Understand the difference between movement and locomotion.
Comprehend the types of movements in the human body.
Analyze the structure and mechanism of muscle contraction.
Differentiate between skeletal, visceral, and cardiac muscles.
Study the organization of the skeletal system and its role in movement.
Movement is one of the vital characteristics of living beings. It is exhibited across plants and animals in various forms. Simple forms, like streaming of protoplasm in unicellular organisms such as Amoeba, are fundamental examples of movement. Other more complex movements include those of cilia, flagella, and tentacles, as seen in various organisms. Humans can move limbs, jaws, eyelids, and even the tongue. Some movements result in the change of place or location, which is termed locomotion. Walking, running, flying, swimming, and climbing are all forms of locomotion.
Locomotory structures often have dual functions. For instance, cilia in Paramecium aid both in food movement through the cytopharynx and in locomotion. Similarly, tentacles in Hydra are used for prey capture and for moving around. Human limbs are utilized both for changing body postures and for locomotion, illustrating that movement and locomotion are interconnected concepts. Thus, while all locomotions are movements, not all movements qualify as locomotion.
Types of Movement
Human body cells exhibit three primary types of movement: amoeboid, ciliary, and muscular.
Amoeboid Movement: Some specialized cells, like macrophages and leucocytes in blood, display this type of movement. It occurs through pseudopodia formed by the streaming of protoplasm. Cytoskeletal elements such as microfilaments assist in amoeboid movement.
Ciliary Movement: Found primarily in internal tubular organs lined by ciliated epithelium, ciliary movements help remove dust particles from the trachea and facilitate the movement of the ova through the female reproductive tract.
Muscular Movement: Movements like those of limbs, jaws, and the tongue require muscular contraction. This type of movement involves a coordinated activity between muscular, skeletal, and neural systems.
Muscle: Structure and Types
Muscles are specialized tissues of mesodermal origin. They contribute 40-50% of the body weight of an adult human. Muscles exhibit properties such as excitability, contractility, extensibility, and elasticity.
Muscles are categorized based on location, appearance, and nature of regulation:
Skeletal Muscles: These are closely associated with skeletal components. They appear striated under a microscope and are under voluntary control, involved primarily in locomotory actions and changes in body posture.
Visceral Muscles: Found in the inner walls of hollow visceral organs, they appear smooth (hence, nonstriated). They are under involuntary control and are involved in functions like the transportation of food in the digestive tract.
Cardiac Muscles: These are the muscles of the heart. Though striated in appearance, they function involuntarily.
Structure of Skeletal Muscle
A skeletal muscle is made up of several muscle bundles or fascicles, held together by fascia (a collagenous connective tissue layer). Each muscle fiber is a syncytium, lined by the sarcolemma and filled with sarcoplasm, which contains multiple nuclei. Sarcoplasmic reticulum in muscle fibers serves as a reservoir of calcium ions. Myofibrils, which are parallel filaments, are present within each fiber and exhibit alternating dark and light bands due to the distribution of actin and myosin proteins.
I-bands (Isotropic bands) contain actin and are light.
A-bands (Anisotropic bands) contain myosin and are dark.
The region between two successive Z-lines is the sarcomere, the functional unit of contraction. In a relaxed state, the H-zone in the center of the A-band is free from overlapping actin filaments.
Important Note:
Sarcomere is the fundamental contractile unit in muscle fibers.
Mechanism of Muscle Contraction
Muscle contraction is explained by the sliding filament theory: the thin filaments (actin) slide over the thick filaments (myosin), resulting in contraction.
Signal Initiation: A signal from the central nervous system (CNS) is transmitted via a motor neuron. The junction between the motor neuron and the muscle fiber is called the neuromuscular junction.
Neurotransmitter Release: The arrival of a neural signal triggers the release of acetylcholine, which generates an action potential in the muscle fiber.
Calcium Ion Release: The action potential spreads, releasing calcium ions (Ca²⁺) into the sarcoplasm.
Troponin Activation: Calcium binds to troponin, unmasking the active sites on actin for myosin binding.
Cross-Bridge Formation: Myosin heads bind to the exposed active sites on actin, forming cross-bridges.
Power Stroke: The Z-lines attached to the actin filaments are pulled inward, shortening the sarcomere and causing contraction. The I-bands shorten, while the A-bands remain unchanged.
Important Note:
ATP is critical for cross-bridge cycling and muscle contraction.
Red and White Muscle Fibers
Muscles are categorized into red fibers and white fibers based on the presence of myoglobin:
Red Fibers: High myoglobin content gives them a reddish appearance. They possess many mitochondria and rely on aerobic respiration for energy.
White Fibers: Low myoglobin content makes them pale. These fibers rely more on anaerobic respiration for energy and have fewer mitochondria.
Skeletal System
The skeletal system is composed of bones and cartilage, which provide structural support, protection, and facilitate movement. Bones are rigid due to calcium salts, while cartilage is more pliable due to chondroitin.
The human skeletal system contains 206 bones and is divided into two main parts: axial and appendicular skeleton.
Axial Skeleton: Consists of 80 bones, including the skull, vertebral column, ribs, and sternum.
The skull is made up of 22 bones, 8 of which are cranial and 14 facial bones.
The vertebral column is composed of 26 vertebrae, arranged in regions: cervical (7), thoracic (12), lumbar (5), sacral (1 fused), and coccygeal (1 fused).
Important Note:
The first vertebra is called the atlas, which articulates with the occipital condyles.
Appendicular Skeleton: Composed of limb bones and girdles. Each limb contains 30 bones.
Pectoral Girdle: Comprised of the clavicle and scapula, which connect the upper limb to the axial skeleton.
Pelvic Girdle: Consists of two coxal bones, formed by the fusion of the ilium, ischium, and pubis.
Joints
Joints facilitate the movement of bony parts. Synovial joints, in particular, allow considerable movement and play a major role in locomotion.
Fibrous Joints: Fixed, non-movable joints, such as those found in the skull.
Cartilaginous Joints: Permit limited movement, as seen between vertebrae.
Synovial Joints: Characterized by the presence of a synovial cavity. Examples include:
Ball and Socket Joint (e.g., between humerus and pectoral girdle)
Hinge Joint (e.g., knee joint)
Pivot Joint (e.g., between atlas and axis)
Saddle Joint (e.g., carpal and metacarpal of the thumb)
Disorders of Muscular and Skeletal System
Myasthenia Gravis: An autoimmune disorder affecting the neuromuscular junction, leading to muscle weakness and paralysis.
Muscular Dystrophy: A genetic disorder causing progressive muscle degeneration.
Tetany: Caused by low calcium levels, leading to muscle spasms.
Arthritis: Involves joint inflammation.
Osteoporosis: Characterized by a reduction in bone mass, leading to fragility. It is often caused by decreased levels of estrogen.
Gout: Results from the accumulation of uric acid crystals in joints, causing inflammation.
MCQ:
Which muscle type is voluntary, striated, and primarily involved in locomotion?