CHAPTER 10: Sensory Physiology
Sensory Physiology ! !
How we perceive our environment ! Six sensory systems Three common steps associated with any sense: ! !
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3. Formulation of “perception”, or our conscious experience of that sensation
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Touch
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Proprioception Temperature
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Pain
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Itch
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1. A 1. A physical stimulus 2. Sensory transduction: transformation of sensory input into nerve impulses
Somatosensory
Visual Auditory Vestibular Olfactory Gustatory *forms of nociception occur in the others, too. Just less often
Sensory Physiology !
Four basic types of information conveyed by each sensory system: !
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1. Modality of stimulus: type of receptor 2. Intensity of stimulus: AP frequency 3. Time course of stimulus: phasic vs tonic 4. Location of stimulus
The sensory cells ! !
Some are themselves neurons Most are specialized epithelial cells that synapse on adjacent sensory neurons
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Four functional classes of sensory receptors ! ! ! !
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1. Mechanoreceptors 2. Chemoreceptors 3. Thermoreceptors 4. Photoreceptors
Sensory receptors at the protein level: !
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Channels (e.g. stretch receptors) GPCR’s (e.g. photoreceptors of retina)
Modalities & sensory receptors !
“The five senses”: ! ! ! ! !
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Taste Touch (a somatic sense) Smell Sight Sound
Additionally, we include: !
Other somatic senses ! ! ! !
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Pain Temperature Itch Proprioception
Sense of balance (vestibular system)
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Each sensory receptor responds to a particular modality The sensory receptor transduces the external stimulus into internal electrical impulses ! !
Stimulus transduction Receptor potentials are generated across their dendritic membranes in response to stimuli (cable like properties)
Sensory Adaptation !
Duration of sensation is in part encoded by adaptation of receptors Phasic receptors ! Fast adapting; ex: touch your arm and feel it instantly. After a while you don’t even notice it More abundant Tonic receptors ! Slow adapting Sensory systems detect contrasts !
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FIGURE 10.1
The Receptive Field The receptive field of a sensory neuron encodes the location of the stimulus In sensory systems, the receptive field is oftentimes comprised of a “center” and a “surround;” magnitude of sensation and contrast between center/ surround are directly correlated Overlapping between receptive fields occurs
Ganglion cell receptive fields: detection of contrast - “On-center” GCs are most stimulated (*) by central illumination & darkness in the surround -“Off-center” GCs are most stimulated (*) by surround illumination & darkness in the center *change in the light received -> change in the action potentials being fired at the ganglia. - Ganglia sees many receptive fields from different polar cells, gets complicated.
Somatosensory Perception !
Receptor types (2) 1. Cutaneous (skin) R’s ! Touch/pressure R’s Hot/cold R’s Nociceptors (pain R’s) Examples Free nerve endings – light touch; hot; cold; nocireception ! Merkel’s discs – sustained touch and pressure ! Ruffini corpuscles – sustained pressure ! Meissner’s corpuscles – changes in texture; slow vibration ! Pacinian corpuscles – deep pressure; fast vibrations ! 2. Proprioceptors (joint/movement) ! Muscle spindles Golgi tendon organs Joint R’s ! ! ! !
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The Eye !
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Cornea Where light gets refracted, or “bent” ! Responsible for fine tuning of our images ! Thin transparent layer that covers the eye ! Iris Gives the eye color ! The part of the eye that contracts or dilates to regulate the amount of light passing ! through the pupil Pupil Empty so like can pour through ! Lens Retina Peripheral vision ! Sensitive to light levels but low acuity ! Fovea Focused vision ! Low light sensitivity but high acuity ! Optic Nerve Vitreous Humor
The Eye
Inversion & reversal of the visual field
The light is upside down and switched left/right
Know what is ipsilateral or contralateral
Autonomic Control of Pupil Diameter Radial muscles stimulated via 1-adrenergic receptors
Circular muscles stimulated via muscarinic receptors
The Visual System !
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Photoreceptor cells are G-protein Coupled Receptors (GPCR PRCs). Retina transduce electromagnetic energy, in the form of photons Two types of PRCs !
RODS ! ! !
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Dim-light vision Greater sensitivity to light The light receptor is called Rhodopsin
CONES ! !
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Color vision Greater visual acuity ; incoming light rays encounter fewer “obstacles” (less cell layers and/or cellular processes) The light receptors are called Photopsins
Anatomy of the Retina !
Pigmented epithelium
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Homeostasis of PRCs Visual cycle of retinal; constant shedding of pigmented epithelium ! PRCs (rods & cones) sit at the back of the retina !
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Bipolar cells (BPCs) receive input from PRCs Ganglion cells receive input from BPCs ! Bipolar cells Rods to ganglion cells !
Cones to ganglion cells Interneurons mediate lateral information flow !
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Horizontal cells Connects rods and cones to each other !
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Amacrine cells
Ganglion to ganglion Information flow !
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Hits ganglion axons first, then neuronal cell bodies, through bipolar bodies, then cell bodies of photoreceptor cells " Phototransduction goes In opposite direction, back up to ganglions " "
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Thalamic neurons Cortical neurons
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Occipital lobe (contains visual map)
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FIGURE 10.36
Visual Transduction in the Retina !
