Locomotion and Movement - Comprehensive Study Notes

Fundamental Principles of Movement and Locomotion

Distinction Between Movement and Locomotion:
- Movement: Movement is one of the most significant features displayed by living organisms. It includes streaming of protoplasm in unicellular organisms (e.g., Amoeba), as well as movement of cilia, flagella, tentacles, limbs, jaws, eyelids, and tongue in multicellular organisms.
- Locomotion: Voluntary movements that result in a change of place or location are categorized as locomotion. Examples include walking, running, climbing, flying, and swimming.
- Interrelationship: All locomotions are movements, but all movements are not locomotions.
Locomotry Structures across Taxa:
- Locomotry structures often serve dual roles in other non-locomotry physiological functions.
- In Paramoecium, cilia facilitate both the movement of food through the cytopharynx and organismal locomotion.
- In Hydra, tentacles are utilized for capturing prey as well as for locomotion.
- In humans, limbs are used both to alter body posture and to achieve locomotion.
Biological Purpose of Locomotion:
- Locomotion is generally driven by environmental demands and habitat requirements. Key purposes include:
- Searching for food and shelter.
- Finding a mate.
- Locating suitable breeding grounds.
- Seeking favorable climatic conditions.
- Escaping from enemies or predators.
Systemic Coordination:
- Locomotion requires the synchronized, coordinated interaction of three organ systems: the muscular system, the skeletal system, and the neural system.

Cellular Mechanisms and Types of Movement

Amoeboid Movement:
- Exhibited by specialized human cells such as macrophages and leucocytes (white blood cells).
- Driven by pseudopodia, which are formed by the streaming movement of protoplasm.
- Cytoskeletal elements, particularly microfilaments, actively participate in generating amoeboid movement.
Ciliary Movement:
- Occurs in most internal tubular organs lined by ciliated epithelium.
- Trachea: Coordinated, rhythmic beating of cilia sweeps out dust particles and inhaled foreign substances from the respiratory tract.
- Female Reproductive Tract: Ciliary movements within the fallopian tubes facilitate the passage of the ovum toward the uterus.
Muscular Movement:
- Required for movements involving large body structures such as limbs, jaws, and tongue.
- Leverages the contractility of muscular tissue. Used extensively by humans and most multicellular organisms for locomotion and postural adjustment.
Flagellar Movement:
- Driven by cellular outgrowths of the plasma membrane.
- Essential for:
- Swimming of spermatozoa.
- Maintenance of fluid currents within the canal system of sponges.
- Locomotion in flagellated protists such as Euglena.

Muscle Tissue Classification and Anatomy

Tissue Origin and Quantity:
- Muscle is a specialized tissue derived from the mesoderm.
- Accounts for approximately $40-50\%$ of total body weight in a healthy adult human.
Fundamental Functional Properties:
- Excitability: Ability to respond to neural stimuli.
- Contractility: Ability to shorten forcibly.
- Extensibility: Ability to be stretched.
- Elasticity: Ability to recoil back to resting length.
Classification of Muscles:
- Muscles are classified according to location, microscopic appearance, and physiological regulation:
1. Skeletal Muscle:
  - Location: Closely associated with skeletal bones.
  - Appearance: Displays distinct dark and light cross-striations under light microscopy; termed striated muscle.
  - Regulation: Controlled voluntarily by the somatic nervous system; termed voluntary muscle.
  - Primary Function: Locomotion and maintenance/change of body posture.
2. Visceral Muscle:
  - Location: Situated in the inner walls of hollow internal organs (e.g., alimentary canal, reproductive tract).
  - Appearance: Lacks striations, appearing smooth; termed smooth or non-striated muscle.
  - Regulation: Functioning is involuntary and not under direct conscious neural control; termed involuntary muscle.
  - Primary Function: Transporting materials such as food through the digestive tract and gametes through the genital tract.
3. Cardiac Muscle:
  - Location: Restricted exclusively to the wall of the heart.
  - Structure: Cells assemble in a branching, interconnected pattern.
  - Appearance: Striated.
  - Regulation: Involuntary; regulated via autonomous cardiac pacemakers and autonomic nervous inputs.

