Anatomy of Eye Annexes and Sensory Receptor Functions

Annexes to the Eye

• Eyebrows: Folds of skin covered in hair that direct sweat away from the eyes and protect them from direct sunlight.
• Eyelids: Skin folds that move reflexively to protect the eye from small foreign bodies. At the edges of the two eyelids are Meibomian glands, which prevent the eyelids from sticking together.
• Eyelashes: Associated with small sebaceous glands, they act jointly with the eyelids for protection.
• Stye: An infection of the eyelashes.
• Conjunctiva: A very thin and transparent membrane located below the eyelid and above the eyeball.
• Conjunctivitis: Inflammation of the conjunctiva.
• Lacrimal gland: Produces and sheds tears into the conjunctiva.
• Tear ducts: These remove excess liquid and carry it to the nostrils.
• Lacrimal secretion: Consisting of salts dissolved in water, mucin, and lysozyme (which kills bacteria), it keeps the eye surface moist.
• Lacrimal caruncle: A reddish prominence located at the corner of the eye that contains glands responsible for the secretion of rheum (sleep).

Sensory Receptors

Receptors are specialized neural structures designed to capture external stimuli or the internal environment and translate them into nerve impulses.

Features of Receptors

• Specificity: The ability of receptors to respond to one particular stimulus.
• Adaptation: When a sustained stimulus is applied to a receptor, the discharge rate of the nerve fiber is greater at the beginning and then decreases over time until the receptor fails to generate nerve impulses. In this case, we say the receptor has adapted.
• Excitability: The ability of nerve and muscle cells to respond to a stimulus of threshold intensity.
• Transduction: The ability of receptors to absorb and transform light, mechanical, chemical, or other energy into electrical energy or action potentials.

Classification of Receptors

Based on the source of the stimulus:

• Exteroreceptors: Stimuli coming from the external environment. Examples include: Pacini corpuscles (Pressure), Meissner corpuscles (Touch), Krause corpuscles (Cold), and Ruffini endings (Heat).
• Interoreceptors: Located in the organs and blood vessels, they report on internal factors such as body temperature, composition, pH, and blood pressure. These include Baroreceptors (blood pressure) and Osmoreceptors (blood composition).
• Proprioceptors: Located within the muscles, tendons, and joints, they report the location of the limbs, head, and body movement.

Nature of the Stimulus

• Mechanoreceptors: Receive mechanical stimuli such as pressure or sound. They are found in the skin and the urinary, circulatory, digestive, auditory, and respiratory systems.
• Photoreceptors: Receive light stimuli and are found in the retina. These include Cones (daytime vision) and Rods (night vision).
• Chemoreceptors: Respond to chemical stimuli.
• Thermoreceptors: Respond to temperature variations.
• Nociceptors: Respond to noxious stimuli, such as sudden mechanical forces.