Chapter 5.4: Locomotion and Movement

1. Types of Movement

Movement is one of the significant features of living beings. It can be the movement of a part of the body or the whole organism. When a movement results in a change of place or location, it is called locomotion.

• Ciliary Movement: Occurs in most of our internal tubular organs which are lined by ciliated epithelium. The coordinated movements of cilia in the trachea help us in removing dust particles and some of the foreign substances inhaled along with the atmospheric air. Passage of ova through the female reproductive tract is also facilitated by the ciliary movement.
• Flagellar Movement: The swimming of spermatozoa, maintenance of water current in the canal system of sponges and in locomotion of protozoans like Euglena.
• Muscular Movement: A more complex movement that involves the contraction and relaxation of muscles. It is responsible for the movement of our limbs, jaws, tongue, etc., and for locomotion.

2. Muscles

a) General Properties of Muscles

• Excitability: The ability of a muscle to respond to a stimulus.
• Contractility: The ability of a muscle to shorten forcefully when stimulated.
• Extensibility: The ability of a muscle to be stretched.
• Elasticity: The ability of a muscle to recoil to its original resting length after being stretched.

b) Structure of Skeletal Muscle

• Each organised skeletal muscle in our body is made of a number of muscle bundles or fascicles held together by a common collagenous connective tissue layer called fascia.
• Each muscle bundle contains a number of muscle fibres (muscle cells).
• Each muscle fibre is lined by the plasma membrane called the sarcolemma enclosing the sarcoplasm.
• Muscle fibre is a syncytium as the sarcoplasm contains many nuclei. The endoplasmic reticulum, i.e., sarcoplasmic reticulum of the muscle fibres is the storehouse of calcium ions.
• A characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils.
• Each myofibril has alternate dark and light bands on it. The light bands contain actin and are called I-band (isotropic band), whereas the dark band, called A-band (anisotropic band) contains myosin.
• In the centre of each I-band is an elastic fibre called Z-line which bisects it.
• The portion of the myofibril between two successive Z-lines is considered as the functional unit of contraction and is called a sarcomere.

• In the centre of the A-band, a lighter region called the H-zone is present. A thin dark line called the M-line runs through the centre of the H-zone.

c) Contractile Proteins

• Actin (Thin Filament): Each actin filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state, a subunit of troponin masks the active binding sites for myosin on the actin filaments.
• Myosin (Thick Filament): Each myosin filament is also a polymerised protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM). The HMM component, i.e.; the head and short arm projects outwards at regular distance and angle from each other from the surface of a polymerised myosin filament and is known as cross arm. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

d) Sliding Filament Theory of Muscle Contraction

• Mechanism: Muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments (actin) over the thick filaments (myosin).
  1. A signal sent by the central nervous system (CNS) via a motor neuron reaches the neuromuscular junction or motor end-plate.
  2. Release of a neurotransmitter (acetylcholine) generates an action potential in the sarcolemma.
  3. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm from the sarcoplasmic reticulum.
  4. Increase in Ca++ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin.
  5. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
  6. This pulls the attached actin filaments towards the centre of the A-band. The Z-line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
  7. During shortening of the muscle, or contraction, the I-bands get reduced, whereas the A-bands retain the length. The H-zone disappears.
  8. The myosin, releasing the ADP and Pi, goes back to its relaxed state. A new ATP binds and the cross-bridge is broken. The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding.
  9. When Ca++ ions are pumped back to the sarcoplasmic reticulum, the actin filaments are again masked and this causes relaxation.

e) Chemical Events During Muscle Contraction

• ATP Hydrolysis: ATP → ADP + Pi + Energy (This energy is used for the power stroke).
• Creatine Phosphate: Creatine Phosphate + ADP → Creatine + ATP (This reaction replenishes ATP).

f) Definitions

• Summation: If a muscle is stimulated a second time before it has completely relaxed, the second contraction is stronger than the first. This is called summation.
• Tetanus: If a muscle is stimulated repeatedly at a high frequency, the individual contractions fuse to form a smooth, sustained contraction called tetanus.
• Rigor Mortis: The stiffening of the body after death due to the depletion of ATP, which is required to break the cross-bridges between actin and myosin.

