Receptors and
Sense Organs
Dr Fawzia ALRoug, MBBS, Master, Ph.D
Assistant Professor, Department of
Physiology, College of Medicine, King
Khalid University Hospital, Riyadh,
Saudi Arabia
A sensory System
It is a system which gives information about
sensory stimuli through skin and the body
adjusts its responses through the motor
system.
Components of Sensory System
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Receptors
Peripheral nerves
Spinal cord
Tracts
Thalamus
Thalamocortical projection
Somatosensory cortex
Tracts
Medullary
Nuclei
2nd order
Thalamus
3rd order
Sensory Cortex
SI & SII
(Localization & Perception
of sensation)
Dorsal Horn
Of
Spinal cord
1st order
Peripheral
Nerve
Receptor
Stimulus
How information about internal & external
environment reaches the CNS?
Via sensors i.e., Receptors that are connected to
the CNS by different cables [i.e., ascending tracts].
What are the receptors?
They are transducers that convert various forms of
energy in the environment [internal or external] into
action potentials in the afferent neurons.
What is the difference between a sense
organ and a receptor?
A sensory receptor is a specialized structure or a
cell that generates Ap in neurons.
A sense organ forms when the receptor is
associated with non-neural cells surrounding it.
What are the forms of energy converted by
receptors?
Mechanical
Thermal
Electromagnetic [light]
Chemical energy [odor,
taste, O2 content of blood]
How receptors respond to stimuli [different
forms of energies]?
By converting them into electrical response,
generator potential which when reach a threshold
value will cause as AP. This is what is called
receptor transduction.
Do all receptors respond to the same
stimulus?
NO.
Because each receptor is most sensitive to a
particular form of energy called its adequate
stimulus.
The receptor respond to one particular form of
energy at a much lower threshold than other
receptors respond to this form of energy.
Do receptors respond to forms of energy
other than their adequate stimulus?
YES.
But the threshold for these non-specific responses
is much higher, e.g. pressure on the eyeball will
stimulate rods & cones, but the threshold of these
receptors to pressure is higher than threshold of
pressure receptors in the skin.
Differential Sensitivity of Receptors:
How do two types of sensory receptors
detect different types of sensory stimuli?
This depends on the differential sensitivity of
receptors, i.e., each type of receptor is highly
sensitive to one type of stimulus for which it’s
designed & is not responsive to normal intensities
of other types of stimuli. Normally pain receptors
are not stimulated by touch or pressure in normal
skin, under normal condition
Sensory Modalities. What is this?
A sensory modality is the type(s) of sensation(s) we
experience when a receptor is adequately
stimulated, e.g., pain, touch, sound.
There is what we call submodality, e.g.:
4 different submodalities of taste: sweet, salt, sour,
bitter. Each of them is subserved by distinct type of
receptors.
Classification of sensory receptors:
1. Teleceptors: (distance receivers), detect events at
distance e.g. rods & cones  light; sound: hair cells
2. Exteroceptors: concerned with information of the ext.
environment e.g. skin: pain, touch, temperature
3. Interoreceptors: concerned with internal environment,
e.g. chemoreceptors, baroreceptors, osmoreceptors
4. Proprioceptors: provide information about body
position in space, e.g. receptors in muscle, tendons &
joints: muscle spindles, Goli tendon organs
5. Nociceptors: these are pain receptors
Free nerve ending
Epithelium of Skin
Non adapting
Pain
Another Classification
Cutaneous receptors
Mechanoreceptors:
terminal of AB
myelinated fibers
Thermoreceptors
nociceptors
Musculoskeletal
receptors
proprioceptors
Visceral receptors
e.g.,
chemoreceptors,
baroceptors,
mechanoreceptors
Transduction of sensory stimuli into nerve
impulses:
The generator potentials [=receptor potential]: it’s
the change in the membrane potential of the
receptor when excited by a stimulus.
It’s a non-propagated depolarizing potential
resembling EPSP.
When the magnitude of the generated potential is
10mV  AP is generated in the sensory nerve.
Transduction (receptor potential)
 It is a change in membrane potential of a receptor.
 Due to:
1) Mechanical deformation of receptor membrane.
2) Application of chemical which binds to the receptor.
3) Change in temperature.
4) Application of electromagnetic energy.
 These lead to the opening of selective channels and
depolarization of receptor membrane.
Generator Potential
(Receptor Potential)
• Non propagated depolarizing potential.
• It is produced in un-myelinated nerve
terminal.
 Depolarization of receptor membrane
leads to local circuit which initiates action
potential at the first node of Ranvier on the
nerve attached to the receptor.
RECEPTOR POTENTIAL & ACTION POTENTIAL
Sensory Transduction
Source of generator potential:
The generator potential is produced in the
unmyelinated nerve terminal of the receptor
When pressure stimulus is applied  the terminal
fibers of the corpuscle will be deformed  opening
of Na+ channels  Na+ influx (to the interior of the
fiber)  creates high positivity inside the fiber 
receptor potential  then the generator potential
depolarizes the sensory nerve at the 1st node of
Ranvier. Once the firing level is reached, action
potential is produced.
Once the firing level is reached, action
potential is produced:
The node converts the graded response of the
receptor into action potentials
The frequency of AP is proportionate to the
intensity (magnitude) of the applied stimuli
In other way: the more the receptor potential
rises above threshold level, the greater becomes
the action potential frequency
Ionic basis of receptor potentials:
Opening of channels of receptor membranes, e.g.