Structural Hierarchy of Skeletal Muscle and Sarcomere Anatomy

Gross Anatomical Organization:
- Each skeletal muscle consists of multiple muscle bundles called fascicles.
- Fascicles are bound together by a common layer of tough, collagenous connective tissue called fascia.
- Each fascicle contains numerous individual, elongated muscle cells known as muscle fibres.

Diagrammatic cross sectional view of a muscle showing muscle bundles and muscle fibres

Microscopic Structure of the Muscle Fibre:
- Sarcolemma: The plasma membrane that envelopes the muscle fibre.
- Sarcoplasm: The cytoplasm enclosed by the sarcolemma.
- Syncytium: Muscle fibres are multi-nucleated (syncytial) because the sarcoplasm contains numerous nuclei.
- Sarcoplasmic Reticulum: Specialized smooth endoplasmic reticulum that acts as the primary intracellular storage facility for calcium ions ( $\text{Ca}^{2+}$ ).
Myofibrils and Filament Layout:
- The sarcoplasm is densely packed with parallelly arranged rod-like filaments called myofilaments or myofibrils.
- Alternating light and dark bands along the myofibril give skeletal muscle its striated appearance.
- Striations are created by the organized arrangement of two primary contractile proteins: Actin and Myosin.
Myofilament Types:
- Thin Filaments (Actin): Thinner strands located primarily within the light band.
- Thick Filaments (Myosin): Thicker strands located primarily within the dark band.
- I Band (Isotropic / Light Band): Contains only thin (actin) filaments. Bisected centrally by a dense elastic line called the Z line (Z-disc). Thin filaments are anchored firmly to the Z line.
- A Band (Anisotropic / Dark Band): Contains thick (myosin) filaments along its entire length, overlapping partially with thin filaments at its outer margins. Thick filaments are held together centrally by a thin fibrous membrane called the M line.
The Sarcomere:
- Defined as the segment of a myofibril bounded between two successive Z lines.
- Represents the basic functional unit of muscle contraction.
- Formed of one complete A band flanked on either side by two half-I bands.
The H Zone:
- In a resting, non-contracted muscle fibre, thin filaments overlap the outer ends of thick filaments, leaving the central region of the thick filaments un-overlapped.
- This central un-overlapped portion of the A band is designated as the H zone.

Molecular Structure of Contractile and Regulatory Proteins

Structure of Thin Filaments (Actin):
- Consists of two polymeric strands of F-actin (filamentous actin) spirally wound around each other in a double helix.
- Each F-actin is a polymer of globular monomeric subunits known as G-actin (Globular actin).
- Tropomyosin: Two strands of the fibrous protein tropomyosin run close to and parallel with the F-actin strands along their entire length.
- Troponin: A complex globular protein attached at regular intervals along tropomyosin strands.
- Resting Function: In an unstimulated muscle, a subunit of troponin physically masks the active myosin-binding sites on the G-actin monomers.

Structure of Thick Filaments (Myosin):
- Each thick filament is a polymer composed of monomeric protein units called Meromyosins.
- Anatomy of Meromyosin:
- Heavy Meromyosin (HMM): Consists of a globular head and a short projecting arm. Projects outward at regular distances and angles from the body of the filament, forming the cross arm.
- Light Meromyosin (LMM): Consists of the elongated tail portion.
- Enzymatic and Binding Properties of the Globular Head:
- Functions as an active ATPase enzyme that hydrolyzes ATP.
- Contains specific ATP-binding sites.
- Contains active actin-binding sites designed to bind with exposed active sites on thin filaments.

