Neuromuscular Junction and Muscle Functions

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Questions and Answers

What role do somatic motor neurons play at the neuromuscular junction?

Somatic motor neurons stimulate skeletal muscle fibers to contract by transmitting action potentials.

Explain the function of the synaptic cleft at a neuromuscular junction.

The synaptic cleft is a small gap that separates the somatic motor neuron from the muscle fiber, preventing direct electrical communication.

Identify the primary functions of muscles in the human body.

The primary functions of muscles include movement, stability, control of body openings, heat production, and glycemic control.

How do muscles contribute to posture and stability in the body?

Muscles maintain posture by resisting gravitational forces and stabilizing joints through tension on tendons and bones. Signup and view all the answers

What percentage of body heat is produced by skeletal muscles, and why is this important?

Skeletal muscles produce about 85% of body heat, which is vital for enzyme functioning and metabolism. Signup and view all the answers

Discuss the role of muscles in glycemic control.

Muscles help regulate blood glucose concentration within its normal range through their metabolic actions. Signup and view all the answers

What are myofibrils, and what is their significance in muscle fibers?

Myofibrils are structures within muscle fibers that enable contraction and are responsible for muscle force generation. Signup and view all the answers

Describe the pathways through which muscles facilitate the movement of bodily contents.

Muscles facilitate the movement of bodily contents through actions like breathing, digestion, and circulation. Signup and view all the answers

How does troponin I contribute to muscle contraction regulation in resting skeletal muscle?

Troponin I binds to the myosin binding site on actin, partially covering it and preventing myosin-actin interaction. Signup and view all the answers

What role does ATP play in the cross-bridge cycle of muscle contraction?

ATP binds to the myosin head, breaking the actin-myosin interaction and allowing muscle relaxation. Signup and view all the answers

What causes the elevated concentration of calcium ions in muscle cells after death?

Calcium is not pumped back into the sarcoplasmic reticulum due to the absence of ATP, and calcium ions diffuse from the extracellular fluid into the cytoplasm. Signup and view all the answers

Describe the function of the troponin-tropomyosin complex during muscle contraction.

The troponin-tropomyosin complex acts as a relaxing protein that inhibits the interaction between myosin and actin, preventing contraction. Signup and view all the answers

What are myofibrils surrounded by and what invaginates them?

Myofibrils are surrounded by sarcoplasmic reticulum and invaginated by transverse tubules (T tubules). Signup and view all the answers

What happens to the muscle fibers during rigor mortis?

The muscle fibers become stiff due to the inability of myosin heads to detach from actin, caused by the lack of ATP. Signup and view all the answers

Explain the effect of muscle membrane inactivity on calcium ion levels post-mortem.

The inactive muscle membrane cannot maintain the calcium ion gradient, leading to increased intracellular calcium concentration. Signup and view all the answers

Describe the composition and structure of thick filaments in muscle fibers.

Thick filaments are primarily composed of myosin, which consists of six polypeptide chains forming heavy and light chains, with heavy chains coiling to form a tail and globular heads on the ends. Signup and view all the answers

What role does tropomyosin play in muscle contraction?

Tropomyosin blocks the myosin-binding sites on actin when the muscle is at rest, preventing interaction between actin and myosin. Signup and view all the answers

What is the significance of the sliding filament model in muscle contraction?

The sliding filament model explains how muscle contraction occurs through the sliding of actin and myosin filaments past each other, shortening the sarcomere. Signup and view all the answers

How does the presence of calcium ions facilitate muscle contraction?

Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin binding sites on actin, allowing cross-bridge formation. Signup and view all the answers

What initiates the conformational change that allows muscle contraction?

The binding of Ca2+ to troponin C initiates the conformational change, moving tropomyosin and exposing the myosin-binding sites on actin. Signup and view all the answers

What are the three proteins that compose thin filaments and their function?

Thin filaments are composed of actin, tropomyosin, and troponin, where actin provides the binding sites for myosin, and tropomyosin and troponin regulate the interaction between actin and myosin. Signup and view all the answers

Explain the structure of actin in the context of muscle fibers.

Actin exists as G-actin in a globular form and polymerizes into F-actin, which forms two twisted strands to create the filamentous structure. Signup and view all the answers

What is the significance of myosin ATPase in muscle contraction?

Myosin ATPase hydrolyzes ATP, providing the energy required for the cross-bridge cycle during muscle contraction. Signup and view all the answers

How does the arrangement of thick and thin filaments contribute to the banding pattern in striated muscle?

The interdigitating arrangement of thick and thin filaments in sarcomeres creates a distinct banding pattern, which is visually apparent in striated muscle. Signup and view all the answers

What distinguishes slow-twitch fibers from fast-twitch fibers in terms of contraction speed and ATP production?

Slow-twitch fibers contract slowly and primarily produce ATP through oxidative phosphorylation, while fast-twitch fibers contract quickly and mainly generate ATP through glycolysis. Signup and view all the answers

What role does the phosphagen system play in muscle metabolism?

The phosphagen system provides nearly all the energy for short bursts of intense activity, such as during weight lifting or sprinting. Signup and view all the answers

How does the body shift from immediate to short-term energy sources during intense exercise?

