Sensory Receptors and Transduction

Question 1

INTRO - Overview

Answer 1

• neurons in the brain and spinal cord do not respond when they are exposed to sensory stimuli such as touch
• instead, each form of energy but be transduced by a populaton of specialised cells —> these receptor cells convert the stimulus from a sensory into an electric signal for the neurons to interpret
• there are distinct receptor types which are selective for that particular energy form. Even in one sensory system, there are different types!
  
  • relationship between number of receptor types in a system and number of stimuli types that system is able to detect
• sensory transduction involves changes in membrane conductance
• two important processes
  
  • sensory receptors and stimulus QUANTITY—> sensitivity
    
    • extracting information on the magnitude/intensity of a stimulus
  • sensory receptors and stimulus QUALITY —> selectivity
    
    • extracting information about the quality of stimulus

Question 2

Detection

Answer 2

• receptors are specialised and only detect a specific type of stimulus energy

Visual receptors

• photoreceptors in retina convert light into electrical signals, triggering a change in the cell’s membrane potential
• two types or photoreceptors
  
  • rods: extremely sensitive, necessary for dark vision
  • cones: less sensitive, require brighter light; three different types respond to different wavelengths

Auditory and vestibular receptors

• inner and outer hair cells on the basilar membrane in the cochlear
• outer: amplify low-level sounds
• inner hair cells: transform sound vibrations in the fluids of the cochlea into electrical signals

Olfactory receptors

• located both in olfactory sensory neurons and the epithelium of the human airway
• display affinity for a range of odour molecules
• where odour molecules bind, depends on physio-chemical properties

Question 3

Amplification (Sensitivity)

Answer 3

• all receptors transduce energy to which they are sensitive into a change in membrane potential
• receptor transmits voltage change to a class of neurons, typically ganglion cells, which then signal to the brain or spinal cord
• two types of receptors
  
  • metabotropic: act through second messenger (relevant for visual and olfactory system)
  • ionotropic: channels that allow ions to pass through membrane in response to the binding of a chemical (auditory and vestibular system)

Metabotropic Receptor examples

Visual system

• odopsin receptors becomes altered in response to a photon being absorbed —> becomes excited —> activates g-protein cGMP
• cGMP synthesis decreases —> transduction channels close —> hyperpolarisation
• produces electrical signal

Olfactory System

• chemoreceptor interacts with odorant molecule —> activates receptor —> rise of cAMP and Ca2+ —> Ca2+ causes Cl- channels to open —> amplify receptor potential by 10-fold, helps excite the neuron

Ionotropic Receptors

Auditory and vestibular system

• direct activation of ion channels by the sensory stimulus
• example of mechanoreceptors: hair cells in inner ear
  
  • transduction channels are found at the top of each hair —> when hair bundle is moved (increased tension), opens ion channels —> depolarises cell
  • each receptor responds to a specific direction, if hairs are moved into the opposite direction, this reduced tension, channels will not open
  —> opening and closing of transduction channels follow the wave form of the sound stimulus

Question 4

Selectivity

Answer 4

• the dynamic range of any given neuron is limited
• neurons have refractory periods, so there is a rate limit —> limits range of firing rate that can encode variation in the amplitude of the stimulus
• but, different solutions for this have evolved, such as different receptors operating at different ranges, leading to stimulus selectivity

Visual system

• rods and cones selectively respond to different light wave lengths
  
  • rods respond to low levels of light
  • cones respond to higher levels of light and also responsible for seeing colour and for high spatial acuity
    
    • different types of cones are further selective for specific wavelengths

Auditory System

• high frequency sounds stimulate the base of the cochlea
• low frequency sounds stimulate the tip
• hair cells are arranged in rows along the cochlea and sense the motion of the basilar membrane
• in this way, the auditory system has a tonotopic organisation

Question 5

Sensory Adaptation

Answer 5

• a constant stimulus does not always generate a constant response
• when the rate of firing decreases progressively after initial burst —> adaptation occurs
• change in strength of a stimulus more important than the absolute value
  
  • e.g. if a hand is rested on a table, the table is initially felt against the skin but over time sensory adaptation occurs and the sensation of the table against the hand becomes unnoticeable
• all sensory and neural systems have a form of adaptation in order to constantly detect changes in the environemnt
• Ca2+ ions play a key role in that they send negative feedback that lead to receptor cells opening or closing channels in response to changes in ion flow
• mechanoreceptors use Ca2+ influx to physically move proteins to open or close channels

Visual System

• example: ‘afterimage’ - an image that continues to appear in the eye after one is no longer exposed to it
• explanation: photoreceptors are constantly exposed to the same stimulus which will eventually exhaust their supply of photopigment, resulting in a decrease of signals to the brain
• usually such overstimulation is avoided by constant microsaccades

Auditory System

• adaptation takes place in the auditory nerve fibers

—> initial firing rate slows down to a lower constant rate (spike frequency adaptation)

—> allows humans to adapt to sounds in their environment where they are no longer consciously perceived

Somatosensory System

• mechanoreceptors in the skin are either slow- or fast-adapting
• slow-adapting receptors are transmit important information and are highly sensitive e.g. to avoid further damage —> painful stimuli, temperature and light touch