About This Chapter • General properties of sensory systems • Somatic senses • Chemoreception: smell and taste • The ear: hearing • The ear: equilibrium • The eye and vision © 2016 Pearson Education, Inc. Sensory Pathways • Stimulus as physical energy sensory receptor – Receptor acts as a transducer • Intracellular signal usually change in membrane potential • Stimulus threshold action potential to CNS • Integration in CNS cerebral cortex or acted on subconsciously © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Receptors to Particular Forms of Energy • Naked (“free”) nerves • Complex neural receptors encased in connective tissue capsules • Smell receptors are neurons • Non-neural receptors for four special senses © 2016 Pearson Education, Inc. Receptors to Particular Forms of Energy • Chemoreceptors respond to chemical ligands: taste, smell • Mechanoreceptors respond to mechanical energy pressure and sound: hearing • Thermoreceptors respond to temperature • Photoreceptors for vision respond to light © 2016 Pearson Education, Inc. Figure 10.1 Simple, complex, and nonneural sensory receptors Simple receptors are neurons Complex neural receptors have nerve Most special senses receptors are cells with free nerve endings. They endings enclosed in connective tissue capsules. that release neurotransmitter onto sensory may have myelinated or This illustration shows a Pacinian corpuscle, neurons, initiating an action potential. The unmyelinated axons. which senses touch. cell illustrated is a hair cell, found in the ear. Stimulus Stimulus Stimulus Enclosed nerve Free nerve endings Specialized receptor ending cell (hair cell) Layers of connective tissue Synaptic vesicles Synapse Unmyelinated axon Myelinated axon Myelinated axon Cell body Cell body Cell body of sensory neuron © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Sensory Transduction • Stimulus energy converted into information processed by CNS – Ion channels or second messengers initiate membrane potential change • Adequate stimulus: form of energy to which a receptor is most responsive • Threshold: minimum stimulus • Receptor potential: change in sensory receptor membrane potential © 2016 Pearson Education, Inc. Integration by CNS • Sensory information – Spinal cord to brain by ascending pathways – Directly to brain stem via cranial nerves • Visceral reflexes integrated in brain stem or spinal cord usually do not reach conscious perception. © 2016 Pearson Education, Inc. Integration by CNS • Perceptual threshold: level of stimulus necessary to be aware of particular sensation • Habituation: decreased perception through inhibitory modulation – Falls below perceptual threshold • Each major division of the brain processes one or more types of sensory information © 2016 Pearson Education, Inc. Figure 10.3 Sensory pathways in the brain Primary somatic sensory cortex Gustatory cortex Olfactory cortex Olfactory bulb Auditory cortex Visual cortex Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum Most sensory pathways project Nose Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium Equilibrium pathways project primarily to the cerebellum. Tongue FIGURE QUESTION Which sensory pathways shown do not synapse in the thalamus? Somatic senses © 2016 Pearson Education, Inc. Figure 10.3 Sensory pathways in the brain Slide 1 Primary somatic sensory cortex Gustatory cortex Olfactory cortex Olfactory bulb Auditory cortex Visual cortex Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum Most sensory pathways project Nose Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium Equilibrium pathways project primarily to the cerebellum. Tongue Somatic senses © 2016 Pearson Education, Inc. Figure 10.3 Sensory pathways in the brain Slide 2 Primary somatic sensory cortex Gustatory cortex Olfactory cortex Olfactory bulb Auditory cortex Visual cortex Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum Nose Thalamus Sound Brain stem Equilibrium Tongue Somatic senses © 2016 Pearson Education, Inc. Figure 10.3 Sensory pathways in the brain Slide 3 Primary somatic sensory cortex Gustatory cortex Olfactory cortex Olfactory bulb Auditory cortex Visual cortex Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum Most sensory pathways project Nose Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium Tongue Somatic senses © 2016 Pearson Education, Inc. Figure 10.3 Sensory pathways in the brain Slide 4 Primary somatic sensory cortex Gustatory cortex Olfactory cortex Olfactory bulb Auditory cortex Visual