Sensory Receptors: An Introduction

• Intro to Sensory Receptors

• Sensation is a conscious awareness of incoming sensory information.

• Sensation can only occur if the sensory input reaches the cerebral cortex.

Receptors

• Receptors respond to stimuli and initiate sensory input to the CNS.
• There is a range in the complexity of receptors.
• Stimuli are changes in the sensory information that receptors detect.

Receptors as transducers

• Transducers change energy from one form to the next.
• The original energy form is what the receptor detects.
• This energy is converted into electrical or chemical energy.
• Receptors establish and maintain a resting membrane potential across their plasma membrane.
• Receptors contain modality-gated channels in their plasma membranes.

Receptive Field

• The receptive field is the area through which a stimulus is detected.
• Receptive field size inversely correlates with the density of receptors.
• More receptors means more frequent and smaller fields.
• The larger the receptive field, the less precise the exact spot of stimulation.

Tonic Receptors

• Tonic receptors respond continuously to stimuli at a constant rate.
• Tonic receptors respond to both the presence and intensity of the stimulus.
• Receptor sensitivity stays constant over time or slowly adapts.

Phasic receptors

• Phasic receptors deal with temperature, pressure, and light touch.
• Phasic receptors detect new stimuli or changes in the state of the detected stimulus, along with onset and offset.
• They undergo rapid adaptation, with a reduction in sensitivity to a continually applied stimulus.

General and Special Senses

• General sense receptors are typically simple in structure.
• Somatic sensory receptors are housed within the skin and monitor tactile sensations.
• Somatic sensory receptors are also housed within muscles and joints, monitoring stretch and movement.
• Visceral sensory receptors are housed in the walls of the viscera.
• Visceral sensory receptors respond to temperature, chemicals, stretch, and pain.
• Special senses involve specialized, complex sense organs.
• Special senses include gustation, olfaction, audition, vision, and equilibrium.

Classification by stimulus origin

• Exteroceptors detect stimuli of external origin and are located near body surfaces.
  • Tactile receptors are found in the skin and mucous membranes.
• Interoceptors detect stimuli of internal origin.
  • Visceral sensory receptors detect changes within the visceral organs and report temperature, pressure, chemical changes, and perceived pain.
• Somatic sensory receptors are part of musculoskeletal structures that detect the position of bones and muscles.
• Propioceptors are located in muscles, tendons, and joints.
  • They detect body movement, skeletal muscle contraction and stretch, and enable awareness of body position.

Classifying general sensory receptors by modality

• Thermoreceptors are 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 stretch or distention within body structures.
  • Osmoreceptors detect changes in solute concentration of bodily fluids.
  • Proprioceptors detect the position of the body in space.
• Nociceptors respond to painful stimuli and they detect chemical, heat, or mechanical damage.

Thermoreceptors

• Thermoreceptors detect changes in temperature.
• There are six times more cold receptors than warm receptors.
• Thermoreceptors cannot detect temperatures below 10°C.
• Receptors are transient receptor potential cation (TRP) channels.
  • TRP channels respond at different temperatures.
  • TRP channels allow Ca2+ to depolarize the cell.
  • Some TRP channels also respond to chemicals.
    • Menthol activates TRPM8, detecting 25 and 28°C.
  • Capsaicin activates TRPV1 & 2, detecting >43°C & >52°C, respectively.

Nociceptors

• Nociceptors are a subtype of free nerve endings, concentrated in areas more prone to injury.
• Nociceptors adapt very slowly or not at all.
• Visceral nociceptors detect internal damage within the viscera.
• Somatic nociceptors detect chemical, temperature or mechanical changes at the body surface, joints, or skeletal muscles.

Nociceptors respond to:

• Cellular damage
• Noxious chemicals
• Cellular signals

Classification by structure

• Encapsulated receptors have nerve endings embedded within connective tissue.
• Unencapsulated receptors have free nerve endings.
• Sensory cells can be coupled to neurons, where the sensory cell possesses receptors to detect stimuli and stimulates a peripheral neuron.
• Neurons can also possess receptors and respond directly to stimuli via peripheral processes.

Tactile Receptors

• Tactile receptors are the most numerous type of receptor and are mechanoreceptors.
• Most tactile receptors are located in the dermis and subcutaneous layer.
• Tactile receptors can be simple (unencapsulated) or complex (encapsulated).

Tactile sensation

• Touch provides information about location, texture, size, shape, and movement.
• Pressure results from deformation of deeper tissues.
• Vibrations are rapid and repetitive sensory signals.

Unencapsulated tactile receptors

• Dendritic ends of sensory neurons that lack a protective coat.
• Free Nerve Endings, Root Hair Plexuses, and Tactile Discs (Merkel discs).
• Abundant in epithelia and CT
• Most are unmyelinated.

Free Nerve Endings and Root Hair Plexus

• Terminal branches of dendrites that are the least complex.
• Capable of slow to rapid adapting.
• Projections to the CNS are known as Αα, Αδ, and C fibers.
• Lie close to the surface in stratum granulosum
• Respond to hair movement and can be called root hair plexus.
• Polymodal. Detect temperature, touch, pressure, stretch and cell damage.

Merkel (or tactile) cells

• Flattened nerve endings associated with specialized sensory cells, abundant in the tips of fingers and lips.
• Located in the stratum basale.
• Are tonic receptors
• Detect fine touch and distinguish texture and shapes of objects.

