Chapter 12,13,14,15 Check your Understanding

Muscle Types and Contraction Mechanisms

Comparison of Skeletal, Smooth, and Cardiac Muscle Contraction

Muscle tissue in the human body is classified into three main types: skeletal, smooth, and cardiac muscle. Each type has unique structural and functional characteristics, especially regarding contraction time and fatigue.

• Skeletal Muscle: Voluntary, striated muscle attached to bones; contracts rapidly but fatigues quickly.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs; contracts slowly and is resistant to fatigue.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart; contracts at a moderate speed and is highly resistant to fatigue.

Contraction Time and Fatigue:

• Skeletal: Fast contraction, quick fatigue due to anaerobic metabolism.

• Smooth: Slow contraction, sustained for long periods with little fatigue.

• Cardiac: Intermediate contraction speed, does not fatigue under normal conditions due to rich blood supply and abundant mitochondria.

Detailed Comparison: Smooth vs. Skeletal Muscle Contraction

• Initiation: Skeletal muscle contraction is initiated by somatic motor neurons; smooth muscle by autonomic nerves, hormones, or local factors.

• Calcium Source: Skeletal muscle relies on sarcoplasmic reticulum; smooth muscle uses both extracellular and sarcoplasmic reticulum calcium.

• Regulatory Proteins: Skeletal muscle uses troponin-tropomyosin complex; smooth muscle uses calmodulin and myosin light-chain kinase (MLCK).

• Contraction Mechanism: Skeletal muscle contraction is regulated by actin; smooth muscle by myosin phosphorylation.

• Relaxation: Both require removal of calcium, but smooth muscle relaxation is slower.

Anatomy of Skeletal Muscle and Comparison to Smooth Muscle

Skeletal Muscle Anatomy:

• Composed of long, cylindrical, multinucleated fibers.

• Striated appearance due to organized sarcomeres.

• Voluntary control via the somatic nervous system.

Smooth Muscle Anatomy:

• Spindle-shaped, single nucleus per cell.

• No striations; actin and myosin arranged irregularly.

• Involuntary control via the autonomic nervous system.

Action Potentials: Cardiac vs. Skeletal Muscle

• Skeletal Muscle: Short action potential (~2 ms), rapid depolarization and repolarization, no plateau phase.

• Cardiac Muscle: Longer action potential (~200-300 ms), includes a plateau phase due to prolonged calcium influx, prevents tetanus.

Steps in the Skeletal Muscle Contraction Cycle

1. Action Potential: Motor neuron releases acetylcholine at neuromuscular junction.

2. Depolarization: Muscle fiber membrane depolarizes, triggering calcium release from sarcoplasmic reticulum.

3. Cross-Bridge Formation: Calcium binds to troponin, exposing binding sites on actin; myosin heads attach to actin.

4. Power Stroke: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere.

5. Detachment: ATP binds to myosin, causing it to detach from actin.

6. Reactivation: ATP is hydrolyzed, re-cocking the myosin head for another cycle.

7. Relaxation: Calcium is pumped back into the sarcoplasmic reticulum, and the muscle relaxes.

Sarcomere Structure and Function

• Sarcomere: The basic contractile unit of striated muscle, defined as the segment between two Z lines.

• During Contraction: Sarcomeres shorten as actin and myosin filaments slide past each other (sliding filament theory).

• During Relaxation: Sarcomeres return to their resting length.

Cardiac Physiology

Electrocardiogram (ECG) Events

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization (and atrial repolarization, which is masked).

• T wave: Ventricular repolarization.

• Clinical Use: ECGs are used to diagnose arrhythmias, myocardial infarction, and other cardiac conditions.

Cardiac Volumes and Output

• Stroke Volume (SV): The amount of blood ejected by a ventricle in one contraction.

• End-Diastolic Volume (EDV): The volume of blood in a ventricle at the end of filling (diastole).

• End-Systolic Volume (ESV): The volume of blood remaining in a ventricle after contraction (systole).

• Cardiac Output (CO): The volume of blood pumped by each ventricle per minute.

Formulas:

• Stroke Volume: $SV = EDV - ESV$

• Cardiac Output: $CO = SV \times HR$

Preload: The degree of stretch of cardiac muscle fibers at the end of diastole (related to EDV).

Regulation: Cardiac output is regulated by heart rate, stroke volume, preload, afterload, and contractility.

Cardiovascular System: Blood Vessels, Pressure, Flow, and Resistance

Blood Vessels

• Arteries: Carry blood away from the heart; thick, elastic walls.

• Veins: Return blood to the heart; thinner walls, contain valves.

• Capillaries: Sites of exchange between blood and tissues; very thin walls.

Pressure, Flow, and Resistance

• Blood Pressure (BP): The force exerted by circulating blood on vessel walls.

• Blood Flow (Q): The volume of blood moving through a vessel per unit time.

• Resistance (R): The opposition to blood flow, mainly determined by vessel diameter, length, and blood viscosity.

Relationship:

• Blood flow is directly proportional to the pressure gradient and inversely proportional to resistance:

$Q = \frac{\Delta P}{R}$

Regulation: Blood vessel diameter (vasoconstriction/vasodilation), blood viscosity, and total vessel length affect resistance and thus blood flow and pressure.

Summary Table: Comparison of Muscle Types

Feature	Skeletal Muscle	Smooth Muscle	Cardiac Muscle
Control	Voluntary	Involuntary	Involuntary
Striations	Present	Absent	Present
Location	Attached to bones	Walls of hollow organs	Heart
Contraction Speed	Fast	Slow	Intermediate
Fatigue Resistance	Low	High	Very High