Sensory Systems

Sensory Systems

• Sensory systems comprise sensory receptors that detect stimuli and send information to an integrating center
• Sensory receptors transform incoming stimuli into changes in membrane potential
• The general properties of sensory reception are the same across all the senses

Classifying Sensory Receptors By Their Stimulus Modality

• Chemoreceptors sense the presence of chemicals
• Mechanoreceptors sense pressure and movement, including proprioception
• Photoreceptors sense light
• Thermoreceptors sense temperature
• Electroreceptors sense electric fields
• Magnetoreceptors sense magnetic fields

Stimulus Encoding

• Sensory receptors convert information about the stimulus into action potentials

Stimulus Modality and Location

• Receptor location encodes stimulus modality and location
• The integrating center interprets modality and location
• Modality uses labeled lines which are discrete pathways from sensory cell to integrating center

Adequate Stimulus

• An adequate stimulus is a particular afferent neuron associated with one modality of receptor and stimulus
• Perception is based on the pathway activated, not the stimulus identity

Polymodal Receptors

• Polymodal receptors have more than one adequate stimulus
• They are sensitive to more than one stimulus modality
• Nociceptors are an example, detecting potentially damaging stimuli

Receptive Field and Localization of Stimulus

• Receptive fields encode stimulus location via labeled line
• A receptive field is a region of the sensory surface that causes a response when stimulated
• Smaller fields allow more precise localization of the stimulus, or greater spatial acuity

Overlapping Receptive Fields

• Overlapping fields can enhance localization, increase sensitivity, and improve contrast
• However, they can lead to redundant information

Lateral Inhibition

• Signals from neurons at the center of the receptive field inhibit neurons in the surround
• This enhances contrast, decreases noise and improves edge boundaries

Encoding Signal Intensity

• Sensory neurons code stimulus intensity by changes in action potential frequency
• For example, strong stimuli equate with high action potential frequency

Dynamic Range

• The dynamic range is the range of stimulus intensities over which a receptor exhibits an increased response
• The threshold of detection is the weakest stimulus that will affect a response in a receptor
• Saturation is the top of the dynamic range, meaning the maximal response

Dynamic Range vs Discrimination

• There is a trade-off between dynamic range and discrimination
• A large dynamic range results in poor sensory discrimination
• A narrow dynamic range leads to good sensory discrimination
• With a large there will be a small change in the action potential frequency

Range Fractionation & Logarithmic Encoding

• Sensory discrimination improves by distributing responses amongst the receptor population
• Logarithmic encoding of intensity allows for a compromise between dynamic range and discrimination

Tonic Receptors

• Tonic receptors respond for the entire duration of a stimulus, meaning they are typically slow adapting
• Sensory adaptation sees a decreased response to stimulus as duration increases
• For prolonged stimuli, sensory adaptation can continue until the stimulus is tuned-out

Phasic receptors

• These receptors encode changes in stimulus

Chemosensation

• Involves the basic principles and varieties of chemosensation

Mechanoreception

• Describes the function of mechanoreception in touch and proprioception

Hearing and Balance

• Involves the hearing and balance

Chemoreceptors

• These mediate detection of chemicals
• Exteroceptors include olfaction (smell), gustatory (taste), and nociception (pain)
• Interoceptors include blood pH, chemosensors in stomach, and internal nociception (pain)

Mammalian System (Smell)

• The olfactory epithelium is located in the roof of the nasal cavity
• A mucus layer moistens olfactory epithelium and dissolve odorants
• Odorant binding proteins allows lipophilic odorants to dissolve in mucus
• Olfactory receptor cells are ciliated bipolar neurons

Odorant Receptor Neurons

• Olfactory receptor neurons (ORNs) express G-coupled receptors that lead to depolarization and action potential firing
• A binding of odorant to an odorant receptor causes a conformational change
• The activated G protein, Golf, moves through the membrane and activates adenylate cyclase
• Adenylate cyclase converts ATP into cAMP and cAMP opens cAMP-gated ion channels
• Ca2+ and Na+ enter the cell, causing a generator potential
• The Ca2+ also opens Ca2+-activated CI channels, causing CI to leave, increasing the depolarization
• The generator potential opens voltage-gated Na+ channels, triggering action potentials

Odorant Receptors

• Each olfactory neuron expresses only one type of odorant receptor protein
• Each receptor can recognize more than one odorant
• Each odorant can stimulate more than one receptor
• An odorant receptor is linked to a g-protein, which causes the formation of cAMP and the subsequent opening of ion channels that leads to depolarization
• Odors are perceived in the brain using combinatorial code in which each perceivable odor activates a unique set of receptors and olfactory neurons

