muscle growth & plasticity
Post on 30-Dec-2015
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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.
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