Chapter 10a

1. Chapter 10a Sensory Physiology

2. About this Chapter • General properties of sensory systems • Somatic senses • Chemoreception: smell and taste • The ear: hearing • The ear: equilibrium • The eye and vision

3. General Properties: Sensory Division Table 10-1 (1 of 2)

4. General Properties: Sensory Division Table 10-1 (2 of 2)

5. 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

6. Somatosensory Receptors Stimulus Free nerve endings Unmyelinatedaxon Cell body (a) Figure 10-1a

7. Somatosensory Receptors Stimulus Enclosed nerveending Layers of connectivetissue Myelinated axon Cell body (b) Figure 10-1b

8. Somatosensory Receptors Stimulus Specialized receptorcell (hair cell) Synaptic vesicles Synapse Myelinated axon Cell body ofsensory neuron (c) Figure 10-1c

9. Sensory Receptors Table 10-2

10. Sensory Transduction • Stimulus energy converted into information processed by CNS • Ion channels or second messengers initiate membrane potential change • Adequate stimulus: Preferred form of stimulus • Threshold: Minimum stimulus • Receptor potential: Change in sensory receptor membrane potential

11. Receptive Fields of Sensory Neurons Primary sensoryneurons The primary sensory neuronsconverge on one secondarysensory neuron. Information from thesecondary receptivefield goes to the brain. Secondarysensoryneuron The receptive fields of three primary sensory neuronsoverlap to form one large secondary receptive field. SECTION THROUGH SPINAL CORD Figure 10-2

12. Sensory Neurons: Two-Point Discrimination • Two-point discrimination varies with the size of the secondary receptive field (a) Compass with pointsseparated by 20 mm Skin surface Primarysensoryneurons Secondarysensoryneurons One signal goes to the brain. Figure 10-3a

13. (b) Compass with pointsseparated by 20 mm Skin surface Primarysensoryneurons Secondarysensoryneurons Two signals go to the brain. Sensory Neurons: Two-Point Discrimination • Two-point discrimination varies with the size of the secondary receptive field Figure 10-3b

14. 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 • Perceptual threshold: level of stimulus necessary to be aware of particular sensation

15. Sensory Pathways • Each major division of the brain processes one or more types of sensory information

16. Sensory Pathways Primary somaticsensory cortex Gustatory cortex Olfactory cortex Olfactory bulb Auditorycortex Visualcortex 1 Olfactory pathways fromthe nose project throughthe olfactory bulb to theolfactory cortex. Eye 2 Cerebellum Most sensory pathways projectto the thalamus. The thalamusmodifies and relays informationto cortical centers. 2 1 Thalamus Nose Sound Brainstem Equilibrium 3 3 Equilibrium pathways projectprimarily to the cerebellum. Tongue Somaticsenses Figure 10-4

17. Properties of Stimulus: Modality • Indicated by where • Sensory neurons are activated • Neurons terminate in brain • Specific to receptor type • Labeled line coding • 1:1 association of receptor with sensation

18. Properties of Stimulus: Location • According to which receptive fields are activated • Auditory information is an exception • Sensitive to different frequencies • Lateral inhibition • Increases contrast between activated receptive fields and inactive neighbors • Population coding • Multiple receptors functioning together

19. Properties of Stimulus: Location • The brain uses timing differences rather than neurons to localize sound Figure 10-5

20. Properties of Stimulus: Location • Lateral inhibition enhances contrast and makes a stimulus easier to perceive Stimulus Stimulus Pin Skin A B C Frequency of action potentials Tonic level Primary neuronresponse is proportionalto stimulus strength. Primarysensoryneurons Pathway closest tothe stimulus inhibitsneighbors. Secondaryneurons A B C Frequency of action potentials Inhibition of lateralneurons enhancesperception of stimulus. Tonic level Tertiaryneurons A B C Figure 10-6

21. Properties of Stimulus • Intensity • Coded by number of receptors activated and frequency of action potentials • Duration • Coded by duration of action potentials • Some receptors can adapt or cease to respond • Tonic receptors versus phasic receptors

22. Properties of Stimulus • Sensory neurons use action potential frequency and duration to code stimulus intensity and duration Cell body Transduction site Trigger zone Myelinated axon Axon terminal Stimulus Amplitude 20 0 -20 Membrane potential (mV) -40 Threshold Duration -60 -80 (a) Moderatestimulus 0 5 10 0 5 10 0 5 10 Time (sec) 20 0 -20 Membrane potential (mV) -40 -60 -80 0 5 10 0 5 10 0 5 10 (b) Longer andstrongerstimulus Neurotransmitterrelease varies withthe pattern of actionpotentials arrivingat the axon terminal. Receptor potentialstrength andduration vary withthe stimulus. Receptor potentialis integrated at thetrigger zone. Frequency of actionpotentials is proportionalto stimulus intensity.Duration of a series ofaction potentials isproportional to stimulusduration. 1 2 3 4 Figure 10-7

23. Tonic and Phasic Receptors Figure 10-8a

24. Tonic and Phasic Receptors Figure 10-8b

25. Somatic Senses: Modalities • Touch • Proprioception • Temperature • Nociception • Pain • Itch

26. Somatic Senses Pathways Sensations are perceivedin the primary somaticsensory cortex. 4 4 Sensory pathwayssynapse in the thalamus. 3 3 THALAMUS MEDULLA 2 Fine touch, vibration,and proprioceptionpathways cross themidline in the medulla. 2 Fine touch,proprioception,vibration KEY Pain, temperature, andcoarse touch cross themidline in the spinal cord. 1 1 Nociception,temperature,coarse touch Primary sensory neuron Secondary sensory neuron Tertiary neuron Figure 10-9 SPINAL CORD

27. The Somatosensory Cortex Figure 10-10

28. Touch Receptors in the Skin Meissner’s corpuscleresponds to flutter andstroking movements. Merkel receptorssense steady pressureand texture. Hair Free nerveending Free nerve ending ofnociceptor respondsto noxious stimuli. Free nerve endingof hair root senseshair movement. Hair root Sensory nervescarry signals tospinal cord. Pacinian corpusclesenses vibration. Ruffini corpuscleresponds to skinstretch. Figure 10-11

29. 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

30. Nociceptors • Free nerve ending • Respond to strong noxious stimulus that may damage tissue • Modulated by local chemicals • Substance P is secreted by primary sensory neurons • Mediate inflammatory response • Inflammatory pain

31. Nociceptors Pathways • Reflexive protective response • Integrated in spinal cord • Withdrawal reflex • Ascending pathway to cerebral cortex • Becomes conscious sensation (pain or itch)

32. Somatosensory Nerve Fibers Table 10-5

33. Nociceptors: Pain and Itch • Itch • Histamine activates C fibers causing itch • Pain • Subjective perception • Fast pain • Sharp and localized—by A fibers • Slow pain • More diffuse—by C fibers

34. The Gate-Control Theory of Pain Figure 10-12a

35. The Gate Control Theory of Pain Modulation Figure 10-12b

36. The Gate Control Theory of Pain Modulation Figure 10-12c

37. Referred Pain Skin(usual stimulus) Primary sensoryneurons Kidney(uncommon stimulus) Secondarysensoryneuron Ascending sensorypath to somatosensorycortex of brain (b) Figure 10-13b

38. Pain • Ischemia • Lack of adequate blood flow • Chronic pain is a pathological pain • Analgesic drugs • Aspirin • Inhibits prostaglandins and slows transmission of pain to site of injury