Lactate Removal, Energy Sources & RER

Lactate Removal and Fates

• Lactate accumulated during exercise is removed post-exercise.
• Lactate levels during light exercise are lower than at rest.
• Intracellularly, some lactate converts to muscle glycogen.
• About 25% of lactate is used in gluconeogenesis.
• Gluconeogenesis involves lactate traveling via blood to the liver and kidneys.
• Most (75%) of lactate is used in muscle aerobic metabolism.
• Fast-twitch muscle cells excel at lactate removal.
• Muscles efficient at producing lactate can also produce glycogen.

Energy Sources and Respiratory Exchange Ratio (RER)

• Cells can use proteins for ATP production, but it's not ideal, they mainly produce carbs and lipids.
• Fast-twitch muscles prefer carbohydrates.
• Lactate hinders the release of fatty acids into the blood.
• RER (VCO2/VO2) ranges from 0.7 to 1.0, indicating the ratio of carbon dioxide produced to oxygen consumed.
• As the R-value increases, lipid contribution decreases, and carbohydrate contribution increases.
• Higher exercise intensity corresponds to a higher R-value.
• Glycogen becomes more important with increased exercise intensity.
• The body can store more lipids than glycogen.

Neuroendocrinology Overview

• The nervous and endocrine systems control the body and communicate using chemical messengers: neurotransmitters (nervous) and hormones (endocrine).
• Neurons are the primary cells of the nervous system.
• Synapses are communication points between neurons, involving presynaptic and postsynaptic neurons.

Neuronal Function

• In a neuron's resting state, the outside is negative, and the inside is positive.
• Stimulation of a neuron causes Na+ to enter the cell, potentially leading to an action potential.
• During an action potential, the inside of the neuron becomes more positive than the outside.
• Action potentials signal neurons to communicate.
• When an action potential reaches the axon terminal, it triggers the release of neurotransmitters.

Nervous System Structure

• The nervous system comprises the Central Nervous System (CNS) and Peripheral Nervous System (PNS).
• The CNS (brain & spinal cord) acts as the control center.
• The PNS includes sensory & motor divisions, further divided into somatic & autonomic systems, and then sympathetic & parasympathetic branches.

Brain Control Levels

• Most movement signals originate in the brain.
• Basal nuclei (2nd level of control): scattered throughout the brain, strongly connected to the cerebral cortex, and coordinate learned acts like posture.
• Damage to the basal nuclei can lead to uncontrollable shaking and involuntary movements, such as in Parkinson’s Disease.
• Cerebellum (3rd level of control): takes feedback from muscles to refine new movement signals, regulating posture and balance.

Spinal Motor Tracts

• Direct (pyramidal) tracts originate in the cortex for voluntary movement.
• Indirect (extrapyramidal) tracts originate below the cortex and control balance and equilibrium.
• Signals travel down the spinal cord (descending) to neurons that carry signals to the rest of the body.
• Somatic motor neurons, starting in the spinal cord, branch out to skeletal muscles.

Autonomic Motor System

• Receives sensory input from chemoreceptors, osmoreceptors, mechanoreceptors, and hormone receptors.
• Integrating centers, mainly the hypothalamus and medulla, regulate electrolyte/fluid balance, metabolism/fuel use, blood pressure/cardiac output, and temperature.
• Sympathetic function: fight/flight response, adrenal medulla, mass or individual activation.
• Parasympathetic function: resting functions, vagus nerve, individual activation.

Hormones and Transport

• Hormones, secreted by endocrine glands, enter the bloodstream to bind to target cells.
• Transports: Endocrine Gland -> Bloodstream (water) -> Cell Membrane (lipid) -> Cytoplasm (water).
• Hormones must detach from carriers to leave the blood and take effect.
• Hormone entry increases concentration, while hormone exit decreases concentration.
• Hormone secretion increases concentration, while hormone metabolism lowers it.
• The liver and kidneys remove excess hormones.

Hormone Receptors

• Effective hormone action requires correct receptors and receptor amounts.
• More receptors indicate higher sensitivity.
• Down-regulation: high hormone levels reduce receptors.
• Up-regulation: low hormone levels add receptors.

