Smooth Muscle Physiology - Block 3

Neurotransmitters and hormones have the ability to either stimulate or inhibit smooth muscle. This means that they can either cause smooth muscle to contract or relax. Therefore, it is correct to say that neurotransmitters and hormones can excite or inhibit smooth muscle.

Multi-unit smooth muscle refers to a type of smooth muscle where individual cells are not electrically connected to each other. This means that each cell must receive its own input or stimulation in order to contract. This type of smooth muscle is found in various locations in the body, including lung airways, large blood vessels, eye muscles, and hair follicles. Therefore, the statement that multi-unit smooth muscle cells are not electrically connected and each cell must receive input is true.

Calcium is sourced from the extracellular space and sarcoplasmic reticulum. The extracellular space refers to the area outside of the cell, where calcium ions can enter the cell through specific channels. The sarcoplasmic reticulum, on the other hand, is an internal membrane system within muscle cells that stores and releases calcium ions during muscle contraction. Together, these two sources provide the necessary calcium ions for muscle function.

Ligand-gated channels are proteins found in the cell membrane that open or close in response to the binding of a specific molecule, or ligand. When these channels open, calcium ions can enter the cell, leading to depolarization of the membrane. This depolarization can then trigger the opening of voltage-gated calcium channels, allowing even more calcium to enter the cell. In smooth muscle, this influx of calcium can lead to the generation of an action potential, which is a brief electrical signal that allows the muscle to contract. Therefore, the statement is true.

This statement is true because a partially contracted state in muscle fibers can be maintained as long as there is a threshold level of sacroplamic calcium present to keep a few myosin heads still active. This allows the muscle to remain at a certain level of tension even when it is not fully contracted.

The explanation for the given correct answer is that neurotransmitters, such as norepinephrine, bind to specific receptors and open ligand-gated channels. This process can occur either directly or through a series of signaling events. The example provided in the explanation is norepinephrine binding to alpha 2 adrenergic receptors. This supports the statement that if the stimulus is a neurotransmitter, it will bind to a specific receptor and open the ligand-gated channel. Therefore, the answer is true.

Smooth muscles are found in the lining of hollow organs, such as the stomach, intestines, and blood vessels. These muscles are responsible for regulating blood flow and assisting in the digestion process. Therefore, the correct answer is "all the above" as smooth muscles are found in the lining of hollow organs, regulate blood flow, and are present in the digestive tract.

Calmodulin is a protein that plays a crucial role in calcium signaling within the cytoplasm. When calcium ions bind to calmodulin, it undergoes a conformational change, allowing it to interact with various target proteins, including protein kinases, enzymes, and ion channels. This interaction regulates their activity and initiates specific cellular processes. Therefore, calcium in the cytoplasm binds to calmodulin, enabling it to modulate various cellular functions.

Smooth muscle is a type of muscle found in the walls of organs and blood vessels. Myosin ATPase is an enzyme that catalyzes the breakdown of ATP to provide energy for muscle contraction. The statement suggests that the speed of Myosin ATPase in smooth muscle is slower compared to that in cardiac or skeletal muscle. This is because smooth muscle has a slower contraction and relaxation rate compared to the other two types of muscle. Therefore, the given answer, True, is correct.

Smooth muscles are found in various organs such as the intestines, blood vessels, and uterus. When these muscles are stretched, it triggers the opening of mechanosensitive ion channels. These ion channels allow the flow of ions into the smooth muscle cells, leading to membrane depolarization. Depolarization is an essential step in the contraction of smooth muscles, as it initiates the generation of action potentials and subsequent muscle contraction. Therefore, the statement that stretching opens mechanosensitive ion channels in smooth muscles, resulting in membrane depolarization, is true.

Calcium-induced calcium release (CICR) is a physiological process in which the entry of extracellular calcium into a cell triggers the release of calcium from intracellular stores, specifically the sarcoplasmic reticulum (SR). This release is mediated by the activation of ryanodine receptors located on the SR membrane, which in turn open the calcium channels. Therefore, the given statement accurately describes the process of calcium-induced calcium release.