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Rods & cones share many key elements of phototransduction Photoreceptors: GPCR’s !
Outer segment contains stacks of ! ! !
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Rhodopsin in rods Photopsins in cones Photopigment is retinal
Photoreceptors convert photons into G-protein (transducin)
“Dark Current” ! ! !
PRCs are depolarized in dark PRCs are hyperpolarized in light Dark current is a positive inward (depolarizing) current
FIGURE 10.37
Phototransduction in a Rod ! ! ! ! ! ! !
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1. " (photon)
activates Rhodopsin 2. Rhodopsin activates Gt (transducin) 3. G!(GTP) activates cGMP-dependent phosphodiesterase (PDE) 4. PDE decreases [cGMP]i 5. Decreased [cGMP]i closes Na+ channels 6. Decreased Na+ influx hyperpolarizes the rod cell 7. Hyperpolarization means decreased neurotransmitter (glutamate) release at PRC/BPC synapse to inhibitory receptor 8. Decreased NT binding to the inhibitory metabotropic receptor on BPC means the nerve terminal on the opposite side of the BPC releases more excitatory NT to the LGIC on the ganglion cell Pink = retinal ! Common to cones and rods (photopsin/rhodopsin) ! When light hits it, it transforms to all trans and then is no longer bound
The Chemical Senses: Gustation & Olfaction TASTE !
Sensory receptors Taste buds ! Gustatory/taste cells (specialized epithelia; perform the sensory transduction) Supporting cell Five submodalities ! Salty Na+ ion through LGIC causing depolarization, opens Ca2+ channels, releases ! neurotransmitter Sour H+ ion through LGIC causing depolarization, opens Ca2+ channels, releases ! neurotransmitter Sweet and Umami Sugars bind to GPCRs, 2nd messenger closes K+ channels, depolarization, ! releases neurotransmitter Bitter Quinine bind to GPCRs, 2nd messenger induces Ca2+ release from endoplasmic ! reticulum, depolarization, releases neurotransmitter One sensory neuron per taste submodality ! !
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Taste Cells
FIGURE 10.8
The Chemical Senses: Gustation & Olfaction SMELL ! ! !
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Sensory cells are bipolar neurons intercalated among olfactory epithelia Each sensory neuron has one type of receptor Dendritic cilia contain odorant R’s (GPCR); odorant binds " ! G-protein dissociates " ! ! Binds Adenylate cyclase " ! Activates CAMP " Binds to channel " ! ! Opens a Ca2+/Na+ channel (both flow in) " ! Depolarization excites bipolar neuron in the olfactory ! Axonal end of the bipolar neuron synapses at glomeruli bulb " ! These neurons, with the help of the interneurons tuft and mitral , send information to prefrontal cortex Smell and taste have in common: all chemoreceptors
Auditory System ! ! ! !
Sense of sound Ear is the sensory organ Auditory transduction occurs in the cochlea of the inner ear Auditory receptor cells are hair cells
Hair Cells Pitch (frequency of sound waves) encoded by location of stimulated hair cells in cochlea. Specific location determines the pitch. Just like motor maps, a pitch map exists Loudness (intensity of sound waves) encoded by degree of bending of hair bundle " determines frequency of action potential The stereo cilia are connected at the tip, which is key to AP frequency
Potassium, as opposed to Na, moves in and depolarizes it. This is due to unique concentration in the ear. K+ moves with it’s electrochemical gradient
Ears and Shit !
Ears Outer Ear ! Consists of the auricle and auditory canal Collects longitudinal sound waves and channels them to the tympanic membrane, which is the beginning of the middle ear Middle Ear ! Has the tympanic membrane, which vibrates back and forth due to the vibrations from the outer membrane, and pushes the ossicles back and forth Ossicles (also in middle ear): ! 1. Malleus ! 2. Incus ! 3. Stapes The ossicles transmit information to the oval window of the fluid-filled inner ear The ossicles use a reduced surface area to amplify the force from the tympanic membrane twenty fold The muscles tensor tympani and stapedius insert onto the ossicles during loud explosions to protect the inner ear ! !
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Ears and Shit !
Ears !
Inner ear Contains the oval window, as well as the: ! 1. Cochlea contains hair cells The vibration of the ossicles on the oval window causes fluid waves in the inner ear that depolarize the hair cells of the cochlea Hair cells: are responsible for the transduction mechanism that generates an electrical signal the nervous system can interpret The action potentials from the hair cells travel to the auditory nerve to the brain ! 2. Semicircular canals The semicircular canals are important for balance; 3 semicircular canals exist in each ear 1. X Plane (3) 2. Y Plane (4) 3. Z Plane (2) Endolymph Fluid in the semicircular canals Movement of endolymph in the canals puts pressure on the hair cells inside Perilymph Present in the scala vestibula and scala tympani Difference in concentration gradients between this and endolymph is crucial for generating electrical signals !
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Ears and Shit !
Ears
Ears and Shit !
Ears
