Important Diagrams: Locomotion and Movement | Biology Class 11 - NEET PDF Download

1. Structure of Skeletal Muscle

Skeletal muscles consist of muscle bundles (fascicles) held together by fascia, containing muscle fibres lined by sarcolemma and rich in myofibrils with alternating dark and light bands due to actin and myosin proteins. The sarcoplasmic reticulum stores calcium ions, and the muscle fibre is multinucleated.

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

2. Sarcomere

Diagrammatic representation of (a) anatomy of a muscle fibre showing a sarcomere (b) a sarcomere

3. Structure of Contractile Proteins

(a) An actin (thin) filament (b) Myosin monomer (Meromyosin)

• Sliding Filament Theory: Muscle contraction occurs as thin actin filaments slide over thick myosin filaments.
• Initiation:A signal from the central nervous system (CNS) travels via a motor neuron. The motor neuron and connected muscle fibres form a motor unit. The junction between the neuron and muscle is called the neuromuscular junction or motor-end plate.
• Neural Signal: The signal reaches the junction, releasing acetylcholine, a neurotransmitter. This generates an action potential in the muscle fibre's sarcolemma (membrane). The action potential spreads, causing the release of calcium ions (Ca²⁺)into the sarcoplasm.
• Calcium and Troponin: Increased calcium binds to troponin on actin filaments, exposing myosin-binding sites by removing the mask.
• Cross-Bridge Formation: Myosin heads bind to the exposed actin sites, forming cross-bridges using energy from ATP hydrolysis. Myosin heads pull actin filaments toward the center of the A-band, shortening the sarcomere (contraction).
• Changes During Contraction: I-bands shorten, while A-bands retain their length.
• Relaxation: Myosin releases ADP and P₁, detaching from actin, returning to its relaxed state. New ATP binds to myosin, breaking the cross-bridge, and the cycle continues until calcium is pumped back into the sarcoplasmic reticulum. The sarcomere returns to its original position, causing muscle relaxation.
• Fatigue: Repeated muscle activation leads to lactic acid accumulation due to anaerobic glycolysis, causing fatigue.
• Muscle Types: Red fibres: High in myoglobin, many mitochondria, use oxygen for aerobic respiration, and appear reddish. White fibres: Low in myoglobin, fewer mitochondria, rely on anaerobic processes for energy, and appear pale.

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

Sliding-filament theory of muscle contraction (movement of the thin filaments and the relative size of the I band and H zones)

5. Axial Skeleton

The axial skeleton consists of 80 bones along the body’s main axis, including the skull, vertebral column, sternum, and ribs. The skull has 22 bones, with 8 cranial bones protecting the brain and 14 facial bones forming the front. The hyoid bone is located at the base of the buccal cavity, and each ear contains three ear ossicles (malleus, incus, stapes). The vertebral column has 26 vertebrae, divided into cervical, thoracic, lumbar, sacral, and coccygeal regions, protecting the spinal cord. The sternum is a flat bone, and the rib cage consists of 12 pairs of ribs, with 7 true ribs, 3 false ribs, and 2 floating ribs, all contributing to the thoracic structure.

Diagrammatic view of human skull

Vertebral column (right lateral view)

Ribs and rib cage

6. Appendicular Skeleton

Q1:  Which of the following statements are correct regarding skeletal muscle? (NEET 2023)
A. Muscle bundles are held together by collagenous connective tissue layer called fascicle.
B. Sarcoplasmic reticulum of muscle fibre is a store house of calcium ions.
C. Striated appearance of skeletal muscle fibre is due to distribution pattern of actin and myosin proteins.
D. M line is considered as functional unit of contraction called sarcomere.
Choose the most appropriate answer from the options given below:
(a) A, B and C only 
(b) B and C only
(c) A, C and D only 
(d) C and D only
Ans: (b)

Option (B) is the correct answer because statements B and C are only correct statements while A and D are incorrect statements.Muscle bundles are held together by collagenous connective tissue layer called fascia. Muscle bundles are called fascicles. The portion of the myofibril between two successive ‘Z’ lines is considered as functional unit of contraction called sarcomere.

Q2: According to the sliding filament theory:   (NEET 2022 Phase 2)
(a) The actin filaments slide away from A-band resulting in shortening of sarcomere.
(b) Actin and myosin filaments slide over each other to increase the length of the sarcomere.
(c) Length of A-band does not change
(d) I-band increases in length.
Ans: (c)