscle.
Explain the two types of smooth muscle.
(1) Unitary/syncytial/visceral: Unitary (single unit) smooth muscle is present in the gastrointestinal tract, bladder, uterus, and ureter. The term unitary does not mean single muscle fibers. Instead, it means a mass of hundreds to thousands of smooth muscle fibers that are arranged as bundles or sheets and are adherent to each other at multiple points on the cell membrane. The smooth muscle in these organs contracts in a coordinated fashion because the cells are linked by gap junctions.
(2) Multi-unit: This type of smooth muscle is present in the iris, in the ciliary muscles of the lens, and in the vas deferens. It is composed of discrete, separate, smooth muscle fibers. Each muscle fiber behaves as a separate motor unit (similar to skeletal muscle), and there is little or no coupling between cells. Multiunit smooth muscle cells are densely
innervated by postganglionic fibers of the parasympathetic and sympathetic nervous systems, and it is these innervations that regulate function. Furthermore, the outer surfaces of these fibers, like those of skeletal muscle fibers, are covered by a thin layer of basement membrane–like substance, a mixture of fine collagen and glycoprotein that helps insulate the separate fibers from one another. The major characteristic of this type of muscle is that fibres can contract independently.
Briefly discuss the smooth muscle neuromuscular junction.
☯︎ Neuromuscular junctions of the highly structured type found on skeletal muscle fibers do no occur in smooth muscle. Instead, the autonomic nerve fibers that innervate smooth muscle generally branch diffusely on top of a sheet of muscle fibers.
☯︎ The axons that innervate smooth muscle fibers do not have the typical branching end feet of the type found in the motor end plate on skeletal muscle fibers. Instead, most of the fine terminal axons have multiple varicosities distributed along their axes.
☯︎ In the varicosities are vesicles similar to those in the skeletal muscle end plate that contain transmitter substance. However, in contrast ot the vesicles of skeletal muscle junctions, which always contain acetylcholine, the vesicles of the autonomic nerve fiber endings contain acetylcholine in some fibers and norepinephrine in others and occassionally other substances as well.
☯︎ There are two types of smooth muscle NMJs: (1) Diffuse [for single unit muscle fibres] and (2) Contact [for multi-unit muscle fibres]. See more details in the video below.
☯︎ [11-minute video]: The Smooth Muscle NMJ according to Ganong
