Receptor

RECEPTORS
DR. SARAN AJAY
DEPT. OF PHYSIOLOGY, GMCM
2

DEPT. OF PHYSIOLOGY, GMCM 3
Specific Learning Objectives
• Introduction
• Classification of Receptors
• Cutaneous Mechanoreceptors
• Cutaneous Nociceptors
• Cutaneous Thermoreceptors
• Receptor Potential
• Sensory Coding
• Properties of Receptor
• Difference between AP and RP
• Summary

Information about internal and external environment reaches
CNS through a variety of sensory receptors.

Receptors detect sensory stimuli such as
• Touch and Pain
• Sound and Light
• Cold and Warmth
• Blood Pressure and Linear Acceleration

These sensations are conveyed to CNS by means of
sensory nerve impulses.

Receptors
• Receptors are transducers.
• That convert various forms of energy in the
environment into action potentials in the sensory
neurons
• i.e. mechanical, thermal, electromagnetic, chemical.

Sensory receptor may be
1. Specialized dendritic endings of sensory nerve fibers
2. Free nerve endings

Sense Organ
Receptor is sometimes associated with non neuronal cells
that surround it and forms a sense organ. e.g. Eye

Sensory Modality
Type of energy transmitted by the stimulus
e.g.: touch, pain, temperature etc.

• Introduction
• Sensory Coding
• Summary

A. Classification– Traditional
1. Special senses
• Vision ,hearing, smell, taste, rotational and linear
acceleration (vestibular apparatus)
• Information carried by cranial nerves
• Located close to CNS

2. Cutaneous senses
• Receptors in the skin
• Touch-pressure, pain ,warmth, cold
• Information is carried by cutaneous branches of
spinal nerves.

a. Epicritic
• Mild/light sensations, perceived more accurately
• Fine touch, tactile localization, tactile discrimination
b. Protopathic
• Crude type of sensations
• Pressure, pain, extremes of temperature

3. Deep senses
• from deep body tissues
• e.g. from joints, muscles and tendons
• Carried by spinal or cranial nerves
4. Visceral senses
• those concerned with perception of internal environment
• Carried by autonomic nerves
• Pain from viscera

B. Classification– Type of Stimulus
1. Mechanoreceptors
2. Thermoreceptors
3. Nociceptors
4. Electromagnetic (photo) receptors
5. Chemoreceptors

1. Mechanoreceptors
Activated by mechanical distortion (compression or
stretching) of receptor or of tissues adjacent to the
receptor.

• Cutaneous receptors for touch –pressure
• Proprioceptors
• Sound receptors of cochlea
• Vestibular receptors
• Baroreceptors of carotid sinuses and aorta

2. Thermoreceptors
Respond to temperature changes
• Cold –cold receptors
• Warmth –warm receptors

3. Nociceptors
• Free nerve endings
• Mediates potentially harmful stimuli
• Pain, extreme heat, extreme cold

4. Electromagnetic receptors
(Photoreceptors)
• Respond to light.
• Rods & cones in retina

5. Chemoreceptors
Receptors stimulated by a change in the chemical
composition of the environment in which they are
located.
• Taste and Smell
• Osmolality of blood (osmoreceptors in HT)

C. Classification – location of stimulus
1. Teleceptors
2. Exteroceptors
3. Interoceptors
4. Proprioceptors

1. Teleceptors
Concerned with events at a distance.
• Visual receptors
• Auditory receptors
• Receptors for smell

2. Interoceptors
Concerned with internal environment.
• Pulmonary stretch receptors – alveoli, bronchioles
• Central chemoreceptors –medullary
• Peripheral chemoreceptors- aortic & carotid body
• Osmoreceptors –HT

3. Proprioceptors
Provide information about the position of body in space
at any given instant
• Muscle spindle - muscle length
• Golgi tendon organ - muscle tension

4. Exteroceptors
• Concerned with external environment near at hand.
• Cutaneous receptor
• They are free nerve endings, expanded nerve
endings or encapsulated nerve endings seen in
skin or subcutaneous tissues

4 Cutaneous senses
1. Touch – Pressure mechanoreceptors
2. Pain – Nociceptors
3. Cold – cold receptors
4. Warmth –warm receptors

• Introduction
• Sensory Coding
• Summary

Cutaneous Mechanoreceptors
Touch
• Stimulation of tactile receptors in skin or in tissues
immediately beneath the skin
Pressure is sustained touch
• Deformation of deeper tissues

1. Meissner’s corpuscle
• Endings of myelinated Type Aβ
sensory nerve fibers
• Encapsulated
• Rapidly adapting

Location
• Abundant in fingertips, lips, nipple
• Small receptive field – precise
localization of sensation
Stimulus
• Movement of objects over skin
• Slow vibration

