Muscle Growth & Plasticity

Embryology All muscles derive from the MESODERM of the GASTRULA Remember? Morula then Blastula then Gastrula From its mesoderm layer: A) striated or voluntary muscles B) cardiac muscle or scalariform C) smooth muscle (of GI tract, Urinary, etc)

Anatomy of Voluntary Muscles 1) bundles of fibers surrounded by a thin layer of connective tissue: Sarcolemma 2) hooked to the bony plates of the skeleton by strong tendons 3) receiving stimuli from nerves originating from the anterior horns of the spinal cord

Composition Muscles represent: 50% of the weight of the adult human 20% of that weight is PROTEIN Remaining parts are salts and water

Histology Each fiber being a multinucleated cell consists of myofibrils in bundles with a large number of mitochondria and a myoglobin (pigmented protein)

Importance of the Nervous System Autonomic nervous system controls smooth and cardiac muscles Central nervous system controls the voluntary muscles

Contractility Secondary to the sliding characteristic of the 2 main proteins of the myofibrils: MYOSIN ACTIN (thinner)

The Functional Unit of the Muscle Fiber Sarcomere: A) Myosin-thick filaments B) Actin-thin filaments Transverse striations: M, Z, and I

Steps in Contraction 1. Discharge of motor-neuron at the myo-neural plate with release of the neurotransmitter acetylcholine (Ach) 2. The binding of Ach to its receptors increases Na+ and K+ conductance, generating an action potential 3. The sarcoplasmic reticulum releases Ca++, activating the enzyme troponin, which uncovers the myosin binding sites allowing cross- linkages between actin and myosin to form Steps in Relaxation Ca++ pumped back in the sarcoplasmic reticulum, and the release of Ca++ from the troponin induces cessation of interaction between actin and myosin.

Electrical Phenomena Resting muscle = charged positively Where contraction starts, the charge changes to negative Hence, there is a difference in potential Completely contracted muscle = charged negative As relaxation returns there is a difference in potential and, again, the resting muscle is charged positively in total Electromyography can study these changes

THE FUEL GLUCOSE, 6 carbon (C) compound, enters the muscle +/- insulin Broken into 3C compound: Lactate and Pyruvate Lactate to pyruvate and/or back to circulation for gluconeogenesis

TYPES of FIBERS Type 1: reddish Slow Oxidative (SO) Type 2: pale and divided into Fast Oxidative Glycolytic (FOG) Fast Glycolytic (FG)

The number and type of fibers is dictated by genetic determinants but ……..ALAS!!! Appropriate stimulations may induce an almost complete change Hence….. MYOPLASTICITY

Myoplasticity: Concept Ability of the muscle to alter the quantity and the type of its proteins in response to stimulations Modalities of stimulations: 1) Physical activities leading to an increase in its cross- sectional area 2) Increase in the muscular mass with changes in the myosin type Muscle plasticity may involve: Change in the amount of protein Change in the type of protein Combination of both

Myoplasticity Due to Exercise Endurance exercise increases the oxidative metabolism of the muscle Resistance training increases the cross-sectional area due to true hypertrophy of the single cells Inactivity induces rapid regression

Muscle Fiber Number Virtually Fixed at Birth The increase in mass (hypertrophy, sometimes as much as 50%) is due to increase in length and in the cross-sectional area of the muscle fibers. This is due to an increase in the number of myofibrils (from 75 to over 1000) The capacity for regeneration and plasticity is a response to neural, hormonal and nutritional differences

Whole Body Growth Since birth, growth follows an S shaped curve good increase up to age 6-10 big spurt from age to All conditioned by: a) genetic prints b) environmental exposure

Gender Differences Boys: Testosterone, and to a lesser extent GH, provide potential for maximal gain in muscle strength Girls: Estrogens have a significant effect on adipose tissue distribution/amount and bones; Androgens (androstenedione) are secreted in the moderate levels by the adrenal glands ALERT “protect certain characteristics of the female body” (world distance records have been set by women with over 15% of body fat) For women swimmers, a higher body fat % provides a significant advantage

Ergogenic Aids = Dreams, Failures, Tragedies Doping substances: stimulants (amphetamines, ephedrine, excess of caffeine); ß-2 agonists (ventolin, albuterol) and hormones (gonadotropin, corticotropin, & erythropoietin EPO) Anabolic steroids: Testosterone and its derivatives. Positive effects: increase in protein synthesis, muscle size, etc.); **ALERT for the THG** Negative effects: liver toxicity, higher BP, increased LDL, acne, etc.