# Structure

The human skeleton is designed for structure, support and protection. The first picture on the right shows the structure of the human skeleton, mostly made of bone, with some cartilage. Bones provide the structure, whereas cartilage covers the end of bones are moveable joints, for lubrication and ease of movement.

### Bone Histology

Bone and cartilage in humans are living tissues, which require nutrients and oxygen same as any other cell. Most of the long shafts of bone in the body are compact bones. Compact bones have many concentric circular arrangements, all close together, and each circle is known as a Haversian system.
In the haversian systems are osteocytes (which begin their life as osteoblasts), which synthesise and secrete the fibrous protein tropocollagen, and secrete it outside their plasma membrane. Tropocollagen molecules link up end to end to form collagen fibres, and between these fibres calcium phosphate is deposited. Cells become totally surrounded by and trapped within a matrix collagen and calcium phosphate.
This means bone is a composite material, with collagen giving it tensile strength since collagen fibres are very strong, whereas calcium phosphate gives rigidity and compressive strength.
Living bone is broken down by osteoclasts, allowing bone structure to be altered if need be, for example to repair damage.

#### Cartilage

Cartilage in the human body is known as hyaline cartilage, and is a translucent tissue that is found covering the ends of bones at moveable joints. The cells inside the cartilage are called chondrocytes, and are responsible for producing and maintaining the matrix where they are. The secretion is 75% water and 25% collagen, leading to a extremely smooth and slippery bone end at joints.

### Axial Skeleton

The axial skeleton is the middle of the skeleton - the skull, vertebral column, ribs and sternum. The vertebral column is made from 33 vertebrae; 7 cervical (support head/neck), 12 thoracic (chest/rib articulation), 5 lumbar (large muscles of lower back attach to), 5 fused sacral vertebrae, and the 4 fused tail vertebrae - the coccyx.
The lumbar vertebrae (pictured right) consists of:
* Centrum: Load-bearing part
* Neural Arch: Holds neural canal - spinal column
* Transverse processes: Project outwards/sideways - powerful lower back muscles attach here, requiring the transverse process.
* Neural spine - Backwards pointing process, larger than in thoracic vertebrae.
* Articular processes - On the upper and lower surface, in contact with the vertebrae either side.
The thoracic vertebrae (pictured right) articulate with the ribs and are similar to the lumbar vertebrae except;
* Smaller

### Appendicular Skeleton

The appendicular skeleton is the limb bones, pectoral girdle and pelvic girdle. Numerous small bones make up the ends of limbs, and there are five digits in each hand/foot, a pendactyl limb system - evidence that all terrestrial vertebrates evolved from a common ancestor.

# Muscles and Movement

Muscles are specialised tissues whose purpose is to exert force when they contract. There are three types - smooth, cardiac and skeletel. Smooth muscle is found in the walls of the alimentary canal and is able to contract slowly and for long periods of time. Cardiac muscle contracts/rhythmically throughout its life. Muscle connected to skeleton, which is responsible for movements under conscious control is skeletal muscle.

## Joints

When two bones meet a joint is formed, and sometimes movement is allowed, sometimes not, for example in the cranium. However, some joints allow a great degree of freedom.

### Structure

The limb joints allow a great degree of freedom - ball-socket joints are at femur and pelvis, and arm and pectoral girdle, hinge joints at the elbow and knee. Both these types of joints are known as synovial joints, where bones are able to move substantially.
A synovial joint consists of a capsule formed from collagen (may also contain ligaments made from collagen) lined with a thin synovial membrane, cells which secrete small quantities of a clear viscous fluid that reduces friction between the bone ends. The ends of each bone at a joint are covered with thin, extremely smooth hyaline cartilage known as articular cartilage. Articular cartilage working with synovial fluid provides an almost friction free movement.

### Elbow movement

The elbow has antagonistic muscles, attached by tendons (collagen fibres), that contract to move the bones. Antagonistic muscles refers to the fact that they 'antagonise' each other or that there are pairs of muscles arranged so that one pair can pull in one direction, the other pair in the opposite direction.

### Levers

A force multiplier is one which multiplies the force exerted by the distance from the pivot - but our biceps do not work like this. Our muscles use a distance multiplier, meaning that the force the biceps have to exert to lift the weight is much greater than the weight itself. The reason for this is that muscles cannot contract over long distances.

