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The hypaxial muscles are a group of muscles in the trunk of jawed vertebrates, including humans, that are located on the ventral side of the body, often below the horizontal septum. They are innervated by the ventral ramus of the spinal nerves and include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles. The serratus posterior inferior and serratus posterior superior are hypaxial muscles that are specifically innervated by the ventral primary ramus. The hypaxial muscles are defined by their pattern of motor innervation from the spinal cord and exhibit highly differentiated morphology and function.
| Characteristics | Values |
|---|---|
| Location | Ventral side of the body |
| Innervation | Ventral ramus (branch) of the spinal nerves |
| Muscle types | Vertebral muscles, diaphragm, abdominal muscles, limb muscles, tongue/infrahyoid (hypobranchial) muscles |
| Examples | Serratus posterior inferior, serratus posterior superior |
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What You'll Learn
- Hypaxial muscles are innervated by the ventral ramus of spinal nerves
- They include vertebral muscles, the diaphragm, abdominal muscles, and limb muscles
- The serratus posterior superior and serratus posterior inferior are innervated by the ventral primary ramus
- Hypaxial muscles are defined by the pattern of motor innervation from the spinal cord
- Lamprey HBMs are innervated by the hypoglossal nerve
Hypaxial muscles are innervated by the ventral ramus of spinal nerves
The hypaxial muscles are a group of muscles in the trunk of jawed vertebrates, including humans. They are located on the ventral side of the body, often below the horizontal septum of the vertebrae. The hypaxial muscles include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles.
The hypaxial muscles are innervated by the ventral ramus (or branch) of the spinal nerves. This is in contrast to the epaxial muscles, which lie dorsal to the septum and are innervated by the dorsal ramus of the spinal nerves. The epaxial muscles include other (dorsal) muscles associated with the vertebrae, ribs, and base of the skull.
In amniotes such as mammals, birds, and reptiles, the muscle layers at the dorsal and ventral sides of the body are categorized as epaxial and hypaxial muscles, respectively. These muscle groups are innervated by the dorsal and ventral rami of spinal motor nerves, respectively. The epaxial and hypaxial distinction is more noticeable in cartilaginous and bony fish, where the two compartments are separated by the horizontal myoseptum, a connective tissue sheet located on either side of the notochord.
The hypaxial muscles are highly variable in morphology and function, including muscles associated with paired limbs, shoulder girdles, and tongue/infrahyoid (hypobranchial) muscles. Lampreys, which are jawless vertebrates, lack many of the derived traits of gnathostomes, such as jaws and paired fins, but they possess the primitive form of the hypobranchial muscle. Lamprey HBMs (hypobranchial muscles) are innervated by the hypoglossal nerve, which is homologous to the 12th cranial nerve in mammals.
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They include vertebral muscles, the diaphragm, abdominal muscles, and limb muscles
The hypaxial muscles are located on the ventral side of the body, often below the horizontal septum of the vertebrae. They include vertebral muscles, the diaphragm, abdominal muscles, and limb muscles.
Vertebral Muscles
The vertebral muscles are those that are connected to the vertebrae, which are the bones that make up the spinal column. These muscles help to support the torso and enable movement, including bending over, twisting, turning the head, and extending the back. They also play a role in breathing by expanding and contracting the chest during inhalation and exhalation.
Diaphragm
The diaphragm is a dome-shaped muscle that separates the chest cavity from the abdominal cavity. It plays a crucial role in breathing by contracting and relaxing to facilitate inhalation and exhalation. The diaphragm is innervated by the ventral ramus, which arises from the spinal nerves.
Abdominal Muscles
The abdominal muscles consist of five main pairs of muscles. Two are vertical muscles located towards the middle of the body, and three are flat muscles stacked on top of each other, situated towards the sides of the trunk. The rectus abdominis, one of the vertical muscles, helps hold internal organs in place and keeps the body stable during movement. The external and internal obliques are the flat muscles, allowing the trunk to twist and turn.
Limb Muscles
The limb muscles include both upper and lower limb muscles. The upper limb muscles act on the joints of the hand, arm, and shoulder, providing stability and allowing precise fluid movement. The muscles of the upper limb receive their arterial supply primarily from the axillary artery and its branches, including the brachial artery and the profunda brachii. The lower limb muscles are responsible for movements of the hip, knee, and ankle joints, enabling actions such as walking, running, and jumping.
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The serratus posterior superior and serratus posterior inferior are innervated by the ventral primary ramus
The serratus posterior superior and serratus posterior inferior are hypaxial muscles, which are located on the ventral side of the body. In adult vertebrates, the trunk muscles can be divided into hypaxial and epaxial muscles. The former lies ventral to the horizontal septum of the vertebrae, while the latter lies dorsal to the septum.
The hypaxial muscles include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles. They are innervated by the ventral ramus (or branch) of the spinal nerves. The serratus posterior superior and serratus posterior inferior are innervated by the ventral primary ramus and are, therefore, hypaxial muscles.
The serratus posterior superior originates on the supraspinal ligament, the ligamentum nuchae, the spinous processes of the upper two to three thoracic vertebrae (T01–T03), and the seventh cervical vertebra. It inserts on the upper borders of ribs 2–5 and is innervated by intercostal nerves 2–5.
