+1 (315) 557-6473 

Spring 2022 Mid-Term Exam Solution on Muscles from the Massachusetts Institute of Technology (MIT)

This article features a commonly examined question that rarely misses appearing in an exam setting. The question's keen interpretation indicates our ability to take your anatomy exam. We base our services on concrete research to ensure you get a top grade. So, trust our biology exam help services and get higher grades this season. Contact us to make a booking.

What Is the Length of a Muscle, and How Do Muscular Fibres Work in the Body?

All animal movement is the work of muscles. Muscles can only work by pulling, never by pushing. However much a man may push down a wall, every muscle doing work is doing it by pulling; the body's engineering sees that the pulling becomes pushing. Human muscle tissue is laid down before birth or shortly afterwards. In this aspect, it is similar to nervous tissue. An infant's complement of muscle fibres is its complement for life; it will probably acquire no more. Strength comes after the expansion of each fibre and after it has been made to do work.

For the same reason, the limbs of a blacksmith and a girl are contasimilar of fibres, although he may have several times her strength. Big people are customarily stronger than small people, but per pound of body weight, bantam-class champions can lift about half a pound more than heavyweight champions. In terms of maximum energy output, it has been calculated that the theoretical limit for man is about six horsepower, that the highest recorded is 4-5 horsepower, that 0-6 horsepower can be sustained for five minutes, and that 0-5 horsepower can be sustained indefinitely. (Which makes one wonder about the sustained horsepower of one horse.)

Muscle fibres can be long-up to 14 ins. (4 cms.), and they can be minute - - a millimetre or less, but their diameter is always far smaller. Most fibres are from a tenth of a millimetre to a hundredth across. Many individual fibres comprise each distinct anatomical muscle, and about 656 muscles in the body or over three times as many muscles as bones. Some 42% of the male weight is a muscle, and some 36% of the female weight. Most muscles attached to the skeleton are linked to a bone either at one end by a tendon or, less frequently, at both ends. These tendons (or ligaments) may be small or more than a foot long in man. Ligaments are also composed of strong fibrous tissue, but their role is to bind bones together.

Each muscle fibre obeys the same all-or-none principle as the nerve fibre; it either does contract or does not. However, movement is not a series of robot-like jerks because each muscle contains innumerable fibres, and each separate fibre has to be triggered to affect its pull. Should the stimulus be strong, as when a hand touches a hot surface, every fibre will pull, and the result will be a jerk? Should the nervous stimuli contract to continue to arrive very rapidly at the muscle, it will not contract and relax again, as generally happens. Still, it will remain contracted in a state of tetanus.

The impulses which can cause a fibre to twitch are electrical, mechanical, thermal, and chemical. The time between the arrival of a stimulus at the fibre and the start of that fibre's contraction is between two and four thousand a second. A muscle works by converting chemical energy into mechanical energy; only about 25% of the potential energy is correctly converted. The remaining 75% is assumed to be lost as heat, assisting the rising temperature of someone suddenly doing strenuous work. Maximum efficiency of 25% is similar to an internal combustion engine; that, too, loses most of its energy as heat, and systems are necessary to dissipate this heat.

However much a muscle may either contract or fail to contract on an all-or-none basis. However, much of this rule may be generally applicable; there is no such uniformity about the total time taken either for a contraction or between regular contractions. On heartbeat alone, there is great diversity: an elephant will contract every two and a half seconds, and a canary will contract seventeen times every second. The human heart, as everyone knows, can change from a solemn beat of forty-five a minute to a furious pace three or four times faster during extreme exercise. Even a fast heart is not the fastest muscular rhythm achievable by the body. Finger tapping can be far faster, and the tongue, teeth, and eyelids can all move with a far swifter tremor. The wing beats of many insects completely outclass such human speeds. Butterflies are ponderous as they flap their casual course. Still, the wings of beetles can oscillate up to 175 times a second, of bees up to 247 cycles a second, of mosquitoes up to 587 cycles, and one midge has been recorded with a wing beat faster than 1,000 cycles a second. Such vibrations are only possible because the wing muscles do not conform to the conventional relationship between nerve and muscle. Almost always, a voluntary muscle twitches once in response to a single nerve impulse. The remarkably high-speed insect wing twitches many times in response to each stimulus. It is unique in this respect, for the muscle does not go through the normal relaxing routine after contracting. This phenomenon means that weight for weight, the wing muscle of some insects generates more energy than any other animal tissue, certainly much more than human muscle tissue.

