Anatomy and Physiology: Sensory Receptors and Vision

General Function of Sensory Receptors

• Provide information about external and internal environments (stimulus)
• Each type of receptor responds to a specific type of stimulus:
  • Light energy (eye receptors)
  • Sound energy (ear receptors)
• Transducers convert stimulus energy into electrical energy:
  • Receptors have a resting membrane potential
  • Modality gated channels respond to specific stimuli

General Structure of Sensory Receptors

• Sensory receptors convey signals to the CNS via sensory neurons.
• Receptive Field: Area where dendritic endings of a single sensory neuron are distributed.
  • Smaller fields allow for more precise localization of stimuli; require more receptors.

Sensory Information Provided by Sensory Receptors

1. Sensory Input travels from receptors to CNS for interpretation.
2. Sensation: A stimulus we are consciously aware of (only a fraction of stimuli lead to sensations).
   • Example: Blood pressure signals relayed to the brainstem initiate responses without conscious awareness.

Characteristics of Stimulus Information

• Modality (type of stimulus)
• Location (active receptive field)
• Intensity (determined by frequency of nerve signals)
• Duration (receptors become less sensitive to constant stimulus)
  • Example: Brain interprets optic nerve signals to the occipital lobe.

Receptor Adaptation

• Tonic Receptors: Limited adaptation, continuously respond (e.g., pain receptors)
• Phasic Receptors: Adapt quickly, respond only to new stimuli (e.g., pressure receptors)

Sensory Receptor Classification

1. By Distribution:

   • General sense receptors: Simple structures throughout the body
   • Somatic sensory receptors (tactile receptors of skin, proprioceptors)
   • Visceral sensory receptors (found in internal organs)
   • Special sense receptors: Complex organs in the head
   • 5 special senses: olfaction, gustation, vision, audition, equilibrium

2. By Stimulus Origin:

   • Exteroceptors: External environment stimuli (skin, special senses)
   • Interoceptors: Internal organ stimuli (visceral sensory)
   • Proprioceptors: Body and limb movements (muscle, tendon, joint receptors)

3. By Modality:

   • Chemoreceptors: Detect chemicals (smell, oxygen in blood)
   • Thermoreceptors: Detect temperature changes
   • Photoreceptors: Detect light intensity, color, movement
   • Mechanoreceptors: Detect distortion of cell membranes (touch, pressure)
   • Nociceptors: Detect painful stimuli (damage)

Tactile Receptors

• Abundant mechanoreceptors for touch, pressure, vibration, and stretch.
• Two types: Unencapsulated and Encapsulated tactile receptors.

Unencapsulated Tactile Receptors

1. Free Nerve Endings:
   • Simplest receptors, located close to skin surface.
   • Detect pain, temperature, light touch.
2. Root Hair Plexuses:
   • Wrap around hair follicles, located in deeper dermis.
   • Detect hair displacement (phasic receptors).
3. Tactile Discs:
   • Flattened endings in epidermis, respond to light touch (tonic receptors).

Encapsulated Tactile Receptors

1. End Bulbs (Krause Corpuscles):
   • Located in dermis, detect pressure and low-frequency vibration.
   • Tonic receptors.
2. Lamellated (Pacinian) Corpuscles:
   • Found deep in dermis, detect deep pressure and high-frequency vibration (phasic receptors).
3. Bulbous (Ruffini) Corpuscles:
   • Located in dermis/subcutaneous layer, detect continuous deep pressure.
   • Tonic receptors.
4. Tactile (Meissner) Corpuscles:
   • Found in dermal papillae; sensitive regions, detect discriminative light touch (phasic receptors).

Proprioceptors

• General sensory receptors in muscles, tendons, and joints.
  • Function: Relays sensory information about body position and movement (the sixth sense).
  • Types:
  1. Muscle Spindle: Detects stretch in skeletal muscle
  2. Golgi Tendon Organ: Detects tension in tendons
  3. Joint Kinesthetic Receptors: Detect stretch in articular capsules

Referred Pain

• Inaccurate localization of sensory signals, where visceral pain is perceived as originating from skin or muscle
  • Many sensory neurons send signals via the same ascending tracts within the spinal cord.

