Characteristics of the Smooth (Involuntary) Muscles

Smooth muscles are present in the iris (pupil contraction), skin (hair muscles that cause goose bumps), walls of vessels and hollow internal organs: the bronchi, lower part of esophagus, stomach, gallbladder, intestine, reproductive and urogenital organs, glands and their ducts. The connective tissue is made up of proteins collagen and elastin. An average smooth muscle cell is 25-50 micrometers long and 2-5 micrometers wide (1 micrometer = 1/1000 of a millimeter). Muscle protein molecules actin and myosin are not organized in sarcomeres, so no striations are visible under a light microscope. Smooth muscle cells are pretty non-organized and meet each other in different angles. In the vessels they are arranged circumferentially, so they can change vessel’s diameter by contracting. Smooth muscles are innervated by the sympathetic and parasympathetic (autonomous) nerves. It takes about 500 ms (one half of a second) for a smooth muscle to reach peak contraction.


A stimulus for a smooth muscle contraction comes from:

  • The muscle cells (peristalsis pacemaker cells in the intestinal wall)
  • Intestinal wall distension caused by food
  • Autonomic nervous system (sympathetic nerves cause contraction, parasympathetic nerves cause relaxation)
  • Hormones (adrenalin causes smooth muscle relaxation, vasopressin causes vasoconstriction, oxytocin causes contraction of uterus during childbirth).

We can not consciously control contraction of the smooth muscles.

Other substances that affect smooth muscle contraction: potassium, calcium, magnesium, serotonin, prostacyclin, nitric oxide (NO2), histamine, medications (nitroglycerin), drugs (cocaine causes vasoconstriction, morphine causes vasodilatation) and some alkaloids and poisons.

Smooth muscle cells obtain energy for contraction from aerobic breakdown of glucose and fatty acids. Energy from anaerobic metabolism is used for trans-membrane transport.

Smooth muscles are of two types: tonic muscles in vessels sustain long contraction, phasic muscles in the intestine contract rapidly.

Regeneration of the Smooth Muscles

A repairing process in the smooth muscle includes hypertrophy only. Formation of new smooth cells after a smooth muscle damage is not usual.

The Cardiac (Heart) Muscle

Cardiac muscle is striated and can not be directly voluntary controlled. Latin word for the whole heart muscle is myocard. While the heart contracts, it pumps the blood through the circulatory system. The heart does not get oxygen from the blood flowing through it, but from two coronary arteries arising from the aorta. If these are obliterated for 20-40 minutes (for example, due to a blood clot), or if the heart muscle demand for oxygen exceeds oxygen delivery, a myocardial infarct can occur and a part of the heart will be permanently damaged, since the cardiac muscle fibres can not regenerate and a scar will form. This is called a heart attack.

Heart muscle fibres are about 10-20 µm wide and 50-100 µm long. They can use glucose, fatty acids, amino acids and lactate as a source of energy. The cardiac muscle cells are rich in mitochondria and get highly oxygenated blood so all the above mentioned substances are broken down in an aerobic way even during a hard exercise. The heart grows only by enlarging of the heart muscular cells (hypertrophy) and not by their multiplying.

Heart Beat

Cardiac muscle contraction is myogenic – it stimulates its own contraction.A single cardiac cell contracts at a steady rate that is independent of the nervous system. If two cardiac cells are in a contact, whichever contracts first, will stimulate the other one to contract (via intercalated discs in the cell wall). Sinoatrial (SA) node in the wall of the right atrium is a group of cardiac cells, which act as a primary pacemaker. A stimulus for contraction of the heart originates in the SA node (not in the brain). The sympathetic nerve fibres increase heart rate and the parasympathetic nerve fibres decrease heart rate. Excitation wave from SA node propagates through the electrical conduction system toward other heart cells. If a SA node is impaired, some other cardiac muscle cell will start to act like a pacemaker eventually resulting in derangement of a heart beat - arrhythmia.

An average heart rate in an adult at rest is 72/minute. A heart rate between 60-100/min at rest is considered as normocardia, over 100/min as tachycardia, below 60 as bradycardia. Bradycardia as low as 38/min can be normal in athletes. For a cardiac muscle, time to peak contraction may be around 150 ms. A heart beats even when taken out of the body (like during transport for transplantation).

Effects of Adrenalin, Potassium, Calcium and Magnesium on the Muscles

Adrenalin

Adrenalin lowers a threshold of excitation of motor units, so the skeletal muscle is excited faster, and it reaches peak contraction early and the contraction is stronger. It dilates bronchi thus enabling additional oxygen flow into the lungs. It dilates arteries in the skeletal muscles and constricts the arteries in the smooth muscles in the gut and skin, thus redirecting the blood flow towards the skeletal muscles. It relaxes the gut muscles (may lead to constipation). It dilates pupils, increases the heart rate and stroke volume (volume of the blood expelled from the heart in one beat). It enhances the breakdown of glycogen in the liver thus increasing blood glucose concentration. It enhances glycolysis and the breakdown of fatty acids in skeletal and heart muscle cells.

Potassium

Potassium enables the propagation of excitation in the nerve and muscle fibres.

Both hypo- and hyperkalemia cause muscle weakness and cramps and derangement of a heart beat – arrhythmia; if severe, it may be lethal. Conditions listed can occur in a kidney or adrenal gland disease, in irregular diet (small potassium intake while fasting), in alcohol abuse or after taking oral or intravenous potassium.

Calcium

Calcium is essential for contraction of all muscle types. Hypercalcemia (from renal disease, cancer, lithium pills) can cause muscle weakness and hypertension. Excess calcium taken orally (antacids) might cause constipation in some people, but this is not proven. Hypocalcemia (from a parathyroid gland failure or lack of vitamin D) causes numbness and, when severe, tetanic muscle cramps and hypotension because of the heart failure.

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