The Skeletal and Muscular System
Anatomy & Physiology –Skeletal, Articular and Muscular Systems
The Skeletal, Articular and Muscular Systems
a. Analyse the effects of hormones, diet and activity on bone growth.
Bone comprises various types of tissue including bone tissue, cartilage, dense connective tissue, epithelium, adipose tissue and nervous tissue. Bone is continuously growing, remodelling and repairing itself. The process of formation of bone is called as ossification which occurs under the following four conditions:
a. Initial formation of bones in an embryo or foetus. In an embryo or foetus there are two types of ossification, intramembranous ossification and endochondral ossification. Intramembranous ossification is the formation of the flat bones of the skull and mandible or lower jawbone. Endochondral ossification involves replacement of cartilage by bone and most bones in the body are formed in this way.
b. Growth of bones during infancy, childhood and adolescence until adult bone sizes are reached. Long bones grow in length and bones throughout the body grow in thickness during infancy, childhood and adolescence.
c. The remodelling of bones is the replacement of old bone tissue by new bone tissue throughout life of the individual. It involves bone resorption, which is the removal of minerals and collagen fibres from the bone, and bone deposition, which is the addition of minerals and collagen fibres to the bone.
d. Repairs of fractures or breaks in bones throughout life. The repair of fractures or breaks sometimes can take months. This is due to the bone slow and gradual deposition of calcium and phosphate needed to strengthen and harden the new bone, and bones cells generally grow and reproduce slowly.
There are a number of factors that affect the development, growth and repair of bones. The following three factors will be discussed below:
Hormones secreted by various glands in the body such as the pituitary gland, thyroid gland and parathyroid glands affect the growth and development of bones. For example, the pituitary gland secrets growth hormone, which stimulates division of cartilage cells during ossification. Some of the bone conditions caused due to hormonal imbalance in the body are as follows:
a. Dwarfism. A condition caused by low levels of growth hormone during childhood causing the body to under-develop. This leads to an individual who is short height with limbs that are in proportion to the rest of the body.
b. Gigantism. A condition caused by abnormally high levels of growth hormone during childhood before the bone growth plate has closed. This causes the child to grow abnormally tall.
c. Acromegaly. A condition caused by an excess in production of growth hormone after the bone growth plate has closed. It causes abnormal enlargement of bones and tissues, most commonly in the limbs, it occurs gradually over numerous years.
d. Cretinism is the stunted body growth and mental development appearing in the first years of life resulting the inappropriate development of the thymus gland or inadequate maternal intake of iodine during pregnancy, which affects the production of Thyroxine.
Diet or nutrition are very important for the formation and maintenance of healthy bones. Foods rich in minerals such as calcium and phosphorous, which are the main components of the bone, are essential for bones to be strong. For instance, such as rickets in children. The following are bone conditions that are caused by poor diet:
a. Rickets. It is a condition that affects children in which the bones are softer and weaker due to lack of calcium and vitamin D, which is necessary for the absorption of calcium in the small intestine and in its absence calcium is poorly absorbed in the body. This causes the bone to lack in calcium, can causes the bone to soften and thereby cause rickets.
b. Osteoporosis. It is a condition affecting mostly older people in which the bones become less dense that makes them weak and more prone to breaks and fractures. Healthy diet rich calcium and vitamin D is beneficial to prevent osteoporosis.
c. Osteoarthritis. A condition which causes pain and stiffness in joints. Food supplements such as glucosamine, chondroitin, avocado and soyabean is beneficial in some cases.
Activity is essential for maintaining healthy bones. A lack of physical activity can cause the bones to lose density and become weak and prone to fractures. Exercise helps stimulate bone formation cells and strengthen the bones. In addition it also gives strength to the muscles supporting the bones. Regular weight-bearing exercises are particularly good for increasing bone density.
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b. Explain how bones are joined together to form a framework and list any common distortions.
Bones are connected together by flexible connective tissues called ligaments. Ligaments are tough, elastic bands of connective tissue connecting bones or cartilage at a joint or they hold organs in position. They are made of dense fibrous tissue formed by parallel bundles of collagen fibres, which give strength and stability to the joint. Stability is provided by controlling the range of motion of a joint, for example the ligaments prevent the elbow from bending backwards. Cartilage is a tough, elastic connective tissue which is found in the linings of the joints and acts as a cushion against shock. It made up of a dense network of collagen and elastic fibres. It is also found is in other parts of the body such as the nose and the external ear. The type of cartilage found in joint surfaces is called as hyaline cartilage. Hyaline cartilage is the most abundant type of cartilage which is made up of a bluish-white, shiny ground substance with fine collagen fibres and chondrocytes, which are connective tissue cell within the cartilage matrix. Hyaline cartilage is located at the ends of long bones, the anterior ends of ribs, the nose, and parts of the larynx, trachea, bronchi, bronchial tubes and embryonic and foetal skeleton.
