In the name of Allah,In the name of Allah,
the most Beneficent,the most Beneficent,
the most Mercifulthe most Merciful
• Sensory receptors respond to stimuli and
transmit data about them to the brain.
• In the skin, receptors detect touch, pressure,
vibration, temperature, and pain.
• Elsewhere in the body, more specialized
receptors detect light (eye works), sound
(mechanism of hearing), smell, and taste.
Internal receptors called proprioceptors sense
body position and the location of body parts
in relation to each other.
• In a sensory system, a sensory receptor is a
sensory nerve ending that responds to a
stimulus in the internal or external
environment of an organism.
• In response to stimuli the sensory receptor
initiates sensory transduction by creating
action potentials in the same cell or in an
Receptor Category Name of Receptor Response to Stimulus
Mechano receptor Pacinian corpuscle Pressure
Meissner’s corpuscle Touch
Merkel cell Light touch
Cochlea (organ of Corti) Sound waves
Semicircular canals Positional changes – balance
Muscle spindles Proprioception
Golgi tendon organ Proprioception
Taste buds Chemicals in saliva
Olfactory tract Chemicals in inhaled air
Oxygen in blood
Pain due to trauma
In physiology, transduction is the conversion of
a stimulus from one form to another.
Transduction in the nervous system typically
refers to synaptic events where in an
electrical signal, known as an action
potential, is converted into a chemical one
via the release of neurotransmitters.
Classification of Sensory Receptors
Based on stimulus location
Telereceptors – detect distant stimuli, e.g., vision
Exteroceptors – detect stimuli on the outside of
body, e.g., pressure and temperature
Interoceptors – detect stimuli inside the body,
e.g., blood pressure and blood oxygen
Classification by receptor complexity:
Free nerve endings are dendrites whose
terminal ends have little or no physical
Encapsulated nerve endings are dendrites
whose terminal ends are enclosed in a
capsule of connective tissue.
Classification of Sensory Receptors
Based on type of stimuli the receptors can detect
Chemoreceptors – chemicals, e.g., smell and taste
Mechanoreceptors – pressure and movement, e.g., touch,
hearing, balance, blood pressure
Photoreceptors – light, e.g., vision; detect photons
Electroreceptors – electrical fields
Magnetoreceptors – magnetic fields
Thermoreceptors - temperature
Classification by location
• Sensory receptors can be classified by
• Cutaneous receptors are sensory receptors
found in the dermis or epidermis.
• Muscle spindles contain mechanoreceptors
that detect stretch in muscles
Classification by morphology
• Somatic sensory receptors near the surface of
the skin can usually be divided into to groups
based on morphology:
• Free nerve endings characterize the
nociceptors and thermoreceptors and are
called thus because the terminal branches of
the neuron are unmyelinated and spread
throughout the dermis and epidermis.
• Encapsulated receptors comprise of the
remaining types of cutaneous receptors.
Encapsulation exists for specialized
• In biochemistry, a receptor is a protein on the cell
membrane or within the cytoplasm or cell nucleus
that binds to a specific molecule (a ligand), such as
a neurotransmitter, hormone, or other substance,
and initiates the cellular response to the ligand.
Ligand-induced changes in the behavior of receptor
proteins result in physiological changes that
constitute the biological actions of the ligands
Binding and activation
• Ligand binding to a receptor is an equilibrium process: Ligands
bind to an empty receptor and they dissociate from it
(according to the law of mass action):
• (the brackets stand for concentrations)
• fits into a given receptor is the binding affinity which is
measured as the dissociation constant Kd (good fit means
high affinity and a low Kd). The activation of the second
messenger cascade and the final biological response is
achieved only when at a certain time point a significant
number of receptors are activated by bound ligands.
Agonists versus antagonists
• Not every ligand that binds to a receptor also
activates the receptor. The following classes
of ligands exist:
• (Full) agonists are able to activate the
receptor and result in a maximal biological
response. Most natural ligands are full
• Partial agonists are not able to activate the
receptor maximally, resulting in a partial
biological response compared to a full agonist.
• Antagonists bind to the receptor but do not
activate it. This results in a receptor blockade
that inhibits the binding of agonists.
• Inverse agonists are antagonists that are able
to further reduce the receptor activation by
decreasing its basal activity
• A mechanoreceptor is a sensory receptor that responds
to mechanical pressure or distortion.
• There are four main types in the skin of humans:
• Pacinian corpuscles,
• Meissner's corpuscles,
• Merkel's discs,
• and Ruffini corpuscles.
• There are also mechanoreceptors in the hairy skin, and
the hair cells in the cochlea are the most sensitive
mechanoreceptors in tranducing air pressure waves into
All stimuli are ultimately converted into action
potentials in the primary afferent neurons
Sensory receptors encode four types of information
YouTube - How the Body Works _ The Sensory Cortex and Touch_1.flv
• Pacinian corpuscles are one of the four major types
of mechanoreceptor. They are nerve endings in the
skin, responsible for sensitivity to pain and pressure.
• The Pacinian corpuscle was named after its
discoverer, Italian anatomist Filippo Pacini.