Smooth muscle can be classified into two types: multi-unit and single-unit. In multi-unit smooth muscle, each individual cell contracts independently in response to a specific stimulus. In contrast, single-unit smooth muscle cells are interconnected by gap junctions, allowing for the spread of electrical impulses and coordinated contraction of all cells as a unit. Therefore, the correct answer is single unit, as it describes the type of smooth muscle where spontaneous electrical impulses spread through gap junctions and cause all cells to contract together.

Smooth muscle contractions require extracellular calcium because the sarcoplasmic reticulum (SR), which is responsible for storing and releasing calcium ions during muscle contraction, does not have enough calcium to sustain prolonged contractions. Therefore, the statement that prolonged contractions of smooth muscles need extracellular calcium is true.

Extracellular calcium is absolutely necessary for contraction because it plays a crucial role in the process of muscle contraction. When a muscle receives a signal to contract, calcium ions are released from the extracellular space into the muscle cell. These calcium ions bind to the protein troponin, causing a conformational change that allows the myosin heads to bind to actin and initiate the sliding of the muscle filaments. Without extracellular calcium, this process cannot occur, and muscle contraction would be impossible.

Dephosphorylation of the light chain prevents actin and myosin interaction and causes relaxation. When the light chain is phosphorylated, it allows actin and myosin to interact, leading to contraction. However, when the light chain is dephosphorylated, this interaction is inhibited, resulting in relaxation of the muscle.

The correct answer is "all of the above." This is because multiple inputs, such as hormones, nitric oxide, pacemaker cells, and stretching, can collectively excite or inhibit smooth muscle, leading to the generation of action potentials. Each of these inputs can play a role in regulating smooth muscle contraction and relaxation, either by directly affecting the muscle or by influencing the activity of other cells involved in smooth muscle control. Therefore, all of these factors can contribute to the overall regulation of smooth muscle function.

In tonically contracted smooth muscles, such as the esophageal sphincter, the latch state is a specialized variant of the cross bridge cycle that allows the muscle to generate tension with low ATP energy. This latch state occurs when the myosin head remains attached to actin for an extended period of time, maintaining muscle contraction without the need for continuous ATP hydrolysis. This allows the muscle to sustain a contracted state for prolonged periods, such as in the case of maintaining muscle tone in the esophageal sphincter.

Calcium can enter the cell through voltage-gated channels and ligand-gated channels. Voltage-gated channels open in response to changes in the electrical potential across the cell membrane, allowing calcium ions to flow into the cell. Ligand-gated channels, on the other hand, open when specific molecules, called ligands, bind to them, allowing calcium ions to enter the cell. Both of these mechanisms play a role in regulating calcium levels within the cell.

Pacemaker cells are specialized cells found in certain tissues, such as the heart and the gastrointestinal tract, that generate spontaneous electrical activity. This activity is responsible for initiating and coordinating the contractions of these muscles. Hormones, neurotransmitters, and troponin are not directly involved in generating electrical activity in the plasma membrane of muscle cells.

Smooth muscle can be made to contract in the absence of an action potential. ie stretching

In smooth muscle, the altering of properties of myosin itself is known as thick filament regulation. Unlike in skeletal muscle, where the interaction between actin and myosin is controlled by altering the properties of the thick filament via calcium, troponin, tropomyosin interaction.

The correct answer is MLCK; MLCK. The Ca-calmodulin complex binds to MLCK (myosin light chain kinase) and activates the enzyme. Once activated, MLCK phosphorylates the light chain in the head region of the myosin molecule. This phosphorylation causes a conformational change that allows myosin to bind to actin.

Hormones binding to receptors on the cell membrane cause the intracellular formation of inositol 1,4,5-triphosphate (IP3), which binds to IP3 receptors on the SR membrane causing a release of calcium.

