Study Notes on Locomotion and Movement

LOCOMOTION AND MOVEMENT

Introduction to Movement

Movement is a significant feature exhibited by living beings, spanning a variety of actions in both animals and plants. In unicellular organisms such as Amoeba, a simple form of movement is represented through the streaming of protoplasm. Various organisms utilize movements such as cilia, flagella, and tentacles. In humans, the ability to move different body parts like limbs, jaws, eyelids, and tongue contributes to various actions. Notably, some movements lead to a change in position, classified as voluntary movements known as locomotion. Common forms of locomotory movement include walking, running, climbing, flying, and swimming.

It is crucial to recognize that locomotory structures may not necessarily differ from those involved in other types of movements. For instance, in Paramoecium, cilia assist in moving food through the cytopharynx while also aiding in locomotion. Similarly, Hydra use its tentacles for both prey capture and locomotion. Limb movements in humans facilitate changes in body posture and locomotion alike. Thus, movements and locomotion are intertwined concepts, where it can be stated that all locomotions are movements but not all movements qualify as locomotions. The methods of locomotion in animals depend significantly on their habitats and situational demands, typically serving purposes such as searching for food, seeking shelter, finding mates, locating suitable breeding grounds, adapting to favorable climatic conditions, or escaping predators.

Types of Movement

The cells within the human body display three primary types of movements: amoeboid, ciliary, and muscular.

Amoeboid Movement

Specialized cells such as macrophages and leukocytes exhibit amoeboid movement, which is characterized by the formation of pseudopodia from protoplasmic streaming, similar to mechanisms observed in Amoeba. Cytoskeletal components, specifically microfilaments, play a vital role in facilitating amoeboid movements.

Ciliary Movement

Ciliary movement is prevalent in various internal tubular organs lined with ciliated epithelium. The coordinated action of cilia in the trachea helps expel dust and foreign particles inhaled with atmospheric air. Additionally, ciliary movements are responsible for facilitating the passage of ova through the female reproductive tract.

Muscular Movement

The movement of limbs, jaws, tongue, and other body parts necessitates muscular movement. The contractile properties of muscles are harnessed for locomotion and other bodily movements in humans and most multicellular organisms. Effective locomotion results from a well-coordinated interaction among the muscular, skeletal, and neural systems. The following sections delve into the structure and functionality of these muscles.

Muscle

Definition and Function

Muscle is classified as specialized tissue of mesodermal origin. Notably, muscles contribute to 40-50% of an adult human's body weight. Muscles possess specific characteristics, including:

• Excitability: Capacity to respond to stimuli.

• Contractility: Ability to shorten forcibly and produce movement.

• Extensibility: Capability to be stretched without damage.

• Elasticity: Ability to return to original length post-stretching.

Muscle classification can be based on various criteria such as location, appearance, and regulatory mechanisms. Muscles are categorized into three types based on location:

1. Skeletal Muscle

   • Found in association with the skeleton,

   • Exhibits a striated appearance under a microscope, hence termed striated muscles,

   • Voluntary control by the nervous system, making them known as voluntary muscles,

   • Involved mainly in locomotion and postural changes.

2. Visceral Muscle

   • Located within the inner walls of hollow organs like the alimentary canal and reproductive tract,

   • Non-striated and smooth in appearance, thus referred to as smooth muscles,

   • Involuntary control, meaning their actions are not consciously manageable,

   • Functionally significant in aiding transportation processes in various body systems.

3. Cardiac Muscle

   • Comprising the heart's muscle tissue,

   • Exhibits striated appearance, forming a branching pattern,

   • Also involuntary, as nervous system control is not direct.

Structure of Muscle Fibres

Muscle fibres are surrounded by a plasma membrane termed the sarcolemma, which encloses sarcoplasm. Each muscle fibre functions as a syncytium, containing multiple nuclei. The sarcoplasmic reticulum, a specific endoplasmic reticulum in muscle fibres, is crucial as it serves as the reservoir for calcium ions. Within the muscle fibre are numerous parallelly arranged filaments known as myofilaments or myofibrils. Each myofibril demonstrates an alternating pattern of dark and light bands resulting from the arrangement of two key proteins: Actin and Myosin.

• I-band (Isotropic band): Contains actin (thin filaments).

• A-band (Anisotropic band): Contains myosin (thick filaments).

• Z-line: An elastic fibre bisecting the I-band where thin filaments are anchored.

• M-line: A thin fibrous membrane at the center of the A-band holding thick filaments together.

• H-zone: The central section of the A-band that is not overlapped by thin filaments during rest.

