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Muscles of vertebrates is a topic that covers the comparative anatomy, evolution, homologies, and development of the head, neck, pectoral, and forelimb muscles of vertebrates. Vertebrates are one of the most diverse groups of animals, with over 58,000 living species. The study of vertebrate muscles involves understanding their structure, function, and evolution, including the cranial muscles and their motor nerves. This field of study is essential for various disciplines, including zoology, molecular biology, and medicine, as it helps us understand the similarities and differences among different species and contributes to our knowledge of human anatomy and muscular abnormalities.
| Characteristics | Values |
|---|---|
| Number of Living Vertebrate Species | More than 58,000 |
| Muscle Names | Many names were given hundreds of years ago and are still used today |
| Muscle Homology Criteria | Metamerism, Segmentality, Embryonic Origin, and Similarities in Origins and Insertions |
| Muscle Types | Epaxial, Hypaxial, Appendicular, Secondary Appendicular, Intrinsic, Extrinsic, Pectoralis, Supracoracoideus, Constrictors, Levators, and more |
| Muscle Groups | Head and Neck, Pectoral, Forelimb, Abdominal, Vertebral, Cranial, and more |
| Muscle Study Applications | Functional Morphology, Ecomorphology, Evolutionary Developmental Biology, Zoology, Molecular Biology, Embryology, Phylogeny, Medicine |
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What You'll Learn
Muscles of vertebrates: comparative anatomy
The Vertebrata is one of the most speciose groups of animals, comprising more than 58,000 living species. A book by Diogo and Abdala provides a detailed account of the comparative anatomy, development, homologies, and evolution of the head, neck, pectoral, and forelimb muscles of vertebrates. The book includes illustrations and tables showing the homologies between the muscles of all the major extant vertebrate taxa, including lampreys, elasmobranchs, hagfish, coelacanths, and ginglymodians.
The book also provides a list of synonyms used by other authors to designate these muscles. It reviews data from evolutionary developmental biology, molecular biology, and embryology, explaining how this data helps understand the evolution and homologies of vertebrate muscles. The book will be useful to students, teachers, and researchers working in fields such as functional morphology, ecomorphology, and evolutionary developmental biology.
Human muscles were named several hundred years ago, and many of these names are still used today. Based on similarities of origins and insertions, these names were subsequently used for the corresponding muscles of other vertebrates. However, origins and insertions are not reliable criteria for determining homology because natural selection has sometimes favored "shifts" in muscle position. More reliable criteria for determining homologies are whether the muscles are metameric, segmental, or arise from segmental mesodermal somites.
Tetrapods, like fish, have epaxial and hypaxial masses, and these retain some evidence of metamerism even in the highest tetrapods. The epaxials of tetrapods lie along the vertebral column, and hypaxials of the abdomen have no myosepta and form broad sheets of muscle. Reptiles have more numerous and diverse muscles than amphibians, providing better support of the body and increased mobility of the distal segments of the limbs.
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Evolution, homologies and development
The Vertebrata is one of the most speciose groups of animals, comprising more than 58,000 living species. The evolution of vertebrate muscles can be traced back to the emergence of ancestral limb muscles within the first 30% of vertebrate evolution history, approximately 520 million years ago. The fin-to-limb transition occurred rapidly, while limb muscles remained largely unchanged for 85-90% of the paired appendage history. This extended period without significant evolutionary deviation is strong evidence for the robustness of limb muscle development and its singular evolutionary origin.
The evolution of the muscular system in tetrapod limbs has received less attention compared to skeletal evolution. Tetrapod limb muscles develop from diffuse migrating cells derived from dermomyotomes, and the limb-innervating nerves lose their segmental patterns to form the brachial plexus distally. Despite these seemingly disorganized developmental processes, limb muscle homology has been highly conserved in tetrapod evolution, except for the mammalian diaphragm. The limb mesenchyme likely plays a crucial role in the subdivision of the myogenic cell population into individual muscles through the formation of interstitial muscle connective tissues. Interactions with tendons and motoneuron axons are involved in the early and late phases of limb muscle morphogenesis, contributing to the generation of limb muscle homology.
