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Animals have different muscle fibre types, which can be categorized into two types: slow fibres and fast fibres. Slow fibres have a low maximum velocity of shortening, whereas fast fibres have a high maximum velocity of shortening. The muscle structure of animals also varies depending on their body structure. For instance, slugs and worms are invertebrates that lack a skeleton, and their movement is not produced by lever action. On the other hand, vertebrates have body parts with muscles but no skeletal component, such as the tongue.
| Characteristics | Values |
|---|---|
| Muscle structure | Muscle structure varies depending on the animal. For example, a cat's muscle layout will differ from a lizard's. |
| Large muscle groups and their nerves are similar across animals due to evolution and embryology. | |
| Muscle structure also depends on the type of muscle fiber present. Animals with slow muscle fibers have lower maximum velocity, while those with fast muscle fibers have a higher maximum velocity. | |
| Muscle structure also depends on the animal's size, as larger animals require more muscle power to support their weight. | |
| Echinoderms have two main muscular systems, the visceral and the somatic. | |
| Invertebrate animals without a skeleton, such as slugs and worms, have muscle systems based on longitudinal and circular muscle fibers. |
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What You'll Learn
Muscle structure and movement
Types of Muscle Tissue
There are three primary types of muscle tissue found in animal bodies: smooth muscle, skeletal muscle, and cardiac muscle. Smooth muscle, also known as non-striated muscle, lacks the banded appearance of skeletal and cardiac muscles. It is under involuntary control and is responsible for functions like pushing blood through blood vessels and moving food through the digestive tract. Skeletal muscle, on the other hand, is under voluntary control and works in coordination with the skeletal system to enable movement, posture, and balance. Cardiac muscle, found exclusively in the heart, is responsible for the rhythmic contractions that pump blood throughout the body. Cardiac muscle is also involuntary and influenced by the autonomic nervous system to regulate heart rate.
Muscle Structure and Function
The structure of muscles can vary depending on the animal's anatomy and mode of movement. For example, animals like slugs, worms, and many invertebrates lack a skeleton, so their movement is not produced by lever action. Instead, they rely on muscle systems where longitudinal muscle fibres run lengthwise along the body, and circular fibres encircle it. This allows them to deform their bodies into different shapes while maintaining a constant volume. In vertebrates, there are also parts of the body, such as the tongue, that have muscles but no skeletal component.
Some animals, like sea anemones, have all their muscle fibres in the gastrodermis, with a mix of longitudinal and circular fibres. By contracting these muscles, sea anemones can change their shape, growing longer and thinner or shorter and fatter, and bending in any direction. Additionally, the water in their gut cavity acts as a hydrostatic skeleton, providing further flexibility and support for movement.
Evolutionary Tweaks and Performance
Evolution plays a significant role in muscle structure across species. Large muscle groups and their nerves are often similar due to evolutionary "tweaks" and the limitations of embryology. However, these muscle groups can vary in their composition, with some species having a single muscle and others having multiple muscles performing similar functions. For example, a cat's biceps brachii may differ from a human's due to their different physical attributes, such as retractable claws and the ability to stand on all fours.
The specific composition of muscle fibres also influences performance. Some species may have predominantly fast muscle fibres, which are stronger and faster but fatigue quickly, while others may have more slow fibres, which are slower and weaker but have higher endurance. This variation in muscle structure and function allows animals to adapt to their unique ecological niches and perform the necessary tasks for survival.
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Muscle regeneration
Regulatory T cells (Treg) and satellite cells are important players in muscle regeneration, influencing the inflammatory milieu in regenerating muscles. Treg cells are closely related to regenerating fibres during muscle repair, while satellite cells contribute to the maintenance and alteration of a homeostatic environment. Other precursors and stem cell populations, either residing within the muscle or recruited via circulation in response to injury, can also contribute to muscle regeneration. For example, muscle-resident non-myogenic cells, such as fibro-adipogenic progenitors (FAPs), are determinant components of the muscle niche.
The regenerative potential of muscles can vary among different body parts and species, as well as with animal age and body size. Echinoderms, for instance, possess two main muscular systems, the visceral and the somatic, and their muscles retain some epithelial features. Amphioxus exhibits the highest regenerative capacity in the oral cirri and post-anal tail regions.
The study of muscle regeneration has a broad scope, with early observations noting the potential influences of weather conditions and sociopolitical contexts on recovery and tissue repair. While exercise was once believed to be detrimental to muscle regeneration, it is now recognised that potential benefits may exist, although the risk of injury and over-exertion must be considered. Overall, the complex nature of muscle regeneration continues to draw significant scientific attention, with ongoing research aiming to develop effective therapeutic strategies for muscular disorders and improve our understanding of cellular behaviour and molecular pathways during each regenerative stage.
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Muscle fibre types
Slow oxidative fibres, also known as slow-twitch fibres, contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They produce low-power contractions over long periods and are slow to fatigue. The soleus muscle in the leg has a high proportion of slow-twitch fibres.
