Sensory system.
Nose.
Odour.
Olfactory system.
Neural pathways.
Tongue.
Taste.
Other sensation of taste.
Skin.
Skin layers.
Skin sensory receptors.
Sensory perception.
The connectome.
We perceive the world, through our senses.
If we did not have any senses, our mind will be blank.
We will be somewhat like a state of coma.
All of our life’s experiences are the result of sensory experiences.
The human body has special organs, to perceive specialised senses.
Our nose, perceives smell.
Our eyes, perceives vision.
Our ears, perceives sound.
Our tongue, perceives taste.
Our skin, perceives pressure, temperature, pain etc.
These five organs are the traditional sense organs of the body.
We will discuss these organs, in the first part of this module.
To regulate metabolic processes in the body,
the brain has to sense other parameters, like body temperature,
blood pressure, oxygen levels etc.,
We will also discuss these functionalities, in the second part of this module.
Nose is a sensory organ, involved in the sense of smell.
The sense of smell is called olfaction.
This functionality of the nose is called as the olfactory system.
The nose also performs other important functions.
It is involved in breathing.
Hairs inside the nose, prevent large particles from entering the lungs.
It warms and humidifies the air, before it enters the lungs.
These important functions, are not part of the olfactory system.
The olfactory system is involved only in the sense of smell, or olfaction.
Odour is typically caused by volatile chemical compounds.
When we breathe in air, these chemicals reach the receptor cells, in the nose.
This is one way, odour reaches the nose.
When we chew on food, a few food molecules, get released.
There is a passage connecting the mouth and the nose.
The odour molecules, from the food we are chewing,
can reach the same odour receptors in the nose.
We can smell food, by just chewing it.
This is one more way, that we can smell food.
The olfactory system, senses odour, from both these channels.
Nose has receptors to smell odours.
These receptors are chemoreceptors.
These chemoreceptors are located in the top of the nose,
in the olfactory epithelium.
The olfactory epithelium acts like a sensory pad for odours.
Odours from the mouth also reach the olfactory epithelium.
There are different chemoreceptors for different type of chemicals.
One chemoreceptor, will be sensitive to only one type of chemical.
Chemoreceptors act as transducers, which translate chemical signals,
into action potentials or nerve impulses.
The binding of the particular odour molecule,
with the corresponding chemoreceptor,
results in generating the action potential.
Afferent nerve fibres, carry signals from the olfactory sensory neurons, to the brain.
The cranial nerve 1, is the neural pathway from the nose, to the brain.
The nerve fibres, first converge in the olfactory bulb,
and then travels to the olfactory cortex, and other association areas of the brain.
The olfactory system, is the only human sense that bypasses the thalamus.
The olfactory bulb has regions, in the outer layer, called glomeruli.
In the inner layer, it has regions called the mitral cells.
The nerve fibres from the olfactory epithelium converge, in the glomeruli,
in the outer areas, of the olfactory bulb.
These in turn, connect to a few mitral cells, in the inner layer.
Some level of information integration takes place, in the olfactory bulb.
Nerve fibres from the mitral cells, exit via the olfactory tract.
They eventually reach five major regions of the brain.
Each of these regions in the brain, perform some specialised functions.
One region identifies the chemical structure of the molecule,
and categorises different odours.
For example, it might categorise it as woody, citrus, minty, etc.
Another region is involved in conscious perception of the odour.
The amygdala is involved in the emotional response to the odour.
The hippocampus is involved in association with memories.
Like all senses, the olfactory sense, is related to other senses.
For example, the sight, the smell, and the taste,
contributes to the overall experience of the food.
The olfactory sense is also strongly related to long term memory.
When an odourant is detected by the receptors, the information is broken down,
based on the chemical constituents of the odour.
The brain synthesises the information, for detection and perception.
The brain is capable of detecting an amazing number of odours.
Our sense of olfaction, is still not as keen as some other animals.
Dogs for example, have a very keen sense of olfaction.
From the evolutionary view point, the sense of olfaction,
is one of the oldest senses, that we developed.
This is likely because animals, use the sense of olfaction,
to smell food, mates, and prey or predator.
The wiring circuit in the brain for olfaction, is slightly different, from other senses.