The evolutionary rate of vertebrate morphology has been variable, with morphological deviations and alterations occurring unevenly throughout history. The integration of paleontology and evolutionary developmental biology can enhance our understanding of how morphological homologies arise and why they are conserved in subsequent generations. However, the challenge lies in the fact that only post-embryonic morphology is typically observable in fossils, making it difficult to attribute observed changes solely to evolutionary processes.
The study of muscle homologies in vertebrates involves examining the similarities and differences in muscle origins and insertions across species. While muscle names in humans were assigned several hundred years ago, these names have been applied to corresponding muscles in other vertebrates based on apparent similarities. However, origins and insertions alone are not reliable criteria for determining homology due to shifts in muscle position influenced by natural selection. More reliable criteria for determining homologies include metamerism, segmental embryonic origin, and development from segmental mesodermal somites.
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Head and neck muscles
The head and neck muscles in vertebrates vary across species, from fish to modern humans, and play a crucial role in various functions, including movement, breathing, swallowing, and facial expressions.
Neck Muscles
The neck is a complex region with around 30 muscles in humans that work together to support and stabilise the head, neck, and upper spine. These muscles enable movements such as nodding, twisting, and tilting the head. Some specific neck muscles include:
- Transversospinalis muscles: These muscles help move the head forward, backward, and side to side, and they also stabilise the spine.
- Scalene muscles: These muscles control the movement of the first two ribs during breathing and assist in head movements and stabilising the cervical spine.
- Infrahyoid muscles: These four muscles are located below the hyoid bone and move the larynx up and down.
- Lateral neck muscles: These muscles control head movements from the base of the skull and enable twisting and tilting of the cervical spine.
Head Muscles
The head muscles, particularly those in the face, play a significant role in facial expressions and non-verbal communication. Some of the muscles involved in facial expressions include:
- Frontalis
- Orbicularis oris
- Laris oculi
- Buccinator
- Zygomaticus
Additionally, there are four pairs of muscles responsible for chewing or mastication, with the temporalis and masseter being the most prominent.
Vertebrate Muscles in Different Species
The structure and function of vertebrate muscles vary across different species:
- Fish: Fish exhibit metamerism, with axial muscles used for locomotion.
- Amphibians: In aquatic amphibians, axial muscles are employed for swimming, while in tetrapods like urodeles (salamanders), epaxial muscles are referred to as dorsalis trunci.
- Reptiles: Reptiles showcase more diverse and numerous muscles than amphibians, providing better body support and increased mobility in distal limb segments.
- Birds: Birds exhibit enlarged pectoralis (downstroke muscle) and supracoracoideus (upstroke muscle) muscles.
- Mammals: Mammals display a range of muscle structures, including the rectus abdominis muscle, which extends from the sternum to the pelvic girdle.
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Appendicular muscles
The appendicular muscles are those that are required to operate the limbs. They are divided into two groups: the primary appendicular muscles, whose original function was to operate appendages, and the secondary appendicular muscles, which did not originally function to operate appendages.
The appendicular muscles of the lower body position and stabilize the pelvic girdle, which serves as a foundation for the lower limbs. The pelvic girdle has a limited range of motion because it was designed to stabilize and support the body. The body's centre of gravity is in the area of the pelvis, and if it were not fixed, standing up would be difficult. The leg muscles may have a limited range of motion and versatility, but they make up for it in size and power, facilitating the body's stabilization, posture, and movement.
Most muscles that insert on the femur (the thigh bone) and move it, originate on the pelvic girdle. The psoas major and iliacus make up the iliopsoas group, which are the major flexors of the hip. Some of the largest and most powerful muscles in the body are the gluteal muscles or gluteal group. The gluteus maximus is the largest, and it is one of the major extensors of the thigh at the hip. Deep to the gluteus maximus is the gluteus medius, and deep to the gluteus medius is the gluteus minimus, the smallest of the trio.
The anterior muscles of the femur extend the lower leg and aid in flexing the thigh. The posterior muscles of the femur flex the lower leg and aid in extending the thigh. A combination of gluteal and thigh muscles also adduct, abduct, and rotate the thigh and lower leg. The tensor fascia latae is a thick, squarish muscle in the superior aspect of the lateral thigh. It acts as a synergist of the gluteus medius and iliopsoas in flexing and abducting the thigh. It also helps stabilize the lateral aspect of the knee by pulling on the iliotibial tract (band), making it taut. Deep to the gluteus maximus are the piriformis, obturator internus, obturator externus, superior gemellus, inferior gemellus, and quadratus femoris, which laterally rotate the femur at the hip. The adductor longus, adductor brevis, and adductor magnus can both medially and laterally rotate the thigh depending on the placement of the foot.