Contractile Proteins

Within each actin (thin) filament are two F (filamentous) actins helically wound to each other, connected by two filamentous proteins known as tropomyosin. Notably, troponin, a complex regulatory protein, is distributed at intervals along the tropomyosin. In the resting state, a subunit of troponin masks the active binding sites on the actin filaments for myosin binding.

Conversely, each myosin (thick) filament comprises numerous polymerized proteins termed meromyosins that consist of:

• Heavy Meromyosin (HMM): The portion with a globular head and short arm projecting outward.

• Light Meromyosin (LMM): The tail section of the myosin filament.
  The globular head functions as an active ATPase enzyme with binding sites for ATP and active sites for actin, forming cross-bridges during muscle contraction.

Mechanism of Muscle Contraction

The sliding filament theory elucidates that muscle contraction occurs via the sliding of thin filaments over thick filaments. The process initiates with neural signals dispatched from the central nervous system (CNS) via motor neurons. A motor unit comprises a motor neuron and its associated muscle fibres. The interface between a motor neuron and the muscle fibre's sarcolemma is termed the neuromuscular junction or motor-end plate.

Upon neural signal arrival, acetylcholine (a neurotransmitter) is released, generating an action potential across the muscle fibre. This action potential propagates, triggering the release of calcium ions into the sarcoplasm. An increase in calcium ion concentration allows calcium ions to bind to the troponin subunit, lifting the blockade on active sites for myosin on the actin filaments. Exploiting energy from ATP hydrolysis, myosin heads connect to the now-exposed active sites on actin, creating cross-bridges that pull the actin filaments inward toward the centre of the A-band.

This inward pull also draws the Attached Z-lines, causing contraction and shortening of the sarcomere. While muscle contraction effectively reduces the length of the I-bands, the A-bands maintain their dimensions. Upon ATP hydrolysis, myosin releases ADP and inorganic phosphate (P1) and returns to its relaxed state. A new ATP molecule binds to the myosin head, leading to breakage of the cross-bridge, after which the ATP undergoes hydrolysis, refueling the contraction cycle.
The contraction process persists until calcium ions are transported back to the sarcoplasmic cisternae, facilitating the masking of actin filaments and allowing Z-lines to revert to their original position leading to relaxation. Especially in repeated muscle activation, lactic acid may accumulate due to anaerobic glycolysis of glycogen, contributing to fatigue.
Muscle fibres also contain myoglobin, a red-hued oxygen-storing pigment, found particularly in red fibres of muscles, imparting a reddish appearance. Such fibres are abundant in mitochondria that efficiently use available oxygen to produce ATP. Conversely, white fibres possess lower myoglobin content, rendering them pale, and rely more on anaerobic metabolism for energy generation.

SKELETAL SYSTEM

Structure and Function

The skeletal system forms the structural framework of bones and some cartilages, facilitating bodily movement. Bone and cartilage represent specialized connective tissues. Bone has a rigid matrix fortified with calcium salts, whereas cartilage contains pliable matrices composed of chondroitin salts. In humans, this system has 206 bones and comprises two principal divisions: the axial skeleton and the appendicular skeleton.

Axial Skeleton

The axial skeleton consists of 80 bones arranged along the body's main axis, inclusive of the skull, vertebral column, sternum, and ribs. The skull entails two sets of bones: cranial and facial which total 22 bones. The cranial portion comprises 8 bones, creating a protective enclosure for the brain. Facial bones constitute 14 components making up the skull's anterior region along with the hyoid bone, which is U-shaped and resides at the base of the buccal cavity. Each middle ear houses three small bones known as the Ear Ossicles: Malleus, Incus, and Stapes. The connection of the skull with the vertebral column is facilitated by two occipital condyles.

The vertebral column, also part of the axial skeleton, consists of 26 arranged vertebrae, positioned dorsally. It spans from the skull base, serving as the central trunk framework while protecting the spinal cord. The first vertebra, termed the atlas, articulates with the occipital condyles. Vertebrae are classified as cervical (7), thoracic (12), lumbar (5), sacral (1, fused), and coccygeal (1, fused), with cervical vertebrae being universally seven across most mammals.

Appendicular Skeleton

The appendicular skeleton comprises limb bones and their girdles, each limb containing 30 bones. The forelimb or hand includes the humerus, radius, ulna, carpals (8 bones), metacarpals (5 bones), and phalanges (14 bones). The hind limb comprises the femur (longest bone), tibia, fibula, tarsals (7 bones), metatarsals (5 bones), and phalanges (14 bones). The patella serves as the knee cap covering the knee ventrally. Pectoral and pelvic girdles facilitate the connection of upper and lower limbs to the axial skeleton, respectively, with each girdle containing two halves.

