Muscle & Neural Physiology and Bioenergetics

Vocabulary-style flashcards covering the basic structures of the nervous system, excitation-contraction coupling, bioenergetics, cardiovascular responses, and exercise adaptations.

Central Nervous System (CNS)

Consists of the brain and spinal cord.

Peripheral Nervous System (PNS)

Includes all nerves outside the brain and spinal cord.

Afferent Division

The division of the PNS that carries sensory information TOWARD the CNS.

Somatic Motor Division

The division of the PNS that carries motor commands AWAY from the CNS to skeletal muscle.

Excitation-Contraction Coupling

The correct order of events is: ACh release → action potential → $Ca^{2+}$ release → cross-bridge formation.

Acetylcholine (ACh)

The primary neurotransmitter for motor neurons that innervate skeletal muscle; its release is triggered by an action potential arriving at the axon terminal.

Motor End-Plate

The location on the plasmalemma of the muscle fiber where acetylcholine binds to initiate depolarization.

T-tubules

Extensions of the plasmalemma (sarcolemma) that carry the action potential deep into the muscle fiber to trigger calcium release from the sarcoplasmic reticulum.

Sarcoplasmic Reticulum (SR)

Structure that functions primarily to store and release calcium ions ($Ca^{2+}$).

Troponin

The protein that binds calcium released from the SR, causing tropomyosin to move off the myosin-binding site on actin.

Tropomyosin

The protein that, at rest, covers the myosin-binding site on actin to block cross-bridge formation.

Cross-bridge

Formed when the globular heads of myosin bind to actin during muscle contraction.

Sliding Filament Theory

States that during contraction, filaments slide past each other as myosin pulls actin toward the center of the sarcomere.

Calcium Fate (Post-Contraction)

When action potentials stop, calcium is actively pumped back into the sarcoplasmic reticulum using ATP (active transport).

Synapse

The gap between two neurons where communication occurs via neurotransmitters.

Presynaptic Neuron

The neuron that releases neurotransmitters into the synapse.

Norepinephrine

The primary neurotransmitter for most sympathetic neurons.

Excitatory Postsynaptic Potential (EPSP)

Causes depolarization of the postsynaptic membrane.

Inhibitory Postsynaptic Potential (IPSP)

Causes hyperpolarization of the postsynaptic membrane.

Motor Unit

Consists of one alpha motor neuron and all the muscle fibers it innervates.

Size Principle

States that motor units are recruited in order of size, with small Type I motor units recruited first and large Type IIx recruited last.

Myelin Sheath

Lipid-rich insulating layers from Schwann cells that increase the speed of nerve impulse conduction.

Saltatory Conduction

The process by which an action potential "jumps" from one Node of Ranvier to the next in myelinated nerves.

Resting Membrane Potential (RMP)

Approximately $-70\,mV$ in a neuron, where the inside is negatively charged relative to the outside.

Sodium-Potassium Pump

Maintains RMP by actively transporting $3\,Na^+$ out of the cell and $2\,K^+$ into the cell.

Depolarization

Occurs when the inside of a neuron becomes less negative (e.g., from $-70\,mV$ to $-60\,mV$).

Hyperpolarization

Occurs when the inside of a neuron becomes more negative (e.g., from $-70\,mV$ to $-80\,mV$).

Graded Potentials

Localized changes in membrane potential that vary in strength, differing from the all-or-none nature of action potentials.

All-or-None Principle

States that once the threshold is reached, an action potential fires at maximum strength; otherwise, no action potential occurs.

ATP Hydrolysis ($ATP \rightarrow ADP + P_i$)

Provides the energy for the myosin head power stroke and the reuptake of calcium into the SR.

Muscle Spindles

Sensory receptors sensitive to muscle length and rate of change of length; triggering reflexive contraction when stretched.

Golgi Tendon Organs (GTOs)

Located in the tendon, these are sensitive to changes in muscle tension (force) and inhibit the contracting muscle to prevent injury.

Force-Velocity Relationship

In concentric contractions, maximal force decreases as velocity increases; in eccentric contractions, maximal force increases at higher speeds.

