Muscular and Skeletal Disorders

NCERT grounding

NCERT Class 11, Chapter 17 closes with §17.5 — a single page titled “Disorders of Muscular and Skeletal System”. It is the shortest section in the chapter, but the six conditions it lists are the only ones NEET tests directly. The text gives each disorder in one line — a definition, the affected system, and the underlying cause. Nothing else from this section is examinable.

“Myasthenia gravis: auto-immune disorder affecting neuromuscular junction leading to fatigue, weakening and paralysis of skeletal muscle.”

— NCERT Class 11 · §17.5

The NIOS biology lesson (Chapter 16) groups the same six disorders into hereditary, joint and metabolic categories, and adds that osteoporosis can also reflect calcium and vitamin-D deficiency in addition to the oestrogen mechanism. Where NIOS and NCERT differ, NEET follows NCERT phrasing — particularly the auto-immune label on myasthenia gravis and the genetic label on muscular dystrophy.

The six disorders — definitions and causes

The most efficient way to internalise §17.5 is the NCERT line on each disorder, paired with the system it damages and the mechanism class. NEET stems almost always test one of these three fields. The compact grid below holds every NCERT-listed fact verbatim; the deeper sub-sections that follow add only the explanatory context students need to answer matching and assertion–reason questions.

Muscular disorders in depth

Myasthenia gravis

Myasthenia gravis is the textbook example of an auto-immune attack on a synapse. The body produces antibodies that bind to and destroy acetylcholine receptors on the post-synaptic sarcolemma at the neuromuscular junction. Recall from §17.2.2 that a motor neuron releases acetylcholine, which generates an action potential in the sarcolemma; if the receptors are blocked or removed, the action potential fails, Ca²⁺ is not released from the sarcoplasmic reticulum, no cross-bridges form, and the muscle fails to contract. Clinically the patient experiences early fatigue (the few intact receptors are quickly exhausted), progressive weakening through the day and, in severe cases, paralysis of voluntary muscles including those that move the eye and the jaw.

NCERT puts myasthenia gravis in the auto-immune column. NEET 2024 used this exact framing — it asked which of five disorders are auto-immune and accepted myasthenia gravis, rheumatoid arthritis and SLE as the correct set, while explicitly excluding gout (metabolic) and muscular dystrophy (genetic).

Muscular dystrophy

Muscular dystrophy is a group of inherited disorders in which the skeletal muscle progressively degenerates. NCERT gives only the one-line summary — “progressive degeneration of skeletal muscle mostly due to genetic disorder” — and that single fact is sufficient for every NEET stem so far. The condition is the canonical answer to the “which of the following is inherited” question (NEET 2019). It is not auto-immune, even though it superficially resembles myasthenia gravis in producing weakness, and it is not caused by calcium imbalance.

Tetany

Tetany is a calcium-deficiency disorder, not a fixed disease state. When extracellular Ca²⁺ falls (hypocalcaemia), the sodium permeability of motor neuron membranes rises and they fire repetitively even without a CNS signal. The result is rapid, involuntary spasms — described in NCERT as “wild contractions”. Tetany is reversible: restore calcium levels and the spasms stop.

Skeletal and joint disorders in depth

Arthritis

NCERT defines arthritis in a single line — “inflammation of joints”. The umbrella term covers several distinct entities. Osteoarthritis (wear-and-tear cartilage loss) is the most common form. Rheumatoid arthritis is an auto-immune attack on synovial joint linings that NEET groups with myasthenia gravis. Gouty arthritis is the specific joint inflammation caused by uric-acid crystals — which NCERT lists separately as gout. Students need only know the umbrella definition; specific forms come up only when a question explicitly names them.

Osteoporosis

Osteoporosis is the age-related disorder of bone in which bone mass falls and the fracture risk rises. NCERT singles out one cause: decreased levels of oestrogen. Oestrogen normally restrains the activity of bone-resorbing osteoclasts and supports calcium retention in the skeleton. After menopause, oestrogen drops sharply, osteoclast activity outpaces osteoblast deposition, and the trabecular bone thins — vertebrae compress, hips fracture from minor falls. NIOS adds that calcium and vitamin-D deficiency contribute, but NEET 2022 tested only the oestrogen mechanism: the assertion that osteoporosis means decreased bone mass and increased fracture risk is correct, but the reason that names increased oestrogen is wrong. Read the reason clause carefully.

Gout

Gout is a metabolic disorder of purine catabolism. Uric acid — the end product of purine breakdown — normally dissolves in plasma and is excreted by the kidney. When plasma uric acid rises above its solubility limit, monosodium-urate crystals deposit in cooler peripheral joints (classically the metatarsophalangeal joint of the big toe), and they trigger acute, intensely painful inflammation. NCERT positions gout as a separate disorder from arthritis even though it produces joint inflammation. The distinction matters: NEET 2024 listed gout as a non-auto-immune disorder, ruling it out of the auto-immune set.

Classification axes NEET tests

NEET stems pivot on three implicit classification axes that students should be able to apply to any of the six disorders the instant a name appears in an option. Memorising the table below converts these to one-step recalls.

Worked examples

Common confusion & NEET traps