The neural pathways to the brain, are shorter and more direct.
There is a strong link between the limbic system, and the sense of olfaction.
Emotions and memory can be strongly associated with smells.
Human beings have inherited this design.
Even now we can experience a smell, evoking a memory or a emotion.
The tongue is the primary organ for taste.
It also assists in the mastication of food.
The tongue is covered with small nipple or hair like structures,
on the upper surface.
These projections are called lingual papillae.
This gives the tongue its characteristic rough texture.
Most of the lingual papillae, act as taste buds.
These taste buds are involved in tasting the food we eat.
The sensory impression made, by the chemicals, in food or other substances,
on the tongue, is perceived as taste.
Taste is one of the 5 traditional senses.
The perception of taste, is also known as gustation.
Taste along with olfaction, results in perceiving flavour.
The tongue is covered with thousands of papillae, involved in taste.
Within each of this papilla, there are hundreds of taste buds.
Each taste bud contains 50 to 100 taste receptor cells.
Each taste receptor cell, is sensitive to one type of chemical.
These type of receptors, are called as chemoreceptors.
When a chemical substance, comes into contact with the corresponding receptor cell,
it generates an action potential, or electric signal.
This signal is transmitted to the brain.
Taste buds differentiate tastes, by detecting interaction with different molecules, or ions.
The tongue can detect 5 basic tastes:
Sweetness.
Sourness.
Saltiness.
Bitterness.
Umami.
Sweetness is usually regarded as a pleasurable sensation.
It is associated with energy rich foods.
Sweetness is often connected to aldehydes and ketones.
These chemicals contain a carbonyl group.
A carbonyl group is a carbon oxygen double bond.
Specific taste receptors, detect these chemicals and create a sensation, of sweetness.
Sugars are the obvious examples, of sweetness.
Sourness is the taste which detects acidity.
A weak acid will have a sour taste.
Lemon has citric acid.
Tamarind has tartaric acid.
These kind of substances, taste sour to us.
Saltiness is a taste, produced primarily by the presence of sodium ions.
Other alkali metal ions, also measure saltiness.
Sodium chloride, or common salt, is the most widely used salty substance.
Bitterness is one of the most sensitive taste.
Typically bitter taste, is perceived as unpleasant.
Many poisonous substances have a bitter taste.
Perhaps ancient human beings, learnt to avoid these foods, with this taste.
It is possible that we learnt to appreciate some flavours of this taste.
Coffee and cocoa have a component of bitter taste.
However these foods are widely appreciated today.
Umami is a savoury or meaty taste.
It can be tasted in foods like cheese or soy sauce.
Monosodium glutamate or Ajinomoto, has this taste.
Sweetness, sourness, saltiness, bitterness and umami,
are considered as basic tastes.
Specific chemoreceptors in the tongue identify these tastes.
The tongue also has nerve fibres called the trigeminal nerve fibres.
These nerve fibres can sense the properties like texture, pain, and temperature.
These properties are also sensed, by sensors in the tongue.
This sensations are perceived along with other taste.
Using these sensors, the tongue can identify other sensations, like spiciness or coolness.
Some substances like capsaicin, cause a burning sensation in the tongue.
We call this sensation as spiciness, pungency or hotness.
It is not a basic taste, but a sensation perceived by the tongue.
This burning sensation is induced by a trigeminal nerve reaction,
together with normal taste perception.
Capsaicin is present in capsicum, chilli pepper, black pepper etc.
This particular sensation caused by these spices, are not a taste in the technical sense.
The sensation does not arise, from the taste buds.
Foods like chilli pepper, stimulate the somatosensory fibres,
which sense pain and temperature.
These sensory fibres are also present in the tongue.
This sensation is perceived as spiciness or hotness by us.
Spiciness is used in many cuisines.
Some substances stimulate cold trigeminal receptors.
This results in a “cool”, or “minty” sensation.
Even though the temperature is not low, we perceive it as a cool sensation.
Peppermint, menthol etc cause this sensation.
Olfaction, taste, and the trigeminal receptors, together contribute to flavour.
The human tongue can distinguish only five basic taste.
The olfactory system can detect a large number of odours.
