Microscopic Structure of a Muscle Fiber & Muscle Physiology

I would like to make a series of reviews such as is one. An example would be: We will now look more closely at a muscle fiber, keeping in mind that there are thousands of these cylindrical cells in one muscle. Each muscle fiber has its own motor nerve ending; the neuromuscular junction is where the motor neuron terminates on the muscle fiber.

Phill A. Smith M.D.

Microscopic Structure of a Muscle Fiber
By Valerie C. Scanlon, PhD and Tina Sanders on April 21st 2008
We will now look more closely at a muscle fiber, keeping in mind that there are thousands of these cylindrical cells in one muscle. Each muscle fiber has its own motor nerve ending; the neuromuscular junction is where the motor neuron terminates on the muscle fiber.

The axon terminal is the enlarged tip of the motor neuron; it contains sacs of the neurotransmitter acetylcholine (ACh). The membrane of the muscle fiber is the sarcolemma, which contains receptor sites for acetylcholine, and an inactivator called cholinesterase. The synapse (or synaptic cleft) is the small space between the axon terminal and the sarcolemma.

Within the muscle fiber are thousands of individual contracting units called sarcomeres, which are arranged end to end in cylinders called myofibrils. The Z lines are the end boundaries of a sarcomere. Filaments of the protein myosin are in the center of the sarcomere, and filaments of the protein actin are at the ends, attached to the Z lines. Myosin filaments are anchored to the Z lines by the protein titin.

Myosin and actin are the contractile proteins of a muscle fiber. Their interactions produce muscle contraction. Also present are two inhibitory proteins, troponin and tropomyosin, which are part of the actin filaments and prevent the sliding of actin and myosin when the muscle fiber is relaxed.

Surrounding the sarcomeres is the sarcoplasmic reticulum, the endoplasmic reticulum of muscle cells. The sarcoplasmic reticulum is a reservoir for calcium ions (Ca+2), which are essential for the contraction process.

All of these parts of a muscle fiber are involved in the contraction process. Contraction begins when a nerve impulse arrives at the axon terminal and stimulates the release of acetylcholine. Acetylcholine generates electrical changes (the movement of ions) at the sarcolemma of the muscle fiber.

These electrical changes initiate a sequence of events within the muscle fiber that is called the sliding filament mechanism of muscle contraction. We will begin our discussion with the sarcolemma.

Energy Source for Muscle contraction:
By Valerie C. Scanlon, PhD and Tina Sanders on April 21st 2008
Before discussing the contraction process itself, let us look first at how muscle fibers obtain the energy they need to contract. The direct source of energy for muscle contraction is ATP. ATP, however, is not stored in large amounts in muscle fibers and is depleted in a few seconds.

The secondary energy sources are creatine phosphate and glycogen. Creatine phosphate is, like ATP, an energy-transferring molecule. When it is broken down (by an enzyme) to creatine, phosphate, and energy, the energy is used to synthesize more ATP. Most of the creatine formed is used to resynthesize creatine phosphate, but some is converted to creatinine, a nitrogenous waste product that is excreted by the kidneys.

The most abundant energy source in muscle fibers is glycogen. When glycogen is needed to provide energy for sustained contractions (more than a few seconds),it is first broken down into the glucose molecules of which it is made. Glucose is then further broken down in the process of cell respiration to produce ATP,and muscle fibers may continue to contract.

Glucose + O2 = CO2 + H2O + ATP + heat

Look first at the products of this reaction. ATP will be used by the muscle fibers for contraction. The heat produced will contribute to body temperature,and if exercise is strenuous, will increase body temperature. The water becomes part of intracellular water, and the carbon dioxide is a waste product that will be exhaled.

Now look at what is needed to release energy from glucose: oxygen. Muscles have two sources of oxygen. The blood delivers a continuous supply of oxygen from the lungs, which is carried by the hemoglobin in red blood cells.

Within muscle fibers themselves there is another protein called myoglobin, which stores some oxygen within the muscle cells. Both hemoglobin and myoglobin contain the mineral iron, which enables them to bond to oxygen. (Iron also makes both molecules red, and it is myoglobin that gives muscle tissue a red or dark color.)

During strenuous exercise, the oxygen stored in myoglobin is quickly used up,and normal circulation may not deliver oxygen fast enough to permit the completion of cell respiration. Even though the respiratory rate increases, the muscle fibers may literally run out of oxygen.

This state is called oxygen debt,and in this case, glucose cannot be completely broken down into carbon dioxide and water. If oxygen is not present (or not present in sufficient amounts),glucose is converted to an intermediate molecule called lactic acid, which causes muscle fatigue.

In a state of fatigue, muscle fibers cannot contract efficiently, and contraction may become painful. To be in oxygen debt means that we owe the body some oxygen. Lactic acid from muscles enters the blood and circulates to the liver, where it is converted to pyruvic acid, a simple carbohydrate (three carbons, about half a glucose molecule).

This conversion requires ATP, and oxygen is needed to produce the necessary ATP in the liver. This is why, after strenuous exercise, the respiratory rate and heart rate remain high for a time and only gradually return to normal.

Another name proposed for this state is recovery oxygen uptake, which is a little longer but also makes sense. Oxygen uptake means a faster and deeper respiratory rate. What is this uptake for? For recovery from strenuous exercise.

Muscle sense:
By Valerie C. Scanlon, PhD and Tina Sanders on April 21st 2008
When you walk up a flight of stairs, do you have to look at your feet to be sure each will get to the next step? Most of us don’t (an occasional stumble doesn’t count), and for this freedom we can thank our muscle sense. Muscle sense (proprioception) is the brain’s ability to know where our muscles are and what they are doing, without our having to consciously look at them.

