Chapter 40: Human Musculoskeletal System

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Structure and function of the human skeleton

  • Shape and support
  • Protection: of internal organs
  • Movement: muscles use skeleton as levers
  • Production of red and white blood cells
  • Hearing: ossicles vibrate and amplify vibrations
  • Ingestion and digestion: physical digestion in the mouth
  • Storage: of minerals (calcium and phosphorus) that can be taken by the body when needed

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Axial Skeleton
There are 80 bones in the axial skeleton, including:

  • Skull
  • Ossicles
  • Spine
  • Ribs
  • Sternum

Skull:
Also known as the cranium, consisting if 8 fused bones and facial bones

Vertebral column:
Consists of 33 small bones which appears as only 26 due to fusion of the vertebrae of the sacrum (5 fused vertebrae) and coccyx (4 fused vertebrae)

Vertebrae:

  • Protect spinal cord
  • Give support to thoracic cavity and abdomen

Invertebral discs:

  • Located between each vertebra – held in place by ligaments
  • Elastic and compressible
  • Provide flexibility and act as shock absorbers
  • Can “slip” – where soft centre of disc bulges out. It is called prolapsed invertebral disc. Can push against nerve often causing severe pain.

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Ossicles:
Consist of three small bones – the smallest in the human body

  • Hammer (malleus)
  • Anvil (incus)
  • Stirrup (stapes)

Ribs:

  • 12 pairs originate from the vertebral column
  • 7 pairs are “true ribs” (attached directly to sternum)
  • 3 pairs are “false ribs” (attached indirectly to the sternum by only cartilage)
  • 2 pairs are “floating ribs” (attached to spine only)

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Appendicular Skeleton
Consists of 126 bones involving:

  • Pelvic girdle (hips)
  • Pectoral girdle (shoulders):

– Scapula
– Clavicle (collar bone)

  • Limbs:

– Arms: humerus, radius, ulna, carpals, metacarpals, and phalanges
– Legs: femur, tibia, fibula, tarsals, metatarsals, and phalanges

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Macroscopic Anatomy of a Long Bone
Bone is mixture of organic (35%) and inorganic (65%) materialPeriosteum
Thin layer of connective tissue that covers the outer surface of bone in all places except at joints (which are protected by articular cartilage)

Compact bone
Dense and forms the surface of bones (80% of the weight of human skeleton)
Hard with very few gaps
Function: support and protection

Medullary cavity
Central cavity of the bone shaft where yellow marrow (adipose tissue) is stored
Located in the main shaft of the bone (diaphysis)
In children this area is also involved in the formation of red blood cells, so red marrow is present
Function: storage of fat and formation of RBCs in children

Spongy bone
Found at the expanded heads of long bones
Low density and low strength but very high surface area
Fills the inner cavity of long bones
In some bones the spaces are filled with red bone marrow where the production of blood cells occurs
Contains most of the arteries and veins
Functions: production of RBCs and delivery of oxygen and nutrients

Cartilage
Type of dense connective tissue
Composed of collagenous fibers and/or elastic fibers which are embedded in a firm gel-like ground substance called the matrix
Cartilage is avascular (contains no blood vessels) and nutrients can only diffuse through the matrix
Cartilage is found in many places in the body including the joints, the rib cage, the ear, the nose, the bronchial tubes and between intervertebral discs
Functions: provides framework upon which bone deposition can begin and supplies smooth surfaces for the movement of articulating bones

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Joints

  • Immovable – fused joints of the skull
  • Slightly movable – vertebral joints
  • Free-moving/Synovial:

– ball-and-socket joints of the shoulder and hip
– hinge joints such as the elbow and knee

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Ligaments and Tendons

  • Ligaments are strong, slightly elastic bands of fibrous tissue (collagen) – they control the range of movement of a joint
  • Tendons are extremely strong, inelastic bands/cords of collagen – they attach muscles to bone

Skeletal Muscle

  • Contractile tissue – has ability to shorten and generate a pulling force
  • Voluntary muscles – we can consciously control contractions of skeletal muscle
  • Function: movement and temperature regulation in the body as they generate a lot of heat during movement

Antagonistic Muscle Pairs

  • Antagonistic muscle are pairs of muscles where the action of each pulls the attached bone in the opposite direction to the other
  • Most of the body’s 700 muscles operate as part of antagonistic muscle pairs
  • e.g. biceps and triceps of the upper arm
  • e.g. hamstrings and quadriceps of the upper leg

Musculoskeletal Disorder – osteoporosis

  • Brittle-bone disease – bones are porous/less dense
  • Often results in compression fractures in the vertebrae and in the neck of the femur (broken hip)
  • Possible cause: Bone replacement has slowed down
  • Prevention: Physical exercise during puberty reduces – risk of developing the disease in old age
  • Calcium-rich diet and drinking fluoridated water maintains bone density
  • Treatment: Females use HRT (but bone material – already lost is not replaced!
  • Physical exercise, dietary calcium, and – vitamin D are recommended

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Musculoskeletal Disorder – Rheumatoid Arthritis

  • Joint (fingers, wrists, ankle most common) become stiff and sore (inflammation occurs)
  • Articular cartilage has been destroyed and bones fuse
  • Possible cause: Immune reaction against the body’s own cartilage
  • Prevention: No known preventative measure yet
  • Treatment: Pain-killers and anti-inflammatory drugs are administered
  • Badly affected areas, such as hip or knee can be replaced with artificial joints

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Growth and Development in Bones

  • Skeleton of early embryo is mostly cartilage
  • Bone formation begins just before week 8
  • Specialised cells called osteoblasts secrete a thick layer of bone around the cartilage of the diaphysis – a tube of bone now encircles the diaphysis
  • Cartilage continues to grow at ends – epiphyses – bone elongates
  • After birth the epiphyses start to calcify
  • Cartilage remains at the junctions between the diaphyses and epiphyses – this area is called the growth plate
  • The growth plates enable the bones to elongate during childhood and puberty and eventually calcify and are replaced by bone at the end of puberty then fully grown – 18 in females and 21 in males

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Bone Renewal

  • Bone is a living organ and is capable of self-renewal
  • Bone is constantly broken down and replaced (parathormone involved)
  • Skeleton is completely replaced every 7 years and complete replacement slows down with age
  • Osteoclasts remove calcium and osteoblasts lay calcium down
  • Renewal of bones is affected by the hormone parathormone (secreted by the parathyroids) but also by exercise levels and dietary calcium