Types Of Movement

Cells in the human body exhibit three main types of movements:

• **Amoeboid movement:** Similar to the movement of *Amoeba*, effected by the streaming of protoplasm and formation of **pseudopodia** (false feet). Exhibited by specialized cells like macrophages and leucocytes (WBCs) in blood, allowing them to move within tissues and engulf foreign particles. Cytoskeletal elements like microfilaments are involved.
• **Ciliary movement:** Occurs in organs lined by ciliated epithelium, due to the coordinated beating of cilia. Helps in the movement of substances (e.g., removing dust/foreign particles from the trachea, movement of ova through the female reproductive tract).
• **Muscular movement:** Requires the contraction and relaxation of muscle cells. Used for movement of limbs, jaws, tongue, etc., and for locomotion in humans and most multicellular organisms.

Flagellar movement, similar to ciliary movement but typically involves longer structures, is also found in some cells (e.g., tail of spermatozoa for swimming) and organisms (*Euglena* for locomotion).

Muscle

**Muscle** is a specialized tissue of **mesodermal origin** that contributes significantly to body weight (40-50% in adult humans). Muscles have unique properties: **excitability** (respond to stimuli), **contractility** (shorten forcibly), **extensibility** (can be stretched), and **elasticity** (return to original shape after stretching/contracting).

Muscles are classified based on location, appearance, and regulation:

• **Skeletal muscles:** Closely associated with skeletal components. Appear striped (**striated**) under a microscope. Under **voluntary control** of the nervous system. Involved in locomotion and changing body postures.
• **Visceral muscles:** Located in inner walls of hollow visceral organs (alimentary canal, reproductive tract). Lack striations (**smooth**). Under **involuntary control**. Assist in substance transport (e.g., food through digestion, gametes through reproductive tract).
• **Cardiac muscles:** Muscles of the heart. Appear **striated** but are **involuntary** in nature. Cardiac muscle cells are branched.

Let's examine skeletal muscle structure and contraction mechanism in detail:

Each skeletal muscle is made of **muscle bundles (fascicles)** held by fascia (connective tissue). Each fascicle contains muscle fibers (muscle cells). Each muscle fiber is a syncytium (multinucleated cytoplasm - **sarcoplasm**) lined by **sarcolemma** (plasma membrane). Sarcoplasmic reticulum (endoplasmic reticulum) stores calcium ions. Sarcoplasm contains many parallelly arranged filaments called **myofilaments** or **myofibrils**.

Each myofibril has alternate dark and light bands due to the arrangement of two main proteins: **Actin** (thin filaments) and **Myosin** (thick filaments).

• **I-band (Isotropic band):** Light band, contains **actin** filaments. Bisected by an elastic fiber called the **Z line**, to which thin filaments are attached.
• **A-band (Anisotropic band):** Dark band, contains **myosin** filaments (thick filaments). In the center of the A-band, a thin fibrous membrane (M line) holds thick filaments together. The central part of the A-band not overlapped by thin filaments is the **H zone**.

The portion of the myofibril between two successive Z lines is the functional unit of muscle contraction called the **sarcomere**.

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Structure Of Contractile Proteins

Thin filament (Actin): Made of two filamentous (F) actins helically wound. Each F actin is a polymer of monomeric globular (G) actins. Two filaments of **tropomyosin** run along F actins. **Troponin**, a complex protein, is distributed at intervals on tropomyosin. In resting state, a troponin subunit masks the active binding sites for myosin on actin.

Thick filament (Myosin): Polymerized protein made of many monomeric **meromyosins**. Each meromyosin has a globular head with a short arm (Heavy Meromyosin, HMM) and a tail (Light Meromyosin, LMM). HMM component (head and short arm) projects outwards as a **cross arm**. The globular head is an active ATPase enzyme with binding sites for ATP and active sites for actin.

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Mechanism Of Muscle Contraction (Sliding Filament Theory)

Muscle contraction is explained by the **sliding filament theory**: thin actin filaments slide over thick myosin filaments, shortening the sarcomere.

Steps:

1. Signal from CNS via motor neuron reaches the **neuromuscular junction** (motor-end plate). Neurotransmitter (Acetylcholine) is released.
2. Acetylcholine generates an action potential in the sarcolemma, which spreads through the muscle fiber.
3. Action potential causes release of **calcium ions (Ca$^{++}$)** from sarcoplasmic reticulum into the sarcoplasm.
4. Increased Ca$^{++}$ level binds to troponin on actin filaments, changing its conformation and **unmasking the active binding sites** for myosin on actin.
5. Myosin head binds to exposed active sites on actin, forming a **cross bridge**. This utilizes energy from ATP hydrolysis (myosin head is an ATPase).
6. Cross bridge pulls the attached actin filaments towards the center of the A band (power stroke). Z lines attached to actins are pulled inwards, shortening the sarcomere (contraction). I bands reduce; A bands remain same length.
7. Myosin releases ADP and Pi, goes to relaxed state. A new ATP molecule binds to myosin head, breaking the cross bridge.
8. ATP is hydrolysed, myosin head cocks back, ready for next cycle. Cross bridge formation and breakage cycle repeats, causing further sliding.

Process continues until Ca$^{++}$ ions are pumped back into sarcoplasmic reticulum, causing masking of actin binding sites by troponin. Cross bridges are broken, and muscles relax. The Z lines return to original position, sarcomere lengthens.

