1. Sensory Functions
• General Sensations
Mechanical – touch, pressure, vibration,
stretch
Chemical
• Special Sensations
Vision
Hearing
Taste
Smell

2. Sensory area
in the brain
FF
A
NT
RE
E
Touch stimulus
Receptor
Sensory
modality
Ascending
Sensory pathway
Central Connections
Sensory nerve

3. • stimulation of a receptor usually
produces only one sensation
modality specific
• But some receptors are stimulated
by more than one sensory modality
eg. free nerve endings

4. Classification of receptors
• Mechanoreceptors
• Thermoreceptors
• Nociceptors
pain
• Chemoreceptors
taste, smell, visceral
• Electromagnetic receptors
visual
Guyton p.496

5. Mechanoreceptors
• Mainly cutaneous
Touch
Pressure
Vibration
• Crude or Fine mechanosensations
• Others: auditory, vestibular, stretch

6. Mechanoreceptors
• Pacinian corpuscle
• Meissner’s corpuscle
• Krause’s corpuscle
• Ruffini’s end organ
• Merkel’s disc
• Hair end organ
• Free nerve endings
Guyton p.496

7. Mechanoreceptors
• Pacinian corpuscle
deep, pressure sensitive, fast adapting
• Ruffini’s end organ
deep, tension sensitive, slow adapting
• Merkel’s disc
superficial, sensitive to deformation of
skin, slowly adapting
Guyton p.496

8. Mechanoreceptors
• Meissner’s corpuscle
superficial, sensitive to sideways
movements
• Krause’s endings
• Hair end organ
• Free nerve endings
Guyton p.496

12. Pacinian Corpuscle
Nerve fibre
Capsule

13. What happens inside a
receptor?
• TRANSDUCTION
Stimulus energy is converted to action
potentials
 Inside the nervous system signals are always action
potentials
 Language of the nervous system contains only 1
word: action potentials
• At the brain opposite happens in order to
feel the sensation
PERCEPTION

14. Transduction
Stimulus
Receptor potential
(Generator potential)
Action potential

15. Receptor potentials
• are graded
• do not follow all-or-none law
• amplitude depends on the strength
•
of the stimulus
when reaches the threshold: triggers
the action potentials

16. Action Potentials
Threshold
Resting
Membrane
Potential

18. Resting

19. Receptor potentials
• mechanical transformation on the
capsule

20. Physical Stimulus

21. Receptor potentials
• Opens up Na+ channels
• Na+ influx

22. Physical Stimulus

23. Receptor potentials
• Membrane inside the capsule:
•
depolarisation
thus receptor potential is generated

24. Physical Stimulus
local current

25. Physical Stimulus
Action Potentials
are generated

26. • Stimulus strength is coded as the
•
•
frequency of AP
Higher the stimulus more frequent
are the APs
Amplitude of AP is constant

27. Action
potentials
Receptor
potentials
Stimulus

28. Adaptation
• “getting used to”
• after a period of time sensory receptors adapt
partially or
completely
• different types
fast adapting receptors
slowly adapting receptors

29. Adaptation
• after a period of time sensory
•
receptors adapt partially or
completely
different types
fast adapting receptors
slowly adapting receptors

30. Impulses per second
Pain
Muscle
spindle
Pacinian
corpuscle
Time

31. Mechanism of adaptation
• In the Pacinian corpuscle
mechanical deformation is transmitted
throughout the capsule and pressure
redistributes
Na+ channels inactivates after some
time

32. Stimulus
Impulse
Redistribution of pressure inside the capsule
Stimulus
No
Impulse

33. • Rapidly adapting receptors
phasic or rate or movement receptors
 detect changes in stimulus strength
 eg. Pacinian corpuscle, hair end-organ
• Slowly adapting receptors
tonic receptors
 detect continuous stimulus strength
 eg. muscle spindles, Golgi tendon organ,
baroreceptors, Ruffini endings and Merkel’s discs,
pain receptors

34. Sensory area
in the brain
Ascending
Sensory pathway
Central Connections
Sensory nerve
Touch stimulus
Receptor
Sensory
modality

35. Two ascending pathways
• Dorsal column - medial lemniscus
pathway
fast pathway
• Spinothalamic pathway
slow pathway
These two pathways come together at the level of thalamus

36. Posterior (dorsal)
Dorsal root ganglion
Dorsal root
Dorsal columns
Dorsal horn
Spinothalamic
tracts
Anterior (ventral)

