Support and Movement notes

Support and Movement in Humans

The human body needs support to maintain its shape and to protect delicate organs. Movement occurs when muscles pull on bones across joints. The skeleton acts as a framework, while muscles and joints work together as the locomotory system.

The human skeleton is also involved in mineral storage and blood cell formation. Bones are living organs with blood vessels, nerves and bone cells. Cartilage provides flexible support at several parts of the body.

• The human skeleton provides support, protection, shape and sites for muscle attachment.
• Bones act as levers, while joints act as points around which movement occurs.
• The skeleton is divided into the axial skeleton and the appendicular skeleton.
• The axial skeleton includes the skull, vertebral column, ribs and sternum.
• The appendicular skeleton includes the bones of the limbs and the pectoral and pelvic girdles.
• The skeleton also stores mineral salts, especially calcium and phosphate.
• Bone marrow is involved in the formation of blood cells.

Human Skeleton

The human skeleton is made mainly of bones and cartilage. Bones are hard and strong, whereas cartilage is flexible and less rigid. The hardness of bone is due to the deposition of mineral salts in its matrix.

The skull protects the brain and forms the framework of the face. The vertebral column supports the body and protects the spinal cord. The ribs and sternum protect the heart and lungs.

The skull of an adult human is made up of 22 bones. Most bones of the skull are joined by immovable fibrous joints called sutures.

• The human skull has 22 bones.
• The skull consists of cranial bones and facial bones.
• Cranial bones surround and protect the brain.
• The joints between the flat bones of the skull are sutures.
• Sutures are immovable fibrous joints.
• Bone matrix is hardened mainly by calcium phosphate and other calcium salts.
• Cartilage is present in the nose, external ear and many other flexible parts.
• Nose and ears are mainly supported by cartilage, not hard bone.

Vertebral Column and Thoracic Cage

The vertebral column, or backbone, extends from the neck to the lower part of the trunk. It supports the head and trunk and protects the spinal cord. It is made of a series of vertebrae with cartilage between many adjacent vertebrae.

The vertebral column normally contains 33 vertebrae. Some vertebrae are separate, while the sacral and coccygeal vertebrae are fused in adults. The cervical vertebrae form the neck region.

The thoracic cage consists of the thoracic vertebrae, ribs and sternum. It protects the heart and lungs and also helps in breathing movements.

• There are 7 cervical vertebrae in the neck.
• There are 12 thoracic vertebrae in the chest region.
• There are 5 lumbar vertebrae in the lower back.
• There are 5 fused sacral vertebrae.
• The coccyx is formed by fused coccygeal vertebrae.
• The joints between the bodies of adjacent vertebrae are slightly movable.
• Intervertebral discs are made of fibrocartilage and help absorb shocks.
• Humans have 12 pairs of ribs, giving a total of 24 ribs.
• The 7 pairs of ribs attached directly to the sternum are called true ribs.
• The number of true ribs is 14, or 7 pairs.
• The remaining ribs are called false ribs; the last two pairs are floating ribs.

Bones, Cartilage and Types of Tissue

Bone is a connective tissue with a hard matrix. Its hardness results from inorganic calcium salts, especially calcium phosphate. Collagen fibres in the matrix provide some flexibility and resistance to breaking.

Cartilage is also a connective tissue, but its matrix is softer and more flexible than bone. It supports structures such as the nose and ear and reduces friction at the ends of bones.

A simple epithelium consists of a single layer of cells. These cells make a definite layer and may be involved in protection, absorption, secretion or diffusion, depending on their form and location.

• Bone cells are called osteocytes and are present in spaces within the bone matrix.
• Calcium phosphate gives bone its hardness.
• Collagen fibres help prevent bones from being completely brittle.
• Cartilage has cartilage cells called chondrocytes.
• Cartilage has no blood vessels within its matrix and receives nutrients by diffusion.
• Simple epithelium consists of one cell layer and makes a definite layer.
• Bone is rigid, while cartilage is flexible and less mineralised.
• Cartilage at the ends of bones helps reduce friction during movement.

Muscles and Muscle Fibres

Muscles are contractile tissues. They shorten or develop tension and produce movement by pulling on bones or other body structures. A muscle is made of many muscle fibres, and each muscle fibre is a specialised cell.

The cell membrane of a muscle fibre is called the sarcolemma. Its cytoplasm is called sarcoplasm. The sarcoplasm contains myofibrils, which are long structures responsible for contraction.

There are three main types of muscles: skeletal, smooth and cardiac. Their structures, locations and methods of control are different.