• Pacinian corpuscles are larger and
fewer in number than both
Merkel cells and Meissner's
• The Pacinian corpuscle is oval shaped
and approximately 1 mm in length.
• It has 20 to 60 concentric lamellae
separated by gelatinous material. The
lamellae are very thin, flat, modified
• In the center of the corpuscle is the
inner bulb, a fluid-filled cavity with a
single afferent unmyelinated
The tip of the central fiber inside theThe tip of the central fiber inside the
capsule is unmyelinated, but the fibercapsule is unmyelinated, but the fiber
does become myelinated shortly beforedoes become myelinated shortly before
leaving the corpuscle to enter aleaving the corpuscle to enter a
peripheral sensory nerve.peripheral sensory nerve.
• All sensory receptors have one feature in
common. Whatever the type of stimulus that
excites the receptor, its immediate effect is to
change the membrane electrical potential of
the receptor. This change in potential is called
a receptor potential
• by compression of the corpuscle, ion channels opened in the membrane,
allowing positively charged sodium ions to diffuse to the interior of the
• This creates increased positivity inside the fiber, which is the "receptor
• The receptor potential in turn induces a local circuit of current flow, that
spreads along the nerve fiber.
• At the first node of Ranvier, the local current flow depolarizes the fiber
membrane at this node, which then sets off typical action potentials that
are transmitted along the nerve fiber toward the central nervous system.
(Increasing "Stimulus Strength")(Increasing "Stimulus Strength")
applied to the central core of aapplied to the central core of a
pacinian corpuscle the amplitudepacinian corpuscle the amplitude
increases rapidly at first but thenincreases rapidly at first but then
progressively less rapidly at highprogressively less rapidly at high
stimulus strength.stimulus strength.
that very intense stimulation of thethat very intense stimulation of the
receptor causes progressively lessreceptor causes progressively less
and less additional increase inand less additional increase in
numbers of action potentials.numbers of action potentials.
This is an exceedingly importantThis is an exceedingly important
principle that is applicable toprinciple that is applicable to
almost all sensory receptors.almost all sensory receptors.
It allows the receptor to beIt allows the receptor to be
sensitive to very weak sensorysensitive to very weak sensory
experience and yet not reach aexperience and yet not reach a
maximum firing rate until themaximum firing rate until the
sensory experience is extreme.sensory experience is extreme.
This allows the receptor to have anThis allows the receptor to have an
extreme range of response, fromextreme range of response, from
very weak to very intense.very weak to very intense.
Relation Between Stimulus Intensity
and the Receptor Potential.
Adaptation of Receptors• receptors adapt either
partially or completely to
any constant stimulus
after a period of time.
• That is, when a
stimulus is applied, the
receptor responds at a
high impulse rate at first
and then at a progressively
slower rate until finally the
rate of action potentials
decreases to very few or often
to none at all.
• Meissner's corpuscles (or tactile corpuscles) are a type of
• They are a type of nerve ending in the skin that is
responsible for sensitivity to light touch.
• In particular, they have highest sensitivity (lowest
threshold) when sensing vibrations lower than 50 Hertz.
They are rapidly adaptive receptors.
• Meissner's corpuscles were discovered by the
anatomist Georg Meissner (1829-1905).
• Papilla of the hand, treated with acetic acid. Magnified 350 times.
A. Side view of a papilla of the hand.
a. Cortical layer.
b. Tactile corpuscle.
c. Small nerve of the papilla, with neurolemma.
d. Its two nervous fibers running with spiral coils around the tactile corpuscle.
e. Apparent termination of one of these fibers.
• They are distributed throughout the skin, but
concentrated in areas especially sensitive to light
touch, such as the fingertips, palms, soles, lips,
tongue, face and the skin of the male and female
genitals (non hari skin) .
• They are primarily located just beneath the
epidermis within the dermal papillae
• Any physical deformation in the corpuscle will cause an
action potential in the nerve. Since they are rapidly
adapting, the action potentials generated quickly decrease
and eventually cease.
• If the stimulus is removed, the corpuscle regains its shape
and while doing so (ie: while physically reforming) causes
another volley of action potentials to be generated.
• (This is the reason one stops "feeling" one's clothes.)
Because of their superficial location in the dermis, these
corpuscles are particularly sensitive to touch and vibrations
Merkel nerve ending
• Merkel nerve endings are
mechanoreceptors found in the
skin and mucosa that provide touch
information to the brain.
• Each ending consists of a Merkel
cell in close apposition with an
enlarged nerve terminal. This is
sometimes referred a Merkel disk
• A single afferent nerve fibre
branches to innervate up to 90 such
endings. They are classified as
slowly adapting type I
• In mammals, Merkel nerve endings have a wide
distribution. Merkel nerve endings are found in the
basal layer hairy skin, in hair follicles, and in oral
and anal mucosa.
• In humans, Merkel cells (along with Meissner's
corpuscles) occur in the superficial skin layers, and
are found clustered beneath the ridges of the
fingertips that make up fingerprints.
