Muscular System Overview

Muscular System Overview

• The muscular system is a complex system composed of three main types of muscle: skeletal, smooth, and cardiac.
• Cardiac muscle is specifically for the heart.

Learning Outcomes

• Students will be able to understand the structure, function, and inner workings of cardiac and smooth muscle.
• Students will be able to differentiate among skeletal, smooth, and cardiac muscle

Cardiac Muscle: Contractile Mechanism

• The heart is made up of three major types of cardiac muscle: Atrial, Ventricular, and Specialized Excitatory & Conductive Muscle.
• Atrial and Ventricular muscle contract similarly to skeletal muscle, but the contraction duration is longer.
• Specialized excitatory and conductive muscle fibers contract more weakly due to fewer contractile fibrils.

Cardiac Muscle Fibers

• Cardiac muscle fibers are striated in appearance.
• The functional unit is a sarcomere.
• Fibers branch and connect at intercalated discs.
• Intercalated discs contain gap junctions.
• Nuclei are centrally located.
• The sarcoplasmic reticulum is less abundant in comparison to skeletal muscle, but more dense than smooth muscle.
• Sarcolemma has ion channels, including voltage-gated Ca2+ channels, which aren't found in skeletal muscle.
• Fibers are not anchored at ends, facilitating sarcomere shortening and lengthening.

Cardiac Muscle: Intercalated Discs

• Cell membranes of intercalated discs fuse to form "communicating junctions" (gap junctions).
• Gap junctions allow free ion diffusion.
• Ions move easily through the longitudinal axes of cardiac muscle fibers, enabling easy action potential transmission from one cardiac cell to the next.
• This interconnectedness creates a syncytium, allowing the action potential to spread throughout all heart muscle cells.

Cardiac Muscle Contraction

• Cardiac muscle contraction is described as isometric (occurs without significant shortening) or isotonic (occurs without significant change in force).
• Intercalated discs form functional syncytia.
• Two types of membrane junctions within these discs: desmosomes and gap junctions.

Cardiac Muscle Cells

• The heart beats rhythmically due to action potentials generated within the heart itself (autorhythmicity).
• Two types of cardiac muscle cells:
  • Contractile cells: make up 99% of cardiac muscle and do the pumping work.
  • Autorhythmic cells: do not contract but are specialized to initiate and conduct action potentials for working cells' contraction.

Cardiac Muscle Properties

• Automaticity: ability to spontaneously generate an electrical impulse.
• Excitability: ability to respond to an electrical impulse.
• Conductivity: ability to transmit the electrical impulse to other cardiac cells.
• Contractility: ability to contract after an electrical impulse.
• Rhythmicity: ability to send electrical impulses in a regular manner.

Smooth Muscle

• Smooth muscle is a primitive type of muscle compared to skeletal and cardiac muscle.
• Smooth muscle lacks striations and sarcomeres.
• It is highly resistant to fatigue, and its cross-sectional area strength is comparable to skeletal muscle.
• Smooth muscle fibers are smaller, involuntary, and have tapered ends and a centrally positioned nucleus.
• Sarcoplasm contains more thin than thick filaments with dense bodies similar to Z-disks anchoring the thin filaments.

Smooth Muscle Contraction

• Sliding filament mechanism involves thick and thin filaments
• No troponin complex.
• Actin and myosin pull on the dense bodies attached to the sarcolemma.
• Contraction causes lengthwise shortening and bulges in the fiber.
• Shortening is a corkscrew-like twisting of the fiber helix.

Types of Smooth Muscle

• Single-unit smooth muscle: large aggregates of cells acting as a single unit. Found in hollow organs and blood vessels.
• Multi-unit smooth muscle: individual cells with a single nerve connection, found in specific areas like the iris and arrector pili muscles.

Smooth Muscle Contractile Mechanism

• Smooth muscle contraction begins slower and lasts longer compared to striated muscle.
• Contractions can extend for hours or days.
• They can shorten and stretch significantly more.
• Initiation is triggered by Ca2+ influx.
• Calcium movement is slow, owing to the sparse sarcoplasmic reticulum and lack of T-tubules.

Smooth Muscle Stimulation

• Smooth muscle responds to nerve impulses, hormones, and mechanical stimulation.
• Key hormones influencing smooth muscle include norepinephrine, epinephrine, acetylcholine, angiotensin II, oxytocin, vasopressin, serotonin, and histamine.