Within muscles are receptors called stretch receptors (proprioceptors or muscle spindles). The general function of all sensory receptors is to detect changes. The function of stretch receptors is to detect changes in the length of a muscle as it is stretched. The sensory impulses generated by these receptors are interpreted by the brain as a mental “picture” of where the muscle is.

We can be aware of muscle sense if we choose to be, but usually we can safely take it for granted. In fact, that is what we are meant to do. Imagine what life would be like if we had to watch every move to be sure that a hand or foot performed its intended action. Even simple activities such as walking or eating would require our constant attention.

At times, we may become aware of our muscle sense. Learning a skill such as typing or playing the guitar involves very precise movements of the fingers, and beginners will often watch their fingers to be sure they are moving properly.

With practice, however, the movements simply “feel” right, which means that the brain has formed a very good mental picture of the task. Muscle sense again becomes unconscious, and the experienced typist or guitarist need not watch every movement.

All sensation is a function of brain activity, and muscle sense is no exception. The impulses for muscle sense are integrated in the parietal lobes of the cerebrum (conscious muscle sense) and in the cerebellum (unconscious muscle sense) to be used to promote coordination.

By Valerie C. Scanlon, PhD and Tina Sanders on April 21st 2008
Good muscle tone improves coordination. When muscles are slightly contracted, they can react more rapidly if and when greater exertion is necessary. Muscles with poor tone are usually soft and flabby, but exercise will improve muscle tone.

There are two general types of exercise: isotonic and isometric. In isotonic exercise, muscles contract and bring about movement. Jogging, swimming, and weight lifting are examples. Isotonic exercise improves muscle tone, muscle strength, and, if done repetitively against great resistance (as in weight lifting), muscle size.

This type of exercise also improves cardiovascular and respiratory efficiency, because movement exerts demands on the heart and respiratory muscles. If done for 30 minutes or longer, such exercise may be called aerobic, because it strengthens the heart and respiratory muscles as well as the muscles attached to the skeleton.

Isotonic contractions are of two kinds, concentric or eccentric. A concentric contraction is the shortening of a muscle as it exerts force. An eccentric contraction is the lengthening of a muscle as it still exerts force.

Imagine lifting a book straight up (or try it); the triceps brachii contracts and shortens to straighten the elbow and raise the book, a concentric contraction. Now imagine slowly lowering the book. The triceps brachii is still contracting even as it is lengthening, exerting force to oppose gravity (which would make the book drop quickly). This is an eccentric contraction.

Isometric exercise involves contraction without movement. If you put your palms together and push one hand against the other, you can feel your arm muscles contracting. If both hands push equally, there will be no movement; this is isometric contraction. Such exercises will increase muscle tone and muscle strength but are not considered aerobic.

When the body is moving, the brain receives sensory information about this movement from the joints involved, and responds with reflexes that increase heart rate and respiration. Without movement, the brain does not get this sensory information, and heart rate and breathing do not increase nearly as much as they would during an equally strenuous isotonic exercise.

Many of our actions involve both isotonic and isometric contractions. Pulling open a door requires isotonic contractions of arm muscles, but if the door is then held open for someone else, those contractions become isometric. Picking up a pencil is isotonic; holding it in your hand is isometric. Walking uphill involves concentric isotonic contractions, and may be quite strenuous.

Walking downhill seems easier, but is no less complex. The eccentric isotonic contractions involved make each step a precisely aimed and controlled fall against gravity. Without such control (which we do not have to think about) a downhill walk would quickly become a roll. These various kinds of contractions are needed for even the simplest activities.

Anabolic steroids are synthetic drugs very similar in structure and action to the male hormone testosterone. Normal secretion of testosterone, beginning in males at puberty, increases muscle size and is the reason men usually have larger muscles than do women.

Some athletes, both male and female, both amateur and professional, take anabolic steroids to build muscle mass and to increase muscle strength. There is no doubt that the use of anabolic steroids will increase muscle size, but there are hazards, some of them very serious. Side effects of such self-medication include liver damage, kidney damage, disruption of reproductive cycles, and mental changes such as irritability and aggressiveness.

Female athletes may develop increased growth of facial and body hair and may become sterile as a result of the effects of a male hormone on their own hormonal cycles.

Muscle synergist:
By Valerie C. Scanlon, PhD and Tina Sanders on April 20th 2008
Synergistic muscles are those with the same function, or those that work together to perform a particular function. Recall that the biceps brachii flexes the forearm. The brachioradialis, with its origin on the humerus and insertion on the radius, also flexes the forearm. There is even a third flexor of the forearm, the brachialis.

You may wonder why we need three muscles to perform the same function, and the explanation lies in the great mobility of the hand. If the hand is palm up, the biceps does most of the work of flexing and may be called the prime mover. When the hand is thumb up, the brachioradialis is in position to be the prime mover, and when the hand is palm down, the brachialis becomes the prime mover.

If you have ever tried to do chin-ups, you know that it is much easier with your palms toward you than with palms away from you. This is because the biceps is a larger, and usually much stronger, muscle than is the brachialis.

Muscles may also be called synergists if they help to stabilize or steady a joint to make a more precise movement possible. If you drink a glass of water, the biceps brachii may be the prime mover to flex the forearm.

At the same time, the muscles of the shoulder keep that joint stable, so that the water gets to your mouth, not over your shoulder or down your chin. The shoulder muscles are considered synergists for this movement because their contribution makes the movement effective.

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