Muscle Terminology and Structure

Roots and Origins of Muscle Terms

The study of muscle anatomy begins with understanding the terminology derived from Latin and Greek roots. These roots help describe muscle structure and function.

• Latin root 'mus' and Greek root 'mys' or 'myo' both mean 'muscle'.

• Muscle comes from the Latin word meaning 'little mouse', referring to the appearance of muscles under the skin.

• Muscle tissue makes up about 40% of the body's mass.

• All muscle functions stem from the ability to contract ("to shorten").

Muscle Cell Structure

Skeletal and smooth muscle cells are called muscle fibers because of their elongated shape.

• Muscle cells contain actin and myosin, the two main types of contractile proteins.

• Prefixes referring to muscle include myo- and sarco-.

Features of Muscle Cells

Muscle cells have specialized structures for contraction and support.

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasmic reticulum: The specialized endoplasmic reticulum that stores calcium ions.

• Sarcoplasm: The cytoplasm of a muscle cell.

Muscle Contraction Types and Graphs

Types of Muscle Contractions

Muscle contractions can be classified by their speed and force.

• Slowest to fastest contractions:

  1. Smooth muscle

  2. Cardiac muscle

  3. Skeletal muscle

• Graph axes:

  • X-axis: Time

  • Y-axis: Tension

• Most forceful contractions: Skeletal muscle contracts most forcefully but tires easily and must rest.

• Rhythmic contractions: Cardiac and smooth muscle types have rhythmic contractions.

Connective Tissue Coverings of Muscle

Types of Connective Tissue Wrappings

Muscles are organized and protected by three main connective tissue layers:

• The end of the epimysium blends to form a tendon or sheet-like aponeurosis.

• A bundle of muscle fibers is called a fascicle.

Labeling Muscle Connective Tissue

Muscle tissue is wrapped in three layers: endomysium (innermost), perimysium (middle), and epimysium (outermost).

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

Each muscle fiber contains myofibrils, which are made up of repeating contractile units called sarcomeres.

• Thick filament: Myosin

• Thin filament: Actin

• Sarcomere: The basic contractile unit of muscle

• Dark band (A band): Contains both actin and myosin

• Middle of sarcomere: M line

• Sarcoplasmic reticulum: Stores and releases calcium ions, the "go" signal for contraction

The Nerve Stimulus and Action Potential

Neuromuscular Junction

Muscle contraction is initiated by nerve impulses at the neuromuscular junction.

• One neuron and all the skeletal muscle fibers it stimulates is called a motor unit.

• The neurotransmitter that stimulates muscle fibers is acetylcholine (ACh).

• Synaptic vesicles in the axon terminals are filled with ACh.

• There is a gap between the nerve ending and the muscle fiber called the synaptic cleft.

Events at the Neuromuscular Junction

• A nerve impulse reaches the axon terminal.

• Calcium channels open, and calcium enters the axon terminal.

• ACh is released from synaptic vesicles.

• ACh binds to receptors on the sarcolemma, triggering muscle contraction.

Sliding Filament Model of Muscle Contraction

Mechanism of Contraction

Muscle contraction occurs through the sliding filament model, where actin and myosin filaments slide past each other.

• In a relaxed sarcomere, regulatory proteins block the binding sites on actin.

• Upon stimulation, calcium ions bind to regulatory proteins, exposing binding sites for myosin.

• Myosin heads attach to actin, pulling it toward the center of the sarcomere.

• ATP provides energy for myosin heads to detach and reset.

• Contraction continues as long as calcium and ATP are present.

Contraction of Skeletal Muscle as a Whole

Muscle Fiber and Whole Muscle Contraction

• Muscle fiber contraction: All-or-none principle; a fiber contracts completely or not at all.

• Whole muscle contraction: Depends on the number of fibers stimulated and the frequency of stimulation.

• Graded responses are produced by changing the frequency and number of muscle fibers stimulated.

Types of Muscle Responses

• Twitch: Single, brief contraction

• Summation: Multiple contractions summed together

• Incomplete tetanus: Rapid stimulation with some relaxation

• Complete tetanus: Rapid stimulation with no relaxation

Muscle Response to Stronger Stimuli

• Force of contraction depends on the number of motor units activated and the number of muscle cells stimulated.

• If a muscle does not contract, it will shrink or atrophy.

Types of Muscle Contractions

Isotonic and Isometric Contractions

• Isotonic contraction: Tension stays the same, and the muscle shortens. Example: Lifting a weight.

• Isometric contraction: Tension increases, but the muscle does not shorten. Example: Holding a weight steady without moving it.

Attachment Points and Classification of Muscles

Muscle Attachments

• Origin: Attachment to an immovable or less movable bone.

• Insertion: Attachment to a movable bone.

• When a muscle contracts, the insertion moves toward the origin.

Classification of Muscles

Muscle Movements, Roles, and Names

Types of Body Movements

Muscles produce various types of movements, such as flexion, extension, abduction, adduction, and rotation, depending on their attachment points and joint structure.

• Flexion: Decreases the angle between two bones

• Extension: Increases the angle between two bones

• Abduction: Moves a limb away from the midline

• Adduction: Moves a limb toward the midline

• Rotation: Moves a bone around its longitudinal axis

Key Equations and Concepts

• Muscle contraction equation:

• Sliding filament model:

Additional info:

• Some context and terminology were inferred from standard Anatomy & Physiology textbooks to ensure completeness and clarity.