Smooth Muscle Structure and Function

Striated muscle tissue with sarcomeres; requires nerve stimulation to contract; denervation leads to atrophy; stimulated independently.

Striated muscle tissue with sarcomeres; can contract without nerve stimulation; action potentials originate in pacemaker cells.

Non-striated muscle; relies less on nerve stimulation; maintains tone in the absence of stimulation; uses calmodulin, not troponin.

Basic units of striated muscle; organized arrangement of actin and myosin filaments.

Protein in the thin filaments of striated muscle, involved in regulating muscle contraction.

Protein involved in smooth muscle contraction, activated by Ca2+.

Specialized network of membranes in muscle cells that stores and releases calcium ions.

Connections between muscle cells that allow for the rapid transmission of signals.

Smooth muscle contraction differs from striated muscle, utilizing a biochemical cascade instead of a steric mechanism, and involving calmodulin to regulate the process.

Smooth muscle cells have a significantly different ratio of thin to thick filaments (approximately 16:1) compared to striated muscles (2:1).

Unlike striated muscles, smooth muscle has longer thin filaments that attach to dense bodies, analogous to striated muscles' Z-discs.

Smooth muscle myosin is arranged vertically, allowing cross-bridges along the entire thick filament length. This differs from the horizontal arrangement in striated muscles.

Smooth muscle lacks sarcomeres, organized for greater extensibility and the ability to contract forcefully even with significant stretch.

Smooth muscles can contract effectively, even when significantly stretched, like those in the bladder and uterus.

The predominant type of smooth muscle in the body; organized as a syncytium, found in digestive and excretory systems, enabling coordinated contractions.

Striated muscle loses its ability to contract when stretched to a point that actin and myosin filaments no longer overlap.

Smooth muscle cells are spindle-shaped, arranged in layers around hollow organs and blood vessels. They lack sarcomeres and are therefore non-striated.

Smooth muscle contraction is slow and sustained, due to slower ATPase activity compared to skeletal muscle, and can contract even when stretched significantly.

Smooth muscle contraction is involuntary and doesn't require nerve stimulation (like skeletal muscle does) to initiate or modulate. It can maintain tone in absence of stimulation.

Smooth muscle action potentials are varied. They can be triggered by different factors, including local humoral factors, circulating hormones, and mechanical stimulation (like stretching).

Smooth muscle contraction involves calcium influx, activating calmodulin, which regulates the interaction between actin and myosin filaments. The process does not use troponin.

Smooth muscle in the digestive tract, ureters, and other tubes are often arranged circularly and longitudinally. Contraction of these layers produces peristaltic waves.

Smooth muscle cells do not have sarcomeres, but they do have dense bodies which are analogous to Z-disks in skeletal muscles and serve in the regulation of the myofilament organization.

Smooth muscle regulates movement within hollow organs by adjusting the size and pressure of these organs, enabling functions like digestion, urination, and childbirth.

Smooth muscle is categorized into low-resistance (single-unit) and multiunit types. Single-unit muscle contracts in a coordinated manner, while multiunit muscle contracts independently.

Smooth muscle type characterized by gap junctions that facilitate synchronized contraction.

Smooth muscle type where each fiber is innervated and contracts independently; no gap junctions.

Smooth muscle contraction is triggered by calcium influx, activating calmodulin and myosin light-chain kinase, leading to cross-bridge formation and muscle shortening.

Rhythmic fluctuations in membrane potential that act as a pacemaker, leading to spike potentials in single-unit smooth muscle, initiating contraction.

While some smooth muscle has intrinsic pacemaker activity, autonomic nervous system input significantly influences its contraction and relaxation.

Calcium combines with calmodulin to activate other proteins initiating contraction.

An enzyme activated by the Ca2+-CaM complex that phosphorylates myosin, causing contraction.

Nerves that control smooth muscle activity in multiple organs simultaneously; sympathetic and parasympathetic nerves affect many organs in the body together.

A response triggered by sympathetic nerves, causing various bodily changes like dilated pupils and rapid heartbeat to prepare for action.

Connections forming between autonomic nerve fibers and smooth muscle cells along the nerve fiber, not just at a specific junction as with skeletal muscle.

Bulges along autonomic nerve fibers where neurotransmitters are released to stimulate smooth muscle cells.

Skeletal muscles have a direct, single connection to motor neurons, while smooth muscles have multiple points of contact along the nerve fiber, which releases neurotransmitters.

Neurotransmitter receptors found on the entire surface of smooth muscle cells

Specific region of contact between a motor neuron and skeletal muscle; not involved in autonomic stimulation of smooth muscles

Nerve stimulation that generally opposes the effects of sympathetic stimulation