8 muscles physiology - its - boston college · pdf file8 muscles physiology: muscle structure...
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8 Muscles Physiology: MUSCLE STRUCTURE
SLIDING FILAMENT THEORY
(or, 6[ish] easy steps to contraction)
3 TESTS OF THE SLIDING FILAMENT MODEL
Where hath all my skin
gone?
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Weekly Quiz 2: regrade Q4
Review session: Tomorrow, 5:00 or 6:00?
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Striated muscle fibrils
fiber
fibril
sarcoplasmic reticulum in x-section
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Myofilament organization dictates the macroscopic appearance of muscle.
A = dark
I =
light
Z disc
Q: through which parts of the fiber were these sections made?
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The Sliding Filament Theory of muscle contraction
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The neuromuscular junction is the point of synaptic contact between the axon terminal of a motor neuron and the muscle fiber it controls.
Action potentials in the motor neuron cause acetylcholine release into the neuromuscular junction.
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DHP “receptor” senses t-tubule voltage->ryanodine receptor
Q: Where in the muscle contraction cycle do ligand-gated channels occur? Voltage-gated?
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ryanodine receptor releases Ca2+ into cytosol.
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Ryanodine
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Relaxed: tropomyosin blocks the cross-bridge binding site on actin.
5 Ready: Ca2+ binds to troponin: -tropomyosin “moves” -myosin binding sites exposed.
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6 another 6 substeps involving ATP hydrolyzation
6.1
6.2
6.3
6.4
6.5
6.5
A-M binding, ready
ATP binds to actin, release A-M bond
ATP partially hydrolysis
Actin head “cocked”
“power stroke” powered by P release
ADP released, binding site clear
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at rest
THREE TESTS OF THE SLIDING FILAMENT MODEL Q: Do sarcomeres change in length and structure when they contract? If so, which regions contract?
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at rest
contracted
THREE TESTS OF THE SLIDING FILAMENT MODEL Q: Do sarcomeres change in length and structure when they contract? If so, which regions contract?
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Furaptra = a fluorescent calcium indicator
Q: Does concentration of myoplasmic free calcium change during contraction?
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Furaptra = a fluorescent calcium indicator
Q: What is happening between the time of the action potential and the peak of myoplasmic calcium?
Q: Why is the contractile force generated AFTER the spike in myoplasmic free calcium?
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Q: Does the force generated by muscles differ in muscles of different resting length?
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HINT: ARE FILAMENTS INFINITELY LONG?
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Q: Are all contractions functionally similar?Q: Which ones make you most sore?
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Q. How do muscles generate variable amounts of tension?
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CONTRACTION ENERGETICS
Slow oxidative (SO),
fast glycolytic (FG), and
fast oxidative glycolytic (FOG)
fibers have distinctive traits visible even in histologic section
Three metabolic pathways supply ATP for muscle contraction
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Q: How might dominant fiber type differ among athletes with different specialties?
?
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Proportion SO vs FG up to 83% of vastus lateralis fibers are FOG and FG (PK = pyruvate kinase / LDH = lactate dehydrogenase)
Glycolytic enzymes PK and LDH levels are elevated CS = citrate synthase / HOAD = 3-hydroxyactyl-CoA dehydrogenase Oxidative enzymes CS and HOAD levels are depressed
Mitochondria 2-4% of volume of muscle fiber
Myoglobin concentration within range of other sprinters and endurance runners
Fiber types in the running muscles of the cheetah (Williams et al. 1997)
MUSCLE CELL TRADE-OFFS
Q: Is there a downside to being the fastest mammal? http://www.youtube.com/watch?v=jMwefq1ZtNI
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Muscle cell volume as a zero-sum system
From: Rome and Lindstedt. 1998. News Physiol. Sci 13:261-268.
Q: Predict the effects of maximizing:
muscle protein
mitochondria
sarcoplasmic reticulum
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If cell volume is constant, tradeoffs exist.
Mitochondria(“fuel tank”)
Myofibrillar(“horse power”)
Sarcoplasmicreticulum
(“accelerator”)
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Q. How does Ca2+ get back within SR?
sarco/endoplasmic reticulum calcium-ATPase (SERCA) actively pumps calcium back into the SR
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Calcium transients for three different fibers taken from the oyster toadfish. SO fibers FG fibers Superfast twitch swimbladder fibers
Q: which is which? Q: what can you predict about cell composition?
seconds
http://www.dosits.org/audio/fishes/oystertoadfish/
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≈ 90 ≈ 5 ≈ 5
50 30 4
31 26 26
myofibrils SR mitochondria (force) (Ca++ uptake) (ATP supply)
Q: Which of the muscle profiles belongs to each of the muscles pictured?
toadfish swim bladder typical skeletal muscle rattlesnake rattle
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2
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8.5 - Food and Fuel
Hydrolysis vs. cellular respiration
Digestive tract anatomy and function
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Feeding
Mechanical breakdown
Storage
Chemical breakdown = hydrolysis
Absorption of monomers
(elimination)
DIGESTIVE TRACT ANATOMY AND FUNCTION
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Transported and assembled into more complex molecules after absorption
HEADGUT - food capture, reception, mechanical breakdown, some storage (usually mouth, teeth, lips, tongue, salivary glands)
FOREGUT - food conduction, storage, chemical digestion, limited maceration (usually esophagus and stomach)
MIDGUT - chemical digestion and absorption (usually small intestine)
HINDGUT - water and mineral absorption, waste storage and elimination (usually large intestine / rectum)
Sequential processing of food
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Examples of EXOCRINE processes in digestion (monomers of macromolecules)
bile is an emulsifier (not an enzyme)
pepsin, a protease
salivary amylase, a carbohydrase
bicarbonate adjusts pH (not an enzyme)
lipases, carbohydrases, and proteases make up “pancreatic juice”
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Examples of ENDOCRINE processes in digestion.
secretin stimulates pancreatic release of bicarbonate
cholecystokinin simulates release of pancreatic enzymes (and bile)
gastrin stimulates pepsinogen and acid release
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Review: glucose homeostasis is regulated (in part) by pancreatic hormones
If blood glucose is high…..
If blood glucose is low …
α-cells are red
β-cells are blue
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Q: How can external conditions affect glucose levels?
The HPA axis
CRH = corticotropin releasing hormone
ACTH = adrenocorticotropic hormone
Glucocorticoids (e.g. cortisol, corticosterone)
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CARNIVORY vs HERBIVORY
Q: Which diet is “higher quality” – carnivorous or herbivorous ?
Hume, 1989
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Slicing dentition
Small stomach
Short midgut
Large liver (bile production)
Little storage space
Short transit time
Q: What makes meat so easy to digest? Q: How is diet reflected in digestive tract anatomy?
Carnivores
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QQ1: What is so problematic about a plant diet?
Q: What digestive strategies are available to overcome these difficulties?
1.
2.
3.