2. Pacinian Corpuscle
• Encapsulated
• Unmyelinated dendritic ending of sensory nerve fiber (Aβ)
-2mm in diameter
• Surrounded by concentric lamellae of connective
tissue –appearance of onion

• 1st node of Ranvier – inside the capsule
• 2nd near the point at which the nerve fiber leaves the capsule

Location
• Dermis of glabrous and hairy skin
• Intramuscular connective tissues
• Periosteum and mesentery
Stimulus
• Deep pressure
• Fast tissue vibrations
Adaptation

3. Ruffini’s Corpuscle/ End organ
• Multi branched ,enlarged, dendritic ending of Aβ fibers
• Elongated capsule
Location
• Deeper layers of skin & internal tissues
• Joint capsule

Stimulus
• Sustained touch & pressure
• Signals degree of joint rotation
• Slowly adapting
Nerve

4. Merkel’s disc /cells
• Expanded dendritic ending
• Innervated by Ab fibers
Location
• Epidermis of glabrous skin, moderate
numbers in hairy parts of the skin.

Stimulus
• Sustained pressure & touch
• Helps to determine continuous touch
of objects on the skin
Adaptation
• Slowly adapting

5. Krause’s End Bulb
• Spherical mechanoreceptors
• Afferent fibers –Ad group
• Encapsulated
• Respond to touch & pressure
Location - Conjunctiva , Papillae of lips
and tongue, Skin of genitalia

6. Hair End Organ
• Constituted by each hair & its basal
nerve fiber - Aβ
Stimulus
• Detects movement of objects on
surface of the body
Nerve
Hair

Free Nerve Endings
• Found everywhere in skin & many
other tissues
• Terminal branches of thin,
unmyelinated Ad & C fibers.
• Detect touch, pain & temperature

Receptor Sensation Location Features Adaptation
Meisner's
Corpuscle
Slow vibration
Movement of
object over skin
Epidermis of
glabrous skin
Abundant in
fingertips, lips,
nipple
Encapsulated
branched
dendrites in
connective tissue
Fast Adapting
Pacinian
Corpuscle
Fast vibration,
deep pressure
Dermis of
glabrous and
hairy skim
Largest receptor,
encapsulated
nerve ending
Fast Adapting
Merkel's Disc Touch, Sustained
Pressure
Epidermis of
glabrous skin
Expanded
dendritic endings
Slow Adapting
46

Receptor Sensation Location Features Adaptation
Ruffini’s endings Sustained touch
and pressure
Signals degree of
joint rotation
Dermis of
glabrous skin
Enlarged dendritic
endings,
multibranched
within capsule
Slow adapting
Krause End Bulb Touch and
Pressure
Conjunctiva,
tongue, skin of
genitalia
Spherical
encapsulated
mechanoreceptors
Fast Adapting
Hair end organs Detects movt. of
objects on body
surface
hair & its basal
nerve fiber - Aβ
Rapidly adapting
Free Nerve
Endings
Pain, Temp,
Touch
Everywhere in
skin
Terminal branches Slow adapting
47

• Introduction
• Sensory Coding
• Summary

Free (Naked) nerve endings
• Endings of Aδ and C fibers

Location
• Widespread in superficial layers of skin
• Certain internal tissues
Periosteum
Arterial walls
Joint surfaces
• Most other deep tissues → only sparsely supplied with
pain endings

Stimulus
• Respond to noxious stimuli
• Polymodal receptors
• Pain can be elicited by multiple types of stimuli
mechanical, thermal & chemical
Adaptation
• Non adapting

• Introduction
• Sensory Coding
• Summary

Thermoreceptors
• Discriminate thermal gradations
• Cold receptors
• Warm receptors
• Cold & warmth receptors are sub-epithelial located at
discrete spots.
Innocuous

1. Cold Receptors
• Dendritic endings of Aδ & C fibers
• Inactive at temp. of 40 ° C
• Steadily ↑ their firing rate as skin temperature falls to about
24°C.
• Further ↓in temperature - firing rate ↓ until 10 ° C
• Below that receptors inactive & cold becomes a local
anesthetic.

2. Warm Receptors
• On C fibers
• The firing rate of warm receptors can ↑as the skin
temperature reaches about 45°C.
• They become silent if skin temperature is further ↑ to
induce a sensation of pain.

• Introduction
• Sensory Coding
• Summary

• When a stimulus is applied certain changes occur in the
receptor.
• Immediate effect is the change in membrane
permeability to ions.
• It leads to change in membrane potential of the
receptor.
Receptor Potential

• Non propagated depolarizing potential called Receptor
Potential.
• It resembles EPSP.
• This leads to the generation of action potential in the
sensory neuron.

Generation of impulses in Cutaneous Receptors
Pacinian corpuscle has been studied in detail
1. Large size.
2. Easily accessible in the mesentery of experimental
animals.
3. Can be isolated, studied with microelectrodes &
subjected to microdissection.