When we chew food, some of the molecules reach the nose.
This happens in the exhalation phase.
Flavour is an unique combination of taste and olfaction.
The skin is the outer covering of the body.
It is the largest organ, of the body, in terms of surface area.
It is the most important part of the integumentary system.
The integumentary system, is the outer most part of the body,
comprising of skin, hair, etc.
A typical human being, has about 1.5 to 2 square meters of skin.
It is about 2 to 3 millimetres thick.
A sample of 6.5 square centimetres of skin, typically contains,
20 blood vessels.
650 sweat glands.
60000 melanocytes.
and 1000 nerve endings.
The skin guards the underlying, muscles, bones,
ligaments and internal organs, of the body.
The skin makes the body water proof.
It protects the body from pathogens.
It provides insulation, and temperature regulation.
It involves in the synthesis of vitamin D.
The skin is an important sensory organ of the body.
It provides the sense of touch, pressure, temperature, pain etc.
All these senses are perceived by the brain, in the somatosensory cortex.
The skin protects us from harmful ultra violet rays, in sunlight.
The skin is pigmented by a substance known as melanin.
This helps absorb some of the harmful ultra violet or UV rays.
Melanin is produced when the skin is exposed to sunlight.
People living in tropical areas, are exposed to more sunlight.
The skin of these people have more melanin.
The melanin gives the skin, its characteristic colour.
The somatosensory system is a complex sensory system.
It is comprised of many senses.
The sense of touch, pressure, temperature, pain, etc,
are all part of the somatosensory system.
The receptors for these senses are located in the skin.
These receptors gives us the typical sensation we feel from the skin.
Some receptors are also located in the skeletal muscles, bones, joints etc.
These receptors give us a sense, where our body parts are currently located.
The skin comprises of three main layers.
The epidermis.
The dermis.
The hypodermis.
The epidermis is the top, or outer most layer of the skin.
It forms the water proof outer surface of the body.
This layer serves as a barrier to infection.
The epidermis has no blood supply.
The cells of the epidermis are generated in deeper layers,
and pushed upwards.
These cells have a limited life cycle.
The top layers are keratinised.
Keratin is a fibrous structural protein.
The keratinised top most layer of the skin, makes it water proof,
and acts as a barrier to infection.
The cells in the upper layer die, and are discarded.
These cells are replaced by fresher cells, from the lower layers.
A large number and variety of microbes, thrive on the surface of the skin.
Most of them are harmless, or beneficial to the skin.
Sunlight, water and air, play an important part in keeping the skin healthy.
The dermis is the middle layer of the skin.
It is below the epidermis, and above the hypodermis.
The dermis cushions the body from stress and strain.
The dermis has hair follicles, sweat glands, sebaceous glands,
lymphatic vessels and blood vessels.
Blood vessels provide nourishment for the cells.
Sweat glands are involved in producing sweat, which helps to cool down the body.
The sebaceous gland produce an oily and waxy matter,
which lubricates and water proofs the skin.
Many sensory nerve endings are situated in the dermis.
The dermis has two layers.
The papillary region.
The reticular region.
The papillae in the papillary region provides a bumpy surface.
This provides the strong connection between the layers.
When the papillae protrudes into the epidermis,
we can see these projections.
For example, this happens in the palm of the hand.
Finger prints are a good example, of papillae projections.
The reticular region is below the papillary region.
It comprises of collagenous, elastic, and reticular fibres.
These protein fibres provide the strength and elasticity of the skin.
The hypodermis is below the dermis.
It is not considered as part of the skin.
The main function of the hypodermis, is to attach the skin,
to the underlying muscles and bones.
The hypodermis has loose connective tissue, and elastin.
The hypodermis also has adipose tissue.
Adipose tissue stores fat.
Fat acts as an energy reserve.
Fat also provides an insulating layer, for the skin.
Skin is sensitive to more than one sense.
The skin can sense light touch, pressure, vibrations, temperature and pain.
The skin has different sensors, to detect different senses.
The sensors are located at the end of nerve fibres.
The sensors can be mechanoreceptors, which senses mechanical signals.
The sensors can be chemoreceptors, which senses chemical signals.
