Alec Titan
The human body is a complex system, and understanding its muscular composition is essential for various fields, from medicine to sports science. One aspect of this muscular system is the presence of epaxial muscles. Epaxial muscles are a specific group of muscles found in vertebrates, including humans, that play a crucial role in our anatomy and movement. These muscles are distinct from other muscle groups, such as hypaxial muscles, based on their location, function, and innervation. The study of epaxial muscles can provide valuable insights into human development, evolution, and the treatment of muscular conditions.
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What You'll Learn
- Epaxial muscles are the intrinsic (deep) back muscles in vertebrates
- Humans only have a few epaxial muscles, including the erector spinae
- Epaxial muscles are derived from the medial halves of somites
- The epaxial myomeres are located in the anterior trunk region
- The development of epaxial muscles in the human embryo has been studied using Amira3D® reconstruction and Cinema4D® remodelling software
Epaxial muscles are the intrinsic (deep) back muscles in vertebrates
In adult vertebrates, trunk muscles can be divided into hypaxial and epaxial muscles. The epaxial muscles are the intrinsic (deep) back muscles in vertebrates. They are located dorsal to the septum of the vertebrae. The hypaxial muscles, on the other hand, lie on the ventral side. The serratus posterior inferior and serratus posterior superior are hypaxial muscles, innervated by the ventral primary ramus.
In humans, the erector spinae, the transversospinales (including the multifidus, semispinalis, and rotatores), the splenius, and the suboccipital muscles are the only epaxial muscles. The erector spinae, also known as the spinal erectors, are a group of muscles that run alongside the spine and help to keep the body upright. They are essential for maintaining posture and facilitating movement. The transversospinales are a group of deep muscles that run transversely across the spine and provide stability and support. The splenius muscles are located in the neck and upper back and are responsible for various movements of the head and neck. The suboccipital muscles are a group of small muscles located below the skull, and they control fine movements of the head and neck.
The development of epaxial muscles in the human embryo is a complex process. Studies have shown that at Carnegie Stage (CS)15, the epaxial portions of the myotomes become identifiable laterally to the developing vertebrae. As the embryo develops further, the longitudinal muscle mass segregates into medial and lateral columns. The medial column further segregates into intermediate and medial columns, resulting in three longitudinal muscle columns that form the basic architectural design of the intrinsic muscles of the back. This process is influenced by craniocaudal and lateromedial gradients in development.
The differentiation of hypaxial and epaxial muscles is thought to have evolved as a new trait in vertebrate animals. This differentiation is not only based on their relative position to the vertebral column but also on their innervation patterns. The epaxial muscles are innervated by the dorsal branches of the spinal nerves, while the hypaxial muscles are innervated by the ventral branches. The absence of PAX3, a transcription factor, results in impaired muscle development, with hypaxial muscles being more affected than epaxial muscles.
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Humans only have a few epaxial muscles, including the erector spinae
In adult vertebrates, trunk muscles are divided into hypaxial and epaxial muscles. The hypaxial muscles lie ventral to the horizontal septum of the vertebrae, and include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles. On the other hand, epaxial muscles lie dorsal to the septum and include other (dorsal) muscles associated with the vertebrae, ribs, and base of the skull.
In humans, the epaxial muscles are limited to the erector spinae, the transversospinales (including the multifidus, semispinalis and rotatores), the splenius, and suboccipital muscles. The erector spinae, in particular, form the basic architectural design of the intrinsic muscles of the back. They consist of three longitudinal muscle columns, which run from the head to the tail in other vertebrates.
The development of epaxial muscles in the human embryo has been studied using Amira3D® reconstruction and Cinema4D® remodeling software for visualization. At Carnegie Stage (CS)15, the epaxial portions of the myotomes become identifiable laterally to the developing vertebrae. The medial (multifidus) group acquires its transversospinal course during the closure of the vertebral arches in the early fetal period. The anatomical ontology of the epaxial muscles is determined by craniocaudal and lateromedial gradients in development.
The epaxial and hypaxial muscles have also been studied in salamanders, where they act synergistically to produce the travelling waves of lateral bending during swimming. Salamanders also utilise specific forms of locomotion, such as crawling as a second terrestrial gait.
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Epaxial muscles are derived from the medial halves of somites
Humans have epaxial muscles, which lie dorsal to the septum. These include the erector spinae, the transversospinales (including the multifidus, semispinalis and rotatores), the splenius, and suboccipital muscles.
The dermomyotome contains the reservoir of proliferative skeletal muscle progenitor cells. In mice, the absence of PAX3 causes impaired muscle development, specifically a loss of limb and some trunk muscles. However, the epaxial-derived muscles are less affected, indicating that hypaxial and epaxial muscles have different requirements for PAX3.
