Musculoskeletal System & Locomotion

Introduction to the Musculoskeletal System

This lecture introduces the musculoskeletal system, focusing on aspects relevant to small animal anatomy.

Learning Outcomes

By the end of this lecture, you should be able to:

  1. Understand locomotor system adaptations in cursorial animals.

  2. Describe limb movements relative to the trunk.

  3. Recognize bone types and describe long bone anatomy, relating structures to function.

  4. Explain bone formation from ossification centers and bone growth.

  5. Identify growth plates on radiographs and differentiate between young and mature animal radiographs.

  6. Describe skeletal muscle types and gross features.

Introduction to the Locomotor System

The locomotor system, also known as the musculoskeletal system, enables animal movement. It comprises:

  • Bones (Osteology): Provide rigid support.

  • Joints (Arthrology): Allow movement between bones.

  • Muscles (Myology): Produce movement at joints between bones.

Cursorial Adaptations of the Locomotor System

Cursorial animals, adapted for running, exhibit specific adaptations:

  1. Reduced Pectoral Skeleton:

    • Allows more cranial/caudal movement.

    • The clavicle is often absent or vestigial in cursorial animals.

    • Absence of the clavicle means no skeletal attachment of the forelimb to the trunk. The joint is purely muscular and called a synsarcosis. Synsarcosis means syn-together, sarco = flesh. It's also called the omothoracic junction.

  2. Elongation at Proximal End of Limb:

    • The scapula lies laterally.

    • This increases leg length, contributing to stride length.

    • The scapula has moved from lying flat against the back of a broad chest to lying flat against the sides of a deep narrow chest.

  3. Elongation at Distal End of Limb:

    • Long metacarpals.

    • Animals walk on toes (digitigrade) or hooves (unguligrade) instead of the whole foot (plantigrade).

    • Increasing leg length increases stride length, an energy-efficient way to cover ground quickly.

Locomotion

Interaction between Limbs and Trunk:

  • Folding limb = flexion

  • Moving limb forwards = protraction

  • Stretching limb = extension

  • Moving limb backwards OR fixing limb and moving trunk forwards relative to limb = retraction

  • Result = forward movement of animal

Trunk Contribution Varies by Species:

Locomotion and diet are linked in evolution.

  • Herbivores (e.g., Horse):

    • Roughage diets and bulky abdominal viscera.

    • Less flexible trunks.

    • Rely more on limb elongation for locomotion.

    • Relatively rigid vertebral column.

    • Most propulsion/stride length results from limbs.

    • Flexed suspension phase.

  • Carnivores (e.g., Cat):

    • Small-volume, meat diet.

    • Run fast.

    • Flexible trunks, multipurpose limbs.

    • Flexible vertebral column allowing limbs to swing more.

    • Acts like a bow to increase propulsion/stride length from limbs.

    • Flexed & extended suspension phases.

Gait

Gait = sequence of limb movements

  • Walk (4-beat gait): Each limb moves forward individually and sequentially, hitting the ground separately.

  • Trot (2-beat gait): Diagonal gait where two limbs hit the ground simultaneously. Foot fall sequence is LF+ RH & RF+ LH.

  • Canter (3-beat gait): Three beats with one period of suspension.

    • Left Lead: Foot fall sequence RH > LH+RF > LF

    • Right Lead: Foot fall sequence LH > RH+LF> RF

  • Gallop (4-beat gait): Fastest gait with one period of suspension.

    • Left Lead: Foot fall sequence RH > LH> RF > LF

    • Right Lead: Foot fall sequence LH > RH> LF> RF

Skeleton

Domestic species are vertebrates.

  • Internal Skeleton:

    • Axial skeleton - bones of vertebral column, skull and ribcage

    • Appendicular skeleton - all bones of fore- and hind-limbs

Bone Types

  1. Flat (e.g., Scapula, Pelvis):

    • Large surface area for muscle attachment.

    • Protection of underlying structures.

  2. Long (e.g., Humerus, Femur):

    • Support & leverage in limbs.

  3. Short (e.g., Carpus, Tarsus):

    • Large range of movement.

    • Anti-concussive.

  4. Sesamoid Bones:

    • Embedded in tendons.

    • Change in angle over bony surface.

    • Redirect forces.

    • Reduce friction so prevent damage.