Deep fascia in the thigh separates it into medial, anterior, and posterior compartments. The muscles in the medial compartment of the thigh are responsible for adducting the femur at the hip. Along with the adductor longus, adductor brevis, adductor magnus, and pectineus, the strap-like gracilis adducts the thigh in addition to flexing the leg at the knee. The muscles of the anterior compartment of the thigh flex the thigh and extend the leg. This compartment contains the quadriceps femoris group, which comprises four muscles that extend and stabilize the knee. The rectus femoris is on the anterior aspect of the thigh, the vastus lateralis is on the lateral aspect of the thigh, the vastus medialis is on the medial aspect of the thigh, and the vastus intermedius is between the vastus lateralis and vastus medialis and deep to the rectus femoris. The tendon common to all four is the quadriceps tendon (patellar tendon), which inserts into the patella and continues below it as the patellar ligament. The patellar ligament attaches to the tibial tuberosity. In addition to the quadriceps femoris, the sartorius is a band-like muscle that extends from the anterior superior iliac spine to the medial side of the proximal tibia. This versatile muscle flexes the leg at the knee and flexes, abducts, and laterally rotates the leg at the hip. This muscle allows us to sit cross-legged. The posterior compartment of the thigh includes muscles that flex the leg and extend the thigh. The three long muscles on the back of the knee are the hamstring group, which flexes the knee. These are the biceps femoris, semitendinosus, and semimembranosus. The tendons of these muscles form the popliteal fossa, the diamond-shaped space at the back of the knee.
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Axial muscles
The axial musculoskeletal system is central to the vertebrate body, playing a crucial role in locomotion and existing in all craniates. Axial muscles can be broadly divided into two groups: epaxial and hypaxial. These two groups are initially separated by a horizontal myoseptum. Epaxial muscles reside dorsal to the myoseptum and include muscle groups associated with the vertebral column and the base of the skull. In contrast, hypaxial muscles are located ventral to the mysoseptum and give rise to various muscle groups, including abdominal and intercostal muscles, as well as the diaphragm in mammals.
In fish and amphibians, the distinction between dorsal and ventral axial muscles is maintained into adulthood. However, in tetrapods, many of these positional differences have been lost. In tetrapods, hypaxial muscles also generate all the muscle in the limb. The axial muscles in tetrapods have evolved unique functions essential for life on land, including maintaining spinal alignment and breathing.
The evolution of the axial system is marked by significant changes in its morphology and function. The increasing differentiation of its muscular, neural, and skeletal components contributes to the diversity of locomotor mechanics among craniates. The transition to land, coupled with the evolution of extremities, led to increased complexity in the planes of axial movement and profound changes in axial muscle function. The axial system became progressively appendage-based, and axial muscles took on the additional role of stabilising the trunk against gravitational and inertial forces.
In aquatic vertebrates, the rhythmic contraction of axial muscles is vital for generating propulsive force during swimming. In contrast, land vertebrates' axial circuits have evolved to enable new motor capabilities, dissociating from locomotor functions. The axial neuromuscular system in fish and tetrapods shares many anatomic features and early developmental programs despite their distinct motor functions.
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Frequently asked questions
Yes, all vertebrates have muscles.
The book provides a detailed account of the comparative anatomy, development, homologies, and evolution of the head, neck, pectoral, and forelimb muscles of vertebrates.
Some examples of vertebrate muscles include the cranial muscles, pectoral and forelimb muscles, pelvic girdle muscles, and the rectus abdominis muscle.
In reptiles, there are muscles such as the pectoralis and supracoracoideus. Birds have enlarged pectoralis and supracoracoideus muscles as well. In fish, there are epaxial and hypaxial muscles.
Vertebrate muscles were named several hundred years ago, and these names were based on similarities of origins and insertions. While origins and insertions may not be reliable criteria for determining homology due to shifts in muscle position, other criteria include metamerism, segmental origin, and embryonic origin.
