1. Chapter 10a
Sensory Physiology

2. General properties of sensory systems Somatic senses
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. Stimulus as physical energy  sensory receptor
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
Unmyelinated axon
Cell body
(a)
Figure 10-1a

7. Somatosensory Receptors
Stimulus
Enclosed nerve ending
Layers of connective tissue
Myelinated axon
Cell body
(b)
Figure 10-1b

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

9. Sensory Receptors
Table 10-2

10. Stimulus energy converted into information processed by CNS
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 sensory neurons
The primary sensory neurons converge on one secondary sensory neuron.
Information from the secondary receptive field goes to the brain.
Secondary sensory neuron
The receptive fields of three primary sensory neurons overlap 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 points separated by 20 mm
Skin surface
Primary sensory neurons
Secondary sensory neurons
One signal goes to the brain.
Figure 10-3a

13. Sensory Neurons: Two-Point Discrimination
Two-point discrimination varies with the size of the secondary receptive field
Two signals go to the brain.
Compass with points separated by 20 mm
Primary sensory neurons
Skin surface
Secondary sensory neurons
(b)
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 Figure 10-4 Primary somatic sensory cortex
Gustatory cortex
Olfactory cortex
Olfactory bulb
Auditory cortex
Visual cortex 1
Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex.
Eye 2
Cerebellum 2
Most sensory pathways project to the thalamus. The thalamus modifies and relays information to cortical centers. 1
Nose
Thalamus
Sound
Brain stem
Equilibrium 3 3
Equilibrium pathways project primarily to the cerebellum.
Tongue
Somatic senses
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 neuron response is proportional to stimulus strength.
Primary sensory neurons
Pathway closest to the stimulus inhibits neighbors.
Secondary neurons A B C
Frequency of action potentials
Inhibition of lateral neurons enhances perception of stimulus.
Tertiary neurons
Tonic level 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
Transduction site
Trigger zone
Myelinated axon
Cell body
Axon terminal
Stimulus
Amplitude 20
-20
Membrane potential (mV)
-40
Threshold
Duration
-60
-80
(a) Moderate stimulus 5 10 5 10 5 10
Time (sec) 20
-20
Membrane potential (mV)
-40
-60
-80
(b) Longer and stronger stimulus 5 10 5 10 5 10 1
Receptor potential strength and duration vary with the stimulus. 2
Receptor potential is integrated at the trigger zone. 3
Frequency of action potentials is proportional to stimulus intensity. Duration of a series of action potentials is proportional to stimulus duration. 4
Neurotransmitter release varies with the pattern of action potentials arriving at the axon terminal.
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 Pathways4 4
Sensations are perceived in the primary somatic sensory cortex. 3 3
Sensory pathways synapse in the thalamus.
THALAMUS
MEDULLA 2 2
Fine touch, vibration, and proprioception pathways cross the midline in the medulla.
Fine touch, proprioception, vibration
KEY 1 1
Pain, temperature, and coarse touch cross the midline in the spinal cord.
Nociception, temperature, coarse touch
Primary sensory neuron
Secondary sensory neuron
Tertiary neuron
SPINAL CORD
Figure 10-9

27. The Somatosensory Cortex
Figure 10-10

28. Touch Receptors in the Skin
Meissner’s corpuscle responds to flutter and stroking movements.
Merkel receptors sense steady pressure and texture.
Hair
Free nerve ending
Free nerve ending of nociceptor responds to noxious stimuli.
Free nerve ending of hair root senses hair movement.
Hair root
Sensory nerves carry signals to spinal cord.
Pacinian corpuscle senses vibration.
Ruffini corpuscle responds to skin stretch.
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. Respond to strong noxious stimulus that may damage tissue
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. Reflexive protective response
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
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. Primary sensory neurons Kidney (uncommon stimulus)
Referred Pain
Skin (usual stimulus)
Primary sensory neurons
Kidney (uncommon stimulus)
Secondary sensory neuron
Ascending sensory path to somatosensory cortex of brain
(b)
Figure 10-13b

38. Chronic pain is a pathological pain Analgesic drugs
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