1. PNS – Afferent Division Sensory Physiology Part I

2. Peripheral Nervous System
PNS – all neural structures outside the brain and spinal cord
Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings
Provides links to and from the external environment

3. Organization of the Nervous System
Figure 8-1: Organization of the nervous system

4. Properties of Sensory Systems
Stimulus
Internal
External
Energy source
Receptors - Afferent pathway
Sense organs
Transducer
CNS integration

5. From Sensation to Perception
Survival depends upon sensation and perception
Sensation is the awareness of changes in the internal and external environment
Perception is the conscious interpretation of those stimuli

6. Sensory Receptors: Transducers
Transduction - stimulus energy converted into information processed by CNS
Sensory receptors are structures specialized to respond to stimuli, activation results in
Ion channels or second messengers that initiate membrane potential change is sensory receptors
Depolarizations trigger impulses to the CNS
The realization of these stimuli, sensation and perception, occur in the brain

7. Sensory Receptor Types

8. Receptor Classification
Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch
Thermoreceptors – sensitive to changes in temperature
Photoreceptors – respond to light energy (e.g., retina)
Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry)
Nociceptors – sensitive to pain-causing stimuli
Osmoreceptors – detect changes in concentration of solutes, osmotic activity

9. Receptor The receptor must have specificity for the stimulus energy
The receptor’s receptive field must be stimulated
Stimulus energy must be converted into a graded potential
A generator potential in the associated sensory neuron must reach threshold

10. Conversion of Receptor and Generator Potentials into Action Potentials
Occur in specialized nerve endings
Stimulus opens ion channels in receptor causing local current flow
Local current flow opens ion channels in afferent neuron AP generating region
If threshold reached, AP is generated  
Receptor potentials
Occur in separate receptor cells
Stimulus opens ion channels in receptor causing graded membrane potential
Receptor cell releases chemical messenger
Chemical messenger opens ion channels in afferent neuron AP generating region
If threshold reached, AP is generated
Receptor Potential
Generator Potential

11. 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

12. Sensory Pathways – External Stimuli
Vision
Hearing
Taste
Smell
Equilibrium
Somatic Senses

13. Somatic Senses – Internal Stimuli
Touch
Temperature
Pain
Proprioception
Figure 10-10: The somatosensory cortex

14. Somatic Pathways
First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from the skin to the spinal cord or brain stem
Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to the thalamus or cerebellum
Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex of the cerebrum
Figure 10-9: Sensory pathways cross the body’s midline

15. Sensory Coding Modality – type of stimulus Location Intensity
Coded by site of the stimulated receptor
Precision of location called acuity,
Receptive field
Lateral inhibition
Intensity
Increased stimulus results in a larger receptor potential leading to a higher frequency of action potential
Stronger stimuli also affect a larger area and recruit a larger number of receptors
Duration - Adaptation
Tonic receptors
Phasic receptors

16. Receptive Fields of Sensory Neurons
Figure 10-2

17. Receptive Field: Two-point discrimination

18. Lateral Inhibition
Figure 10-6: Lateral inhibition

19. Sensory Coding: Stimulus Intensity & Duration
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
Duration
Amplitude
Time (sec) 5 10
Threshold
(a)
(b)
Stimulus 20
-20
-40
-60
-80
potential (mV)
Membrane
Longer and
stronger
stimulus
Figure 10-7

20. Figure 10-7: Sensory coding for stimulus intensity and duration

21. Adaptation
Adaptation occurs when sensory receptors are subjected to an unchanging stimulus
Receptor membranes become less responsive
Receptor potentials decline in frequency or stop
Tonic receptors – do not adapt or adapt very slowly
Phasic receptors – readily adapt

22. Sensory Adaptation Tonic receptors (Pain): Phasic receptors:
Produce constant rate of firing as long as stimulus is applied
Phasic receptors:
Burst of activity but quickly reduce firing rate (adapt) if stimulus maintained.
Sensory adaptation: cease to pay attention to constant stimuli.

23. Adaptation
Receptors responding to pressure, touch, and smell adapt quickly
Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles
Pain receptors and proprioceptors do not exhibit adaptation

24. Touch (pressure) Mechanoreceptors Free nerve endings
Lamellated (Pacinian) corpuscles - rapidly adapting skin receptor that detects pressure and vibration.
Corpuscle of touch (Meissner‘s) - receptor for discriminative touch
Type I cutaneous (Merkel) receptors for discriminative touch
Type II cutaneous(Ruffini) receptor for continuous touch sensation
Baroreceptors – receptors to detect pressure changes

25. Proprioceptors Muscle spindle Golgi tendon organ Joint receptors
In muscles
Sense stretch
Golgi tendon organ
Near tendon
Sense force
Joint receptors
Sense position & pressure

26. Muscle Spindle Structure
Consist of collections of specialized muscle fibers known as intrafusal fibers
Lie within spindle-shaped connective tissue capsules parallel to extrafusal fibers
Each spindle has its own private efferent and afferent nerve supply
Play key role in stretch reflex

27. Stretch Reflex
Primary purpose is to resist tendency for passive stretch of extensor muscles by gravitational forces when person is standing upright
Classic example is patellar tendon, or knee-jerk reflex

28. Pain
Nociceptors
Reflexive path
Fast pain
Slow pain

29. Nociceptive Transmission Pathway
A-Delta
Small, thinly myelinated.
10 % sensory pain fibers.
Conduct at 5-30 m/sec.
Mechanical and thermal stimuli.
Sensations of sharp, pricking pain.
C Fibers
Small, unmyelinatd fibers.
90% of afferent sensory fibers.
Conduct at m/sec.
Mechanical, thermal, chemical.
Long lasting, burning pain.
Thalamus
Sensory
Receptor
axon
Spinal
cord
DRG

30. Fibers A-Delta C Fibers Small, thinly myelinated.
10 % sensory pain fibers.
Conduct at 5-30 m/sec.
Mechanical and thermal stimuli.
Sensations of sharp, pricking pain.
C Fibers
Small, unmyelinatd fibers.
90% of afferent sensory fibers.
Conduct at m/sec.
Mechanical, thermal, chemical.
Long lasting, burning pain.

31. Ad and C Nociceptors Mediate Pain
First
pain
Second
Time
Pain
intensity
C-fiber
Ad fiber

32. Neurotransmitters in Spinal Cord
Key nociceptor transmitter is substance P.
Activates ascending pathways that transmit nociceptor impulses.
Glutamate:
Binds to AMPA receptors, increases permeability, increasing likelihood of AP.
Binds to NMDA receptors increases excitability of dorsal horn neurons.

33. Spinal Cord: Excitatory Transmitters
Spinothalamic
Tract
DRG
1o Afferent fiber
Substance P
Glutamate
2nd Order
Neuron