BSC 215 Lecture Notes - Lecture 32: Levator Palpebrae Superioris Muscle, Lacrimal Punctum, Fibrous Tunic Of Eyeball
11 Jun 2018
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The Somatic Senses
The somatic (general) senses collect information about cutaneous sensations
(tactile sensations on the surface of the skin) and proprioceptive sensations.
The following stimuli are detected:
• Tactile stimuli are detected by mechanoreceptors and produce
sensations of touch and pressure:
• Merkel discs are receptors with free nerve endings that detect surface
pressure (light touch). They are located deep in the epidermis.
• Root hair plexuses are receptors with free nerve endings that surround
hair follicles and detect hair movement.
• Corpuscles of touch (Meissner's corpuscles) are receptors with
encapsulated nerve endings located in the dermal paillae (near the
surface) of the skin that detect surface pressure (light touch).
• Pacinian corpuscles are encapsulated nerve receptors that detect deep
pressure and are located in the subcutaneous layer (below the skin).
• Thermal stimuli are detected by free nerve ending thermoreceptors
sensitive to heat or cold.
• Pain stimuli are detected by free nerve ending nociceptors.
• Proprioceptive stimuli are detected by the following receptors:
• Muscle spindles are mechanoreceptors located in skeletal muscles.
They consist of specialized skeletal muscle fibers enclosed in a spindle‐
shaped capsule made of connective tissue.
• Golgi tendon organs are mechanoreceptors located at the junctions of
tendons and muscles.
• Joint kinesthetic receptors are mechanoreceptors located in synovial
joints.
Vision
The eye is supported by the following accessory organs:
• The eyebrows shade the eyes and help keep perspiration that
accumulates on the forehead from running into the eyes.
• The eyelids (palpebrae) lubricate, protect, and shade the eyeballs.
Contraction of the levator palpebrae superioris muscle raises the upper
eyelid. Each eyelid is supported internally by a layer of connective
tissue, the tarsal plate. Tarsal (Meibomian) glands embedded in the
tarsal plate produce secretions that prevent the upper and lower
eyelids from sticking together. The inner lining of the eyelid, the
conjunctiva, is a mucous membrane that produces secretions that
lubricate the eyeball. The conjunctiva continues beyond the eyelid,
folding back to cover the white of the eye.
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• The eyelashes, on the borders of the eyelids, help protect the eyeball.
Nerve endings at the base of the hairs initiate a reflex action that
closes the eyelids when the eyelashes are disturbed.
• The lacrimal apparatus produces and drains tears. Tears (lacrimal
fluid) are produced by the lacrimal glands, which lie above each eye
(toward the lateral edge). In each eye, tears flow across the eyeball
and enter two openings (lacrimal puncta) into lacrimal canals that lead
to the lacrimal sac. From here, the tears drain through the
nasolacrimal duct into the nasal cavity. Tears contain antibodies and
lysozyme (a bacteria‐destroying enzyme).
• Six extrinsic eye muscles provide fine motor control for the eyeballs.
These are the lateral, medial, superior, and inferior rectus muscles and
the inferior and superior oblique muscles.
The eyeball is a hollow sphere whose wall consists of three tunics (layers),
shown in Figure 1.
The three tunics of the eye are described below:
• Fibrous tunic: The outer fibrous tunic consists of avascular connective
tissue called the sclera. The forward 1/6 portion of this tunic is the
cornea, a transparent layer of collagen fibers that forms a window for
entering light. The remainder of the fibrous tunic is the sclera.
Consisting of tough connective tissue, the sclera maintains the shape
of the eyeball and provides for the attachment of the eye muscles. The
visible forward portion of the sclera is the white of the eye.
• Vascular tunic: The middle vascular tunic (uvea) consists of three highly
vascularized (as the name implies), pigmented parts (the iris, the
ciliary body, and the choroid):
• The iris is the colored portion of the eye that opens and closes to
control the size of its circular opening, the pupil. The size of the pupil
regulates the amount of light entering the eye and helps bring objects
into focus.
• The ciliary body lies between the iris and the choroids (the remainder
of the vascular tunic). The ciliary processes that extend from the
ciliary body secrete aqueous humor, the fluid that fills the forward
chamber of the eye. The suspensory ligament between the ciliary
processes and the lens holds the lens in place, while ciliary muscles (in
the ciliary body) that pull on the suspensory ligament control the
shape of the lens to focus images.
• The choroid connects with the ciliary body at a jagged boundary and
forms the remaining portion (5/6) of the vascular tunic. The choroid is
dark brown, absorbing light and reducing reflection within the chamber
of the eyeball that would otherwise blur images. The highly
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Document Summary
The somatic (general) senses collect information about cutaneous sensations (tactile sensations on the surface of the skin) and proprioceptive sensations. The following stimuli are detected: tactile stimuli are detected by mechanoreceptors and produce sensations of touch and pressure, merkel discs are receptors with free nerve endings that detect surface pressure (light touch). The eye is supported by the following accessory organs: the eyebrows shade the eyes and help keep perspiration that accumulates on the forehead from running into the eyes, the eyelids (palpebrae) lubricate, protect, and shade the eyeballs. Contraction of the levator palpebrae superioris muscle raises the upper eyelid. Each eyelid is supported internally by a layer of connective tissue, the tarsal plate. Tarsal (meibomian) glands embedded in the tarsal plate produce secretions that prevent the upper and lower eyelids from sticking together. The inner lining of the eyelid, the conjunctiva, is a mucous membrane that produces secretions that lubricate the eyeball.
