Locomotion and Movement

Types Of Movement

Movement is a fundamental characteristic of living organisms. It is defined as a change in position or location. Movement can occur at various levels, from intracellular processes to the movement of the entire organism (locomotion).

The three main types of movement shown by cells of the human body are:

Amoeboid movement:
- Characteristic of some unicellular organisms like Amoeba.
- Involves the formation of temporary cytoplasmic extensions called pseudopodia (false feet) by the streaming of protoplasm (amoeboid movement).
- In humans, cells like macrophages and leucocytes (WBCs) exhibit amoeboid movement to migrate through tissues and engulf pathogens.
- This movement is facilitated by the cytoskeleton components like microfilaments.
Ciliary movement:
- Involves the coordinated beating of small hair-like projections called cilia.
- Cilia are found on the surface of some epithelial cells.
- In humans, ciliary movement helps in:
  - Removing dust particles and mucus from the respiratory passages (cilia in the trachea and bronchi).
  - Movement of ovum through the fallopian tubes (cilia in the lining of fallopian tubes).
Muscular movement:
- Involves the contraction and relaxation of muscle fibres.
- Muscular movement is responsible for most movements in multicellular animals, including locomotion.
- Examples: Movement of limbs, jaws, tongue, movement of food through the digestive tract, beating of the heart.

Locomotion vs. Movement

Movement: A change in position of a part of the body relative to the whole organism.
Locomotion: Movement of the entire organism from one place to another.

All locomotions are movements, but all movements are not locomotions (e.g., the movement of your hand is movement, but not locomotion of your entire body).

Methods of Locomotion in Animals:

Different animals employ various methods for locomotion, adapted to their habitat and needs:

Walking, running, crawling, climbing, swimming, flying.

These methods require structures like limbs, fins, wings, or modifications of the body wall (as in earthworms and snails).

Locomotion requires coordinated activity of the musculoskeletal system (muscles and bones) and the nervous system.

Muscle

Muscle is a specialised tissue of mesodermal origin. Muscle tissue is responsible for movement in animals. Muscle cells (fibres) have the property of contractility, excitability, extensibility, and elasticity.

Types of Muscles:

Based on their structure, location, and function, muscles are classified into three types:

Skeletal Muscle:
- Location: Attached to bones, typically associated with the skeleton.
- Appearance: Striated (show light and dark bands).
- Control: Voluntary (under conscious control).
- Structure: Long, cylindrical, unbranched, multinucleate fibres.
- Function: Locomotion and change in body posture.
Visceral Muscle (Smooth Muscle):
- Location: Found in the walls of internal organs (viscera) like the digestive tract, blood vessels, uterus.
- Appearance: Non-striated (lack striations).
- Control: Involuntary (not under conscious control).
- Structure: Spindle-shaped, uninucleate fibres.
- Function: Involuntary movements like peristalsis, vasoconstriction.
Cardiac Muscle:
- Location: Found only in the wall of the heart.
- Appearance: Striated.
- Control: Involuntary.
- Structure: Branched fibres, joined by intercalated discs, usually uninucleate (sometimes binucleate).
- Function: Pumping of blood (rhythmic contraction).

Structure of Skeletal Muscle:

A skeletal muscle is made up of many muscle bundles (fascicles).
Each muscle bundle contains several muscle fibres.
Each muscle fibre is a long, cylindrical cell. Its plasma membrane is called the sarcolemma, and the cytoplasm is called the sarcoplasm.
Muscle fibres are multinucleate.
The sarcoplasm contains a network of endoplasmic reticulum called the sarcoplasmic reticulum, which is a storehouse of calcium ions ($Ca^{2+}$).
Muscle fibres contain numerous parallelly arranged filaments called myofibrils.

Structure of Myofibril and Striations:

Myofibrils show alternate light and dark bands (striations).

Light bands (I-bands or isotropic bands): Contain mainly the protein Actin (thin filaments).

Dark bands (A-bands or anisotropic bands): Contain mainly the protein Myosin (thick filaments). Myosin filaments overlap with Actin filaments in this region.

In the centre of the A-band is a relatively lighter region called the H-zone (Hensen's line), where only myosin is present (no actin overlap).

The I-band is bisected by a dark line called the Z-line. The Z-line is a fibrous membrane.

The portion of a myofibril between two successive Z-lines is called a sarcomere. The sarcomere is the structural and functional unit of muscle contraction.

