Electromyogram And Disorders Of Skeletal Muscle Notes

Electromyogram And Disorders Of Skeletal Muscle Definition

Electromyography is the study of the electrical activity of the muscle. Electromyogram (EMG) is the graphical registration of the electrical activity of the muscle.

Electromyography Technique

The cathode ray oscilloscope or a polygraph is used to record electromyography. Two types of electrodes are used for recording the electrical activities of the muscle:

1. The surface electrode or skin electrode for studying the activity of a muscle
2. The needle electrodes for studying the electrical activity of a single motor unit.

Read And Learn More: Medical Physiology Notes

Electromyogram

• The structural basis for electromyography is the motor unit. The electrical potential developed by the activation of one motor unit is called motor unit potential. It lasts for 5-8 milliseconds and has an amplitude of 0.5 mV. It is mostly monophasic.
• Electrical potential recorded from the whole muscle shows smaller potentials if the force of contraction is less. When the force increases, larger potentials are obtained due to the recruitment of more and more motor neurons.

Uses Of Electromyogram: Electromyogram is useful in the diagnosis of neuromuscular diseases such as motor neuron lesions, peripheral nerve injury, and myopathies.

Disorders Of Skeletal Muscles – Myopathy

Myopathy is a muscular disorder in which the dysfunction of muscle fibers leads to muscular weakness. Myopathies may be acquired or genetically derived. These diseases may or may not involve the nervous system. The common diseases of skeletal muscles are:

1. Muscular dystrophy
2. Diseases involving muscle tone
3. Fibrillation and denervation hypersensitivity
4. Myasthenia gravis
5. Lambert-Eaton syndrome
6. McArdle’s disease
7. Mitochondrial myopathy
8. Nemaline myopathy.

1. Muscular Dystrophy:

• Muscular dystrophy is a disease characterized by progressive degeneration of muscle fibers without the involvement of the nervous system. Mostly it has a hereditary origin.
• The muscles fail to regenerate, resulting in progressive weakness and confinement to a wheelchair. Eventually, death occurs. Common types of muscular dystrophy are Duchenne muscular dystrophy and Becker’s muscular dystrophy.
  • Duchenne Muscular Dystrophy:
    • It is a sex-linked recessive disorder. It is due to the absence of a gene product called dystrophic in the X chromosome. Dystrophy is necessary for the stability of sarcolemma. This disease is characterized by degeneration and necrosis of muscle fibers. The degenerated muscle fibers are replaced by fat and fibrous tissue.
    • The common symptom is muscular weakness. Sometimes, there is an enlargement of muscles (pseudo-hypertrophy). In severe conditions, respiratory muscles become weak resulting in difficulty in breathing and death.
  • Becker’s Muscular Dystrophy:
    • It is also a sex-linked disorder. It occurs due to the reduction in quantity or alteration of dystrophic. The common features of this disorder are slow progressive weakness of the legs and pelvis, pseudohypertrophy of calf muscles, difficulty in walking, fatigue, and mental retardation.

2. Diseases Involving Muscle Tone:

• Hypertonia: Hypertonia or hypertonicity is a muscular disease characterized by increased muscle tone and the inability of muscles to stretch.
  • Hypertonia Causes:
    • It occurs in upper motor neuron lesions. During the lesion of the upper motor neuron, inhibition of lower motor neurons (gamma motor neurons in the spinal cord) is lost. It causes exaggeration of lower motor neuron activity resulting in hypertonia.
    • In children, hypertonia is associated with cerebral palsy (a permanent disorder caused by brain damage which occurs at or before birth and is characterized by muscular impairment). Here also, the motor pathway is affected. Such children usually have speech and language delays with a lack of communication skills.
  • Hypertonia and spasticity:
    • Hypertonia may be related to spasticity, but it is present with or without spasticity. Spasticity is a motor disorder characterized by stiffness of certain muscles due to continuous contraction. Hypertonicity is one of the major symptoms of spasticity. The paralysis (complete loss of function) of the muscle due to hypertonicity is called spastic paralysis.
    • In hypertonia, there is resistance to passive movement, and it does not depend on velocity (the speed at which the movement occurs). Whereas in spasticity there is an increase in resistance to sudden passive movement. It is velocity dependent, i.e. faster the passive movement, stronger the resistance.
• Hypotonia: Hypotonia is a muscular disease characterized by decreased muscle tone. The tone of the muscle decreased or is lost. The muscle offers very little resistance to stretch. The muscle becomes flaccid (lack of firmness) and the condition is called flaccidity.
  • Hypotonia Causes:
    • The major cause for hypotonia is lower motor neuron lesion. The paralysis of muscle with hypotonicity is called flaccid paralysis and it results in wastage of muscles.
    • Hypotonia may also occur because of central nervous system dysfunction, genetic disorders or muscular disorders.
    • Some of the clinical conditions associated with hypotonia are:
      • Down syndrome chromosomal disorder characterized by physical and learning disabilities
      • Myasthenia gravis (see below)
      • Kernicterus- brain damage caused by jaundice in infants
      • Congenital cerebellar ataxia (incoordination)
      • Muscular dystrophy
      • Congenital hypothyroidism
      • Hypervitaminosis D
      • Rickets
      • Infant botulism – paralysis due to botulinum toxin.
• Myotonia:
  • It is a congenital disease characterized by continuous contraction of the muscle and slow relaxation even after the cessation of voluntary act. The main feature of this disease is muscle stiffness, which is sometimes referred as cramps. Muscle relaxation is delayed.
  • This type of muscular stiffness with delayed relaxation causes discomfort during simple actions like walking, grasping, and chewing. The muscles are enlarged (hyper-trophy) because of the continuous contraction. Myotonia sets in during early to late childhood and it is not progressive.
    • Myotonia Cause: Myotonia is caused by a mutation in the genes of channel proteins in the sarcolemma. Such disorders are called channelopathies.

