Locomotion and Movement — Study Notes

1. Introduction

Movement is one of the most striking features of living organisms. Movement includes the movement of internal parts of the body, such as the beating of the heart, the movement of food through the alimentary canal, and the movement of the limbs and other external parts. Locomotion, on the other hand, is the movement of an organism from one place to another. While all locomotion is movement, all movement is not locomotion; for example, the movement of the cilia in the respiratory tract is movement but not locomotion.

Movement in animals is brought about by specialised tissues. Cilia, flagella and pseudopodia bring about movement in unicellular organisms, while muscular tissue brings about movement in multicellular animals. In humans, the skeletal system provides the framework for the body, and the muscular system, working against this framework, produces movement and locomotion. This chapter describes the types of movement, the structure and functions of the human skeleton, the mechanism of muscle contraction, and the joints that connect the bones.

2. Types of Movement

• Amoeboid movement: Seen in amoeba and in some cells of the human body such as macrophages and leucocytes. It is brought about by the streaming of protoplasm and the formation of pseudopodia. Amoeboid movement is aided by the cytoskeletal elements, the microfilaments and microtubules.
• Ciliary movement: Seen in the ciliated cells lining the respiratory tract and the fallopian tubes. The rhythmic beating of the cilia moves mucus in the respiratory tract and ova in the fallopian tubes.
• Muscular movement: Brought about by the contraction and relaxation of muscles. It is seen in the movement of limbs, jaws, tongue and heart.

3. Muscle

Muscle is a specialised tissue of mesodermal origin that can contract and relax to bring about movement. About 40 to 50 percent of the body weight of a human adult is contributed by muscles. There are three types of muscles: skeletal (striated, voluntary), smooth (non-striated, involuntary) and cardiac (striated, involuntary).

3.1 Skeletal Muscle Structure

Skeletal muscles are attached to bones and are voluntary. The skeletal muscle is made of muscle fibres (myofibrils), which are the functional units. A muscle fibre is a long, cylindrical, multinucleate cell. Each muscle fibre is surrounded by the sarcolemma (cell membrane) and contains the sarcoplasm (cytoplasm), which is rich in mitochondria and the sarcoplasmic reticulum. The striated appearance is due to the presence of alternating light (I bands) and dark (A bands) bands.

The myofibrils are composed of the contractile proteins actin and myosin, arranged in repeating units called sarcomeres. A sarcomere is the functional unit of contraction, extending from one Z line to the next. The I band contains only thin actin filaments, the A band contains thick myosin filaments with some overlapping actin, and the H zone is the middle region of the A band containing only myosin. The M line lies in the middle of the sarcomere.

3.2 Mechanism of Muscle Contraction

The mechanism of muscle contraction is best explained by the sliding filament theory, which states that during contraction, the thin actin filaments slide over the thick myosin filaments, shortening the sarcomere without changing the length of the filaments themselves. The key events are: 1. The signal for contraction arrives at the neuromuscular junction from a motor neuron. 2. Acetylcholine, a neurotransmitter, is released, which triggers an action potential in the sarcolemma. 3. The action potential travels along the T-tubules into the muscle fibre and stimulates the sarcoplasmic reticulum to release calcium ions (Ca2+). 4. The calcium ions bind to troponin, which causes the tropomyosin molecule to move away, exposing the active sites on the actin filaments. 5. The myosin heads bind to the exposed actin sites, forming cross-bridges, and pull the actin filaments towards the centre, using energy from ATP. 6. This shortens the sarcomere, and the muscle contracts. 7. After the stimulus stops, calcium ions are pumped back into the sarcoplasmic reticulum, the tropomyosin covers the active sites again, and the muscle relaxes.

During contraction, the I band and H zone shorten, but the A band remains unchanged in length.

4. The Skeletal System

The human skeletal system consists of 206 bones and the associated cartilage, along with the joints. It is divided into two main parts: the axial skeleton and the appendicular skeleton.

4.1 Axial Skeleton

The axial skeleton consists of 80 bones, which lie along the main axis of the body. It includes the skull, vertebral column and the rib cage. - Skull: Composed of the cranium (8 bones) that encloses the brain and the facial bones (14 bones). The skull has a total of 22 bones. The hyoid bone (U-shaped) is present at the base of the buccal cavity and is not part of the skull proper. The middle ear contains the malleus, incus and stapes (3 ear ossicles each side). - Vertebral column: Composed of 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused as the sacrum) and 4 coccygeal (fused as the coccyx). The vertebral column is slightly curved and is a "vertical column" supporting the head and trunk. The first vertebra, the atlas, supports the head and allows its nodding, while the second vertebra, the axis, allows rotation of the head. - Sternum and ribs: The sternum is a flat bone on the ventral side. The ribs are 12 pairs: 7 pairs are true ribs (attached to the sternum directly), 3 pairs are false ribs (attached to the cartilage of the 7th rib) and 2 pairs are floating ribs (not attached to the sternum).

