Muscles in vertebrates

The ability of vertebrates to move their bodies and their contents is a feature of major importance for their survival. Movements result from the contractions and relaxations of tissues specialized for the active generation of force: the muscles. For many large vertebrates, muscle comprises around half of their body—40 percent in humans, 50 percent in fish, and 40 percent in antelopes. However, some species, like sharks, which have around 85 percent muscle, do not follow this trend. Initially, it may seem sufficient to have only one type of muscle tissue, but a reflection on the functional requirements makes it clear that more than one type of muscular tissue is probably necessary. For example, the movement of the limbs should be under the conscious, voluntary activation and control of the animal. Otherwise, unwanted and uncoordinated limb movements would result at random while vital movements would not be forthcoming. The control of the limb movements should be precise, with as wide a range as possible for the forces that can be generated.

On the other hand, consider the movement of blood through the circulatory system. This job must be continuously performed every minute of the animal’s life. It would obviously be better to have this function run automatically without the need for conscious, voluntary activation and control. The need for precise control of the forces generated here is also not as great as for the limb movements, nor is the need for a wide range of forces as great. The flow of blood to various organs can be much better regulated by varying the diameter of the blood vessels at or near their entrance to the organs, thereby varying blood flow into the organs while the pumping forces generated to propel the blood remain relatively constant.

For the functions of controlling blood flow into organs and the mixing and passage of food through the digestive tract, it is, again, reasonable to have automatic operation of the muscles involved without the need for conscious, voluntary activation and control. To control the huge number of such muscles consciously is an impossible task in any case. Also, these muscles need to be able to change their lengths greatly, and sometimes to maintain a maximal contraction for very extended periods; however, very rapid actions are not as important.

To reasonably accommodate all vertebrates, three types of muscle tissue exist—skeletal muscle tissue, cardiac muscle tissue, and smooth muscle tissue. In nearly all vertebrates, involuntary muscles are smooth, and voluntary muscles are striated. Each muscle type serves an individual purpose and has distinct characteristics.

Skeletal Muscle

The skeletal muscle tissue is usually attached to bones and causes voluntary movements of the skeleton in response to stimuli. Some skeletal muscles are attached to the skin or to other skeletal muscles. They are under the animal’s conscious, voluntary control. Skeletal muscle cells are long, cylindrically shaped cells with rounded ends. When viewed under a microscope, they have thousands of alternating light and dark bands oriented perpendicular to their long axis and multiple nuclei sharing each cytoplasm. Because of these characteristics, skeletal muscle cells are said to be striated and multinucleated. The cells of a muscle are arranged in a parallel fashion, with their long forms mostly following the long axis of the parent muscle.

When a nerve signal stimulates a skeletal muscle cell, the muscle cell contracts relatively quickly and then, just about as quickly, relaxes (a muscle twitch). The contraction results in a shortening and thickening of the cell, which pulls the two ends of the cell, and whatever is attached to them toward each other. The duration of a skeletal muscle twitch varies from muscle to muscle. For example, a muscle cell from one of the muscles controlling eye movements may complete a twitch in about ten milliseconds, while a cell from the soleus muscle (found in the calf of the leg) will complete a twitch about ten times more slowly.

The strength of the contraction of a skeletal muscle depends on two factors—the number of muscle cells that become stimulated and, therefore, contract (called multiple motor unit summation or recruitment), and the frequency at which the stimulations, and, therefore, the contractions, occur (called temporal summation). Since most skeletal muscles are composed of hundreds of individual muscle cells, and since each of the nerve cells (motor neurons) that can stimulate the muscle makes contact with only a relatively limited number of the muscle cells (each motor neuron and the muscle cells it contacts forming a motor unit), it is possible to voluntarily regulate the strength and precision of the muscle’s contractions over a very wide range. This is accomplished by varying the summations—multiple motor units (number of motor units activated) and temporal (the frequency of motor unit activation)—to meet the demands of a particular situation.

When two muscle twitches occur in rapid succession, it is possible for the muscle to begin the second twitch before it has completely relaxed from the first twitch. In this case, the contraction of the second twitch adds its strength to the force developed as a result of the first contraction; therefore, the second contraction develops more force than the first contraction. The result of continuing the frequency of such twitch-evoking stimuli is a somewhat jerky, oscillating contraction called incomplete tetanus. At high frequencies of stimulation (for example, about fifty twitches per second), there is no evidence of the muscle relaxing from any one twitch before the following twitch takes place, and the muscle makes a very smooth and sustained contraction called complete tetanus. If, following an initial single twitch-evoking stimulus of a skeletal muscle cell, a second stimulus is applied too quickly, there will be no further contraction of the muscle cell, regardless of how strong the second stimulus may be. The cell’s ability to respond to this second stimulus is absent until enough time has passed to allow the cell to recover its excitability. The period during which the cell’s excitability is absent is referred to as its refractory period. The refractory period of skeletal muscle cells is quite short (about five milliseconds). Skeletal muscles are responsible for the movements of the skeleton (skull, limbs, fingers, toes, and trunk) and for the variety of facial expressions that humans can produce. They also permit speaking, eye movements, breathing, chewing, and swallowing.

