Locomotor Apparatus Study Notes (Osteology, Arthrology, Myology)

Osteology

• Definition: osteo- (bone) + -logy (study of) = the study of bones.
• Primary Functions of Bones
  • $1$. Support the weight of the body
  • $2$. Movement in concert with joints and muscles
  • $3$. Protection of internal organs
  • $4$. Growth
  • $5$. Storage of minerals (e.g.: $\mathrm{Ca}^{2+}$ and $\mathrm{P}^{+}$)
  • $6$. Storage of fat → marrow
  • $7$. Blood cell formation (hematopoiesis) → marrow

Forelimb region and bones

• Scapular region → Scapula
• Brachium → Humerus
• Antebrachium → Radius and Ulna
• Manus (front paw):
  • Carpus → carpal bones
  • Metacarpus → metacarpal bones
  • Digits → Phalanges & sesamoids

Types of bones according to shape

• Flat bone: scapula
• Long bones: humerus, radius, ulna (longer than wide; shaft and two ends)
• Short bones: carpal bones (roughly cube-shaped; similar length, width, height)
• Irregular bones: vertebrae
• Sesamoid bones: a special type of short bone found within tendons

Sesamoid bones

• Sesamoid bones are found in the tendons of origin or insertion of certain muscles or in a muscle.
• They can develop as a response to strain or as a normal occurrence.
• Functions:
  • $1.$ Provide a smooth surface for tendons to slide over, preventing friction of the associated tendon
  • $2.$ Act as a fulcrum providing extra leverage to the muscle, enhancing the tendon's ability to transmit muscular forces
  • $3.$ Can displace the axis of force by altering the direction of tension in the muscle tendon

Long bones – gross anatomy

• At least $3$ centers of ossification
• Diaphysis = bone shaft → medullary cavity (contains yellow marrow)
• Epiphysis = end regions of bone
• Physeal growth plate: located between epiphyses and diaphysis in young animals; comprised of cartilage cells
• Growth plate (physeal plate) is a primary site for growth and ossification in developing bones

Growth plate relevance

• Primary site for fractures, infections, metastasis, and effects of endocrine bone disorders
• Example: in a dog, age ~$8$ months, hindlimb growth plate fractures are a common concern

Bony prominences

• Scapula, distal view; Scapula, lateral view
• Bones remodel in response to stress placed upon them (Wolf’s law = form follows function)
• Non-articular prominences: attachment sites for muscles; passages of vessels and nerves, etc. (e.g.: spine of the scapula, supraspinous & infraspinous fossae)
• Articular prominences: make up joints (e.g.: glenoid cavity)

Arthrology

• Arthro- (joint) + -ology (study of)
• Provides support and movement to the skeleton (not all joints are movable!)
• A joint is the point of contact, or articulation, between two or more bones/cartilages

Structural classification of joints

1. Fibrous joints – strong connective tissue unites articular surfaces of bones; bones can fuse (synostosis) → almost no movement
2. Cartilaginous joints – cartilage unites articular surfaces of bones → limited movement
3. Synovial joints – joint cavity between articular surfaces of bones filled with synovial fluid → most movable

• Example: intervertebral disc in the human spine

Anatomy of a synovial joint

• Note: a joint may have multiple synovial compartments/sacs within a single joint capsule
• Articular surfaces:
  • Protected by articular cartilage
  • Enclosed within a fluid-filled joint cavity
• Joint capsule:
  • Provides strength and resistance
  • For lubrication and nutrition of the articular cartilages
  • Produces synovial fluid
  • Highly vascularized and innervated

Ligaments, menisci, and intra-articular structures

• Ligaments (lig.) = bands of tough fibrous connective tissue
  • Extracapsular (e.g.: collateral ligaments of the elbow joint)
  • Intracapsular (e.g.: cruciate ligaments of the knee joint)
• Meniscus (pl. menisci) = fibrocartilages located within a synovial cavity; allow articular surfaces to fit together
• Accessory structures of synovial joints (example: knee joint include medial and lateral menisci)
• Bones, tendons, and ligaments interact to stabilize joints

Basic synovial joint movements

• Gliding of two surfaces on each other
• Angular motions:
  • $1$. Flexion: decreasing angle between limb segments
  • $2$. Extension: increasing angle between limb segments
• Abduction vs. Adduction:
  • $1$. Abduction: moving away from the median plane
  • $2$. Adduction: moving toward the median plane
• Circumduction: movement describing a cone shape
• Medial/Lateral Rotation: rolling pin movement on the axis of the limb
• Pronation vs. Supination of the forearm (antebrachium):
  • Consistent pronation in domestic animals

Synovial joint classifications (by features)

• By number of articulating bones:
  • Simple (between 2 bones): e.g., glenohumeral (shoulder) joint
  • Compound (more than 2 bones): e.g., humeroradioulnar (elbow) joint, carpal joint
• By how well the bones fit together:
  • Congruent (articular surfaces fit well): e.g., humeroradioulnar (elbow) joint
  • Incongruent (articular surfaces do not fit well together): e.g., knee joint (in some species; noted in education materials)
• By shape and permitted motions around the joint (several types):
  • Hinge – least versatile; flexion and extension only (e.g., elbow joint)
  • Spheroidal (ball & socket) – most versatile; all range of movements (e.g., shoulder, hip joints)
  • Others: condylar, ellipsoidal, etc.

Myology

• etymology: myo = Greek term for muscle; myo + ology = study of muscle(s)
• Locomotor System (Apparatus) =
  • bones + joints (PASSIVE components)
  • muscles (ACTIVE component)
• Importance: lameness can be caused by muscle/tendon damage, not just bone or joint pathology

Skeletal muscles, tendons, and attachments

• Skeletal muscles attach to bones via tendons
• Tendon of origin (proximal attachment) – relatively fixed point; less movement here than at insertion
• Muscle head or belly – the part that contracts
• Tendon of insertion (distal attachment) – relatively more motile; more movement at the insertion end than at the origin
• Aponeurosis: a flat, sheet-like tendon; allows muscle to have a broader attachment
• Canine forearm (lateral view) – proximal and distal regions identified in imaging studies

Accessory structures of muscles

• Synovial bursa (bursa) = synovial fluid-filled pouch protecting a tendon from a bony surface
• Retinaculum = fibrous band holding down tendons to a bone
• Synovial tendon sheath = synovial fluid-filled sleeve surrounding a tendon (like a sock) to ease gliding between a retinaculum and bone

Ligament, tendon, and retinaculum distinctions

• Ligament: attaches bone to bone (usually part of a joint); can be tendon-to-bone in some cases
• Tendon: attaches muscle to bone (or to another tissue surface)
• Retinacula (pl.) = bands holding down tendons close to the surface of a bone

Fasciae

• Fasciae (pl.) = fibrous layers enveloping and isolating muscle groups and individual muscles
• Superficial fascia = loose connective tissue attaching skin to underlying muscles
• Fasciae/fascial planes: organize muscle groups into layers
• Deep fascia = leaf of dense connective tissue from which some muscles may originate or insert; separates muscle groups into fascial planes
• Example: Thoracolumbar fascia; superficial fascia on dorsum and caudal regions

Use muscle names to your advantage

• By Shape: e.g., trapezius m. = shape-based naming
• By Size: e.g., latissimus dorsi m. = “the widest muscle of the back”
• By Relative position: superficial, middle, and deep gluteal muscles
• By Origin and Insertion: sternocephalicus m. = “sternum to head” muscle
• By Function: extensor carpi radialis m. = “radial extensor of the carpus muscle”
• By Structure: triceps brachii m. = “three headed muscle of the arm”