Chapter 14 Sensory Processes Flashcards
Sensory Receptor Cell Properties
- Sensory receptor cells are specialized cells that transform a stimulus energy into a neural signal
- Receptors on sensory cells are sensitive to specific modalities
- Receptors send signals which are amplified in the cell
- The sensory cell’s output goes to the CNS
3 examples of different sensory modalities:
- sound (bell ringing)
- light (light bulb)
- taste (skunk)
Sensory Receptor Classification
Primary function
- Exteroreceptors- external environment
- Interoreceptors- internal environment (homeostasis)
- Proprioceptors- position of the body in space
Receptor type
- Mechanoreceptors
- Photoreceptors- photons of light
- Chemoreceptors- chemical and olfaction
- Thermoreceptors- body temperature
- Nociceptors- pain
- Electroreceptors- electrical field (non humans)
- Magnetoreceptors- magnetics fields
Sensory Transduction
The process of converting stimulus energy into the energy of a nerve impulse
Intracellular Events in a Receptor Cell
Generator potential-when the receptor potential spreads to a spike initiating zone and generates an action potential
Mechanoreception
- Stretch-activated channels
- Ion channels that can be opened or closed by stretching a cell membrane
- open by tugging or stretching the cell membrane (cytoskeleton)
Mechanoreception: Crayfish Stretch Receptor
- Tetrodotoxin (TTX) prevents the production of action potentials, but not receptor potentials(blocks voltage gated Na+ channels)
- The receptor potentials are similar to EPSP’s
- Dendritic cell membranes are sensitive to stretch and can produce graded receptor potentials
- These are converted to APs in the spike initiation zone
Insect Mechanoreceptors
- The surface of insect bodies are covered with sensory “bristles” (bristle sensillum)
- The no mechanoreceptor potential C (NOMPC) receptor is activated as the bristle bends
- Similar to a v-gated K+ channel, but not v-sensitive
- ANK repeats are attached to the cytoskeleton
- Most stretch-activated receptors are nonselective cation channels
- Usually cause depolarization
Mammalian Mechanoreceptors: Touch
All touch receptors are dorsal root ganglion (DRG) cells
Channels responsible for transduction haven’t been identified
Mammalian Mechanoreceptors: Touch
DRG neurons have 5 types of endings in the skin
- Merkel disk
- Most important for form & texture
- 1 neuron goes to several disks
- Meissner’s corpuscles
- 2-6 endings surrounded by myelin & collagen
- Paccinian corpuscles
- Ruffini endings
- Endings around hair follicles
Receptor Adaptation
Tonic Receptors
- Adapt slowly, if ever
- Muscle stretch & joint proprioceptors
- ex: mocieptor, proprioceptor
Receptor Adaptation Continued
Phasic Receptors
- Adapt quickly
- On-off response
- Tactile stimulus
- can ignore the stimulus after awhile
- ex: sense of touch
Crayfish Stretch Receptor Adaptation
- Phasic receptors (above) only produce AP’s during the beginning of a stimulus.
- Larger depolarizations may cause trains of multiple action potentials.
Mechanisms of Sensory Adaptation
- Mechanical properties may filter the stimulus. Common in mechanoreceptors
- The receptor molecules may “run-down.” Example: bleaching of photopigment
- Enzyme cascade may be inhibited by substance accumulation
- A change in the electrical properties. Example: increased intracellular Ca2+ levels
- The spike initiating zone may become less excitable.
- May take place in higher order cells in the nervous system
Vestibular organs & hearing
- Vestibular organs can be used for equilibrium
- Simplest organ is a statocyst
- Vertebrates use a type of hair cell
- Hearing
- Detection of low frequency vibrations within the air, water, or substrate
- Tympanal organ – most common insect form
- Found in many locations
- Thorax, abdomen, legs, labial palps
- Found in many locations
- Vertebrates use hair cells
Organs of Equilibrium: Invertebrates
The lobster statocyst contains dense “statoliths” (made of sand or calcified secretions) which rest on hair cells connected directly to axons which go to the brain
Organs of Equilibrium: Invertebrates 2
These AP recordings from a single receptor in the statocyst epithelium show how firing rate changes when a specially-trained lobster performs a somersault
Organs of Equilibrium: Vertebrates
Hair cells
- Consist of stereocilia (a type of microvillus
- Some non-mammalian species also have a kinocilium
Organs of Equilibrium: Vertebrates 2
- Hair cells
- Movement may hyperpolarize or depolarize the hair cell
- Produces a receptor potential
- Alters neurotransmitter release
- Regulates sensory neuron response
- Movement may hyperpolarize or depolarize the hair cell
Ear Diagram
2 endolymph-filled inner ear chambers called the sacculus and the utriculus as well as the semicircular canals are the vertebrate organs of equilibrium
Static equilibrium (up, down, left, right)
- Sensed by hair cells within the macula of the sacculus and utriculus
- Movement of mineralized otoliths bends the stereocilia, giving the animal positional information relative to gravity
- Otoliths made of calcium carbonate
- 3 mutually perpendicular semicircular canals arise from the utriculus.
