Comprehensive Study Guide to the Canine Muscular System
Overview of the Canine Muscular System
The muscular system is one of the largest systems in the dog's body.
Composition: It is composed of units of tissue that possess the power to contract, which consequently produces movement.
Muscles: Defined as the contractile organs responsible for movement in an animal.
Functional Cellular Units:
Muscle cells are referred to as fibers.
The term "myofiber" is synonymous with the muscle fiber as the functional cellular unit.
Primary Functions and Forces of the Muscular System
Locomotion or Posture: Providing the force necessary for movement and the maintenance of structural alignment.
Respiration: Facilitating the mechanical process of breathing.
Alimentation: Assisting in the movement of food through the digestive tract.
Circulation: Providing the forces required for the movement of blood through the body.
Indication of Emotional States: Manifested through behavioral and physical actions including:
Barking.
Facial expressions.
Raising the hair (piloerection).
Wagging the tail.
Heat Production: Generation of heat essential for the maintenance of body temperature.
Clinical and Practical Importance: Knowledge of the muscular system is critical for:
Understanding structures involved during surgical procedures.
Meat inspection processes.
Identified specific muscles used for intramuscular injections.
Basic Physiologic Properties of Contractility
Excitability (Irritability): The capacity of the muscle tissue to receive and respond to a stimulus.
Extensibility: The ability of the muscle tissue to be stretched.
Elasticity: The ability of the muscle to return to its original shape following a contraction or after being stretched.
Relationship to Movement: All three properties (excitability, extensibility, and elasticity) are fundamentally related to the muscle's ability to produce movement.
Comparative Classification of Muscle Types
Striated, Voluntary Skeletal (Somatic) Muscle
Cellular Structure: Consists of long, cylindrical, multinucleated cells.
Appearance: Exhibits dark and light bands under a microscope, creating a striated or striped look.
Function:
Allows movement from one place to another (locomotion).
Facilitates movement of one body part relative to another.
Maintains body posture.
Attaches bones to the body to make motion possible.
Nervous System Control: Innervated by the Somatic Nervous System.
Physical Color: Reddish.
Formation: Arranged as individual muscles of variable shapes.
Striated, Involuntary Cardiac Muscle
Cellular Structure: Composed of elongated, branched cells that lie parallel to each other.
Special Features: Presence of intercalated discs located between cell extremities.
Appearance: Striated with dark and light bands; usually contains a single central nucleus.
Function:
Constitutes the working musculature of the heart.
Acts as the impulse-conducting system for cardiac contraction.
Involuntary contraction to propel blood throughout the body.
Nervous System Control: Innervated by the Autonomic Nervous System.
Physical Color: Red-brown.
Formation: Arranged in layers.
Autorhythmic: Yes.
Non-striated, Involuntary Smooth (Visceral) Muscle
Cellular Structure: Spindle-shaped cells with a single central nucleus.
Appearance: Non-striated (unstriated) without stripes or dark/light bands.
Location: Walls of hollow organs, blood vessels, and glands.
Function:
Produces slow contractions to allow the unconscious functioning of internal organs.
Compression of organs, ducts, and tubes.
Nervous System Control: Innervated by the Autonomic Nervous System.
Physical Color: Yellow-gray.
Formation: Arranged in layers.
Cell-to-Cell Attachments: No (unlike cardiac muscle).
Detailed Skeletal Muscle Morphology and Coloration
Color Classification
Red Muscle:
Source of Color: High amounts of myoglobin and cytochrome in the muscle cells.
Location: Found in areas performing sustained and continuous work, generally situated at the deeper aspects of the body.
White Muscle:
Source of Color: Lower levels of myoglobin and cytochrome.
Location: Found in areas performing quick, intermittent movements, generally located superficially.
Microscopic Composition
Sarcolemma: The plasma membrane of a skeletal muscle fiber; it is the site of action potential conduction which triggers contraction.
Myofibrils: Long cylindrical structures lying parallel to the muscle fiber.
Diameter: Approximately .
Quantity: Hundreds to thousands per muscle fiber.
Attachment: They attach to the sarcolemma at their ends; shortening of myofibrils causes the entire cell to contract.
Myofilaments: The building blocks of myofibrils, categorized as:
Thick myofilaments (Myosin).