The sensors act as transducers, which translate the mechanical or chemical signals,
to action potentials.
These nerve fibres, carry the nerve impulses to the brain.
The brain interprets these signals, as sensations of,
touch, pressure, temperature, pain, etc.
Meissner’s corpuscles detects sense of light touch.
It is a mechanoreceptor.
These receptors are located in the epidermis.
These type of receptors are concentrated in areas like the finger and lips.
These areas are specially sensitive to touch.
Even a slight physical deformation will cause the meissner’s corpuscles to fire.
This is the sense, which we use to feel texture.
For example, we can feel the smoothness of silk.
We can also sense braille characters, using these receptors.
These receptors are called rapidly adapting receptors.
If the stimulus is constant, it will not continue to fire.
This is the reason, we don’t feel the clothes we are wearing.
Merkel’s nerve ending are also mechanoreceptors.
They provide a sense of sustained touch.
They are slow adapting receptors.
They will fire as long as the sense of touch is sustained.
Pacinian receptors detect deep pressure.
They have a bulb shaped ending.
They are located lower down in the skin, typically in the dermis.
The mechanoreceptors which detect touch and pressure,
also detect vibrations.
Thermoreceptors detect changes in temperature.
They are free nerve endings.
These nerve endings are specialised in sensing hot or cold.
There are separate receptors for hot and cold.
Nociceptor senses pain.
Pain is a perception, caused when tissue is damaged.
They sense noxious stimuli.
When cells get damaged, they release chemicals,
which are recognised as noxious stimuli.
Pain basically warns the brain, that cells are getting damaged.
This can happen for example, when a pin, is pressed in the skin.
The brain typically will take defensive action.
Often this response can be, a reflex response.
If we touch something warm, it will stimulate the heat nerve endings.
We will feel the warmth.
If we touch something hot enough to burn the tissue,
the nociceptors will get stimulated, and we will feel the pain.
Nociceptors are also located in other parts of the body,
other than the skin.
Example, nociceptors are located in muscles.
When muscles are strained, we get a sensation of pain.
Nociceptors are also situated in other organs and parts of the body.
Though there are many types of sensory receptors,
all of them translate the different sensory inputs, into action potentials.
The brain finally receives only electrical signals.
The brain interprets these signals, depending on which receptor originated it.
If a pressure receptor sends a signal, it perceives it as a pressure sensation.
If a heat receptor sends a signal, it perceives it as a heat sensation.
If a nociceptors send a signal, it perceives it as a pain signal.
It is important to note, the difference between,
the perception of a stimuli, and the stimuli itself.
The brain also is aware from which part of the body, that the stimuli is received.
We can easily differentiate a pressure from a right thumb, and a right little finger.
All the nerve impulses, from the sensory receptors,
eventually reach the somatosensory cortex, via neural pathways.
The somatosensory cortex, interprets the type of signal received.
It also identifies the location, of the origin of the stimuli.
The map of the body is present in the somatosensory cortex.
The brain integrates all the information that it receives, into a meaningful whole.
When we lift a glass of water, pressure signals from all the fingers reach the brain.
It holistically and intelligently perceives these delicate signals.
It uses this information to direct the muscles, in the fingers to apply,
the right amount of pressure on the glass.
The brain can also integrate information from the eyes and the hand.
If we wish to touch the tip of our nose, with our finger,
with our eyes closed, we can easily do so.
We have an innate sense of where our body parts are located.
Sensors are located in all our muscles, tendons and joints.
These sensors are called proprioceptors.
Because, the brain knows where the muscle is, it also knows where the body part is.
The brain integrates information from proprioception, and from the vestibular system,
into an overall sense of body position.
The motor system in the brain, controls the muscles in the body.
To effectively do this, it requires feedback from the sense of proprioception.
When we want to lift a glass of water,
the eye is sending signals, on the location of the glass.
The motor system is sending command signals, to the hand and the fingers.
The sense of proprioception gives feedback on the location of the hand and fingers.
The sense of touch and pressure, gives a feedback of holding the glass.
All this information is integrated and intelligently processed in the brain.
This seemingly simple process, is actually a complex brain processing mechanism,
which works effortlessly and unconsciously.