4.2 Appendicular Skeleton

The appendicular skeleton consists of 126 bones, which include the bones of the limbs and the girdles. - Pectoral girdle: Each girdle has a clavicle (collar bone) and a scapula (shoulder blade). It connects the upper limbs to the axial skeleton. - Pelvic girdle: Made of the fused ilium, ischium and pubis bones, forming the hip bone. It connects the lower limbs to the axial skeleton. - Upper limb: Each upper limb has the humerus (upper arm), radius and ulna (forearm), carpals (wrist, 8 bones), metacarpals (palm, 5 bones) and phalanges (14 bones in the fingers). - Lower limb: Each lower limb has the femur (thigh, the longest bone), tibia and fibula (shin), tarsals (ankle, 7 bones, including the heel bone calcaneus), metatarsals (5 bones) and phalanges (14 bones in the toes).

5. Joints

Joints are the points where two or more bones meet. They are of three main types: - Fibrous joints: Bones are joined by fibrous tissue, and there is no movement between them. They are found in the sutures of the skull. - Cartilaginous joints: Bones are joined by cartilage, allowing limited movement. They are found between the vertebrae and in the pubic symphysis. - Synovial joints: Bones are separated by a fluid-filled cavity called the synovial cavity, allowing free movement. The synovial fluid lubricates the joint. The synovial joints are of several types: ball and socket joints (shoulder and hip, allowing movement in all directions), hinge joints (knee and elbow, allowing movement in one plane), pivot joints (between the atlas and axis, allowing rotation), gliding joints (between the carpals, allowing sliding movement) and saddle joints (between the thumb and metacarpal).

Quick Revision Tables

Table 1: Types of Muscles

Table 2: Sarcomere Bands

Table 3: Axial vs Appendicular Skeleton

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students say all movement is locomotion; locomotion is a subset of movement, so all locomotion is movement but not vice versa.
2. The A band is said to shorten during contraction; the A band remains constant, while the I band and H zone shorten.
3. The H zone is said to contain both actin and myosin; it contains only myosin filaments.
4. The skull is said to have 22 bones plus the hyoid; the hyoid is not counted in the skull, and the ear ossicles are separate.
5. The sternum has 7 true, 3 false and 2 floating ribs; students often misassign the numbers of rib pairs.
6. The atlas is said to allow rotation of the head; it allows nodding, while the axis allows rotation.
7. Calcium ions are said to bind directly to myosin; they bind to troponin, which moves tropomyosin to expose the actin active sites.
8. Smooth muscle is said to be striated; smooth muscle is non-striated and involuntary.

Exam Tips

1. Distinguish movement vs locomotion with examples (beating of cilia is movement, not locomotion).
2. Sliding filament theory: know the sequence of events from the neuromuscular junction to relaxation, with Ca2+, troponin, tropomyosin, ATP.
3. Sarcomere band changes: I shortens, H shortens, A constant; this is a classic difference question.
4. Bone count: 206 total (80 axial + 126 appendicular); skull 22, vertebral column 33.
5. Joint types with examples: ball and socket (shoulder, hip), hinge (knee, elbow), pivot (atlas-axis), gliding (carpals), saddle (thumb).
6. Vertebral regions: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 coccygeal.
7. The three types of movement (amoeboid, ciliary, muscular) and their examples are easy scoring one-mark questions.

Conclusion

Locomotion and movement are fundamental to the life of animals, enabling them to interact with their environment. The three types of movement, amoeboid, ciliary and muscular, reflect the different mechanisms that cells and organisms use to move. The skeletal muscle, built from the sliding filaments of actin and myosin, demonstrates the elegant molecular machinery that converts chemical energy into mechanical work through the sliding filament mechanism. The human skeleton, divided into the axial and appendicular skeletons, provides the framework that supports the body, protects vital organs and, through its joints, allows a remarkable range of movements. Understanding the structure of bones, muscles and joints is essential for physiology and for appreciating the disorders of the musculoskeletal system, and it connects naturally with the nervous control of movement in the following chapter.