13.5: Nervous System Control of Muscle Tension (2023)

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    learning objectives
    • Explain concentric, isotonic, and eccentric contractions
    • Describe the length-tension relationship
    • Describe the three phases of a muscle contraction
    • Define wave summation, tetanus and tripping

    To move an object, called a load, the sarcomeres in the muscle fibers of skeletal muscle must shorten. The force generated by muscle contraction (or sarcomere shortening) is calledmuscle tension. However, muscle tension is also generated when the muscle contracts against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions.

    Emisotonic contractions, where the tension in the muscle remains constant, a load is moved as the length of the muscle changes (shortens). There are two types of isotonic contractions: concentric and eccentric. Aconcentric contractioninvolves muscle shortening to move a load. An example of this is the contraction of the biceps brachii muscle when the weight of the hand is lifted with increased muscle tension. As the biceps brachii contracts, the angle of the elbow joint decreases as the forearm is brought towards the body. Here, the biceps brachii contracts as the sarcomeres in its muscle fibers shorten and cross-bridges form; myosin heads pull actin. Oneeccentric contractionoccurs as muscle tension decreases and the muscle lengthens. In this case, the weight of the hand is lowered in a slow, controlled manner as the amount of cross-bridges 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 body movement and balance.

    Oneisometric contractionoccurs when muscle produces tension without changing the angle of a skeletal joint. Isometric contractions involve shortening the sarcomere and increasing muscle tension, but they do not move a load, as the force produced cannot overcome the resistance provided by the load. For example, if someone tries to lift a very heavy hand weight, there will be activation and shortening of the sarcomere to some extent, and increasing muscle tension, but no change in the angle of the elbow joint. In everyday life, isometric contractions are active in maintaining posture and maintaining bone and joint stability. However, keeping the head upright is not because the muscles cannot move the head, but because the objective is to remain stationary and not produce movement. Most body actions are the result of a combination of isotonic and isometric contractions working together to produce a wide range of results (Figure 1).

    13.5: Nervous System Control of Muscle Tension (1)

    All these muscular 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.

    Motor Units

    As you learned, each 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 themotor unit. The size of a motor unit is variable depending on the nature of the muscle.

    A small motor unit is an arrangement in which a single motor neuron supplies a small number of muscle fibers in a muscle. Small motor units allow very fine motor control of the muscle. The best example in humans are the small motor units of the extraocular eye muscles that move the eyeball. 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 JNMs. This allows for fine control of eye movements so that both eyes can quickly focus on the same object. Small motor units are also involved in many fine movements of the fingers and thumb of the hand for grasping, texting, etc.

    A large motor unit is an arrangement in which a single motor neuron supplies a large number of muscle fibers in a muscle. Large motor units are concerned with simple or “gross” movements, such as powerfully extending the knee joint. The best example is the large motor units of the thigh or back muscles, where a single motor neuron will supply thousands of muscle fibers in a muscle as its axon divides into thousands of branches.

    There is a wide range of motor units in many skeletal muscles, which gives the nervous system a wide range of control over the muscles. Small motor units in muscle will have smaller, lower threshold motor neurons that are more excitable, firing first to skeletal muscle fibers, which also tend to be the smallest. Activation of these smaller motor units results in a relatively small degree of contractile force (tension) generated in the muscle. As more force is required, larger motor units with larger, higher-threshold motor neurons are recruited to activate larger muscle fibers. This increased activation of motor units produces an increase in muscle contraction known asrecruitment. As more motor units are recruited, muscle contraction gets progressively stronger. In some muscles, the largest motor units can generate a contractile force 50 times greater than the smallest motor units in the muscle. This allows a feather to be picked up using the biceps brachii muscle of the arm with minimal force, and a heavy weight to be lifted by the same muscle, recruiting the largest motor units.

    When needed, the maximum number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle, but this cannot last long because of the energy needs to sustain the contraction. To avoid complete muscle fatigue, motor units are generally not all active simultaneously, but some motor units rest while others are active, which allows for longer muscle contractions. The nervous system uses recruitment as a mechanism to efficiently utilize skeletal muscle.

    The length-tension range of a sarcomere

    When a skeletal muscle fiber contracts, the myosin heads attach to actin to form cross bridges followed by the thin filaments sliding over the thick filaments as the heads pull on actin, and this results in the sarcomere shortening, creating tension. of muscle contraction. Cross-bridges can only form where thin and thick filaments already overlap, so the length of the sarcomere has a direct influence on the force generated when the sarcomere shortens. This is called the length-tension relationship.

    13.5: Nervous System Control of Muscle Tension (2)

    The optimal length of a sarcomere to produce maximum tension is between 80% and 120% of its resting length, with 100% being the state in which the medial edges of the thin filaments are exactly at the most medial myosin heads of the thick filaments ( Figure 2 ).