Clinical Relevance

• Example: Heart attack pain may be felt in the pectoral region or medial arm.
• Other examples include kidney pain referred to the inferior abdomen, and visceral pain along sympathetic nerves.

Olfaction: The Sense of Smell

• Detection of Odorants: Volatile molecules dissolved in nasal mucus detected by chemoreceptors.
• Provides information about food, people, and danger; can distinguish thousands of odors.
• Olfactory Epithelium: Located in the superior region of the nasal cavity; contains:
  • Olfactory receptor cells
  • Supporting cells
  • Basal cells (continually replace olfactory receptor cells).

Gustation: The Sense of Taste

• Taste occurs from encountering taste-producing molecules (tastants).
• Taste Buds: Contain gustatory cells (chemoreceptors) that detect tastants.
• Tongue Surface Structure:
  • Filiform: No taste buds (manipulate food).
  • Fungiform: Few taste buds.
  • Foliate: Few taste buds in early childhood.
  • Vallate: Largest and contain most taste buds.

Five Basic Taste Sensations

1. Sweet: Organic compounds (sugars).
2. Salt: Metal ions (Na+, K+).
3. Sour: Acids (e.g., vinegar).
4. Bitter: Alkaloids (e.g., unsweetened chocolate).
5. Umami: Savory taste related to amino acids.

Accessory Structures of the Eye

• Components include:
  • Extrinsic eye muscles, eyebrows, eyelids, eyelashes, conjunctiva, and lacrimal glands.

Eyebrows

• Aid in nonverbal communication and prevent sweat from dripping into the eyes.

Eyelids

• Protective covering over the eye; consist of fibrous core, orbicularis oculi muscle, and thin skin.

Conjunctiva

• Transparent lining that covers the eye and eyelids.

Lacrimal Apparatus

• Produces and drains lacrimal fluid (tears); contains:
  • Lacrimal gland producing fluid
  • Puncta, canaliculus, lacrimal sac, and nasolacrimal duct.

Eye Structure

• 2.5 cm diameter, located in the orbit, contains two cavities (posterior and anterior).
• Tunics of the Eye:
  1. Fibrous (External): sclera and cornea.
  2. Vascular (Middle): choroid, ciliary body, iris.
  3. Retina (Inner): pigmented and neural layers.

Vitreous and Aqueous Humor

• Vitreous Humor: Transparent gel in posterior cavity stabilizing the eye.
• Aqueous Humor: Watery fluid in anterior cavity, nourishing cornea and lens; secreted by ciliary processes.

Retina Structure

• Layers:
  • Pigmented layer: Absorbs stray light, provides vitamin A.
  • Neural layer: Houses photoreceptors (rods and cones) that convert light into nerve signals.

Physiology of Vision

• Refraction of Light: Required for sharp vision; light rays must be bent as they pass through differing media.
• Focusing Light:
  • Distant Objects: Ciliary muscles relaxed, lens flattens.
  • Close Objects: Ciliary muscles contract, lens becomes more spherical.

Phototransduction

• Converts light to electrical signals via photoreceptors (rods and cones).
• Photopigments: Light-absorbing molecules within rods and cones that interact with light to generate a nerve signal.

Auditory System

• Function: Detect sound and head movement via the ear structures.
• Pathway from Sound to Signal:
  • Sound waves vibrate tympanic membrane
  • Auditory ossicles transmit waves to oval window
  • Cochlea transforms waves into nerve signals transmitted via CN VIII.

Cochlear Structure

• Spiral Organ: Contains hair cells that release neurotransmitters to sensory neurons.
• Pathway of Sound: Sound waves cause vibrations leading to fluid movement and activation of hair cells, ultimately exciting sensory neurons.