Thin myofilaments (Actin).
Connective Tissue Elements of Skeletal Muscle
Endomysium: Connective tissue division that attaches to the sarcolemma and covers individual muscle fibers or myofibrils.
Perimysium: Connective tissue that envelops muscle bundles (fascicles). It is composed of reticular and collagenic fibers and separates fascicles from one another.
Epimysium: Dense irregular connective tissue that surrounds the entire muscle (a collection of muscle bundles). It is continuous with the perimysium.
Tendon or Aponeurosis: The epimysium is continuous with these structures, which facilitate attachment over distances.
Fascial Functions: These layers contain blood vessels, nerves, and fat, enabling nourishment, nervous stimulation, and friction-reduced sliding.
Marbling: Visible fat deposits within the connective tissue layers of the meat.
Anatomical Sites of Attachment and Geometry
Origin (Head): The less movable or more fixed point of attachment. In limbs, this is typically the proximal attachment.
Insertion/Termination (Tail): The more movable of the two attachments. In limbs, this is typically the distal attachment.
Belly: The expanded fleshy portion of the muscle.
Slips: Minor divisions of either the origin or termination points.
Attachment Types:
Tendinous: Dense connective tissue (tendon) connecting spindle-shaped or pennate muscles to bone.
Aponeurotic: Flat, tendinous sheets (aponeurosis) associated with flat muscles (e.g., the abdominal wall).
Fleshy: Direct-looking attachment of muscles to bone (e.g., scapular muscles).
Difference Between Tendon and Aponeurosis
Aponeurosis: Acts like a spring to bear extra pressure and tension during flexion or extension. Provides strength and stability.
Tendon: Functions to move a bone when a muscle contracts. Provides flexibility and basic movement capability.
Muscle Function and Dynamics of Contraction
Contraction: Tightening of the muscle; it becomes shorter and thicker. This causes a change in the alignment of bones around a joint.
Muscle fibers contract to a maximum of about half () of their stretched length.
Contractions are initiated by nerve impulses via motor units.
The number of active motor units determines total muscle activity.
Relaxation: The lessening of tension; the muscle returns to its original shape.
Neuromuscular Junction: The contact point between nerve endings and muscle cells.
Tonus (Muscle Tone): Balanced muscle tension. The combining form is "ton/o".
Classification of Muscles by Fiber Direction
1. Parallel Muscles
Characteristics: Fibers are arranged parallel to the line of pull. Fibers are long but fewer in number.
Subtypes:
Strap: Examples include the Sartorius and Sternohyoid.
Strap with Tendinous Intersection: Example includes the Rectus abdominis.
Quadrilateral: Examples include Quadratus lumborum and Quadratus femoris.
Fusiform: Examples include Biceps brachii and Digastricus.
2. Oblique Muscles
Characteristics: Fibers are oblique to the line of pull. Fibers are shorter but more numerous, providing a feather-like appearance.
Pennate Types:
Unipennate: Fleshy fibers slope into one side of the tendon. 1 origin and 1 insertion. Example: Extensor digitorum longus, Peroneus tertius.
Bipennate: Central tendon with fibers sloping in from two sides. 1 origin and 2 insertions. Example: Rectus femoris, Infraspinatus.
Multipennate: Series of bipennate muscles side-by-side with a central tendon. 1 origin and 3 or more insertions. Example: Deltoid, Subscapularis.
3. Other Structural Arrangements
Triangular: Fibers converge from a wide base to an apical tendon. Example: Temporalis, Adductor longus.
Spiral: Fibers are twisted close to their insertion. Example: Latissimus dorsi, Trapezius, Pectoralis.
Cruciate: Fibers arranged in superficial and deep planes that cross in an "X" shape. Example: Masseter, Adductor magnus.
Functional Groupings and Actions
Muscle Group Relations
Prime Mover (Agonist/Synergist): The muscle producing the characteristic movement at a joint. Synergists work together to support movement. Example: Brachialis is the prime mover for elbow flexion.
Antagonist: Muscles arranged in pairs that work opposite to each other. One pair contracts while the other relaxes. Example: Triceps brachii is the antagonist to the brachialis/biceps during flexion.
