General and Special Senses: Sensory Receptors and the Eye

General and Special Senses

Introduction to Sensory Information

The human body is constantly exposed to a variety of sensory stimuli from the environment and within. Sensory systems detect, process, and interpret these stimuli, allowing for conscious awareness and appropriate responses.

• Sensory receptors convert sensory stimuli into nerve impulses.

• Types of sensory information include: temperature, pain, touch, pressure, vibration, equilibrium, vision, olfaction, gustation, and hearing.

Properties and Classification of Sensory Receptors

Properties of Sensory Receptors

Sensory receptors exhibit different properties in response to constant stimuli:

• Tonic receptors: Respond continuously to stimuli without significant adaptation (e.g., pain receptors).

• Phasic receptors: Rapidly adapt to a constant stimulus and stop responding (e.g., tactile receptors for touch).

Classification by Stimulus Location

Sensory receptors are classified based on their location relative to the body:

• Exteroceptors: Located near the body surface; respond to external stimuli (e.g., skin, special senses).

• Interoceptors: Located in viscera and internal organs; monitor internal environment (e.g., smooth muscle, blood vessels).

• Proprioceptors: Found in muscles, tendons, and joints; provide information about body position and movement.

Classification by Stimulus Modality

Receptors can also be classified by the type of stimulus they detect:

• Chemoreceptors: Detect chemicals (e.g., taste, smell, blood pH).

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Detect light (e.g., in the retina).

• Mechanoreceptors: Detect physical deformation such as pressure, vibration, and stretch.

Tactile (Touch) Receptors

Types of Tactile Receptors

Tactile receptors are the most numerous type of sensory receptor, primarily located in the skin and subcutaneous layers. They detect touch, pressure, and vibration.

• Unencapsulated tactile receptors: Free nerve endings, root hair plexuses, and tactile (Merkel) discs; detect light touch and movement of hairs.

• Encapsulated tactile receptors: End (Krause) bulbs, lamellated (Pacinian) corpuscles, bulbous (Ruffini) corpuscles, and tactile (Meissner) corpuscles; detect deep pressure, vibration, and continuous touch.

Table: Comparison of Tactile Receptors

Type	Structure	Stimulus Detected	Location
Free nerve endings	Unencapsulated	Pain, temperature	Dermis, epidermis
Root hair plexuses	Unencapsulated	Hair movement	Base of hair follicles
Tactile (Merkel) discs	Unencapsulated	Light touch	Stratum basale of epidermis
End bulbs	Encapsulated	Light pressure, low-frequency vibration	Dermis, mucous membranes
Lamellated corpuscles	Encapsulated	Deep pressure, high-frequency vibration	Dermis, subcutaneous tissue
Bulbous corpuscles	Encapsulated	Continuous deep pressure	Dermis, subcutaneous tissue
Tactile (Meissner) corpuscles	Encapsulated	Light touch, texture	Dermal papillae of fingertips, lips

Clinical Significance: Pain and Dermatomes

Pain signals from specific regions of the skin are transmitted via spinal nerves, each corresponding to a dermatome (an area of skin supplied by a single spinal nerve). For example, heart attack pain may be referred to the left arm (T1–T5 dermatomes).

Special Senses: The Eye

External Structures of the Eye

• Eyelids (palpebrae): Protect and lubricate the eye.

• Lacrimal apparatus: Produces and drains tears; includes the lacrimal gland, puncta, and nasolacrimal duct.

Eye Lens

• Lens: Biconvex, transparent structure that focuses light onto the retina; held in place by suspensory ligaments.

Eyeball Layers (Tunics)

The wall of the eyeball consists of three main layers, each with distinct functions:

• Fibrous tunic: Outermost layer; includes the sclera (white of the eye) and cornea (transparent anterior part).

• Vascular tunic (uvea): Middle layer; includes the choroid (supplies nutrients and oxygen), ciliary body (controls lens shape), and iris (controls pupil size and eye color).

• Neural tunic (retina): Innermost layer; contains photoreceptors (rods and cones) that convert light into nerve impulses.

Table: Layers of the Eyeball

Tunic	Main Structures	Functions
Fibrous	Sclera, Cornea	Protection, shape, refracts light
Vascular	Choroid, Ciliary body, Iris	Nourishment, controls lens and pupil
Neural	Retina	Photoreception, transmits visual signals

Key Equations and Concepts

• Refraction of Light: The bending of light as it passes through the cornea and lens focuses images on the retina.

• Phototransduction: The process by which photoreceptors convert light into electrical signals.

Additional info:

• Photoreceptors include rods (sensitive to low light, black and white vision) and cones (responsible for color vision and visual acuity).

• The optic disc is the "blind spot" where the optic nerve exits the eye; it contains no photoreceptors.