Answers to end of chapter ‘revise’ questions

Test your knowledge of what you have learned in the book by completing the end of chapter ‘revise’ questions in the book and checking your answers.
 

1. Identify and describe the function of the four types of bone cells.

Answer:

Bone tissue consists of four types of cells: osteogenic cells, osteoblasts, osteocytes and osteoclasts.

Osteogenic cells

These are derived from mesenchymal cells (adult stem cells), they undergo cell division and these cells then develop into osteoblasts. They are found in the periosteum, endosteum and within the canals that contain the blood vessels.

Osteoblasts

These are the bone building cells. They make the bone matrix by synthesising and secreting collagen fibres and other organic components. They also initiate calcification of the matrix.

Osteocytes 

These start as osteoblasts. As they are surrounded by the matrix they are trapped, no longer able to secrete matrix and become osteocytes. Osteocytes are therefore found in mature bone and are the main cell type in bone. Their function is to maintain the daily metabolic function of bone by ensuring exchange of nutrients and waste products with the blood.

Osteoclasts 

These bone cells are formed by the fusion of approximately 50 monocytes (a type of macrophage, white blood cell) and remove old bone. They are very large, multinucleated and are found predominately in the endosteum. The plasma membrane of the osteoclast is folded into deep ruffles and faces the surface of the bone. It secretes powerful lysosomal enzymes and acids that are responsible for dissolving the protein and mineral matrix. This is known as resorption and is part of the normal development, maintenance and repair of bone. The removal of old bone is usually aligned to the production of new bone cells by the osteoblasts.

2. Identify the constituent parts of the axial and appendicular skeleton.

Answer:

The axial skeleton makes up the central bony core of the body (the axis):

  • Skull.
  • Vertebral column.
  • Ribs.
  • Sternum.

The appendicular skeleton consists of the:

  • Shoulder girdle (two scapulae and two clavicles) and bones of the upper limb (humerus, ulna, radius, carpal, metacarpals and phalanges).
  • Pelvic girdle composed of the two hip bones (each composed of three bones: the ilium, ischium and pubis) which are fused in adult life. The two pubic bones fuse anteriorly at the symphysis pubis, and fuse with the sacrum posteriorly. The remaining bones are those of the two lower limbs, each including: the femur, tibia, fibula, patella, tarsals, metatarsals and phalanges. 

3. List the types of synovial joint found in the human body and explain the role of particular structures common to all types.

Answer:

There are six types of synovial joints within the human body:

  • Ball and socket joints, e.g. shoulder and hip – allow wide range of movement.
  • Hinge joints, e.g. elbow and knee – allow for flexion and extension.
  • Gliding joints, e.g. joints between carpal bones in wrist – least movable.
  • Pivot joints, e.g. head rotation – bone or limb rotates.
  • Condyloid joints, e.g. metacarpopharyngeal.
  • Saddle joints, e.g. base of thumb – similar range of movement as condyloid plus opposition of the thumb – promote flexion, extension, abduction, adduction and circumduction.

How the structure of a synovial joint is related to its function

  • Articular cartilage:

o   covers the ends of bones of synovial joints;

o   smooth and strong;

o   allows the two surfaces to move over each other smoothly.

  • Synovial membrane:

o   secretes synovial fluid;

o   covers parts of bone within a joint that are not covered by cartilage.

  • Synovial fluid:

o   thick, sticky fluid – lubricates joint surfaces;

o   provides nutrients for structures in joint cavity;

o   contains phagocytes so removes microbes.

  • Capsular ligament:

o   surrounds and holds the synovial joint together;

a sleeve of fibrous tissue that is loose enough to permit movement of the joint, but strong enough to protect it from injury.

4. Skeletal muscle is under control of which division of the nervous system?

Answer:        

Peripheral nervous system – somatic – voluntary control.

5. A 72 year old lady fell when out walking her dog and sustained a fractured neck of the femur. Describe the stages of bone healing. Include in your answer any nutrients that are necessary to promote the healing process. 

