1. Sensory Function of the Nervous System
Part 3
Sensory Function of the Nervous System

2. I Sensory pathways
Sensory systems allow us to detect, analyze and respond to our environment
“ascending pathways”
Carry information from sensory receptors to the brain
Conscious: reach cerebral cortex
Unconscious: do not reach cerebral cortex
Sensations from body reach the opposite side of the brain

3. 1. Sensory receptors A: Free nerve endings (pain, temperature)B C D
A: Free nerve endings (pain, temperature)
B: Pacinian corpuscle (pressure)
C: Meissner’s corpuscle (touch)
D: Muscle spindle (stretch)

4. Ruffini's endings respond to tension and stretch in the skin

5. 2. Sensory pathways: 3 neurons
1st: enters spinal cord from periphery
2nd: crosses over (decussates), ascends in spinal cord to thalamus
3rd: projects to somatosensory cortex

6. 2.1 Spinothalamic pathway
Carries pain, temperature, touch and pressure signals
1st neuron enters spinal cord through dorsal root
2nd neuron crosses over in spinal cord; ascends to thalamus
3rd neuron projects from thalamus to somatosensory cortex

7. spinothalamic
pathway

8. Spinothalamic Pathway
Primary somatosensory cortex (S1)
Thalamus
Medulla
Small sensory fibres:
Pain, temperature, some touch
Spinothalamic tract
Spinal cord

9. Spinothalamic damage Left spinal cord injury spinothalamic pathway
Loss of sense of:
Touch
Pain
Warmth/cold
in right leg

10. 2.2 Dorsal column pathway
Carries fine touch, vibration and conscious proprioception signals
1st neuron enters spinal cord through dorsal root; ascends to medulla (brain stem)
2nd neuron crosses over in medulla; ascends to thalamus
3rd neuron projects to somatosensory cortex

11. Two-Point Discrimination

12. dorsal
cloumn
pathway

13. Dorsal column pathway Large sensory nerves:
Primary somatosensory cortex (S1) in parietal lobe
Dorsal column
nuclei
Thalamus
Medulla
Medial
lemniscus
Dorsal column
Large sensory nerves:
Touch, vibration, two-point discrimination, proprioception
Spinal cord

14. Dorsal column damage Left spinal cord injury dorsal column pathway
Loss of sense of:
touch
proprioception
vibration
in left leg
Dorsal column damage

15. Dorsal column damage Sensory ataxia
Patient staggers; cannot perceive position or movement of legs
Visual clues help movement

16. Central Pathways

17. 3.3 Spinocerebellar pathway
Carries unconscious proprioception signals
Receptors in muscles & joints
1st neuron: enters spinal cord through dorsal root
2nd neuron: ascends to cerebellum
No 3rd neuron to cortex, hence unconscious

18. Spinocerebellar tract damage
Cerebellar ataxia
Clumsy movements
Incoordination of the limbs (intention tremor)
Wide-based, reeling gait (ataxia)
Alcoholic intoxication produces similar effects!

19. 4. Somatosensory cortex
Located in the postcentral gyrus of the human cerebral cortex.

20. (the exception: the same side of the face).
Spatial orientation of signals.
Each side of the cortex receives sensory information exclusively from the opposite side of the body
(the exception: the same side of the face).

21. whereas the trunk and lower part of the body, relatively small area.
2)The lips, face and thumb are represented by large areas in the somatic cortex,
whereas the trunk and lower part of the body, relatively small area.
Spatial orientation of signals.
3)The head in the most lateral portion, and the lower body is presented medially

22. II . Pain

23. “Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”
International Association for the Study of Pain

24. Why feel pain? Gives conscious awareness of tissue damage Protection:
Remove body from danger
Promote healing by preventing further damage
Avoid noxious stimuli
Elicits behavioural and emotional responses

25. 1. Nociceptors
free nerve endings in skin respond to noxious stimuli

26. Nociceptors
Nociceptors are special receptors that respond only to noxious stimuli and generate nerve impulses which the brain interprets as "pain".

27. Nociopectors
Adequate Stimulation
Temperature
Mechanical damage
Chemicals (released from damaged tissue)
Bradykinin, serotonin, histamine, K+, acids, acetylcholine, and proteolytic enzymes can excite the chemical type of pain.
Prostaglandins and substance P enhance the sensitivity of pain endings but do not directly excite them.

