Muscle Physiology and Cardiac Mechanics Fundamentals

Fundamental Properties of Muscle Tissue

Muscles possess unique characteristics that allow them to function effectively within the body:

• Excitability: They can receive and respond to stimulation from the brain.
• Contractility: They can contract (shorten) following stimulation.
• Extensibility: They can expand (stretch) through the application of force.
• Elasticity: They return to their original shape and length after being contracted or extended.

Structural Hierarchy of Skeletal Muscle

The connection from bone to the functional unit of the muscle follows a specific hierarchy:

Bones → Muscles (via Tendons) → Fascicle → Muscle Fiber → Myofibrils (containing Sarcomeres)

Neural Control of the Knee-Jerk Reflex

During a reflex action, such as a knee tap, the following sequence occurs:

• The tap is sensed by sensory neurons, which send signals to the spinal cord and brain.
• The sensory neuron from the quadricep is coupled to:
  • A motor neuron of the quadricep (causing contraction).
  • A motor neuron of the bicep (via an inhibitory neuron to prevent contraction).
• The net result is the extension of the leg at the knee joint.

Skeletal vs. Cardiac Muscle Characteristics

Both cardiac and skeletal muscles are striated, but they differ in structure:

• Skeletal Muscle: Consists of fibers in bundles forming a syncytium of fused muscles.
• Cardiac Muscle: Composed of cardiomyocytes, which are not a syncytium and typically contain two nuclei.

Sarcomere Structure and Molecular Motors

The sarcomere is the basic unit of muscle contraction, consisting of:

• Thin unit: Actin (in F-actin, the pointed end is negative).
• Thick unit: Myosin.

Molecular motors exhibit different duty ratios (r):

• Kinesin: r = 0.5 (one leg) or r = 1 (two legs).
• Myosin: r = 0.2 (spends less time 'on' and requires time for recovery).

The Biochemical Process of Muscle Contraction

Contraction is triggered by a series of electrochemical events:

1. Brain impulses cause the release of acetylcholine.
2. Acetylcholine binds to receptors on the muscle cell.
3. This causes the entry of Na+ into the muscle cell.
4. Ca++ stored in the sarcoplasmic reticulum is released into the cytosol.
5. This activates myosin to generate force within the sarcomere.
6. The muscle contracts.

Molecular Mechanics of the Sarcomere

Muscle cells are specialized to contract (they do not expand actively). They perform quick, repetitive contractions against opposing forces. The skeletal muscle cell (myofiber) is packed with repeating sarcomeres located between two Z-disks. Each sarcomere can shorten to approximately 70% of its resting length.

The process involves:

• A nerve impulse at the motor neuron increases Ca2+ in the muscle cell.
• Proteins called tropomyosin, which normally block the actin-binding site of myosin, release those sites.
• Myosin binds to actin and hydrolyzes ATP to generate force.
• This causes the Z-disks to move closer to each other, resulting in muscle contraction.

Cardiac Cycle and Pathological Conditions

The cardiac cycle typically operates at 72 beats per minute, totaling approximately 100,000 beats per day, with a duration of 0.83 seconds per beat. Several pathologies can affect heart function:

• Hypertrophic Cardiomyopathy (HCM): Characterized by asymmetric interventricular septal thickening. Ventricular cardiomyocytes increase in size by adding sarcomeres as a compensatory mechanism to increase force.
• Dilated Cardiomyopathy (DCM): Involves the thinning of the ventricular walls.
• Left Ventricular Non-Compaction: Trabeculated ventricular walls create a distinct spongy appearance.