Muscle Structure and Function

Lab and Lecture Exams

  • On the lab exam, expect questions on muscle insertions, actions, and muscle building processes that align with the lecture material.

  • The lecture exam will primarily cover the mentioned topics, along with a focus on muscle tissues.

Overview of Muscle Tissue

  • There are three types of muscle tissues:

    • Skeletal Muscle

    • Cardiac Muscle

    • Smooth Muscle

  • Each type differs in:

    • Structure

    • Location

    • Function

    • Activation

Muscle Cell Structure

  • Muscle cells are often referred to as muscle fibers or muscle cells.

  • Characteristics of muscle fibers include:

    • They are elongated in shape and called muscle fibers;

    • Skeletal muscle fibers are described as cylindrical;

    • Fibers are multinucleated (contain multiple nuclei);

    • Muscle contraction depends on two types of myofilaments:

    • Actin (thin filaments)

    • Myosin (thick filaments)

Terminology Related to Muscle

  • Prefixes to know:

    • Sarco (related to muscle)

    • Myo or Mys (also indicates muscle)

Important Muscle Definitions

  • Sarcolemma: The muscle cell membrane or plasma membrane of muscle cells.

  • Sarcoplasm: Equivalent to cytoplasm of a muscle cell, containing myofibrils.

Muscles and Their Functions

  • Skeletal Muscle:

    • Responsible for locomotion and body movement.

    • Controlled voluntarily.

    • Has striations (visible stripes).

    • Contracts rapidly but fatigues easily.

    • Can exert various force levels.

  • Cardiac Muscle:

    • Responsible for pumping blood through the heart and blood vessels.

    • Controlled involuntarily by the SA node (sinoatrial node) acting as a pacemaker.

    • Contains striations and contracts steadily without fatiguing easily.

  • Smooth Muscle:

    • Found in hollow visceral organs like the stomach and intestines.

    • Moves substances through these channels via peristalsis.

    • Non-striated and involuntary control.

Muscle Functions

  • Major roles of muscle include:

    • Locomotion (skeletal muscle)

    • Circulating blood (cardiac muscle)

    • Moving substances on a cellular level (smooth muscle)

    • Maintaining posture and stabilizing joints

    • Generating heat during movement and contraction

Structural Organization of Skeletal Muscle

  • Connective Tissue Sheaths:

    • Epimysium: Envelops the entire muscle; outermost layer.

    • Perimysium: Surrounds bundles of muscle fibers called fascicles.

    • Endomysium: Surrounds individual muscle fibers or cells.

  • Muscle belly is a key area for muscle location referencing.

Fascicles and Myofibrils

  • Muscle consists of:

    • Fascicles: Bundles of muscle fibers, encased by perimysium.

    • Myofibrils: Long strands inside muscle fibers, responsible for contraction via actin and myosin structures or contractile elements.

Myofilaments and Sarcomeres

  • Sarcomere: The smallest functional unit of muscle, located between Z discs. It contains:

    • A bands: Dark bands composed of thick myosin filaments.

    • I bands: Light bands containing thin actin filaments.

  • H zone: area where thick filaments do not overlap with thin filaments when relaxed.

Functions in Muscle Contraction

  • Muscle contraction involves:

    • Sliding filament model where actin slides over myosin to shorten the sarcomere.

    • Myosin heads form cross-bridges with actin filaments during contraction.

    • Requires ATP to form and break cross-bridges.

Energy for Muscle Contraction

  • Each skeletal muscle fiber requires continuous delivery of oxygen and nutrients through arteries.

  • Removal of waste products via veins is vital for muscle function.

  • Myoglobin: The iron-binding protein in muscle tissue that helps store oxygen.

Neuromuscular Junction and Muscle Contraction

  • Neuromuscular junction involves:

    • The connection between nerve terminals and muscle fibers, allowing control of muscle contraction.

    • Acetylcholine (Ach) is released into the synaptic cleft, leading to muscle fiber stimulation.

    • Receptors on the motor end plate trigger the action potential.

Mechanism of Action Potential

  1. Calcium influx triggers Ach release from nerve endings into the synaptic cleft.

  2. Ach binds to its receptors on the muscle cell membrane (motor end plate).

  3. Binding leads to sodium influx and potassium efflux, creating an action potential.

  4. This action potential travels along the muscle cell, causing muscle contraction.

Breakdown of Acetylcholine

  • After muscle contraction, acetylcholinesterase breaks down acetylcholine in the synaptic cleft to prevent continuous stimulation of the muscle fiber, avoiding sustained contraction which can lead to fatigue.

  • Key Takeaway: Cumulative knowledge of muscle types, structure, function, and neural control is crucial for understanding muscle physiology and its applications in health and exercise.

Focus on memorizing key terms, associated actions, and locations of major muscles for successful exam performance.