cortex Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye Cerebellum Most sensory pathways project Nose Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium Equilibrium pathways project primarily to the cerebellum. Tongue Somatic senses © 2016 Pearson Education, Inc. Properties of Stimulus: Modality • Four properties of a stimulus – Modality – Location – Intensity – Duration © 2016 Pearson Education, Inc. Properties of Stimulus: Modality • Modality indicated by – Which sensory neurons are activated – Where neurons terminate in brain • Each receptor type is most sensitive to a particular modality of stimulus • Labeled line coding – 1:1 association of receptor with sensation © 2016 Pearson Education, Inc. Properties of Stimulus: Location • According to which receptive fields are activated • Auditory information is an exception – Ear neurons sensitive to different frequencies – Brain uses timing to locate © 2016 Pearson Education, Inc. Properties of Stimulus: Location • Lateral inhibition – Increases contrast between activated receptive fields and inactive neighbors • Population coding – Multiple receptors functioning together © 2016 Pearson Education, Inc. Figure 10.4 Localization of sound Source of sound Sound takes longer to reach left ear. Signals coming Left from the right reach Right the brain first. Top view of head © 2016 Pearson Education, Inc. Figure 10.5 Lateral inhibition Stimulus Stimulus Pin Skin A B C Tonic level Primary neuron A B C response is proportional to stimulus strength. Primary Frequency of action potentials sensory neurons Pathway closest to Secondary the stimulus inhibits neurons neighbors. A B C Inhibition of lateral Tonic level Tertiary neurons enhances neurons perception of stimulus. A B C Frequency of action potentials © 2016 Pearson Education, Inc. Properties of Stimulus: Intensity and Duration • Intensity – Coded by number of receptors activated and frequency of action potentials called frequency coding • Duration – Coded by duration of action potentials – Some receptors can adapt, or cease to respond • Tonic receptors versus phasic receptors © 2016 Pearson Education, Inc. Figure 10.6 Coding for stimulus intensity and duration Cell body Transduction site Trigger zone Myelinated axon Axon terminal Stimulus Moderate Stimulus 20 0 Amplitude 20 40 Threshold 60 80 0 5 10 0 5 10 0 5 10 Duration Membrane potential (mV) Time (sec) Longer and 20 Stronger Stimulus 0 20 40 60 80 0 5 10 0 5 10 0 5 10 Membrane Membrane potential (mV) Frequency of action Receptor potential Receptor potential Neurotransmitter potentials is proportional strength and is integrated at the release varies to stimulus intensity. duration vary with trigger zone. with the pattern Duration of a series of the stimulus. of action potentials action potentials is arriving at the axon proportional to stimulus terminal. duration. © 2016 Pearson Education, Inc. Figure 10.7a Receptor adaptation Tonic receptors are slowly adapting receptors that respond for the duration of a stimulus. Stimulus Stimulus Receptor Receptor potential Axon of sensory neuron Action potentials in sensory neuron Time © 2016 Pearson Education, Inc. Figure 10.7b Receptor adaptation Phasic receptors rapidly adapt to a constant stimulus and turn off. Stimulus Stimulus Receptor Receptor potential Axon of sensory neuron Action potentials in sensory neuron Time © 2016 Pearson Education, Inc. Somatic Senses: Modalities • Touch • Proprioception • Temperature • Nociception – Pain – Itch © 2016 Pearson Education, Inc. Temperature Receptors • Free nerve endings • Terminate in subcutaneous layers • Cold receptors – Lower than body temperature • Warm receptors – Above body temperature to about 45°C – Pain receptors activated above 45°C • Thermoreceptors use cation channels called transient receptor potential (TRP) channels © 2016• Pearson Education, Inc. Nociceptors • Respond to strong noxious stimulus that may damage tissue • Free nerve endings • Primary sensory fibers • A fibers • C fibers © 2016 Pearson Education, Inc. Nociceptors • Pain – Subjective perception – Fast pain • Sharp and localized—by A fibers – Slow pain • Duller and more diffuse—by C fibers • Itch – Histamine activates C fibers, causing itch – From skin nociceptors © 2016 Pearson Education, Inc. Nociceptors Pathways • Reflexive protective response – Integrated in spinal cord – Withdrawal reflex • Ascending pathway to cerebral cortex – Becomes conscious sensation (pain or itch) © 2016 Pearson Education, Inc. Nociceptors Pathways • Modulated