Introduction to the Locomotor Apparatus Study Notes

Introduction to the Locomotor Apparatus

• Instructor: W. Brady Little, DVM, MSc, Cert Vet Ed, FHEA.
• Module: Module 1 (LO: 3-11).
• Resources: Available via Canvas within the same module.
• Core Components of the Locomotor System (Apparatus):     * Passive Components: Includes bones and joints.     * Active Component: Consists of the muscles.

Fundamentals of Osteology

• Etymology: The term "Osteology" is derived from "osteo-" (meaning bone) and "-ology" (meaning the study of).
• Primary Functions of Bones:     * Support: Holding the weight of the body.     * Movement: Operating in concert with joints and muscles to facilitate locomotion.     * Protection: Shielding internal organs from damage.     * Growth: Providing a framework for development.     * Mineral Storage: Serving as a reservoir for minerals, specifically Calcium ($Ca^{2+_{}}$) and Phosphorus ($P^{+}$).     * Fat Storage: Stored within the bone marrow.     * Hematopoiesis: The formation of blood cells, which occurs within the bone marrow.

Forelimb Regions and Corresponding Bones

• Scapular Region: Supported by the Scapula.
• Brachium: Supported by the Humerus.
• Antebrachium: Supported by the Radius and Ulna.
• Manus (Front Paw): This region includes several sub-groups of bones:     * Carpus: Carpal bones.     * Metacarpus: Metacarpal bones.     * Digits: Composed of Phalanges and sesamoids.

Classification of Bones by Shape

• Flat Bones: Characterized as being broad and flat. They are constructed of two thin layers of compact bone with a layer of spongy bone sandwiched between them.     * Example: The Scapula.
• Long Bones: Defined as being longer than they are wide. They consist of a shaft and two ends.     * Examples: Humerus, Radius, and Ulna.
• Short Bones: Roughly cube-shaped, having dimensions for length, width, and height that are approximately similar.     * Example: Carpal bones.
• Irregular Bones: Possess complex shapes that do not fit into the other categories.     * Example: Vertebrae.
• Sesamoid Bones: A special type of short bone found within the tendons of origin or insertion of specific muscles.     * Functions of Sesamoids:         * Provide a smooth surface for tendons to slide over, thereby preventing friction for the associated tendon.         * Act as a fulcrum to provide extra leverage to the muscle, enhancing the ability of the tendon to transmit muscular forces.         * Displace the axis of force by altering the direction of tension within the muscle tendon.

Gross Anatomy and Clinical Significance of Long Bones

• General Structure: Long bones typically have at least three centers of ossification.
• Diaphysis: The shaft of the bone. It contains the medullary cavity, which houses yellow marrow.
• Epiphysis: The end regions of the bone.
• Physeal Growth Plate:     * Located between the epiphyses and the diaphysis in young animals.     * Comprised of cartilage cells.
• Growth Plate Relevance:     * The growth plate is the primary site for several pathological issues, including fractures, infections, metastasis, and the effects of endocrine bone disorders.     * Case Example: In a canine of approximately $8\,months$ of age, a normal growth plate should be distinguishable from a fracture in the hindlimb.

Bony Prominences and Wolf’s Law

• Wolf’s Law: Form follows function. Bones are remodeled in response to the physical stresses placed upon them.
• Non-articular Prominences: These serve as attachment sites for muscles or as passages for vessels and nerves.     * Examples found on the Scapula: Spine of the scapula, supraspinous fossa, and infraspinous fossa.
• Articular Prominences: These structures make up the actual joints.     * Example: The glenoid cavity of the scapula.

Principles of Arthrology

• Etymology: Derived from "Arthro-" (meaning joint) and "-ology" (meaning the study of).
• Definition: A joint is the point of contact, or articulation, between two or more bones or cartilages.
• Function: Provides support and movement to the skeleton, though it is important to note that not all joints are moveable.
• Structural Classification of Joints:     1. Fibrous Joints: Articular surfaces of bones are united by strong connective tissue. These may lead to bones fusing (synostosis). They allow for almost no movement or no movement at all.     2. Cartilaginous Joints: Articular surfaces are united by cartilage, providing limited movement. An example is the intervertebral disc in a spine.     3. Synovial Joints: Characterized by a joint cavity between articular surfaces filled with synovial fluid. These are the most movable joints.

