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 Contraction1. 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: ConceptAbility 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 12-14 to 18-22

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.