Sliding Filament Model of Muscle Contraction

Core Concept:
- The mechanism of muscle contraction is described by the Sliding Filament Theory, which establishes that contraction occurs when thin (actin) filaments slide inward over thick (myosin) filaments toward the center of the sarcomere.
Step-by-Step Mechanism of Contraction:
1. Neural Signal Generation:
- Contraction is initiated by a nerve impulse sent from the Central Nervous System (CNS) down a motor neuron.
- Motor Unit: A single motor neuron together with all the individual muscle fibres it innervates.
- Neuromuscular Junction (Motor-End Plate): The synaptic junction between the terminal axon of a motor neuron and the sarcolemma of a muscle fibre.
1. Depolarization and Calcium Release:
- Arrival of the action potential at the axon terminal causes the release of the neurotransmitter Acetylcholine into the synaptic cleft.
- Acetylcholine binding generates an action potential across the sarcolemma.
- The action potential propagates into the muscle fibre along the sarcoplasmic network, causing the release of large quantities of calcium ions ( $\text{Ca}^{2+}$ ) from the sarcoplasmic reticulum into the sarcoplasm.
1. Unmasking Active Sites:
- Sarcoplasmic $\text{Ca}^{2+}$ binds to a specific subunit of troponin on the thin filaments.
- This binding induces a conformational change in troponin, shifting tropomyosin away and exposing the active binding sites for myosin on actin.
1. Cross-Bridge Formation and Power Stroke:
- Energized by the hydrolysis of ATP ( $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$ ), the myosin head attaches to the exposed active site on actin, forming a cross bridge.

Stages in cross bridge formation, rotation of head and breaking of cross bridge

 - The myosin head undergoes a conformational power stroke, pulling the attached actin filaments inward toward the M line at the center of the A band.
 - Attached Z lines are drawn inward, shortening the sarcomere and contracting the muscle cell.

Dimensional Changes in Sarcomere Bands:
- I Band: Shortens significantly.
- H Zone: Decreases in width and vanishes completely during maximal contraction.
- A Band: Maintains its exact baseline length throughout contraction.
Cross-Bridge Detachment and Cycle Repetition:
- Myosin releases $\text{ADP}$ and $\text{P}_i$ , returning to its low-energy state.
- A new ATP molecule binds to the myosin head, inducing detachment of the myosin head from actin and breaking the cross bridge.
- The new ATP is hydrolysed, re-cocking the myosin head.
- This cycle of cross-bridge formation, power stroke, detachment, and re-cocking continues repeatedly as long as sarcoplasmic $\text{Ca}^{2+}$ remains elevated and ATP supply is sufficient.
Muscle Relaxation:
- When motor signals cease, $\text{Ca}^{2+}$ ions are actively pumped back into the sarcoplasmic cisternae.
- Removal of $\text{Ca}^{2+}$ allows troponin to restore its resting conformation, masking the active binding sites on actin.
- Thin filaments slide back outward, Z lines return to their initial position, and the muscle relaxes.

Muscle Metabolism and Fibre Types

Muscle Fatigue:
- Strenuous, repeated muscle contraction leads to fatigue.
- Fatigue is caused by the accumulation of lactic acid, which is generated through the anaerobic breakdown of glycogen stores within the muscle.
Comparative Breakdown of Red vs. White Muscle Fibres:

Overview of the Human Skeletal System

Composition:
- Composed of a structural framework containing $206$ distinct bones and a few cartilages.
Matrix Properties:
- Bone: Contains an extremely hard, rigid matrix heavily impregnated with calcium salts.
- Cartilage: Possesses a firm but pliable matrix reinforced by chondroitin salts.
Anatomical Divisions:
- The skeletal system is divided into two primary structural components:
1. Axial Skeleton: Contains $80$ bones arranged along the main longitudinal body axis.
2. Appendicular Skeleton: Contains $126$ bones comprising upper and lower limbs, along with their supporting pectoral and pelvic girdles.

Detailed Breakdown of the Axial Skeleton

The Skull (22 Bones + Associated Structures):
- Composed of two sets of bones totaling $22$ bones:
- Cranial Bones ( $8$ ): Form the protective braincase (cranium). Consists of:
  - Frontal ( $1$ )
  - Parietal ( $2$ )
  - Temporal ( $2$ )
  - Occipital ( $1$ )
  - Sphenoid ( $1$ )
  - Ethmoid ( $1$ )
- Facial Bones ( $14$ ): Form the anterior structural framework of the face. Consists of:
  - Nasal ( $2$ )
  - Maxilla ( $2$ )
  - Zygomatic / Cheek ( $2$ )
  - Lacrimal ( $2$ )
  - Palatine ( $2$ )
  - Inferior Nasal Conchae ( $2$ )
  - Vomer ( $1$ )
  - Mandible / Lower Jaw ( $1$ )

