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

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?

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

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

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

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/

≈ 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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8.5 - Food and Fuel

Hydrolysis vs. cellular respiration

Digestive tract anatomy and function

Feeding

Mechanical breakdown

Storage

Chemical breakdown = hydrolysis

Absorption of monomers

(elimination)

DIGESTIVE TRACT ANATOMY AND FUNCTION

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

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”

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

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

Q: How can external conditions affect glucose levels?

The HPA axis

CRH = corticotropin releasing hormone

ACTH = adrenocorticotropic hormone

Glucocorticoids (e.g. cortisol, corticosterone)

CARNIVORY vs HERBIVORY

Q: Which diet is “higher quality” – carnivorous or herbivorous ?

Hume, 1989

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

QQ1: What is so problematic about a plant diet?

Q: What digestive strategies are available to overcome these difficulties?

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