Title: The Motor System
Key words: musculo-skeletal, bones, joints, synovial, cartilage, muscles, striated, smooth and cardiac, collagen,
Date: July 2000
Category: 6. The Body
Author: DJE Candlish
The Motor System
The motor or musculoskeletal system has two main functions: providing a rigid structure for the support and protection of soft organs and providing a flexible framework to enable the limbs and body to move effectively.
The motor system consists of bones, joints, cartilage and muscles. The nervous system is also involved in the control of muscles, awareness of body position and co-ordination of activities.
There are 206 bones in the human skeleton. These provide the framework for the body shape and enable the muscles to create movement. There are five types of bone, classified by shape. These are:
Bones have a dense, hard outer layer mainly composed of calcium and phosphates and a spongy, honeycomb-like inner core. Some spaces within the inner core contain bone marrow. This is where blood cells are produced. Bone also contains collagen, a substance that gives it a limited flexibility, to minimise the risk of breaking under stress. Around the bone is a protective coating or fibrous periosteum, to which muscles and ligaments are attached. Bone is well supplied with blood vessels, which pass through from the periosteum.
Although bone appears to be a fixed, stable tissue, it is actually in a constant state of breakdown and renewal. Cells called osteoblasts produce an enzyme that causes calcium phosphate to precipitate out from the blood to form new bone. This takes place below the periosteum and at the ends of bones, the growing sites. Other cells called osteoclasts, dissolve bone, forming tubes down which the osteoblasts deposit the new bone. This process prevents the bones from becoming brittle with age.
Bone growth during childhood and adolescence is controlled by growth hormone from the pituitary gland and by the sex hormones. At puberty, the sex hormones stop bone growth by fusing the growing sites at the end of the long bones of the limbs. The parathroid glands are also involved in bone growth and repair throughout life,as they regulate the calcium levels in the blood.
Where the bones involved in movement meet, they form a joint. The articulating surfaces (at the point of contact) are covered in a layer of cartilage. This provides a smooth, low friction surface which also cushions the bones against shock and impact during movement. Cartilage is a specialised tissue, composed mainly of a matrix of chondrin and collagen fibres. It contains relatively few cartilage cells.
There are three types of connection between bones, fibrous or connective tissue, cartilaginous and synovial. Connective tissue holds the teeth securely in the jawbone and joins the bones of the skull. The connective tissue consists of firm networks of elastic fibres, mainly collagen. This gives a springy, tough connection. The bones of the skull grow so closely together that only limited amounts of connective tissue are present at these joints, which are called sutures.
Cartilaginous junctions simply involve a layer of cartilage connecting the bones. The ribs are connected to the sternum or breastbone by this method, which gives a limited amount of flexibility to the joint. The cartilage discs between the vertebrae represent another type of cartilaginous joint, which allow limited movement.
Synovial junctions are what we call joints in everyday English. The knee is one example. These are very flexible connections in which the two adjoining surfaces have been adapted in shape to match each other almost perfectly. The adjoining surfaces are coated in cartilage as described earlier and encapsulated by two membranes.
The inner or synovial membrane secretes a fluid that reduces friction still further and also supplies nutrients to the cartilage. The outer membrane is tough and fibrous. It can contain bundles of collagen fibres or ligaments, to give extra support to the joint. Disease of these joints is a common and debilitating problem, especially in old age.
There are three types of muscle in the human body:
Voluntary muscles consist of striated muscle tissue. They are controlled and co-ordinated by the central nervous system by conscious effort. Voluntary muscles are involved in movements like walking, picking up objects and other interactions with the external environment.
Involuntary muscles consist of smooth muscle tissue, under the control of the autonomic system. These muscles are involved in internal movements and are found in the walls of the blood vessels, intestines, bronchi and bronchioles. Their activity is not under conscious control.
The cardiac muscle of the heart is striated, like voluntary muscle but is not under conscious control. It has an inherent rhythm of contraction and relaxation which can be altered by adrenaline in the blood stream or by nervous system control from a specific part of the brain - the cardiac centre.
Movement occurs when the voluntary muscles contract and exert force in a specific direction. The force is transmitted via the joints and bones. Muscles are attached extremely firmly to the tough membrane around the bone and even into the bone itself by tendons. These are fibrous extensions of the muscles.
A voluntary muscle consists of many elongated cells which form fibres that can contract or relax in response to stimulation from motor nerves. The muscle fibres are grouped into bundles and enclosed in a fibrous sheath of connective tissue known as the fascia. Muscle contraction is an active process, which uses energy, while relaxation is a passive process. Most muscles are in opposing pairs, so that when one contracts, its opposite number must relax. This is co-ordinated by the nervous system.