What this chapter is about
Locomotion and movement are fundamental properties of living organisms. Movement refers to any change in the position of body parts or internal structures, while locomotion is the voluntary movement of the entire organism from one place to another. This chapter explains how humans and other animals move, focusing on the muscular and skeletal systems that make this possible.
A Class 11 student meets this chapter as part of human physiology because understanding movement requires integrating knowledge of tissues, organs and organ systems. You will learn how muscles contract at the molecular level, the types of muscles present in the body, the structure of the human skeleton, and how joints allow different types of movements.
After studying this chapter, you should be able to describe the structure of a skeletal muscle fibre, explain the sliding filament theory of muscle contraction, name the major bones and joints in the human body, and identify common disorders affecting the musculoskeletal system.
Key ideas
- Movement occurs at cellular, organ and organism levels; locomotion is the coordinated movement of the whole body from place to place, requiring specialised structures like limbs, cilia or flagella.
- Three types of muscle tissue exist in vertebrates: skeletal muscle (voluntary, striated, attached to bones), smooth muscle (involuntary, non-striated, found in visceral organs), and cardiac muscle (involuntary, striated, found only in the heart).
- A skeletal muscle fibre is a multinucleate cell containing myofibrils, which are made of repeating units called sarcomeres; the sarcomere is the functional unit of muscle contraction.
- Muscle contraction follows the sliding filament theory: thin actin filaments slide over thick myosin filaments, shortening the sarcomere; this requires calcium ions, ATP and the regulatory proteins troponin and tropomyosin.
- The human skeleton has 206 bones in adults, divided into the axial skeleton (skull, vertebral column, ribs, sternum) and the appendicular skeleton (limbs and girdles).
- Joints are classified by the degree of movement allowed: fibrous joints (immovable, like skull sutures), cartilaginous joints (slightly movable, like intervertebral discs), and synovial joints (freely movable, like the knee).
- Skeletal disorders include arthritis (joint inflammation), osteoporosis (reduced bone density), gout (uric acid crystals in joints), and muscular dystrophy (progressive muscle weakness).
Formulas and facts to remember
1. Sarcomere boundaries: A sarcomere extends from one Z-line to the next Z-line in a myofibril; it contains A-band (dark), I-band (light) and H-zone.
2. Sliding filament mechanism: Actin filaments slide inward over myosin; the A-band remains constant in length, the I-band and H-zone shorten during contraction.
3. Role of calcium ions: Calcium released from the sarcoplasmic reticulum binds to troponin, exposing active sites on actin for myosin head attachment.
4. ATP requirement: ATP binds to the myosin head, causing it to detach from actin; ATP hydrolysis re-cocks the head for the next power stroke.
5. Axial skeleton count: 80 bones (skull 22, vertebral column 26, ribs 24, sternum 1, hyoid 1, ear ossicles 6).
6. Appendicular skeleton count: 126 bones (upper limbs 60, lower limbs 60, pectoral girdle 4, pelvic girdle 2).
7. Types of synovial joints: Ball-and-socket (shoulder, hip), hinge (elbow, knee), pivot (atlas-axis), gliding (wrist), saddle (thumb base), condyloid (knuckles).
8. Muscle proteins: Actin and myosin are contractile proteins; troponin and tropomyosin are regulatory proteins.
Worked examples
### Example 1: Identifying sarcomere changes during contraction
Question: During muscle contraction, which of the following change: length of A-band, length of I-band, length of H-zone, distance between Z-lines?
Solution:
- The thick myosin filaments define the A-band; since they do not shorten, the A-band length remains constant.
- The I-band contains only thin actin filaments; as actin slides inward, the I-band shortens.
- The H-zone is the central region of the A-band with only myosin; as actin moves inward, the H-zone shortens or may disappear.
- The distance between successive Z-lines (sarcomere length) decreases as contraction pulls Z-lines closer.
Answer: I-band, H-zone and sarcomere length decrease; A-band length stays the same.
### Example 2: Counting vertebrae in the human spine
Question: A student is studying the vertebral column. How many vertebrae are present in each region?
Solution:
- Cervical region (neck): 7 vertebrae.
- Thoracic region (chest): 12 vertebrae, each articulating with a pair of ribs.
- Lumbar region (lower back): 5 vertebrae, large and weight-bearing.
- Sacral region: 5 vertebrae fused into one sacrum in adults.
- Coccygeal region: 4 vertebrae fused into the coccyx (tailbone).
Total: 7 + 12 + 5 + 5 + 4 = 33 vertebrae in a child; in adults the fused sacrum and coccyx count as 2 bones, giving 26 bones.
### Example 3: Identifying joint types
Question: Classify the following joints: (a) joint between skull bones, (b) joint at the elbow, (c) joint at the hip.
Solution:
- (a) Skull bones are joined by fibrous joints called sutures; they allow no movement.
- (b) The elbow is a synovial hinge joint; it permits movement in one plane (flexion and extension).
- (c) The hip is a synovial ball-and-socket joint; the spherical head of the femur fits into the acetabulum, allowing movement in multiple directions.
Answer: (a) fibrous (suture), (b) hinge, (c) ball-and-socket.
Common mistakes
- Thinking the A-band shortens during contraction → The A-band stays constant; only I-band and H-zone shorten.
- Confusing movement with locomotion → Movement is change in position of any body part; locomotion is movement of the whole organism from place to place.
- Believing cardiac muscle is voluntary because the heart beats rhythmically → Cardiac muscle is involuntary; its rhythmic contraction is intrinsic and not under conscious control.
- Counting 206 bones in children → The 206-bone count applies to adults; children have more bones because some fuse later.
- Forgetting calcium's role and only mentioning ATP → Both calcium ions (to expose binding sites) and ATP (for myosin head detachment and re-cocking) are essential for contraction.
Quick revision
- Sarcomere = Z-line to Z-line; contains A-band, I-band, H-zone.
- Sliding filament theory: actin slides over myosin; needs Ca²⁺ and ATP.
- Human adult skeleton: 206 bones (80 axial + 126 appendicular).
- Synovial joints are freely movable; types include ball-and-socket, hinge, pivot.
- Skeletal muscle is voluntary and striated; smooth muscle is involuntary and non-striated; cardiac muscle is involuntary and striated.
- Common disorders: arthritis, osteoporosis, gout, muscular dystrophy.