Repeated muscle activation without sufficient oxygen can cause lactic acid accumulation (anaerobic glycogen breakdown) leading to fatigue.

Muscle fiber types:

• **Red fibres:** Have high myoglobin (oxygen-storing pigment, gives reddish appearance) and many mitochondria. Primarily rely on aerobic respiration for ATP (aerobic muscles). Slower contraction, fatigue resistant.
• **White fibres:** Have less myoglobin (appear pale/whitish) and fewer mitochondria. High sarcoplasmic reticulum. Rely on anaerobic processes for energy. Faster contraction, fatigue quickly.

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Skeletal System

The **skeletal system** provides a framework of bones and cartilages that supports the body and facilitates movement. Bone and cartilage are specialized connective tissues; bone matrix is hard due to calcium salts, cartilage matrix is slightly pliable due to chondroitin salts. The human skeletal system has 206 bones and a few cartilages.

Divisions of the skeletal system:

• **Axial skeleton (80 bones):** Along the main axis of the body. Includes skull, vertebral column, sternum, and ribs.
• **Appendicular skeleton (126 bones):** Bones of the limbs and their supporting girdles. Includes pectoral girdle, pelvic girdle, and limb bones (arms and legs).

Axial Skeleton components:

• **Skull (22 bones + Hyoid + Ear Ossicles):** Composed of 8 cranial bones (cranium) protecting the brain and 14 facial bones forming the face. Includes a single U-shaped hyoid bone at the base of the buccal cavity and three tiny ear ossicles (Malleus, Incus, Stapes) in each middle ear. The skull articulates with the vertebral column via two occipital condyles (dicondylic skull).

• **Vertebral column (26 vertebrae):** Dorsally placed, extends from the base of the skull, forming the main framework of the trunk. Each vertebra has a neural canal for the spinal cord. Differentiated into regions: Cervical (7), Thoracic (12), Lumbar (5), Sacral (1 fused), Coccygeal (1 fused). Atlas (first vertebra) articulates with occipital condyles. Protects spinal cord, supports head, attachment point for ribs and back muscles.

• **Sternum (1 bone):** Flat bone on the ventral midline of the thorax.
• **Ribs (12 pairs):** Thin, flat bones. Dorsally connected to thoracic vertebrae (bicephalic). Ventrally connected to the sternum (except floating ribs).
  • True ribs (1st-7th pairs): Connected directly to the sternum via hyaline cartilage.
  • False ribs (8th, 9th, 10th pairs): Do not articulate directly with sternum but join the 7th rib via hyaline cartilage (vertebrochondral ribs).
  • Floating ribs (11th, 12th pairs): Not connected ventrally.

Thoracic vertebrae, ribs, and sternum form the rib cage.

Appendicular Skeleton components:

• **Limb bones:** Each hand (fore limb) has 30 bones: humerus, radius, ulna, 8 carpals (wrist), 5 metacarpals (palm), 14 phalanges (digits). Each leg (hind limb) has 30 bones: femur (thigh bone - longest), tibia, fibula, 7 tarsals (ankle), 5 metatarsals, 14 phalanges. Patella (kneecap) covers the knee ventrally.

• **Girdles:** Help articulate limbs with the axial skeleton. Each girdle has two halves.
  • Pectoral girdle (shoulder girdle): Each half has a clavicle (collar bone) and a scapula (shoulder blade). Scapula is a large triangular flat bone. Spine projects as acromion, articulating with clavicle. Glenoid cavity (depression) articulates with humerus head to form shoulder joint.
  • Pelvic girdle (hip girdle): Two coxal bones. Each coxal bone formed by fusion of ilium, ischium, and pubis. Acetabulum cavity (at fusion point) articulates with femur head. Two halves meet ventrally at pubic symphysis (fibrous cartilage).

Joints

**Joints** are essential points of contact between bones, or between bones and cartilages. They enable movement of bony parts, with the joint acting as a fulcrum and muscles generating the force. Movability varies between joints. Joints are classified structurally into three types:

• **Fibrous joints:** Do not allow any movement. Bones are fused end-to-end with dense fibrous connective tissues (sutures). Example: Between skull bones forming the cranium.
• **Cartilaginous joints:** Bones joined by cartilages. Permit limited movements. Example: Joint between adjacent vertebrae in the vertebral column.
• **Synovial joints:** Characterized by a fluid-filled **synovial cavity** between the articulating surfaces of bones. Allows considerable movement. Play a significant role in locomotion. Examples: Ball and socket (shoulder, hip), hinge (knee, elbow), pivot (atlas/axis), gliding (carpals), saddle (carpal/metacarpal of thumb).

Disorders Of Muscular And Skeletal System

Various disorders can affect the muscular and skeletal systems:

• **Myasthenia gravis:** Autoimmune disorder affecting neuromuscular junction, leading to fatigue, weakening, and paralysis of skeletal muscles.
• **Muscular dystrophy:** Progressive degeneration of skeletal muscles, often due to genetic disorders.
• **Tetany:** Rapid muscle spasms (wild contractions) caused by low calcium ion (Ca$^{++}$) levels in body fluid.
• **Arthritis:** Inflammation of joints.
• **Osteoporosis:** Age-related disorder causing decreased bone mass and increased fracture risk. Often due to decreased estrogen levels.
• **Gout:** Inflammation of joints caused by the accumulation of uric acid crystals.

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