Muscle Physiology

Question 1

Three Muscle Types

Answer 1

Skeletal - motion/support
Smooth - control digestive/reproductive system, and capillary action*
Cardiac - heart; first muscle to undergo stress
*Capillaries deliver oxygen, glucose, FA, etc. to muscles, and remove heat; stronger capillaries can transport more

Question 2

Muscle Development

Answer 2

Largest tissue of the body (TB - 55% muscle), and develops early on
Day 10 - embryo (ball-shaped) begins producing muscle tissue
Day 30 - muscle is present in the embryo (animal-like)
Day 65 - somites appear along dorsal ridge of embryo (large animal-like), origin for specific muscles
Day 85 - fetus (horse-like)
2nd and 3rd Trimester - specific muscle development

Question 3

Myogenesis

Answer 3

1st myogenesis (10-12 days) - prolific cells (myoblasts) form myotubes, which form mature muscle fibers
Muscle fibers end up with multiple nuclei from all the myoblasts that make up the fiber
2nd myrogenesis - few muscle fibers increase to hundreds/thousands of fibers; born with the number of muscle fibers will have for life, can only get larger/smaller, or change type of fiber

Question 4

Anatomy of Muscles

Answer 4

Location determines form, and form determines function
No muscles below the knee - would be too heavy to lift
Tendons - muscles to bone
Ligament - muscles to muscles
*Tendons and ligaments are used to store energy, “spring-like” energy efficiency
*Superdigital Flexor Tenden is most likely to be damaged

Question 5

Muscle Fiber

Answer 5

Muscle is 90% fiber, 10% nerves, vessels, etc., and is composed of highly organized structure made of bundles of muscle fibers (fasciles)
One muscle fiber is long 10-100 mm, and is composed of chains of sarcomeres/myofibrils made from thick (myosin) and thin (actin) fibers

Question 6

Sarcomere

Answer 6

Composed of a thick (myosin) and thin (actin) filament, it is the basis for contracting the muscle
Tropomyosin forms the chain that actin is bound to, along with troponin, which blocks actin from binding to the myosin heads. Calcium releases troponin from the binding site, allowing actin to bind to the myosin heads, and the myosin head ratchet towards the center of the sarcomere, contracting the muscle fiber.

Question 7

Ultra-structure of Muscle Fiber

Answer 7

Sarcoplasm - the cytoplasm of the muscle cell, contains myoglobin to bind to oxygen and glycolitic enzymes to generate ATP, along with other organelles (mitochondria, RER, etc.)
Sarcoplasmic Reticulum - long tube around muscle fibers, collect and store Ca
T-Tubules - divots in SR that allow action potentials to reach within the muscle cell (not just the surface)
* T-tubules also contain: reanidine receptors to release Ca stored in SR, CaATPases to pump Ca into SR, and calsequestrian to bind to Ca and keep it in the SR

Question 8

Motor Unit

Answer 8

The nerve and the muscle fibers it controls
The fewer number of nerves that control a group of cell fibers, the faster the reaction will be
The greater or faster the release of Ca from the SR, the faster the muscle contraction will be

Question 9

Action Potential on the Muscle Fiber

Answer 9

ACT is released from the neuromuscular junction, causing an action potential along the muscle fiber.
The action potential deactivates the reanidine receptor, releasing Ca into the sarcoplasm.
The Ca then binds to troponin, allowing actin to bind to myosin heads.
The myosin head then ratchets towards the middle of the sarcomere, pulling the muscle fiber and contracting the muscle as a whole.

Question 10

Hyperkalemic Periodic Paralysis (HYPP)

Answer 10

Issue where the Na/K channels are “leaky” and allow more Na to leave the cell than K are pulled back into the cell. This causes a prolonged action potential, making the muscle contract almost constantly.
This can cause tremors, paralysis, and collapsing, as well as high levels of K in the blood which can interfere with the function of the heart.
Genetic test for HYPP; manage with a diet low in K.
Impressive - QH stallion first diagnosed with HYPP

Question 11

Aerobic Pathways

Answer 11

Utilize TCA cycle with glucose and beta oxidation of FA to produce 34-36 ATP
Slow energy
Horses utilize fat well for energy, but cannot be fed over 10-12%

Question 12

Anaerobic Pathways

Answer 12

Creatine - short burst of energy, lasts ~3 seconds, and is the first source of energy
Glycolysis can turn glycose to pyruvate to for 2 ATP, but produces lactate as a by-product
Fast energy
Lactate build up can decrease pH of the blood, denature and destroy proteins, and depletes ATP from the system

Question 13

Muscle Fiber Types

Answer 13

Muscle is heterogeneous; location determines the percent composition of fiber
Fiber 1A - slow twitch , ATP production by mitochondria
Fiber 2A - fast twitch, ATP production by mitochondria
Fiber 2X - fastest twitch, ATP production by anaerobic oxidation
Detection of muscle fiber includes immunohistochemcials to determine mysoin isoform, or histochemicals to determine enzyme (metabolic) activity

Question 14

Fiber Recruitment

Answer 14

The act of changing muscle fiber type by training; must be sequential transition (1A - 2A - 2Ax* - 2aX* - 2X)
*hybrid fibers show how well horse is transitioning
Breed influences the type of fibers a horse will have, and genotype influences how easy those fibers are to change

Question 15

Control of Fiber Identity

Answer 15

Myogenic - genetic based
Extracellular - hormones
Neural inputs - fast/slow contractions
Exercise - changes through training

Question 16

Response to Exercise

Answer 16

Aerobic - sub-maximal (25-60% VO2-Max) - degradation of muscle/liver tissue, increase in epinephrine and FFA
Anaerobic - high intensity (80-100% VO2-Max) - low FA input, accumulation of lactate

Question 17

Response to Training

Answer 17

Increase to muscle size, fiber type transitions, increase to capillary density (transport more), improve buffer capacity (resistance to lower pH)