Exam 8: Muscle Physiology

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Smooth muscle and skeletal muscle both use Ca2+ for contraction, but the sources and involvement of Ca2+ differ between the two types of muscle. In smooth muscle, Ca2+ is primarily sourced from the extracellular space, entering the cell through voltage-gated Ca2+ channels in the plasma membrane. In contrast, skeletal muscle primarily relies on Ca2+ release from the sarcoplasmic reticulum (SR) within the muscle cell.

In terms of involvement, Ca2+ binds to calmodulin in smooth muscle, leading to the activation of myosin light chain kinase (MLCK) and subsequent phosphorylation of myosin, which initiates contraction. In skeletal muscle, Ca2+ binds to troponin C, leading to the exposure of myosin binding sites on actin and the initiation of contraction.

As for the absence of T tubules in smooth muscle, this is due to the structural differences between smooth and skeletal muscle cells. T tubules are invaginations of the plasma membrane that allow for the rapid transmission of action potentials deep into the muscle cell, triggering the release of Ca2+ from the SR. In smooth muscle, the lack of T tubules is compensated for by the presence of caveolae, which are small invaginations of the plasma membrane that also facilitate the entry of extracellular Ca2+.

Overall, the differences in the source and involvement of Ca2+ in smooth muscle and skeletal muscle contraction reflect the distinct physiological and structural characteristics of these two types of muscle.

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Neurogenic multiunit smooth muscle cells and myogenic single-unit smooth muscle cells differ in their mechanisms of excitation and contraction.

In neurogenic multiunit smooth muscle cells, excitation occurs through the release of neurotransmitters from autonomic nerve fibers. These neurotransmitters bind to receptors on the muscle cell membrane, leading to depolarization and the generation of action potentials. This results in the activation of voltage-gated calcium channels and the influx of calcium ions into the cell, which triggers the contraction of the muscle cell.

On the other hand, myogenic single-unit smooth muscle cells are capable of spontaneous depolarization and contraction without the need for external neural input. This is due to the presence of pacemaker cells within the muscle tissue, which generate rhythmic action potentials that spread to neighboring muscle cells through gap junctions. The depolarization of these cells leads to the opening of voltage-gated calcium channels and the subsequent influx of calcium ions, resulting in muscle contraction.

In terms of contraction, both types of smooth muscle cells utilize the calcium-calmodulin complex to activate myosin light chain kinase and initiate the cross-bridge cycling that leads to muscle contraction. However, the regulation of contraction in neurogenic multiunit smooth muscle cells is more dependent on external neural input, while myogenic single-unit smooth muscle cells have a greater degree of intrinsic control over their contraction.

In summary, neurogenic multiunit smooth muscle cells rely on external neural input for excitation and contraction, while myogenic single-unit smooth muscle cells are capable of spontaneous excitation and contraction due to the presence of pacemaker cells and gap junctions. Additionally, the regulation of contraction differs between the two cell types, with neurogenic multiunit smooth muscle cells being more dependent on external input and myogenic single-unit smooth muscle cells having a greater degree of intrinsic control.