In the middle of the A-band, a thin fibrous membrane called the M-line holds the myosin filaments together.

(Image shows a detailed diagram of a sarcomere illustrating the arrangement of actin and myosin filaments and the different zones/lines)

Structure Of Contractile Proteins

The thin and thick filaments are made up of proteins called contractile proteins, primarily Actin and Myosin.

Actin (Thin Filament):

Composed of two filamentous 'F' actin molecules, which are helically wound around each other.

Each F-actin is a polymer of globular 'G' actin monomers.

Two other proteins, Tropomyosin and Troponin, are also present on the thin filament.

Tropomyosin is a fibrous protein that runs along the length of the F-actin, covering the myosin binding sites on actin in a resting muscle.

Troponin is a complex of three subunits, located at intervals on tropomyosin. One subunit binds to tropomyosin, one binds to G-actin, and one has a binding site for $Ca^{2+}$ ions.

(Image shows a diagram illustrating the double helix of F-actin, tropomyosin coiled around it, and troponin complexes positioned along the tropomyosin)

Myosin (Thick Filament):

Each thick filament is a polymer of many monomeric proteins called meromyosins.

Each meromyosin has two parts: a globular head (with a short arm) forming the heavy meromyosin (HMM), and a tail forming the light meromyosin (LMM).

The HMM component (head and short arm) projects outwards at regular distances and angles from the surface of the thick filament. This is the cross arm.

The myosin head is an active ATPase enzyme and has binding sites for ATP and active sites for Actin.

(Image shows a diagram illustrating multiple meromyosins forming a thick filament, highlighting the head and tail regions, and showing the heads projecting out as cross-arms)

Mechanism Of Muscle Contraction

The mechanism of muscle contraction is explained by the Sliding Filament Theory. This theory states that muscle contraction occurs by the sliding of the thin filaments (actin) over the thick filaments (myosin).

Steps Involved in Muscle Contraction:

Neural Signal: Muscle contraction is initiated by a signal from the central nervous system (CNS) via a motor neuron. The junction between a motor neuron and the muscle fibre is called the neuromuscular junction or motor end plate.

Neurotransmitter Release: A neural signal arriving at the neuromuscular junction releases a neurotransmitter (acetylcholine) into the synaptic cleft.

Muscle Fibre Excitation: The neurotransmitter binds to receptors on the sarcolemma, generating an action potential in the muscle fibre.

Release of Calcium Ions: The action potential spreads through the sarcolemma and into the sarcoplasmic reticulum, stimulating the release of Calcium ions ($Ca^{2+}$) into the sarcoplasm.

Binding of $Ca^{2+}$ to Troponin: $Ca^{2+}$ ions bind to the troponin subunit of the thin filament.

Exposure of Myosin Binding Sites: Binding of $Ca^{2+}$ causes a conformational change in troponin, which in turn moves tropomyosin, uncovering the myosin binding sites on the actin filaments.

Formation of Cross Bridges: The myosin head binds to the exposed active sites on actin, forming a cross bridge. This binding requires energy from ATP hydrolysis (myosin head is an ATPase).

Power Stroke: The bound myosin head pulls the actin filament towards the centre of the sarcomere. This pulling action is called the power stroke. During this, ADP and Pi are released from the myosin head.

Breaking of Cross Bridges: A new ATP molecule binds to the myosin head, causing the cross bridge to detach from actin.

Cycling of Cross Bridges: The myosin head hydrolyses the new ATP, reorients, and is ready to form another cross bridge and repeat the cycle, provided $Ca^{2+}$ is still present. The sliding continues as long as $Ca^{2+}$ ions are available and ATP is supplied.

Relaxation of Muscle:

Muscle relaxation occurs when the neural signal stops.

Acetylcholine is broken down by acetylcholinesterase.

$Ca^{2+}$ ions are actively pumped back into the sarcoplasmic reticulum.

Removal of $Ca^{2+}$ from troponin causes tropomyosin to cover the myosin binding sites on actin again.

Cross bridges detach, and the actin filaments slide back to their original positions, lengthening the sarcomere and the muscle fibre.

(Image shows a sarcomere in relaxed and contracted states, and possibly a detailed diagram of the cross-bridge cycle between myosin head and actin filament with steps involving ATP, Pi, ADP, and Ca2+)

Changes in Sarcomere During Contraction:

The length of the sarcomere decreases.

