SENSORY
SYSTEM
Shivani sharma
Associate
professor
SENSATION
• Sensation is the conscious or subconscious awareness
of changes in both internal and external environment.
• It can be defined as conscious awareness of the
stimuli received by sensory receptors .
• This process of sensation begins with sensory
receptors that are located in the various organs of
body such as nose, ears, eyes, tongue and skin also.
TERMS USED
❖Stimulus- must be present that cause change in
internal and external body environment.
❖Receptors- must be capable of detecting the
stimulus and transforming the stimulus into
action potentials.
❖Action potentials / nerve impulses generated
by receptor must be conducted to the CNS.
CLASSIFICATION OF SENSES-
The senses may be classified into two major groups
1. General senses- it includes information about the
internal body organs and its also external
environment.
2. Special senses- these are those with highly localized
receptors that provide specific information about
the external environment like smell, taste, sight
,hearing and balance.
TYPES OF SENSORY RECEPTORS
1. Photoreceptors- detect light and present in the retina of
eye.
2. Thermoreceptors- detect changes in temperature and
present in the skin.
3. Proprioreceptpors-detect changes in tension and present
in the joints.
4. Nociceptors-are pain receptors.
5. Mechanoreceptors- detect mechanical stimuli eg touch,
pressure, vibration.
6. Baroreceptors – detect changes in pressure of blood.
7. Chemoreceptors- detect changes in chemical
environment.
GUSTATION
( sense of taste)
GUSTATORY SYSTEM
• The word taste comes from the Latin word taxare
meaning - to touch, estimate or to judge.
• In humans, the sense of taste is transduced by
taste buds and conveyed via three cranial nerves-
cranial nerve 7 , cranial nerve 9 and 10th .
• Information from these cranial nerves is
processed by the gustatory system.
• All taste buds can respond to all types of tastes
differently.
ANATOMY OF TASTE BUDS
• The taste buds are the sensory receptor organs of
taste that are located primarily in the oral cavity.
• Of nearly 10,000 or so taste buds of a young adult lies
on the tongue.
• A few taste buds are scattered on the soft palate,
pharynx and epiglottis of the larynx but most are
found in papillae, peg like projections of the tongue
mucosa that gives upper surface of the tongue.
• The number of taste buds declines with age.
PAPILLAE
• These are the specialized epithelial
cells.
There are four types of papillae-
1. FILIFORM
2. FUGIFORM
3. FOLIATE
4. CIRCUMVALLATE PAPILLAE.
• Only filiform papillae does not have
taste buds on their surface.
Cont…
1. Fungiform papillae- these are slightly
mushroom shaped papillae which are
scattered irregularly over the entire dorsal
surface of the tongue. These contain 5 buds
and appear as small red dots interspersed
among numerous filiform papillae.
2. Filiform papillae-
these are thin , longer
papillae but longer one
and contains no any
taste buds. These
papillae are mechanical
and not involved in
gestation. These allow
us to experience food
texture and feel.
3. Foliate papillae- these are
ridges and grooves towards
the posterior part of the
tongue.
4. Circumvallate papillae- are
largest and least numerous
papillae type. These are
circular and form inverted
V- shaped row at the
posterior part of the
tongue.
Structure of taste buds
• Each taste bud is a flask shaped structure having
a small pore like opening on the surface of the
tongue, taste pore.
• Each taste bud consists of 50-100 epithelial cells.
• These cells are of three –
1. Gustatory cells,
2. Supporting cells and
3. Basal cells.
Magnified
Papillae
taste
bud
Microscopic
sensory cell view of
taste buds (x100)
food
brain
1. GUSTATORY CELLS
• The neurosensory gustatory cells occupy the central
portion of the bud.
• 50 gustatory cells are present in each bud.
• Long microvilli called gustatory hairs project from the
tips of all gustatory cells and extend through a taste
pore to the surface of the epithelium, where they
bathed by saliva.
• At their base, the receptor cells synapse with dendrites
that form first part of the gustatory pathway.
• A single sensory neuron can be connected to several
taste cells in each of several different taste buds.
2. SUPPORTING CELLS
• These are columnar cells found between the taste
receptor cells.
• Some are found in the interior of the bud
between the gustatory cells and they support
gustatory cells by building envelope around the
buds.
3. BASAL CELLS
• Basal cells found at the periphery of the taste bud
near the connective tissue layer, produce
supporting cells which then develop into
gustatory receptors.
TYPES OF TASTE
• There are five primary taste sensations
➢Salty
➢Sour
➢Sweet
➢Bitter
➢Umami
• Most taste buds respond to two or more taste
qualities and many substances produce a
mixture of basic taste sensations.
- Humans have four kinds of taste buds.
