Cellular Metabolism & Exercise

• Physiology is the science of HOW the body works.
• It studies living mechanisms, from molecular cell function to integrated behavior.
• It studies how molecules, cells, and organs interact to form a whole being.
• It is a branch of biology, dealing with the functions and activities of living organisms, including physical and chemical processes.

Levels of Organization

• Cells are the smallest level.
• Tissues are made of cells.
• Organs are made of tissues.
• Organ Systems are made of organs.
• An organism comprises organ systems.

Na+-K+ Pump, ATPase

• Pumps sodium ions out of the cell and potassium ions into the cell using ATP.

Membrane Transport

• Simple Diffusion: Molecules move across the lipid bilayer.
• Carrier-Mediated transport requires energy.
• Channel-mediated transport is passive.
• Active Transport requires energy.
• Passive Transport does not require energy.

Receptor-Mediated Signalling

• Signaling molecules, like hormones, bind to receptor proteins.
• This binding triggers intracellular signal molecules, affecting target proteins and causing a response.

Sarcoplasmic Reticulum

• Specialized for calcium storage and release in muscle cells.
• It includes triad structures, T-tubules, and terminal cisternae.
• I bands mark the area containing the thin filaments (actin).
• A bands mark the area containing the thick filaments (myosin).
• Z discs define the boundaries of the sarcomere.
• H zone defines the region containing only myosin.

Events Leading to Muscle Action

• An action potential arrives at the axon terminal, releasing acetylcholine (ACh).
• ACh binds to receptors, depolarizing the muscle fiber and signaling the sarcoplasmic reticulum to release Ca2+.
• Ca2+ binds to troponin, exposing active sites on actin.
• Myosin heads bind to actin, forming cross-bridges.
• ATP is hydrolyzed, causing the myosin head to cock back for the power stroke, sliding the thin filament over the thick filament.
• Following nervous stimulation, ACh is broken down, and calcium ions are reabsorbed by the sarcoplasmic reticulum.
• Tropomyosin returns to its blocking position, ceasing muscle contraction.

Muscle Twitch

• A muscle twitch involves a period of contraction.
• The cross-bridges are active, and the muscle shortens if tension overcomes the load.
• Relaxation occurs as Ca2+ is pumped back into the sarcoplasmic reticulum, with muscle tension decreasing to baseline in 10-100 ms.

Muscle Fiber Types

• Type I (slow oxidative) fibers contract weakly.

• Type 1 fibers use aerobic respiration, are fatigue resistant, and have a twitch time of 200 ms.

• Type IIa (fast oxidative-glycolytic) fibers contract more strongly.

• Type IIa fibers use aerobic and anaerobic respiration, are more fatiguable than Type I fibers, and have a twitch time of 100 ms.

• Type IIx (fast glycolytic) fibers contract very strongly.

• Type IIx fibers use anaerobic respiration, are highly fatiguable, and have a twitch time of 50 ms.

Fiber Type Adaptations to Exercise

• Aerobic exercise increases oxidative capacity and capillary number, converting Type IIx to oxidative fibers and decreasing fiber diameter.
• High-intensity exercise decreases oxidative capacity with lower mitochondrial size, converting Type IIa to glycolytic fibers, and increasing fiber diameter.

Length-Tension Relationship

• Muscle contraction strength is intimately related to its length.

Motor Unit Recruitment

• Orderly recruitment means motor units are recruited based on their size.
• The size principle: recruitment is directly related to motor neuron size.

Action Potential Stages

• A resting membrane potential is established due to the Na+/K+ pump
• A depolarizing stimulus is received.
• Membrane depolarizes to threshold.
• Voltage-gated Na+ channels open so Na+ enters the cell
• Cell rapidly depolarizes.
• Na+ channels close, and slower K+ channels open.
• K+ leaves the cell causing repolarization
• K+ channels remain open and additional K+ leaves cell, causing hyperpolarization
• The neuron returns to normal resting membrane potential

Neural Integration

• Spatial and temporal summation are needed to generate an action potential.

Sensory-Motor Integration

• Sensory receptors receive a stimulus.
• The action potential travels to the CNS.
• The CNS interprets the information and determines the motor response.
• The motor action potential travels out from the CNS through motor neurons.
• The action potential reaches the muscles, initiating a response.

Reflex Arc

• Involves a receptor, sensory neuron, integration center, motor neuron, and effector.
• Functions in reactions like flexor reflexes and crossed-extensor reflexes.

Golgi Tendon Reflex

• Protects tendons from excessive tension by inhibiting agonists and exciting antagonists.

Energy Systems

• Anaerobic systems include the ATP-PCr system and glycolysis.
• Aerobic systems include the Krebs cycle and the electron transport chain.
• The goal of each of these systems is to meet the cellular demand for ATP.

Glycolysis

• Occurs in the cytosol, starting with glucose.
• Produces pyruvate, NADH, and a net gain of 2 ATP.

Krebs Cycle

• Occurs in the mitochondria, starting with Acetyl CoA.
• CO2 , NADH, FADH2 and ATP is produced.
• When considering (for each Acetyl CoA): 3 NADH + H+, 1 FADH2, and 2 CO2, resulting in 1 ATP.
• Results in 2 ATP for glucose/glycogen and X 8 for an 16C FFA.

Electron Transport Chain

• Uses energy from NADH + H+ and FADH2 to pump hydrogen ions across the inner mitochondrial membrane.
• Hydrogen ion concentration gradient drives ATP production.

Fat Metabolism

• Fatty acids travel to the sarcolemma, then beta oxidation in the muscle, producing pyruvate
• It Uses the CPT-I and CPT-II system.

ATP Production Pathways

• PCr system provides rapid ATP.
• Glycolysis produces ATP from glucose/glycogen.
• Beta-oxidation breaks down fatty acids.
• The Krebs cycle metabolizes acetyl CoA.
• The electron transport chain generates the most ATP.

Energetics of FA oxidation

• The beta-oxidation of palmitic acid will be repeated 7 cycles
• This produces 8 molecules of acetyl COA.

Energy Expenditure

• Metabolic rate is the rate of energy use by the body.
• Respiratory Exchange Ratio (RER) indicates fuel use, ranging from 0.71 (fats) to 1.00 (carbohydrates).
• Caloric equivalent is around 4.8 kcal/L O2.

Fatigue

• Peripheral fatigue occurs within muscle fibers.
• Central fatigue originates in the central nervous system.

Causes of Fatigue

• Energy systems: inadequate energy delivery/metabolism.
• Accumulation of metabolic by-products like H+ and lactate.
• Neuromuscular failure.
• Central Governor: altered neural control.

Explore ATP production in glycolysis, the role of the electron transport chain, and key enzymes in metabolism. Investigate fatty acid oxidation, metabolic rates, and causes of exercise-related fatigue. Test your knowledge of cellular respiration stages.