The orbitalis muscle is a smooth muscle found in the orbit of the eye. It is a thin, vestigial muscle that plays a minor role in the anatomy of the eye and orbit. Unlike the extraocular muscles, which are skeletal muscles under voluntary control, the orbitalis muscle is smooth muscle, meaning it is under involuntary control. The muscle is also known as Müller’s muscle of the orbit and is considered to be part of the sympathetic nervous system.
Location
The orbitalis muscle is located within the orbit, covering parts of the inferior orbital fissure. It spans the bony walls of the orbit and bridges the gap between the sphenoid bone and the maxilla. The muscle is closely associated with the periosteum of the orbit and lies adjacent to the orbital fat. Though its function is largely rudimentary, its location is significant in relation to the structures that pass through the inferior orbital fissure, including nerves and blood vessels.
Structure and Anatomy
The orbitalis muscle is a unique, thin, and vestigial smooth muscle located within the orbit of the eye. Unlike the voluntary extraocular muscles, it is composed of smooth muscle fibers and is part of the sympathetic nervous system. Below is a detailed description of the anatomy of the orbitalis muscle, including its location, structure, relations, and composition.
Structure
The orbitalis muscle is composed of smooth muscle fibers, which differ significantly from the striated skeletal muscles found in the extraocular muscles. Smooth muscle fibers are elongated, spindle-shaped cells that are not under voluntary control, unlike the extraocular muscles responsible for moving the eye. The orbitalis muscle is extremely thin and delicate, forming a bridge over the inferior orbital fissure without attaching directly to any bone.
Due to its small size and vestigial nature, the muscle is not easily distinguished in most anatomical dissections, and its presence is often subtle. The muscle’s structure is mainly composed of:
- Smooth muscle fibers arranged in a thin layer, which are involuntarily controlled by the autonomic nervous system.
- Connective tissue elements that blend with the periosteum of the orbit and surrounding structures.
Blood Supply
The blood supply to the orbitalis muscle comes from nearby orbital vessels, specifically branches of the ophthalmic artery. The infraorbital artery, a branch of the maxillary artery, also provides blood supply to structures near the inferior orbital fissure, potentially supplying the orbitalis muscle as well. Due to the muscle’s small size and reduced functionality, its blood supply is minimal compared to the larger extraocular muscles.
Nerve Supply
The orbitalis muscle is innervated by the sympathetic nervous system. The sympathetic fibers responsible for innervating the muscle originate from the superior cervical ganglion. These fibers travel along the internal carotid artery and pass through the cavernous sinus, eventually reaching the orbitalis muscle. Because it is smooth muscle, the orbitalis is not under voluntary control; instead, it responds to autonomic signals that regulate involuntary actions within the orbit.
Relations to Other Structures
The orbitalis muscle is anatomically related to several key structures within the orbit:
- Inferior Orbital Fissure: The muscle spans the inferior orbital fissure, a gap between the sphenoid and maxilla bones that serves as a passageway for important nerves and vessels. The infraorbital nerve, a branch of the maxillary nerve (V2), and the infraorbital artery pass through this fissure. The orbitalis muscle’s position over the fissure allows it to interact with these structures.
- Orbital Fat: The orbitalis muscle is surrounded by orbital fat, which cushions and supports the structures within the orbit. This fat helps stabilize the position of the muscle and nearby structures without impeding their function.
- Periosteum: The orbital periosteum is the connective tissue lining the bones of the orbit. The orbitalis muscle is closely associated with the periosteum, and its fibers are connected to it, helping stabilize the muscle in place.
Fascial Attachments
The orbitalis muscle has no direct bony attachments but is closely associated with the surrounding orbital fascia and periosteum. This fascia provides structural support and ensures that the muscle remains positioned over the inferior orbital fissure. The thin nature of the muscle, along with its location within the orbit, means that its fascial connections help stabilize it, even though it does not exert significant force or perform active movement like the other extraocular muscles.