Synovial fluid is secreted by synovial membrane, which is present in the inner layer of the joint capsule. Its functions include lubrication of the joint, shock absorption and nourishment of the joint. They also remove microbes and debris from the joint cavity.
The following are a list of and brief description of common bone distortions:
a. Kyphosis. An abnormal outward curving of the upper spine (aka hunchback). It causes a rounded shoulder appearance and may cause pain and stiffness. It is often caused by bad posture and sometimes by structural problems caused by fractures to the vertebrae or simply by abnormal growth of the spine.
b. Lordosis. An abnormal forward curvature of the lumbar region of the vertebral column (aka hollow back or saddle back). It can be caused during pregnancy or extreme obesity, the main cause is poor posture. It can also be associated with rickets.
c. Scoliosis. The side to side curvature of the vertebral column. The spine bends either to the left or to the right. It is more common in women than men and mostly occurs at the start of adolescence. Normally scoliosis is not a serious condition but if left untreated the curve of the spine can worsen and can damage the spine, chest, pelvis, heart and lungs.
d. Acromegaly. It is a condition caused by the excess production of growth hormone after the bone growth plate has closed. It is usually caused by a tumour affecting the pituitary gland. It causes abnormal enlargement of bones and tissues, most commonly in the limbs and it occurs gradually over many years.
e. Arthritis. An inflammatory condition of the joints that causes pain, swelling, heat, redness and the limitation of movement. Two main types of arthritis are osteoarthritis and rheumatoid arthritis.
f. Gigantism. A condition caused by abnormally high levels of growth hormone during childhood before the bone growth plate has closed. This is usually caused by a tumour affecting the pituitary gland which causes a child to grow abnormally tall.
g. Dwarfism. A condition caused by low levels of growth hormone during childhood and it causes the body to under develop. It can be due to a genetic abnormality or a tumour in the pituitary gland. This leads to a very short height with limbs that are in proportion to the rest of the body.
h. Rickets. It is a condition affecting children caused due to poor development of the bones in the skeleton. It is caused due to severe vitamin D deficiency, which is essential for the development of strong and healthy bones. It causes stunted growth and the child will most likely be short in stature as an adult.
i. Bunions. It is a condition where the metatarsophalangeal joint (the joint just behind the big toe in either foot) is enlarged abnormally. It is caused by abnormal motion of the foot during walking or running, usually resulting from chronic irritation and pressure from poor fitting footwear.
c. Describe the functions of the skeleton.
The following are the six functions of the skeleton:
1. Protection. It protects important internal organs of the body such as heart, lungs, spinal cord and the brain. Examples are the skull which protects the brain, the vertebrae that protect the spinal cord and the ribcage which protects the heart and lungs.
2. Movement. Movement, of either the whole body or a single limb, is possible due to the contraction and relaxation of skeletal muscles that are attached to the bones of the skeleton. When these muscles contract, they pull the bones to produce movement. Muscles, bones and joints provide the mechanics for the movement of the body.
3. Support. The skeleton provides a structural framework to the body. It supports the body and maintains its shape by supporting soft tissues and providing attachment point for the tendons of most skeletal muscles.
4. Manufacture of Blood Cells. Some human bones contain connective tissue called red bone marrow that produces red blood cells, white blood cells and platelets by a process called hemopoiesis. Red bone marrow is present in developing bones of the foetus and in some adult bones such as hip bones, vertebrae, skull, ends of the bones of the arm and thigh, ribs and breastbone.
5. Storage of Phosphorous and Calcium: The bone tissue stores several minerals, mainly calcium and phosphorous that contributes to the strength of the bone. If needed, it releases minerals into the blood to maintain critical mineral balances and distribute minerals to other parts of the body.