Structure of Pacinian Corpuscle
• Central unmyelinated nerve fiber
• Dendritic ending of sensory nerve fiber (Aβ)
• Encapsulated
• Surrounded by multiple concentric lamellae of
connective tissue

• Myelin sheath begins inside capsule
• 1st node of Ranvier inside capsule
• 2nd node – near the point at which nerve fibre
leaves the corpuscle
• Compression anywhere on the outside of capsule
elongate, indent or deform the nerve fibre.

• When a graded pressure is applied to corpuscle.
1. Small amount of pressure → Non propagated
depolarizing potential- Generator potential (GP) or
Receptor potential.
• The receptor converts mechanical energy into
electrical response.

Generator Potential

2. As pressure is increased → Magnitude of
Generator Potential is ↑.

Generator Potentials
Magnitude ↑

The magnitude of GP is proportionate to the intensity
of stimulus – graded RP.

3. When magnitude of RP is >10 mV → an Action
Potential (AP) is generated in sensory nerve.

Generator Potentials
Action Potential

• GP is produced in non-myelinated nerve terminal.
• GP in turn depolarizes the sensory nerve at 1st node of
Ranvier.

• When firing level is reached – AP is produced in the
1st node and the membrane then repolarizes.

• If the GP is great enough neuron fires again as soon as
it repolarizes.
• It continues to fire if the GP is large enough to bring the
membrane potential of the node to the firing level

The node converts the graded response of the receptor into
action potentials.
The frequency of which is proportional to the magnitude of
the applied stimulus.

Mechanism of production of GP & AP in Pacinian Corpuscle
Stimuli causes compression of Pacinian Corpuscle
↓
Mechanical distortion of the lamellas and small area of nerve terminal
↓
Stretch sensitive Sodium Ion channels in membrane are opened
↓
Na+ diffuses to interior
↓
Increased positivity inside fiber
↓
Generator potential
79

Generator potential
↓
Local circuit of current flow
↓
Spreads along nerve fiber
↓
At 1st node of Ranvier, local current flow sets off Action Potential
↓
Transmitted along nerve fiber to CNS

• Introduction
• Sensory Coding
• Summary

Sensory Coding
Converting a sensory stimulus to recognizable sensation
is called sensory coding.

4 attributes of a stimulus
• Modality –type of energy transmitted by stimulus
• Location –site on body or space where stimulus originated
• Intensity –signaled by amplitude of response or frequency
of action potential generated
• Duration –time from start to end of response

Whatever be the stimulus applied, the nerve fiber from
receptor transmits only AP.
AP are similar in all nerves.
How does stimulation of touch receptor evoke touch
sensation?

• How can we differentiate between different intensities
of the same stimulus ?
• How do we localize the stimulus ?

• Introduction
• Sensory Coding
• Summary

Properties of Receptor
1. Adequate stimulus
2. Adaptation
3. Muller’s Doctrine of specific nerve energies
4. Law of projection
5. Receptive field
6. Law of intensity discrimination

1. Law of adequate stimulus
• Specificity of response
• Each receptor is specialized to respond to one particular
form of energy.
• The particular form of energy to which a receptor is
most sensitive –adequate stimulus.

• Receptors can respond to forms of energy other than
their adequate stimulus.
• But the threshold of these receptors for nonspecific
responses is much higher.

• Retinal receptors –light, but deep pressure on eye ball
can also stimulate the receptor
• Muscle spindle –stretch

2. Adaptation / Desensitization
• If a stimulus of constant strength is maintained on a
sensory receptor, it becomes accustomed to the
stimulus.
• The frequency of AP in its sensory nerve declines
over time –receptor adaptation.

• The degree to which adaptation occurs varies with the
type of sense organ.
• Based on this, receptors can be classified as
1. Phasic – rapidly adapting
2. Tonic – slowly adapting

a. Phasic / Rate / Movement receptors
• Rapidly adapting.
• e.g. Pacinian corpuscle, Meissner’s corpuscle
• Stimulated only on change in stimulus strength.
• Cannot be used to transmit a continuous signal, but react
strongly when a change occurs.

b. Tonic receptors
• Slowly adapting.
• Transmit impulses as long as stimulus is present.
• Awareness of the status of the body and its relation to
surroundings.
• Examples – Merkel cells, Ruffini’s end organ,
muscle spindle etc.

Advantages of Adaptation
1. Light touch would be distracting if it were persistent.
2. Slow adaptation of spindle input is needed to maintain
posture.
3. Input from nociceptors provides a warning that would
be lost if the receptor adapted rapidly.