Anatomy and Accessory Structures of Synovial Joints

• Synovial Joint Anatomy:     * Articular Surfaces: These are protected by articular cartilage.     * Joint Cavity: A fluid-filled space enclosed within the joint capsule.     * Joint Capsule: Provides strength and resistance, produces synovial fluid for lubrication and nutrition of articular cartilages, and is highly vascularized and innervated.     * Note: A single joint may contain multiple synovial compartments or sacs within one joint capsule.
• Accessory Structures:     * Ligaments (lig.): Bands of tough fibrous connective tissue.         * Extracapsular: Located outside the capsule, such as the collateral ligaments of the elbow joint.         * Intracapsular: Located inside the capsule, such as the cruciate ligaments of the knee joint.     * Meniscus (pl. menisci): Fibrocartilages located within a synovial cavity that allow articular surfaces to fit together more effectively.

Basic Synovial Joint Movements

• Gliding: Two surfaces sliding over each other.
• Angular Motions:     * Flexion: Decreasing the angle between limb segments.     * Extension: Increasing the angle between limb segments.     * Abduction: Moving a limb away from the median plane.     * Adduction: Moving a limb towards the median plane.
• Other Movements:     * Circumduction: A movement that circumscribes a cone shape.     * Medial/Lateral Rotation: Described as a "rolling pin" movement on the axis of the limb.     * Pronation vs. Supination: Rotational movements of the forearm (antebrachium). In domestic animals, there is a consistent state of pronation.

Functional Classification of Synovial Joints

• By Number of Articulating Bones:     * Simple Joint: Articulation between exactly 2 bones (e.g., glenohumeral/shoulder joint).     * Compound Joint: Articulation involving more than 2 bones (e.g., humeroradioulnar/elbow joint, carpal joint).
• By Congruency (Fit):     * Congruent: Articular surfaces fit well together (e.g., humeroradioulnar/elbow joint).     * Incongruent: Articular surfaces do not fit well together (e.g., knee joint).
• By Shape and Range of Motion:     * Hinge: The least versatile; permits only flexion and extension (e.g., elbow joint, similar to a door hinge).     * Spheroidal (Ball & Socket): The most versatile; allows all ranges of movement (e.g., shoulder joint, hip joint, moves like a sphere).     * Others: Includes condylar and ellipsoidal joints.

Myology and Muscle Anatomy

• Etymology: "Myo" is the Greek term for muscle; "Myo" + "ology" is the study of muscles.
• Clinical Significance: Lameness indicates locomotor system issues, which can be caused by damage to muscles, tendons, or ligaments, not just bone/joint pathology.
• Anatomy of Skeletal Muscle:     * Tendon of Origin (Proximal Attachment): A relatively fixed point with less movement than the insertion end.     * Muscle Head or Belly: The contracting part of the muscle.     * Tendon of Insertion (Distal Attachment): Relatively more motile; more movement occurs at this end.     * Aponeurosis: A flat, sheet-like tendon that allows a muscle to have a broader attachment area.

Accessory Structures of Muscles and Connective Tissues

• Synovial Bursa: A synovial fluid-filled "pouch" or "balloon" that protects a tendon from contact with a bony surface.
• Retinaculum: A fibrous band that holds down one or more tendons to a bone.
• Synovial Tendon Sheath: A synovial fluid-filled "sleeve" or "sock" that completely surrounds a tendon, easing its gliding between a retinaculum and bone.
• Comparison of Connective Tissues:     * Ligament: Attaches bone to bone (usually part of a joint) or sometimes tendon to bone.     * Tendon: Attaches muscle to bone (or to other tissue surfaces).     * Retinacula (pl.): Bands holding down muscle tendons close to the bone surface.
• Fasciae (pl.):     * Fibrous layers that envelop and isolate individual muscles or muscle groups.     * Superficial Fascia: Loose connective tissue that attaches skin to underlying muscles.     * Deep Fascia: A leaf of dense connective tissue from which some muscles originate or insert; it separates muscle groups into distinct fascial planes (e.g., thoracolumbar fascia).

Taxonomy of Muscle Naming

• Shape: e.g., Trapezius m. (referring to its shape).
• Size: e.g., Latissimus dorsi m. ("the widest muscle of the back").
• Relative Position: e.g., Superficial, middle, and deep gluteal mm.
• Origin and Insertion: e.g., Sternocephalicus m. (originates at the sternum and inserts on the head).
• Function: e.g., Extensor carpi radialis m. (radial extensor of the carpus).
• Structure: e.g., Triceps brachii m. ("three-headed muscle of the arm").