6. Endocrine regulation. Osteoblasts in the bone tissue release a hormone called osteocalcin that helps in the regulation of blood glucose and fat deposition. Osteocalcin increases insulin secretion and its sensitivity and reduces fat storage.
d. Compare and contrast the different types of joints
Joints can be classified according to their structure based on their anatomical characteristics and functionally, based on the degree and the type of movement they allow. Structurally, joints either have or do not have a space called the synovial cavity between the articulating bones and the type of connective tissue joining the bones together also differ from one type of joint and another. The following are the types of joints classified structurally:
Fibrous joints do not have a synovial cavity and the bones are joined together by dense irregular connective tissue rich in collagen fibres. These joints are also known as immovable joints. They allow little or no movement and the three types of fibrous joints are sutures, syndesmoses and interosseous membranes.
A suture consists of thin layer of dense irregular connective tissue that are only found between bones of the skull. Syndesmoses have a greater distance between the articulating surfaces and are composed of denser irregular connective tissue than in sutures. The joint between a tooth and socket of a alveolar process is an example of a syndesmosis. Interosseous membranes consist of a large sheet of dense irregular connective tissue. They join together long bones and allow little movement, an example is the joint between the tibia and the fibula.
Cartilaginous joints do not have synovial cavity and the bones are joined together by cartilage instead of connective tissue as described in fibrous joints. These joints are also known as partial moveable joints, they allow little or no movement. The articulating bones in these joints are joined tightly together by either hyaline cartilage or fibrocartilage and their two types are synchondroses and symphyses.
Synchondroses have hyaline cartilage that joins the bones together, the joint between the hip bone and sacrum is an example of synchondrosis. Whilst in a symphyses the articulating bones are covered with hyaline cartilage but it is the broad flat disc of fibrocartilage that joins the bones together. This type of joint are found at the intervertebral joints between the vertebrae.
Unlike fibrous joints and cartilaginous joints, synovial joints have synovial cavity. The bones are joined together by dense irregular connective tissue, like in fibrous joints, of an articular capsule and often by accessory ligaments. These joints are also known as freely moveable joints. Synovial joints are divided into the following sub-groups according to the type of movement they allow:
a. Ball and Socket Joint. They consist of a ball like surface of bone that fits into the hollow cup shaped socket of another bone. Examples are the shoulder and hip joints. These joints allow movement in all directions or in three planes.
b. Hinge Joint. They consist of one convex surfaced bone that fits into another concave surfaced bone. Examples are the knee and elbow joints. They produce an angular, opening and closing motions allowing movement in one plane, similar to a hinged door.
c. Gliding Joint. The bones slide over one another in gliding joints but the amount of movement is very limited. Examples are the joints between the carpal bones, tarsal bones and between the articular processes of successive vertebrae.
d. Pivot Joints. They consist of a rounded or pointed surface of one bone rotating round the axis of another bone. An example is the odontoid process of the axis which allows the head to turn from side to side.
e. Saddle Joints. They consist of a saddle shaped bone, with another articular shaped bone fitting in like a horse rider would sit on a saddle. Examples are the carpometacarpal joint between the trapezium of the carpus and metacarpal of the thumb. They allow movement in three planes.
Muscles are structural tissues with special properties that include contractibility, extensibility, elasticity, and electrical excitability. Muscle fibres or cells consist of sarcoplasm (cytoplasm) and sacrolemma (cell membrane). The three types of muscular tissues:
Striated muscles also known as skeletal muscle are attached to the skeleton or bones and are involved in movement and locomotion. They are attached to the bones via tendons. Ligaments join one bone with another bone and tendons attach muscles to bones. The contractions of the muscle are generally of a voluntary nature (reflex arcs being the exception) and are strong, quick and powerful. They do though fatigue quickly. Striated muscles consist of large number of distinct threads called myofibrils. These threads are connected by connective tissue and they have parallel banding. They are not branched and not connected to each other. The myofibrils are made up of alternating dark and light band. The dark band, known as the anisotropic band, contains the proteins actin and myosin, whilst the light band known as isotropic band only contains actin. There is pale region in the dark band known as the H zone where actin is missing. Membranes passing through the light band are known as Z lines. The distance between one Z line to the adjacent Z line is known as sarcomere. The sacromere shortens when the muscle contracts after being stimulated by a voluntary somatic nerve.
Smooth muscle comprises a large number of small individual cells. They are spindle shaped, narrow at the ends and wide in the middle. Each cell has a nucleus in the middle. There are no striations in the sarcoplasm and there are no dark or light bands present. They are typically found in the walls of the intestine. They consist of longitudinal and radial layers that go in different directions. Smooth muscles are also found in the ciliary muscle and the iris. The ciliary muscle accommodates the lens of the eye and the iris changes the size of the pupil. They are also present in the walls of arteries and veins. Smooth muscles have slow and sustained contractions. These contractions are weak and involuntary in nature.