Fixator (Articular Muscle): Stabilizes the proximal end of a limb while the distal end moves. Example: The Deltoid stabilizes the bicep during a curl.
Specific Muscle Actions
Extensors: Straighten bone alignment or open a joint.
Flexors: Angulate bones or bend the joint.
Adduction: Movement toward the median plane (body) or toward the limb's axis in digits.
Abduction: Movement away from the median plane or away from the limb's axis in digits.
Rotation: Movement around a part's long axis.
Circumduction: Movement of an extremity describing a cone (sequence: flexion-abduction-extension-adduction).
Supination: Rotation of the forearm so the palmar side is upward/forward.
Pronation: Rotation of the palmar side of the paw/foot downward/backward.
Dynamics of Action
Spurt Muscles: Provide acceleration motion to joints. Example: Brachialis.
Shunt Muscles: Provide stabilizing centripetal force on a joint. Example: Flexor carpi radialis.
Accessory and Auxiliary Structures
Synovial Bursa: Simple, thin-walled sac lined with synovial membrane and filled with synovial fluid. Facilitates movement by minimizing friction at pressure points.
Subcutaneous: Usually acquired; located under the skin.
Submuscular: Usually inherited; located at tendinous insertions.
Synovial Tendon Sheath: A sac wrapped around a tendon to improve gliding over bony prominences. Consists of a parietal part and a tendinous part connected by a mesotendon.
Areolar Tissue: Loose, irregular connective tissue often containing fat.
Fasciae: Sheets of dense connective tissue between muscles.
Superficial Fascia: Beneath the skin; thinner, blended with subcutaneous tissue.
Deep Fascia: Thicker, tougher; attaches to bones, ligaments, and tendons; may form intermuscular septa.
Cutaneous Muscles: Pale variant of skeletal muscle with no direct attachment to the skeleton.
Sesamoid Bones: Small rounded nodules in tendons/capsules to:
Protect tendons from bony prominences.
Increase surface area for attachment.
Redirect pull for greater effective force.
Connective Tissue Sheath Functions: Maintain form, permit independent contraction, provide pathways for vessels/nerves, guide direction, and protect against rupture from excessive strength.
Nomenclature and Etymology of Skeletal Muscles
Latinization: Names may be Latinized (e.g., Flexor digitorum profundus) or Anglicized (e.g., Deep digital flexor).
Naming Conventions:
Shape: Deltoid (delta-shaped), Trapezius (trapezoid), Rhomboideus (diamond), Serratus (saw-toothed), Orbicularis (circular), Quadratus (square), Gracilis (slender).
Location: Brachialis (brachium), Zygomaticus (zygomatic bone), Temporalis (temporal bone).
Attachments: Sternothyrohyoideus (sternum, thyroid cartilage, hyoid bone).
Structure/Origin: Biceps (2 heads), Triceps (3 heads), Quadriceps (4 heads).
Function/Action: Supinator, Flexor (reduces angle), Extensor (increases angle), Abductor (away from midline), Adductor (toward midline), Levator (elevates), Depressor (lowers), Sphincter (decreases opening size), Tensor (makes rigid).
Direction of Fibers: Rectus (parallel to midline), Transverse (perpendicular), Oblique (diagonal).
Size: Longus/Longissimus (longest), Latissimus (widest), Magnus (large), Major (larger), Minor (smaller), Bravis (small), Vastus (great/greater).
Dissection Procedure: Shoulder Region
Positioning: Place the dog in right lateral recumbency.
Landmarks for Palpation: Wing of the atlas and the olecranon.
Incision Methodology:
Ventral midline incision: From the transverse plane of the atlas wings to the level of the umbilicus.
Transverse incisions: On the left side, from ventral to dorsal midlines at the level of the atlas and umbilicus.
Circumscribed incision: Around the forelimb at the level of the olecranon (carefully avoid the cephalic vein on the cranial surface of the elbow).
Connection: Connect the elbow and ventral midline incisions via a line from the medial brachium (axilla) to the chest.
Reflection: Reflect skin from the trunk and brachium, leaving it attached along the dorsal midline to prevent desiccation of the cadaver.
Observation: Superficial fascia contains the cutaneus trunci muscle, mammary tissue, and variable fat.