Locomotion and Movement: common doubts, answered
The questions students ask most often about Locomotion and Movement, each with a short answer. For the full chapter, read the Locomotion and Movement notes.
Movement and locomotion
Read this section in the notes →What is the difference between movement and locomotion?
Movement is any change in position of a part of the body or the whole organism, while locomotion is movement that carries the organism from one place to another. So all locomotion is movement, but not all movement is locomotion. Blinking an eye or moving a hand is movement without locomotion, but walking is both.
Types of movement
Read this section in the notes →What are the types of movement shown by human cells?
Human cells show amoeboid, ciliary and muscular movement. Amoeboid movement occurs in macrophages and leucocytes using pseudopodia and cytoskeletal elements. Ciliary movement occurs in the trachea and in the oviduct, where it moves particles or ova. Muscular movement is used for limb movement and many other functions.
Types of muscle
Read this section in the notes →What is the difference between skeletal, smooth and cardiac muscle?
Skeletal muscle is striated and voluntary, attached to bones. Smooth, or visceral, muscle has no striations, is involuntary and lines the walls of hollow organs. Cardiac muscle is striated like skeletal muscle, but it is branched and involuntary, and it forms the wall of the heart.
Structure of skeletal muscle
Read this section in the notes →What is the function of sarcoplasmic reticulum in muscle?
The sarcoplasmic reticulum stores calcium ions in a muscle fibre. When a nerve impulse arrives, calcium is released into the sarcoplasm and begins contraction. During relaxation the calcium is pumped back into the sarcoplasmic cisternae. A muscle fibre is also a syncytium, because it has many nuclei in one cell.
Structure of contractile proteins
Read this section in the notes →What is a sarcomere and what are the I band and A band?
A sarcomere is the portion of a myofibril between two successive Z lines, and it is the functional unit of contraction. The I band is the lighter band, containing only thin actin filaments, and it is bisected by the Z line. The A band is the dark band, made of myosin, with thin filaments overlapping at the ends.
What is the structure of the thin and thick filaments?
A thin filament is made of two F actin strands, each a polymer of G actin, twisted together, with two tropomyosin strands along the groove and troponin at regular intervals. A thick filament is a polymer of many myosin molecules, each with a head and tail. The myosin head has ATPase activity and actin-binding sites.
What is the role of troponin and tropomyosin in muscle contraction?
In resting muscle a subunit of troponin masks the myosin-binding sites on the actin filaments. When calcium is released, it binds to that troponin subunit, the masking is removed and the binding sites are exposed. Myosin heads can then attach to actin and form cross bridges. Troponin sits at regular intervals along tropomyosin, which runs close to the F actin throughout its length.
Mechanism of muscle contraction
Read this section in the notes →What is the sliding filament theory?
According to the theory, muscle contraction occurs when thin actin filaments slide over thick myosin filaments toward the centre of the sarcomere, pulled by the myosin heads, which form cross bridges using ATP energy. The filaments themselves do not shorten. The Z lines are drawn closer, so the sarcomere shortens and the muscle contracts.
Which bands shorten during muscle contraction?
The I band and the H zone shorten during contraction, as the thin filaments slide inward and the Z lines move closer. The A band keeps its length, since the thick filaments do not change in size. This is a common exam point as many students wrongly say the A band shortens.
What is the role of calcium and ATP in muscle contraction and relaxation?
Calcium binds to troponin and exposes the actin binding sites, while ATP is used by the myosin head to form cross bridges and pull actin inward. During relaxation, calcium is pumped back into the sarcoplasmic reticulum, the actin sites are covered again, and the Z lines return to their original position.
Red and white fibres
Read this section in the notes →What is the difference between red and white muscle fibres?
Red fibres contain plenty of myoglobin, which gives the red colour, and many mitochondria, so they use oxygen for aerobic energy production and can work for long periods. White fibres have little myoglobin, few mitochondria and a lot of sarcoplasmic reticulum, and they depend on anaerobic processes for energy.
Axial skeleton
Read this section in the notes →How many bones are there in the human skeleton and how are they divided?
The adult human skeleton has 206 bones, divided into the axial skeleton with 80 bones and the appendicular skeleton with 126 bones. The appendicular part is made of 120 bones in the limbs and 6 in the girdles. The skull, vertebral column, ribs and sternum form the axial skeleton.
How many bones does the skull have?
The skull has 22 bones: 8 cranial bones that protect the brain and 14 facial bones. The U-shaped hyoid bone at the base of the buccal cavity is also counted with the skull, and each middle ear holds three ossicles, the malleus, incus and stapes. So the 22 are cranial plus facial bones, and the hyoid and six ossicles are added to reach the 80 bones of the axial skeleton.
How many vertebrae are there in each region of the vertebral column?
The vertebral column has 26 bones: 7 cervical, 12 thoracic, 5 lumbar, 1 sacral and 1 coccygeal, where the sacral and coccygeal bones are each formed by fusion of several vertebrae. The first cervical vertebra, the atlas, articulates with the occipital condyles of the skull. Almost all mammals, humans included, have seven cervical vertebrae, whatever the length of the neck.
What are true, false and floating ribs?
Humans have 12 pairs of ribs. The first seven pairs are true ribs, joined to the sternum ventrally through hyaline cartilage. The next three pairs, the false or vertebrochondral ribs, do not connect to the sternum directly but join the seventh rib. The last two pairs have no ventral connection and are called floating ribs.
Appendicular skeleton
Read this section in the notes →Which bones form the pectoral and pelvic girdles?
Each half of the pectoral girdle is made of a clavicle and a scapula, and the glenoid cavity of the scapula holds the head of the humerus. The pelvic girdle is made of two coxal bones, each formed by the fusion of the ilium, ischium and pubis, and the acetabulum at their junction receives the head of the femur. The girdles join the limbs to the axial skeleton.
How many bones are in each limb?
Each limb has 30 bones. In the forelimb these are the humerus, radius and ulna, 8 carpals, 5 metacarpals and 14 phalanges. In the hind limb they are the femur, tibia, fibula, 7 tarsals, 5 metatarsals, 14 phalanges and the patella, the knee cap. These counts are asked often.
Joints
Read this section in the notes →What are the types of synovial joints, with examples?
Ball and socket joints, as between the humerus and pectoral girdle, allow movement in all directions. Hinge joints, such as the knee, move in one plane. Pivot joints, such as that between the atlas and axis, allow rotation. Gliding joints are between the carpals, and saddle joints are at the thumb between the carpal and metacarpal.
Disorders of muscular and skeletal system
Read this section in the notes →What is the difference between arthritis, osteoporosis and gout?
Arthritis is inflammation of the joints. Osteoporosis is an age-related disorder with decreased bone mass and a higher chance of fractures, in which decreased levels of estrogen are a common cause. Gout is inflammation of joints caused by accumulation of uric acid crystals. They affect the skeletal system in different ways.
What is the difference between tetany and myasthenia gravis?
Tetany is rapid spasm, or wild contractions, of muscles caused by a low calcium level in body fluids. Myasthenia gravis is an autoimmune disorder that affects the neuromuscular junction, leading to fatigue, weakening and paralysis of skeletal muscle. Muscular dystrophy is a separate genetic progressive degeneration of skeletal muscle.
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