Gustatory System

• Vertebrate gustatory receptors are taste buds located along the tongue
• Taste receptor cells (TRCs) are distributed across the tongue with regional differences in sensitivity to certain modalities (salty, sour, sweet, bitter, umami)
• Signals are transmitted from the TRCs to primary gustatory afferent neurons
• Increased intracellular Ca2+ leads to neurotransmitter release and activation of the GRN
• H+ from sour foods activate pH-sensitive proton channels Otop1
• These enter the cell and lower pH
• Lowered pH inhibits K+ channels and depolarizes the membrane, activating voltage-gated channels

Sweet, Bitter or Umami

• Sweet, bitter, or umami substance will bind to a receptor, and make a conformal change, causing Ca2+ to activate
• ATP is released from the cell and serves as a neurotransmitter by binding to P2X receptors on the afferent neuron

Mechanoreception

• Mechanoreceptors couple mechanical stimuli to ion channels like the: Epithelial Sodium Channel and Transient Receptor Potential (TRP) Channel
• The variations on mechanoreception includes: touch/pressure, proprioception, equilibrium/balance, hearing and baroreception

Touch Receptors

• Touch receptors are not evenly dispersed in the skin
• The hands and face are the most densely innervated regions
• Large fibers and corpuscles are touch receptor fibers

Touch

• There are different ways to detect stimulus
• Large vs small receptive fields and/or Generator vs receptor potentials
• Fast vs slow sensory adaptation

Proprioceptors

• Proprioceptors monitor the position of the body in space
• Spindle fibers respond to length changes is the body
• Golgi tendon organs detect the stretch of muscles and tendons

Hair Cells

• Hair cells are mechanosensors inside in the inner ear
• Their deflections modulate K+ conductance and transmitter release in these cell

Balance

• The mechanoreceptors for equilibrium (balance) are located in the inner eat
• Hair cells of the vestibular apparatus detect movements
• Maculae of the utricle and saccule detect linear acceleration and tilting
• Cristae of the semicircular canal detect angular acceleration
• Excitation vs inhibition depends on the direction of movement and the location of the hair cells

Hearing

• Hearing Involves the outer, middle, and inner ear
• Sound waves enter ear canal and vibrate the tympanic membrane.
• Stereocilia on the hair cells bend causing hair cells to depolarize
• Inner ear cells release neurotransmitter
• Glutamate excites afferent neuron, where round window serves as a pressure valve

Hair Cells for Hearing

• Hair cells in Organ of Corti detect basilar membrane movements
• Basilar membrane is the structural part in mammalian
• Inner hair cells transduce vibration while stereocilia on hair cells bend and depolarize

Amplifying Sounds

• Outer hair cells amplify sounds by somatic electromotility, which refers to its ability to change length and frequency
• This increases basilar membrane deflection and amplifies signals to Inner hair cells

Photoreceptors

• Photoreceptors convert light energy into changes in membrane potential
• The photoreceptive opsins are seven-membrane and span GPCRs
• Opsins localize to membranes in the outer segments of vertebrate photoreceptors

Phototransduction

• Phototransduction occurs through chromophore Isomerization
• Summary of light-induced events:
• Opsins covalently bind vitamin-A derived chromophores
• Photons cause isomerization of the chromophore
• Isomerization of the chromophore leads to changes in the opsin
• These include Conformational change in opsin, Dissociation of chromophore
• G-protein signaling events cause changes in membrane potential

Classes Of Photoreceptor

• Two classes are found throughout the animal kingdom
• Rhabdomeric PRs were recently discovered in vertebrates
• These PRs help entrain our internal clocks to environmental light/dark cycles

Rhabdomeric Photoreceptor Signaling

• These signal through Gq protein to absorb light
• Activated opsin activates a Gq protein which activates PLC, converting PIP₂ to DAG and IP3

Ciliary Photoreceptors

• Ciliary photoreceptors signal through G₁/transducin
• 11-cis Retinal absorbs light and isomerizes into all-trans retinal
• G; Protein transducin decreases cGMP and decreases NA+ which causes a Hyper-polarization

Eye

• In Mammalian version the eye allows for formation of a bright, focused image
• Cornea refracts light and focuses is onto the lens
• Muscle fibers of the iris change the pupil diameter in response to light
• The choroids of nocturnal animals contain a reflective layer called tapetum lucidum

Accomodation

• Refers to the ability of the eye
• Focus point is the point that light travels in when the muscle is relaxed

Rods and Cones

• These cells are located at the back of the retina
• Many rods synapse on a single bipolar cell, which can lead to convergence and a large receptive field

Eye Processing

• Ganglion cells which come from outside the fovea area
• Smaller receptive fields will give a greater acuity result

Sharp Central Vision

• Fovea centralis has several important characteristics
  • Consists of many cones, devoid of capillaries and displaced obstructing cell layers
• Human bodies also vary, and this is demonstrated in the absorption of light and color detection
• Activated horizontal or amacrine cells inhibit neighboring cells

Explore sensory systems in living organisms, receptor types, and sensory discrimination. Understand how sensory receptors convert external stimuli into signals for the nervous system. Logarithmic encoding advantages are also discussed.