Table of contents |
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Locomotion and Movement |
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Types of Movement |
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Muscles |
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Mechanism of Muscle Contraction |
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Skeletal System |
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Movement is a core feature exhibited by living organisms, encompassing a wide array of forms from basic protoplasmic flow in Amoeba to the coordinated motion of cilia, flagella, and tentacles in various species. Human beings, endowed with voluntary muscles, can manipulate their limbs, jaws, eyelids, tongue, and other organs. Among these movements, some involve changing position or location and are termed locomotion. Locomotion encompasses activities like walking, running, climbing, flying, and swimming.
The structures facilitating locomotion often serve dual purposes, contributing to other forms of movement as well. For example, in Paramecium, cilia aid in both food propulsion and locomotion, while Hydra utilizes its tentacles for prey capture and movement. In humans, limbs are instrumental in altering body posture as well as facilitating locomotion. These observations underscore the interconnectedness of movements and locomotion, leading to the assertion that "While all locomotions are movements, not all movements are locomotions."
Animals adapt various methods of locomotion according to their habitats and specific requirements. This diversity in locomotion serves purposes such as foraging for food, seeking shelter, finding suitable breeding grounds, responding to climatic conditions, or evading predators.
The cells within the human body demonstrate three primary types of movement: amoeboid, ciliary, and muscular.
Muscle tissue, derived from the mesoderm, is a specialized component of the human body, constituting approximately 40-50% of the adult body weight. Examining muscle properties reveals their excitability, contractility, extensibility, and elasticity.
Muscles are categorized according to various criteria such as location, appearance, and regulatory nature of their activities. Primarily, muscles are classified into three types based on their location: skeletal muscles, visceral muscles, and cardiac muscles.
Skeletal muscles are structured with muscle bundles, or fascicles, bound together by fascia. Each bundle comprises muscle fibers encased by the sarcolemma and sarcoplasm. Within the sarcoplasm, the sarcoplasmic reticulum serves as a reservoir for calcium ions, while myofibrils, housing myofilaments, impart the striated appearance to the muscle fiber. Actin predominates in the I-band, whereas myosin is chiefly concentrated in the A-band. These filaments run parallel to each other along the longitudinal axis of the myofibril. The Z-line demarcates the I-band, and the M-line anchors the thick filaments within the A-band. Sarcomeres, positioned between Z-lines, act as the functional units governing muscle contraction. During rest, thin filaments partially overlap with thick filaments, leaving the H-zone unobstructed.
The structure of an actin (thin) filament comprises two helically wound "F" (filamentous) actins, each of which is a polymer formed by monomeric "G" (globular) actins. Alongside these "F" actins, two tropomyosin protein filaments run parallel to them. Tropomyosin, evenly distributed along the actin filaments' length, serves as a complex protein. During the resting state, troponin subunits cover or mask the active binding sites on the actin filaments, preventing myosin from binding to them.
The skeletal system comprises bones and cartilage, serving as the body's framework and supporting movement. Bone, a specialized connective tissue, boasts a tough matrix rich in calcium salts, while cartilage possesses a somewhat flexible matrix thanks to chondroitin salts. Human skeletal makeup includes 206 bones and several cartilages, organized into two primary divisions: the axial skeleton and the appendicular skeleton.
The axial skeleton comprises 80 bones running along the body's central axis, including the skull, vertebral column, sternum, and ribs. The skull consists of cranial and facial bones totaling 22, with 8 cranial bones protecting the brain and 14 facial bones shaping the front. The hyoid bone, positioned in the buccal cavity's base, is part of the skull, alongside three ear ossicles in each middle ear: malleus, incus, and stapes.
The vertebral column, situated along the body's dorsal side, encompasses 26 vertebrae extending from the skull base, forming the trunk's primary framework. Each vertebra contains a neural canal housing the spinal cord. Starting from the skull, the column divides into cervical (7), thoracic (12), lumbar (5), sacral (1-fused), and coccygeal (1-fused) regions. Its key functions include safeguarding the spinal cord, supporting the head, and anchoring ribs and back muscles.
The sternum, or breastbone, is a flat bone in the thorax's ventral midline, connecting ribs and shielding internal organs. The 12 pairs of ribs attach dorsally to the vertebral column and ventrally to the sternum. True ribs (1-7) directly connect to the sternum via hyaline cartilage, while false ribs (8-10) attach indirectly. The last two pairs, floating ribs (11-12), lack ventral connections. Together with thoracic vertebrae, ribs, and sternum, they form the rib cage protecting thoracic organs.
The appendicular skeleton encompasses limb bones and girdles. Each limb includes 30 bones. The forelimb contains humerus, radius, ulna, eight carpals, five metacarpals, and fourteen phalanges, while the hind limb features the femur, tibia, fibula, seven tarsals, five metatarsals, and fourteen phalanges, along with the patella guarding the knee joint.
The pectoral and pelvic girdles connect upper and lower limbs to the axial skeleton. Each half of the pectoral girdle comprises a clavicle and scapula, the latter articulating with the humerus via the glenoid cavity. The pelvic girdle, formed by the fusion of ilium, ischium, and pubis, supports the femur via the acetabulum and connects at the pubic symphysis.
Both girdles, along with associated bones, provide stability, support, and mobility, enabling limb articulation with the axial skeleton and various movements.
Bones in the Skeletal System
The skeletal system comprises various bones essential for supporting, protecting, and structuring the body. Here are the key bones in the human skeletal system:
Joints
Several disorders affect the muscular and skeletal systems:
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