The length of the I-band decreases (as actin slides over myosin).

The length of the H-zone decreases and may even disappear at maximum contraction (as actin overlaps completely).

The length of the A-band remains constant (length of myosin filament does not change).

The distance between Z-lines decreases.

Muscle contraction is a complex process requiring coordination between the nervous system, excitation-contraction coupling, and the interaction of contractile proteins utilising ATP energy.

Skeletal System

The skeletal system in humans is a rigid framework of bones and cartilage. It provides structural support, protection, enables movement, stores minerals, and is the site of blood cell production.

Components of the Skeletal System:

The adult human skeleton consists of 206 bones. It is divided into two main parts:

Axial Skeleton: Forms the central axis of the body. Includes the skull, vertebral column, sternum, and ribs. (80 bones).
Appendicular Skeleton: Forms the limbs and the girdles that connect the limbs to the axial skeleton. Includes the bones of the arms, legs, shoulder girdle, and pelvic girdle. (126 bones).

Axial Skeleton (80 bones):

Skull: (29 bones) - Protects the brain. Includes cranial bones (8, enclose the brain), facial bones (14), hyoid bone (1, in the neck), and ear ossicles (6 - Malleus, Incus, Stapes in each ear).

Vertebral Column (Backbone): (26 bones in adult) - Provides support and protects the spinal cord. Extends from the base of the skull to the pelvis. Composed of vertebrae (7 Cervical, 12 Thoracic, 5 Lumbar, 1 Sacrum - fused, 1 Coccyx - fused).

Sternum (Breastbone): (1 bone) - A flat bone on the ventral midline of the thorax.

Ribs: (12 pairs = 24 bones) - Curved bones that form the rib cage, protecting the lungs and heart. Connected to the vertebral column dorsally and sternum ventrally (directly or indirectly).

True ribs (1-7 pairs) - connected directly to sternum by costal cartilage.

False ribs (8-10 pairs) - connected to the sternum indirectly via the cartilage of the 7th rib.

Floating ribs (11-12 pairs) - not connected to the sternum ventrally.

(Image shows a human skeleton highlighting the skull, vertebral column, sternum, and rib cage)

Appendicular Skeleton (126 bones):

Limb Bones: (120 bones)

Upper limbs (60 bones): Humerus (upper arm), Radius and Ulna (forearm), Carpals (8 bones in wrist), Metacarpals (5 bones in palm), Phalanges (14 bones in fingers - 3 per finger, 2 in thumb).

Lower limbs (60 bones): Femur (thigh bone), Patella (kneecap), Tibia and Fibula (lower leg), Tarsals (7 bones in ankle), Metatarsals (5 bones in sole), Phalanges (14 bones in toes - 3 per toe, 2 in big toe).

Girdles: Connect the limbs to the axial skeleton.

Pectoral girdle (Shoulder girdle): (4 bones) - Connects the upper limb to the axial skeleton. Each half consists of a Clavicle (collarbone) and a Scapula (shoulder blade).

Pelvic girdle (Hip girdle): (2 bones) - Connects the lower limb to the axial skeleton. Formed by the fusion of three bones: Ilium, Ischium, and Pubis on each side, forming the coxal bone. The two coxal bones articulate with the sacrum to form the pelvis.

(Image shows a human skeleton highlighting the bones of the arms, legs, shoulder girdle, and pelvic girdle)

Functions of Skeletal System:

Support: Provides the structural framework of the body.
Protection: Protects vital organs (skull protects brain, rib cage protects heart/lungs, vertebral column protects spinal cord).
Movement: Provides attachment points for muscles. Bones act as levers, and joints allow for movement.
Mineral Storage: Bones store calcium and phosphate, releasing them into the blood when needed.
Haemopoiesis (Blood cell formation): Red bone marrow, found within certain bones, is the site of production of blood cells.

Joints

A joint is a point of articulation between two or more bones, or between a bone and cartilage. Joints are essential for movement and flexibility of the skeleton.

Joints can be classified based on the degree of movement they allow:

Fibrous Joints (Immovable Joints):
- Bones are joined by dense fibrous connective tissue.
- They allow little or no movement.
- Example: Sutures between the cranial bones of the skull in adults (immovable).
Cartilaginous Joints (Slightly Movable Joints):
- Bones are joined by cartilage.
- They allow limited movement.
- Example: Joints between adjacent vertebrae (intervertebral discs made of fibrocartilage), pubic symphysis.
Synovial Joints (Freely Movable Joints):
- Characterised by the presence of a synovial cavity between the articulating bones, filled with synovial fluid (lubricant).
- The articulating surfaces of the bones are covered by articular cartilage.
- The joint is enclosed by a fibrous joint capsule.
- These joints allow considerable movement and are crucial for locomotion and most body movements.
- Different types allow different ranges of motion.