As the phosphagen system is exhausted, the muscles switch to anaerobic fermentation to temporarily meet the energy demand. Signup and view all the answers

What is muscular hypertrophy and what causes it?

Muscular hypertrophy is the enlargement of existing muscle fibers caused by increased production of myofibrils and organelles due to forceful, repetitive muscular activity. Signup and view all the answers

Why is aerobic respiration important for long-term energy needs during exercise?

Aerobic respiration delivers sufficient oxygen to meet the ATP demand of muscles after about 40 seconds of exercise, enabling sustained activity. Signup and view all the answers

Contrast fast glycolytic fibers with fast oxidative fibers in terms of ATP production.

Fast glycolytic fibers primarily produce ATP through glycolysis, while fast oxidative fibers use both glycolysis and oxidative phosphorylation for ATP production. Signup and view all the answers

What types of skeletal muscle fibers are predominantly found in the soleus muscle, and what are their characteristics?

The soleus muscle contains mostly slow-twitch fibers, which have a high oxidative capacity and contract slowly. Signup and view all the answers

Explain the significance of myosin ATPase activity in muscle fiber classification.

Myosin ATPase activity helps classify muscle fibers by their contraction speed; fast oxidative fibers have an intermediate activity compared to slow and fast glycolytic fibers. Signup and view all the answers

What distinguishes unitary smooth muscle from multiunit smooth muscle in terms of cell connectivity?

Unitary smooth muscle cells are linked by gap junctions, allowing for coordinated contraction, while multiunit smooth muscle cells have few or no gap junctions and function independently. Signup and view all the answers

Describe the role of gap junctions in unitary smooth muscle.

Gap junctions in unitary smooth muscle allow for the rapid transmission of electrical impulses between cells, leading to coordinated contractions. Signup and view all the answers

What is the significance of spontaneous pacemaker activity in unitary smooth muscle?

Spontaneous pacemaker activity in unitary smooth muscle leads to rhythmic contractions, essential for the functioning of the gastrointestinal tract and similar organs. Signup and view all the answers

How do action potentials in smooth muscle cells differ from those in striated muscle cells?

Action potentials in smooth muscle cells have a prolonged upstroke due to slower opening of Ca2+ channels compared to Na+ channels in striated muscle cells. Signup and view all the answers

What initiates the depolarization phase in the smooth muscle action potential?

The depolarization phase is initiated by the influx of Ca2+ ions through voltage-gated calcium channels in the sarcolemmal membrane. Signup and view all the answers

Contrast the mechanism of contraction in visceral smooth muscle versus multiunit smooth muscle.

Visceral smooth muscle can contract in response to action potentials, while multiunit smooth muscle typically does not generate action potentials and contracts independently under neural control. Signup and view all the answers

In multiunit smooth muscle, how do cells receive signals for contraction?

In multiunit smooth muscle, individual cells receive signals for contraction primarily through autonomic nerve innervation and may respond to hormonal or local chemical stimuli. Signup and view all the answers

What role does potassium play in the repolarization of smooth muscle action potentials?

Potassium channels contribute to repolarization by opening slowly, allowing K+ ions to exit the cell, which helps return the membrane potential to its resting state. Signup and view all the answers

What is junctional potential and how does it affect muscle contraction?

Junctional potential is a local depolarization that spreads along the muscle fiber, leading to calcium influx through voltage-gated Ca++ channels, which ultimately causes muscle contraction. Signup and view all the answers

How do pacemaker potentials differ in visceral smooth muscle compared to cardiac muscle?

In visceral smooth muscle, pacemaker potentials do not originate from a fixed location but can shift between multiple sites, while in cardiac muscle, they are generated at predetermined locations. Signup and view all the answers

What are the two additional mechanisms that can increase intracellular calcium concentration in smooth muscle?

The two additional mechanisms are ligand-gated Ca2+ channels and IP3-gated Ca2+ release channels, both of which permit the entry or release of calcium into the cell. Signup and view all the answers

What role does calmodulin play in the contraction process of smooth muscle?

Calmodulin binds to calcium ions, forming a Ca2+-calmodulin complex that activates myosin-light-chain kinase, which is essential for initiating contraction. Signup and view all the answers

What happens to myosin when intracellular calcium concentration decreases?

When calcium concentration decreases, myosin is dephosphorylated by myosin-light-chain phosphatase, leading to the formation of latch-bridges instead of cross-bridges. Signup and view all the answers

How does the influx of Ca2+ from the extracellular fluid affect smooth muscle cells?

The influx of Ca2+ increases intracellular calcium concentrations, which triggers contraction processes in the smooth muscle cells. Signup and view all the answers

Explain the concept of latch-bridges in smooth muscle.

Latch-bridges are stable attachments between actin and dephosphorylated myosin that allow smooth muscle to maintain tension with minimal energy use. Signup and view all the answers

What triggers the opening of voltage-gated Ca2+ channels in smooth muscle?

Action potentials in the smooth muscle cell membrane trigger the opening of voltage-gated Ca2+ channels. Signup and view all the answers

Flashcards

Somatic Motor Neuron

A neuron that stimulates skeletal muscle fibers to contract.

Neuromuscular Junction (NMJ)

The synapse between a somatic motor neuron and a skeletal muscle fiber.