8 Sensory physiology

8 Sensory physiology - Mechanisms of sensory physiology...

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Unformatted text preview: Mechanisms of sensory physiology Sensory s3mula3on opens s3mulus ­gated ion channels, which changes the membrane poten3al at the sensory ending. Informa3on about the intensity of the s3mula3on is coded in the number and frequency of ac3on poten3als produced in the sensory neuron. The receptor neurons in each sensory modality are specialized to respond to a specific kind of energy: Sensory modality Effec3ve s3mulus Sight (Vision) Light energy (electromagne5c radia5on Hearing (Audi5on) Mechanical vibra5on in air Smell (Olfac5on) Specific chemical species Taste (Gusta5on) Chemical species Pressure Mechanical force Pain Damage to 5ssues Many sensory neurons are located in very elaborate structures, e.g. the human ear. The non ­neuronal structures are cri3cal for the func3on of the organ. A hair cell has no axon, but it releases neurotransmiHer onto a sensory neuron. The amount of transmiHer varies depending on the posi3on of the cilia (or “hairs), and the amount of neurotransmiHer released changes the number and frequency of ac3on poten3als conducted along the axon of the sensory neuron. The human eye is another example of a set of sensory neurons that are surrounded by very elaborate structures. The sensory cells in the vertebrate eye are located in the re3na. All of the rest of the structures provide physical op3cs that guide and focus light onto the re3na. Individual photoreceptors contain stacks of membrane disks, and the protein rhodopsin is densely packed in the membrane of the disks. Rhodopsin absorbs a photon of light Rod Outer segment INSIDE OF DISK Disks cis isomer Light Enzymes Cell body CYTOSOL Synap3c terminal Rhodopsin Re3nal Opsin Fig. 50-20 trans isomer When a rhodopsin molecule absorbs a photon of light (i.e., a unit amount of light energy, the re3nal that is linked to opsin in rhodopsin undergoes a steric change from its cis conforma3on to its trans conforma3on. That structural change IS the ini3al visual event. Rod Outer segment INSIDE OF DISK Disks cis isomer Light Enzymes Cell body CYTOSOL Synap3c terminal Rhodopsin Re3nal Opsin Fig. 50-20 trans isomer The cis – trans isomeriza3on of re3nal triggers a 2nd ­messenger cascade in the cytoplasm outside of the disks, which causes Na channels that have been open to close. INSIDE OF DISK Light EXTRACELLULAR FLUID Disk membrane Phosphodiesterase Ac3ve rhodopsin Plasma membrane CYTOSOL Inac3ve rhodopsin cGMP Transducin GMP Sodium channel Na+ Membrane poten3al (mV) Dark Fig. 50-21 0 –40 –70 Light Hyper ­ polariza3on Time Na+ In the dark, the photoreceptor releases glutamate onto the next neuron along the path. When Na+ channels close, the membrane hyperpolarizes (remember the K+ leak channels), and glutamate release stops. Whether the bipolar cell is depolarized or hyperpolarized depends on the characteris3cs of this postsynap3c cell. Processing of visual informa3on begins in the re3na as signals from photoreceptors travel across synapses to the re3nal ganglion cells, which send the visual signal to the brain. That is, ac3on poten3als are transmiHed along ganglion cell axons and produce synap3c transmission onto postsynap3c cells in the brain. Light enters the eye, changes the behavior of photoreceptor cells, and produces a signal that travels through the re3na, along the op3c nerve, through a thalamic nucleus (the lateral geniculate), and from there to the primary visual cortex. From there, the informa3on is sent to many places in the brain for further processing. ...
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