Comprehensive Study Guide to the Muscular System

Introduction to the Muscular System

• Three Types of Muscle Tissue:

  • Skeletal Muscle Tissue

  • Cardiac Muscle Tissue

  • Smooth Muscle Tissue

• Muscle Volume and Count:

  • There are more than $600$ skeletal muscles that make up the muscular system.

  • Skeletal muscle accounts for approximately $40\%$ of the body's total volume.

Properties of Muscle Tissue

• Four Unique Characteristics:

  1. Excitability: The ability to be input receptive; the tissue can respond to stimuli.

  2. Contractility: The ability of the muscle to shorten or contract when responding to stimuli.

  3. Elasticity: The ability of the muscle to return to its original length; tension is not a problem for the tissue's structural integrity.

  4. Extensibility: The ability to be stretched without damage.

Functions of Skeletal Muscle Tissue

1. Body movement: Moving the bones of the skeleton.

2. Maintenance of posture: Stabilizing joints and maintaining body position.

3. Temperature regulation: Heat is produced as a byproduct of muscle contraction ($\text{thermogenesis}$).

4. Storage and movement of materials: Regulating the passage of substances through internal tracts/sphincters.

5. Support: Protecting internal organs and supporting the weight of visceral structures.

Anatomy and Attachment of Skeletal Muscle

• Skeletal Muscle as an Organ: Each skeletal muscle is considered an organ because it is composed of multiple tissue types (muscle tissue, connective tissue, nervous tissue, and blood vessels).

• Striation: Skeletal muscle appears striated (banded) under a microscope.

• General Attachment: Muscles usually attach to bones via tendons.

• Muscle Attachments (Tendons):

  • Most muscles work over a joint and have attachments to both moving bones.

  • Origin ($\text{Prox}$): The less moveable point of attachment; typically the more proximal attachment point.

  • Insertion ($\text{Distal}$): The more moveable point of attachment; typically the more distal attachment point.

  • Action: Contraction causes one of the articulating bones to move while the other remains stationary.

• Specific Attachment Details (OIAI):

  • Axial Skeleton Attachments:

    • Inferior attachment: The less moveable attachment point.

    • Superior attachment: Often the more moveable attachment point (e.g., pulling the neck toward the torso).

  • Appendicular Skeleton Attachments:

    • Distal (O): Identified as more mobile in specific configurations.

    • Proximal (I): Identified as the less mobile attachment point.

  • Innervation: Muscular contraction is controlled by somatic motor neurons.

• Specific Structures:

  • Aponeurosis: A thin, flat sheet of connective tissue that serves as a muscle attachment point.

    • Examples: Longitudinal and transverse fibers of the palmar aponeurosis, flexor retinaculum, and palmaris brevis muscle.

  • Tendonitis: Inflammation caused by overuse or age-related changes; it may affect the tendon or the surrounding sheath.

Structural Organization of Skeletal Muscle

• Hierarchy of Organization:

  1. Skeletal Muscle: Multiple fascicles containing many muscle fibers, connective tissue, blood vessels, and nerves. Covered by Epimysium.

  2. Fascicle: A bundle of muscle fibers separated from other bundles. Covered by Perimysium.

  3. Muscle Fiber (Muscle Cell): Elongated, multinucleated, cylindrical cell containing myofibrils. Covered by Endomysium.

  4. Myofibril: Long, cylindrical contractile element as long as the cell itself; composed of myofilaments.

  5. Myofilaments: Short contractile proteins—thick (myosin) and thin (actin, tropomyosin, troponin).

• Connective Tissue Components:

  • Endomysium: The innermost layer that surrounds and electrically insulates each individual muscle fiber; composed of delicate areolar connective tissue.

  • Perimysium: Surrounds each fascicle; composed of dense irregular connective tissue.

  • Epimysium: Surrounds the entire skeletal muscle; composed of dense irregular connective tissue.

  • Deep Fascia: External to the epimysium; surrounds each muscle and separates different muscles from one another.

  • Superficial Fascia (Subcutaneous layer): Supports skin and binds it to underlying structures; composed of loose connective tissue.

• Functions of CT Layers: CT provides a framework for nerve and blood vessel distribution and acts as the site for muscle attachment.

Histology of the Muscle Fiber

• General Fiber Characteristics:

  • Fibers are modified for contraction.

  • Contain many mitochondria (up to $300$ per cell) to meet energy demands.

  • Multinucleated cells.

  • Can be quite long, with some fibers reaching $30\,cm$ in length.

• Specialized Terminology:

  • Sarcolemma: The plasma membrane of a muscle fiber; regulates the entry and exit of materials.

  • Sarcoplasm: The cytoplasm of the muscle fiber; site of metabolic processes.

  • Sarcoplasmic Reticulum: Similar to smooth ER; stores $Ca^{2+}$ ions necessary for contraction.

  • Transverse Tubules (T-tubules): Deep passageways of the sarcolemma that extend into the sarcoplasm. They form a network that allows muscle impulses to spread quickly to the interior of the cell.

  • Terminal Cisternae: Expanded ends of the sarcoplasmic reticulum that store and release $Ca^{2+}$. They are in contact with T-tubules.