Answer:

A fracture is any break in the bone, the process of bone healing is an ordered progression of steps.

  • Stage 1. Fracture haematoma – blood clot forms 6–8 hours following the fracture. Blood supply to the bone cells that lie on either side of the fracture is disrupted and they begin to die. Macrophages and osteoclasts start to remove the dead bone and other cells from the fracture site and cause localised swelling and inflammation. Next step is to re-establish the blood supply to the fracture area so new cells can begin to heal the fracture. Blood capillaries grow into the haematoma. This stage may last several weeks. Presence of nicotine can inhibit this capillary ingrowth and therefore smokers have an increased length of fracture healing time
  • Stage 2. Fibrocartilaginous callus – new blood vessels that have grown into haematoma organise into granulation tissue (procallus, i.e. pro–callus). Fibroblasts (from periosteum) and osteogenic progenitor cells from the periosteum, endosteum and red bone marrow start to invade the procallus. Collagen produced by fibroblasts become chondroblasts and connect the two ends of the fracture. Osteogenic progenitor cells enter the bordering regions of healthy bone on the edges of the dead bone, ready to make new bone matrix. This stage lasts about 3 weeks. The procallus and fibrocartilaginous callus are very soft during the first 4–6 weeks of healing and therefore need adequate support.
  • Stage 3. Bony callus – osteogenic progenitor cells that invaded the procallus migrate to the borders of dead tissue and form into osteoblasts and begin to secrete bone matrix. This forms spongy bone trabeculae (not arranged in an ordered way = woven bone). The trabeculae join the living tissue on either side of the fracture. Depending on size of bone and nature of injury it may take up to 3 months for the whole fibrocartilaginous callus to be transformed into bone (ossification) of adequate strength.
  • Stage 4. Bone remodelling – woven bone lattice is rearranged into the normal cortical and spongy bone arrangement. The woven bone is removed gradually and replaced by other osteoblasts. This may take up to a year after injury.

Nutrients needed

  • Calcium and phosphorus – primary minerals in the composition of human bone.
  • Vitamin D – plays an important role in drawing calcium from your blood into the bones.
  • Vitamin C – essential for proper synthesis of the bone collagen protein matrix. It is also one of the most important antioxidants and anti‐inflammatory nutrients.
  • Vitamin K ­– plays a key role in strengthening osteocalcin, a protein component of bone, without increasing the mineral density of bone.
  • Vitamin B6 – modulates the effects of vitamin K on bone through complex biochemical pathways.
  • Protein – growth and repair.

6. Explain the steps involved in muscle contraction.

Answer:

  • Action potential travels along a motor neuron until it reaches a synaptic terminal – vesicles release ACh (acetylcholine) into the synaptic cleft.
  • ACh molecules diffuse across the synaptic cleft and bind to ACh receptors on the sarcolemma.
  • Permeability changes, and allows sodium ions into the sarcoplasm which triggers production of muscle action potential in the sarcolemma.
  • Action potential spreads across the entire surface of the sarcolemma, travels down T tubules to the cisternae.
  • Releases significant amounts of calcium ions – leads to initiation of muscle contraction.
  • Action potential generation ceases as ACh is broken down by AChE (acetylcholinesterase) and concentration of calcium ions in sarcoplasm declines. Once calcium ions return to resting levels, muscle contraction ends = muscle relaxation.
  • Muscle contraction involves the sliding of thick myosin and thin actin filaments over each other to produce shortening of the muscle fibre. It begins with activation of the cross bridges from the myosin filaments, uncovering the tropomyosin-binding sites on the actin filament. When activated by ATP the heads of the myosin filaments swivel in a fixed arc. During contraction each myosin head undergoes its own cycle of movement, forming a bridge attachment, releasing it and moving it to another site where the same sequence occurs. This pulls the thin and thick filaments past each other.