Function
The orbitalis muscle is a vestigial smooth muscle in the orbit, meaning its function in humans is largely diminished compared to other extraocular muscles. Unlike the skeletal muscles of the eye, the orbitalis muscle does not contribute to voluntary eye movements. However, its remaining function is tied to its role within the sympathetic nervous system. Below is a detailed description of its primary functions.
Support and Tension in the Orbit
One of the main roles of the orbitalis muscle is to provide structural support within the orbit. Despite being vestigial, the orbitalis muscle helps maintain the tension of the soft tissues within the orbital cavity. By spanning the inferior orbital fissure, the muscle contributes to the overall stability of the structures that pass through the fissure, including the infraorbital nerve, infraorbital artery, and ophthalmic veins.
While it does not exert significant force like the larger extraocular muscles, its contribution to structural integrity ensures that the contents of the inferior orbital fissure are kept in proper alignment and do not become displaced within the orbital cavity. This support is particularly important in maintaining the architecture of the orbit, especially in relation to the soft tissues surrounding the eye.
Minimal Role in Sympathetic Control
The orbitalis muscle is a smooth muscle that is innervated by the sympathetic nervous system. Historically, it has been linked to minor actions related to the sympathetic innervation of the orbit. The sympathetic system is responsible for involuntary actions such as pupil dilation, eyelid retraction, and vascular regulation. Although the orbitalis muscle itself does not directly participate in these actions, it is believed to have once played a role in modulating the position of the eyeball or influencing the orbit’s vasculature.
In humans, this function is minimal, as the muscle is vestigial. In certain lower mammals, the orbitalis muscle may contribute to controlling the position of the eye in the orbit or adjusting intraorbital pressure. However, in humans, the loss of significant functionality has rendered the orbitalis muscle’s role in sympathetic control negligible.
Prevention of Orbital Pulsation
The orbitalis muscle has been thought to help prevent pulsation of the orbital contents. Due to the proximity of the orbitalis muscle to the infraorbital artery and ophthalmic veins, it is hypothesized that the muscle helps regulate the movement or expansion of these vessels. This regulation may be important in preventing the visible pulsing of blood vessels within the orbit, known as pulsatile exophthalmos, although this is a rare function in humans.
By lightly supporting the vascular structures near the inferior orbital fissure, the muscle may play a small part in minimizing the transmission of pulsations from the nearby blood vessels to the surrounding tissues. However, the contribution of the orbitalis muscle to this function is minimal in humans due to its reduced size and activity.
Remnant of Evolutionary Development
In humans, the orbitalis muscle is considered a vestigial remnant of an evolutionary past when it likely had a more active function. In some lower mammals, the orbitalis muscle plays a more significant role in positioning the eye within the orbit and maintaining the structure of the orbital contents. Over time, as humans evolved, the need for this function diminished, and the muscle’s role became largely redundant.
The muscle’s smooth fibers indicate that it was once part of a broader system of autonomic control within the orbit, potentially helping to fine-tune the position of the eye or regulate intraorbital pressure. In humans, however, this function has been lost, and the orbitalis muscle now serves no major purpose in eye movement or ocular positioning.
Clinical Significance
The orbitalis muscle is a vestigial structure with minimal functional significance in humans, but it may still have clinical relevance in certain rare conditions. The muscle is innervated by the sympathetic nervous system, and its dysfunction may be associated with sympathetic denervation in the orbit. Damage to the orbitalis muscle, typically involving disruption of sympathetic innervation, could contribute to conditions such as Horner’s syndrome, which is characterized by ptosis (drooping of the eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating). While the orbitalis itself does not directly contribute to major eye movements, its relationship with sympathetic pathways may be implicated in cases of sympathetic nerve damage.
Additionally, the muscle’s role in providing structural support to the inferior orbital fissure means that its involvement in certain orbital surgeries or trauma cases might be considered, especially when addressing issues related to intraorbital pressure or vascular pulsation. However, its overall clinical significance is minimal due to its vestigial nature.