Cardiac Muscle

The second type of muscle tissue found in vertebrates is called cardiac muscle. It is the muscle tissue found in the heart. Microscopic examination of cardiac muscle cells reveals them also to possess striations similar to skeletal muscle; however, cardiac muscle is not under voluntary control. Hence, cardiac muscle is referred to as involuntary striated muscle. Cardiac muscle cells also differ from skeletal muscle in that the cardiac fibers branch and form interconnections with one another, forming a network of cells. The individual members of a network are joined by special types of cellular junctions called intercalated disks. The intercalated disks permit the excitation of a single cell, which must occur for the muscle cell to contract, to spread throughout the entire network of cardiac cells; therefore, the contraction of any one cardiac cell will result in the contraction of the entire network of cardiac cells. Thus, the heart’s muscle tissue network operates as a functional unit, which is very important for the efficient development of the pressures necessary to push blood through the body’s circulatory system. Another very important characteristic of cardiac muscle is its ability to contract in a spontaneous, rhythmical fashion without the need for neural or hormonal stimuli (although both nerves and hormones are present and function as modulators of cardiac activity, also at a subconscious level). This property is termed autorhythmicity, and it accounts for the involuntary nature of cardiac muscle. Cardiac muscle twitches are about ten to fifteen times longer than those of skeletal muscle, and the refractory period (about three hundred milliseconds) is about sixty times longer than that of skeletal muscle. The consequences of these traits are important. The long twitch maintains muscular pressure on the blood contents of the heart until most of it has been pumped out of the heart’s chambers. The long refractory period prevents the development of tetanus and allows the heart to relax between beats so that it can be refilled with blood.

Smooth Muscle

The third type of vertebrate muscle is called smooth muscle because of its lack of striations when viewed microscopically. Its basic and most important functions relate to the maintenance of stable internal conditions within vertebrate bodies. Examples of this are the regulation of blood flow to the various organs to supply them with the proper amounts of oxygen and nutrients, the maintenance of blood pressure during postural changes (such as from reclining to standing), the mixing and movement of ingested food within the digestive tract, and the directing of blood flow through or away from the skin to aid in body temperature regulation. Smooth muscle is also usually involuntary and often displays automaticity. Automaticity refers to the fact that smooth muscle often is stimulated to contract simply by being stretched, as, for example, occurs in the stomach following a meal, without the necessity of neural or hormonal stimulation. Nevertheless, involuntary nerves and hormones are both involved in regulating the actions of smooth muscle cells. The functions of smooth muscle are almost always such that contraction is the appropriate response to stretch of the organ containing the smooth muscle (as in the stomach example just mentioned).

Smooth muscle cells are very small: only about eight micrometers in diameter and between thirty and two hundred micrometers long. They are spindle-shaped, tapering toward each end. Although very small, these cells are able to contract to a length which is a much smaller fraction of their resting length than can either cardiac or skeletal muscle cells. Smooth muscle can also remain fully contracted for long time periods and consume very little energy. The speed of contraction of smooth muscle, however, is the slowest of all three muscle types.

Many smooth muscle cells are arranged as functional units, which have the individual cells connected to each other by gap junctions. Gap junctions permit the passage of contraction signals from one cell to the next in the network in much the same way as it occurs in cardiac muscle. The result is that when one cell within the network begins to contract, a wave of contraction spreads throughout the entire network of smooth muscle cells.

Studying Vertebrate Muscle

Light and electron microscopes are frequently used to study muscle tissue. In particular, the electron microscope has made it possible to visualize the intricate and highly specialized internal structures of muscle cells, most of which are impossible to view with a light microscope, given their very small dimensions.

Many studies of muscle cells focus on isolated parts of muscle cells. It is possible to break apart muscle cells and separate many of their components by using centrifugation. Centrifugation is the use of centrifugal force to separate objects by size or density. A centrifuge is a device in which samples of objects (in this case, muscle-cell structural components) that are to be separated are subjected to high centrifugal force generated by spinning the samples at great speed. The centrifugal force impels objects outward from the center of rotation.

Once the subcellular components are isolated, various biochemical tests may be performed to determine what type of molecules they contain (such as proteins, lipids, or carbohydrates), how much of each type they contain, and the exact chemical composition of these molecules (such as the amino acid sequence of a protein). In the case of muscle cells, much scientific interest has centered on the large quantities of the proteins actin and myosin they contain. Using isolated actin and myosin, scientists have learned much about their functions in muscle cells. In particular, it is now known that these proteins are the molecules that generate the contractile forces that muscle cells produce. Actin forms thin filaments and is attached to sarcomere endings, and myosin forms thick filaments inside the sarcomeres—an arrangement that gives the muscle a striated appearance.