g) Red and White Muscles

3. Skeletal System

a) Functions of the Human Skeleton

1. Provides a framework for the body.
2. Provides support and protection for internal organs.
3. Serves as a surface for the attachment of muscles.
4. Plays a role in breathing (rib cage).
5. Stores minerals like calcium and phosphorus.
6. Produces blood cells (in the bone marrow).

b) Human Skeleton

The human skeleton consists of 206 bones and is divided into the axial skeleton and the appendicular skeleton.

i) Axial Skeleton (80 bones)

• Skull (22 bones):
  • Cranial Bones (8): Frontal (1), Parietal (2), Temporal (2), Occipital (1), Sphenoid (1), Ethmoid (1).
  • Facial Bones (14): Nasal (2), Maxilla (2), Zygomatic (2), Lacrimal (2), Palatine (2), Inferior nasal conchae (2), Mandible (1), Vomer (1).
• Hyoid Bone (1): U-shaped bone present at the base of the buccal cavity.
• Ear Ossicles (6): Malleus (2), Incus (2), Stapes (2).
• Vertebral Column (26):
  • Cervical (7)
  • Thoracic (12)
  • Lumbar (5)
  • Sacrum (1 - fused from 5)
  • Coccyx (1 - fused from 4)
• Sternum (1): Flat bone on the ventral midline of the thorax.
• Ribs (12 pairs):
  • True Ribs (1-7): Directly attached to the sternum.
  • False Ribs (8-10): Do not attach directly to the sternum but join the seventh rib.
  • Floating Ribs (11-12): Not connected ventrally.

ii) Appendicular Skeleton (126 bones)

• Pectoral Girdle (4 bones):
  • Clavicle (2) (Collar bone)
  • Scapula (2) (Shoulder blade)
• Upper Limbs (60 bones):
  • Humerus (2)
  • Radius (2)
  • Ulna (2)
  • Carpals (16) (Wrist bones)
  • Metacarpals (10) (Palm bones)
  • Phalanges (28) (Digits)
• Pelvic Girdle (2 bones):
  • Coxal bone (2) (formed by the fusion of ilium, ischium, and pubis)
• Lower Limbs (60 bones):
  • Femur (2) (Thigh bone - longest bone)
  • Tibia (2)
  • Fibula (2)
  • Patella (2) (Knee cap)
  • Tarsals (14) (Ankle bones)
  • Metatarsals (10)
  • Phalanges (28) (Digits)

4. Joints

Joints are essential for all types of movements involving the bony parts of the body. They are points of contact between bones, or between bones and cartilages.

• Fibrous Joints (Immovable): Do not allow any movement. This type of joint is shown by the flat skull bones which fuse end-to-end with the help of dense fibrous connective tissues in the form of sutures, to form the cranium.
• Cartilaginous Joints (Slightly Movable): The bones involved are joined together with the help of cartilages. The joint between the adjacent vertebrae in the vertebral column is of this pattern and it permits limited movements.
• Synovial Joints (Freely Movable): Characterised by the presence of a fluid-filled synovial cavity between the articulating surfaces of the two bones. Such an arrangement allows considerable movement. These joints help in locomotion and many other movements.
  • 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)
  • Gliding Joint: (e.g., between the carpals)
  • Saddle Joint: (e.g., between carpal and metacarpal of thumb)

5. Disorders of Muscular and Skeletal System

• Myasthenia Gravis: An autoimmune disorder affecting the neuromuscular junction leading to fatigue, weakening, and paralysis of skeletal muscle.
• Tetany: Rapid spasms (wild contractions) in muscle due to low Ca++ in body fluid.
• Muscular Dystrophy: Progressive degeneration of skeletal muscle, mostly due to a genetic disorder.
• Arthritis: Inflammation of joints.
• Osteoporosis: Age-related disorder characterised by decreased bone mass and increased chances of fractures. Decreased levels of estrogen is a common cause.
• Gout: Inflammation of joints due to the accumulation of uric acid crystals.