MUSCULOSKELETAL SYSTEM INTRO

ILOS

Discuss the structure of the skeletal system and classification of bones • Discuss the simple classification of joints, giving relevant examples of each type • Describe the structure of bursae and explain their importance • Describe the characteristics of skeletal muscle and the process of contraction • Explain the role of Ca2+ in skeletal muscle function and how muscle force is controlled • Describe the length-tenson relationship in skeletal muscle

Bone

forms most of skeleton

Cartilage

Forms some parts of skeleton (e.g. costal cartilage)

• lines articular surfaces

Tendons and ligaments

• tendons attach muscles, mostly to bone

• ligaments connect 2 or more bones or cartilage

Bones are classified according to their shape

• long

• short

• flat

• irregular

• sesamoid = small bones resembling sesame seeds

• splanchnic = collection of bones in cow heart (ossa cordis), for dogs and cats os penis & clitoral bone in females, rostral bone in pigs

• pneumatic = hollow & containing air

Skeletal system diagram

Bone marrow

• found in medulla and in spaces between trabeculae (thin columns and plates of bone that create spongy network) of cancellous bone

(compact) cortical bone has a much denser structure compared to cancellous (spongy) bone

Where is growth plate found

Metaphysis, between epiphysis and diaphysis

Fibro-cartilaginous joint classification

Fibrous joint = JOINT HELD TOGETHER BY FIBROCARTILAGE (bone-bone)

• → Connect bones of the skull (sutures), connect bones in the body (syndesmosis), and hold teeth in sockets (gomphosis)

Primary cartilaginous joints = JOINT HELD TOGETHER BY HYALINE CARTILAGE (bone - bone)

→ can be found at costochondral joints (connecting ribs to costal cartilage at sternum),

• allows flexibility while maintaining structural integrity as hyaline cartilage helps cushion and support costochondral junction

• primary cartilaginous joints also found in the growth plates of long bones.

Secondary cartilaginous joints = JOINTS HELD TOGETHER BY FIBROCARTILAGE AND HYALINE CARTILAGE (fibrocartilage in the middle and hyaline cartilage on each side between the bones) (bone-bone)

• found at the pubic symphysis and intervertebral discs

Synovial joints

consist of

• shaped articular cartilage surfaces

• joint capsule containing synovial fluid → capsule keeps synovial fluid across surface of these cartilage structures and keeps them lubricated to reduce friction so joints can move smoothly

• supporting ligaments around capsule / as part of capsule → help to hold bones together and provide stability during movement.

• supporting muscles and tendon → must have muscle/tendon attached to a muscle crossing joint to actively move the joint

What does range of motion of joint depend on?

• Shape of articular surfaces

• Amount of muscles and tendons / ligaments across joint

• What the muscles are doing

Bursae

Structures with a capsule and synovial fluid but no joint

• helps to smooth passage of bones, tendons, ligaments during movement

• can become inflamed “bursitis” when there is excessive friction/rubbing / infection

example:

• navicular bursae in equine foot

  → around the navicular bone in equine distal foot

• acts as a pouch of space for things to move smoothly across each other in that space instead of causing friction on the structures

Skeleton provides..

• Support

• Protection

• Structure

but requires active control from muscles for movement functions of the limbs, head and neck

Muscles to maintain body posture

‘Antigravity’ extensor muscles of the limbs and spine

Muscles to move limb joints

Flexor & Extensor muscles

Muscles to move the jaw

Muscles of mastication (trigeminal muscles)

Muscles to shape the lips

Muscles of facial expression

Muscles to move food into the digestive tract

Pharyngeal and palatine muscles

Muscles to support abdominal organs

Abdominal muscles

Skeletal muscle is organised into muscle tendon units

has contractile and non-contractile portion

• contractile muscular ‘belly’ with visible fascicles (each comprising of multiple muscle fibres)

• non-contractile fibrous elastic portion (tendon)

proportion of each depends on function

• will have diff shape of fascicles which also contribute to & dictate muscle function

share a name → Biceps brachii refers to both the muscle and the tendon, so must specify Biceps brachii muscle (belly of Biceps brachii) or Biceps brachii tendon

Muscle Attachments

• origin; ‘start’ of muscle, generally proximal and move less

• Insertion; ‘end’ of muscle, typically distal and move more

mostly attached to bone but can also be attached to fascia (midline), or tendons

• can have multiple insertion and origins per muscle

• determine leverage of the muscle

intrinsic = both attachment points are within limb

extrinsic = from limb to body (e.g. attaching scapula to torso or attaching femur to pelvis)

e.g. Biceps brachii

origin = supraglenoid tubercle

insetion = proximally on the radial tuberosity and ulna

→ crosses 2 separate joints where it inserts and attaches determines how much leveragenit has around each of the joints

Skeletal muscles have

• striated, cylindrical cells, multinucleate, voluntary control

• contraction comes from sarcomere

• fibres are arranged into visible fascicles; parallel (allow muscles to shorten quickly and changed length of muscle in more dramatic way) and pinnate

(fanned out to maximise force production within the muscle cos contraction is pulling tightly in the same direction)

• motor unit = motor neuron & the skeletal muscle fibers it innervates at the neuromuscular junction

→ indiv neurons innervate a portion of the muscle belly, allowing for control cos some muscle fibers are being activated to some level, not entire muscle at once

tendons and ligaments

• tendons connect muscle to bone, ligaments connect bone to bone

• both connect regular connective tissue, ligaments generally have a greater elastic component

• tendons act as strings or springs but for them to act as springs, attached muscle fibres need to develop force so the tendon can be stretched

• reduced muscle fibre content reduces the need for energy e.g. equine superficial digital flexor tendons have an additional elastic tissue component (so more yellow) → used as an elastic storage mechanism that gives energy back in recoil, but need to contract muscle belly to make sure theres tension and muscle belly is not changing length

Skeletal muscle structure

• Tendon holds muscle to bone

• Skeletal muscle surrounded by epimysium / deep fascia

• Individual visible fascicles surrounded by perimysium

• Individual muscle fibers within fascicles surrounded by endomysium

• within individual muscle fibres are myofibrils with sarcomeres

Muscle activation & contraction

MUSCLE ACTIVATION (ELECTRICAL)

• Brain sends signal (AP) down motor neuron.

• Nerve impulse reaches neuromuscular junction, triggering release of acetylcholine,

• Ach binds to receptors on the muscle membrane, triggering an AP in the muscle cell

• AP reaches sarcoplasmic reticulum and calcium is released

MUSCLE CONTRACTION

• `Exposure of active sites:

  Calcium binds to troponins, shifting tropomyosin away and exposing actin myofilament binding sites

• Cross-bridge formation:

  Myosin heads bind to the exposed actin binding sites and form cross-bridges. Phosphates are the released from the myosin heads.

• Power stroke:

  Energy stored in myosin heads is used to move the myosin heads, causing the actin filaments to slide past the myosin myofilaments and ADP is released from the myosin heads resulting in muscle shortening.

• Cross-bridge release:

  An ATP molecule binds to each of the myosin heads, causing them to detach from the actin'

• Hydrolysis of ATP

  The myosin ATPase portion of the myosin heads split ATP into ADP and phosphate, which remain attached to the myosin heads\

• Recovery stroke

  The heads of the myosin molecules return to their resting position and energy is stored in the heads of myosin molecules. If Ca2+ is still attached to troponins, cross bridge formation and movement are repeated and the cycle repeats many times during muscle contraction.

if muscle is deactivated, Ca2+ is pumped back into the sarcoplasmic reticulum. ATP then binds onto the myosin heads to allow it to let go of the actin