Sensory Reception-Introduction PDF

An Introduction to Sensory Systems PH5208 Dr. Moira Jenkins Transduction-A transducer convert one type of energy into another type of energy Sensory receptors- convert stimulus intensity into voltage change across membrane Proportional/graded to stimulus intensity Sensory neuron carries the ener...

An Introduction to Sensory Systems PH5208 Dr. Moira Jenkins Transduction-A transducer convert one type of energy into another type of energy Sensory receptors- convert stimulus intensity into voltage change across membrane Proportional/graded to stimulus intensity Sensory neuron carries the energy (in the form of action potential) to CNS for processing and response Each sensory system “transduces” a particular form of stimulus energy into a bioelectric signal for nervous transmission from the periphery to the brain The continual flow of sensory information into the brain serves to “inform” the brain of the current state of the body, and of the surrounding environment terminology: • “periphery”  outside of the central nervous system • “stimulus”  some form of physical energy to which a peripheral sensory receptor will respond • “transduction”  a neurochemical process through which a sensory receptor neuron transforms the stimulus energy into a bioelectric signal • “bioelectric signal”  the action potentials that originate from a sensory neuron Modality Stimulus “modality” refers to the type of stimulus energy “5 senses” (smell, taste, sight, touch, hearing) subcatagories • vision: color and brightness • taste: bitter, sweet, salty, sour Stimuli share common attributes: • intensity  the strength of a particular stimulus • duration  the span of time that a stimulus is sensed • location  of the sensory receptor upon or within the body, and/or of the origin of the stimulus from outside of the body Somatosensory modalities Touch How men is it Pressure/Vibration Proprioception Pain Temperature Quantitative Somatosensory modalities Touch Pressure/Vibration Howbad does it Or hurt How Hot is Proprioception Pain it Temperature Qualitative Is quality Sensory Receptors The dendritic endings of a sensory neuron OR Specialized bipolar neurons that can detect stimulus information and transmit it to the dendrites of a sensory (afferent) neuron, some SPECIAL senses Ka: nonspiking neurons, neuroepithelial cells Have voltage gated ion channels, no action potential since the channels are within the decay range Somatosensory structure: peripheral receptor (modified “nerve ending”) periphery cell body (dorsal root ganglion) primary afferent fiber (peripheral nerve) dorsal root synapse (dorsal horn) CNS Somatosensory receptors transduce the stimulus energy into trains of action potentials that encode the stimulus energy for transmission into the CNS stimulus energy applied to the receptor leads to depolarization of a “trigger zone”, that is analogous to the axon hillock region of a neuron Sensory Receptors All sensory receptors are “stimulated” by some form of energy: mechanical: pressure; sound electromagnetic: light thermal: heat chemical: acidity; concentration; molecular structure General Receptor Names •Tactile receptors - touch •Nociceptors/Pain – respond to pain or noxious stimuli •Chemoreceptors – respond to chemicals (O2) •Thermoreceptors – respond to temperature changes •Mechanoreceptors – respond to compression, bending, stretching, etc. •Many of the general sensory receptors are found in the skin Special Receptors • Hair Cells ear • Photoreceptors • Taste cells tounge • Olfactory cells/epithelium eyes Nose Nonspiking Sensory Receptors Some sensory receptors are modified epithelial cells, capable of releasing neurotransmitter, no axon so no action potential Discrimination between separate modalities is accomplished via modality-specific receptors • photoreceptors • chemoreceptors • mechanoreceptors • thermoreceptors • nociceptors auditory, vestibular, & proprioceptive receptors are specialized to detect mechanical displacements caused by sound energy or gravity Kandel & Schwartz Fig. 21-1 Depolarization can lead to an action potential on the sensory neuron  towards the CNS Generator/Receptor Potential The somatosensory “generator potential” is the responsive depolarization of the trigger zone of the sensory nerve terminal The stimulus energy at the receptor will cause gated ion channels to alter the membrane conductance, generating a graded depolarization that will be proportional to the stimulus intensity y intensity generator (or receptor) potential:this is the change in membrane potential of the receptor that occurs in response to the stimulus (analogous to the PSP in synaptic neurotransmission) stimulus threshold:this refers to the intensity of the stimulus energy the stimulus threshold is the minimum intensity that will cause a sufficiently large generator potential to trigger “signaling” via the generation of action potentials The stimulus-induced generator potential must exceed the threshold potential of the trigger zone generatorpotential k changeinmembrane potentialin responseto