Chapter Notes

Locomotion and Movement

Movement is a fundamental characteristic of all living things. It can be as simple as the streaming of protoplasm inside a cell, like in Amoeba, or as complex as the coordinated actions of limbs, jaws, and tongue in humans. When a voluntary movement results in a change of location, we call it locomotion. Walking, running, swimming, and flying are all forms of locomotion.

It's important to understand the relationship between movement and locomotion: all locomotions are movements, but all movements are not locomotions. For example, blinking your eyelids is a movement, but it doesn't move you from one place to another.

Organisms use locomotion for various essential purposes, such as searching for food, finding shelter, mating, locating suitable breeding grounds, migrating to better climates, or escaping from predators.

Types of Movement

The cells within the human body show three main kinds of movement:

• Amoeboid Movement: This is a crawling-like movement achieved by pushing out temporary projections called pseudopodia, which are formed by the streaming of protoplasm. This is seen in specialized cells like macrophages and leucocytes (white blood cells) in our blood. Cytoskeletal elements called microfilaments are involved in this process.
• Ciliary Movement: This movement is caused by the coordinated, rhythmic beating of cilia. It occurs in our internal tubular organs that are lined with ciliated epithelium.
  • In the trachea: Cilia help sweep away dust particles and foreign substances that we inhale.
  • In the female reproductive tract: Ciliary movement helps move the ova (eggs) along the tract.
• Muscular Movement: This is the most complex type of movement in multicellular organisms and is responsible for most of our body's movements, including the movement of limbs, jaws, and tongue. It relies on the contractile property of muscles. Locomotion is a result of the perfectly coordinated activity of the muscular, skeletal, and neural systems.

Muscle

Muscle is a specialized tissue of mesodermal origin that makes up about 40-50% of an adult human's body weight. Muscles have several key properties:

• Excitability: They can respond to a stimulus.
• Contractility: They can shorten forcefully.
• Extensibility: They can be stretched.
• Elasticity: They can return to their original length after being stretched.

Muscles are classified based on their location, appearance, and how their activity is controlled. There are three types of muscles:

1. Skeletal Muscle:

   • Location: Attached to the bones of the skeleton.
   • Appearance: They have a striped or striated appearance under a microscope.
   • Control: They are under conscious or voluntary control.
   • Function: Primarily involved in locomotion and changing body posture.

2. Visceral Muscle:

   • Location: Found in the inner walls of hollow visceral organs like the alimentary canal and reproductive tract.
   • Appearance: They are smooth and do not have striations (nonstriated).
   • Control: They are not under conscious control, making them involuntary.
   • Function: Assist in transporting substances, such as food through the digestive tract or gametes through the genital tract.

3. Cardiac Muscle:

   • Location: Found only in the heart.
   • Appearance: They are striated and have a branching pattern.
   • Control: They are involuntary; their activity is not directly controlled by the nervous system.
   • Function: Pumping blood throughout the body.

Structure of Skeletal Muscle

A typical skeletal muscle is an organized structure.

• It consists of many muscle bundles or fascicles.
• These fascicles are held together by a connective tissue layer called fascia.
• Each fascicle contains numerous muscle fibres.

A single muscle fibre is a long, cylindrical cell with specific features:

• Sarcolemma: The plasma membrane of the muscle fibre.
• Sarcoplasm: The cytoplasm of the muscle fibre.
• Syncitium: A muscle fibre is a syncitium, meaning it contains many nuclei.
• Sarcoplasmic Reticulum: The endoplasmic reticulum of the muscle fibre, which is a storehouse for calcium ions ($Ca^{++}$).
• Myofibrils (or Myofilaments): The sarcoplasm contains a large number of parallelly arranged filaments called myofibrils.

The characteristic striated appearance of skeletal muscle comes from the pattern of alternating dark and light bands on the myofibrils. This pattern is created by the arrangement of two important contractile proteins: Actin (thin filaments) and Myosin (thick filaments).

The Sarcomere: The Functional Unit of Contraction

• I-band (Isotropic band): The light band, which contains only actin filaments.
• A-band (Anisotropic band): The dark band, which contains myosin filaments. The ends of the actin filaments overlap with the myosin filaments in the A-band.
• Z-line: An elastic fibre in the center of each I-band that bisects it. The thin actin filaments are firmly attached to the Z-line.
• M-line: A thin fibrous membrane in the middle of the A-band that holds the thick myosin filaments together.
• H-zone: The central part of the thick filament (A-band) that is not overlapped by thin filaments in a resting muscle.
• Sarcomere: The portion of the myofibril between two successive Z-lines. It is considered the functional unit of muscle contraction.