## Structure/Function of Striated Muscle

Striated muscle is known as such because it appears striped under a microscope.

### Histology

Muscle is formed from many fibres laying parallel, and each fibre is formed from several cells, forming one multinucleate cell surrounded by a plasma membrane know as a sarcolemma. Each fibre cell is filled with parallel structures, each banded with light and dark staining 'fibrils'. Each cell also has many mitochondria for muscle contraction, and the endoplasmic reticulum is comparatively orderly, and is known as the sarcoplasmic reticulum. Their cisternae lies at right angles to channels called transverse tubules that are formed from the deep infolds of the sarcolemma.

### Fibril

Each part of the stripes in a fibril have their own letters. The stripes are known as filaments and are made from two different proteins - myosin and actin. Myosin forms the thick filaments and each filament is made up of many of them lying side by side. Each myosin molecule has a head, and the filaments branch out from the 'M' line where the tails meet.
Actin forms the thin filaments, and is a globular protein that links up to form long chains, twisted around each other forming a thin filament that's firmly anchored in the Z lines. Two other proteins form the thin filament structure, tropomyosin and troponin. Tropomyosin forms a long thin molecule which lies in the groove between the two actin chains. Troponin is a globular protein which binds to the actin chains are regular intervals along them.

### Contraction

When a muscle contracts, the thin actin filaments slide in between the thick myosin filaments, shortening the sarcomere. The heads of the myosin molecules act as enzymes that catalyse the hydrolysis of ATP to ADP and phosphate, an ATPase. A relaxed muscle fibre has ADP and phosphate attached to each head.
A nerve impulse arriving at the muscle introduces calcium ions that bind to the troponin, thus changing its shape. This causes troponin and tropomyosin to move away from the myosin binding site, allowing the head of the myosin molecule to bind to its neighbouring actin filament. The head then tilts through 45 degrees, pulling the actin filament towards the centre of the sarcomere. The tilting causes ADP and phosphate to be released and ATP to take their place. This is then hydrolysed, and the energy generated detaches the myosin head from the actin molecule and flips it back to its original position, pulling it further along. This cycle happens over and over during contraction.

#### Force Generated

The millions of myosin molecules acting on millions of actin filaments creates quite a large force - around 40-50N per cm$^2$ / CSA. The more myosin that acts on the actin the larger force is generated.

#### Nerve Impulses

So how does a nerve impulse cause a muscle contraction? At the end of a motor neurone there is a neuromuscular junction, a special type of synapse. The motor neurone axon divides into several branches forming a motor end-plate.
    1. The action potential causes an uptake of Calcium ions at the motor end plate
    2. The calcium ions cause vesicles containing acetylcholine to fuse with the presynaptic membrane, diffuse across the cleft and bind with the receptors in the sarcolemma, opening sodium channels.
    3. Sodium Ions flood in through open channels in the sarcolemma, depolarising the membrane and initiating a new action potential.
    4. This depolarisation spreads down the channels in the sarcolemma (T-tubules formed by the infolding)
    5. Calcium channels open and calcium diffuses out of the sarcoplasmic reticulum.
    6. Calcium binds to troponin, causing it to change shape, making tropomyosin move to expose myosin binding sites on actin filaments.
    7. Myosin heads bind and filaments slide. (See previous section).

# Effects of ageing
### Osteoporosis

Osteoporosis is a degenerative disease that results from a breakdown in the normal repair function in bones, and it affects the bones directly. Bone density gradually decreases, as the rate of the osteoblasts that build bone becomes less than that of the osteoclasts that break it down. This bone mass loss results in bones that are much more likely to break - it is a widespread and common disease, especially in woman.
Prevention is the usual things - good diet - lack of vitamin D and calcium can increase the risk, exercise, avoiding cigarettes. Women are more likely to develop osterporosis simply because their bone mass is less throughout their lives, but the lack of oestrogen in women that have reached the menopause is through to contribute.

### Osteoarthritis

Osteoarhtritis is a condition in which cartilage at the joints becomes rougher, making joint movement harder and sometimes very painful, causing a loss of mobility. It is due to changes that occur in the collagen and glycoproteins that help to give cartilage its resilience, slowly breaking them down.