1. Nervous System PhysiologyBy
Dr. SHAHAB SHAIKH
PhD MD MBBS
Lecture – 3: Sensory system - 1
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Faculty of Medicine
Al Maarefa Colleges of Science & Technology

2. Sensory system Sensations: Touch Pressure Position Vibration
Fine
Crude
Pressure
Position
Vibration
Two point discrimination
Pain
Temperature
Cold
Warm
Stereognosis

3. Thalamocortical projection
Receptors ↓
Peripheral nerves
Spinal cord
Tracts
Brain stem
Thalamus
Thalamocortical projection
Somatosensory cortex
Sensory Conduction

4. Sensory Receptors
These are transducers that convert various forms of Stimuli energy into action potentials .
Every receptor is most sensitive to a particular modality of stimuli. That particular form of energy to which the receptor is most sensitive is called as its Adequate Stimulus.
The receptor is almost non-responsive to the normal intensities of other forms of energy.
e.g. rods & cones are stimulated by light not heat, sound etc

5. Sensory receptors Sensory receptors can be subdivided into:
1) Mechanoreceptors.
2) Thermoreceptors.
3) Nociceptors.
4) Proprieceptors.
5) Visceral receptors
6) Special senses.

6. Temperature receptors (carotid) Touch Magnetoreception
Sensory receptors
Based on Signal source, Sensory receptors can be subdivided into:
Exteroceptors
Interoceptors
vision pH
audition
baroreceptors
Smell
CO2/O2
Taste
Temperature receptors (carotid)
Touch
Magnetoreception

7. Pressure

8. Tactile Receptors in the Skin

9. Skin Receptors

11. Receptor Properties Receptor potential or Generator Potential
Specificity of Response
Adaptation
Tactile Localization
Two Point Discrimination
Law of Projection
Recruitment

12. Receptor Properties 1.Receptor potential or Generator Potential
The depolarization produced in the receptor membrane on an appropriate stimulus is called Generator potential or Receptor potential.
Non propagated depolarizing potential.

13. Difference Between Receptor potential In the Receptor Graded
Doesn’t obey all or none rule
Can be summated
Unpropagated
Action potential
In the Sensory Nerve fiber
Not Graded
Obeys all or none rule
Not summated
Propagated

14. Stimulus > Receptor potential > Action potential

15. Relation between Receptor potential and Action potential
If stimulus to the receptor is sub threshold then no action potential is generated in sensory nerve fiber
Higher the Receptor potential – more is the No. of Action potentials Generated

16. Sensory Transduction

17. Adaptation or Desensitization
When stimulus of constant strength is applied to a receptor, some receptors can diminish the extent of their depolarization inspite of sustained stimulus. This is called adaptation. Degree of adaptation varies in different receptors.
Thus receptors may be ….
Phasic Receptors: Rapidly Adapting Receptors
E.g. pacinian corpuscles, Meissners corpusle.
For Touch, Pressure & Smell etc.
Tonic Receptors: Slow Adapting receptors
E.g. Nociceptors, muscle spindles
For Pain, Body position etc.

18. ADAPTATION OF RECEPTOR
Tonic
Phasic

19. Tactile Localization
It is the capacity of a person to localize where a stimulus is applied
It is due to the Doctrine of specific nerve energies.
Specific receptors.
Specific sensory pathways.
Specific part of the brain they activate.
Areas with maximum Tactile Localization ability are
Lips
Fingers
Tip of Nose
Ear Lobes

20. Receptor field
Receptor field of a sensory unit is the area from which a stimulus produces response in that unit.
Smaller the receptive field – More precise the information e.g. Finger tips
Larger the receptive field- less precise the information e.g. Back, arms, legs.

21. Two Point Discrimination
It is the capacity of a person to distinguish two tactile stimulus as two separate stimuli
It is dependent on the Receptor field
Smaller the receptive field, higher will be the ability for two point discrimination
Areas with maximum Two Point Discrimination ability are
Lips, Fingers
Areas with less Two Point Discrimination ability are
Back, Legs etc.

22. Receptor Field & Two Point Discrimination

23. Law of projection
If we stimulate sensory pathway along its course to the sensory cortex, the conscious sensation produced is perceived to be from the location of receptors. This principle is called as Law of projection.
If we stimulate sensory cortex area which receives impulses from left hand, patient reports sensation in the left hand, not in the head.
Phantom limb:
Patients with an amputed limb may complain of pain or other sensation in the limb which is absent due to law of projection.

24. Recruitment of sensory units
Weak stimulus activates receptors with lowest threshold.
Strong stimulus activates those receptors also which have high threshold .

25. Receptors

26. Physiological classification of nerve fibers

27. Physiological classification of Sensory nervesIa Ib II
III IV
Muscle spindle - annulo-spiral ending.
Golgi tendon organ.
Muscle spindle - flower-spray ending, touch, pressure
Pain and cold receptors; some touch receptors
Pain, temperature, and other receptors A A A C
Number
Origin
Fiber type

28. Sensory Pathways

29. Sensory Pathway
Impulses generated by the receptors are carried by sensory nerves.
The sensory nerve have cell bodies in dorsal root ganglia.
Primary afferent fibers which carry cutaneous sensations are
Large myelinated fiber Aα – proprioception
Large myelinated fiber Aβ – touch, pressure.
Small myelinated fiber Aδ – fast pain, Temp
Small unmyelinated C fibers – slow pain, Temp