Structure of Contractile Proteins

• Actin (Thin Filament): An actin filament is made of two helical 'F' (filamentous) actins. Each 'F' actin is a polymer of 'G' (Globular) actins. Two other proteins are also part of the thin filament:

  • Tropomyosin: Two filaments of this protein run along the F-actins.
  • Troponin: A complex protein distributed at regular intervals on the tropomyosin. In a resting muscle, a subunit of troponin masks the active binding sites for myosin on the actin filaments.

• Myosin (Thick Filament): A myosin filament is a polymer of many monomeric proteins called Meromyosins. Each meromyosin has two parts:

  • Heavy Meromyosin (HMM): Consists of a globular head and a short arm. This part projects outwards from the filament and is known as the cross arm. The head is an active ATPase enzyme and has binding sites for ATP and actin.
  • Light Meromyosin (LMM): The tail portion of the meromyosin.

Mechanism of Muscle Contraction

The mechanism of muscle contraction is explained by the sliding filament theory. This theory states that muscle contraction occurs when the thin filaments (actin) slide past the thick filaments (myosin).

The process involves a series of coordinated steps:

1. Signal Initiation: The central nervous system (CNS) sends a signal via a motor neuron. A motor unit consists of a single motor neuron and all the muscle fibres it connects to.
2. Neurotransmitter Release: The signal reaches the neuromuscular junction (or motor-end plate), the synapse between the motor neuron and the muscle fibre's sarcolemma. A neurotransmitter, Acetylcholine, is released.
3. Action Potential: The acetylcholine generates an action potential (an electrical signal) in the sarcolemma.
4. Calcium Ion Release: The action potential spreads throughout the muscle fibre and causes the sarcoplasmic reticulum to release calcium ions ($Ca^{++}$) into the sarcoplasm.
5. Unmasking of Active Sites: The increase in $Ca^{++}$ levels causes calcium to bind to a subunit of troponin on the actin filaments. This binding changes the shape of troponin, which in turn moves tropomyosin, exposing the active sites on the actin filaments that myosin can bind to.
6. Cross-Bridge Formation: The myosin head, energized by the hydrolysis of ATP into ADP and $P_i$, binds to the exposed active site on the actin, forming a cross bridge.
7. Power Stroke: The binding causes the myosin head to pivot. This pulls the attached actin filament towards the center of the A-band. Since the Z-lines are attached to the actin filaments, they are also pulled inwards. This causes the sarcomere to shorten, which we see as muscle contraction. During contraction, the I-bands get reduced, while the A-bands retain their length.
8. Cross-Bridge Detachment: A new ATP molecule binds to the myosin head, causing the cross-bridge to break.
9. Reactivation of Myosin: The ATP is again hydrolyzed by the ATPase activity of the myosin head, re-energizing it for the next cycle.
10. Relaxation: The cycle of cross-bridge formation and breakage continues as long as $Ca^{++}$ ions and ATP are present. When the nerve signal stops, $Ca^{++}$ ions are pumped back into the sarcoplasmic reticulum. This causes the troponin-tropomyosin complex to once again mask the active sites on actin. The Z-lines return to their original position, and the muscle relaxes.

Muscle Fatigue and Fibre Types

• Fatigue: Repeated activation of muscles can lead to the accumulation of lactic acid from the anaerobic breakdown of glycogen, causing fatigue.
• Myoglobin: Muscles contain a red-colored, oxygen-storing pigment called myoglobin.
  • Red Fibres: These muscles have high myoglobin content, giving them a reddish look. They also contain plenty of mitochondria and rely on aerobic respiration for ATP production. They are suited for sustained activity.
  • White Fibres: These muscles have very little myoglobin and appear pale or whitish. They have few mitochondria but a high amount of sarcoplasmic reticulum. They depend on anaerobic processes for energy and are suited for fast, short-duration contractions.

Skeletal System

The skeletal system provides a framework of bones and a few cartilages. It plays a crucial role in movement, provides support and protection, and is essential for actions like chewing and walking.

• Bone: A specialized connective tissue with a very hard matrix due to calcium salts.
• Cartilage: A specialized connective tissue with a slightly pliable matrix due to chondroitin salts.

The human skeleton consists of 206 bones and is divided into two main parts: the axial skeleton and the appendicular skeleton.

Axial Skeleton

The axial skeleton (80 bones) forms the main axis of the body. It includes the skull, vertebral column, sternum, and ribs.

• The Skull (22 bones):

  • Cranial Bones (8): Form the cranium, the hard protective covering for the brain.
  • Facial Bones (14): Form the front part of the skull.
  • Hyoid Bone (1): A U-shaped bone at the base of the buccal cavity.
  • Ear Ossicles (3 in each ear): Tiny bones in the middle ear – Malleus, Incus, and Stapes.
  • The skull articulates with the vertebral column via two occipital condyles (a dicondylic skull).