- They can detect four kinds of tastes:
sweet, sour, salty and bitter.
-.
bitter
sour
salty
sweet
Mechanism of taste transduction
• Salty and sour work with ion channels but
sweet,bitter and umami work with signal through
a G protein coupled receptor.
• Salty- salty taste is produced by metal ions. At
least one of the receptors for salty substances-
NaCl is an ion channel that allows sodium Na+ to
enter directly into the cells.
• Sour- sour taste is produced by acids specifically
their hydrogen ion H+ in the solution.
• Sweet- the sweet taste is evoked by many organic
substances including sugar, alcohal, some amino
acids and some lead salt.
• Bitter- the bitter taste is caused by alkaloids such
as quinine, nicotine, caffeine, morphine as well as
non alkaloids substances such as aspirin.
• Umami – umami is the response to salts of
glutamic acid like monosodium glutamate a
flavour enhancer used in many processed food
and in many asian dishes.
PHYSIOLOGY OF TASTE
Gustatory stimuli (sweet, sour ,salt or bitter )
Perceived by various areas of tongue
Transduction of gustatory stimuli into electrical signal
is intiated at the level of receptors
Taste receptors are chemoreceptors which are
stimulated by substances dissolved in the mouth by
saliva
Dissolved substances act on microvilli of taste buds
Development of receptor potential in the
receptor cells which generates action potential
in sensory nerves
Transmission of information about taste to
gustatory cortex
Encoding of taste information by unique pattern
of inputs from many separate fibres
Sensation of flavors is a combination of gustatory
,olfactory and somatosensory inputs
ABNORMALITIES OF TASTE
SENSATIONS
• AGEUSIA- refers to absence of taste sensation.
• HYPOGEUSIA- refers to diminished taste
sensitivity.
• DYSGEUSIA – refers to distributed sense of
taste.
OLFACTORY SYSTEM
(sense of smell)
INTRODUCTION
• The sense of smell or olfaction is well
developed in animals like dog and rabbit to
give warning of environmental changes.
• In humans, the sense of smell is less
developed but still plays important role in
pleasure and enjoying the taste of food.
SITE OF OLFACTION-
• The olfactory stimuli are detected by
specialized receptors located on the free
nerve endings of the olfactory nerves
which are located in the olfactory mucosa
of nose in the human beings
OLFACTORY MUCOSA
• In humans, the olfactory mucosa
is confined to upper one third of
nasal cavity.
• It consists of three types of cells
receptor cells, supporting cells
and basal cells.
• Receptor cells are bipolar neurons
which lie between the supporting
cells.
NERVE SUPPLY TO THE OLFACTORY
MUCOSA-
• Special sensory nerves innervating the
olfactory mucosa are 15-20 bundles of
olfactory nerve fibres, 1st cranial nerve that
conveys sense of smell.
• General sensory nerve supplying the olfactory
mucosa are branches of 5th cranial nerve
named as trigeminal nerve.
OLFACTORY PATHWAYS
• Olfactory pathways comprises
1. Olfactory nerves- about 15-30 nerve
filaments which consists of the axons of
the bipolar olfactory neurons.
Cont..
2. Olfactory bulb – it is an oval flattened strip of gray
matter lying on the cribriform plate which receives
the olfactory nerve filaments.
3. Olfactory tract- it lies in the olfactory sulcus on
the orbital surface of the frontal lobe.
• Olfactory cortex- it includes anterior olfactory
nucleus, prepiriform cortex, olfactory tubercle
and amygdala. All these are part of limbic
system.
PHYSIOLOGY OF OLFACTION
Odoriferous stimuli ( any smell producing molecules enter into
nasal cavity )
Dissolve in the mucus layer of the lining of olfactory mucosa in
the nose
Reached to the olfactory receptors where the cilia of the
olfactory neurons perceive the odorants
Odorants molecules bind to receptors linked to proteins in
plasma membrane called G proteins
It activates the enzyme adenylate cyclase and produce
adenosine monophosphate (CAMP)
Opening of sodium (Na+) channels
Inflow of Na+ and activates generator potential
Generation of nerve impulses in the olfactory
receptors
Processing of olfactory sensation in the olfactory
receptors
Information send to olfactory cortex
Perceived type of smell
Factors influencing olfactory functions
1. Intensity of odour – the concentration of
odorant must be change by about 30%
before a difference can be detected .
2. Threshold of olfactory receptors – it
varies from substance to substance.
3. Adaptation – olfactory sensation adopts
very rapidly with continued exposure to an
odour.
Abnormalities of olfactory system
• Anasmia- Total loss of sense of smell.
• Hyposmia - refers to diminished olfactory
sensitivity.
• Paraosmia – refers to distributed sense of
smell means person incorrectly interprets the
odour .