30. Sensory Pathways Sensory fiber go to dorsal horn in the spinal cord.
Dorsal horns are divided into laminae I – VII.

31. Sensory Pathways Are part of Ascending tracts
Carry information all the way from Receptors to CNS
Are 3 Neuron System:
1st order, 2nd order & 3rd order neuron
Sensory Pathways:
Dorsal Column Medial Lemniscal system
Antero-Lateral Spino-Thalamic Tract

32. Ascending Tracts Carry Afferent Impulses from Spinal Cord to Brain
Major Ascending Tracts are . . .
Dorsal Column Tract (DCML)
Anterior (Ventral) Spinothalamic Tract
Lateral Spinothalamic Tract
Dorsal Spinocerebellar Tract
Ventral Spinocerebellar Tract
Spino Olivary Tract
Spino TectalTract
Spino ReticularTract
Spino PontineTract
Sensory Pathways are part of Ascending tracts

33. Sensory Pathways Dorsal Column Medial Lemniscal system Also called as
Posterior column Tract or
Tracts of Goll & Burdach or
Fasciculus Gracilis and Cuneatus
Carries sensations of fine touch, position, vibration, two point discrimination & stereognosis.
Anterolateral system
Consists of …
Anterior (Ventral) spinothalamic tract
carries crude touch and pressure,
Lateral spinothalamic tract
carries pain and temperature.

34. Spinal Tracts

35. Sensory Pathways

36. Dorsal Column System
1st order neurones have Cell body in Dorsal Root Ganglion and run Ipsilateraly (on the same side) & synapse with 2nd order neurones in the nuclei in Medulla Oblongata.
2nd order nerve fibres arise from cell bodies in Nucleus Gracilis & Cuneatus and cross to the opposite side (Decussate). These ascend through the medial leminiscus & synapse with 3rd order neuron in Thalamus
3rd order nerve fibers from VPL nucleus of thalamus project to the Primary somatosensory cortex (S1) in parietal lobe (Post Central Gyrus) of Cerebral Cortex.

37. Homunculus of Sensory Cortex
The area of sensory cortex devoted to a body region is relative to the number of sensory receptors.

38. Dorsal column Injury
dorsal column
pathway
Left
spinal cord injury
Loss of sense of:
touch
proprioception
vibration
in left leg
Spinal cord lesions affecting the dorsal column (e.g., vitamin B12 neuropathy, tabes dorsalis) result in ipsilateral sensory deficits below the lesion, because the pathway does not decussate until it is at the level of the medulla.

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

40. Anterolateral System
1st order neurones have Cell body in Dorsal Root Ganglion & synapse with 2nd order neurones in the nuclei in Spinal Cord Ipsilaterally.
2nd order nerve fibres arise from cell bodies in Spinal cord and cross to the opposite side (Decussate) mostly. These ascend through Ventral or Lateral tracts & synapse with 3rd order neuron in Thalamus. Some terminate in RAS also.
3rd order nerve fibres from thalamus project to the Primary somatosensory cortex (S1) in parietal lobe (Post Central Gyrus) of Cerebral Cortex.

41. Dorsal Column
Anterolateral System

42. ?

43. DORSAL COLUMN PATHWAY
Carries fine touch, position, pressure, vibration, two point discrimination, stereognosis
Afferent sensory fibers aβ type.
Very fast velocity 30 – 70 m/s
ANTEROLATERAL PATHWAY
Carries pain & temperature (lat. Sp.Th)
Crude touch & pressure (vent, sp. Th)
Afferent sensory fibers aδ
6 – 30 m/s (myelinated) fast pain
C fibers – 0.5 – 2 m/s (unmyelinated) slow pain

44. TEMPERATURE
Skin has cold sensitive area & heat sensitive area. Receptors are free nerve endings.
Cold receptors are 4 – 10 times more than warm sensitive spots.
Cold receptor responds from 10 – 38 oC.
Warm receptor responds from 30– 45 oC.
Afferent from cold receptors – Aδ & C fibers.
Afferent from warm receptors – C fibers.
Temperature sensation is carried via lateral spinothalamic tract.

45. THERMORECPTOR

46. PAIN Pain receptors or Nociceptors – free nerve endings.
Pain sensation are carried by two types of nerve fibers.
Aδ (myelinated) -2 – 5 μm. in diameter, conduction velocity 12 – 30 m/sec.
For fast pain (sharp localized)
C fibers (unmyelinated) 0.4 – 1.2μm. in diameter, conduction velocity 0.5 – 2 m/sec.
For slow pain (dull, diffuse)
Both Aδ & C fibers terminate in dorsal horn.

47. PERCEPTION
Perception is conscious interpretation of external world by Brain due to sensory impulses delivered to Brain from sensory receptors.
The primary somatosensory cortex relies upon various association areas of the cortex to properly process sensory information
Factors affecting Perception include:
Receptor adaptation and afferent processing
Emotions and experiences
Not all stimuli give rise to a conscious sensation
For example: stretch receptors monitor blood pressure arteries
Lack of receptors for certain stimuli
For example: radio waves
Damaged neural pathways
Drugs
Further perceptual processing involves arousal, attention, learning, memory, language, and emotions, and also comparison of the information presented via one type of sensation with that presented through another.

48. References
Human physiology by Lauralee Sherwood, 8th edition
Text Book Of Physiology by Guyton & Hall, 11th edition

49. THANK YOU