Mechanism of Adaptation
Adaptation occurs in 2 ways in mechanoceptors
a. Receptors
b. Nerve Fiber

a. Receptor Phenomenon
e.g. in Pacinian Corpuscle and Rods and Cones
• Pacinian Corpuscle is a viscoelastic structure.
• When distorting force is applied to one side of corpuscle, it
is transmitted to the nerve fiber, eliciting a GP.
• Immediately, the fluid within the corpuscle redistributes,
and GP is no longer elicited.

In eye, rods and cones adapt by changing the concentration
of light – sensitive chemicals.

b. Nerve fibre accommodation
• Slower
• Tip of nerve fiber gradually become accommodated to
the stimulus
• Results from progressive inactivation of Na+
channels in nerve fibre membrane.

3. Muller’s doctrine of specific nerve energies
• Law of specific energies
• Specific sensory pathways are discrete from sense organ
to cortex.
• Achieved early during development of CNS.

• When a sense organ or nerve pathway from a particular
sense organ is stimulated, the sensation evoked is that for
which the receptor is specialized.
• No matter how or where along the pathway, activity is
initiated.

• This was 1st established by Muller in1835.
• If the sensory nerve from a Pacinian corpuscle in the
hand is stimulated by pressure at the elbow the
sensation evoked is touch.

Labeled Line Principle
• Specific nerve fibers transmit only one modality of
sensation
• The specificity of nerve fibers for transmitting only one
modality of sensation –Labeled Line Principle

4. Law of Projection
No matter where a particular sensory pathway is
stimulated along its course to the cortex, the conscious
sensation produced is referred to the location of the
receptor.

During neurosurgical procedures on conscious patients when
the cortical area receiving impulses from left hand is
stimulated the patient report of sensation in the left hand,
not in the head.

Phantom Limb
• Seen in amputees.
• 50 -80 % of amputees experience phantom sensations in
the region of their amputated limb.
• Patient complains of pain and proprioceptive
sensations in absent limb.

• The ends of nerve cut at the time of amputation -
neuroma.
• Discharge spontaneously or when pressure is put on
them.

Since the nerve fibers previously came from the sense
organ in the amputated limb, sensation is projected to
where the receptors used to be.

• Newer evidence suggests plasticity in sensory systems
within the CNS as the reason for phantom limb
phenomenon.
• Remapping of somatosensory cortex occurs when sensory
input is cut off.

5. Receptive Field
• A single sensory nerve fiber and its peripheral
branches is a sensory unit.
• Receptive field of a sensory unit is the area from
which a stimulus produces a response in that unit

• A receptor fires only when the skin close to its
receptive field is stimulated.
• Receptor fields differ in size and response.
• The area supplied by one unit may overlap and
interdigitate with areas supplied by other.

Merkel cells and Meissner corpuscles provide the most
precise localization as they have the smallest receptive
fields.

5. Law of Intensity Discrimination
How is it possible to tell whether touch is light or heavy
or whether pain is mild , moderate or severe?

Two mechanisms
1. By variation in the frequency of AP generated by
activity in a given receptor.
2. By variation in the number of receptors activated.

• Intensity of sensation felt is determined by the
intensity of stimulus applied to a receptor.
• Greater the intensity of applied stimulus, larger will be
the magnitude of Receptor Potential (RP).
1.

• Greater the magnitude of RP, greater will be the
frequency of Action Potentials in sensory nerves.
• These are interpreted in brain as an increase in
intensity of sensation.

• Greater intensity of stimulus will also activate receptors
with higher threshold in the same sensory unit.
• Recruits more receptors in surrounding areas into the
receptive field –recruitment of sensory units.
2.

As the strength of stimulus is increased leads to
recruitment of sensory units.
• Weak stimulus- activate receptors with low threshold
• Strong stimulus - activate receptors with higher
threshold as well

Weber Fechner Law
The magnitude of sensation felt is proportionate to the
log of intensity of the stimulus.

Perceived Intensity = K. log S
K = constant
S = intensity of the stimulus

Stevens Power Law
Now, it is known that a power function more accurately
describes this relation.

R = K.SA
R = sensation felt
S = intensity of stimulus
K and A are constants for any sensory modality

• Introduction
• Sensory Coding
• Summary

Action Potential Receptor Potential
Produced by threshold stimulus By subthreshold stimulus
Always depolarising Can be de/hyper polarising
Propagated along entire length of
cell without decrement
Can be conducted with
decrement for a short distance
Has constant size and shape as it
travels along the fiber
Size of the potential decreases
with distance

Obeys “All or None” law
Size of AP does not ↑ with ↑in
stimulus strength
Does not obey
Amplitude of GP is proportionate to
strength of stimulus
Exhibits refractory period
So cannot be summated
Do not exhibit refractory period
So they can be summated

• Introduction
• Sensory Coding
• Summary

saran.adhoc@gmail.com

Receptor

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