28. Hyperalgesia:
The skin, joints, or muscles that have already been damaged are unusually sensitive. A light touch to a damaged area may elicit excruciating pain;
Primary hyperalgesia occurs within the area of damaged tissue;
Secondary hyperalgesia occurs within the tissues surrounding a damaged area.

29. 2. Localization of Pain
Superficial Somatic Pain arises from skin areas
Deep Somatic Pain arises from muscle, joints, tendons & fascia
Visceral Pain arises from receptors in visceral organs
localized damage (cutting) intestines causes no pain
diffuse visceral stimulation can be severe
distension of a bile duct from a gallstone
distension of the ureter from a kidney stone

30. 3. Fast and Slow Pain
Most pain sensation is a combination of the two types of message.
If you prick your finger you first feel a sharp pain which is conducted by the A fibres,
and this is followed by a dull pain conveyed along C fibres.

31. Fast pain (acute) Slow pain (chronic)
occurs rapidly after stimuli (.1 second)
sharp pain like needle puncture or cut
not felt in deeper tissues
larger A nerve fibers
Slow pain (chronic)
begins more slowly & increases in intensity
in both superficial and deeper tissues
smaller C nerve fibers

32. Impulses transmitted to spinal cord by
nociceptor
Aδ nerve
C nerve
spinothalamic
pathway
to reticular
formation
Impulses transmitted to spinal cord by
Myelinated Aδ nerves: fast pain (80 m/s)
Unmyelinated C nerves: slow pain (0.4 m/s)

33. Impulses ascend to somatosensory cortex via:
reticular
formation
spinothalamic
pathway
thalamus
somato-
sensory
cortex
Impulses ascend to somatosensory cortex via:
Spinothalamic pathway (fast pain)
Reticular formation (slow pain)

34. 4. Visceral pain Notable features of visceral pain:
Often accompanied by strong autonomic and/or somatic reflexes
Poorly localized;
may be “referred”
Mostly caused by distension of hollow organs or ischemia (localized mechanical trauma may be painless)

35. Afferent innervation of the viscera.
Often anatomical separation nociceptive innervation (in sympathetic nerves) from non-nociceptive (predominantly in vagus).
Many visceral afferents are specialized nociceptors, as in other tissues small (Ad and C) fibers involved.
Large numbers of silent/sleeping nociceptors, awakened by inflammation.
Nociceptor sensitization well developed in all visceral nociceptors.

36. Referred pain
Pain originating from organs perceived as coming from skin
Site of pain may be distant from organ

37. Referred pain Convergence theory:
This type of referred pain occurs because both visceral and somatic afferents often converge on the same interneurons in the pain pathways.
Excitation of the somatic afferent fibers is the more usual source of afferent discharge,
so we “refer” the location of visceral receptor activation to the somatic source even though in the case of visceral pain.
The perception is incorrect.
Referred pain
The convergence of nociceptor input from the viscera and the skin.

38. 5. “Pain Gate” Theory Melzack & Wall (1965)
A gate, where pain impulses can be “gated”
The synaptic junctions between the peripheral nociceptor fiber and the dorsal horn cells in the spinal cord are the sites of considerable plasticity.
A “gate” can stop pain signals arriving at the spinal cord from being passed to the brain
Reduced pain sensation
Natural pain relief (analgesia)

39. descending nerve fibers from brain pain pathways “THE PAIN GATE”
axons from touch receptors
axons from nociceptors
“THE PAIN GATE”
opioid-releasing
interneuron
pain pathways

40. How does “pain gate” work?
The gate = spinal cord interneurons that release opioids.
The gate can be activated by:
Simultaneous activity in other sensory (touch) neurons
Descending nerve fibers from brain

41. Applications of pain gate
Stimulation of touch fibres for pain relief:
TENS (transcutaneous electrical nerve stimulation)
Acupuncture
Massage
Release of natural opioids
Hypnosis
Natural childbirth techniques

42. 6. Pain Relief
Aspirin and ibuprofen block formation of prostaglandins that stimulate nociceptors
Novocain blocks conduction of nerve impulses along pain fibers
Morphine lessen the perception of pain in the brain.