Epimysium

The connective tissue layer that surrounds the entire muscle.

Perimysium

The connective tissue layer surrounding individual fascicles (bundles of fibers).

Endomysium

The connective tissue layer surrounding individual muscle fibers.

Sarcomere

The repeating contractile unit of a myofibril, composed of actin (thin) and myosin (thick) filaments.

Titan

A third myofilament that acts as a spring, stabilizes the sarcomere, and prevents overstretching.

Beta-oxidation

The aerobic breakdown of free fatty acids into acetyl-CoA occurring in the mitochondria.

ATP-PCr System

The energy system that produces ATP at the fastest rate but only sustains maximal exercise for $3-15\,seconds$.

Glycolytic System

An anaerobic system that can sustain high-intensity exercise for approximately $15\,seconds$ to $2\,minutes$.

Oxidative System

The aerobic system that produces the most ATP per molecule of substrate and can sustain exercise for hours.

Gluconeogenesis

The process of converting protein (amino acids) or other non-carbohydrate sources into glucose.

Creatine Kinase (CK)

The rate-limiting enzyme for the ATP-PCr system, activated when ATP levels are low and ADP levels are high.

Phosphofructokinase (PFK)

The rate-limiting enzyme for glycolysis, inhibited by high levels of ATP.

Isocitrate Dehydrogenase

The rate-limiting enzyme for the Krebs cycle (oxidative system).

Net ATP from Glucose

The oxidative system produces a net of $32\,ATP$ from one molecule of glucose ($2\,ATP$ from glycolysis, $2\,ATP$ from the Krebs cycle, and the rest from the ETC).

Electron Transport Chain (ETC)

A series of proteins in the mitochondria that create an $H^+$ gradient to synthesize ATP; yields $2.5\,ATP$ per NADH and $1.5\,ATP$ per $FADH_2$.

Oxygen (O₂)

The final electron acceptor in the electron transport chain.

Direct Calorimetry

A method of measuring energy expenditure by measuring heat production directly.

Indirect Calorimetry

Estimates energy expenditure by measuring oxygen consumed ($VO_2$) and carbon dioxide produced ($VCO_2$).

Oxygen Deficit

The lag in oxygen consumption at the start of exercise where oxygen demand exceeds consumption, requiring anaerobic ATP production.

Excess Postexercise Oxygen Consumption (EPOC)

The increased oxygen consumption after exercise used to replenish ATP/PCr stores, convert lactate to glycogen, and replenish oxygen in blood/muscle.

Intercalated Disks

Structures in cardiac muscle containing desmosomes and gap junctions that allow for rapid conduction of action potentials between cells.

AV Node Delay

A delay in the electrical impulse that allows the atria to contract and the ventricles to fill completely before they contract.

Vagus Nerve

Carries parasympathetic stimulation to the heart, releasing acetylcholine to decrease heart rate.

Stroke Volume (SV)

The volume of blood pumped per beat, calculated as $EDV - ESV$.

Cardiac Output ($Q$)

The total volume of blood pumped per minute, calculated as $HR \times SV$; resting value is approximately $5\,L/min$.

Ejection Fraction (EF)

The percentage of blood pumped out of the left ventricle per beat, calculated as $SV / EDV$; normal resting value is ~$60\%$.

Mean Arterial Pressure (MAP)

The average driving pressure for tissue perfusion, calculated as $DBP + \frac{1}{3}(SBP - DBP)$; depends on cardiac output and total peripheral resistance.

Arterioles

Known as "resistance vessels," they are the primary site of resistance and are responsible for $70-80\%$ of the pressure drop in the vasculature.

Functional Sympatholysis

The inhibition of sympathetic vasoconstriction in active muscle during exercise to allow for increased blood flow.

Fick Equation

States that $VO_2 = Q \times (a-\bar{v})O_2\text{ difference}$, integrating the cardiovascular, respiratory, and skeletal muscle systems.

Primary Factor for VO2max Increase

Increased maximal stroke volume (and thus maximal cardiac output) due to endurance training.

Interference Effect

The phenomenon where endurance training blunts strength gains when strength and endurance training are performed concurrently (cross-training).