Fast oxidative fibres, also known as fast-twitch fibres, have relatively fast contractions and primarily use aerobic respiration to generate ATP. They produce higher-tension contractions than slow oxidative fibres. The extraocular muscles that position the eyes have a high proportion of fast-twitch fibres.
Fast glycolytic fibres, also known as Type II fibres, primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess large volumes of glycogen, which is used to generate ATP quickly. Due to their reliance on anaerobic metabolism, these fibres do not possess a substantial number of mitochondria, resulting in a limited capillary supply and a white coloration for muscles containing large numbers of these fibres. Fast glycolytic fibres fatigue quickly and are only used for short periods, but they can produce rapid, forceful contractions associated with quick, powerful movements.
The different types of muscle fibres are present in varying proportions in most skeletal muscles, and they have the ability to adapt to changing demands by changing size or fibre type composition. This plasticity serves as the basis for physical therapy interventions designed to increase a patient's force development or endurance. For example, endurance training can modify slow fibres to make them more efficient by producing more mitochondria and increasing aerobic metabolism and ATP production. On the other hand, high-intensity resistance training can lead to changes in fibre type similar to those seen with endurance training, although muscle hypertrophy also plays a role in producing strength gains.
It is worth noting that the major bones, muscles, organs, and nerves in humans are generally present in approximately the same locations in companion mammal species. However, the specific muscle layout and structure may vary between different species. For example, a lizard and a cat may not have the exact same muscle layout, but their muscle lever arms and anatomy can be compared to determine which has a stronger hind limb.
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Muscles and body size
Muscle anatomy varies across different animal species, with some species having unique muscle structures. However, large muscle groups and their nerves are generally similar across species due to evolutionary "tweaking". This means that while a cat's muscle layout may not be identical to a human's or a lizard's, they share similar major bones, muscles, organs, and nerves located in approximately the same places.
The functions and performance of muscles can differ across species. For instance, a cat's biceps brachii may be toned differently to facilitate standing on all fours. Additionally, some species may have predominantly fast muscle fibres, which are stronger and faster but fatigue quickly, while others may have more slow fibres, which are slower and weaker but have higher endurance.
The size and structure of muscles are influenced by an animal's body size and shape. For example, the human gluteus maximus muscle is larger relative to overall body size when compared to apes and monkeys. This may be an evolutionary adaptation to the demands of being bipedal. Similarly, the latissimus dorsi, or "lats," are the largest human muscles in terms of surface area.
Invertebrates like slugs, worms, and many other animals lack a skeleton, and their movement is not produced by lever action. Instead, they have muscle systems based on longitudinal and circular muscle fibres that allow them to deform their bodies into different shapes while maintaining a constant volume. Sea anemones, for instance, can change their body shape through the interaction of longitudinal and circular muscles.
Amphioxus, a type of invertebrate, possesses almost exclusively striated muscles, with the most prominent being the segmental axial muscles that provide force for burrowing and swimming. Echinoderms, on the other hand, have two main muscular systems: the visceral and the somatic. Despite differences in anatomical location, their muscles share a similar structure.
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Muscles in invertebrates
The muscular cells of invertebrates can be divided into three major classes based on their striation pattern: transversely striated, obliquely striated, or smooth muscle. Smooth muscle has been observed in coelenterates, annelids, molluscs, brachiopods, and echinoderms, but not in arthropods.
Invertebrate thin filaments are similar to vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, on the other hand, are very different from vertebrate striated thick filaments and show great variation within invertebrates. This diversity is partly due to differences in paramyosin content, which is absent in vertebrate muscles. The thick (myosin) myofilaments in invertebrates also show variation in length and width.
The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates, this process is understood on a near-atomic level. Invertebrate muscles are dually regulated, with thin filaments regulated by tropomyosin and troponin, and thick filaments primarily regulated by direct Ca++ binding to myosin.
Some examples of muscle structures in invertebrates include the shell muscle of the abalone Haliotis, which connects the domed shell to its adhesive foot. When the muscle shortens, the shell is pulled down to protect the animal. When the muscle lengthens, the shell is raised, allowing respiratory water currents to circulate. Another example is the sea anemone, which has both longitudinal and circular muscle fibres in the gastrodermis. When the mouth of the sea anemone is closed, the water in the gut cavity acts as a hydrostatic skeleton, allowing the animal to change its shape.
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Frequently asked questions
Most animals have different types of muscles depending on their species. For example, a cat does not have the same bicep muscles as a human, and a lizard does not have the same muscle layout as a cat. However, evolution has conserved the major bones, muscles, organs, and nerves in mammals, which are generally present in approximately the same locations.
Yes, all animals have muscles, but they vary in structure and function. For example, slugs and worms have no skeleton, so their movement is not produced by lever action. Echinoderms, such as sea stars and sea urchins, have two main muscular systems: the visceral and the somatic.
Yes, animals with fast muscle fibres are stronger and faster but fatigue quickly, while those with slow muscle fibres are slower and weaker but have extremely high endurance. For example, monkeys are strong but slower, whereas humans have weaker muscle strength but higher speed and endurance.
