If a muscle twitch is just a single quick contraction followed immediately by relaxation, how do we explain the smooth continued movement of our muscles when they contract and move bones through a large range of motion? The answer lies in the ordering of the firing of the motor units. If all of the motor units fired simultaneously the entire muscle would quickly contract and relax, producing a very jerky movement.
Instead, when a muscle contracts, motor units fire asynchronously, that is, one contracts and then a fraction of a second later another contracts before the first has time to relax and then another fires and so on. So, instead of a quick, jerky movement the whole muscle contraction is very smooth and controlled.
Even when a muscle is at rest, there is random firing of motor units. This random firing is responsible for what is known as muscle tone. So, a muscle is never "completely" relaxed, even when asleep. However, if the neuron to a muscle is cut, there will be no "muscle tone" and this is called flaccid paralysis. There are several benefits of muscle tone: First it takes up the "slack" in the muscle so that when it is asked to contract, it can immediately begin to generate tension and move the limb.
If you have ever towed a car you know what happens if you don't take the slack out of the tow rope before starting to pull. The second thing muscle tone does is deter muscle atrophy.
Muscle contractions are described based on two variables: force tension and length shortening. Isometric contractions are important in maintaining posture or stabilizing a joint. Furthermore, isotonic contractions can be classified based on how the length changes. If the muscle generates tension and the entire muscle shortens than it is a concentric contraction. An example would be curling a weight from your waist to your shoulder; the bicep muscle used for this motion would undergo a concentric contraction.
In contrast, when lowering the weight from the shoulder to the waist the bicep would also be generating force but the muscle would be lengthening, this is an eccentric contraction. Eccentric contractions work to decelerate the movement at the joint. Additionally, eccentric contractions can generate more force than concentric contractions. Think about the large box you take down form the top shelf of your closet. You can lower it under total control using eccentric contractions but when you try to return it to the shelf using concentric contractions you cannot generate enough force to lift it back up.
Strength training, involving both concentric and eccentric contractions, appears to increase muscle strength more than just concentric contractions alone. However, eccentric contractions cause more damage tearing to the muscle resulting in greater muscle soreness. If you have ever run downhill in a long race and then experienced the soreness in your quadriceps muscles the next day, you know what we are talking about. Muscle size is determined by the number and size of the myofibrils, which in turn is determined by the amount of myofilament proteins.
Thus, resistance training will induce a cascade of events that result in the production of more proteins. Often this is initiated by small, micro-tears in and around the the muscle fibers. If the tearing occurs at the myofibril level the muscle will respond by increasing the amount of proteins, thus strengthening and enlarging the muscle, a phenomenon called hypertrophy.
This tearing is thought to account for the muscle soreness we experience after a workout. As mentioned above, the repair of these small tears results in enlargement of the muscle fibers but it also results in an increase in the amount of connective tissue in the muscle.
When a person "bulks up" from weight training, a significant percent of the increase in size of the muscle is due to increases in the amount of connective tissue.
It should be pointed out that endurance training does not result in a significant increase in muscle size but increases its ability to produce ATP aerobically. Obviously our muscles are capable of generating differing levels of force during whole muscle contraction.
Some actions require much more force generation than others; think of picking up a pencil compared to picking up a bucket of water. The question becomes, how can different levels of force be generated? Multiple-motor unit summation or recruitment : It was mentioned earlier that all of the motor units in a muscle usually don't fire at the same time. One way to increase the amount of force generated is to increase the number of motor units that are firing at a given time.
Here, the biceps brachii contracts as sarcomeres in its muscle fibers are shortening and cross-bridges form; the myosin heads pull the actin. An eccentric contraction occurs as the muscle tension diminishes and the muscle lengthens. In this case, the hand weight is lowered in a slow and controlled manner as the amount of cross-bridges being activated by nervous system stimulation decreases.
In this case, as tension is released from the biceps brachii, the angle of the elbow joint increases. Eccentric contractions are also used for movement and balance of the body. An isometric contraction occurs as the muscle produces tension without changing the angle of a skeletal joint.
Isometric contractions involve sarcomere shortening and increasing muscle tension, but do not move a load, as the force produced cannot overcome the resistance provided by the load. For example, if one attempts to lift a hand weight that is too heavy, there will be sarcomere activation and shortening to a point, and ever-increasing muscle tension, but no change in the angle of the elbow joint. In everyday living, isometric contractions are active in maintaining posture and maintaining bone and joint stability.
However, holding your head in an upright position occurs not because the muscles cannot move the head, but because the goal is to remain stationary and not produce movement. Most actions of the body are the result of a combination of isotonic and isometric contractions working together to produce a wide range of outcomes [link]. All of these muscle activities are under the exquisite control of the nervous system. Neural control regulates concentric, eccentric and isometric contractions, muscle fiber recruitment, and muscle tone.
A crucial aspect of nervous system control of skeletal muscles is the role of motor units. As you have learned, every skeletal muscle fiber must be innervated by the axon terminal of a motor neuron in order to contract. Each muscle fiber is innervated by only one motor neuron. The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit.