    This length maximizes the overlap of actin binding sites and myosin heads. If a sarcomere is stretched beyond this ideal length (beyond 120%), the thick and thin filaments do not overlap sufficiently, which results in less tension being produced. If a sarcomere is shortened beyond 80%, the overlap zone is shortened with the thin filaments projecting beyond the last myosin head and shrinks the H zone, which is normally composed of myosin tails.

    Eventually there is nowhere else for the thin filaments to go and the amount of tension decreases. If the muscle is stretched to the point where the thick and thin filaments do not overlap, no cross bridges can be formed and no tension is produced in this sarcomere. This amount of stretching usually does not occur, as accessory proteins and connective tissue oppose extreme stretching.

    The frequency of motor neuron stimulation

    A single action potential from a motor neuron will produce a single twitch in the muscle fibers of its motor unit. This isolated contraction is calledMuscular contraction. A muscle contraction can last a few milliseconds or 100 milliseconds, depending on the type of muscle. The tension produced by a single contraction can be measured by amyogram, an instrument that measures the amount of tension produced over time (Figure 3). Each contraction goes through three phases.

    • The first phase is thelatent period, during which the action potential is being propagated along the sarcolemma and Ca++ions are released from the SR. This is the phase during which excitation and contraction are coupled, but contraction has not yet occurred.
    • Ocontraction phaseoccurs next. THE CA++ions in the sarcoplasm have bound to troponin, tropomyosin has moved away from the actin-binding sites, cross-bridges have formed, and the sarcomeres are actively shortening to the point of peak tension.
    • The last phase is therelaxation phase, when tension decreases as contraction stops. Here++ions are pumped from the sarcoplasm into the SR and the cross-bridge cycle stops, returning the muscle fibers to their resting state.
    13.5: Nervous System Control of Muscle Tension (3)

    Although a person can feel a “muscle twitch”, a single muscle twitch does not produce any significant muscular activity in a living body. A series of action potentials for muscle fibers is required to produce a muscle contraction that can produce work. Normal muscle contraction is more sustained and can be modified by nervous system stimuli to produce varying amounts of force; this is calledgraded muscle response. The frequency of action potentials (nerve impulses) from a motor neuron and the number of motor neurons that transmit action potentials affect the tension produced in skeletal muscle.

    The rate at which a motor neuron fires action potentials affects the tension produced in skeletal muscle. If the fibers are stimulated while a previous contraction is still occurring, the second contraction will be stronger. This answer is calledwave summation, because the excitation-contraction coupling effects of successive motor neuron signaling are summed or summed (Figure 4a). At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ions, which become available to activate additional sarcomeres while the muscle is still contracting after the first stimulus. The sum results in greater contraction of the motor unit.

    If the frequency of motor neuron signaling increases, the sum and subsequent muscle tension in the motor unit continues to increase until it reaches a peak. The tension at this point is about three to four times greater than the tension in a single muscle contraction, a condition known as incomplete tetanus. During incomplete tetanus, the muscle goes through rapid contraction cycles with a short relaxation phase for each one. If the stimulus frequency is so high that the relaxation phase disappears completely, the contractions become continuous in a so-called complete process.tetanus(Figure 4b).

    13.5: Nervous System Control of Muscle Tension (4)

    During tetanus, the concentration of Ca++Ions in the sarcoplasm allow virtually all sarcomeres to cross-bridge and shorten so that contraction can continue uninterrupted (until the muscle tires and can no longer produce tension).


    13.5: Nervous System Control of Muscle Tension (5)

    When a skeletal muscle is inactive for a long time and then activated to contract, all other things being equal, initial contractions generate about half the force of later contractions. Muscle tension builds up in a gradual way that for some feels like a flight of stairs. This voltage increase is calledstaircase,a condition in which muscle contractions become more efficient. It is also known as the “ladder effect” (Figure 5).

    It is believed that the treppe results from a higher concentration of Ca++in the sarcoplasm resulting from the constant flow of signals from the motor neuron. It can only be maintained with adequate ATP.

    Muscle tone

    Skeletal muscles are rarely completely relaxed or flaccid. Even if a muscle is not producing movement, it contracts a little bit to keep its proteins contractile and producemuscle tone. Tension produced by muscle tone allows muscles to continuously stabilize joints and maintain posture.

    Muscle tone is achieved by a complex interaction between the nervous system and skeletal muscles that results in the activation of a few motor units at a time, probably in a cyclical fashion. In this way, the muscles never fully tire, as some motor units can recover while others are active.

    The absence of low-level contractions that lead to muscle tone is referred to ashypotonyor atrophy, and can result from damage to parts of the central nervous system (CNS), such as the cerebellum, or from loss of innervation to a skeletal muscle, as in poliomyelitis. Hypotonic muscles have a flabby appearance and have functional impairments such as weak reflexes. On the other hand, excessive muscle tone is referred to ashypertension, accompanied by hyperreflexia (excessive reflex responses), often the result of damage to upper motor neurons in the CNS. Hypertonia can manifest with muscle rigidity (as seen in Parkinson's disease) or spasticity, a phasic change in muscle tone, where a limb "bounces" due to passive stretching (as seen in some strokes).

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