Muscle Physiology and Metabolism: Energy Systems and Fiber Types

Core Concepts in Muscle Physiology and Metabolism

Energy Systems and Regulation

400m Athlete Energy Use

The 400m athlete primarily utilizes the anaerobic alactic system.

Glycogenolysis

Glycogenolysis is the decomposition of glycogen stored by the cell for reuse as glucose. The catalytic enzyme is phosphorylase.

Negative Modulators of Glycolysis

The negative modulator of glycolysis is Phosphofructokinase (PFK). PFK controls the response to Citric Acid (a product of the Krebs cycle), ATP, and pH. PFK slows down glycolytic action when large numbers of any of these modulators are present.

Carnitine and Fatty Acid Transport

Carnitine transports fatty acids and is responsible for introducing them into the mitochondria.

Muscle Anatomy and Function

Myofibril Structure

The Myofibril is the contractile element of muscles. Long filaments appear, composed of small subunits called sarcomeres.

Intrafusal Fibers

Intrafusal fibers are innervated by the gamma motor neuron.

Acute Muscle Inflammation

Acute muscle inflammation is an inflammatory response produced immediately after a workout. It is due to the accumulation of water and lactate (hydrogenic ions).

Pachymeter Use

A Pachymeter (caliper) is used for measuring small diameters, such as adipose folds.

Storage Proteins and Amino Acid Fate

Storage proteins are used for forming structures. When all necessary structures are formed, the remaining amino acids (AAs) change function. Their contribution may result in excessive fat or sugar storage, or they may be removed.

Detailed Physiological Processes

Muscle Fiber Types: Slow Twitch vs. Fast Twitch

Slow Twitch (ST) Fibers (Type I, Red)

• Slow form of Myosin ATPase.
• Large number of mitochondria and high oxidative capacity.
• Smaller cell body, innervates 10–180 muscle fibers.
• Great endurance.

Fast Twitch (FT) Fibers (Type II, White)

• Fast Myosin ATPase.
• Increased Sarcoplasmic Reticulum (SR) for Ca++ storage.
• Large cell body, supplies 300–800+ muscle fibers.
• Low resilience (endurance).
• Requires less irrigation because it relies less on oxygen.

Lipid Metabolism and Beta-Oxidation

Lipids are stored in adipocytes as triglycerides. Fatty acids can also be found in the liver and muscle.

The triglycerides are broken down into glycerol and fatty acids. Fatty acids bind to albumin for transport through the blood. The fatty acid enters the sarcoplasm, changes its morphology, and requires 2 ATP. The fatty acid then joins carnitine to enter the mitochondria, where it undergoes the process of Beta-oxidation.

This process yields Acetyl CoA, which enters the Krebs cycle.

Steps of Lipid Utilization:

1. Mobilization
2. Transport
3. Muscle Uptake
4. Action (Entry into Sarcoplasm)
5. Beta-oxidation

Motor Impulse Transmission and Muscle Contraction

To initiate muscle contraction, an impulse must be sent. The neurotransmitter is Acetylcholine.

1. The neuron reaches the sarcolemma, causing depolarization and the release of Calcium (Ca++).
2. Ca++ binds to the C subunit of Troponin.
3. Troponin changes shape, dragging Tropomyosin and freeing the binding sites on Actin and Myosin.
4. When Myosin binds to Actin, it generates the power stroke (or "wing coup").
5. The thin filaments slide closer together, resulting in contraction.