Special senses

Special Senses

Overview

• Our bodies are constantly exposed to sensory information called stimuli.

• Sensation: The sensory information arriving at the CNS (Central Nervous System).

• Perception: Conscious awareness of a sensation.

Types of Senses

• General Senses:

  • Sensations include pain, temperature, pressure, vibration, touch, and proprioception (body position).

  • General senses arrive at the primary somatosensory cortex.

• Special Senses:

  • Include taste (gustation), smell (olfaction), sight, hearing, and balance (equilibrium).

Receptors

• Receptors act as transducers, changing one form of energy into another.

• Receptor specificity is dictated by structure.

• Receptive field: The area monitored by a single receptor.

• Adaptation: A reduction in sensitivity due to constant stimulation.

Receptive Fields

• Small Receptive Field: High density of receptors; fine sensory discrimination.

• Large Receptive Field: Lower density; less precision in sensory perception.

Criteria for Classifying Receptors

• Receptor Distribution (Body Location):

  • General Senses:

    • Scattered throughout the body, structurally simple.

    • Somatic sensory receptors (in skin, joints, muscles, tendons).

    • Visceral sensory receptors (in walls of viscera).

  • Special Senses:

    • Located only in the head, structurally complex.

• Stimulus Origin (Location of Stimulus):

  • Exteroceptors: Detect stimuli in the external environment.

  • Interoceptors: Detect stimuli within the body.

  • Proprioceptors: Detect stimuli relating to skeletal muscles, tendons, and joints.

• Modality of Stimulus (Stimulating Agent):

  • Chemoreceptors: Detect dissolved chemicals in fluids.

  • Thermoreceptors: Detect changes in temperature.

  • Photoreceptors: Detect changes in light intensity, color, and movement of light rays.

  • Mechanoreceptors: Sensitive to deformation due to touch, pressure, vibration, and stretch.

  • Baroreceptors: Respond to stretch or distension of an organ.

  • Nociceptors: Respond to painful stimuli.

General Sense Receptors

• Scattered throughout the body and simple in structure.

• Four Types:

  • Nociceptors: Responsible for pain sensation from tissue damage.

  • Thermoreceptors: Sensitive to changes in temperature.

  • Mechanoreceptors: Respond to touch and pressure.

  • Chemoreceptors: Respond to chemicals or molecules.

Nociceptors

• Free nerve endings with large receptive fields.

• Found in surface skin, joint capsules, periosteum of bones, and blood vessels.

• Referred Pain: Pain perceived at a location different from its source.

• Three Types of Nociceptors:

  • Temperature: Respond to extremes of heat and cold.

  • Mechanical Damage: Respond to physical damage.

  • Chemical: Respond to hazardous substances.

Thermoreceptors

• More abundant for cold than hot.

• Consisting of free nerve endings; adaptation occurs with continued stimulation.

Mechanoreceptors

• Detect stretch, compression, twisting, or distortion.

• Three Classes:

  • Tactile Receptors: Response to touch, pressure, and vibration.

  • Baroreceptors: Monitor pressure changes in blood vessels and organs.

  • Proprioceptors: Sense the position of joints and muscles.

Chemoreceptors

• Transduce chemical substances to generate biological signals.

• Crucial for the sense of smell, e.g., detecting rotten food and signaling avoidance.

Touch Sensation

• Meissner’s Corpuscle (Tactile Corpuscle): Responds to light touch.

• Pacinian Corpuscle (Lamellated Corpuscle): Responds to deep touch stimulation.

Tactile Receptors

• Most numerous receptor type.

• Mechanoreceptors that react to touch, pressure, and vibration.

• Located in the dermis and subcutaneous layers of the skin.

Unencapsulated Tactile Receptors

• Tactile Disc: Senses light touch.

• Free Nerve Ending: Detects pain and temperature.

Encapsulated Tactile Receptors

• Meissner's Corpuscle: Light touch detection.

• End Bulb: Senses pressure in skin.

• Bulbous Corpuscle: Senses continuous pressure.

• Root Hair Plexus: Detects hair movement.

• Pacinian Corpuscle: Senses deep pressure and vibration.

Taste (Gustation)

• Fungiform Papillae: Contain cranial nerve VII (facial).

• Filiform Papillae: No taste buds present.

• Circumvallate Papillae: Contain cranial nerve IX (glossopharyngeal).

• Foliate Papillae: Contain cranial nerves VII and IX.

Tongue Histology

• Key components include epithelium, taste buds, and supporting stem cells.

• Taste sensations include sweet, umami, bitter, salty, and sour.

Olfactory System

• The Olfactory Epithelium is responsible for smell perception.

• Olfactory receptors, supported by basal and supporting cells, interact with odor molecules in the nasal cavity.

Eye Anatomy - Accessory Structures

• Eyelashes and Eyebrows: Protect the eye.

• Lacrimal Apparatus: Involves the lacrimal gland and ducts responsible for tear production and drainage.

Muscles of the Eye

• Superior Rectus Muscle: Moves the globe upward.

• Inferior Rectus Muscle: Moves the globe downward.

• Medial Rectus Muscle: Moves the globe medially.

• Lateral Rectus Muscle: Moves the globe laterally.

• Superior Oblique Muscle: Associated with downward rotation.

• Inferior Oblique Muscle: Associated with upward rotation.

Internal Eye Anatomy

• Fibrous Tunic: Comprises the sclera and cornea.

• Vascular Tunic: Consists of the iris, ciliary body, and choroid.

• Retina: Contains photoreceptors (rods and cones), bipolar cells, and ganglion cells, which process visual signals.

• Optic Nerve: Carries visual information to the brain.

Cochlea and Hearing

• Cochlea: A crucial structure in hearing, where sound vibrations are transduced into neural signals.

• Organ of Corti: Contains hair cells that respond to sound waves.

Sound Wave Pathways through the Ear

1. Sound waves enter the ear and vibrate the tympanic membrane.

2. Vibrations move through auditory ossicles (malleus, incus, stapes) amplifying sound.

3. The stapes generates pressure waves in the cochlea.

4. Hair cells in the cochlear duct initiate a nerve signal in response to pressure waves.

Balance and the Vestibular System

• Utricle and Saccule: Involved in static equilibrium; detect head position.

• Semicircular Canals: Involved in dynamic equilibrium; detect rotational movements.

• Cristae Ampullares: Hair cells within the ampulla detect acceleration and head movement.