Locomotion & Movement – Study Notes (NEET Biology)
Chapter 12 – Locomotion & Movement
Syllabus:
• Locomotion in animals – fish, amphibians, reptiles, birds, mammals
• Human movement – muscle contraction, neuromuscular junction, motor unit, types of joints & movements
1. Simple Explanation
Locomotion is the ability of an organism to change its position or to move from one place to another.
In animals it evolves into different modes:
- Swimming (fish, amphibians) – undulatory, side‑to‑side waves of body & fins.
- Walking/Running (mammals, birds) – alternating heel‑to‑toe steps, balance by the vestibular system.
- Flying (birds, bats) – rapid wing beats, lift and thrust.
In humans, locomotion is made possible by skeletal muscles acting on the skeleton through joints. Muscle contraction is initiated by an action potential that travels through the motor neuron to the muscle fiber (neuromuscular junction) → release of Ca²⁺ → actin–myosin cross‑bridge cycling → shortening of the muscle.
2. Key Points
| Category | Important Facts |
|---|---|
| Locomotion in animals | • Fish – fin beats, undulatory body waves. • Amphibians – “frog‑leg” jump, tunnelling. • Reptiles – lateral undulation, sidewinding. • Birds – wing beat produces lift & thrust. • Mammals – bipeds vs quadrupeds, gait analysis. |
| Human muscle contraction | • Excitation–Contraction Coupling (action potential → Ca²⁺ release → cross‑bridge). • Motor Unit – one motor neuron + all fibers it innervates. |
| Muscle types | • Skeletal – voluntary, striated, single nucleus. • Cardiac – involuntary, striated, intercalated discs. • Smooth – involuntary, non‑striated, single nucleus. |
| Neuromuscular Junction (NMJ) | • Motor end‑plate, acetylcholine (ACh) release → AChR → Na⁺ influx → depolarisation → action potential. • ACh‑esterase degrades ACh, ending the signal. |
| Motor Unit | • Size (number of fibers) determines strength vs fine control. • Recruitment follows Henneman’s size principle. |
| Joint Types | • Skeletal joints: fibrous (syndesmosis, sutures), cartilaginous (synchondrosis, symphysis), synovial (ball‑and‑socket, hinge, pivot, saddle, gliding, condyloid). |
| Movement types | • Flexion/Extension – decrease/increase angle at a joint. • Abduction/Adduction – away from / toward the body mid‑line. • Rotation – around a vertical axis. • Circumduction – circular motion. |
| Eye & Neck movement | • Eye: 6 extraocular muscles for 3‑dimensional movement (convergence, divergence). • Neck: 7 cervical vertebrae, 7‑joint system, allows flexion/extension, lateral flexion, rotation. |
3. Worked Example
How does a skeletal muscle fibre contract?
-
Neuronal stimulus
- Motor neuron fires → action potential travels down axon to neuromuscular junction.
-
Neuromuscular junction (NMJ)
- Acetylcholine (ACh) released → binds to ACh receptors on sarcolemma.
- Na⁺ influx → depolarisation → creates an action potential in the muscle fibre.
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Propagation along the fibre
- Action potential travels along the sarcolemma → invaginates into the fibre via transverse (T) tubules.
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Calcium release
- Depolarisation activates voltage‑gated Ca²⁺ channels (dihydropyridine receptor) → Ca²⁺ released from sarcoplasmic reticulum (SR) into cytosol.
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Excitation‑contraction coupling
- Ca²⁺ binds to troponin‑C → conformational change shifts tropomyosin off actin’s myosin‑binding sites.
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Cross‑bridge cycle
- Myosin heads (ATPase) bind to actin → power stroke (hydrolysis of ATP → ADP + Pi).
- Actin–myosin cross‑bridges repeatedly cycle, shortening the sarcomere (and the whole muscle).
-
Relaxation
- Ca²⁺ pumped back into SR by SERCA ATPase → troponin‑C releases Ca²⁺ → tropomyosin blocks actin → fibre relaxes.
Result: The muscle shortens, generating force that pulls on the bone, producing movement at the joint.
4. Quick Revision Summary
| Topic | Essential Points |
|---|---|
| Locomotion in animals | Fish – undulation; amphibians – jump; reptiles – side |