2. LECTURE OBJECTIVES
1- Discuss the classification of sensory receptors.
2- Define labeled line principle.
3- Explain receptor potential with example of Pacinian corpuscle.
4- Describe the mechanism of receptor adaptation.
5- Distinguish between tonic and phasic receptors according to
adaptation.
3. COMPONENTS OF SENSORY SYSTEM
The sensory division of the human nervous system includes following components:
• Sensory receptors: These are specialized cells that transduce stimulus energy into
neural signals.
• Afferent neurons: These carry sensory impulses to the sensory cortex and constitute
the neural pathway. Sensory neural pathway consists of:
First-order neurons,
Second-order neurons and
Third-order neurons.
• Sensory cortex: It includes the sensory areas of cerebral cortex. It is formed by
fourth-order sensory neurons. The information received in the sensory cortex results in
a conscious perception of the stimulus, i.e. a sensation.
4. RECEPTORS
• Sensory receptors are specialized cells
that receive stimuli from the external
or internal environment and transduce
these signals into neural signals (action
potential).
• Transmitting the information to brain
for the perception of sensation.
• Receptors can be classified differently
5. CLASSIFICATION OF RECEPTORS
DEPENDING ON THE SOURCE OF STIMULUS
(Sherrington’s classification) location of stimuli
1. Exteroceptors, i.e. the receptors which receive stimuli from immediate surrounding
outside the body, e.g. Cutaneous receptors for pain, touch and temperature.
2. Enteroceptors, receive stimuli from within the body, e.g. chemoreceptors,
baroreceptors, proprioceptors, osmoreceptors and glucoreceptors,
• Visceroceptors – receive stimuli from viscera (distension of viscera, visceral pain)
• Proprioceptors – provide information about the position of the body in the space
Located in musculoskeletal organs (muscle, tendons and skin around joints)
3. Telereceptors, i.e. the receptors that receive stimuli from the distance, e.g. visual
receptors, cochlear receptors and olfactory receptors.
6. DEPENDING ON TYPE OF STIMULUS
CLASSIFICATION OF RECEPTORS
Cutaneous receptors,
proprioceptors,
baroreceptors & cochlear
receptors
cold & warm receptors extremes of mechanical,
thermal and chemical stimuli
Rods and Cones of retina
taste, olfaction, CO2, O2, pH
& osmoceptors & glucoreceptors
8. DIFFERENTIAL SENSITIVITY OF RECEPTORS
• Each type of receptor is highly sensitive to one type of stimulus for
which it is designed & is completely nonresponsive to other types of
stimuli.
• Rods & Cones of the eye only responsive to light and never to
temperature or pressure change on eyeball.
• Pain receptors are not stimulated by touch or pressure in normal skin,
under normal condition.
9. MODALITY OF SENSATION
• Each type of sensation is called Modality of sensation, like; pain,
temperature, pressure, touch, sound etc.
• Type of sensation that we experience: what is perceived after a
stimulus.
Sub modality:
Taste has 4 sub modalities: sweet, salt, sour, bitter. Each of them is
detected by distinct type of receptors.
• Nerve fibers transmit only impulses
• So how different modalities of sensation are transmitted by nerve fibers
10. LABELED LINE PRINCIPLE
• Specific receptors sense only specific
modalities
• Each nerve tract terminates at a
specific point in the CNS
• The type of sensation is determined
by the specific point in the brain
which has received the fiber
• The specificity of nerve fibers for
transmitting only one modality of
sensation is called the
labeled line principle. Modality specific receptors > specific nerve fibers >
modality specific cortex areas
11. Important point to remember
• No matter how and wherever the
pathway is stimulated one will feel that
sensation
• If pain (blue) pathway is stimulated, pain
sensation will be perceived.
• The stimulus can be crushing, overheating
or electricity damaging the tissue and
stimulating the pain nerve ending.
• LLP also helps the brain to locate the
area from where the sensation is coming
12. • Receptor stimulation changes the membrane potential of a receptor (opens ion channels),
this is called Receptor potential.
• Mechanism of Receptor potentials
mechanical stimulus
chemicals
temperature
Electromagnetic wave
By either way, the receptor is stimulated, the basic cause of receptor potential
will be change in the membrane permeability of the receptor
SENSORY TRANSDUCTION
(STIMULI INTO ELECTRICAL IMPULSES)
14. RELATION OF THE RECEPTOR POTENTIAL TO
ACTION POTENTIALS
• When the receptor potential rises above the threshold, action
potentials appear at first node of Ranvier and the receptor is
active.
• The greater the intensity of the stimulus, the greater the receptor
potential above the threshold, and the greater the rate of action
potential generation. (action potential frequency)
• So this shows that the receptor potential is graded potential
15. THE EFFECT OF STIMULUS STRENGTH ON RECEPTOR
POTENTIAL
This features allows the receptor to have an extreme range of response, from very
weak to very strong.
16. ADAPTATION OF RECEPTORS
When a continuous stimulus is applied, receptors
respond rapidly at first, but response declines until all
receptors stop firing. Rate of adaptation varies with
type of receptor.
Therefore, receptors respond when a change is taking
place (i.e., think of the feel of clothing on your skin.)
Some receptors adapt within hundredths of a
second
Some may take 2 days to adapt (baroreceptors)
Some like chemoreceptors, proprioceptors and pain
receptors never adapt
(NON ADAPTING RECEPTORS).
17. • Rapidly adapting receptor will discharge at the beginning of stimulus, ceases if
the stimulus persisted, may also discharge at the end of stimulus.
• Slowly adapting receptors discharge continuously for some time (Merkel’s disc.
Ruffini’s corpuscles)
• Non adapting receptors: never stop sending signal
18. TONIC RECEPTORS (SLOWLY ADAPTING RECEPTORS)
Continue to transmit impulses to the brain for long periods of time
while the stimulus is present
Since continue to transmit information for hours so called tonic
receptors
keep brain apprised of the status of the body with respect to its
surrounding
these receptors include: muscle spindle, golgi tendon apparatus,
Ruffini’s endings, Merkels discs, Macula, chemoreceptors
19. PHASIC RECEPTORS (RAPIDLY ADAPTING RECEPTORS)
Respond only when change is taking place. Cease to pay attention to
constant stimuli.
Rate and Strength of the response is related to the Rate and Intensity
of the stimulus
important for predicting the future position or condition of the body
very important for balance and movement
types of rapidly adapting receptors: pacinian corpuscle, Meissner's
corpuscles , semicircular canals in the inner ear
20. Adaptation reduces the amount of information
reaching the cerebral cortex
only 1% of sensory information
coming in, reaches our awareness
