Section 3 Flashcards
Describe what is meant by motor unit recruitment.
Describe the length-tension relationship for skeletal muscle.
Describe what is meant by twitch summation and tetanus.
Compare and contrast the different types of skeletal muscle contractions.
How do whole muscles contract in terms of muscle fibers?
Whole muscles are composed of various numbers of muscle fibers. Smaller muscles may have a few hundred fibers, while larger muscles, like those in the leg, can have hundreds of thousands of individual fibers.
What is the basic unit of muscle contraction?
The basic unit of muscle contraction is a single muscle fiber’s contraction, known as a “twitch.”
Why is a single muscle fiber’s twitch not sufficient to generate the contraction of an entire muscle?
A single muscle fiber’s twitch is not sufficient to contract the entire muscle because it represents only a localized response. For a muscle to develop tension and contract as a whole, more muscle fibers need to be involved.
What are the two ways by which a muscle can develop tension by involving more muscle fibers?
Muscle tension can be developed by involving more muscle fibers through either “motor unit recruitment” or by “increasing the frequency of stimulation.”
What is motor unit recruitment in muscle contraction?
Motor unit recruitment is the process by which more motor units (a motor neuron and the muscle fibers it innervates) are activated to increase muscle force and contraction.
How does increasing the frequency of stimulation affect muscle contraction?
Increasing the frequency of stimulation involves rapidly firing action potentials, which can lead to a sustained and stronger muscle contraction as more motor units are activated more frequently.
What is the relationship between the number of muscle fibers recruited and muscle tension in muscle contraction?
The greater the number of muscle fibers recruited to contract, the greater the muscle tension.
What is a motor unit in muscle physiology?
A motor unit is a motor neuron and the multiple muscle fibers it innervates. When a motor neuron is activated, all the muscle fibers in that motor unit contract simultaneously.
Are the muscle fibers of a motor unit adjacent to each other?
No, the muscle fibers of a motor unit are not adjacent; they are spread throughout the entire muscle. Activation of a single motor unit results in a weak contraction, and to achieve a stronger contraction, multiple motor units need to be recruited.
How does the body prevent muscle fatigue during sustained contractions?
To prevent muscle fatigue during sustained contractions, the body can selectively rotate the activation of motor units. This allows some motor units to rest while others take over, helping to maintain contractions for a longer duration.
How does the frequency of stimulation affect muscle contraction?
The frequency of stimulation can affect muscle contraction in several ways:
- Restimulating a muscle fiber AFTER complete relaxation results in a second twitch of the same magnitude as the first.
- Restimulating a muscle fiber BEFORE complete relaxation leads to twitch summation, where the second twitch adds on to the first.
- Rapid stimulation of muscle fibers with overlapping twitches can cause tetanic contraction, which can be unfused (where the muscles fibres do not completely relax before the next stimulus because they are being stimulated at such a high rate) or fused (where there is no relaxation of the muscles fibres between stimuli), with fused tetanic contraction being the strongest single-unit twitch in muscle contraction. This state is known as tetanus.
Explain what would occur if you held a 10 pound weight with a straight arm for 5 minutes.
When holding a heavy load, motor units are stimulated by motor neurons at very high frequency.
Each stimulus causes a twitch. If you were to hold the 10 pound weight for 5 continuous minutes with no
period of relaxation, the twitches would overlap and cause what is known as fused tetanus.
This maximal contraction would occur until fatigue. At this point, the 10 pound weight would be dropped or the arm position would change in order to recruit other muscles.
What is the relationship between muscle length and the amount of tension that can be generated in muscle contraction?
The amount of tension that can be generated in muscle contraction, especially during tetanus, is greatly dependent on the length of the muscle at the onset of contraction. This relationship is known as the length-tension relationship.
What is the optimal length (Io) of a muscle fiber, and why is it important in force generation?
Every muscle fiber has an ideal or optimal length (Io) at which maximal force can be generated. This length provides the best conditions for cross-bridge binding and maximal force production.
How does muscle length affect the efficiency of contraction and tension when it is less than the optimal length?
When muscle fiber length is less than the optimal length, thin filaments overlap the part of thick filaments without cross-bridges, reducing the efficiency of contraction and resulting in decreased tension. Further shortening can lead to thin filaments from opposite sides of the sarcomere overlapping and thick filaments contacting the Z-lines.
What happens to muscle fiber tension when it is at the optimal length?
At the optimal length, the maximal number of cross-bridge binding sites are available for cross-bridges to bind. The thin filaments do not overlap the central region of the thick filaments, which lacks cross-bridges. This results in the muscle fiber’s ability to generate maximal tension.
How does muscle length affect tension when it is greater than the optimal length?
When muscle fibers are stretched beyond the optimal length, the distance between Z-lines increases, decreasing the overlap of thick and thin filaments. With less overlap, fewer cross-bridges are available, leading to reduced tension. When muscle length exceeds approximately 70% longer than the optimal length, there is no overlap between thick and thin filaments, preventing contraction altogether.
What is the significance of muscle fibers generally being at the optimal length at rest?
Muscle fibers are generally at the optimal length at rest, which means they can achieve maximal contraction when needed. This ensures that muscles are prepared to generate the most force when required.
What is the relationship between muscle tension and bones in the context of muscle contraction?
The relationship between muscle tension and bones is vital for movement. Muscles are connected to bones through tendons, and the contraction or relaxation of muscles allows for the movement of bones around a joint.
How are muscles typically attached to bones around a joint?
Muscles are usually attached to two bones surrounding a joint. Connective tissues surrounding the muscles form tendons, making a physical connection to the bone.