Scientists struggle to program the same functionality into a robot.
Sensory receptors are located at the end of nerve fibres.
The nerve fibres convey the action impulses to the brain.
In the body the spinal nerve fibres travel via the spinal cord to the brain.
In the face the nerve fibres travel via the cranial nerves, to the brain.
Spinal nerves carry sensory, motor and autonomic signals,
between the spinal cord and the body.
Afferent nerve fibres carry sensory information,
from the receptors, to the spinal cord.
The somatosensory system is concerned, with this sensory information.
The spine is called as the vertebral column.
The spinal cord, is housed in the vertebral column.
It connects all the spinal nerves to the brain.
The spinal cord extends from the medulla, in the brainstem,
to the lumbar region, in the spine.
The spinal cord acts like a information highway to and from the brain.
All the spinal nerves travel via the spinal cord, to the brain.
The spinal cord, is a bundle of nerve fibres.
It contains efferent fibres, carrying information from the sensory nerves,
to the brain.
It also contains the afferent fibres, carrying motor information,
from the brain, to the various muscles and parts of the body.
The spinal cord, also performs a limited reflex function.
The spinal cord, is a critically important part, of the nervous system.
It is also very delicate.
It is surrounded by protective layers, and cerebrospinal fluid.
The spine is comprised of a number of bone discs, which are hollow.
The spinal cord, passes through these hollow bone discs.
The bony structure of the spine, protects the spinal cord.
We have 31 pairs of spinal nerves.
One nerve from each pair, goes to the left side,
and the other, to the right side of the body.
The spinal nerves are classified as,
Cervical nerves.
Thoracic nerves.
Lumbar nerves.
Sacral nerves.
Coccygeal nerves.
Eight pairs of cervical nerves, emanate from the cervical region.
Twelve pairs of thoracic nerves, emanate from the thoracic region.
Five pairs of lumbar nerves, emanate from the lumbar region.
There are five pairs of sacral nerves.
There is one pair of coccygeal nerves.
These nerves form the peripheral nervous system.
They comprise of both motor and sensory nerves.
The sensory nerves form the main sensory system.
The cranial nerves also have sensory nerves.
The spinal nerves emerging from the spinal cord,
extensively branch out, into many nerve fibres.
These nerve fibres reach out to all parts of the body.
The motor nerves carry motor signals, from the brain, to most parts of the body.
The sensory nerves, have a sensory receptor in the end, of each nerve fibre branch.
They carry sensory information from all parts of the body, to the brain.
Every neuron has a cell body.
The cell bodies of the afferent neurons, are located in the dorsal root ganglion,
located in the back of the spinal column.
This is the first neuron, or the primary neuron, of the spinal nerve.
The afferent fibre from the receptors, from different parts of the body,
first reach this neuron cell body.
The first neuron connects to a second neuron, or the secondary neuron,
in the spinal cord, or in the brain stem.
Nerve fibres carry the impulses through the spinal cord, to the brain.
Some of them go to the cerebellum, or other parts of the brain.
Most of these nerve fibres reach the thalamus.
Finally most of these somatosensory impulses reach the somatosensory cortex.
This is the primary processing centre, for senses in the brain.
Cranial nerves originate from the brain.
They do not travel via the spinal cord, to reach the brain.
They directly connect to areas in the brain.
Some cranial nerves are motor nerves.
They innervate muscles in the brain.
Muscles involved in speaking, facial expressions, eye movement etc.,
are innervated by motor cranial nerves.
Some are pure sensory nerves.
Some cranial nerves carry both sensory and motor nerve fibres.
We will discuss the cranial nerves, which carry sensory information.
Cranial nerve 1, is the olfactory nerve.
It carries odour nerve impulses to the brain.
Cranial nerve 2, is the optic nerve.
It carries visual nerve impulses, from the retina to the brain.
Cranial nerve 4, is the trigeminal nerve.
The trigeminal nerve is involved in sensations from the face.
It also has motor functions like biting and chewing.
Cranial nerve 7, is the facial nerve.
It is involved in the sensation of taste, from the front of the tongue.
The facial nerve also controls the muscles, involved in facial expressions.
Cranial nerve 8, is the vestibulocochlear nerve.
