Overview
This lecture covered the structure, types, and function of muscle tissue, including cellular organization, connective tissue wrappings, contraction mechanism, and motor unit control.
Types of Muscle Tissue
- Muscle tissue is one of four principal tissue types and specializes in contraction.
- Three types: smooth muscle, cardiac muscle, and skeletal muscle.
- Smooth muscle: spindle-shaped cells, found in organs/tubes (e.g., intestines, blood vessels), involuntary control, can divide.
- Cardiac muscle: branched cells, only in the heart, involuntary, cannot divide.
- Skeletal muscle: long, cylindrical cells, attach to skeleton, voluntary control, cannot divide—cells only grow by getting larger.
Muscle Fiber Organization
- "Muscle fibers" and "muscle cells" are interchangeable terms.
- Whole muscles are made of thousands of muscle cells/fibers.
- Each muscle contains a mix of slow-twitch (endurance) and fast-twitch (powerful, fatiguable) fibers.
- Slow-twitch: small, red, high mitochondria, resist fatigue, slow contraction.
- Fast-twitch: large, white, fewer mitochondria, fast, powerful, fatigue quickly.
Muscle Structure & Connective Tissue Wrappings
- Muscle belly: red, contractile part of muscle.
- Tendons: attach muscle belly to bone.
- Origin: tendon attachment to less mobile bone.
- Insertion: attachment to more mobile bone.
- Connective tissue layers (from innermost): endomysium (loose CT around each cell), perimysium (dense irregular CT around bundles/fascicles), epimysium (dense irregular CT around the whole muscle), fascia (dense irregular CT around groups of muscles).
- Tendons are extensions of connective tissue layers that become more organized (dense regular CT) as they reach the bone.
Microscopic Anatomy & Muscle Contraction
- Organization: whole muscle > muscle cell (fiber) > myofibril > sarcomere.
- Myofibrils are made of repeating sarcomeres.
- Sarcomere: functional contractile unit, bounded by Z-discs.
- Proteins: thick filaments (myosin), thin filaments (actin), regulatory proteins (troponin-tropomyosin complex, TTC).
- At rest, TTC blocks myosin-binding sites on actin.
- Calcium binds to TTC, exposing binding sites.
- Myosin binds actin, performs a power stroke (contraction), releases using ATP.
- Sarcomeres shorten, causing the whole muscle to contract.
- ATP is required for both contraction and relaxation (removal of calcium).
Motor Units & Muscle Control
- Motor unit: a motor neuron plus all muscle fibers it controls.
- Neuromuscular junction: site where neuron communicates with muscle cell.
- All-or-nothing: when a neuron fires, all its muscle fibers contract fully.
- Muscles are divided into many motor units to allow graded contraction.
- "Motor unit recruitment": engaging more motor units produces greater force.
- Sedentary individuals can voluntarily recruit ~70% of motor units; training or adrenaline can increase this.
Key Terms & Definitions
- Muscle fiber/cell — the basic contractile unit of a muscle.
- Sarcomere — the smallest contractile unit within a myofibril.
- Myofibril — a bundle of sarcomeres within a muscle cell.
- Endomysium — loose connective tissue surrounding each muscle fiber.
- Perimysium — dense irregular connective tissue wrapping muscle fascicles.
- Epimysium — dense irregular connective tissue wrapping the whole muscle.
- Fascia — connective tissue surrounding groups of muscles.
- Origin — less mobile tendon attachment.
- Insertion — more mobile tendon attachment.
- Motor unit — one motor neuron and all muscle fibers it innervates.
- Neuromuscular junction — the synapse between a motor neuron and a muscle cell.
Action Items / Next Steps
- Review diagrams of muscle structure and connective tissue layers.
- Memorize key definitions and distinctions between muscle tissue types.
- Prepare for questions on muscle organization, contraction mechanism, and motor unit function.
- Read about muscle architecture and function as assigned.