The size of a motor unit is variable depending on the nature of the muscle. A small motor unit is an arrangement where a single motor neuron supplies a small number of muscle fibers in a muscle.
Small motor units permit very fine motor control of the muscle. The best example in humans is the small motor units of the extraocular eye muscles that move the eyeballs. There are thousands of muscle fibers in each muscle, but every six or so fibers are supplied by a single motor neuron, as the axons branch to form synaptic connections at their individual NMJs. This allows for exquisite control of eye movements so that both eyes can quickly focus on the same object.
Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for grasping, texting, etc. A large motor unit is an arrangement where a single motor neuron supplies a large number of muscle fibers in a muscle. The best example is the large motor units of the thigh muscles or back muscles, where a single motor neuron will supply thousands of muscle fibers in a muscle, as its axon splits into thousands of branches.
There is a wide range of motor units within many skeletal muscles, which gives the nervous system a wide range of control over the muscle. The small motor units in the muscle will have smaller, lower-threshold motor neurons that are more excitable, firing first to their skeletal muscle fibers, which also tend to be the smallest. Activation of these smaller motor units, results in a relatively small degree of contractile strength tension generated in the muscle.
As more strength is needed, larger motor units, with bigger, higher-threshold motor neurons are enlisted to activate larger muscle fibers.
Lactic acid removal by the cardiovascular system improves with training which increases the anaerobic capacity. Even so, the glycolysis-lactic acid system can produce ATP for active muscle cells for only about a minute and a half. Aerobic metabolism - ultimately, the product of glycolysis, pyruvic acid, must be metabolized aerobically. Aerobic metabolism is performed exclusively in the mitochondria. Pyruvic acid is converted to a molecule called an acetyl group and put into a pathway known as the Krebs Cycle.
Energy is released in the form of ATP and, especially, as high energy electrons. These high energy electrons are sent to a process within the mitochondria known as the electron transport system which produces the vast majority of the ATP.
The waste products of aerobic metabolism are CO 2 and H 2 O. The reactant other than glucose is O 2. Aerobic metabolism is used for endurance activities and has the distinct advantage that it can go on for hours.
Aerobic training increases the length of endurance activities by increasing the number of mitochondria in the muscle cells, increasing the availability of enzymes, increasing the number of blood vessels, and increasing the amount of an oxygen-storing molecule called myoglobin.
The types of muscle cells. Different types of cells perform the differing functions of endurance activities and speed- strength activities. There are three types, red, white, and intermediate. The main differences can be exemplified by looking at red and white fibers and remembering that intermediate fibers have properties of the other two.
White Fibers Fast twitch Large diameter, used for speed and strength. Depends on the phosphagen system and on glycolysis-lactic acid. Stores glycogen for conversion to glucose. Fewer blood vessels. Little or no myoglobin. Red Fibers Slow twitch Small diameter, used for endurance. Eccentric contractions are also used for movement and balance of the body.
An isometric contraction occurs as the muscle produces tension without changing the angle of a skeletal joint. Isometric contractions involve sarcomere shortening and increasing muscle tension, but do not move a load, as the force produced cannot overcome the resistance provided by the load. For example, if one attempts to lift a hand weight that is too heavy, there will be sarcomere activation and shortening to a point, and ever-increasing muscle tension, but no change in the angle of the elbow joint.
In everyday living, isometric contractions are active in maintaining posture and maintaining bone and joint stability. However, holding your head in an upright position occurs not because the muscles cannot move the head, but because the goal is to remain stationary and not produce movement. Most actions of the body are the result of a combination of isotonic and isometric contractions working together to produce a wide range of outcomes Figure 1.
Figure 1. Types of Muscle Contractions. During isotonic contractions, muscle length changes to move a load. During isometric contractions, muscle length does not change because the load exceeds the tension the muscle can generate. All of these muscle activities are under the exquisite control of the nervous system. Neural control regulates concentric, eccentric and isometric contractions, muscle fiber recruitment, and muscle tone. A crucial aspect of nervous system control of skeletal muscles is the role of motor units.
As you have learned, every skeletal muscle fiber must be innervated by the axon terminal of a motor neuron in order to contract. Each muscle fiber is innervated by only one motor neuron. The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit.
The size of a motor unit is variable depending on the nature of the muscle. A small motor unit is an arrangement where a single motor neuron supplies a small number of muscle fibers in a muscle. Small motor units permit very fine motor control of the muscle. The best example in humans is the small motor units of the extraocular eye muscles that move the eyeballs. There are thousands of muscle fibers in each muscle, but every six or so fibers are supplied by a single motor neuron, as the axons branch to form synaptic connections at their individual NMJs.
This allows for exquisite control of eye movements so that both eyes can quickly focus on the same object. Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for grasping, texting, etc. A large motor unit is an arrangement where a single motor neuron supplies a large number of muscle fibers in a muscle. The best example is the large motor units of the thigh muscles or back muscles, where a single motor neuron will supply thousands of muscle fibers in a muscle, as its axon splits into thousands of branches.
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