Studies of the electrical properties of muscle cells are performed using very sensitive amplifiers connected to fine glass pipette microelectrodes. These electrodes are formed from thin glass tubes which have been heated and pulled apart to produce a tapering of the wall of the tube. The final tip of the tapered tube is much smaller than a muscle cell, so it can be inserted into the cell without killing it. When the electrodes are filled with a solution capable of conducting electrical signals, they can record the electrical activity that takes place in living, contracting, and relaxing muscle cells: their electrophysiological properties. These studies have revealed that all muscle cells possess an electrical voltage difference between their interior region and the surrounding environment. Just before a muscle cell begins to contract, this voltage difference rapidly decreases toward a zero value. Combining such electrophysiological studies of muscle cells with simultaneous biochemical studies can be very rewarding.

Other techniques used to study muscle tissue include the observation of the relationships that various muscles have with other muscles, nerves, and bones using X-ray and computed tomography (CT) scans. Strain gauges are also used to measure the strength of muscles when different experimental conditions occur, such as temperature changes, blood-flow variations, and pharmacological treatments.

The Importance of Muscle

Vertebrate muscle tissue is involved in almost every bodily function. Any function requiring the movement of some body part or parts needs to have a source of motive power. For this purpose, evolution has resulted in the development of the specialized muscle tissues found in all vertebrates, which account for around 50 percent of their total body weight.

Because of the muscle-tissue characteristics of excitability (the ability to respond to stimuli), extensibility (the ability to be stretched), contractility (the ability to shorten actively and thereby generate force for work), and elasticity (the ability to return to original length following extension or contraction), vertebrates have a wide range of important capabilities available which assist them in their survival. Muscles in vertebrates exhibit an all-or-none response, meaning the strength of a stimulus is independent of the resulting response in a muscle. This principle was proposed by Henry P. Bowditch in 1871 following his work with heart muscles. Further research revealed that this all-or-nothing law also applies to skeletal muscle fibers and nerves. This is unique to vertebrates.

For the muscular system, disease states or disorders resulting from improper nutrition, injury, or toxic substances are very often life-threatening conditions. This is most obvious for disturbances of the respiratory muscles or of the muscles involved in eating and swallowing; however, muscular disorders involving the heart, the circulatory system, the digestive tract, or any major group of skeletal muscles may also prove to be life-threatening.

Many heart problems are caused by the reduction or blockage of the blood supply to the heart muscle. Reduced blood supply usually is the cause of a reduced oxygen supply. The insufficient oxygen supply weakens the heart muscle cells, causing the condition of ischemia. Complete interruption of the blood supply to an area of cardiac muscle tissue usually results in necrosis (death) of the affected muscle cells; the condition is referred to as myocardial infarction. The dead muscle cells do not regenerate but are replaced by scar tissue, which is not contractile. This results in decreased pumping efficiency by the heart. Depending on the size and location of the dead area, the result may range from barely noticeable to sudden death.

Some bacterial infections, like streptococci, staphylococci, and enterococci, cause the lining of the heart muscle to become inflamed. This can result in abnormal irritation of some heart muscle cells. These cells can then disrupt the normal autorhythmicity of the heart. Irregular heartbeats or uncoordinated contractions among the heart muscle cells ensue. These conditions are very serious and can lead to death.

Principal Terms

Involuntary: Functioning automatically; not under conscious control

Motor Neuron: A nerve cell that transmits impulses from the central nervous system to an effector such as a muscle cell

Motor Unit: A motor neuron together with the muscle cells it stimulates

Tissue: A group of similar cells that executes a specialized function

Twitch: A rapid muscular contraction followed by relaxation that occurs in response to a single stimulus

Voluntary: Capable of being consciously controlled

Bibliography

Fritzsch, Bernd. “Evolution and Development of Extraocular Motor Neurons, Nerves and Muscles in Vertebrates.” Annals of Anatomy, vol. 253, 2024, doi:10.1016/j.aanat.2024.152225. Accessed 20 Sept. 2024.

Hildebrand, Milton. Analysis of Vertebrate Structure. 6th ed., John Wiley & Sons, 2015.

Kardong, Kenneth V. Vertebrates: Comparative Anatomy, Function, Evolution. 8th ed., McGraw-Hill, 2019.

Keynes, R. D. Nerve and Muscle. 4th ed., Cambridge UP, 2011.

Millane, Rick P., and Pradeep K. Luther. “The Vertebrate Muscle Superlattice: Discovery, Consequences, and Link to Geometric Frustration.” Journal of Muscle Research and Cell Motility, vol. 44, no. 3, 2023, pp. 153–63. doi:10.1007/s10974-023-09642-8. Accessed 20 Sept. 2024.

Netter, Frank H. Musculoskeletal System: Anatomy, Physiology, and Metabolic Disorders. CIBA-GEIGY, 1987.

Tortora, Gerard J., et al. Principles of Anatomy and Physiology. 3rd ed., John Wiley & Sons Australia, 2022.