Cardiac muscle is found exclusively in the heart. It is striated in nature with both dark and light band. The myofibrils in the cardiac muscle branch and have thick dark structures identified as intercalated discs that run across the light band. The contractions in cardiac muscle are quick, fairly strong, and involuntary in nature and they do not fatigue easily. The contraction is called myogenic, that is it can contract automatically without external nerve stimulation unlike skeletal muscle. It has a inbuilt pacemaker cells in the right atrium which stimulate the heart muscle to contract.
Analyse the role of antagonist, agonist and synergist muscles and their contractions.
Antagonist muscles work in opposition to each other and are paired, when one muscle contracts the other relaxes and vice versa. Each muscle (or group of muscles) produce a movement of a certain part of the body, there is another muscle (or group of muscles) that generate an opposite movement. An example of antagonist muscles is the bicep and triceps muscles. The bicep is antagonistic to the triceps. They work in opposite directions. The bicep muscle is situated at the front of the arm in front of the humerus and triceps is situated at the back of the arm behind the humerus. When the arm is moved towards the body, the biceps contracts while the triceps relaxes, bending at the elbow. At this point the biceps muscle becomes thicker and shorter while the triceps muscle becomes thinner and longer. When the arm is straightened, the triceps muscle contracts, becoming thicker and shorter while the biceps muscle relaxes, becoming thinner and longer.
Agonist muscles are a group of skeletal muscles that causes specific movement or several movements to occur through contraction. They are the muscles primarily responsible for movement, they are known as prime movers.
The Muscles of the Upper Leg
Synergist muscles are skeletal muscles that cause or assist in causing the same set of joint movement as the agonist muscles. These muscles help to neutralize extra motions produced by the agonists to make sure that the force generated works within the desired plane of movement. Since they cancel out the extra motion produced by agonists, they are also known as neutralizers. Synergist muscles act on moveable joints.
Agonist muscles triggers the antagonist muscles. Therefore, an agonist and an antagonist function as a pair and produce opposite effects. On the other hand, synergist muscles are a group of muscles working together to produce the same effects. Another difference is that an agonist muscle is the primary move, the one doing the work whereas a synergist muscle is a helper muscle where they assist the primary mover in doing the work.
Explain the ‘sliding filament hypothesis’ of muscle contraction referring to the ultra structure of striated muscle and the utilisation of energy in muscle contraction.
The skeletal muscles have thick and thin filaments. Thick filaments are made up of myosin and the thin filaments are made up of actin. The dark band (also called anisotropic band or A band) has a mixture of thick and thin filament. There is a pale region in the middle of the dark band called as the H zone. This zone is due to thin filament not touching and it has gaps. Running down the centre of the H zone is the M line, which is a cell membrane. The light band only contains thin filaments and is also called the isotropic band or I band. Passing across the light band are Z lines which are membrane like structure. The distance between one Z line to the adjacent Z line is called sarcomere.
Muscles must be stimulated by voluntary nerves. When they are stimulated a wave of electrical impulses spreads over the muscle surface membrane. The Ca ions (Ca2+) rush into the muscle cells from external tissue fluid. Ca2+ ions are stored in two different microtubule systems, T-system and sarcoplasmic reticulumn. The Ca2+ ions join up with myosin, which is then converted into an enzyme, which splits the ATP releasing energy. The released energy contracts the muscle. This process is called a contraction excitation coupling.
In the 1930s Huxley and Hanson studied the mechanism of muscle contraction. They came up with the sliding filament theory. The sliding filament theory suggests that the thin filaments slide towards each other. The H zone narrows or disappears completely. The sarcomere becomes shorter as the Z lines move towards each other. Subsequently, it has been discovered that another protein, actomyosin is found between the thick filaments. These protein connections are called cross bridges. They act as a ‘ratchet mechanism’ and assist the thin filament to slide.
When the muscle relaxes, Ca2+ ions become detached from the myosin and join up with chemicals called relaxing factors. The Ca2+ ions then leave the muscle and diffuse out into the tissue fluids. Due to muscles’ elastic properties the thin filaments separate from each other, the H zone appears again, the Z lines move away from each other and the sarcomere returns to its original length.