Muscle Physiology and Organization

A highly organized microfilament-based structure that contracts to pull an external structure or squeeze a fluid. They produce force, generate movements, and constitute a significant portion of body weight in many animals.

Skeletal muscle, cardiac muscle, and smooth muscle.

Muscle contraction enables purposeful locomotory movement, manipulation of external objects, propulsion and emptying of contents in organs, production of heat for thermoregulation, and even sound production.

The functional unit of a muscle, defined as the area between two Z lines.

A stacked set of thick filaments with a portion of thin filaments overlapping on both ends.

The remaining portion of thin filaments that do not project into the A band.

A flat cytoskeletal disc located in the middle of each I band, marking the boundary between sarcomeres.

The lighter area in the middle of the A band where thin filaments do not reach.

Made up of supporting proteins, extending vertically down the middle of the A band within the center of the H zone.

Myosin filaments are thick filaments composed of multiple myosin molecules arranged lengthwise, with globular heads (cross bridges) protruding at regular intervals.

Thin filaments consist of two chains of spherical actin molecules twisted together, along with troponin and tropomyosin molecules that regulate muscle contraction.

Calcium ions play a crucial role in initiating muscle contraction by binding to troponin, causing a conformational change that exposes myosin binding sites on actin, allowing for cross-bridge formation.

During contraction, the thin filaments slide closer together between the thick filaments, pulling the Z lines closer together, resulting in a shortening of the sarcomere.

All three muscle types possess specialized contractile apparatus (thin and thick filaments), require calcium for contraction, and directly use ATP as the energy source for cross-bridge cycling.

They differ in their structure, organization of fibers, mechanism of excitation and contraction, and contractile response.

Skeletal muscle is striated and voluntary, with large, cylindrical, multinucleated cells that extend the full length of the muscle. Smooth muscle is small, unstriated, and involuntary, composed of elongated, spindle-shaped cells with a single nucleus.

Phasic smooth muscle contracts in bursts, triggered by action potentials, while tonic smooth muscle is partially contracted at all times, maintaining a tone.

Multi-unit smooth muscle is neurogenic, requiring nerve stimulation for contraction, while single-unit (visceral) smooth muscle is myogenic, capable of spontaneous electrical activity.

Cardiac muscle cells are found only in the heart, are striated, form functional syncytium, have intercalated disks, rely on aerobic metabolism, and have a long refractory period.

A specialized synapse where a motor neuron communicates with a muscle fiber, triggering muscle contraction.

A modified endoplasmic reticulum in muscle cells that stores and releases calcium ions, regulating muscle contraction.

T-tubules are membranous extensions of the surface membrane that dip deep into muscle fibers. They help transmit the action potential rapidly throughout the muscle, ensuring synchronized contraction.

A series of events involving the interaction of myosin and actin filaments, driven by ATP, that results in muscle contraction. This cycle involves attachment, power stroke, detachment, and re-energizing.

ATP provides the energy for the power stroke, detachment of cross bridges, and active transport of calcium back into the sarcoplasmic reticulum.

ATP is produced through three main metabolic pathways: phosphagen system, oxidative phosphorylation, and glycolysis.

The process by which an action potential in a motor neuron leads to the contraction of muscle fibers. This involves the release of acetylcholine at the neuromuscular junction, depolarization of the muscle fiber, calcium release from the sarcoplasmic reticulum, and interaction of myosin and actin to generate force.

Factors influencing muscle contraction include neuromuscular block (e.g., hypocalcemia, curare, succinylcholine), muscle relaxants, length of the fiber at the onset of contraction, extent of fatigue, thickness of the fiber, and frequency of stimulation.

Muscle contraction involves the sliding filament theory, where thin filaments (actin) slide past thick filaments (myosin) due to the formation and breakdown of cross bridges between these filaments.

The process by which an action potential in a cardiac muscle cell triggers contraction. This involves the influx of calcium from both the extracellular fluid and sarcoplasmic reticulum, leading to the interaction of myosin and actin and subsequent contraction.

Cardiac muscle is found only in the heart, is striated, forms functional syncytium, relies on aerobic metabolism, has a long refractory period, and exhibits pacemaker activity.

An ECG is a recording of the electrical currents generated by the heart during depolarization and repolarization, which spread into the tissues surrounding the heart and body fluids.

A normal ECG displays three distinct waves: the P wave representing atrial depolarization, the QRS complex representing ventricular depolarization, and the T wave representing ventricular repolarization.

The sequence of events that occurs during a single heartbeat, encompassing both contraction (systole) and relaxation (diastole) of the heart chambers.

The amount of blood pumped by the heart per minute, determined by heart rate (beats per minute) and stroke volume (volume pumped per beat).

Cardiac output is primarily controlled by the autonomic nervous system. Sympathetic stimulation increases heart rate and contractility, while parasympathetic stimulation decreases heart rate.