When a human fingertip, for example, is tapped rapidly on a table in a feeble attempt to emulate the insects, it will soon tire. Eventually, suffering paralysis of its own, the finger becomes immobile. The muscle, it seems, can do no more. If that muscle is then stimulated electrically and externally, as with those massage machines used by therapists, the finger will start tapping again. It is, therefore, not the tired muscle, for that it still responsive; nor is it the nerve which is tired, for that, is still capable of transmitting the customary stimuli; rather, it is the gap between the two, the synapse, which fails to conduct the stimulus any more from nerve to the muscle. Of course, such finger-tapping paralysis is short-lived. Within a few moments, the synapse will have had its capabilities restored, and tapping can begin again. Muscular fatigue itself exists quite independently of any synapse fatigue. It is believed to be due to an accumulation of lactic acid from the temporary lack of oxygen.

How do muscles work?

The subject was opened up notably after World War Two and is still in a state of exciting development. The decade of the fifties ended with a generality that muscle contracts because it possesses two kinds of filament. One is thick, and one is thin, and they slide past each other to produce the shortening. It was known that this contraction is extremely rapid - each fibre shortens at speed equal to several times its length in a second, and it was known that the power of its tension is 40 lbs. per sq. inch of muscle cross-section. It is now known that the thin filaments (composed of the protein actin) are drawn further and further in between the thick filaments (composed of the protein myosin), and it was felt valid in the 1950s to compare the system to a rachet action. Those doing this work still like the comparison. They like it because the sliding action appears in a series of distinct notches.

Biochemistry is extremely complex, but it is understood that the energy for pushing each filament on a notch comes from ATP (or adenosine triphosphate), which gets energy from glucose. ATP uses up its energy in the notch-advancement process and becomes ADP (or adenosine diphosphate) when it is a protein which acted as an enzyme to split off the phosphate group, the act which turns the triphosphate into the diphosphate. Such a remark is easy to make, but how proteins act as enzymes- which they do all the time in every bodily activity- is another matter. Anyhow, glucose, the simplest sugar, then supplies the energy to reverse the process and return that phosphate group- another easy remark of hideous complexity. Each molecule of glucose can recharge many molecules of ADP to form ATP.

Even if this oversimplified version is understandable, it is not the whole story. The full story is about how proteins work, catalyze biochemical reactions, and how are unaffected yet unchanged. Such questions are not just unanswered problems of muscular contraction. They are among the fundamental questions of the chemistry of living tissue. ATP is not just another set of initials; they stand for the molecule that is the universal carrier of energy in the living cell.

Work on muscular contraction has taken such a leap forward since its critical strides in the 1950s (which also led to Nobel prizes) that it may solve not only many of its problems but others equally fundamental to all forms of tissue. The biochemical examination of contraction and the two proteins myosin and actin have already shown to be a favourable platform for examining protein as a whole. As with some tortuous knot, carefully unravelling one small part of it can suddenly expose the simplicity of the remainder. However, it has not done so yet.

Cramps, stiffness and stitch, have been painfully encountered by most people. Although common, they are certainly not fully understood. Cramp, for example, is certainly a muscular contraction and may well be caused by the nervous system, notably without recourse to the higher control centres; but there is cramp and cramp. It frequently comes during sleep, when there has been discomfort, when a body is starved of salt, or when cold and exertion must be many possible causes, each leading to different as in swimming- combine to induce it. These varieties of cramps or even the same punitive and spasmodic kind of contraction.

Stiffness is equally emphatic in its symptoms and equally vague in its causes. It may affect joints, ligaments, tendons or muscles. It usually occurs after exceptional exercise; it may be either an accumulation of products of contraction, whatever they may be or merely the manifestation of slight injury. The tissue between muscle fibres may have been torn or pulled, causing modest pain and impairment.

A stitch can give the sharpest pain of the lot. Generally, it is felt below the ribs and generally on the left. It is frequently the accompaniment of jolting exercise, like running with a loaded stomach. Cross-country running, with all its irregularities, is more of a jolt. In Fitness for the Ordinary Man, Sir Adolphe Abrahams says that exposure to cold can help to bring it on, that wearing mittens has sometimes proved beneficial, and that it helps if the stomach is empty. That disappointment usually follows attempts at prevention. Some people do seem to have a constitutional tendency for stitch.

No comments yet be the first one to post a comment!
Post a comment