NPB101 Midterm 2

Study Guide Material

What types of muscle are striated, and what control systems regulate them?

Skeletal and cardiac muscle are striated due to their organized contractile elements. Skeletal muscle is under voluntary control, while cardiac and smooth muscle are involuntary and regulated by the autonomic nervous system (ANS).

Why is smooth muscle not striated?

Smooth muscle lacks the organized sarcomere structure found in skeletal and cardiac muscle, so it appears non-striated under the microscope.

What are the levels of muscle organization from largest to smallest?

Bulk muscle → Muscle fiber (single cell) → Myofibrils → Sarcomeres → Thick and thin filaments (basis of the sliding filament mechanism).

What is excitation–contraction coupling?

It is the process linking muscle fiber excitation (action potential) to contraction via calcium release and actin–myosin interaction.

What are the key steps in excitation–contraction coupling?

• Synaptic transmission at the neuromuscular junction (ACh release → depolarization).

• Action potential spreads along the muscle membrane and into T-tubules.

• DHP receptors in T-tubules detect voltage change → activate ryanodine receptors.

• Ryanodine receptors release Ca²⁺ from the sarcoplasmic reticulum.

• Ca²⁺ binds troponin → moves tropomyosin → exposes myosin-binding sites on actin.

• Actin–myosin cross-bridge cycling occurs using ATP (power stroke).

• Ca²⁺ is pumped back into SR → relaxation.

What is the “power stroke” in muscle contraction?

The power stroke is when the myosin head pivots, pulling the actin filament toward the center of the sarcomere, powered by ATP hydrolysis.

What defines an isotonic contraction?

The muscle changes length while generating tension.

Differentiate between concentric, eccentric, and isometric contractions.

• Concentric: Muscle shortens (tension > load).

• Eccentric: Muscle lengthens (load > tension).

• Isometric: Muscle length stays constant while generating tension.

How are tension and load related?

• If tension > load → muscle shortens.

• If load > tension → muscle lengthens.

What is a muscle twitch?

A muscle twitch is the tension produced by a single action potential in a muscle fiber.

What does a typical isometric twitch measure?

It measures the time course of tension development without length change.

What does a concentric twitch measure?

It measures the change in muscle length during shortening in response to one action potential.

What are the two major tension relationships in muscle physiology?

• Frequency–tension relationship: More frequent stimulation increases tension (summation).

• Length–tension relationship: Optimal sarcomere length produces maximal tension (important for the Frank–Starling law).

What explains the Frank–Starling law in cardiac muscle?

it’s based on the length–tension relationship — increased stretch (preload) leads to stronger contraction due to optimal actin–myosin overlap.

How can strength increase without an increase in muscle mass?

Through improved neuromuscular efficiency — better recruitment and synchronization of motor units.

What causes strength increases accompanied by muscle mass growth?

Addition of new myofibrils within existing muscle fibers (hypertrophy).

What causes muscle mass increase without strength gain?

Increase in non-contractile elements (sarcoplasm), known as sarcoplasmic hypertrophy.

Define a motor unit.

A motor unit consists of one motor neuron and all the muscle fibers it innervates.

How many axons innervate a single muscle fiber?

Exactly one motor axon per muscle fiber.

How does motor unit size affect muscle control?

Small motor units (5–10 fibers) allow fine control; large motor units (~1,000 fibers) generate powerful movements but less precision.

How are motor units recruited?

Recruitment follows the size principle — smaller motor units activate first; if more force is needed, larger units are recruited.

What are the three main muscle fiber types?

• Slow-oxidative (Type I): Low myosin ATPase, high oxidative capacity, fatigue-resistant (endurance).

• Fast-oxidative-glycolytic (Type IIa): High myosin ATPase, high oxidative + moderate glycolytic capacity.

• Fast-glycolytic (Type IIx): High myosin ATPase, high glycolytic capacity, fatigues quickly (sprinting).

Which muscle fiber type is best for endurance activities?

Type 1 (slow-oxidative) fibers.

Which muscle fiber type is best for sprinting or explosive movements?

Type 2x (fast-glycolytic) fibers

Can training change fiber type distribution?

Training can modify fiber properties (e.g., increase oxidative capacity), but relative fiber type proportions are largely genetically determined.

What are the main components of the cardiovascular system?

The heart, blood vessels, and blood. Blood consists of plasma, red blood cells, and immune components. Plasma transports electrolytes, nutrients, wastes, gases, and hormones throughout the body.

What are the two major circulatory loops?

• Systemic circulation: Carries oxygenated blood from the left heart to the body.

• Pulmonary circulation: Carries deoxygenated blood from the right heart to the lungs.

What are the main chambers and valves of the heart?

• Chambers: Right atrium, right ventricle, left atrium, left ventricle.

• Valves: Tricuspid, pulmonary semilunar, mitral (bicuspid), aortic semilunar

What structural features make the heart a functional syncytium?

Cardiac cells are connected by desmosomes (mechanical linkage) and gap junctions (electrical coupling), allowing coordinated contraction.

What percentage of cardiac cells are contractile vs. pacemaker cells?

99% are contractile cells; 1% are pacemaker (autorhythmic) cells.

What is the ionic basis of the pacemaker cell action potential?

