ELECTROMYOGRAM AND DISORDER OF SKELETAL MUSCLE.
INTRODUCTION
– The study of electrical activity of the muscle is called Electromyography.
– The graphical representation of electrical activity of muscle is called Electromyogram.
• Electromyography Technique :
-Electromyogram is recorded by the use of cathode ray oscilloscope or a polygraph.
– Electrodes used in this technique are ;
(1) Surface electrode or skin electrode -for studying activity of muscle.
(2) Needle electrodes – for studying activity of single motor unit.
• Electromyogram :
– Motor unit is the structural basis for Electromyogram.
– Motor unit potential is the electrical potential developed by the activation of the motor unit .
– Duration : 5 to 8 milliseconds.
– Amplitude : 0.5 mV.
– It is monophasic.
• Uses of Electromyogram :
– Neuromuscular diseases such as motor neuron lesions , peripheral nerve injury and myopathies can be diagnosed with the help of Electromyogram.
• Disorders of skeletal muscles – Myopathy
– Muscular weakness caused due to dysfunction of muscle fiber is called as Myopathy.
– It can be acquired or genetically derived.
– It may or may not involve nervous system.
✓ Common diseases of Skeletal muscle 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 :
– It is characterized by the progressive degeneration of muscle fibers , without the involvement of nervous system.
– Hereditary in origin.
– Muscle fails to regenerate.
– Progressive weakness confine to wheelchair.
– Eventually death can occur.
– Types of muscular dystrophy ;
(1) Duchenne muscular dystrophy.
(2) Becker muscular dystrophy.
*Duchenne muscular dystrophy :
✓ It is Sex- linked disorder characterized by degeneration and necrosis of muscle fibers.
✓ Caused due to absence of the dystrophin in X chromosome.
✓ Dystrophin is necessary for sarcolemma stability.
✓ Muscular weakness is the common symptom.
✓ In severe cases the respiratory muscles becomes weak which results in difficulty in breathing and death.
* Becker muscular dystrophy :
✓ It is a sex linked disorder occurs due to reduction in quantity of dystrophin .
✓ Features ;
– Slow progressive weakness of legs and pelvis .
– Pseudohypertrophy of calf muscles
– Difficulty in walking
– Fatigue
– Mental retardation.
2) Diseases involving muscle tone :
– It includes ;
√ Hypertonia
√ Hypotonia
√ Myotonia
* Hypertonia :
– Also called hypertono city , characterized by increased muscle tone and inability of muscle to stretch.
– Causes :
✓ During upper motor neuron lesion , inhibition of lower motor neuron is lost and causes exaggeration of lower motor neuron activity resulting in hypertonia.
✓ In children it is associated with cerebral palsy and the motor pathway is affected.
Such children usually have speech and language delays , with lack of communication skills.
– Hypertonia and spasticity :
✓ Hypertonia can occur with or without spasticity.
✓ Spasticity is a motor disorder and due to continuous contraction stiffness of certain muscle can occur .
✓ Paralysis occurs due to hypertono city is called spastic paralysis.
✓ Hypertonia resists passive movements.
* Hypotonia :
– Decrease in muscle tone is called as Hypotonia .
– muscle provide little resistance to stretch.
– Flaccidity can be seen .
– Causes ;
✓ LMN lesion is the main cause.
✓ Flaccid paralysis results in wasting of muscles.
✓ It can occur due to central nervous system dysfunction , genetic disorders or Muscular disorders.
– Clinical condition associated with Hypotonia ;
(1) Down syndrome
(2) Myasthenia gravis
(3) Kernicterus
(4) Congenital cerebellar ataxia
(5) Muscular dystrophy
(6) Congenital hypothyroidism
(7) Hypervitaminosis D
(8) Rickets
(9) Infant botulism
*Myotonia :
– It is a Congenital disease.
– Characterized by continuous contraction of muscle and slow relaxation even after cessation of voluntary act.
– Feature : muscle stiffness sometimes referred as cramps.
✓ More common type of myotonia
✓ Autosomal recessive disorder produced by defective genes contributed by both the parents.
3) Fibrillation and denervation hypersensitivity:
– Skeletal muscle degeneration causes Fibrillation with flaccid paralysis and denervation hypersensitivity.
√ Fibrillation : Fine irregular contraction of individual muscle fibers.
√ Denervation hypersensitivity : Due to degeneration , muscle becomes highly sensitive to acetylcholine released from neuromuscular junction.
4) Myasthenia Gravis :
– Autosomal disease of neuromuscular junction caused by antibodies to cholinergic receptors .
– Grave weakness of muscle due to inability of neuromuscular junction to transmit impulse from nerve to muscle.
– Causes : Due to development of autoantibodies ( IgG autoantibodies ) against the receptors of acetylcholine.
– Symptoms :
✓ Slow and weak muscular contraction
✓ Inability to maintain prolonged skeletal muscle contraction.
✓ Quick fatigability.
✓ Weakness and fatigability of arms and legs.
✓ Double vision and droopy eyelids
✓ Difficulty in swallowing
✓ Difficulty in speech .
✓ In severe cases , there is a paralysis of muscles .
✓ Patient can die due to paralysis of respiratory muscles.
– Treatment : Administration of cholinesterase inhibitors such as neostigmine and pyridostigmine.
5) Lambert – Eaton syndrome :
– It is a Neuromuscular disorder caused by development of antibodies against calcium channel in nerve terminal .
– This disease associated with carcinoma is called carcinomatous myopathy.
– Patient have blurred vision and dry mouth.
6) McArdle’s disease :
– Glycogen storage disease due to mutation of genes.
– It involves muscle glycogen phosphorylase, necessary for breakdown of glycogen in muscles.
– Common features : muscular pain and stiffness .
7) Mitochondrial myopathy :
– It is a inherited disorder occurs due to defect in mitochondria.
8) Nemaline myopathy :
– It is a Congenital myopathy characterized by microscopic changes and small rod like structures can be seen in muscle fibers.
– Also called Nemaline rod myopathy.
– Features : Delayed development of motor activities and weakness of muscles.
