This document provides information about receptors. It defines receptors as biological transducers that convert stimuli into electrical potentials. Receptors are classified based on their location (exteroreceptors and interoreceptors), type of stimulus detected (touch, temperature, chemicals, etc.), and adaptation rate (tonic and phasic). The document discusses the properties of receptors including specificity, adequate stimulus, and receptive fields. It also describes how receptor potentials lead to action potentials and the transmission of sensory information to the central nervous system.

1. Receptor
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com

2. Introduction
A stimulus is a change detectable by the body.
 Stimuli exist in various energy forms, or modalities, such as
heat, light, sound, pressure, and chemical changes.
Afferent neurons have sensory receptors (receptors for short)
at their peripheral endings that respond to stimuli in both the
external world and the internal environment.

3. Definition
Receptors are biological transducers that convert any form of
energy into electrical potentials.
The conversion of stimulus energy into a receptor potential is
known as sensory transduction

4. Classification
According to the location
Exteroreceptors
Interoreceptors

5. Classification
Exteroreceptors
Telereceptors- collect information about disturbance in the
environment taking place at a distance
Smell, vision, audition

6. Classification
Exteroreceptors
Cutaneous receptors - These collects information about
disturbances in the environment that are in close contact
with the skin
Touch, pain, cold, warmth, pressure etc.

7. Classification
Interoreceptors
Visceral receptors
proprioceptors

8. Classification
Visceral receptors
Baroreceptors
Chemoreceptors
Osmoreceptors
Visceral pain receptors

9. Classification
Proprioceptors
Muscle spindle
Golgi tendon organ
Vestibular apparatus
Joint receptors

10. Classification-II
 Based on type of stimulus
 Touch- touch receptor
 Temperature- thermos receptors
 Chemicals- chemoreceptors
 Pain- noci ceptors
 Light – Rods and cones 9photo receptors)
 Muscle tendon and joint sensation - Proprioceptors

11. Classification-III
Based on how rapidly they adapt
Tonic receptors – adopt slowly –pain receptors
Phasic receptors – adopt rapidly – touch receptors, olfactory
receptors

12. Cutaneous receptors
Receptors present in skin
Mechanoreceptors
Thermoreceptors
Nociceptors
proprioceptors

13. Cutaneous receptors
 Mechanoreceptors
 Meissner’s corpuscle
 Nerve endings around hair follicle
 Merkel’s disc
 Ruffin’s end organs
 Pacinian corpuscles
 Naked nerve endings

15. Cutaneous receptors
Although not yet fully accepted as a receptor category, itch
specific receptors were recently discovered in the skin

16. Uses for Information Detected by Receptors
The information detected by receptors is conveyed via afferent
neurons to the CNS, where it is used for various purpose
Afferent input is essential for control of efferent output, both for
regulating motor behavior in accordance with external
circumstances and for coordinating internal activities directed at
maintaining homeostasis.

17. Uses for Information Detected by Receptors
Processing of sensory input by the reticular activating system in
the brain stem is critical for cortical arousal and consciousness
Central processing of sensory information gives rise to our
perceptions of the world around us
Selected information delivered to the CNS may be stored for
future reference

18. Uses for Information Detected by Receptors
 Sensory stimuli can have a profound effect on our emotions.
 The smell of just-baked apple pie,
 the sensuous feel of silk,
 the sight of a loved one,
 the sound of someone sharing bad news
 sensory input can gladden, sadden, arouse, calm, anger, frighten, or
evoke a range of other emotions

19. Stimulation of the receptors
A receptor may be either
 (1) a specialized ending of the afferent neuron or
 (2) a separate receptor cell closely associated with the
peripheral ending of the neuron.

20. Stimulation of the receptors
Stimulation of a receptor
alters its membrane permeability,
usually by opening channels that permit an inward flux of Na+
depolarizes the receptor membrane
This local depolarization, the receptor potential, is a graded
potential

21. Stimulation of the receptors
Stimulation of a receptor
alters its membrane permeability,
usually by opening channels that permit an inward flux of Na+
depolarizes the receptor membrane
This local depolarization, the receptor potential, is a graded
potential

22. Stimulation of the receptors
Also, receptor potentials have no refractory period, so
summation in response to rapidly successive stimuli is
possible
Because the receptor region has few to no voltage-gated Na+
channels and thus has a high threshold, action potentials do
not take place at the receptor itself

23. Receptor potentials may initiate action
potentials in the afferent neuron
If a receptor potential is large enough,
it may trigger an action potential in the afferent neuron membrane
next to the receptor
by promoting the opening of voltage-gated Na+ channels
In myelinated afferent fibers, this trigger zone is the node of
Ranvier closest to the receptor