Hyoid Bone: A single U-shaped bone positioned at the base of the buccal cavity; does not articulate directly with any other bone.
Ear Ossicles ( $6$ total): Each middle ear housing contains three tiny bones: Malleus, Incus, and Stapes ( $3 \times 2 = 6$ ).
Dicondylic Articulation: The skull articulates with the first vertebra via two rounded projection processes called occipital condyles (termed a dicondylic skull).
- The Vertebral Column (26 Units):
Positioned dorsally; formed by $26$ serially arranged bony vertebrae extending from the skull base to the tail region.
Neural Canal: A hollow central canal running through each vertebra through which the spinal cord passes.
Atlas: The first cervical vertebra ( $\text{C}_1$ ); articulates directly with the occipital condyles.
Regional Differentiation:
- Cervical Vertebrae: $7$ in number (consistently $7$ across almost all mammals, including humans).
- Thoracic Vertebrae: $12$ in number; articulate with ribs.
- Lumbar Vertebrae: $5$ in number; support the lower back.
- Sacral Vertebra: $1$ fused unit (formed from $5$ fused embryonic sacral vertebrae).
- Coccygeal Vertebra: $1$ fused unit (formed from $4$ fused embryonic coccygeal vertebrae, forming the tailbone).

Functions: Encloses and protects the spinal cord, provides structural support for the head, and serves as an attachment point for ribs and back musculature.
- The Sternum (1 Bone):
A single flat bone situated along the ventral midline of the thoracic cage.
- The Ribs (12 Pairs / 24 Bones):
Thin, flat curved bones attached dorsally to thoracic vertebrae and extending ventrally toward the sternum.
Bicephalic Nature: Each rib features two distinct articulation surfaces on its dorsal head end, making it bicephalic.
Structural Categorization of Ribs:
- True Ribs (Pairs 1–7): Attach dorsally to thoracic vertebrae and ventrally directly to the sternum via strips of hyaline cartilage.
- Vertebrochondral / False Ribs (Pairs 8, 9, and 10): Do not articulate directly with the sternum; their anterior ends fuse with the hyaline cartilage of the 7th rib.
- Floating Ribs (Pairs 11 and 12): Possess no ventral attachment; remain unattached anteriorly to protect the kidneys.

Rib Cage: Formed by the combined structure of the 12 thoracic vertebrae, 12 pairs of ribs, and the sternum.

Detailed Breakdown of the Appendicular Skeleton

Bones of the Forelimb (30 Bones per Arm = 60 Total):
- Humerus ( $1$ ): Upper arm bone.
- Radius ( $1$ ) & Ulna ( $1$ ): Forearm bones.
- Carpals ( $8$ ): Wrist bones.
- Metacarpals ( $5$ ): Palm bones.
- Phalanges ( $14$ ): Digit bones (finger formula: $2$ in thumb, $3$ in each of the four fingers).

Bones of the Hindlimb (30 Bones per Leg = 60 Total):
- Femur ( $1$ ): Thigh bone; the longest, strongest bone in the human body.
- Patella ( $1$ ): Knee cap; a cup-shaped sesamoid bone that protects the knee joint ventrally.
- Tibia ( $1$ ) & Fibula ( $1$ ): Lower leg bones.
- Tarsals ( $7$ ): Ankle bones.
- Metatarsals ( $5$ ): Sole/foot bones.
- Phalanges ( $14$ ): Toe bones.