• Phase 4: Slow Na⁺ influx (funny current) + reduced K⁺ efflux → gradual depolarization.

• Phase 0: Ca²⁺ influx (via L-type channels) → depolarization.

• Phase 3: K⁺ efflux → repolarization.

What is the cardiac conduction pathway?

SA node → internodal/interatrial pathways → AV node → bundle of His → right and left bundle branches → Purkinje fibers.

Which structure sets the heart’s pace under normal conditions?

The SA node (sinoatrial node), normally at ~70 beats per minute.

What are the intrinsic pacing rates of the SA node, AV node, and Purkinje fibers?

• SA node: ~70 bpm

• AV node: ~50 bpm

• Purkinje fibers: ~30 bpm

Why is there a delay at the AV node?

The AV node slows conduction (~100 ms) to ensure atrial contraction completes before ventricular contraction begins.

What is the ionic basis of the action potential in contractile cells?

• Rapid Na⁺ influx → depolarization.

• Plateau phase due to Ca²⁺ influx (L-type) balanced by K⁺ efflux.

• K⁺ efflux → repolarization.

How does excitation–contraction coupling differ in cardiac muscle vs skeletal muscle?

• Cardiac EC coupling depends on Ca²⁺-induced Ca²⁺ release from the SR.

• Skeletal muscle EC coupling relies on direct mechanical coupling via DHP and ryanodine receptors.

Why do cardiac contractile cells have a long refractory period?

To prevent tetanus and allow sufficient time for ventricles to fill before the next contraction.

What does an ECG measure?

The summed electrical activity of the heart (bulk currents), not individual cell depolarizations or hyperpolarizations.

What are the main phases of the cardiac cycle?

• Ventricular filling

• Isovolumetric contraction

• Ventricular ejection

• Isovolumetric relaxation

What causes the “lub-dub” heart sounds?

• S1 (“lub”): Closure of AV valves at start of ventricular systole.

• S2 (“dub”): Closure of semilunar valves at start of diastole.

What is cardiac output, and how is it calculated?

Cardiac Output (CO) = Heart Rate × Stroke Volume

What regulates heart rate and stroke volume?

• Heart rate: Both branches of the ANS (sympathetic ↑, parasympathetic ↓).

• Stroke volume:

  • Extrinsic: Sympathetic stimulation.

  • Intrinsic: Venous return (Frank–Starling mechanism).

How does the sympathetic system increase heart rate?

Norepinephrine → β₁ receptors → ↑ cAMP → ↑ funny current (Na⁺) and Ca²⁺ influx → faster depolarization (steeper slope of pacemaker potential).

How does the parasympathetic system decrease heart rate?

ACh → M₂ muscarinic receptors → ↑ K⁺ efflux and ↓ Ca²⁺ influx → hyperpolarization and slower depolarization rate.

How does sympathetic stimulation affect ventricular contractility?

Increases Ca²⁺ entry during action potential → stronger cross-bridge cycling → increased contractility and stroke volume.

What is the Frank–Starling mechanism?

Increased venous return stretches cardiac muscle fibers, optimizing actin–myosin overlap, which increases stroke volume (preload-dependent).

What are the major types of systemic blood vessels (in order)?

Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava.

How is flow related to pressure and resistance?

Flow = Pressure Gradient / Resistance
Resistance ∝ 1 / r⁴ → Small changes in radius cause large changes in flow.

What is the main factor regulating vessel resistance?

Vessel radius, controlled by smooth muscle contraction or relaxation.

What is the role of muscular arteries?

Constrict or dilate to regulate blood flow distribution to specific organs.

What is the role of elastic arteries?

Act as pressure reservoirs, maintaining continuous blood flow during diastole.

How does a sphygmomanometer measure blood pressure?

By occluding an artery and detecting Korotkoff sounds as pressure is released to determine systolic and diastolic pressures.

What controls arteriole constriction and dilation?

• Intrinsic controls: Local metabolic and myogenic factors.

• Extrinsic controls: Sympathetic nervous system and hormones.

How are substances exchanged at capillaries?

• Small water-soluble: Diffuse through pores.

• Lipid-soluble: Pass through cell membranes.

• Proteins: Move via vesicular transport.

Define diffusion, bulk flow, ultrafiltration, and reabsorption.

• Diffusion: Movement down concentration gradient.

• Bulk flow: Movement due to pressure differences.

• Ultrafiltration: Fluid leaves capillaries (outward pressure).

• Reabsorption: Fluid enters capillaries (inward pressure).

What is the function of venous valves?

Prevent backflow of blood in veins and ensure one-way return to the heart.

What are the skeletal and respiratory pumps?

• Skeletal muscle pump: Muscle contractions compress veins to push blood upward.

• Respiratory pump: Inhalation decreases thoracic pressure, pulling venous blood toward the heart.

What are baroreceptors, and where are they located?

Stretch-sensitive pressure sensors located in the carotid sinus and aortic arch (not CNS), which detect changes in blood pressure.

How do baroreceptors regulate blood pressure?

• increased BP → increased baroreceptor firing → increased parasympathetic, decreased sympathetic output → lower HR, contractility, and vasoconstriction.

• decreased BP → decreased firing → increased sympathetic tone → raise HR, contractility, and vasoconstriction.