26. Initiation site of action potentials
 the initiation site of action potentials in an afferent neuron differs from the
site in an efferent neuron or interneuron.
 In the latter two types of neurons, action potentials are initiated at the
axon hillock located at the start of the axon next to the cell body
 By contrast, action potentials are initiated at the peripheral end of an
afferent nerve fiber next to the receptor, a long distance from the cell
body

28. Properties of Receptors

30. Intensity discrimination
The intensity of the stimulus is reflected by the magnitude of
the receptor potential.
The larger the receptor potential, the greater the frequency of
action potentials generated in the afferent

31. Weber-Fechner law
Frequency of action potential produced in nerve fiber is
directly proportional to log intensity of stimulus

32. Intensity discrimination
A larger receptor potential cannot bring about a larger action
potential (because of the all-or-none law),
 but it can induce more rapid firing of action potentials

33. Receptors may adapt slowly or rapidly
Some receptors diminish the extent of their depolarization despite
sustained stimulus strength, a phenomenon called adaptation
 Subsequently, the frequency of action potentials generated in the
afferent neuron decreases—that is, the receptor “adapts” to the
stimulus by no longer responding to it to the same degree.

34. Receptors may adapt slowly or rapidly
 Tonic receptors do not adapt or adapt slowly
 These receptors are useful when it is valuable to maintain information about a stimulus.
 Examples of tonic receptors are muscle stretch receptors, which monitor muscle length,
and joint proprioceptors, which measure the degree of joint flexion
 To maintain posture and balance, the CNS must continually get information about the
degree of muscle length and joint position
 It is important, therefore, that these receptors do not adapt to a stimulus but continue to
generate action potentials to relay this information to the CNS

35. Receptors may adapt slowly or rapidly
Phasic receptors are rapidly adapting receptors.
The receptor quickly adapts by no longer responding to a
maintained stimulus.
Some phasic receptors, most notably the Pacinian corpuscle,
respond with a slight depolarization called the off response
when the stimulus is removed

36. Receptors may adapt slowly or rapidly
 Many tactile (touch) receptors that signal changes in pressure on the skin
surface are phasic receptors
 Because these receptors adapt rapidly, you are not continually conscious of
wearing your watch, rings, and clothing
 When you put something on, you soon become accustomed to it because of
these receptors’ rapid adaptation
 When you take the item off, you are aware of its removal because of the off
response

37. Mechanism of Adaptation in the Pacinian
Corpuscle
 It is a specialized receptor ending that consists of concentric layers of connective tissue
resembling layers of an onion wrapped around the peripheral terminal of an afferent
neuron.
 When pressure is first applied to the Pacinian corpuscle, the underlying terminal
responds with a receptor potential of a magnitude that reflects the intensity of the
stimulus.
 As the stimulus continues, it results in redistribution of forces within the layers of the
corpuscle
 Pressure on nerve ending is relieved

38. Excitability
When stimulus is applied
Change in the polarized state occurs
Development of receptor potential
Local potential
Leads to development of AP (action potential)
AP propagates

39. Adequate stimulus
It is just enough strength of stimulus to excite receptor to
produce receptor potential that is sufficient to produce AP in
the afferent fibre

40. Specificity
Each group of receptors are specialized to respond to a particular
type of stimulus very easily
Provided the same receptor can also get stimulated by the other
stimulus also but the strength of stimulus should be very high
Photo receptors are most sensitive to light
But application of pressure on eye ball also can stimulate them

41. Muller’s law of specific nerve energy
Whenever receptor is stimulated with adequate stimulus
Development of AP in the afferent nerve fiber
This AP reaches to brain
Particular sensation is perceived

42. Labelled lines
 The afferent neuron with its peripheral receptor that first detects the stimulus is known as
a first-order sensory neuron.
 It synapses on a second-order sensory neuron, either in the spinal cord or the medulla,
depending on which sensory pathway is involved.
 This neuron then synapses on a third-order sensory neuron in the thalamus, and so on.
 A particular sensory modality detected by a specialized receptor type is sent over a
specific afferent and ascending pathway
 to excite a defined area in the somatosensory cortex—that is, a particular sensory input is
projected to a specific region of the cortex

44. Receptive field
 Each somesthetic sensory neuron responds to stimulus information only
within a circumscribed region of the skin surface surrounding it; this
region is called its receptive field
 The size of a receptive field varies inversely with the density of receptors
in the region
 The smaller the receptive field is in a region, the greater its acuity or
discriminative ability

48. Receptive field
 First, humans have receptors that detect only a limited number of
existing energy forms.
 We perceive sounds, colors, shapes, textures, smells, tastes, and
temperature but are not informed of magnetic forces, polarized light
waves, radio waves, or X-rays because we do not have receptors to
respond to the latter energy forms.
 What is not detected by receptors, the brain will never know.

49. Do you “see” a white square that is not really there?

50. THANK YOU