IV. Biosensors
Biological Sensors: Study of various corpuscles like Pacinian, functions and modelling,
Chemoreceptor, hot and cold receptors, baro- receptors, sensors for smell, sound, vision,
osmolality and taste.
Biosensors: Introduction, Advantages and limitations, various components of Biosensors,
Biocatalysts based biosensors, bio-affinity based biosensors & microorganisms based
biosensors, Types of membranes used in biosensor constructions, Electronic Nose.
4.1 Biological Sensors: Study of various corpuscles like Pacinian, functions and modelling
• Pacinian corpuscle- an encapsulated ending of a sensory nerve that acts as a receptor
for pressure and vibration.
• They are nerve endings in the skin responsible for sensitivity to vibration and pressure.
• They respond only to sudden disturbances and are especially sensitive to vibration.
• Pacinian corpuscles are also found in the pancreas, where they detect vibration and
possibly very low frequency sounds.
• Pacinian corpuscles act as very rapidly adapting mechanoreceptors.
• The human skin is highly sensitive to changes in the external environment.
• These receptors detect the external stimulus and signal the CNS to take necessary
actions to cope with the changing external environment.
• Pacinian corpuscles or lamellar corpuscles
• They are not only found in skin but are also seen in the walls of organs like the pancreas,
urinary bladder, and rectum, etc. here, these corpuscles detect the pressure created by
distortion of the surrounding tissue and make the higher centers aware of it.
Figure 4.1 Pacinian Corpuscle
2
,• Pacinian corpuscles are the capsulated endings of sensory neurons. They are large oval
structures that are 0.5 to 1 mm in diameter. These corpuscles are found deep in the skin
within the layers of reticular dermis and hypodermis.
• Pacinian corpuscles have a single axonal fiber at the center surrounded by 15-20 lamella
arranged in a concentric pattern. The entire structure is also surrounded by a connective
tissue capsule. A fluid of certain nature is also present in between the lamella of the
corpuscle.
• The myelinated fiber enters a peripheral sensory nerve.
• Pacinian corpuscles are responsible for detecting pressure and vibration stimuli.
• Any pressure or change in pressure is detected by the change in the position or shape
of the lamella of Pacinian corpuscles.
• When pressure is applied to the skin, the lamella of Pacinian corpuscles gets deformed.
• This causes stress on the membrane of sensory neuron and potential is generated, called
the generator potential or receptor potential.
• The basic reason of receptor potential is the change in the membrane permeability of
different ions caused by the stress on the sensory axon.
• It causes the ions to diffuse into the cell at a different rate, resulting in a change in
transmembrane potential.
4.1.1 Generation of Action Potential
• compressed anywhere, it will result in elongation, bending, or deformation of the
central fiber.
• When a small area of the axonal fiber is compressed due to the deformation of the
corpuscle, certain ion channels open causing the positively charged sodium ions to
diffuse into the axon.
• These positive ions cause depolarization of the fiber creating a potential called receptor
potential.
• This receptor potential also creates local circuits of current flow that spread throughout
the length of the fiber.
• The first node of Ranvier is located inside the capsule.
• When the local current circuits reach this node, the membrane becomes depolarized.
• The action potential now leaves the corpuscle and travels from node to node, along the
sensory nerve.
• The action potential is only generated when the receptor potential is greater than the
threshold potential of the sensory fiber.
3
, • The strength of the receptor potential is related to the intensity of the stimulus
• The increased influx of sodium ions causes a stronger receptor potential.
• The receptor has an extreme range of responses, from very weak to very strong. The
receptor is more sensitive to the weak sensory stimuli.
• If a continuous stimulus is applied to a sensory receptor, it first generates impulses at a
very rapid rate. The response rate progressively decreases finally reaching a level when
only a few or no impulses are generated. This phenomenon is known as adaptation.
• Pacinian corpuscles are a good example of rate receptors or phasic receptors.
• The rapidly adapting receptors cannot be used to send continuous signals as they are
stimulated only when the strength of the stimulus changes.
• As they react only in response to an actual change, they are termed as rate receptors,
phasic receptors, or movement receptors.
• When a sudden pressure is applied to a tissue, it excites the Pacinian corpuscles for a
few milliseconds.
• The excitation is over soon, although the pressure is still there. A signal is transmitted
again when the pressure is released.
• Thus, it keeps the nervous system aware of the changing deformation of tissues but is
useless in the case of the constant deformation state of the body.
4.1.2 Functions of Pacinian Corpuscles
(i) Detection of Pressure and Vibration Changes
• It is the primary function of Pacinian corpuscles.
• They detect any pressure change applied to the skin.
