extraocular muscles

Examples of extraocular muscles in the following topics:

• Oculomotor (III) Nerve

  • It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the levator palpebrae superiors muscle.
  • The muscles it controls are the striated muscle in the levator palpebrae superioris and all extraocular muscles, except for the superior oblique muscle and the lateral rectus muscle.
  • The Edinger-Westphal nucleus supplies parasympathetic fibers to the eye via the ciliary ganglion, and controls the pupillae muscle (affecting pupil constriction) and the ciliary muscle (affecting accommodation).
  • Sympathetic postganglionic fibers also join the nerve from the plexus on the internal carotid artery in the wall of the cavernous sinus and are distributed through the nerve, for example, to the smooth muscle of levator palpebrae superioris.
  • It then divides into two branches that enter the orbit through the superior orbital fissure, between the two heads of the lateral rectus (a muscle on the lateral side of the eyeball in the orbit).

• Strabismus

  • It typically involves a lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely affect depth perception.
  • Strabismus can be either a disorder of the brain in coordinating the eyes, or of one or more of the relevant muscles' power or direction of motion.
  • A strabismus caused by a lesion in either of these nerves results in the lack of innervation to eye muscles and results in a change of eye position.
  • Surgery does not change the vision; it attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles It is frequently the only way to achieve cosmetic improvement.

• How Skeletal Muscles Are Named

  • Parallel muscles are characterized by fascicles that run parallel to one another, and contraction of these muscle groups acts as an extension of the contraction of a single muscle fiber.
  • Convergent muscles have a common point of attachment, from which the muscle fascicles extend outward, not necessarily in a specific spatial pattern, allowing the muscle to cover a broad surface.
  • In Pennate muscles, the tendon runs through the length of the muscle.
  • However, these muscles tend to have relatively more muscle fibers than similarly sized parallel muscles, and thus carry more tension.
  • If the central tendon branches within a pennate muscle, the muscle is called multipennate.

• Impacts of Exercise on Muscles

  • Sustained, repeated overload of a muscle group leads to hypertrophy and strengthening of those muscles.
  • In the short term muscle can become fatigued and sore for reasons like impaired blood flow, ion imbalance within the muscle, nervous fatigue, loss of desire to continue exercising, and most importantly, the accumulation of lactic acid in the muscle.
  • Muscle hypertrophy, or the increase in muscle mass due to exercise , particularly weight training, is a noticeable long-term effect of exercise.
  • Exercise of specific muscles can often result in hypertrophy in the opposite muscles as well, a phenomenon known as cross education.
  • With sufficient training the metabolic capacity of a muscle can change, delaying the onset of muscle fatigue.

• Types of Muscle Tissue

  • The function of muscles is movement, but the types of movement elicited differ between skeletal, cardiac, and smooth muscle.
  • Skeletal muscle fibers are the longest muscle fibers and have stripes on their surface.
  • Cardiac muscle is found in the walls of the heart.
  • Although cardiac muscle is involuntary in nature, it is structurally different from smooth muscle.
  • Cardiac muscle is striated, similar to skeletal muscle, but beats involuntarily.

• Arrangement of Fascicles

  • Skeletal muscles are grouped into fascicles, which are bunches of muscle fibers surrounded by a perimysium.
  • Skeletal muscle tissue is composed of numerous muscle fibers which are separated from adjacent muscles and other tissues by a layer of dense, elastic connective tissue termed the fascia.
  • This fascia can project beyond the end of the muscle and attach to bones, other muscles, and other tissues.
  • The fascia surrounding a muscle or muscle group does not contain many blood vessels, but is rich with sensory receptors.
  • It extends inwards and becomes the perimysium, then into the muscle separating muscle fibers into small bundles termed fascicles.

• Types of Muscle Contractions: Isotonic and Isometric

  • Muscle contractions are defined by changes in the length of the muscle during contraction.
  • Isotonic contractions maintain constant tension in the muscle as the muscle changes length.
  • Cross-bridge cycling occurs, shortening the sarcomere, muscle fiber, and muscle.
  • Cross-bridge cycling occurs even though the sarcomere, muscle fiber, and muscle are lengthening, controlling the extension of the muscle.
  • In both instances, cross-bridge cycling is maintaining tension in the muscle; the sarcomere, muscle fibers, and muscle are not changing length.

• Abnormal Contractions of Skeletal Muscle

  • Involuntary muscle contractions are referred to as spasms, and can be due to abnormal activity of the nerve or the muscle.
  • In medicine, a spasm is a sudden, involuntary contraction of a muscle, a group of muscles, or a hollow organ, or a similarly sudden contraction of an orifice .
  • Examples of spasms include muscle contractions due to abnormal nerve stimulation, or abnormal activity of the muscle itself.
  • In this case, the hypertonic muscle tone is excessive and the muscles are unable to relax.
  • Hypertonic muscle spasms is the state of chronic, excessive muscle tone, or tension in a resting muscle – the amount of contraction that remains when a muscle is not actively working.

• How Skeletal Muscles Produce Movements

  • Muscles exist in groupings that work to produce movements by muscle contraction.
  • For muscle pairings referred to as antagonistic pairs, one muscle is designated as the extensor muscle, which contracts to open the joint, and the flexor muscle, which acts opposite to the extensor muscle.
  • The majority of muscles are grouped in pairs, with an antagonist to each agonist muscle.
  • Exceptions include those muscles such as sphincter muscles that act to contract in a way that is opposite to the resting state of the muscle.
  • Synergist muscles act around a moveable joint to produce motion similar to or in concert with agonist muscles.

• Structure and Function of the Muscular System

  • Some skeletal muscle can attach directly to other muscles or to the skin, as seen in the face where numerous muscles control facial expression.
  • As with skeletal muscle, cardiac muscle is striated; however it is not consciously controlled and so is classified as involuntary.
  • Smooth muscle is non-striated and involuntary.
  • The body contains three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle, visualized here using light microscopy.
  • Visible striations in skeletal and cardiac muscle are visible, differentiating them from the more randomised appearance of smooth muscle.