(a) Muscle cramps. Muscle cramps are persistent involuntary
contractions of the skeletal muscles. Muscle cramps can be caused by over-exercise,
lack of blood flow, or severe cold.
(b) Myasthenia gravis. Myasthenia gravis is a major disorder of the
skeletal muscle system. Muscle weakness and excessive fatigue characterize it. In
myasthenia gravis, the muscular system is marked by progressive paralysis of the
muscles, which is caused by an abnormal condition at the neuromuscular junction due
to a lack of acetylcholine or an excess of cholinesterase. If there is either too little
acetylcholine or an excess of cholinesterase, a contraction will not occur.
Cardiac Muscle. The muscles of the heart are called cardiac muscles.
(1) Anatomy. Cardiac muscle is made up of branched, striated fibers and
responds to stimuli as if it were a single muscle fiber. Cardiac tissue is responsible for
the propulsion of blood through the circulatory system. The contraction and relaxation
of the heart move the blood.
(2) Physiology. In order for an individual to live (without the assistance of
life-support equipment), his heart must never stop beating. Cardiac muscle must
maintain a steady rhythm and not become fatigued. Cardiac muscle does not become
fatigued because it can use both glucose and lactic acid, its waste product. The
contraction of the cardiac muscle is involuntary and does not directly respond to any
nervous stimulation. This property is referred to as inherent rhythmicity. The heart rate
may be modified by the autonomic nervous system. Sympathetic or adrenergic
stimulation will increase heart rate and parasympathetic or cholinergic stimulation will
decrease heart rate. To ensure rhythmical contractibility, cardiac muscle must be
supplied with appropriate ions in proper concentrations. These ions are supplied in the
blood. Too little sodium leads to weak and rapid heart contractions. Too much
potassium makes the cardiac muscle cells lose their excitability and complete heart
blockage can occur. Excessive levels of calcium in the blood can lead to increased
contractibility of the cardiac muscle. Extremely high levels of the calcium ion in the
heart tissue can cause the heart to remain in a state of contraction.
(3) Disorders. An irregular heart beat pattern is called an arrhythmia.
There are different types of cardiac arrhythmias (that is, flutter or fibrillation).
Arrhythmias can sometimes be treated with drugs. More specific information on
arrhythmias and the drugs used to treat them will be given to you in another subcourse
(MD0806, Pharmacology III).