  • Triad: A three-part structure consisting of two terminal cisternae and one T-tubule.

Molecular Structure of Myofilaments

• Thick Filaments (Myosin):

  • Approximately $11\,nm$ in diameter.

  • Composed of stacks of myosin molecules.

  • Each myosin molecule has an elongated tail and a head.

  • Heads form crossbridges with thin filaments and contain ATP and ATPase binding sites.

• Thin Filaments (Actin):

  • Approximately $5-6\,nm$ in diameter.

  • Composed of two strands of Actin molecules twisted into a spiral (helix).

  • G-actin: Globular subunits of actin.

  • F-actin: Filamentous actin.

  • Regulatory Proteins:

    1. Tropomyosin: Double-stranded protein that covers active sites on actin to prevent myosin binding when at rest.

    2. Troponin: Regulatory protein that holds tropomyosin in place; contains binding sites for $Ca^{2+}$.

• Accessory Proteins:

  • Connectin (Titin): Single molecular filament of a giant protein that helps provide passive tension and organize the sarcomere.

  • Nebulin: A giant protein filament thought to regulate the length of the thin filament.

The Sarcomere and Sliding Filament Theory

• Functional Unit: The sarcomere is the functional contractile unit of a skeletal muscle fiber.

• Boundaries: A sarcomere is defined as the area between two adjacent Z discs (or Z-lines).

• Sarcomere Zones and Bands:

  • A band: Dark band containing the entire length of the thick filaments. Its width remains constant during contraction.

  • H zone: The center of the A band where only thick filaments are present; disappears during full contraction.

  • M line: A protein structure in the middle of the H zone that serves as an attachment site for thick filaments.

  • I band: Light band containing only thin filaments; narrows and can disappear during contraction.

  • Z disc: Anchor point for thin filaments and connectin.

• Sliding Filament Mechanism:

  • During contraction, thick and thin filaments slide past each other; they do not change in actual length.

  • Z discs move closer together.

  • The sarcomere narrows.

  • Interaction between filaments generates tension.

Neuromuscular Junction (NMJ)

• Definition: The site where a motor neuron makes contact with a muscle fiber.

• Components:

  1. Synaptic knob: The expanded tip of the axon.

  2. Synaptic vesicles: Sacs filled with the neurotransmitter Acetylcholine (ACh).

  3. Motor end plate: A specialized region of the sarcolemma with folds to increase surface area for ACh receptors.

  4. Synaptic cleft: The narrow space separating the synaptic knob and the motor end plate.

  5. ACh receptors: Receptors on the motor end plate that bind ACh.

  6. Acetylcholinesterase (AChE): An enzyme in the synaptic cleft that rapidly breaks down ACh to stop the signal.

Mechanism of Muscle Contraction

1. Stimulation: A nerve impulse triggers the release of ACh from synaptic vesicles into the synaptic cleft.

2. Impulse Initiation: ACh binds to receptors on the motor end plate, initiating a muscle impulse (voltage potential) along the sarcolemma and down the T-tubules.

3. Calcium Release: The impulse triggers the release of $Ca^{2+}$ from the terminal cisternae into the sarcoplasm.

4. Binding: $Ca^{2+}$ binds to troponin, causing a shape change that moves tropomyosin away from the active sites on actin.

5. Crossbridge Formation: Myosin heads attach to the exposed active sites on actin.

6. Power Stroke: Myosin heads pivot, sliding the thin filaments toward the center of the sarcomere (M line). This requires ATP.

7. Cycling: Myosin detaches (requires ATP), resets, and repeats the cycle (attach-pivot-detach-return) as long as $Ca^{2+}$ is present.

8. Relaxation: When the nerve signal stops, $Ca^{2+}$ is actively transported back into the sarcoplasmic reticulum. Tropomyosin re-covers the active sites.

• Clinical Note: "Botox" works by interfering with the neuromuscular junction signals.

Muscle Tone and Contraction Types

• Muscle Tone: The constant tension in a resting muscle. Motor units are stimulated randomly to avoid fatigue.

• Types of Contraction:

  1. Isometric Contraction: Muscle tension is less than the resistance (\text{Tension} < \text{Resistance}). The muscle does not shorten, and no movement occurs (e.g., trying to lift an $80\,lb$ weight that is too heavy).

  2. Isotonic Contraction: Muscle tension is equal to or greater than the resistance ($\text{Tension} \ge \text{Resistance}$). The muscle shortens, and movement occurs (e.g., lifting a $10\,lb$ weight).

Exercise and Muscle Adaptation

• Muscle Atrophy: Wasting of tissue resulting in reduced muscle size, tone, and power; often caused by lack of stimulation/disuse.

• Muscle Hypertrophy: Increase in muscle fiber size due to repetitive stimulation.

  • Increased number of mitochondria (higher ATP capacity).

  • Increased number of myofibrils and myofilaments.

Skeletal Muscle Fiber Types

Motor Units

• Definition: A single motor neuron and all the muscle fibers it innervates.

• Innervation Ratios:

  • Average range: $1:100$ to $1:150$ (neuron to fibers).

  • Precise control: Ratio closer to $1:10$.

  • Gross movement: Ratio closer to $1:500$ (e.g., massive thigh muscles).