While the pectoral girdle consists of a clavicle and scapula, the pelvic girdle is made of two coxal bones which are formed by the fusion of three components: ilium, ischium, and pubis. The acetabulum, a cavity where these bones unite, serves as the attachment point for the thigh bone.

JOINTS

Definition and Classification

Joints are critical for enabling movements that involve bony structures in locomotor activities. As points of contact between bones or between bones and cartilages, the movements at joints can vary widely depending on several factors.

Three Types of Joints

Joints can be categorized into three main structural forms:

1. Fibrous Joints

   • These joints permit no movement, exemplified by skull bones that fuse through dense fibrous connective tissues forming sutures.

2. Cartilaginous Joints

   • These joints unify bones through cartilages, allowing limited movements as observed in the joints between adjacent vertebrae in the vertebral column.

3. Synovial Joints

   • Distinguished by a synovial cavity filled with fluid between articulating bones, synovial joints enable considerable movements, facilitating locomotion. Examples include:

   • Ball and Socket Joint (e.g., humerus and pectoral girdle)

   • Hinge Joint (e.g., knee joint)

   • Pivot Joint (e.g., between atlas and axis)

   • Gliding Joint (e.g., between carpal bones)

   • Saddle Joint (e.g., between carpal and metacarpal of the thumb).

DISORDERS OF MUSCULAR AND SKELETAL SYSTEM

A review of common disorders affecting the muscular and skeletal systems includes:

• Myasthenia Gravis: An autoimmune condition impacting neuromuscular junctions, leading to muscle fatigue and paralysis.

• Muscular Dystrophy: A progressive degenerative disorder of skeletal muscles, often tied to genetic inheritance.

• Tetany: Characterized by rapid muscle spasms due to low calcium levels in body fluids.

• Arthritis: Joint inflammation that can affect mobility and comfort.

• Osteoporosis: An age-related condition marked by reduced bone mass, increasing fracture risk, often due to reduced estrogen levels.

• Gout: Involves joint inflammation due to uric acid crystal accumulation.

SUMMARY

Movement is fundamental to all living beings, encompassing forms like protoplasmic streaming, ciliary action, and locomotion through fins, limbs, and wings. Locomotion specifically denotes voluntary movements leading to spatial changes, primarily for seeking food, shelter, mating, better climatic conditions, or protection. Human cellular movements include amoeboid, ciliary, and muscular actions necessitating coordination. Three muscle types—skeletal, visceral, and cardiac—are integral to these processes, exhibiting characteristics such as excitability, contractility, extensibility, and elasticity. Furthermore, muscle contraction operates via a finely balanced sliding filament theory involving actin and myosin proteins, initiated by neuromuscular signals. The skeleton, comprised of bones and cartilages, is categorized into axial and appendicular segments, with various joint types facilitating movement. Understanding these systems is fundamental to grasping the complexities of biological locomotion.

EXERCISES

1. Draw the diagram of a sarcomere of skeletal muscle showing different regions.

2. Define the sliding filament theory of muscle contraction.

3. Describe the important steps in muscle contraction.

4. Write true or false. If false, change the statement to make it true.

   • (a) Actin is present in the thin filament.

   • (b) The H-zone of striated muscle fibre represents both thick and thin filaments.

   • (c) The human skeleton has 206 bones.

   • (d) There are 11 pairs of ribs in humans.

   • (e) The sternum is present on the ventral side of the body.

5. Write the difference between:

   • (a) Actin and Myosin

   • (b) Red and White muscles

   • (c) Pectoral and Pelvic girdle

6. Match Column I with Column II:

   • Column I

     • (a) Smooth muscle

     • (b) Tropomyosin

     • (c) Red muscle

     • (d) Skull

   • Column II

     • (i) Myoglobin

     • (ii) Thin filament

     • (iii) Sutures

     • (iv) Involuntary

7. What are the different types of movements exhibited by cells in the human body?

8. How do you distinguish between skeletal muscle and cardiac muscle?

9. Name the type of joint between the following:

   • (a) Atlas/Axis

   • (b) Carpal/Metacarpal of thumb

   • (c) Between phalanges

   • (d) Femur/Acetabulum

   • (e) Between cranial bones

   • (f) Between pubic bones in the pelvic girdle.

10. Fill in the blank spaces:

    • (a) All mammals (except a few) have __ cervical vertebrae.

    • (b) The number of phalanges in each limb of humans is __.

    • (c) The thin filament of myofibril contains 2 ‘F’ actins and two other proteins namely and .

    • (d) In a muscle fibre, Ca++ is stored in __.

    • (e) and pairs of ribs are termed floating ribs.

    • (f) The human cranium consists of __ bones.

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