ch_06_lecture_presentation

Chapter 6 - The Muscular System

Introduction

• Muscles are responsible for various types of body movements.

• Three basic types of muscle:

  • Skeletal muscle

  • Cardiac muscle

  • Smooth muscle

Characteristics of Muscles

• Skeletal and smooth muscle fibers are elongated (muscle cell = muscle fiber).

• Muscle contraction and shortening are due to microfilament movement.

• Common terminology:

  • Prefixes myo and mys indicate "muscle."

  • Prefix sarco refers to "flesh."

Types of Muscle Tissue

Comparison of Skeletal, Cardiac, and Smooth Muscles

Skeletal Muscle Characteristics

• Skeletal muscles are attached by tendons to bones and are predominantly striated.

• Cells are multinucleate and can be consciously controlled.

• Functions include:

  • Movement production

  • Posture maintenance

  • Joint stabilization

  • Heat generation

Connective Tissue Wrappings of Skeletal Muscle

• Endomysium: Surrounds individual fibers.

• Perimysium: Wraps around a fascicle (bundle of fibers).

• Epimysium: Covers the entire muscle.

• Fascia: On the outside of the epimysium.

Muscle Attachments

• Connective tissue attachments:

  • Tendons: Cord-like structures mainly composed of collagen fibers.

  • Aponeuroses: Sheet-like structures attaching muscles indirectly to bones or other tissues.

Smooth Muscle Characteristics

• Lacks striations and has spindle-shaped cells with a single nucleus.

• Involuntary muscle found in the walls of hollow organs.

Cardiac Muscle Characteristics

• Striated, branched cells typically with a single nucleus.

• Involuntary contractions, found only in the heart walls.

Microscopic Anatomy of Skeletal Muscle

• Sarcolemma: Specialized plasma membrane.

• Myofibrils: Long organelles within muscle fibers, aligned to form distinct bands.

• Sarcoplasmic reticulum: Stores and releases calcium for contractions.

Sarcomere Organization

• The sarcomere is the contractile unit of a muscle fiber, consisting of:

  • Thick filaments (myosin).

  • Thin filaments (actin).

• Z disc: Anchors thin filaments and defines the boundary of a sarcomere.

Contraction Mechanism

• Sliding Filament Theory:

  • Myosin heads attach to actin filaments and pull them toward the sarcomere center, leading to muscle shortening.

• Stimulation of motor neurons is needed for contraction:

  • Excitability, Contractility, Extensibility, and Elasticity are key muscle properties.

Nerve Stimulus and Action Potential

• A motor unit consists of one motor neuron and all the skeletal muscle cells it stimulates.

• Neuromuscular junction: Site where motor neuron axon terminals meet muscle fibers.

• Synaptic cleft: A gap between nerve and muscle filled with interstitial fluid.

• Action potential causes calcium ion entry, leading to neurotransmitter release (acetylcholine - ACh).

• ACh binds to receptors, initiating action potentials in muscle fibers, leading to contraction.

Types of Muscle Contractions

• Isotonic contractions: Muscle shortens and movement occurs.

• Isometric contractions: Muscle tension increases without shortening.

• Graded responses in muscle contraction vary by the frequency of stimulation and the number of fibers recruited.

Energy for Muscle Contraction

• Muscles initially use stored ATP for energy. Then:

  • Creatine Phosphate: Transfers energy to regenerate ATP.

  • Anaerobic respiration: Produces ATP but leads to lactic acid formation.

  • Aerobic respiration: Efficient ATP production occurs in the mitochondria, utilizing oxygen.

Muscle Fatigue and Recovery

• Fatigue occurs when muscles cannot contract despite stimulation, primarily due to oxygen debt and accumulated lactic acid.

Exercise Effects on Muscles

• Regular exercise results in increased muscle size, strength, and endurance.

  • Aerobic exercise: Improves flexibility and metabolism.

  • Resistance exercise: Increases muscle size and strength.

Muscle Naming and Movements

• Muscles are named based on various criteria (e.g., location, shape, action).

• Types of movements include:

  • Flexion/Extension: Decrease/increase joint angle.

  • Abduction/Adduction: Movement away/toward the body's midline.

  • Circumduction: Circular movement of a limb.

  • Special Movements: Such as dorsiflexion, plantar flexion, supination, and pronation.

Types of Muscles in the Body

• Prime mover: Main muscle responsible for an action.

• Antagonist: Opposes prime mover action.

• Synergist: Aids prime movers and stabilizes joints.

• Fixator: Stabilizes the origin of a prime mover.