Bone: General Features

  1. Smooth Surface Areas:

    • Passage of structures / tendons

    • Articular surfaces: Hyaline cartilage at weight-bearing surfaces are resistant to deformation. They have no blood vessels, nerves and periosteum.

  2. Specific Bony Contours:

    • Tuberosities / tubers / tubercles /processes provide sites for attachment of important structures.

  3. Rough Surface Areas:

    • General muscle attachment

Bone Structure

Important features include:

  • Periosteum: Covers outer surface of bone (except articular surfaces). Contains blood vessels and nerves, important for fracture healing.

  • Nutrient Foramen: Enters medullary cavity.

  • Endosteum

Bone: X-Rays / Radiographs

  • Bone = white (mineralized)

  • Diaphysis = shaft (wall = cortex)

  • Medullary cavity = hollow centre

  • Epiphysis = end which incorporates articular surfaces

  • Metaphysis = transition zone

Bone Growth

Limb bones are formed from centers of ossification:

The process involves:

  1. Mesenchyme

  2. Cartilage

  3. Proliferating chondrocytes

  4. Hypertrophic chondrocytes

  5. Osteoblasts (bone).

Short and sesamoid bones usually have one center per bone. Flat and long bones usually have more than one Center of ossification (CoOs) per bone.

Attachment sites for important structures may have separate centers.

Cartilage between centers appears black on radiographs = physis / growth plate, which shouldn't be mistaken for fractures!

Clinical Significance

  • Young Animal: Open physes/growth plates produce an increase in bone length.

  • Mature Animal: Closed growth plates = physeal scars.

  • Time of closure depends on the contribution of specific physis to limb length.

  • Can estimate age from growth plate closure times.

Adult dogs have the weakest point at the shaft = fractures; while Young dogs have the weakest point at the growth plate = damage/separation, which can cause premature closure of all or part of growth plate.

Subsequent problems depend on the timing and location of the damage.

Limb Directional Terminology

  • Cranial = towards animals head; Becomes dorsal at carpus / tarsus

  • Caudal = towards animals tail; Becomes palmar at carpus (FL); Becomes plantar at tarsus (HL)

  • Proximal = top of limb

  • Distal = bottom of limb

  • Medial = towards median plane

  • Lateral = away from median plane

Skeletal Muscle

  • Individual muscle fibers are arranged in parallel lines, collected into bundles, and covered by fibrous tissue (deep fascia).

  • Attached to rigid structures (usually bone).

  • Origin - usually proximal

  • Insertion - usually distal

Skeletal Muscle - Classification

  • Examples: Wide muscle filled with aponeurosis (Rectus abdominis), Wide muscle with aponeurosis, Sphincter m (Circular m, Orbicularis oculi), Spindle-shaped/fusiform muscles, Multipennate, Bipennate, Two bellied m (Two headed, Single headed, Deltoideus), Biceps, Rectus femoris

Skeletal Muscle | Attachment

Attachments can be stationary vs mobile end of muscles (Origin or insertion).

  • Directly to bone:

    • Size of area depends on muscle

    • Rough surface

    • Bony tuber

  • Via tendons:

    • Condensation of deep fascia into cords

    • Less bulky across joints

    • Small area of attachment

    • May have separate centre of ossification

  • Via aponeurosis:

    • Limited access to bone

    • Fibrous tissue arranged as a thin sheet of tissue

    • Condensation of deep fascial into sheets

    • Long area of attachment

    • Muscles in areas with limited access to bone

    • Cervico-thoraco-lumbar fascia

    • Superficial fascia includes subcutaneous and loose connective tissue and fat.

Skeletal Muscle | Actions

Muscle action means the belly contracts & shortens muscle, pulls points of attachment closer together and pivots at joints to decrease angle (flexion) or increase angle (extension).

Action depends on which joints are crossed and the aspect of joint crossed

Antagonistic pairs of muscles have opposite actions.

Skeletal Muscle - Classification

  • Intrinsic muscles: Origin and insertion within the appendicular skeleton, resulting in movement of limb joints (flexion and extension).

  • Extrinsic muscles: Origin on the axial skeleton and insertion in the appendicular skeleton, resulting in movement of limb relative to trunk (protraction and retraction of limb).

Locomotion - Domestic Species

In domestic animals, no joint is present between the forelimb and trunk, only muscular attachments. The scapula glides over the thorax with a pivotal point (PP) midway down the scapula.