- Movement of the head will cause endolymph to distort the hair-cell containing gelatinous cupula lining the inside of the semicircular canals thereby causing a receptor potential.
- This gives the animal information about dynamic equilibrium.
Vertebrate Hearing: The Cochlea
Vertebrate Hearing: The Cochlea Continued
- Cochlear cross-section shows 3 chambers (scala)
- The hair-cell containing Organ of Corti sits on the basilar membrane of the scala media.
- Birds: no organ of corti
- Reptiles: no tectorial membrane
- Amphibians: no tectorial/basilar membrane
Vertebrate Hearing: The Organ of Corti
- Vibration of the ossicles is converted to pressure waves in the perilymph which cause distension of the basilar membrane and deflection of cilia in the overlying hair cells
- Inner hair cells responsible for audition
- Outer cells responsible for amplification (electromotility)
Vertebrate Hearing: The Organ of Corti Continued
- Stretching the ciliary cell membrane opens ion channels causing receptor potentials.
- The unusual ionic composition of endolymph means that K+ is a depolarizing ion in this situation.
Vertebrate Hearing: Fish and Amphibians
- Weberian apparatus transfers vibrations from the swim bladder to the inner ear
- APs are generated by hair cells within the inner ear
Chemical Senses: Taste and Smell
- Chemoreceptor: the general term for sensory receptors in this family
- Can be extremely sensitive: the male silkworm moth can detect the pheromone bombykol at concentrations of 1 molecule/ 1017th!
- Gustatory receptors are taste receptors
- Taste receptors are divided into 4 categories:
- (1) sweet (2) sour (3) salt (4) bitter (5) umami
- Taste receptors are divided into 4 categories:
- Olfactory receptors are smell receptors
- There are hundreds of different types of olfactory receptors
- Richard Axel and Linda Buck won the Nobel Prize in Medicine and Physiology for their discovery of olfactory receptors (2004)!
The Vertebrate Taste Bud
- The taste bud consists of the receptor cell, basal cells (which generate new receptor cells), and supporting cells
- Transduction takes place across the apical membrane
- Receptor cells synapse with afferent neurons which travel to the CNS
Taste Bud and Nerve
Mechanisms of Taste Sensations
Signal transduction of a generic gustatory cell
Mechanisms of Taste Sensations Salty and Sour
Na+ travels through a Na+ channel causing direct depolarization of the apical membrane
H+ travels through cation channels, depolarizing the apical membrane. H+ ions block K+ channels and allow a slow depolarization from Na+ leaking into the cell
Mechanisms of Taste Sensations: Sweet
- Alanine (for sweet—like aspartame) works through a G-protein/cAMP-mediated mechanism to close K+ channels allowing for a slow depolarization.
- Synthetic sweeteners like saccharine activate a receptor which works through phospholipase C to increase DAG and IP3
- Arginine (for sweet) binds to a ligand-gated nonspecific cation channel.
Mechanisms of Taste Sensations Bitter and Umami
Olfaction
- Olfactory receptors have a variety of locations:
- Antennae (sensilla)
- nasal cavities (most vertebrates)
- Mammals have an olfactory epithelium
- Some mammals have an additional olfactory area called the vomeronasal organ which is important for communication (territory, reproductive pheromones)
- vomeronasal receptors are physiologically and molecularly distinct from olfactory receptors in the nasal cavity
Olfaction
Animals that are particularly dependent upon olfaction have turbinates which are complex nasal cavities
Vertebrate and Insect Olfactory Receptors
- Both vertebrate and insect olfactory receptor cells extend cilia into a mucous layer
- Depolarization is initiated in the cilia that extend from the dendrite
- Unlike taste buds, olfactory receptor axons directly carry messages to the CNS
Olfactory Receptors
- Odorants bind to receptors that act via G-protein mediated 2nd messengers (cAMP) to open ligand-gated cation channels causing membrane depolarization
- There are about 1000 known olfactory receptor proteins
- The olfactory cell contains one type of receptor, but it may bind to several odorants
- The vomeronasal organ uses a different family of receptor proteins
Transmission of Olfactory Information
Related receptors send their axons to the same part of the olfactory bulb in the brain
Vomeronasal receptor axons go to glomeruli in the accessory olfactory bulb
