4 (Final): Bioenergetics, Muscle Twitch and Fiber Types

Comprehensive vocabulary based on lecture notes covering muscle bioenergetics, metabolic pathways, motor units, and skeletal muscle fiber types.

Bioenergetics

The study of the transformation of energy in living organisms — how the body breaks down nutrients and converts them into $ATP$.

Adenosine Triphosphate (ATP)

A high energy, unstable molecule composed of three phosphate groups attached to a nitrogenous base of adenine, acting as a rechargeable battery for cellular processes.

Nutrient

A substance in food that the body uses to promote normal development, growth, maintenance, and repair.

Metabolism

The sum of all chemical reactions occurring in the body, resulting from the balance of anabolic and catabolic reactions.

Anabolic Reactions

Chemical reactions that build complex molecules from simpler ones.

Catabolic Reactions

Chemical reactions that break down complex molecules into simpler ones, releasing energy used to make $ATP$.

Macronutrients

Nutrients required in relatively large amounts, including Carbohydrates ($CHO$), Lipids (fats), and Proteins.

Gluconeogenesis

The process occurring in the liver and kidneys where amino acids are converted into glucose.

Glycogenolysis

The process of breaking down glycogen into glucose when needed by skeletal muscle and liver cells.

Glycogenesis

The conversion of lactic acid or glucose into glycogen for storage in liver cells.

Kilocalorie (kcal)

The amount of heat energy needed to raise the temperature of $1$ kilogram of water $1^{\circ}\text{C}$. One gram of $CHO$ or protein contains $4\,kcal$, while lipids contain $9\,kcal$.

Micronutrients

Vitamins and minerals needed in small amounts that are essential for survival, acting as coenzymes or regulating fluid balance.

Cellular Respiration

A group of catabolic reactions involving glycolysis, the creation of Acetyl CoA, the citric acid cycle, and oxidative phosphorylation to produce $ATP$.

Creatine-Phosphate (CP) System

A system where creatine kinase transfers a phosphate group from $CP$ to $ADP$ in the cytosol, providing energy for under $15$ seconds of vigorous activity.

Glycolysis

A multi-step anaerobic process in the cytosol that converts one glucose molecule into $2$ pyruvic acid, $2\,ATP$, and $2\,NADH + H^+$.

Anaerobic Cellular Respiration

The pathway where $NADH + H^+$ unloads hydrogens onto pyruvic acid, converting it to lactic acid when $O_2$ is absent; yields $2\,ATP$ per glucose.

Aerobic Cellular Respiration

The sequence of glycolysis, Acetyl CoA formation, Krebs cycle, and the Electron Transport Chain occurring in mitochondria to produce $\sim 30-32\,ATP$ using oxygen.

NAD+ and FAD

Coenzymes that act as shuttles for hydrogen atoms during cellular respiration to deliver them to the electron transport chain.

Acetyl CoA

The intermediary molecule formed from the conversion of pyruvic acid, fatty acids, or amino acids that enters the citric acid cycle.

Electron Transport Chain (ETC)

A series of enzyme complexes in the inner mitochondrial membrane that use redox reactions to establish an electrochemical gradient of protons.

Chemiosmosis

The movement of $H^+$ ions through $ATP$ synthase down their electrochemical gradient to drive the phosphorylation of $ADP$ into $ATP$.

Oxidative Phosphorylation

The synthesis of $ATP$ by phosphorylating $ADP$ via the Electron Transport Chain within the mitochondria.

Aerobic Endurance

The length of time a muscle can continue to contract using aerobic pathways.

Anaerobic Threshold

The point at which muscle metabolism converts from aerobic processes to anaerobic glycolysis.

Motor Unit

A single motor neuron and all the muscle fibers it innervates; when it fires, all associated fibers contract simultaneously.

Muscle Twitch

The contraction of a single motor unit generated by a single action potential.

Latent Period

The first few milliseconds after stimulation when $Ca^{2+}$ diffuses from the SR, cross bridges form, and muscle 'slack' is removed.

Relaxation Phase

A period of $10-100\,msec$ where $Ca^{2+}$ is pumped back into the SR, cross-bridges deactivate, and force declines.

ATPase

The enzyme on myosin that hydrolyzes $ATP$ into $ADP + Pi$; its speed determines the rate of cross-bridge cycling and twitch speed.

Slow Oxidative (SO) Fibers

Type I fibers that contract slowly, use aerobic respiration, are fatigue-resistant, and appear red due to high myoglobin and capillary supply.

Fast Glycolytic (FG) Fibers

Type IIB fibers that contract quickly, rely on anaerobic glycolysis, fatigue fast, and appear white with large diameters and high glycogen stores.

Fast Oxidative Glycolytic (FOG) Fibers

Type IIA intermediate fibers that are red and rely on both aerobic and anaerobic systems; they can adapt properties based on training stimulus.

Myoglobin

A protein that binds $O_2$ within muscle fibers to provide a supply to mitochondria when needed.