The serratus posterior inferior originates on the supraspinal ligament and the spinous processes of the upper two to three lumbar vertebrae (L01–L03) and the lower two thoracic vertebrae (T11–T12). It inserts on the lower borders of ribs 9–12 and is innervated by intercostal nerves 9–12.
The epaxial muscles, on the other hand, are innervated by the dorsal ramus. In humans, the only epaxial muscles are the erector spinae, the transversospinales (including the multifidus, semispinalis, and rotatores), the splenius, and the suboccipital muscles.
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Hypaxial muscles are defined by the pattern of motor innervation from the spinal cord
The hypaxial muscles are defined by the pattern of motor innervation from the spinal cord. In vertebrates, skeletal muscles are categorised into epaxial and hypaxial muscles based on their innervation and position relative to the vertebral column. The hypaxial muscles are located on the ventral side of the body, often below the horizontal septum, and are innervated by the ventral rami of the spinal nerves. This is in contrast to the epaxial muscles, which are located on the dorsal side and are innervated by the dorsal rami.
The hypaxial group includes muscles with highly differentiated morphology and function, such as the muscles associated with paired limbs, shoulder girdles, tongue, and infrahyoid (hypobranchial) muscles. These muscles are found in jawed vertebrates, which possess two distinct muscle groups in the trunk. The hypaxial muscles include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles.
The development of hypaxial muscles has been studied in various species, including cyclostomes, chondrichthyans, and amniotes. In amniotes such as mammals, birds, and reptiles, the trunk skeletal muscles consist of multiple layers categorised as epaxial or hypaxial, each innervated by the respective rami of spinal motor nerves. This distinction is more pronounced in cartilaginous and bony fish, where the epaxial and hypaxial portions are separated by the horizontal myoseptum, a connective tissue sheet. Their coordinated contraction allows for S-shaped swimming movements.
At the limb levels, hypaxial muscle precursors migrate into the limb bud, proliferate, and generate limb muscles. This process is regulated by various transcription factors, such as Myf5 and Myod, which are essential for distal migration and limb muscle formation. The rhomboid muscles, for example, are situated in the epaxial domain but are innervated by a ventral branch, suggesting their hypaxial origin.
In summary, the hypaxial muscles are defined by their pattern of motor innervation from the ventral rami of the spinal nerves. They exhibit a wide range of functions and morphologies and play a crucial role in the movement and coordination of the body, especially during activities like swimming.
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Lamprey HBMs are innervated by the hypoglossal nerve
Lampreys are jawless vertebrates that possess bilateral hypaxial muscles (HBMs) on the ventrolateral wall of the pharynx. Lamprey HBMs are innervated by the hypoglossal nerve, which is homologous to the 12th cranial nerve in mammals. This innervation pattern suggests that lamprey HBMs are homologous to the tongue and infrahyoid muscles, which originate from the hypaxial portion of anterior somites.
The hypaxial muscles are located on the ventral side of the body and are innervated by the ventral ramus of the spinal nerves. In contrast, epaxial muscles are located on the dorsal side and are innervated by the dorsal ramus. This distinction is more conspicuous in cartilaginous and bony fish, where the two muscle groups are separated by the horizontal myoseptum.
Lampreys, as cyclostomes, lack many of the derived traits of jawed vertebrates, including jaws, paired fins, and the distinct epaxial and hypaxial musculature of the trunk. However, lampreys possess the primitive form of the hypobranchial muscle. Lamprey HBMs have been observed to develop during embryogenesis, with the earliest differentiation of somitic skeletal muscles occurring at the early neurula stage, while HBMs differentiate later at the pre-ammocoete larval stage.
The hypoglossal nerve plays a crucial role in the development and function of lamprey HBMs. During embryogenesis, the hypoglossal nerve extends anteriorly, and its innervation pattern coincides with the distribution of precursor cells of HBMs. The hypoglossal nerve is also associated with the formation of the hypobranchial cord, which is observed during the development of HBMs in sharks and lampreys.
In summary, lamprey HBMs are innervated by the hypoglossal nerve, which is homologous to the 12th cranial nerve in mammals. This innervation pattern supports the homology of lamprey HBMs to the tongue and infrahyoid muscles derived from the hypaxial portion of anterior somites. Lampreys, as cyclostomes, exhibit unique anatomical characteristics and developmental trajectories compared to jawed vertebrates, providing insights into the evolutionary mechanisms underlying the morphological diversity of hypaxial musculature.
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Frequently asked questions
Hypaxial muscles are one of the two distinct groups of muscles in the trunk of jawed vertebrates, the other being epaxial muscles. Hypaxial muscles include some vertebral muscles, the diaphragm, the abdominal muscles, and all limb muscles.
Hypaxial muscles are innervated by the ventral ramus (branch) of the spinal nerves.
Examples of hypaxial muscles include the serratus posterior inferior and serratus posterior superior, the tongue, and infrahyoid (hypobranchial) muscles.
Epaxial and hypaxial muscles are primarily defined by the pattern of motor innervation from the spinal cord. Epaxial muscles are innervated by the dorsal ramus, while hypaxial muscles are innervated by the ventral ramus.
Hypaxial muscles develop from the ventrolateral compartment of somites, which give rise to two different cell populations. The lateral dermomyotome (hypaxial domain) is the source of progenitor cells of the limbs, tongue, intercostal, and abdominal muscles.