Encapsulated tactile receptors are:

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

Tactile corpuscles

• Tactile corpuscles are also known as Meissner's corpuscles
• Consist of intertwined dendrites enclosed by neurolemmocytes arranged as horizontal lamellae, surrounded by dense irregular CT.
• Located in the papillary dermal layer, lips, palms, eyelids, nipples, and genitalia.
• Phasic receptors that detect light touch and vibrations (10 – 50 MHz).
• Used to distinguish texture and shape of objects.

Bulbous corpuscles

• Bulbous corpuscles are also known as Ruffini's corpuscles.
• Consist of spindle-shaped dendritic endings ensheathed in CT.
• Nerve endings are intertwined with collagen fibers encased within a capsule.
• Located in the dermal layer and surrounding the fingernails to monitor the slippage of objects.
• Detect skin distortion and continuous deep pressure, important for our ability to grasp and hold things with our fingers.
• Tonic receptors that do not exhibit adaptation

End bulbs

• Also known as Krause's corpuscles
• Dendritic endings ensheathed in CT
• Locations: Reticular dermal layer, Mucous membranes of oral, nasal, vaginal, & anal cavity
• Detects: Light pressure, Temperature

Lamellated corpuscles

• Lamellated corpuscles are also known as Pacinian corpuscles
• Capsule: Outer CT sheath
  • Concentric layers of collagen fibers
  • Inner core of neurolemmocyte
• Locations: Hairless skin, Subcutaneous layer of palms and soles, breasts and external genitalia, Deep reticular layer of dermis
• Rapidly adapting
• Detects Deep pressure, High frequency vibration

Referred Pain

• Perception of sensory nerve signals of the viscera in dermatomes of the skin.
• May cause a false sense of origin.
• Believed to be the results from shared ascending tracts.
• Useful in medical diagnosis.

Olfaction

• Olfaction is the sense of smell.
• It involves the detection of airborne chemicals by chemoreceptors within the nasal cavity
• Olfaction Allows us to sample our environment: For food, Identification of other individuals, Danger.
• Human olfaction is not as sensitive or developed as other organisms.

Olfactory epithelium

• The olfactory epithelium is found in the superior region of the nasal cavity.
• The epithelium has three distinct cell types:
  • Olfactory receptor cells: An afferent neuron,Detects odors.
  • Supporting cells: Secrete mucus, Support the olfactory receptor cells.
  • Basal cells: Neural stem cell, Give rise to new olfactory receptors cell (regenerate about every 40 - 60 days).
• Olfactory glands are found within lamina propria and produces mucus.

Olfactory receptor cell

• Olfactory receptor cells are bipolar neurons.
  • Single axon that projects to CNS.
  • Single dendrite projecting into overlying mucous.
• Dendrites possess olfactory hairs.
  • Olfactory hairs possess the chemoreceptors
    • Each chemoreceptor in that one cell is the same type.
    • Each chemoreceptor detects a specific chemical shape and thus a specific odorant.
• Axons form fascicles of the olfactory nerves (CN I).
  • Nerves project through the cribiform plate into the olfactory bulb
• Each receptor responds to only one discrete component of the odor (odorant molecule) rather than the whole odor.

Olfactory bulb

• Terminates to the olfactory tract.
• Olfactory nerves synapse with two types of secondary neurons:
  • Mitral and tufted cells
• Forms the olfactory glomerulus
  • About 2000 glomeruli that are highly organized within the olfactory bulb
• Secondary neurons from the olfactory tracts:
  • Project to primary olfactory cortex
  • Project to Hypothalamus and Amygdala (limbic system)
  • Do not project to thalamus

Characteristics of an odor

• Consists of multiple molecules in various concentrations
• Molecules that are detected are called odorants
• To be detected, an odorant must be:
  • Volatile - Easily vaporized
  • Sufficiently water-soluble - Must be able to dissolve in the mucus

Detecting smells

• Most inhaled air does not pass across the olfactory epithelium.
• Deep breathing:
• Inhaled air becomes agitated as it passes over the nasal conchae and moves toward the superior aspect of the nasal cavity.
• Odorants:
  • Diffuse into the mucus overlying the ORCs.
  • Bound by odorant-binding proteins which assist in odorant-receptor coupling.

Olfactory transduction

• Involves a G-protein coupled receptor: the odorant receptor
  • About 1000 different genes account for the different types
• G-protein is termed Golf
• Stimulation of odorant receptor activates a cascade through Golf
• Results in opening ion channels
• Leads to depolarization of an olfactory receptor cell

Stimulation the olfactory receptor cell

• Activation of a specific olfactory receptor varies by what can bind it.
  • Some odorant molecules can bind various receptors with different affinities
  • Some receptors can bind various odorants with different affinities
• The variation of odors give rise to different patterns of stimulation within the glomeruli
• The pattern of glomerular stimulation is submitted to the olfactory cortex and gives rise to the perception of smell

Olfactory detection

• All ORCs that possess the same olfactory receptors terminate in the same glomeruli
• Glomeruli are responsible for separating distinct components of the odor and organizing scent perceptions
• Mitral cells refine the smell signals & relay them to the brain for further processing

Where olfactory signals go

• Olfactory bulb along the olfactory tract
• Cerebral Cortex
  • Allows for conscious perception of smell
  • Identification of smell
• Limbic system
  • Hypothalamus: Controls visceral reaction to smell
  • Amygdala: Recognition of odors, Integrating odor to emotion