• Their rigid structure, and the fact that they are not
encapsulated, causes them to have a sustained
response (in the form of action potentials or spikes)
to mechanical deflection of the tissue. They are the
most sensitive of the four main types of
• Because of their sustained response to pressure,
Merkel nerve endings are classified as slowly
• Merkel nerve endings are extremely sensitive
to tissue displacement, and may respond to
displacements of less than 1 μm
Ruffini ending temperature
• Named after Angelo Ruffini,
the Ruffini ending is a class
of slowly adapting
to exist only in the glabrous
(without hair)dermis and
subcutaneous tissue of
• This spindle-shaped
receptor is sensitive to skin
stretch, and contributes to
the kinesthetic sense of and
control of finger position
Free nerve ending
• A free nerve ending (FNE) is an specialized,
afferent nerve ending, meaning it brings
information from the body's periphery toward
• They function as cutaneous receptors and
• Free nerve endings are unencapsulated and
have no complex sensory structures, unlike
those found in Meissner's or Pacinian
corpuscles. They are the most common type
of nerve ending, and are most frequently
found in the skin. They penetrate the
epidermis and end in the stratum granulosum.
• Free nerve ending have different rate of
adaptation, stimulus modalities and fiber
Rate of adaption
• Different types of FNE can be rapidly adapting,
intermediate adapting, or slowly adapting. A δ
fibres are fast-adapting, while C fibers are
• Free nerve endings can detect temperature, mechanical
stimuli (touch, pressure, stretch) or pain (nociception). Thus,
different free nerve endings work as thermoreceptors,
cutaneous mechanoreceptors and nociceptors. In other
words, they express polymodality
• nociceptor is a sensory receptor that responds to potentially
damaging stimuli by sending nerve signals to the spinal cord
and brain. This process, called nociception, usually causes the
perception of pain
Receptors and stimulus
Location: Can distinguish the location of the
stimulus (touch, light or odour)
Duration: Determine length of stimulus by
responding to the stimulus for the duration of
Intensity: Increase in action potential
frequency or increase in neurotransmitter
Sensitivity to Multiple Modalities
• Adequate stimulus – preferred or most sensitive stimulus
• Many receptors can also be excited by other stimuli, if
sufficiently large, e.g., pressure on eyelid perceive bright
• Nociceptors – sensitive to strong stimuli, e.g., pain; many are
• Only a small fraction of the most important sensory
information usually causes immediate motor response.
• Much of the information is stored for future control of
motor activities and for use in the thinking processes.
• Most storage occurs in the cerebral cortex, but even the
basal regions of the brain and the spinal cord can store
small amounts of information.
• The storage of information is the process we call memory,
and this, too, is a function of the synapses. That is, each
time certain types of sensory signals pass through
sequences of synapses, these synapses become more
capable of transmitting the same type of signal the next
time, a process called facilitation.
• After the sensory signals passed through the
synapses for multiple times, the synapses become
so facilitated that signals generated within the brain
itself can also cause transmission of impulses
through the same sequences of synapses, even when
the sensory input is not excited.
• This gives the person a perception of experiencing
the original sensations, although the perceptions are
only memories of the sensations.
"Second Messenger" System in the
• The process of memory-require prolonged changes in
neurons for seconds to months after the initial transmitter
substance is gone.
• The ion channels are not causing prolonged postsynaptic
neuronal changes because these channels close within
milliseconds after the transmitter substance is no longer
• prolonged postsynaptic neuronal excitation or inhibition is
achieved by activating a "second messenger" chemical system
inside the postsynaptic neuronal cell itself, and then it is the
second messenger that causes the prolonged effect.
• The most common types of second messenger systems, use a group of proteins called G-proteins.
• G-protein is attached to the portion of the receptor that protrudes into the interior of the cell.
• The G-protein in turn consists of three components: an alpha (α) component that is the activator
portion of the G-protein, and beta (β) and gamma (γ) components that are attached to the alpha
component and also to the inside of the cell membrane adjacent to the receptor protein. On
activation by a nerve impulse, the alpha portion of the G-protein separates from the beta and
gamma portions and then is free to move within the cytoplasm of the cell.
• Inside the cytoplasm, the separated alpha component performs four
changes. They are as follows:
1 Opening specific ion channels through the postsynaptic cell
membrane. a potassium channel that is opened in response to the G-
protein; this channel often stays open for a prolonged time, in
contrast to rapid closure of directly activated ion channels that do
not use the second messenger system.
• Activation of cyclic adenosine monophosphate (cAMP) or cyclic guanosine
monophosphate (cGMP) in the neuronal cell. Recall that either cyclic AMP
or cyclic GMP can activate highly specific metabolic machinery in the
neuron and, therefore, can initiate any one of many chemical results,
including long-term changes in cell structure itself, which in turn alters
long-term excitability of the neuron.
• Activation of one or more intracellular enzymes. The G-
protein can directly activate one or more intracellular
enzymes. In turn the enzymes can cause any one of many
specific chemical functions in the cell.
• Activation of gene transcription. This is one of the most important effects
of activation of the second messenger systems because gene transcription
can cause formation of new proteins within the neuron, thereby changing
its metabolic machinery or its structure. Indeed, it is well known that
structural changes of appropriately activated neurons do occur, especially
in long-term memory processes.