lecture 9

receptors

Respond to a stimuli and initiate sensory input to the CNS
• Range in complexity

stiumul:Changes in the sensory information that our receptors detect (internal and external)

sensation

Conscious awareness of incoming sensory information
• Can only occur if the sensory input has reached the cerebral cortex

receptors are transducers

Transducers changes energy from one form to the next
• Original energy form is what is detected by the receptor
• Converted into electrical energy
• Results in a graded potentials called a receptor potential – must initiate an AP to transmit the signa


features:
Receptors establish and maintain a resting membrane potential across their plasma membrane
• Receptors contain modality-gated channels in their plasma membranes

types of receptors

Chemoreceptors – sensitive to specific chemicals
• Mechanoreceptors – sensitive to mechanical energy
• Thermoreceptors – sensitive to heat and cold
• Photoreceptors – respond to visible wavelengths of light
• Osmoreceptors – detect changes in solute concentration in body fluids
• Nociceptors (pain receptors) – sensitive to tissue damage: pinching, burning, distortion

wetness

wetness – there are no wetness receptors
• Touch receptors
• Pressure receptors
• Thermal receptors

gustation

sense of taste
Detection of chemicals by chemoreceptors within the oral cavit
• Aided by our olfactory sense

Papillae possess taste buds
• Found on the anterior surface of the tongue
• Found on the soft palate
• Taste bud :Onion-shaped structure containing a variety of gustatory cells and support cells
• Taste buds have taste pores where dissolved taste-producing chemicals (tastants) come into contact with the variety of gustatory cell


Gustatory cells are the taste receptor cells

gustatory cells

Specialized neuroepithelium
• Dendritic ending is formed by the gustatory microvilli
• Contacts the saliva and the environment of the oral cavity
• Tastants interact with receptors on the microvillus

4 types

type 1

Respond to Na+ ions (salt)

type 2

transduce sweet, umami and bitter
• Use GPCRs to detect tastants

type 3

Respond to sour stimuli

type 4

Serve as the stem cell

5 primary tatses

salt, sweet, umami, sour, bitter

salty

Stimulated by chemical salts.
• Direct entry of Na+ through specialized Na+ channels, called ENaC channels
• Found on Type I gustatory cell

sour

Caused by free H+.
• H+ binds to and blocks K+ channels (called PKD2L1 receptor) in the receptor membrane reducing passive movement of K+
• Found on Type III gustatory cells

sweet

specific configuration of glucose.
• Activates a GPCR to cause depolarization through second messenger cascade
• Found on Type II gustatory cell

umami

Triggered by amino acids, esp. glutamate.
• Activates a GPCR to cause depolarization through second messenger cascade
• Found on Type II gustatory ce

bitter

Chemically diverse group including alkaloids
• Activates a GPCR to cause depolarization through second messenger cascade
• Found on Type II gustatory cell
• These cells contain ~50-100 differing bitter taste receptors to respond to different bitter flavor

Carbon Dioxide

carbonic Anhydrase IV in Type III cell

fatty receptors

FFA1 (found in Type I cells)

tongue map

false

olfaction

sense of smell
• Detection of airborne chemicals by chemoreceptors within the nasal cavity
• Allows us to sample our environment, food
• Identification of other individuals
• Danger
• Human olfaction is not as sensitive or developed as other organism

Olfactory epithelium

Found in the superior region of the nasal cavity
• Three distinct cell types

Olfactory glands found within lamina propia
• Produces mucus

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 (regeneration is about every 40 – 60 days)

Olfactory receptor cell

Bipolar neurons:
Single axon that projects to CNS
• Single dendrite projecting into the overlaying mucus
• Dendrites possesses olfactory hairs

Olfactory hairs possess the chemoreceptors
• Each chemoreceptor in that one cell are the same type and
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

odors

Consists of multiple molecules in various concentrations
• Molecules that are detected are called odorants
• In order for an odorant to be detected it 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
• Moves inhaled air towards 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= Called the odorant receptor
• About 1000 different genes accounting 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 the olfactory receptor cell

olfactory bulb

Terminal end of olfactory tract
• Olfactory nerves synapse with two types of secondary neurons
• Mitral and Tufted cells
• Form the olfactory glomerulus

