Part A

Human Anatomy and Physiology Tenth Edition Chapter 15 Part A: The Special Senses

Special Senses Overview

• General Senses vs Special Senses:

  • The sense of touch is classified as one of the general senses, which are mediated by general receptors (discussed in Chapter 13).

  • The special senses of the body include:

  • Vision

  • Taste

  • Smell

  • Hearing

  • Equilibrium

Special Sensory Receptors

• All special senses utilize specialized sensory receptors that are distinct receptor cells located specifically in the head region.

• These receptors are fundamentally different from the modified nerves found in general receptors.

The Eye and Vision

• Approximately 70% of the body's sensory receptors are located in the eyes.

• About half of the cerebral cortex is dedicated to processing visual information.

Structure of the Eye

• The eye is a small sphere, with only one-sixth of its surface visible to the observer.

• Most of the eye is protected and encased by a fat cushion and bony orbit.

• The eye consists of two main components:

  • Accessory structures

  • The eyeball itself

Accessory Structures of the Eye

• Accessory structures play crucial roles in protecting the eye and enhancing its functionality.

• The key accessory structures include:

  • Eyebrows:

  • Positioned over the supraorbital margins.

  • Function to shade the eyes from sunlight and prevent perspiration from reaching the eye.

  • Eyelids (Palpebrae):

  • Thin, skin-covered folds that protect the eye anteriorly.

  • Separated by the palpebral fissure (slit) and meet at the medial and lateral commissures.

  • Contains the lacrimal caruncle at the medial commissure, which houses oil and sweat glands.

  • Composed of tarsal plates that provide structural support, anchoring the orbicularis oculi and levator palpebrae superioris muscles.

  • Eyelids blink reflexively every 3–7 seconds to protect the eye and distribute secretions across the surface.

  • Eyelashes:

  • Follicles with nerve endings that initiate reflex blinking as a protective mechanism.

  • Lubricating Glands:

  • Tarsal (Meibomian) Glands: Modified sebaceous glands that secrete oily substances to lubricate the eyelids and eye surface.

  • Ciliary Glands: Located between hair follicles and are modified sweat glands.

Conjunctiva

• A transparent mucous membrane responsible for producing lubricating mucous secretion.

• Palpebral Conjunctiva: Lines the underside of the eyelids.

• Bulbar Conjunctiva: Covers the white part of the eyeball (excluding the cornea) and contains small blood vessels visible in “bloodshot” eyes.

• Conjunctival Sac: The space between palpebral and bulbar conjunctiva, which serves as the resting place for contact lenses.

Lacrimal Apparatus

• The lacrimal apparatus consists of the lacrimal gland and associated ducts that drain into the nasal cavity.

• Lacrimal Gland: Located within the orbit above the lateral end of the eye, it produces tears (lacrimal secretion), a dilute saline solution that contains mucus, antibodies, and antibacterial enzyme lysozyme.

• Blinking spreads tears toward the medial commissure, from where they enter the lacrimal canaliculi through the lacrimal puncta.

• Each tear drains into the lacrimal sac and the nasolacrimal duct, eventually emptying into the nasal cavity.

Extrinsic Eye Muscles

• Six strap-like muscles responsible for eye movement, originating from the bony orbit and inserting onto the eyeball:

  • Rectus Muscles:

  • Consist of superior, inferior, lateral, and medial rectus.

  • Their names indicate their respective actions.

  • Oblique Muscles:

  • Superior and inferior oblique muscles; they assist in moving the eye in a vertical plane and help rotate the eyeball.

Muscle Actions and Innervation Summary

Clinical - Homeostatic Imbalances

• Chalazion: A cyst that forms from an infected tarsal gland.

• Sty: Inflammation of smaller sebaceous glands near the eyelid.

• Conjunctivitis: Inflammation of the conjunctiva leading to reddened and irritated eyes, commonly known as pinkeye, and is typically caused by contagious infections.

• Nasal Inflammation: Can result in lacrimal duct swelling, leading to “watery” eyes due to the obstruction of tears.

• Diplopia (Double Vision): Arises when the movements of the external muscles of the two eyes are not well-coordinated, resulting in two images from the same visual field.

• Strabismus (Cross-Eye): A congenital condition where external muscles are weak, causing misalignment of the eyes. This can lead to functional blindness if untreated in early life.

Structure of the Eyeball

• The wall of the eyeball consists of three main layers:

  • Fibrous Layer:

  • Outermost layer composed of dense avascular connective tissue. It includes:

    • Sclera: Opaque posterior region that protects and shapes the eyeball and serves as the anchor for extrinsic eye muscles.

    • Cornea: Transparent anterior one-sixth of the fibrous layer that forms a clear window allowing light to enter.

  • Vascular Layer (Uvea):

  • Middle layer which includes:

    • Choroid: Posterior portion that supplies blood to all layers of the eyeball and contains brown pigment to absorb scattered light.

    • Ciliary Body: Thickened tissue that surrounds the lens, comprised of smooth muscle that controls lens shape and secretes aqueous humor.

    • Iris: The colored part of the eye located between the cornea and lens, containing sphincter pupillae (circular muscles) and dilator pupillae (radial muscles) that control pupil size for light regulation.

  • Inner Layer (Retina):

  • Originates as an outpocketing of the brain, consisting of two layers:

    • Pigmented Layer: Absorbs light, prevents scattering, phagocytizes fragments of photoreceptors, and stores vitamin A.

    • Neural Layer: Composed of photoreceptor cells, bipolar neurons, and ganglion cells; processes light signals which converge into the optic nerve at the optic disc.