It has 2 parts.
The vestibular nerve carries position and balance information,
from the vestibular system, in the ear.
The cochlear nerve carries auditory information,
from the cochlea, in the ear.
Cranial nerve 9, is the glossopharyngeal nerve.
This nerve provides sensory information, from the back of the tongue.
It also carries some important unconscious sensory information.
For example, it carries the impulses of pressure, from the baroreceptors,
from the carotid sinus.
These impulses reach the vasomotor centre in the brain stem.
This helps to maintain a consistent blood pressure.
Cranial nerve 10, is the vagus nerve.
It innervates various organs.
It also conveys sensory information, about the state of the body’s organs,
to the central nervous system.
It is involved in the parasympathetic control,
of the heart and digestive tract.
We have discussed the important sensory information,
carried by the cranial nerves.
Cranial nerves are located in the face area.
They communicate directly to the brain.
Cranial nerves also have important motor functions.
For example, eye movement, facial expression, speaking by tongue movement,
are some of the motor functions, effected through motor nerves.
Somatosensory cortex is located in front of the parietal lobe, in the brain.
Each sensory impulse, comes from a specific part of the body.
The location of the body is mapped in the somatosensory cortex.
Each location in our body, has a corresponding location in the somatosensory cortex.
This is called as the sensory homunculus.
The sensory homunculus, looks like a map of the human body in the brain.
The homunculus, is not directly proportional to the body.
Some parts of the body, like the palms and the face,
is represented by a very large area in the homunculus.
Some parts of the body like the thighs, and the back,
is represented by a very small area in the homunculus.
Parts of the body, which are more sensitive, and which perform more complex tasks,
are represented by a relatively much larger area.
Within the face, the tongue and the lips, are represented by a much larger area.
The map of the human body, looks very distorted in the homunculus.
But, actually it is a very sensible design.
The more sensitive parts are assigned more area, in the somatosensory cortex.
This means that more neurons work to process the sensory information,
from the more sensitive areas.
All the physical movements of the body, are controlled by motor nerves.
Most of the motor nerves, emanate from the motor cortex.
The somatosensory cortex, works closely with the motor cortex.
Control of motor movements, involve sensory feedback.
It is natural, that both work together to achieve required functionality.
The motor cortex is very conveniently located adjacent to the somatosensory cortex.
The somatosensory cortex, is like a band, at the front of the parietal lobe.
The motor cortex is like a band, situated at the back of the frontal lobe.
The proximity of these two interrelated areas,
make communication between them very fast and easy.
Let us imagine a situation in a cricket game,
where you are running to take a catch.
Your sensory system has to work in close coordination, with your motor system,
to achieve this task.
This is just an example, of numerous day to day activities that we might perform,
which require close coordination between the somatosensory cortex,
and the motor cortex.
The human brain has distinctive anatomical features.
Most physical parts of the brain, can be clearly identified.
Initially our understanding, of the brain was limited to an anatomical brain.
Our understanding since then, has dramatically improved.
We now know that there are functional parts of the brain.
Each functional part specialises in one or more specialised functions.
The most important functional parts are now identified.
These functional parts give us a much more clear idea of the functioning of the brain.
Is the brain just the sum, of the functional parts?
The answer is a vehement, “No”.
The brain has about hundred billion neurons.
Each neuron is connected to hundreds and thousands of other neurons.
We have an estimated hundred trillion connections, in the human brain.
A comprehensive map of all the connections, represents the connectome,
of the human brain.
The next step in a more advanced understanding, of the human brain,
will come from understanding the neural connections in the brain.
Scientists are already making progress, in mapping the neural pathways,
between the functional parts of the brain.
We are beginning to understand the complex relationships,
between the functional parts of the brain.
Research is ongoing in this field.
The study of all the connections is called connectomics.
Scientists have undertaken a massive ambitious project,
to map all the connections in the human brain.
It is called the “human connectome” project.
Results of this research, will help to have a deeper understanding of the brain.
The ultimate challenge, is to understand the “brain as a whole”.
Understanding the parts, and even the connectome,
will only be a stepping stone, to a holistic understanding of the brain.
This makes neuroscience a very exciting science of the future.