Myotonia is of two types:

1. Becker type myotonia
2. Thomsen-type myotonia.

• Becker-type myotonia is also called generalized myotonia, and it is more common than Thomsen-type myotonia. It is an autosomal recessive disorder produced by defective genes contributed by both parents.
• Thomsen-type myotonia is relatively rare, and it is an autosomal recessive disorder produced by a defective gene contributed by one parent.

3. Fibrillation And Denervation Hypersensitivity:

The denervation of a skeletal muscle (lower motor neuron lesion) causes fibrillation with flaccid paralysis and denervation hypersensitivity.

• Fibrillation: Fibrillation means fine, irregular contractions of individual muscle fibers.
• Denervation Hypersensitivity: After denervation, the muscle becomes highly sensitive to acetylcholine, which is released from the neuromuscular junction. It is called denervation hypersensitivity.

4. Myasthenia Gravis: Myasthenia gravis is an autoimmune disease of neuromuscular junction caused by antibodies to cholinergic receptors. It is characterized by grave weakness of the muscle due to the inability of the neuromuscular junction to transmit impulses from the nerve to the muscle. It is a serious and sometimes fatal disease.

• Myasthenia Gravis Causes:
  • Myasthenia gravis is due to the development of auto-antibodies (IgG autoantibodies) against the receptors of acetylcholine. That is, the body develops antibodies against its own acetylcholine receptors.
  • These antibodies destroy the acetylcholine receptors. So, though the acetylcholine release is normal, it cannot act because of the destruction of the receptors
• Myasthenia Symptoms:
  • Slow and weak muscular contraction because of the defective neuromuscular activity
  • Inability to maintain the prolonged contraction of skeletal muscle
  • Quick fatigability when the patient attempts repeated muscular contractions
  • Weakness and fatigability of arms and legs
  • Double vision and droopy eyelids due to the weakness of ocular muscles
  • Difficulty in swallowing due to weakness of throat muscles
  • Difficulty in speech due to the weakness of muscles of speech

In severe conditions, there is paralysis of muscles. The patient dies mostly due to the paralysis of the respiratory muscles.

5. Lambert-Eaton Syndrome:

• It is a disorder of neuromuscular junction caused by the development of antibodies against the calcium channels in the nerve terminal resulting in a reduction in the release of quanta of acetylcholine. This disease is commonly associated with carcinoma.
• So, it is also called carcinomatous myopathy. This disease is characterized by several features of myasthenia gravis. In addition, Fibrillation means fine irregular contractions of individual patients who have blurred vision and dry mouth.
  muscle fibers.

6. Mcardle’s Disease: McArdle’s disease is a glycogen storage disease (accumulation of glycogen in muscles) due to the mutation of genes involving the muscle glycogen phosphorylate necessary for the breakdown of glycogen in muscles. Muscular pain and stiffness are the common features of this disease.

7. Mitochondrial Myopathy: It is an inherited disease due to the defects in the mitochondria (which provide critical source of energy) of muscle fibers.

8. Nemaline Myopathy: It is a congenital myopathy characterized by microscopic changes and the formation of small rod-like structures in the muscle fibers. It is also called nemaline rod myopathy. The features are delayed development of motor activities and weakness of muscles.