The embryonic origin of the rhomboid muscles has been studied in quail-chick chimeras, which revealed that these muscles are made up of cells derived mainly from the lateral portion of the somite. This means that the rhomboid muscles, which lie within the epaxial domain of the body, originate from the hypaxial domain of the somites.
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The epaxial myomeres are located in the anterior trunk region
In adult vertebrates, trunk muscles can be divided into hypaxial and epaxial muscles. The epaxial muscles lie dorsal to the septum and include other (dorsal) muscles associated with the vertebrae, ribs, and skull base. Humans have several epaxial muscles, including the erector spinae, the transversospinales, the splenius, and the suboccipital muscles.
Epaxial myomeres are V-shaped muscular blocks, with the point of the V directed anteriorly. They are separated by connective tissue partitions called myosepta. In salamanders, the axial musculature consists of two main muscle groups: the epaxial and hypaxial muscles. The main epaxial muscle (m. dorsalis trunci) extends from the head to the tail, completely segmented into myomeres, with each myomere corresponding to a vertebra.
The epaxial myomeres located in the anterior trunk region exhibit a double burst pattern during each terrestrial stepping cycle. This additional burst is more consistent for myomeres near the pectoral girdle, and their activation may help stabilise the trunk region during stepping. The double burst pattern was observed in EMG recordings of salamanders, which showed alternating unilateral activity independent of the surface or direction of progression.
In tetrapods, the epaxial muscles are known as dorsalis trunci, and they are involved in promoting dorsoventral bending of the spine. The epaxial myomeres in the caudal region of fish are important for propulsion and swimming, while those in the cranial regions are related to feeding processes. The epaxial muscles in tetrapods have undergone some modification to facilitate dorsoventral bending of the spine, which is rare in fishes.
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The development of epaxial muscles in the human embryo has been studied using Amira3D® reconstruction and Cinema4D® remodelling software
In adult vertebrates, the trunk muscles are divided into hypaxial and epaxial muscles. Humans do have epaxial muscles, which include the erector spinae, the transversospinales (including the multifidus, semispinalis and rotatores), the splenius, and the suboccipital muscles. These epaxial muscles are dorsal to the horizontal septum of the vertebrae and are associated with the vertebrae, ribs, and base of the skull.
The development of epaxial muscles in the human embryo is a complex process that has been studied using advanced imaging and analysis techniques. Amira3D® reconstruction and Cinema4D® remodelling software have been particularly useful in this regard. Amira, developed by Zuse Institute Berlin, is an extendable software system for scientific visualisation, data analysis, and presentation of 3D and 4D data. It offers a flexible user interface and modular architecture, making it a versatile tool for processing and analysing data from various modalities, such as micro-CT, PET, and Ultrasound. Amira's capabilities in image segmentation and geometry reconstruction, coupled with Cinema4D's 3D modelling prowess, have likely provided valuable insights into the intricate formation of epaxial muscles during embryonic development.
The differentiation of hypaxial and epaxial muscles is hypothesised to have evolved as a new trait in vertebrates. This evolution has resulted in the distinct arrangement of muscles dorsal and ventral to the horizontal septum of the vertebrae. The epaxial muscles, in particular, have important functions associated with the vertebrae, ribs, and skull, contributing to posture, movement, and structural support.
By utilising Amira3D® reconstruction and Cinema4D® remodelling software, researchers can visualise and analyse the intricate details of epaxial muscle development. This includes studying the cellular changes, growth patterns, and interactions with surrounding structures. Such software tools enable a deeper understanding of the complex processes that occur during human embryonic development, providing valuable insights into the formation of the muscular system and its associated functions.
The application of advanced imaging and analysis software, such as Amira3D® and Cinema4D®, in studying epaxial muscle development, highlights the importance of technological advancements in embryology and developmental biology. These tools enable researchers to non-invasively explore the intricate details of embryonic muscle formation, leading to a more comprehensive understanding of human development and potentially informing clinical practices and interventions related to muscular health.
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Frequently asked questions
Yes, humans have epaxial muscles. In adult vertebrates, trunk muscles are divided into hypaxial and epaxial muscles. The erector spinae, the transversospinales (including the multifidus, semispinalis and rotatores), the splenius, and suboccipital muscles are the only epaxial muscles in humans.
Epaxial muscles are the intrinsic (deep) back muscles. They are innervated by the dorsal branches of the spinal nerves.
The epaxial portions of the myotomes become identifiable during the development of human embryos (5-10 weeks), specifically at Carnegie Stage (CS)15.