Types of Synovial Joints (based on structure and movement):

Ball and Socket Joint: A ball-shaped end of one bone fits into a cup-shaped socket of another bone. Allows movement in multiple planes (multiaxial). Example: Shoulder joint, Hip joint.

Hinge Joint: Allows movement in primarily one plane (uniaxial), like a door hinge. Example: Elbow joint, Knee joint, Ankle joint, finger joints.

Pivot Joint: A rounded bone process articulates with a ring-shaped structure (bone and ligament). Allows rotation around a central axis (uniaxial). Example: Joint between atlas and axis vertebrae (allowing head rotation), radioulnar joints.

Gliding Joint (Plane Joint): Articulating surfaces are relatively flat, allowing bones to slide or glide over each other. Allows limited movement in multiple planes (multiaxial, but limited range). Example: Joints between carpals (wrist bones), joints between tarsals (ankle bones).

Saddle Joint: Both articulating surfaces are saddle-shaped. Allows movement in two planes (biaxial). Example: Joint between the carpel (trapezium) and metacarpal of the thumb.

Condyloid Joint (Ellipsoidal Joint): An oval-shaped condyle of one bone fits into an elliptical cavity of another bone. Allows movement in two planes (biaxial - flexion/extension and abduction/adduction), but no rotation. Example: Wrist joint (radiocarpal joint), metacarpophalangeal joints (knuckles).

(Image shows illustrations of the structure of a synovial joint and diagrams of typical examples for each type of synovial joint)

Synovial joints, with their diverse forms, are responsible for the wide range of movements possible in the appendicular skeleton.

Disorders Of Muscular And Skeletal System

Various conditions can affect the normal functioning of the muscles and the skeletal system, leading to pain, reduced mobility, and disability.

Disorders of Muscular System:

Myasthenia Gravis: An autoimmune disorder affecting the neuromuscular junction. Antibodies block or destroy acetylcholine receptors, leading to muscle weakness and fatigue, especially of muscles in the face, eyes, and throat.
Muscular Dystrophy: A group of genetic disorders causing progressive degeneration of skeletal muscle tissue. Most common type is Duchenne muscular dystrophy (DMD). Leads to increasing muscle weakness and loss of mobility.
Tetany: Rapid spasms (involuntary muscle contractions) due to low calcium levels in the body fluid ($Ca^{2+}$ ions are essential for muscle contraction).
Muscle twitching: Involuntary brief contraction of muscle fibres.
Muscle spasm: Sudden, involuntary contraction of a muscle or group of muscles.

Disorders of Skeletal System:

Arthritis: Inflammation of joints. There are various types:
- Rheumatoid Arthritis: An autoimmune disease causing chronic inflammation of the joints, leading to pain, swelling, stiffness, and potential joint deformation.
- Osteoarthritis: Degenerative joint disease caused by the breakdown of articular cartilage, common in older age. Affects weight-bearing joints like knees, hips, and spine.
- Gouty Arthritis (Gout): Caused by the accumulation of uric acid crystals in joints, leading to severe pain and inflammation, often in the big toe.
Osteoporosis: A disease characterised by decreased bone mass and density, making bones fragile and prone to fractures. Common in older age, especially in post-menopausal women (due to decreased oestrogen levels). Can be caused by calcium or Vitamin D deficiency, hormonal imbalances.
Rickets: A condition in children caused by Vitamin D deficiency, leading to inadequate mineralisation of bones and resulting in soft, weak bones and skeletal deformities (e.g., bowed legs). The equivalent condition in adults is Osteomalacia.

Other Related Disorders:

Sprain: Injury to a ligament, often caused by stretching or tearing.
Strain: Injury to a muscle or tendon, often caused by overstretching or tearing.
Fracture: A break in a bone.
Dislocation: Displacement of bones at a joint.

Maintaining a healthy diet (rich in calcium and Vitamin D), regular exercise, and avoiding smoking and excessive alcohol consumption can help promote musculoskeletal health and reduce the risk of these disorders.