Sensation and Sensory Receptors Overview

Sensation

• Conscious awareness of incoming sensory information
• Occurs when sensory input reaches the cerebral cortex

Receptors

• Respond to stimuli and trigger sensory input to the central nervous system (CNS)
• Vary in complexity
• Detect changes in sensory information

Receptors as Transducers

• Receptors change one form of energy into another
• Original energy (detected by receptor) gets converted to electrical/chemical energy

Receptive Field

• Area of detection for a stimulus
• Size is inversely related to receptor density
• More receptors lead to smaller receptive fields
• Larger receptive fields hinder precise localization of stimulation

Tonic and Phasic Receptors

• Tonic Receptors*

• Respond continuously to stimuli at a constant rate

• Maintain sensitivity to sustained stimuli

• Sensitivity can adjust over time or remain slowly adaptive

• Phasic Receptors*

• Respond to new stimuli or changes in existing stimuli

• Adapt rapidly, experiencing reduced sensitivity to ongoing stimuli.

General and Special Senses

• General Sense Receptor*

• Simple structure

• Somatic receptors sense touch, stretch in muscles and joints

• Visceral receptors detect changes like temperature, chemicals, and pressure in internal organs

• Special Senses*

• Complex sense organs

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

Classification by Stimulus Origin

• Exteroceptors*

• Detect stimuli from the external environment, like touch, pressure, temperature, and pain

• Interoceptors*

• Detect stimuli from internal organs (viscera), sensing things like stretch, chemical changes, temperature

• Proprioceptors*

• Detect body position and movement, including skeletal muscle contraction and stretch

Classifying General Sensory Receptors by Modality

• Thermoreceptors: Sensitive to changes in temperature
• Chemoreceptors: Detect chemicals or specific molecules dissolved in fluid
• Mechanoreceptors: Respond to touch, pressure, vibration, and stretch
• Baroreceptors: Detect changes in pressure
• Osmoreceptors: Detect changes in solute concentration
• Nociceptors: Detect potentially damaging stimuli (e.g., painful stimuli, extremes of temperature)

Thermoreceptors

• Detect changes in temperature
• More cold receptors than warm receptors
• Cannot detect below a certain threshold (e.g., 10°C)
• Different types of thermoreceptors respond to varied temperature ranges
• Respond to chemicals

Nociceptors

• Free nerve endings
• Concentrated in areas prone to injury
• Adapt slowly or not at all
• Two main types
  • Visceral detects internal damage
  • Somatic detect skin/muscle damage
• Respond to cellular damage, noxious chemicals, and cellular signals

Classification by Structure

• Encapsulated*

• Receptors with tissue wrapping

• Unencapsulated*

• Simple nerve endings (lack a wrapping)

Tactile Receptors

• Most numerous type of mechanoreceptor
• Located in the dermis and subcutaneous layer
• Simple (unencapsulated) or complex (encapsulated)

Tactile Sensation

• Touch: Provides information about location, texture, size, shape, and movement
• Pressure: Results from the deformation of deeper tissues
• Vibration: Rapid and repetitive sensory signals

Unencapsulated Tactile Receptors

• Dendritic ends of sensory neurons without a protective coat
• Three types
  • Free nerve endings
  • Root hair plexuses
  • Tactile discs (Merkel discs)
• Abundant in epithelia and connective tissue
• Most are unmyelinated

Free Nerve Endings & Root Hair Plexus

• Least complex terminal branches of dendrites
• Slow to rapidly adapting
• Important for detecting touch, pressure, and movement around hair follicles.

Merkel (or Tactile) Cells

• Flattened nerve endings that work with specialized sensory cells
• Located in the stratum basale
• Tonic receptors, which means they adapt slowly
• Respond to fine touch and differentiate texture and shapes

Encapsulated Tactile Receptors

• Wrapped by connective tissue or surrounded by glial cells
• Four types
  • Tactile corpuscles (Meissner's corpuscles)
  • Bulbous corpuscles (Ruffini's corpuscles)
  • End bulbs (Krause's corpuscles)
  • Lamellated corpuscles (Pacinian corpuscles)
• Almost all are mechanoreceptors

Tactile Corpuscles

• Meissner's corpuscles: Intertwined dendrites and respond to light touch and vibrations
• Located in dermal papillae and areas needing fine touch

Bulbous Corpuscles

• Ruffini's Corpuscles: Spindle-shaped dendritic endings surrounded by CT
• Sensitive to continuous deep pressure and skin distortion.
• Tonic receptors so no adaptation occurs

End Bulbs

• Krause's corpuscles: Dendritic endings enclosed within CT in mucous membranes
• Respond to light pressure and temperature

Lamellated Corpuscles

• Pacinian corpuscles: Capsule has concentric layers of collagen fibers and a core of neurolemmocyte
• Detect deep pressure and high-frequency vibrations

Referred Pain

• Sensation of pain in the skin that originates from internal organs caused by shared nerve pathways
• Useful in medical diagnosis as it aids in identifying internal issues.

Olfaction (Smell)

• Detection of airborne chemicals by chemoreceptors in nasal cavity
• Allows sampling of the environment, including identification of food and other individuals
• Identification of danger
• Not as developed as other organisms

Olfactory Epithelium

• Found in the upper nasal cavity
• Three types of cells
  • Olfactory receptor cells (detect odors)
  • Supporting cells (support ORCs)
  • Basal cells (neural stem cells, regenerate ORCs)
• Contains mucus-producing olfactory glands that help move and stimulate odorants.

Olfactory Receptor Cell

• Bipolar neuron
• Single axon projecting to the CNS
• Dendrites bear olfactory hairs with chemoreceptors
• Each receptor reacts to a specific odorant shape
• Sensory nerves project to the olfactory bulb in the brain.

Olfactory Bulb

• Terminal end of the olfactory tract
• Contains mitral and tufted cells, and highly organized glomeruli.
• Glomeruli group similar olfactory receptor cells
• Secondary neurons relay signals to various areas including the cortex, amygdala, and hypothalamus

Characteristics of an Odor

• Multiple molecules in various concentrations
• Odorants are molecules detected
• Volatile, easily vaporized, and dissolve in the mucus

Detecting Smells

• Deep breathing brings air through nasal conchae
• Odorants diffuse into mucus
• Odorant-binding proteins help with odorant-receptor coupling

Olfactory Transduction

• Involves G-protein-coupled receptors (Golf)
• About 1000 different receptor types
• Stimulation of receptors initiates a cascade leading to ion channel opening and cell depolarization

Stimulation of Olfactory Receptor Cell

• The degree of receptor activation varies based on which odorants are present and their affinity for different receptors.
• Different patterns of stimulation in glomeruli create unique olfactory experiences.

Olfactory Detection

• ORCs with the same type of receptors converge on the same glomerulus
• Glomeruli separate distinct odor components and organize scent perceptions
• Mitral cells refine signals and conduct them to the brain

Where Olfactory Signals Go

• Signals travel along the olfactory tract to the olfactory bulb and then to the olfactory cortex, allowing conscious detection of smells.
• The limbic system is involved in odor recognition and emotional association with scents, including the hypothalamus for visceral responses and amygdala for emotional responses.