stimulus stimulusthreshold b intensity ofstimulus I b Generator Potential - Trigger Zone Stimulus intensity− b Once the stimulus intensity reaches the stimulus threshold, the generator potential will be sufficiently large to depolarize the trigger zone past its threshold potential, triggering the generation of action potentials largegenerationpotential TriggerZone with further increments in stimulus intensity, the frequency of action potentials will then vary in direct proportion to the magnitude of the stimulus intensity and generator potential frequency magnitudeOp theStimulusintensity (or, receptor potential) “stimulus threshold” Rhoades & Bell Fig. 4.3 Neural encoding of the sensory input …  stimulus intensity →  generator potential →  signaling frequency →  neurotransmitter release a a Kandel & Schwartz Fig. 2-10 at the synaptic junction between the primary sensory afferent fiber with the first sensory relay neuron, encoding of the stimulus is preserved as neurotransmitter release from the primary afferent nerve ending, which will vary in proportion to the patterning and frequency of the train of action potentials triggered by the stimulus:  the frequency of action potentials encodes stimulus intensity  the duration of the train of action potentials encodes stimulus duration Receptor Adaptation K All sensory receptors demonstrate “adaptation” to a sustained stimulus rapid slow b stimuluschanges as timegoeson b on off Kandel & Schwartz Fig. 23-4 rapidly adapting receptors: • generate signaling that encodes “stimulus on” and “stimulus off” • more generally, generates signals as the stimulus intensity is changing, but not while it is sustained slowly adapting receptors: • generate signaling that encodes “stimulus is applied” vs. “no stimulus” • more generally, generates sustained signaling as long as the stimulus is applied, with a gradual reduction in the frequency receptor “adaptation” presents as a decreasing generator (or receptor) potential during a sustained stimulus  gradual decline in the frequency of action potentials from that receptor in response to a sustained stimulus X sensory receptors are categorized as being either slowly adapting: SUSTAINED gradual decay of the generator potential triggers a sustained train of action potentials with a gradually decreasing frequency for the duration that the stimulus is applied rapidly adapting: ON/OFF after an initial burst of action potentials (brief duration at high frequency) there is either a rapid fall in the frequency of action potentials, or else a complete cessation of action potentials for the remainder of the stimulus duration Rhoades & Bell Fig. 4.4 Receptive Field • Bundles of sensory afferent fibers are topographically arranged, and each fiber will “map” to a specific receptive field “receptive field” is the spatial domain to which an applied stimulus will trigger signaling along a particular fiber • the capability to localize a stimulus to a particular point upon the body is limited to the surface area of individual receptive fields • likewise, the capability to distinguish between two separate stimuli is also limited by the area of the individual receptive fields “topographic arrangement” implies an ordered projection of the sensory surface through the spatial arrangement of the afferent fibers, and the relay neurons to which they synapse  the capability to identify the specific location upon (or within) the body is dependent upon the specificity of “mapping” of each afferent fiber back to its own specific receptive field Kandel & Schwartz Fig. 21-9 2 Point Discrimination The smaller the receptive field, the better the 2-point discrimination Labelled Line Peripheral somatosensory information is transmitted to the brain via a multi-neuron “labelled line” synaptic pathway first order neuron: • sensory receptor neuron in the periphery that transduces the stimulus energy into a train of action potentials • its axon is a “primary sensory afferent fiber” that enters the CNS via a dorsal root of the spinal cord second order relay neuron: • located within the dorsal horn of the spinal cord OR brainstem • is innervated by primary sensory afferent fibers from the periphery • serves as a relay by projecting a “secondary fiber” via an ascending tract through the spinal cord and brainstem third order relay neuron: • located within the thalamus • has synapse from secondary sensory afferent fibers that ascend from the spinal cord • serves as a relay by projecting a “tertiary fiber” into the primary sensory cortex of the brain Cramer & Darby Fig. 9.9 Labelled Line and Topographic Mapping Somatotopic mapping is maintained along these ascending tracts, and into specific regions of the sensory cortex, in order to enable identification of the location to which a particular stimulus was applied 2 main tracts Dorsal Column Anterolateral Spinothalamic The labeled line pathway from each receptive field into the brain provides the basis for “somatotopic mapping” **Each labelled line maps back to one specific receptive field on the surface or within the body.