Right pelvic girdle and lower limb bones

Pectoral Girdle (4 Bones):
- Connects upper limbs to the axial skeleton. Composed of two identical symmetrical halves.
- Each Pectoral Half Consists Of:
- Clavicle (Collar Bone): An elongated, slender bone featuring two distinct curvatures.
- Scapula: A large, flat triangular bone positioned on the dorsal surface of the thorax between the $2\text{nd}$ and $7\text{th}$ ribs.
  - Spine: A prominent, elevated ridge on the dorsal flat triangular body of the scapula.
  - Acromion: A flattened, expanded process extending from the spine that articulates with the clavicle.
  - Glenoid Cavity: A shallow depression located below the acromion; articulates with the head of the humerus to form the multi-axial ball-and-socket shoulder joint.
Pelvic Girdle (2 Bones):
- Connects lower limbs to the axial skeleton. Consists of two symmetrical coxal bones (hip bones).
- Composition of Coxal Bone: Each coxal bone is formed by the developmental fusion of three separate bones:
- Ilium
- Ischium
- Pubis
- Acetabulum: A deep, cup-shaped socket formed at the fusion point of the ilium, ischium, and pubis; articulates with the head of the femur to form the hip joint.
- Pubic Symphysis: The ventral junction where the two coxal bones meet; contains a pad of fibrocartilage (fibrous cartilage).

Structural and Functional Classification of Joints

Definition and Role:
- Joints are essential points of contact between bones, or between bones and cartilages.
- They function as fulcrums around which muscle forces are applied to produce movement.
Structural Types:
1. Fibrous Joints (Immovable / Synarthroses):
- Permit no movement between articulating bones.
- Articulating bone edges are interlocking and bound tightly by dense fibrous connective tissue forming sutures.
- Example: Flat cranial bones of the skull fusing to enclose the braincase.
1. Cartilaginous Joints (Slightly Movable / Amphiarthroses):
- Articulating bones are connected securely via pads of cartilage.
- Permit limited, restricted flexibility and movement.
- Example: Intervertebral joints between adjacent vertebrae in the spinal column.
1. Synovial Joints (Freely Movable / Diarthroses):
- Characterized by a fluid-filled synovial cavity enclosed within a fibrous capsule between articulating bone surfaces.
- Synovial fluid reduces friction, providing smooth, free movement essential for locomotion.
Subtypes and Examples of Synovial Joints:

| Synovial Joint Subtype | Structural Description | Anatomical Examples | | :--- | :--- | :--- | | Ball and Socket Joint | Spherical head fits into a cup-like depression | Between Humerus and Glenoid cavity of Pectoral girdle; between Femur and Acetabulum of Pelvic girdle | | Hinge Joint | Convex surface fits into concave surface; motion in single plane | Knee joint; Elbow joint; Interphalangeal joints | | Pivot Joint | Rounded bone rotates within a ring formed by bone and ligament | Between Atlas ( $\text{C}_1$ ) and Axis ( $\text{C}_2$ ) vertebrae | | Gliding Joint | Articulating surfaces are flat, sliding over one another | Between individual Carpals in the wrist; between Tarsals in the ankle | | Saddle Joint | Saddle-shaped bone fitting concave-convex mating bone | Between Carpal and Metacarpal of the thumb |

Pathophysiology and Clinical Disorders

Myasthenia Gravis:
- An autoimmune neuromuscular disorder.
- Antibodies target and impair acetylcholine receptors at the neuromuscular junction.
- Clinical Manifestations: Progressive fatigue, muscle weakness, and paralysis of skeletal muscle.
Muscular Dystrophy:
- A genetic disorder characterized by progressive degeneration and wasting of skeletal muscle tissue.
Tetany:
- Marked by rapid, involuntary muscle spasms and wild contractions.
- Etiology: Uncontrolled neural hyperexcitability caused by low levels of calcium ions ( $\text{Ca}^{2+}$ ) in extracellular body fluids.
Arthritis:
- Inflammatory condition affecting one or more joints, leading to pain, stiffness, and joint destruction.
Osteoporosis:
- An age-related skeletal disorder characterized by severe reduction in bone mineral density and bone mass.
- Bone matrix becomes porous and fragile, dramatically increasing susceptibility to fractures.
- Common Cause: Deficit in circulating estrogen levels, frequently seen in post-menopausal women.
Gout (Gouty Arthritis):
- Form of joint inflammation caused by hyperuricemia.
- Etiology: Deposition and accumulation of sharp uric acid crystals within synovial joint cavities, triggering severe inflammation and joint pain.