Muscle diseases or disorders are medical conditions, which affect the muscular system in our body. Diseases that originate and due to direct impairment of muscles are primary muscular diseases, whereas muscle diseases, which are due to manifestation of other diseases are known as secondary muscular diseases. Muscle weakness or atrophy (wasting of muscles) are the common features of muscular diseases. Let’s learn about some of the common muscle diseases, their causes, symptoms and pathogenesis.
1. Myasthenia gravis
It is a neuromuscular disease causing weakness in skeletal muscles. Myasthenia gravis is an autoimmune disease, where our own immune system attacks the healthy cells.
Here, the transmission of nerve impulses to muscles is interrupted at the neuromuscular junction. Antibodies alter, block or destroy receptors for the acetylcholine (a neurotransmitter) at the neuromuscular junction leading to impaired muscle contraction. Antibodies are produced against the acetylcholine receptors or sometimes against other proteins such as muscle-specific-kinase.
It is often caused due to thymus glands, which remain large and active in adults or may develop tumours of the thymus gland.
Facial muscles are mostly affected. It may lead to drooping of eyelids, double vision, difficulty in swallowing, chewing, talking, etc. Onset may be sudden. In severe cases, it may result in shortness of breath and respiratory failure.
2. Muscular dystrophy
Muscular dystrophy is a group of muscle disorders characterized by progressive wasting of muscles and weakening. It is a genetic disorder, which is either inherited or caused due to mutation.
They have been categorised into 9 main types. Duchenne muscular dystrophy or DMD is the most common type of muscular dystrophy accounting for around half of the cases.
Duchenne muscular dystrophy (DMD) is an inherited X-linked recessive disorder. It is due to the defect in the gene coding for dystrophin. Dystrophin protein present in muscle fibres acts as a shock absorber and provides mechanical stabilization. It also regulates the Ca ion levels, which are essential for muscle contraction.
The gene for dystrophin is present on the X chromosome. The disease is more common in males as males have only one X chromosome, the defective gene leads to a lack of protein dystrophin. It results in muscle degeneration and eventually leading to death.
Females are mostly carriers of the disease, as they have a pair of the X chromosome, so even if a gene on one of the chromosomes is defective the other one suffices the requirement of protein. It is generally not possible for females to have both the X chromosomes with defective genes as males mostly die before being reproductively active.
Muscle weakness usually starts with upper arms and legs and progresses over time affecting other parts of the body. Even normal everyday tasks become difficult to perform. It shortens the lifespan due to heart and respiratory complications as the disease progresses.
Tetanus is caused due to bacterial infection. The causative organism is Clostridium tetani. The disease is also known as ‘lockjaw’ and characterized by painful muscle contractions or spasms.
The bacterium is present in dust, soil, manure and enters the body through a cut or wound in the skin or puncture by a contaminated object. It releases a toxin that interferes with normal muscle contractions and causes spasms.
Tetanus neurotoxin or TeNT blocks the release of neurotransmitters from inhibitory neurons resulting in an uncontrolled firing of motor neurons and difficulty in relaxation of muscles. It causes spasms and spastic paralysis.
Mostly it starts with the jaw, making it difficult to open the mouth or swallow. The spasms continue to the rest of the body. Spasms may be so severe that they can even cause fractures. Other symptoms include fever, headache, sweating, high b.p. and faster heart rate.
The disease can be prevented by vaccination.
Tetany is a metabolic disorder characterized by involuntary and continued muscle contractions. It is mainly caused due to calcium deficiency or hypocalcemia.
Impaired functioning of the parathyroid gland may also be one of the causes. Low levels of potassium (hypokalemia) and magnesium (hypomagnesemia) can also cause spasms.
Symptoms include muscle twitching, cramping, pain, spasms of larynx, vomiting, convulsion and breathing difficulties.
This was a brief note on Disorders of the Muscular System. Explore notes on Disorders of Bones and other important concepts related to NEET, only at BYJU’S.