10.6: Muscle Tension at the Fiber Level
I. General Information about Classes of Skeletal Muscle Fibers
A. There are two primary classes of skeletal muscle fibers, which are categorized as Type I (slow oxidative) and Type II (fast glycolytic) fibers.
B. These classifications are based on various factors, including the activity of myosin ATPase—an enzyme critical for muscle contractions—and the predominant energy source each fiber utilizes to produce ATP.
C. It is important to note that most skeletal muscles are not composed of only one type of fiber; instead, they generally contain a combination of both Type I and Type II fibers. The proportion of each fiber type varies significantly depending on the specific function and activity demands of the muscle group.
II. Classes of Skeletal Muscle Fibers
A. Type I fibers / Slow Oxidative fibers
Type I fibers are characterized by their small diameter and slow-twitch properties, which allow them to contract slowly, producing less force over an extended period. They are ideally suited for endurance activities.
These fibers exhibit low myosin ATPase activity, which contributes to their slower contraction rates.
Type I fibers primarily rely on oxidative catabolism for ATP production, meaning they utilize aerobic metabolism. This metabolic pathway is more efficient in producing ATP due to the ample presence of oxygen.
These fibers contain a high density of mitochondria, a well-developed blood supply, and an abundance of myoglobin. The presence of myoglobin is critical as it facilitates oxygen storage and transport, enhancing aerobic metabolism.
a. As a result of these characteristics, Type I fibers have a distinct dark red appearance, often referred to as “dark muscle,” due to the high levels of myoglobin and vascularization.
These fibers predominantly operate in muscles that are vital for maintaining posture and sustaining contractions over lengthy durations, such as the muscles of the back or legs responsible for standing.
B. Type II fibers / Fast Glycolytic fibers
In contrast, Type II fibers are larger in diameter and exhibit fast-twitch properties. They are built for short bursts of power and speed, enabling rapid contractions that ultimately fatigue more quickly than Type I fibers.
These fibers display high levels of myosin ATPase activity, which allows for more rapid contraction rates. However, this also means they rely mainly on glycolytic catabolism for ATP production, which is anaerobic and generates energy more quickly but less efficiently than oxidative pathways.
Type II fibers contain fewer mitochondria, lower levels of myoglobin, and a less extensive blood supply compared to Type I fibers. This results in their lighter color, commonly referred to as “white muscle.”
Because of their reliance on the anaerobic metabolic pathway, Type II fibers are more suited for activities requiring powerful, explosive movements, but they fatigue rapidly, thus limiting their use for prolonged activities.
Examples of Type II fibers include those found in eye muscles and other muscles involved in quick and precise movements; they play a crucial role in tasks necessitating strength and speed, such as sprinting or heavy lifting.
Has two subtypes: Fast Oxidative Glycolytic, and Fast Glycolytic
type IIx fibers are even more powerful
Twitch Contraction
Latent Period: This is the initial phase following the stimulation where there is no observable contraction. It lasts a few milliseconds as the muscle prepares for contraction by releasing calcium ions from the sarcoplasmic reticulum and generating the necessary force.
Contraction Period: During this phase, the muscle fibers shorten, and tension increases as myosin heads pull on actin filaments, forming crossbridges. The duration of this period varies depending on the muscle fiber type and the strength of the stimulus.
Relaxation Period: This phase occurs after the contraction where the tension decreases as calcium ions are reabsorbed into the sarcoplasmic reticulum and the muscle returns to its resting state.
During the start of the latent period and the start of the contraction period, there is a 5 ms interval that the fiber cannot respond to any other stimulus, called the refractory period
Tension Production and the Timing and Frequency of Stimulation
Repeated stimulation of a muscle fiber leads to greater contractions. This occurs because calcium ions cannot be fully removed from the cytosol back into the sarcoplasmic reticulum, resulting in higher concentrations of calcium ions in the cytosol. This phenomenon is known as Wave Summation.
The number of times a muscle fiber is stimulated per second influences its contraction state, which can lead to two different types of tetanus:
Unfused Tetanus: In this state, there is partial relaxation between stimuli. The muscle fiber produces a sustained but wavering contraction, reflecting a series of stimuli that do not allow full relaxation. Occurs if the fiber is stimulated about 50 times per second
Fused Tetanus: This occurs when stimuli are delivered at a high frequency, leading to a sustained contraction without any relaxation. The muscle achieves maximum tension, and this type of contraction is crucial in maintaining posture or producing maximal effort in activities that require sustained force. Occurs if the fiber is stimulated 80 to 100 times per second
V: Length Tension Relationships
Number of crossbridges that can form within a sarcomere also determines the amount of tension produced from a contraction, which depends on the length prior to contraction
Optimal Length is the sarcomere length at which the most crossbridges can form, generating nearly 100% of possible tension
Elastic filaments allow the muscle fibers to stretch, thus, for example, when the wrist is flexed, and the forearm is shortened, there is a great zone of overlap. However when in a neutral state, there are in an optimal state, as there is a small zone of overlap, leading it to have greater force output, but stretched leads to little zone of overlap, leading to lower muscle tensions. The myosin heads in a flexed position have little more room to pull on the actin subunits; the myosin heads in a stretched position have too little room to grip the actin subunits