Calcium-induced calcium release (CICR) is a process in which the entry of calcium ions into the cytoplasm of a muscle cell triggers the release of additional calcium ions from the sarcoplasmic reticulum. This mechanism is important for muscle contraction. In cardiac muscle, CICR plays a crucial role in regulating the strength and duration of contractions, allowing for efficient pumping of blood. Smooth muscle and skeletal muscle also utilize CICR, but the question specifically asks for an option where CICR is "exactly the same," and the only option that satisfies this is cardiac muscle. Metabolism of chylomicrons and lipoproteins are unrelated to CICR.

Parasynthetic neurons release acetylcholine in the Baroreceptor Pathway, which is excitatory.

Extracellular Ca and intracellular Ca are controlled independently of each other. This means that the levels of Ca outside the cell do not directly affect the levels of Ca inside the cell, and vice versa. The regulation of extracellular Ca concentration is mainly controlled by hormones such as parathyroid hormone and calcitonin, while intracellular Ca levels are regulated by various mechanisms including Ca channels, pumps, and exchangers. Therefore, the control of these two Ca pools is independent and not directly influenced by each other.

The correct answer is Sarcoplasmic reticulum calcium ATPase. This enzyme is responsible for actively transporting calcium ions from the cytoplasm back into the sarcoplasmic reticulum, thereby reducing the intracellular calcium concentration in smooth muscle. This mechanism is crucial for muscle relaxation and preventing sustained contraction.

When the stimulus initiates smooth muscle contraction, it causes a change in the membrane potential. This change in membrane potential is what opens the L-type Ca channels. These channels are voltage-gated, meaning they open in response to changes in the electrical potential across the cell membrane. Therefore, when the stimulus changes the membrane potential, it triggers the opening of the L-type Ca channels, allowing calcium ions to enter the cell and initiate the smooth muscle contraction.

SERCA, Plasma membrane calcium ATPase, and Na/Ca exchanger actively uptake calcium from the cytoplasm. These proteins are involved in maintaining calcium homeostasis within the cell. The cytoplasm is the region within the cell where most of the cellular activities occur, and calcium plays a crucial role in various cellular processes such as muscle contraction, signal transduction, and enzyme activation. Therefore, the active uptake of calcium from the cytoplasm helps regulate its concentration and ensures proper cellular function.

The correct answer is skeletal muscle. Both cross bridge and power stroke in smooth muscle are similar to those in skeletal muscle. In both types of muscle, the cross bridge forms between myosin and actin filaments, causing the power stroke which generates force and shortens the muscle fiber. This process is facilitated by the ATPase activity of myosin. Therefore, the cross bridge and power stroke mechanisms are the same in both smooth and skeletal muscle.

Intracellular calcium concentration controls the activity of MLCK (myosin light chain kinase) and MLCP (myosin light chain phosphatase). MLCK phosphorylates the myosin light chain, leading to muscle contraction, while MLCP dephosphorylates the myosin light chain, leading to muscle relaxation. Therefore, reducing calcium will decrease the activity of MLCK and increase the activity of MLCP, shifting the balance in favor of MLCP and muscle relaxation.

the influx is of Calcium not Na.

In the Baroreceptor pathway from the sympathetic division, the postsynaptic neurons release norepinephrine as the neurotransmitter. Norepinephrine is known to have an inhibitory effect, meaning it reduces or inhibits the activity of the target neuron. Therefore, norepinephrine acts as an inhibitory neurotransmitter in this pathway.

they will proceed through the cross bridge cycle. They proceed through the cycel very slowly, slowing the dissociation of the crossbridges and thereby keeping the muscle in a partially contracted state.

Myosin light chains are dephosphorylated by myosin light chain phosphatase.

Smooth muscle contraction is regulated by the thick filaments. Thick filaments are composed of myosin, a motor protein that interacts with actin filaments to generate force. When calcium ions are present, they bind to regulatory proteins on the thick filaments, causing a conformational change that allows myosin to interact with actin. This interaction leads to the sliding of actin and myosin filaments, resulting in muscle contraction. Therefore, thick filament regulation plays a crucial role in controlling smooth muscle contraction.