37. Spinothalamic pathway
Dorsal column pathway
Lateral
Spinothalamic
tract
Anterior
Spinothalamic
tract

38. Dorsal column pathway
•
•
•
•
•
•
touch: fine degree
highly localised touch
sensations
vibratory sensations
sensations signalling
movement
position sense
pressure: fine degree
Spinothalamic pathway
•
•
•
•
•
•
Pain
Thermal sensations
Crude touch &
pressure
crude localising
sensations
tickle & itch
sexual sensations

39. n
se
x
te
or
internal capsule
c
ry
so
thalamocortical tracts
3rd
order
neuron
thalamus
Medial lemniscus
Dorsal column nuclei
2nd
order
neuron
(cuneate & gracile nucleus)
Dorsal column
1st
order
neuron

41. Dorsal column medial lemniscus pathway
sensory cortex
thalamocortical tracts
thalamus
M
ed
ul
la
Medial lemniscus
3rd
order
neuron
cross over
Dorsal column nuclei
2nd order neuron
(cuneate & gracile nucleus)
Dorsal column
Dorsal root
Dorsal root ganglion
1st order neuron

42. dorsal column - medial lemniscus
pathway
• after entering the spinal cord
lateral branch: participates in spinal cord
reflexes
medial branch: turns upwards
• forms the dorsal columns
• spatial orientation:
medial: lower parts of the body
lateral: upper part of the body

43. dorsal column - medial lemniscus
pathway
• synapse in the dorsal column nuclei
nucleus cuneatus & nucleus gracilus
• 2nd order neuron cross over to the
•
•
opposite side and ascends upwards as
medial lemniscus
as this travels along the brain stem fibres
from head and neck are joined (trigeminal)
ends in the thalamus (ventrobasal
complex)
 ventral posterolateral nuclei

44. dorsal column - medial lemniscus
pathway
• spatial orientation in the thalamus
medial: upper part of the body
lateral: lower part of the body

45. n
se
x
te
or
internal capsule
c
ry
so
thalamocortical tracts
3rd
order
neuron
thalamus
Spinothalamic
tracts
2nd
order
neuron
1st
order
neuron

46. Spinothalamic pathway
sensory cortex
thalamocortical tracts
3rd
order
neuron
thalamus
anterior
lateral
2nd order neuron
Spinothalamic tracts
cross over
Dorsal horn
Dorsal root
Dorsal root ganglion
1st order neuron

47. spinothalamic pathway
• after entering the spinal cord
synapse in the dorsal horn
• cross over to the opposite side
• divide in to two tracts
lateral spinothalamic tract:
 pain and temperature
anterior spinothalamic tract
 crude touch

48. spinothalamic pathway
• spatial orientation
medial: upper part of the body
lateral: lower part of the body

49. Note the spatial orientation of

51. Spinothalamic pathway
Dorsal column pathway
Lateral
Spinothalamic
tract
Anterior
Spinothalamic
tract

52. thalamocortical tracts
• from the thalamus 3rd order neuron
•
•
ascends up through the internal capsule
up to the sensory cortex
thalamocortical radiation
tracts diverge

53. Sensory cortical areas
• parietal cortex
• a distinct spatial orientation exists

56. Sensory cortex
• different areas of the body are represented
in different cortical areas in the sensory
cortex
• sensory homunculus
somatotopic representation
not proportionate
distorted map
upside down map

57. Sensory homunculus

58. Brodmann areas

59. Sensory cortical areas
• primary somatosensory cortex (SI)
postcentral gyrus
(Brodmann areas 3a, 3b, 1, 2)
• secondary somatosensory cortex (SII)
parietal cortex behind postcentral gyrus
(Brodmann areas 5, 7)

60. somatosensory cortex
Functions
• Localisation of somatic sensations
• to judge critical degree of pressure
• identify objects by their weight, shape,
form - stereognosis
• to judge texture of materials
• localisation of pain & temperature

61. Abnormalities
• Sensory loss
• Anaesthesia
absence of sensation
• Paraesthesia
abnormal sensation
• Hemianaesthesia
Loss of sensation of one half of the body
• Astereognosis

62. Localisation of the abnormality
• Peripheral nerve
part of a limb is affected
• Roots
dermatomal pattern of sensory loss
• spinal cord
a sensory level
• internal capsule
one half of the body