• Sarcoplasm is the cytoplasm of a muscle fibre.
• Sarcolemma is the cell surface membrane of a muscle fibre.
• Skeletal muscles are usually attached to bones by tendons.
• Skeletal muscles are striated and usually under voluntary control.
• Smooth muscles are involuntary and non-striated.
• Smooth muscle occurs in the walls of organs such as the intestine and blood vessels.
• Cardiac muscle forms the wall of the heart and is involuntary and striated.
• Skeletal muscle fibres are long and usually multinucleate.
• A large amount of haemoglobin is not a characteristic component of human skeletal muscle fibres.

Sarcomere and Contractile Proteins

Each myofibril contains repeating units called sarcomeres. The sarcomere is the functional unit of contraction in a muscle fibre. It lies between two successive Z lines or Z discs.

Sarcomeres contain thin actin filaments and thick myosin filaments. During contraction, the actin filaments slide towards the centre of the sarcomere between the myosin filaments. The filaments themselves do not become shorter.

Tropomyosin is a regulatory protein found as two strands twisted around actin. Troponin is associated with tropomyosin and helps regulate the interaction between actin and myosin.

• The sarcomere is the unit of contraction of a muscle fibre.
• A sarcomere extends from one Z line to the next Z line.
• Actin forms the thin filaments.
• Myosin forms the thick filaments.
• Two strands of tropomyosin twist around the actin chain.
• The M line is a disc-like protein structure found centrally in a sarcomere.
• The Z lines move closer together during muscle contraction.
• The actin and myosin filaments slide past one another; they do not shorten.
• The H zone becomes smaller and may disappear during contraction.

Mechanism of Muscle Contraction

Muscle contraction begins when a nerve impulse stimulates a muscle fibre. This stimulation causes calcium ions to be released inside the fibre. Calcium binds with regulatory proteins and exposes binding sites on actin.

The heads of myosin bind to actin and form cross bridges. Using energy from ATP, the myosin heads pull the actin filaments towards the centre of the sarcomere. Repeated cycles of attachment, pulling and detachment produce contraction.

ATP is needed for several steps. In particular, ATP is used to break the cross bridge between myosin and actin. Without ATP, the cross bridges cannot detach normally.

• Muscle contraction depends on nervous stimulation, calcium ions and ATP.
• Calcium ions expose active binding sites on actin.
• Myosin heads form cross bridges with actin.
• The myosin head pulls actin towards the M line during the power stroke.
• ATP provides energy for the contraction cycle.
• ATP is used for breaking the cross bridge between actin and myosin.
• New ATP binding causes the myosin head to detach from actin.
• The sarcomere becomes shorter during contraction.
• The actin filaments slide inward, while the myosin filaments remain in place.
• When stimulation stops, calcium ions are returned to storage and the muscle relaxes.

Joints and Their Movements

A joint is a place where two or more bones meet. Joints hold bones together and allow different degrees of movement. They may be classified according to the amount of movement they permit or according to their structure.

Fibrous joints are held together by strong fibrous connective tissue. Sutures of the skull are fibrous and immovable. Cartilaginous joints contain cartilage between bones and usually allow limited movement.

Synovial joints are freely movable joints. They contain a joint cavity filled with synovial fluid. Ligaments strengthen the joint, while cartilage covers the ends of bones and reduces friction.

• Immovable joints do not allow movement, as in skull sutures.
• Slightly movable joints permit limited movement, as between the bodies of adjacent vertebrae.
• Freely movable joints are usually synovial joints.
• A synovial joint has a joint cavity and synovial fluid.
• Synovial fluid lubricates the joint and reduces friction.
• Ligaments join bone to bone and strengthen joints.
• Tendons attach muscles to bones.
• The glenoid cavity is a shallow socket in the scapula.
• The glenoid cavity articulates with the head of the humerus.
• The shoulder is a ball-and-socket joint and allows movement in several directions.
• The elbow and knee are examples of hinge joints.

Arthritis and Joint Disorders

Arthritis is inflammation of one or more joints. It may cause pain, swelling, stiffness and difficulty in movement. Different forms of arthritis have different causes and effects on the joint.

In osteoarthritis, the cartilage at the ends of bones may wear away. In rheumatoid arthritis, the immune system attacks tissues of the joints. Treatment depends on the type and severity of the disorder.

If a joint is severely damaged, surgical replacement may be required. An artificial joint, called a prosthesis, can restore some movement and reduce pain.

• Arthritis means inflammation of joints.
• Common symptoms include joint pain, swelling, stiffness and reduced movement.
• Damage to articular cartilage can increase friction between bones.
• Osteoarthritis is associated with breakdown of joint cartilage.
• Rheumatoid arthritis is an autoimmune disorder affecting joints.
• Severe arthritis may require joint replacement surgery.
• A badly damaged joint can be replaced with an artificial prosthesis made of materials such as metal and plastic.
• Regular medical assessment is needed to diagnose the type and treatment of arthritis.