Human Skeleton: MDCAT Biology notes
Human skeleton MDCAT notes: cartilage types, bone cells and matrix, axial and appendicular skeleton, skull, vertebral column, girdles and limb bones.
Skeletal system
The skeletal system is made up of all the bones in the body plus the tissues that connect them: cartilage, ligaments and tendons. Tendons join muscle to bone, and ligaments join bone to bone. The skeleton supports the body, protects organs (for example the cranium encloses the brain), allows movement, and stores minerals.
Cartilage
Cartilage is a connective tissue. It is firm but much softer and more flexible than bone. Its living cells, the chondrocytes, sit in spaces called lacunae. They secrete the flexible matrix, which contains type II collagen. Cartilage is avascular (it has no blood vessels), so it heals very slowly. It covers the ends of bones at joints.
| Type | Features | Found in |
|---|---|---|
| Hyaline | The most abundant type; smooth and glassy | Nose, trachea, larynx, ends of long bones, ribs |
| Elastic | Contains elastic fibres | Pinna (ear flap), epiglottis |
| Fibrocartilage | Dense collagen fibres, very strong | Intervertebral discs, knee joint (menisci), pubic symphysis |
Bone
- Bone's matrix is about 65% inorganic (calcium phosphate salts, which give hardness) and about 35% organic (collagen, which gives flexibility). Bone cells contain only about 20% water.
- Osteoblasts: bone-forming cells that lay down the matrix.
- Osteocytes: mature bone cells trapped in lacunae.
- Osteoclasts: break down bone (resorption) during remodelling.
- Osteons (Haversian systems) are the structural units of compact bone.
Axial skeleton
The axial skeleton is the skull, vertebral column, ribs and sternum.
- Cranial bones: the paired ones are the parietal and temporal bones. The single ones are the frontal, occipital, sphenoid and ethmoid. The vomer is a single facial bone.
- Vertebral column: 33 vertebrae.
| Region | Number | Note |
|---|---|---|
| Cervical | 7 | 1st is the atlas, 2nd is the axis |
| Thoracic | 12 | Carry the ribs |
| Lumbar | 5 | Largest vertebrae |
| Sacral | 5 fused | Form the sacrum |
| Coccygeal | 4 fused | Form the coccyx (tail bone) in the pelvic region |
Cervical plus thoracic vertebrae come to 7 + 12 = 19.
Appendicular skeleton
- Pectoral girdle: the clavicle (collar bone) and the scapula (shoulder blade).
- Upper limb: humerus, radius and ulna, carpals, metacarpals, phalanges.
- Pelvic girdle: the hip bones (ilium, ischium, pubis).
- Lower limb:femur (the longest bone), patella, tibia and fibula, tarsals, metatarsals, phalanges.
Muscle attachment
A skeletal muscle's origin is fixed to the stationary bone. Its insertion is on the movable bone. The hamstrings at the back of the thigh arise mainly from the pelvic girdle and insert on the tibia (and fibula). They flex the knee.
Common MDCAT traps
- Cartilage has no blood vessels, so "extensive blood supply" is the false statement.
- Osteoblasts build bone, osteoclasts break it down, and osteocytes sit in lacunae.
- The femur is in the leg, not the upper limb.
- The nose and trachea are hyaline cartilage. The pinna and epiglottis are elastic. Intervertebral discs are fibrocartilage.
- The temporal bone is paired; the frontal and occipital bones are single.
Quick revision
- The sacrum is formed by 5 fused vertebrae.
- The axis is the 2nd cervical vertebra.
- The clavicle belongs to the pectoral girdle.
- The femur is the longest bone.
- Bone matrix is about 65% inorganic.