Secondary neurons form the olfactory tracts
• Project to primary olfactory cortex
• Project to Hypothalamus and Amygdala (limbic system)
• Do not project to thalamu

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

activation

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

variation

The variation of odors give rise to different patterns of stimulation within the glomeruli


• The pattern of glomerular stimulation is what is submitted to the olfactory cortex and what gives rise to the perception of smell

adaptation

Receptor no longer responds to a stimulus to the same degree

2 types

slow-adapting/tonic

Do not adapt at all or adapt slowly
• Present where maintaining information about the stimulus is important

fast-adapting/phasic

When stimuli stops – slight depolarization called an off response
• Important where changes in stimulus intensity are detecte

Mechanoreceptors of the skin

Most numerous types of receptors
• Responsible for somasthesia
• Located in dermis and subcutaneous layer

• Can be simple (unencapsulated) or complex (encapsulated)

Unencapsulated tactile receptor

Dendritic ends of sensory neurons lack a protective coat
• General sensations: thermoreceptors, nociceptors, light touch receptors
(mechanoreceptors), chemoreceptors (itch, pH)
• Most are unmyelinated
• Abundant in epithelia and CT

Three types
• Free Nerve endings
• Root hair plexuses (not shown)
• Merkel discs

Encapsulated tactile receptors

Wrapped by connective tissue or surrounded by glial cells
• Almost all are mechanoreceptors

4 types

krause bulbs

ermal of mucous membranes
• Light pressure & low frequency vibration
• Rapidly adapting

paicins corpucles

Subcutaneous skin (glabrous & hairy)
• Deep pressure & high frequency vibration
• Rapidly adapting

ruffinis corcuples

Subcutaneous skin (glabrous & hairy)
• Fluttering vibrations
• Slow adapting

messiners corcuples

Papillary ridges of glabrous skin
• Light touch, texture and shape of objects
• Rapidly adapting

receptive fields

area through which a stimulus is detected
•
One receptive field is associated with one primary sensory neuron which synapses with a secondary sensory neuron resulting in a larger secondary receptive field

Receptive fields can overlap
•
The larger the receptive field, the less we can localize the exact spot of stimulation

• Spatial discrimination is dependent on receptive field size

nocireceptors

Mechanical respond to pressure from sharp objects
types
• Thermal respond to burning heat of damaging cold
• Chemical are mechanically insensitive but respond to a variety of chemical agents
• Polymodal respond to a combination of mechanical, thermal and chemical stimuli

Modulated by a variety of chemicals which lower the activation threshold: either activating them or sensitizing the

noiceptor fibers

Ab fibers
• Large, myelinated
• Specific mechanical stimuli
• Some nociception

Ad fibers
• Small, myelinated
• Fast pain (sharp initial response)
• Specific mechanical (touch pressure) or cold stimuli

C fibers
• Small, unmyelinated
• Slow pain (dull persisting ache)
• Specific mechanical; heat and cold stimuli

Analgesia

suppression of the pain response
• Anesthetic is lack of sensation

Dependent on opiate receptors
• Bind endogenous opiates (endorphins, enkaphalins, dynorphin)
• Opiates inhibit the presynaptic terminal of afferent terminal (presynaptic inhibition

Proprioception

Awareness of body position in space
• Enables us to judge external objects through palpation
• Necessary for guiding movements

proprioception
recepotors

both are mechanosensitive
Golgi tendon organ
• Measures the force generated by a muscle through tension

Muscle spindle fiber
• Measure the length and rate of stretch of a muscle

stretch relfex

monitors and regulates muscle length/rate of stretch

• Monitored by stretch receptors called muscle spindles
When Muscle spindle is stretched sensory neurons detect the stretch of the intrafusal muscle fibers, Signal sent to spinal cord (CNS)

indirectly involved in reciprocal inhibition: inhibition of the antagonistic muscle

Golgi tendon reflex

mesures tension force

Prevents excessive contraction in response to increased tension

When a muscle contracts, the tendon stretches
• Golgi tendon organ detects the stretch
• Sends the sensory information to the CNS
• Synapse with interneurons in spinal cord
• Interneurons inhibit alpha motor neurons to prevent muscle contraction

Sensory neuron also stimulates (reciprocal activation) the alpha motor neuron in the antagonistic muscle