No response is seen if the magnitude of the stimulus is below this threshold value. The sliding filament theory explains the contraction of muscles when the threshold stimulus is achieved.
Wave summation. Also called temporal summation. Phenomenon seen when another stimulus is applied to a muscle before the previous relaxation period is complete, resulting in a stronger contraction. May be due to greater calcium availability in stimulated muscle cells.
This form of tetanus is perfectly normal and in fact is the way you maintain a sustained contraction. Treppe is not a way muscles exhibit graded contractions. There are two types of summation: spatial summation and temporal summation that occur between neurones.
Skip to content Common questions. April 22, Joe Ford. The last phase is the relaxation phase , when tension decreases as contraction stops. A series of action potentials to the muscle fibers is necessary to produce a muscle contraction that can produce work.
Normal muscle contraction is more sustained, and it can be modified by input from the nervous system to produce varying amounts of force; this is called a graded muscle response. The frequency of action potentials nerve impulses from a motor neuron and the number of motor neurons transmitting action potentials both affect the tension produced in skeletal muscle.
The rate at which a motor neuron fires action potentials affects the tension produced in the skeletal muscle. If the fibers are stimulated while a previous twitch is still occurring, the second twitch will be stronger. This response is called wave summation , because the excitation-contraction coupling effects of successive motor neuron signaling is summed, or added together [link] a. Summation results in greater contraction of the motor unit. If the frequency of motor neuron signaling increases, summation and subsequent muscle tension in the motor unit continues to rise until it reaches a peak point.
The tension at this point is about three to four times greater than the tension of a single twitch, a state referred to as incomplete tetanus. During incomplete tetanus, the muscle goes through quick cycles of contraction with a short relaxation phase for each. If the stimulus frequency is so high that the relaxation phase disappears completely, contractions become continuous in a process called complete tetanus [link] b. When a skeletal muscle has been dormant for an extended period and then activated to contract, with all other things being equal, the initial contractions generate about one-half the force of later contractions.
The muscle tension increases in a graded manner that to some looks like a set of stairs. This tension increase is called treppe , a condition where muscle contractions become more efficient. It can only be maintained with adequate ATP. Skeletal muscles are rarely completely relaxed, or flaccid. Even if a muscle is not producing movement, it is contracted a small amount to maintain its contractile proteins and produce muscle tone. The tension produced by muscle tone allows muscles to continually stabilize joints and maintain posture.
Muscle tone is accomplished by a complex interaction between the nervous system and skeletal muscles that results in the activation of a few motor units at a time, most likely in a cyclical manner. In this manner, muscles never fatigue completely, as some motor units can recover while others are active. The absence of the low-level contractions that lead to muscle tone is referred to as hypotonia , and can result from damage to parts of the central nervous system CNS , such as the cerebellum, or from loss of innervations to a skeletal muscle, as in poliomyelitis.
Hypotonic muscles have a flaccid appearance and display functional impairments, such as weak reflexes. Conversely, excessive muscle tone is referred to as hypertonia , accompanied by hyperreflexia excessive reflex responses , often the result of damage to upper motor neurons in the CNS.
The number of cross-bridges formed between actin and myosin determines the amount of tension produced by a muscle. The length of a sarcomere is optimal when the zone of overlap between thin and thick filaments is greatest. Muscles that are stretched or compressed too greatly do not produce maximal amounts of power. A motor unit is formed by a motor neuron and all of the muscle fibers that are innervated by that same motor neuron. A single contraction is called a twitch. A muscle twitch has a latent period, a contraction phase, and a relaxation phase.
The motor units within a given muscle appear to be ranked. The same motor units are recruited for a given force production. Units with higher rankings are recruited as the force needed to perform an action increases. The sarcoplasmic reticulum SR constitutes the main intracellular calcium store in striated muscle and plays an important role in the regulation of excitation-contraction-coupling ECC and of intracellular calcium concentrations during contraction and relaxation.
T-tubules are not required to reach the interior of the cell and therefore not necessary to transmit an action potential deep into the fiber. The force—frequency relationship refers to the phenomenon in which the isometric force produced by a muscle increases with the frequency of activation, resulting in a sigmoidal curve which plateaus at the high frequencies.
Skip to content Miscellaneous. 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. This increasing activation of motor units produces an increase in muscle contraction known as recruitment. As more motor units are recruited, the muscle contraction grows progressively stronger. In some muscles, the largest motor units may generate a contractile force of 50 times more than the smallest motor units in the muscle.
This allows a feather to be picked up using the biceps brachii arm muscle with minimal force, and a heavy weight to be lifted by the same muscle by recruiting the largest motor units.
When necessary, the maximal number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle, but this cannot last for very long because of the energy requirements to sustain the contraction. To prevent complete muscle fatigue, motor units are generally not all simultaneously active, but instead 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 a skeletal muscle.
When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-bridges followed by the thin filaments sliding over the thick filaments as the heads pull the actin, and this results in sarcomere shortening, creating the tension of the muscle contraction. The cross-bridges can only form where thin and thick filaments already overlap, so that the length of the sarcomere has a direct influence on the force generated when the sarcomere shortens.
This is called the length-tension relationship.
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