• The changing pressure stimuli create a sense of vibration.
• They have pressure-sensitive sodium channels that are opened in response to the
changing pressure on the skin.
• An action potential is initiated that is carried to the higher centers of the brain via the
ascending pathways in the spinal cord.
(ii) Detection of Pressure in Internal Organs
• Pacinian corpuscles are also present in the walls of some viscera like the rectum and
urinary bladder.
• Here, the detect the pressure created due to the filling of the organ. As a result, a
signal is sent to the CNS that makes us aware of the filled state of the organ.
4
, • The CNS also make necessary arrangements to empty the filled urinary bladder or
rectum. Thus, they have an important role in processes like urination and defecation.
4.2 Chemo receptors
• Chemoreceptors are stimulated by a change in the chemical composition of their
immediate environment.
• There are many types of chemoreceptor spread throughout the body which help to
control different processes including taste, smell and breathing, control the pH, partial
pressure of oxygen (pO2) and partial pressure of carbon dioxide (pCO2) within our
blood
4.2.1 Peripheral Chemoreceptors
• Located in both the carotid body and the aortic body, these receptors detect large
changes in pO2 as the arterial blood supply leaves the heart.
• If an abnormally low pO2 is detected, afferent impulses travel to the respiratory centres
in the brainstem.
• A number of responses are then coordinated which aim to increase the pO2 again.
• These include:
• Increasing the respiratory rate and tidal volume, to allow more oxygen to enter the lungs
and subsequently diffuse into the blood
• Directing blood flow towards the kidneys and the brain (as these organs are the most
sensitive to hypoxia)
• Increasing Cardiac Output in order to maintain blood flow, and therefore oxygen supply
to the body’s tissues.
4.2.2 Central Chemoreceptors
• Located in the medulla oblongata of the brainstem, these receptors are more sensitive
and detect smaller changes in arterial pCO2.
• controls our respiratory system
• An increase in pCO2 leads to an increase in ventilation. This results in more CO2 being
blown off and so the pCO2 returns to normal
• A decrease in pCO2 leads to a decrease in ventilation. This results in more CO2 being
retained in our lungs and so the pCO2 returns to normal.
• these receptors actually detect changes in the pH of the Cerebral Spinal Fluid (CSF).
• The pH of the CSF is established by the ratio of pCO2 : [HCO3–].
5
Biological Sensors: Study of various corpuscles like Pacinian, functions and modelling,
Chemoreceptor, hot and cold receptors, baro- receptors, sensors for smell, sound, vision,
osmolality and taste.
Biosensors: Introduction, Advantages and limitations, various components of Biosensors,
Biocatalysts based biosensors, bio-affinity based biosensors & microorganisms based
biosensors, Types of membranes used in biosensor constructions, Electronic Nose.
4.1 Biological Sensors: Study of various corpuscles like Pacinian, functions and modelling
• Pacinian corpuscle- an encapsulated ending of a sensory nerve that acts as a receptor
for pressure and vibration.
• They are nerve endings in the skin responsible for sensitivity to vibration and pressure.
• They respond only to sudden disturbances and are especially sensitive to vibration.
• Pacinian corpuscles are also found in the pancreas, where they detect vibration and
possibly very low frequency sounds.
• Pacinian corpuscles act as very rapidly adapting mechanoreceptors.
• The human skin is highly sensitive to changes in the external environment.
• These receptors detect the external stimulus and signal the CNS to take necessary
actions to cope with the changing external environment.
• Pacinian corpuscles or lamellar corpuscles
• They are not only found in skin but are also seen in the walls of organs like the pancreas,
urinary bladder, and rectum, etc. here, these corpuscles detect the pressure created by
distortion of the surrounding tissue and make the higher centers aware of it.
Figure 4.1 Pacinian Corpuscle
2
,• Pacinian corpuscles are the capsulated endings of sensory neurons. They are large oval
structures that are 0.5 to 1 mm in diameter. These corpuscles are found deep in the skin
within the layers of reticular dermis and hypodermis.
• Pacinian corpuscles have a single axonal fiber at the center surrounded by 15-20 lamella
arranged in a concentric pattern. The entire structure is also surrounded by a connective
tissue capsule. A fluid of certain nature is also present in between the lamella of the
corpuscle.
• The myelinated fiber enters a peripheral sensory nerve.
• Pacinian corpuscles are responsible for detecting pressure and vibration stimuli.
• Any pressure or change in pressure is detected by the change in the position or shape
of the lamella of Pacinian corpuscles.
• When pressure is applied to the skin, the lamella of Pacinian corpuscles gets deformed.
