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The extraocular muscles are a group of specialized skeletal muscles that control eye movements and allow for precise visual tracking and alignment. There are seven extraocular muscles in total: four rectus muscles, two oblique muscles, and the levator palpebrae superioris. These muscles have unique structural, physiological, and molecular characteristics, including high levels of calcium and oxygen-handling enzymes, which enable them to finely control their contractile properties. While extraocular muscles are generally considered striated muscles, recent studies have identified smooth muscle markers within mouse extraocular muscles, adding a layer of complexity to our understanding of their composition.
| Characteristics | Values |
|---|---|
| Number of extraocular muscles | 6 or 7 |
| Types of extraocular muscles | Rectus, oblique, levator palpebrae superioris |
| Rectus muscles | Superior, inferior, medial, lateral |
| Oblique muscles | Superior, inferior |
| Voluntary muscles | All voluntary muscles are striated |
| Involuntary muscles | All involuntary muscles are smooth except for cardiac muscle |
| Nerve supply | Third (oculomotor), fourth (trochlear), sixth (abducent) nerve |
| Blood supply | Muscular branches of the ophthalmic artery, the lacrimal artery, and the infraorbital artery |
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What You'll Learn
Extraocular muscles are a specialised group of muscles
The extraocular muscles are a highly specialised group of muscles, with unique structural, cellular, molecular, and gene-level characteristics. They are skeletal muscles that play a crucial role in controlling eye movements, allowing for precise visual tracking and alignment. There are six true extraocular muscles in each orbit that move the eye, and they are innervated by three distinct motor nuclei originating in three different locations in the brainstem. These are the abducens nerve, the trochlear nerve, and the oculomotor nerve. The abducens nerve innervates the lateral rectus muscle, the trochlear nerve innervates the superior oblique muscle, and the oculomotor nerve innervates the remaining four extraocular muscles.
The extraocular muscles are divided into two groups: the voluntary and the involuntary muscles. The voluntary muscles include seven extraocular muscles that control the movements of the eye and eyelids and are essential for binocular single vision. These comprise four rectus muscles, two oblique muscles, and the levator palpebrae superioris. The four recti include the superior, inferior, medial, and lateral rectus muscles, while the two oblique muscles include the superior and inferior oblique muscles. The levator palpebrae superioris is responsible for eyelid elevation. The involuntary muscle group includes the superior tarsal muscle (Muller's muscle), the inferior tarsal muscle, and the orbitalis.
The extraocular muscles have a large ratio of nerve fibres to skeletal muscle fibres, with a ratio of 1:3 to 1:5, compared to 1:50 to 1:125 in other skeletal muscles. They exhibit various fibre types, including slow-tonic types that resist fatigue and saccadic (rapid) muscle fibres. The extraocular muscles also display unusual properties, such as complex co-expression of myosin heavy chain isoforms, allowing for precise control over the amount of muscle contraction. Additionally, high levels of calcium and oxygen-handling enzymes further enhance their ability to finely control contractile properties.
The extraocular muscles are involved in several types of eye movements, including ductions, versions, and torsional movements. Ductions refer to monocular eye movements, where nasal movement is adduction, and temporal movement is abduction. Elevation and depression of the eye are called sursumduction (supraduction) and deorsumduction (infraduction), respectively. Incycloduction (intorsion) is the nasal rotation of the vertical meridian, while excycloduction (extorsion) is the temporal rotation. Versions refer to coordinated eye movements, where both eyes move simultaneously to maintain visual stability during gaze shifts and follow moving objects smoothly.
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They control eye movements and eyelids
Extraocular muscles are specialised skeletal muscles that play a crucial role in controlling eye movements and eyelid elevation. They are responsible for precise visual tracking and alignment, allowing the eyes to focus on a specific object or target. There are seven extraocular muscles: four rectus muscles, two oblique muscles, and the levator palpebrae superioris.
The four rectus muscles are the medial rectus, lateral rectus, superior rectus, and inferior rectus. These muscles work together to control horizontal and vertical eye movements. For example, the medial rectus pulls the eye towards the nose (adduction), while the lateral rectus pulls the eye away from the nose (abduction). The superior rectus and inferior oblique contract together to elevate the eye, while the inferior rectus and superior oblique contract to depress the eye.
The two oblique muscles are the superior oblique and inferior oblique. Unlike the rectus muscles, they do not originate from the common tendinous ring. Instead, they take an angular approach to the eyeball. The oblique muscles are involved in both horizontal and vertical eye movements, as well as eye rotation.
The levator palpebrae superioris is responsible for eyelid elevation. It does not contact the globe directly but elevates the eyelid through its attachment to the superior tarsal plate. This muscle is innervated by the oculomotor nerve (CN III).
The extraocular muscles are innervated by three cranial nerves: the oculomotor nerve (CN III), the trochlear nerve (CN IV), and the abducens nerve (CN VI). These nerves coordinate the movements of the extraocular muscles to achieve specific gaze directions. For example, during a right gaze, the right lateral rectus and left medial rectus function as yoke muscles, receiving equal and simultaneous innervation to achieve coordinated movement.
