Module 7: Muscle System

Muscle characteristics

• Contractability

• Elasticity

• Excitability

• Extensibility

Muscle Functions

• Producing movement

• Stabilising joints

• maintaIn posture/body position

• Heat production

Skeletal Muscle - Location

Attached to bones across joints, e.g., biceps, triceps, intercostal muscles between ribs

Skeletal Muscle - Anatomy

Striated fibers due to sarcomere/myofibril arrangement; cylinder-shaped; multinucleated.

Skeletal Muscle - Control

Voluntary, conscious (somatic) control.

Smooth Muscle - Location

Surrounds hollow organs/structures like stomach, bladder, bronchioles, uterus.

Smooth Muscle - Anatomy

Non-striated fibers; spindle-shaped; single (uninucleated) nucleus.

Smooth Muscle - Control

Involuntary (autonomic) control; some under pacemaker control (e.g., uterus at term, intestines).

Cardiac Muscle - Location

Found only in the heart muscle.

Cardiac Muscle - Anatomy

Striated fibers (sarcomeres); branched; one or two nuclei; connected by gap junctions and intercalated discs.

Cardiac Muscle - Control

Involuntary (autonomic) control; contraction rate set by pacemaker cells.

Skeletal Muscle (Organ)

An organ made of muscle fibers, connective tissue layers, tendons, blood vessels, and nerves.

Muscle Fiber

A single elongated muscle cell; the basic contractile unit of skeletal muscle.

Fascicle

A bundle of muscle fibers grouped together within a skeletal muscle.

Muscle Belly

The thick, central part of a skeletal muscle formed by all the fascicles together.

Epimysium

Outer connective tissue layer that surrounds the entire muscle belly.

Perimysium

Connective tissue layer that wraps each fascicle within the muscle.

Endomysium

Delicate connective tissue surrounding each individual muscle fiber.

Tendon

Dense connective tissue that connects muscle to bone and transmits force.

Neurovascular Bundle

A nerve, artery, and vein that enter/exit the muscle belly near its midpoint and branch throughout the muscle.

Blood Vessels in Muscle

Arteries and veins that supply oxygen, nutrients, and remove waste from muscle fibers.

Nerve Supply in Muscle

Motor and sensory nerves that control muscle contraction and relay feedback.

Direct Attachment

The muscle’s epimysium (outer connective layer) fuses directly with the bone’s periosteum, anchoring the muscle without a distinct tendon.

Indirect Attachment

The muscle’s connective tissue continues past the muscle belly as a tendon, which then attaches to the bone.

Skeletal Muscle Attachments

Every skeletal muscle attaches to at least two bones and crosses the joint(s) between them, allowing it to generate movement and stabilize those joints.

Origin

The attachment site on the stationary bone that does not move when the muscle contracts.

Insertion

The attachment site on the moving bone that shifts position when the muscle contracts.

Glycosomes

Little packets of stored glycogen (a chain of glucose) for energy.

Myoglobin

A red protein that holds extra oxygen inside the cell.

Myofibrils

Long, thread-like bundles that contain the muscle’s “machinery” for contracting and make up most of the cell’s volume.

Sarcoplasmic Reticulum (SR)

A smooth endoplasmic reticulum network wrapped around each myofibril; stores Ca²⁺ ions and releases them to trigger muscle contraction.

Sarcolemma & T-Tubules

The muscle cell’s plasma membrane (sarcolemma) which dives inward as T-tubules to encircle myofibrils, carrying action potentials deep into the fiber.

AP Transmission

The cell membrane plunges into the fiber so an electrical impulse can reach all contractile units at once, ensuring coordinated contraction.

Sarcomere

The smallest contractile unit inside a myofibril, stacked end-to-end. Each sarcomere shortens when the muscle contracts.

Thick Filaments

Composed of myosin protein; anchored at the center of each sarcomere (M-line) and pull on thin filaments during contraction.

Thin Filaments

Made of actin plus regulatory proteins (troponin & tropomyosin); anchored at sarcomere ends (Z-lines) and slide past thick filaments.

M-Line

The central “line” in a sarcomere where thick filaments are anchored.

Z-Line

The boundary of each sarcomere where thin filaments attach.

Sliding Filament Mechanism

Overlapping thick and thin filaments slide past each other (thanks to myosin–actin interactions) to shorten sarcomeres and contract the muscle.

Neuromuscular Stimulation - Muscle contraction

A lower motor neuron fires an action potential to the neuromuscular junction, releasing acetylcholine (ACh) into the synaptic cleft.

Muscle Action Potential Initiation - Muscle contraction

ACh binds to sarcolemma receptors, depolarizes the membrane, and triggers an action potential that travels along the sarcolemma and down T-tubules.

Calcium Release & Regulatory Shift - Muscle contraction

The AP in T-tubules causes the sarcoplasmic reticulum to release Ca²⁺, which binds troponin and moves tropomyosin off actin’s myosin-binding sites.

Crossbridge Cycling & Contraction - Muscle contraction

Energized myosin heads attach to exposed actin sites, release Pi and ADP for the power stroke, then bind ATP to detach and re-cock—repeating to shorten sarcomeres.

Relaxation - Muscle contraction

Motor neuron firing stops, Ca²⁺ is pumped back into the SR (ATP-dependent), troponin–tropomyosin block actin again, crossbridge cycling ceases, and the muscle relaxes.

Motor Unit Recruitment - LO13

Activating more motor units (a motor neuron + its muscle fibers) increases the total number of fibers contracting, thus boosting overall muscle force.

Size Principle - LO13

During gradual force increases, small, fatigue-resistant motor units are recruited first; as demand grows, larger, more powerful (but more fatigable) units join in.

Firing Frequency (Rate Coding) - Lo13

Increasing the rate of action potentials in active motor units raises intracellular Ca²⁺, allowing more cross-bridges per fiber and greater tension.

Twitch, Summation & Tetanus - LO13

• Twitch: Single AP → brief contraction (low force)

• Summation: Twitches overlap at higher frequency → greater tension

• Incomplete Tetanus: Rapid stimuli → sustained but wavering tension

• Complete Tetanus: Maximal, smooth tension when stimuli fuse contractions

Combined Impact on Strength - LO13

• Recruitment determines how many fibers contribute.

• Frequency determines how strongly each fiber pulls.
  Together they allow graded control from a gentle hold to a maximal contraction.