Notes:🔗👇🏻

https://mednotesbyrazaullah.blogspot.com/2026/08/inhibition-and-activation-of-actin.html

Isometric vs. Isotonic Contraction

Muscle contractions are classified as either isometric or isotonic, depending on whether the muscle changes length or develops tension while doing physical work.

In an isometric contraction, the muscle does not shorten during contraction. Instead, tension increases significantly while the overall muscle length remains completely constant. This occurs when the load or resistance applied to the muscle is greater than the maximum force the muscle can generate, such as pushing against a solid wall or holding a heavy weight steady in a fixed position. At the microscopic level, the cross-bridges still cycle and generate internal tension, but because the ends of the muscle are fixed, the cross-bridge force merely stretches the elastic components (like tendons and titin filaments) within the muscle unit without shortening the sarcomeres enough to move the load.

In an isotonic contraction, the muscle actually shortens while maintaining a constant degree of tension throughout the movement. This happens when the force generated by the muscle exceeds the load, allowing the muscle to perform external work such as lifting a dumbbell or bending the elbow. The muscle first builds up enough isometric tension to match the resistance; once that threshold is crossed, the sarcomeres shorten continuously as myosin cross-bridges pull the actin filaments inward, sliding past one another while the load is lifted at a steady tension. Most real-world body movements are actually a combination of both mechanisms, usually starting with an initial isometric phase to build required tension before transitioning into an isotonic phase to produce movement.

• Muscle twitches occur via quick electrical stimulation of nerves.
• Isometric: Muscle does not shorten during contraction.
• Isotonic: Muscle shortens while maintaining constant tension.
• Isometric systems measure force changes without shortening.
• Isometric systems best compare functional characteristics of different muscles.

Isometric Twitches in Different Muscles

• Muscle sizes and fiber diameters vary widely across bodies.
• Mechanical characteristics differ based on muscle function.
• Ocular muscle: Contraction lasts under 1/50 second for rapid movement.
• Gastrocnemius: Contraction lasts ~1/15 second for running and jumping.
• Soleus: Contraction lasts ~1/5 second for long-term posture support.

Slow vs. Fast Muscle Fibers

Slow Fibers (Type I, Red Muscle):

• Smaller muscle fibers and smaller nerve fibers.
• Extensive blood vessels and capillaries for high oxygen delivery.
• Increased mitochondria to support high oxidative metabolism.
• Rich in iron-containing myoglobin, giving a reddish appearance.

Fast Fibers (Type II, White Muscle):

• Large fibers for high strength of contraction.
• Extensive sarcoplasmic reticulum for rapid calcium release.
• Large amounts of glycolytic enzymes for fast energy release.
• Less blood supply and fewer mitochondria.
• Deficit of myoglobin gives a white appearance.

Motor Units

• A motor unit is one nerve fiber and all its innervated muscle fibers.
• Small, precise muscles have fewer fibers per motor unit.
• Large muscles like the soleus have several hundred fibers per unit.
• The average motor unit contains about 80 to 100 muscle fibers.
• Overlapping motor unit fibers allow smooth cooperative contraction.

Multiple Fiber Summation & Size Principle

• Summation adds individual twitches to increase overall muscle force.
• Multiple fiber summation: Recruits more motor units simultaneously.
• Size principle: Small motor units trigger first, followed by larger ones.
• Small motor nerve fibers are more excitable than larger ones.
• Asynchronous motor unit firing produces smooth overall contractions.

Frequency Summation & Tetanization 

• Increasing stimulation frequency adds new contractions onto preceding ones.
• High frequencies fuse individual twitches into smooth, continuous contraction (tetanization).
• Tetanization occurs when sustained cytosolic calcium maintains full contraction.
• Maximum tetanic contraction strength averages 3 to 4 kg/cm².

Staircase Effect (Treppe)

• Resting muscles gain strength over initial 10 to 50 twitches.
• Caused by accumulating cytosolic calcium from successive action potentials.

Muscle Tone and Fatigue

• Muscle tone: Rest tautness caused by low-rate spinal nerve impulses.
• Brain signals and muscle spindles control resting nerve impulses.
• Muscle fatigue: Results directly from muscle glycogen depletion.
• Fatigue reflects inability of metabolic processes to sustain work output.

Muscle Hypertrophy and Atrophy:

• Muscle hypertrophy means an increase in total muscle mass.
• Muscle atrophy means a decrease in total muscle mass.
• Hypertrophy increases actin and myosin filaments per fiber.
• Forceful contraction under load triggers significant muscle hypertrophy.
• Hypertrophy develops within 6 to 10 weeks.
• Contractile protein synthesis increases up to 50 percent.
• Enlarged myofibrils split to form new myofibrils.
• Energy enzyme systems and glycolytic enzymes also increase.
• Unused muscle degrades proteins faster than replacement.
• Ubiquitin-proteasome pathway degrades targeted proteins during atrophy.

Adjustment of Muscle Length:

• Stretching muscle adds new sarcomeres at tendon ends.
• Sarcomeres can be added several per minutes.
• Continually shortened muscle loses sarcomeres at fiber ends.
• Sarcomere adjustments remodel muscles to ideal lengths.

Muscle Fiber Hyperplasia:

• Hyperplasia is an increase in muscle fiber count.
• Occurs rarely under extreme muscle force generation.
• Caused by linear splitting of previously enlarged fibers.

Effects of Muscle Denervation:

• Loss of nerve supply causes immediate muscle atrophy.
• Degenerative fiber changes begin after about 2 months.
• Full recovery is possible within 3 months if nerve regrows.
• Return capability disappears after 1 to 2 years.
• The final stage destroys fibers, replacing them with fat/fibrous tissue.
• Fibrous tissue shortens over months, creating debilitating contractures.
• Physical therapy uses daily stretching to prevent contractures.

Poliomyelitis and Macromotor Units:

• Remaining nerves branch out to innervate paralyzed fibers.
• Forms macromotor units with 5 times normal fibers.
• Restores muscle strength but reduces fine motor control.

Rigor Mortis:

• Rigor mortis is post-mortem muscle contraction and rigidity.
• Occurs several hours after death without action potentials.
• Caused by total loss of ATP for cross-bridge detachment.
• Lysosomal enzyme autolysis relieves rigor after 15–25 hours.
• Higher temperatures speed up the rigor process.