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They are divided into voluntary and involuntary muscles
Extraocular muscles are a specialised form of skeletal muscle that control eye movements, allowing for precise visual tracking and alignment. There are seven extraocular muscles: four rectus muscles, two oblique muscles, and the levator palpebrae superioris.
These muscles can be classified as voluntary or involuntary. Voluntary muscles are skeletal muscles that contract and relax under conscious control. They are involved in the movement of body parts and locomotion. Examples include the biceps, triceps, quadriceps, diaphragm, pectoral muscles, abdominals, and hamstrings.
On the other hand, involuntary muscles are not under conscious control. They contract and relax automatically and receive signals from the autonomic nervous system, which regulates internal bodily functions. Involuntary muscles are involved in the movement of internal organs and aid in the passage of fluids and food in the digestive system. They are also called smooth muscles or non-striated muscles, as they lack striations when viewed under a microscope. Examples include the cardiac muscle and smooth muscle lining the intestinal tracts, blood vessels, urogenital tracts, and respiratory tracts.
In vertebrates, all voluntary muscles are striated, and all involuntary muscles are smooth, except for cardiac muscle, which is striated. The cardiac muscle is composed of individual heart muscle cells called cardiomyocytes, which are joined by intercalated discs. This muscle is unique to the heart and is responsible for pumping blood throughout the body.
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They are preferentially spared in muscular dystrophies and motor neuron diseases
The extraocular muscles are a very specialised group of muscles, with six extraocular muscles moving the eye in each orbit. They are the only muscles in the human body that have complex structural and physiological characteristics adapted to meet the motility requirements of fovea-based binocular vision. They are also the fastest and the most fatigue-resistant muscles in the body.
Extraocular muscles have a large ratio of nerve fibres to skeletal muscle fibres. The ratio is 1:3 to 1:5 compared to other skeletal muscles, which are 1:50 to 1:125. They are richly innervated and also have high levels of calcium and oxygen-handling enzymes. These characteristics likely developed in response to the specialised demands placed on the extraocular muscles, such as tonic position-maintaining contractures, conjugate smooth pursuit and saccades, and dysconjugate vergence movements.
Extraocular muscles are preferentially spared in a number of diseases that affect almost all other body skeletal muscles, specifically muscular dystrophies and motor neuron diseases such as amyotrophic lateral sclerosis. For example, in patients with advanced Duchenne's or Becker's muscular dystrophy, the extraocular muscle function is preserved, and peak velocities of saccades are normal. This suggests that fast-twitch fibres in extraocular muscles possess properties that protect against degeneration.
Recent data has shown that extraocular muscles of terminal ALS donors and mice models of ALS maintain their morphology and well-preserved neuromuscular junctions until the end stages of the disease. This is in contrast to limb muscles, which are severely affected and lose contact with supplying axons early in the course of ALS. These differences in the response of extraocular muscles to ALS may be due to intrinsic differences in neurotrophic factors and Wnt isoforms.
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They are the only muscles affected in myasthenia gravis
Extraocular muscles are specialised skeletal muscles that play a crucial role in controlling eye movements, allowing for precise visual tracking and alignment. There are seven extraocular muscles: four rectus muscles, two oblique muscles, and the levator palpebrae superioris. The rectus muscles include the medial rectus, lateral rectus, superior rectus, and inferior rectus, while the oblique muscles comprise the superior and inferior obliques.
Now, onto myasthenia gravis and its effects on the body. Myasthenia gravis is an autoimmune disease that affects the neuromuscular junction, leading to muscle weakness and fatigability that worsens throughout the day. It interferes with how nerves communicate with muscles, causing variable muscle weakness that is more pronounced after physical activity and improves with rest. While myasthenia gravis can affect various muscles in the body, let's focus on its impact on the extraocular muscles specifically.
Myasthenia gravis often presents with ocular symptoms, making it known as ocular myasthenia gravis. The disease affects the extraocular muscles, causing ptosis (drooping eyelids), diplopia (double vision), ophthalmoplegia, and strabismus. The extraocular muscles are smaller compared to limb muscles, and a small amount of muscle weakness in these ocular muscles can become symptomatic.
It is important to note that myasthenia gravis does not solely affect the extraocular muscles. In fact, approximately 85% of patients with ocular symptoms will eventually develop systemic myasthenia gravis, where the condition progresses to impact other voluntary muscles in addition to the ocular muscles. This progression typically occurs within two years of the initial ocular presentation. Therefore, while the extraocular muscles are often affected in myasthenia gravis, it is not the only muscle group impacted by this disease.
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Frequently asked questions
Extraocular muscles are a group of specialised skeletal muscles that control eye movements and eyelids. There are six true extraocular muscles that move the eye in each orbit.
There are seven extraocular muscles, including four rectus muscles, two oblique muscles, and the levator palpebrae superioris.
The functions of extraocular muscles include ductions, versions, and torsional movements. They also help in maintaining stable visual perception during gaze shifts.
Extraocular muscles are considered striated muscles. They have unique structural, cellular, molecular, and genetic characteristics that differentiate them from other striated muscles.
