PHSL 3051 energy metabolism and muscle fiber types

ATP production in skeletal muscle

1. create kinase

2. glycolysis

3. oxidative phosphorylation

create kinase

• one step reaction

• transfers phosphate from Creatine-phosphate to ADP to regenerate ATP

• creatine phosphate provides a reservoir of high energy phosphate which is accumulated during rest in muscle cells

• the action of Creatine Kinase provides ATP during the first few seconds of a contraction

• the ATP production is rapid, but limited by the amount of Creatine phosphate stored in the cell

• powers very short periods of muscle activity

glycolysis

• multistep reaction

• anaerobic

• converts 1 glucose to 2 ATP

anaerobic:

• can produce ATP from glucose in the absence of O2 - kicks in during high intensity exercise

• in the presence of large amounts of glucose, can produce large quantities of ATP

• glucose can come from the blood or from breakdown of muscle glycogen

• powers relatively short periods of muscle activity

oxidative phosphorylation

• multistep reaction

• requires oxygen (O2) and mitochondria

• converts 1 glycose to 36 ATP

• converts fatty acids to ATP

• converts amino acids to ATP

• phosphorylation of ADP occurs in mitochondria

• multi-enzyme pathway that requires O2

• initially (first 5-10 mins of activity), glucose from glycogen is the major fuel

• next 30 mins, blood borne glucose and fatty acids contribute fuel, eventually giving way to mostly fatty acids

• powers extended periods of muscle activity

muscle fiber types

1. classified according to maximal velocity of shortening (ex. how quickly myosin can hydrolyze ATP/fast or slow twitch)

2. major pathway they use to form ATP

• slow oxidative

• fast oxidative

• fast glycolytic

slow oxidative fibers

• small diameter = less tension

• slow myosin so tension development is slow

• mostly oxidative phosphorylation which requires O2

• many mitochondria and many capillaries to provide O2

• large amounts of myoglobin to aid in O2 diffusion and to store O2 (dark meat)

• resistant to fatigue 

• ex. postural muscles

fast oxidative fibers

• larger diameter = more tension

• fast myosin ATPase so tension development is faster

• mostly oxidative phosphorylation which requires O2

• many mitochondria and capillaries to provide O2

• large amounts of myoglobin to aid in O2 diffusion and to sore O2 (dark meat) - less myoglobin than slow oxidative muscle

• somewhat resistant to fatigue

fat glycolytic fibers

• larger diameter = more tension

• fast myosin ATPase so tension development is fast

• mostly glycolytic metabolism (does not depend on O2)

• large amounts of glycogen to provide fuel for glycolysis

• fewer capillaries and mitochondria

• essentially no myoglobin (white meat)

• prone to fatigue 

motor units

motor unit - motor neuron and the population of muscle fibers it innervates

• two motor units can generate more tension than one

• when a large amount of tension needs to be generated, more motor neurons (and hence muscle fibers) are recruited

mechanism of fatigue

fatiguability of skeletal muscles - slow oxidative

• slow myosin ATPase

• least amount of tension (least amount of force)

• least likely to fatigue

fatiguability of skeletal muscles - fast oxidative

• fast myosin ATPase

• moderate amount of tension (moderate force)

• moderately likely to fatigue

fatiguability of skeletal muscles - fast glycolytic

• fast myosin ATPase

• greatest amount of tension (high force)

• most likely to fatigue rapidly

factors affecting how much tension a whole muscle can produce

1. the number of active motor units

2. the number of muscle fibers in each motor unit

3. the fiber types of the activated motor units

recruitment

motor unit 1: slow oxidative

• small diameter muscle fibers

• few fibers per motor unit

• generates least tension

motor unit 2: fast oxidative

• midsized muscle fibers

• moderate # of fibers per motor unit

• generates moderate tension

motor unit 3 - fast glycolytic

• large muscle fibers

• many fibers per motor unit

• generates most tension, quickly

lever actions of muscle and bones

• the arrangement of muscles sometimes creates a mechanical disadvantage, meaning that the force required to move an object is greater than the weight of the object itself

• these disadvantages are typically offset by increased maneuverability due to lever actions around the joints

• the physical levers means that small changes in muscle length can translate into much larger movements of the body

• the speed of the movement is also amplified

actions of muscles attached to the skeleton (arm)

• some of the muscle attachments to the skeleton are arranged as antagonistic pairs

• the biceps and triceps of the upper extremity (arm and forearm) are a useful example

• the contraction of the either muscle opposed the action of its partner

actions of muscles attached to the skeleton (leg)

• the actions of antagonists depend on the joint in the act on and state of the antagonist pair

• when relaxed, concentration of the gastrocnemius of the leg flexes the leg at the knee

• when both muscles are contracted simultaneously, the knee is locked by their action and the foot extends and can raise the body on the toes