• This causes stress on the membrane of sensory neuron and potential is generated, called
the generator potential or receptor potential.
• The basic reason of receptor potential is the change in the membrane permeability of
different ions caused by the stress on the sensory axon.
• It causes the ions to diffuse into the cell at a different rate, resulting in a change in
transmembrane potential.
4.1.1 Generation of Action Potential
• compressed anywhere, it will result in elongation, bending, or deformation of the
central fiber.
• When a small area of the axonal fiber is compressed due to the deformation of the
corpuscle, certain ion channels open causing the positively charged sodium ions to
diffuse into the axon.
• These positive ions cause depolarization of the fiber creating a potential called receptor
potential.
• This receptor potential also creates local circuits of current flow that spread throughout
the length of the fiber.
• The first node of Ranvier is located inside the capsule.
• When the local current circuits reach this node, the membrane becomes depolarized.
• The action potential now leaves the corpuscle and travels from node to node, along the
sensory nerve.
• The action potential is only generated when the receptor potential is greater than the
threshold potential of the sensory fiber.
3
, • The strength of the receptor potential is related to the intensity of the stimulus
• The increased influx of sodium ions causes a stronger receptor potential.
• The receptor has an extreme range of responses, from very weak to very strong. The
receptor is more sensitive to the weak sensory stimuli.
• If a continuous stimulus is applied to a sensory receptor, it first generates impulses at a
very rapid rate. The response rate progressively decreases finally reaching a level when
only a few or no impulses are generated. This phenomenon is known as adaptation.
• Pacinian corpuscles are a good example of rate receptors or phasic receptors.
• The rapidly adapting receptors cannot be used to send continuous signals as they are
stimulated only when the strength of the stimulus changes.
• As they react only in response to an actual change, they are termed as rate receptors,
phasic receptors, or movement receptors.
• When a sudden pressure is applied to a tissue, it excites the Pacinian corpuscles for a
few milliseconds.
• The excitation is over soon, although the pressure is still there. A signal is transmitted
again when the pressure is released.
• Thus, it keeps the nervous system aware of the changing deformation of tissues but is
useless in the case of the constant deformation state of the body.
4.1.2 Functions of Pacinian Corpuscles
(i) Detection of Pressure and Vibration Changes
• It is the primary function of Pacinian corpuscles.
• They detect any pressure change applied to the skin.
• The changing pressure stimuli create a sense of vibration.
• They have pressure-sensitive sodium channels that are opened in response to the
changing pressure on the skin.
• An action potential is initiated that is carried to the higher centers of the brain via the
ascending pathways in the spinal cord.
(ii) Detection of Pressure in Internal Organs
• Pacinian corpuscles are also present in the walls of some viscera like the rectum and
urinary bladder.
• Here, the detect the pressure created due to the filling of the organ. As a result, a
signal is sent to the CNS that makes us aware of the filled state of the organ.
4
, • The CNS also make necessary arrangements to empty the filled urinary bladder or
rectum. Thus, they have an important role in processes like urination and defecation.
4.2 Chemo receptors
• Chemoreceptors are stimulated by a change in the chemical composition of their
immediate environment.
• There are many types of chemoreceptor spread throughout the body which help to
control different processes including taste, smell and breathing, control the pH, partial
pressure of oxygen (pO2) and partial pressure of carbon dioxide (pCO2) within our
blood
4.2.1 Peripheral Chemoreceptors
• Located in both the carotid body and the aortic body, these receptors detect large
changes in pO2 as the arterial blood supply leaves the heart.
• If an abnormally low pO2 is detected, afferent impulses travel to the respiratory centres
in the brainstem.
• A number of responses are then coordinated which aim to increase the pO2 again.
• These include:
• Increasing the respiratory rate and tidal volume, to allow more oxygen to enter the lungs
and subsequently diffuse into the blood
• Directing blood flow towards the kidneys and the brain (as these organs are the most
sensitive to hypoxia)
• Increasing Cardiac Output in order to maintain blood flow, and therefore oxygen supply
to the body’s tissues.
4.2.2 Central Chemoreceptors
• Located in the medulla oblongata of the brainstem, these receptors are more sensitive
and detect smaller changes in arterial pCO2.
• controls our respiratory system
• An increase in pCO2 leads to an increase in ventilation. This results in more CO2 being
blown off and so the pCO2 returns to normal
• A decrease in pCO2 leads to a decrease in ventilation. This results in more CO2 being
retained in our lungs and so the pCO2 returns to normal.
• these receptors actually detect changes in the pH of the Cerebral Spinal Fluid (CSF).
• The pH of the CSF is established by the ratio of pCO2 : [HCO3–].
5