Mechanisms of Hormone Action

• Lipophilic hormones (steroid hormones and thyroxine) stimulate protein synthesis by moving through the cell membrane.
• They bind to receptors in the cytoplasm (then move into the nucleus) or in the nucleus directly.
• The hormone-receptor protein complex attaches to genes to stimulate protein synthesis.
• Many hormones use 2nd messengers to activate intracellular components.
• Three signal transduction methods for lipophilic hormones: channel-linked, enzyme-linked, and G-protein-linked receptors.
• Enzyme-linked receptors: hormone-receptor interaction activates enzymes, which then activate other enzymes to alter metabolism.
• G-Protein-Linked Receptors: hormone-receptor interaction activates the G-protein, which either opens/closes a channel or activates a 2nd messenger.

Hormone Interactions

• Synergistic: hormones work together (e.g., epinephrine and norepinephrine on the heart increase HR).
• Permissive: one hormone enhances another's effect (e.g., thyroxine and epinephrine).
• Antagonistic: one hormone negates the effects of another (e.g., insulin and glucagon on blood glucose).

Hormones and Exercise - Pituitary Gland

• Produces thyroid-stimulating hormone.
• Growth hormone is released in response to hypoglycemia, exercise, sleep, and stress.
• Growth Hormone leads to: increased protein synthesis, lipid utilization, and bone growth; decreased carbohydrate utilization.
• Antidiuretic hormone is released in response to dehydration.
• Antidiuretic Hormone leads to: an increase in water reabsorption from the kidney and a reduction in urine output.

Hormones and Exercise - Thyroid Gland

• Thyroxine is released in response to exercise, cold exposure, and stress.
• Thyroxine leads to: an increase in metabolic rate, carb utilization, lipid utilization, and heat production.
• Thyroxine is the biggest contributor to metabolic rate.

Hormones and Exercise - Adrenal Cortex

• Cortisol is released in response to hypoglycemia, exercise, and stress.
• Cortisol leads to increased lipid utilization and protein utilization (gluconeogenesis) and decreased carb utilization.
• High cortisol levels contribute to muscle breakdown and can block inflammation.

Hormones and Exercise - Gonads

• Sex steroids include testosterone (most important androgen) and estradiol (most important estrogen).

Hormones and Exercise - Adrenal Medulla

• Epinephrine and norepinephrine are released in response to sympathetic stimulation.
• Epinephrine and norepinephrine lead to: increased carb utilization, lipid utilization, cardiac output, and muscle blood flow.
• These are crucial for exercise response (fight or flight).

Hormones and Exercise - Pancreas

• Insulin (feasting hormone) is released in response to hyperglycemia and high blood amino acid content.
• Insulin leads to increased glucose uptake (GLUT insertion), glycogen synthesis, lipid synthesis, and protein synthesis and decreased lipid utilization.
• Glucagon (fasting hormone) is released in response to hypoglycemia.
• Glucagon leads to increased blood glucose, liver glycogenolysis, and gluconeogenesis.

Carbohydrate Utilization

• The liver and skeletal muscles store glycogen.
• Blood glucose is crucial for nervous system function.
• Glycogenolysis (glycogen to glucose) is stimulated by epinephrine and Ca++.
• Hormone levels increase with exercise intensity and duration to maintain blood glucose.
• Calcium and insulin cause GLUT proteins insertion into the cell membrane.
• Glucose uptake increases in active muscle cells during exercise.

Water and Electrolyte Levels

• Sodium (Na+) is the most important electrolyte.
• Osmosis: water moves to areas of high solute concentration (high osmolarity).
• Osmolarity (Osm) = Ratio of solute to H20 critical to osmosis.
• Water moves across a membrane from a solution of lower osmolarity (hypo-osmotic) to a solution of higher osmolarity (hyper-osmotic).
• Normal plasma osmolarity = 300 mOsm.
• Osmoreceptors measure plasma osmolarity.
• Stretch receptors are affected by changes in blood volume.
• Aldosterone promotes Na+ reabsorption and K+ secretion and excretion.
• Low Na+ levels lead to less water retention.

Skeletal Muscle Structure

• Muscles attach via connective tissue.
• Organization: Fascicle > muscle fibers > myofibrils > sarcomeres.
• Myofibrils are individual muscle fibers.
• Sarcolemma is the membrane.
• The sarcoplasmic reticulum stores calcium.
• Actin makes up thin filaments.
• Dystrophin acts as an anchor.

Sliding Filament Theory

• Myosin heads attach to and pull on thin filaments to generate force.
• The H zone gets smaller as myosin heads pull filaments closer together.
• Filaments slide over one another.