• The Vertebral Column (26 vertebrae):

  • Forms the main framework of the trunk, extending from the base of the skull.
  • Each vertebra has a hollow central portion (neural canal) through which the spinal cord passes.
  • It protects the spinal cord, supports the head, and serves as an attachment point for ribs and back muscles.
  • The regions of the vertebral column are:
    • Cervical (7 vertebrae): In the neck. Almost all mammals have 7 cervical vertebrae. The first vertebra is the atlas.
    • Thoracic (12 vertebrae): In the chest region.
    • Lumbar (5 vertebrae): In the lower back.
    • Sacral (1, fused): Formed by the fusion of vertebrae.
    • Coccygeal (1, fused): The tailbone, formed by fused vertebrae.

• Sternum and Ribs:

  • Sternum: A flat bone on the ventral midline of the thorax.
  • Ribs (12 pairs): Thin, flat bones connected dorsally to the thoracic vertebrae.
    • Each rib is bicephalic, meaning it has two articulation surfaces on its dorsal end.
    • True Ribs (Pairs 1-7): Attach directly to the sternum via hyaline cartilage.
    • Vertebrochondral (False) Ribs (Pairs 8-10): Do not attach directly to the sternum but join the seventh rib.
    • Floating Ribs (Pairs 11-12): Are not connected ventrally at all.
  • Rib Cage: Formed by the thoracic vertebrae, ribs, and sternum.

Appendicular Skeleton

The appendicular skeleton consists of the bones of the limbs and their girdles.

• Bones of the Limbs (Each limb has 30 bones):

  • Forelimb (Arm/Hand): Humerus, radius and ulna, carpals (wrist bones - 8), metacarpals (palm bones - 5), and phalanges (digits - 14).
  • Hindlimb (Leg): Femur (thigh bone, the longest bone), tibia and fibula, tarsals (ankle bones - 7), metatarsals (5), and phalanges (digits - 14).
  • Patella: A cup-shaped bone that covers the knee ventrally (the kneecap).

• Girdle Bones:

  • Pectoral Girdle (Shoulder Girdle): Connects the upper limbs to the axial skeleton. Each half consists of:
    • Clavicle: The collar bone, a long slender bone with two curvatures.
    • Scapula: A large, triangular flat bone on the dorsal side of the thorax. It has a ridge called the spine that projects as the acromion. The clavicle articulates here. Below the acromion is the glenoid cavity, which articulates with the head of the humerus to form the shoulder joint.
  • Pelvic Girdle (Hip Girdle): Connects the lower limbs to the axial skeleton. It consists of two coxal bones.
    • Each coxal bone is formed by the fusion of three bones: ilium, ischium, and pubis.
    • Acetabulum: A cavity at the point of fusion where the thigh bone (femur) articulates.
    • Pubic Symphysis: The point where the two halves of the pelvic girdle meet ventrally, containing fibrous cartilage.

Joints

Joints are points of contact between bones, or between bones and cartilages. They are essential for all types of movement. Muscles generate force, which is used to cause movement at the joints, where the joint acts as a fulcrum. Joints are classified into three major types based on their structure and movability.

• Fibrous Joints:

  • Movement: Do not allow any movement.
  • Structure: Bones are fused end-to-end with the help of dense fibrous connective tissues.
  • Example: The sutures between the flat skull bones that form the cranium.

• Cartilaginous Joints:

  • Movement: Permit limited movement.
  • Structure: The bones are joined together with cartilage.
  • Example: The joints between adjacent vertebrae in the vertebral column.

• Synovial Joints:

  • Movement: Allow considerable movement.
  • Structure: Characterized by a fluid-filled synovial cavity between the articulating surfaces of the two bones.
  • Function: Crucial for locomotion and many other movements.
  • Examples:
    • Ball and Socket Joint: Between humerus and pectoral girdle (shoulder).
    • Hinge Joint: Knee joint.
    • Pivot Joint: Between the atlas and axis vertebrae in the neck.
    • Gliding Joint: Between the carpals (wrist bones).
    • Saddle Joint: Between the carpal and metacarpal of the thumb.

Disorders of Muscular and Skeletal System

• Myasthenia Gravis: An autoimmune disorder that affects the neuromuscular junction, leading to fatigue, weakening, and paralysis of skeletal muscles.
• Muscular Dystrophy: Progressive degeneration of skeletal muscle, mostly due to a genetic disorder.
• Tetany: Rapid spasms (wild, uncontrolled contractions) in muscles due to low $Ca^{++}$ levels in the body fluid.
• Arthritis: Inflammation of one or more joints.
• Osteoporosis: An age-related disorder characterized by decreased bone mass and increased chances of fractures. A common cause is decreased levels of estrogen.
• Gout: Inflammation of joints caused by the accumulation of uric acid crystals.