Na+ channels as mentioned
G proteins: as in rods & cones
Properties of receptors:
1- Adaptation:= desensitization
It’s the decrease in the frequency of APs in a
sensory nerve overtime when a maintained stimulus
of constant strength is applied to its receptor.
This phenomenon depends on the type of the
receptor or sense organ.
According to this we have:1- Phase receptors:These are rapidly adapting receptors.
e.g. touch receptors
2- Tonic receptors:These are slowly adapting receptors: its generator
potential is prolonged and decays very slowly.
e.g. muscle spindles, cold & pain receptors, lung
inflation receptors
Mechanism of adaptation:
Accommodation of the sensory nerve fiber to the
generator potential.
RESPONSE OF PHASIC & TONIC MECHANORECEPTOR
ADAPTATION OF RECEPTOR
2- Excitability:
Response of a receptor to an adequate stimulus by
producing a non-propagated potential = receptor
potential.
If magnitude is sufficient  AP will propagate along
the sensory nerve.
How a receptor be stimulated?
1. Mechanical deformation  open ion channels
2. Application of a chemical to the receptor
membrane  open ion channels
3. Change of temperature  change permeability
4. Effect of electromagnetic radiation  (+) G
protein
Give examples for receptors!
3.Adequate stimulus
Each type of receptor is most sensitive to a specific
form of energy, called 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
What is the difference between generator potential and
action potential?
Generator potential
Action potential
Graded
Doesn’t obey all or none rule
Can be summated
Unpropagated
Not
Obeys all or non rule
Not summated
Propagated
Sensory receptors in the skin:
[Mechanoreceptors]
1- Expanded ends of sensory nerve fibers:
- Ruffini endings
- Merkel’s discs
2- Encapsulated endings:
- Meissner’s corpuscles
- Pacinian corpuscles
- Krause’s end-bulbs
3- Naked nerve endings between cells in tissues.
Coding of sensory information
If we know that:
APs in the nerve from a touch receptor are
essentially identical to those in the nerve from
warmth receptor.
So, why
Stimulation of a touch receptor causes a sensation
of touch and not warmth? And how possible to tell
that touch is light or heavy?
1- Doctrine of specific nerve energies: [Modality
discrimination]
The sensation evoked (=produced) by impulses generated
in a receptor depends on the specific part of the brain they
ultimately activate.
Because specific sensory pathway are discrete, from
sense organ to cortex, when nerve pathways from certain
sense organ are stimulated  the sensation produced is
that for which the receptor is specialized no matter how or
where along the pathway the activity is initiated. This is
the doctrine of specific nerve energies. So, if a sensory
nerve from Pacinian corpuscle in the hand is stimulated at
elbow, sensation produced is touch. If stimulated at dorsal
column of sp. cord  touch.
2- Projection: [Locality discrimination]
This principle is that:
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.
Examples:
When the cortical receiving area for impulses from the
left hand is stimulated  patient reports sensation in
the hand not the head.
Phantom limb: amputee complain of pain and
proprioceptive sensation in the absent limb (phantom
limb) as if it were still in place.
Mechanisms: The ends of nerves cut at time of
amputation often forms neuromas. They may
discharge spontaneously or when pressure put on
them, impulses are generated in nerve fibers that
previously come from sense organs in the amputated
limb & sensation produced is projected to where
receptors used to be.
3- Intensity discrimination
Intensity (strength) discrimination depends on:
1- Number of receptors stimulated: stronger stimuli
stimulate more receptors
2- Frequency of action potentials (generated by
activity in a given receptor) reaching the cortex.
R = KSA
R = sensation
S = intensity of the stimulus
K & A = constants
N.B.
Tactile localization:
- one-point localization: ability to localize one-point of touch
[locality discrimination].
- tactile discrimination: two-point discrimination, ability to feel
two points touched simultaneously as two separate points
depends on:1- Number of receptors stimulated
2- degree of overlap in receptive field
 Overlap   discrimination
It’s accurate on tips of fingers, lips & tongue, WHY?
1- greater number of receptors
2- greater number of afferents
3- less convergence
4- more area of cortical representation
Note:
Labeled line principle:
The specificity of nerve fibers for transmitting only one
modality of sensation is called “labeled line principle”
e.g. if a touch fiber is stimulated by exciting a touch
receptor electrically or in any other way, the person
perceives touch because touch fibers leads to specific
touch areas in the brain.
So, the sensation felt depends on which area in the NS
the fibers leads [ultimately stimulated].
RECEPTIVE FIELD
Sensory Pathways
DORSAL COLUMN
PATHWAY
• CARRIES FINE TOUCH,
POSITION, PRESSURE,
VIBRATION, TWO POINT
DESRIMINATION
stereognosis
• AFFERENT SENSORY
FIBERS Aβ TYPE.
• VERY FAST VELOCITY
30 – 70 m/s
• 3 NEURON SYSTEM
(SEE THE DIAGRAM)
ANTEROLATERAL PATHWAY
• CARRIES PAIN &
TEMPRATURE (lat. Sp.Th)
• CRUDE TOUCH &
PRESSURE ( VENT, Sp. Th)
• AFFERENT SENSORY
FIBERS Aδ (MYELINATED)
FAST PAIN
• C FIBERS( UNMYELINATED)
SLOW PAIN
• RELATIVELY SLOW
VELOCITY Aδ – 6 – 30 m/s.
C – 0.5 – 2 m/s.
• 3 NEURON SYSTEM
(SEE THE DIAGRAM)
Spinal Tracts