4. SENSORY
SYSTEM
• The sensory system detects signals from
the outside environment and
communicates it to the body via the
nervous system.
• The sensory system relies on
specialized sensory receptor
cells that transduce external stimuli into
changes in membrane potentials.
5. SENSORY RECEPTOR
Specialized neurons (the receptor cell is also
a neuron)
specialized sensory cells which synapse
with a neuron (the receptor cell secretes
neurotransmitters to stimulate changes in
membrane potential in the synapsed
neuron)
6. SENSORY RECEPTOR
• Sensory receptor cells transduce (convert into changes in membrane potential)
incoming signals and may either depolarize or hyperpolarize in response to
the stimulus, depending on the sensory system.
• In vertebrates, each sensory system transmits signals to a different specialized
portion of the brain such as the olfactory bulb (smell) or occipital lobe (sight),
where the signal is integrated and interpreted to effect some sort of response
(often motor output) via the PNS.
• Different sensory receptor cells are specialized for different types of stimuli,
and are categorized by the type of stimulus they detect.
7. TYPES OF
SENSORY RECEPTORS
Based on the energy they transduce, sensory receptors fall into five
categories
1. Mechanoreceptors
2. Thermoreceptors
3. Chemoreceptors
4. Photoreceptors
5. Pain receptors
8. MECHANORECEPTORS
• Mechanoreceptors are receptors in
the skin and on other organs that
detect sensations of touch.
• They are called mechanoreceptors
because they are designed to detect
mechanical sensations or differences
in pressure.
9. THERMORECEPTORS
• Thermoreceptors are specialized
nerve cells that are able to detect
differences in temperature.
Temperature is a relative measure of
heat present in the environment.
• Thermoreceptors are able to detect
heat and cold and are found
throughout the skin in order to allow
sensory reception throughout the
body.
• The location and number of
thermoreceptors will determine the
sensitivity of the skin to temperature
changes.
10. CHEMORECEPTORS
• A chemoreceptor, also known
as chemosensor, is a specialized sensory
receptor cell which converts a chemical
substance and generates a biological signal.
• A chemosensor detects toxic or hazardous
chemicals in the internal or external
environment of the human body and
transmits that information to the central
nervous system, in order to expel the
biologically active toxins from the blood,
and prevent further consumption of alcohol
and/or other acutely toxic recreational
intoxicants.
11. PHOTORECEPTORS
• Special cells in the eye’s retina that are
responsible for converting light into signals
that are sent to the brain.
• Photoreceptors give us our color vision and
night vision.
12. • Rods are a type of photoreceptor cell in
the retina.
• They are sensitive to light levels and help
give us good vision in low light.
• They are concentrated in the outer areas of
the retina and give us peripheral vision.
• Rods are 500 to 1,000 times more sensitive to
light than cones.
• The retina has approximately 120 million
rods and 6 million cones.
TYPES OF PHOTORECEPTORS
CELLS
13. • Cones are a type of photoreceptor cell in
the retina.
• They give us our color vision.
• Cones are concentrated in the center of
our retina in an area called
the macula and help us see fine details.
• The retina has approximately 120
million rods and 6 million cones.
TYPES OF PHOTORECEPTORS
CELLS
14. PAIN RECEPTORS
• A nociceptor ("pain receptor") is
a sensory neuron that responds to
damaging or potentially damaging
stimuli by sending “possible
threat” signals to the spinal cord
and the brain.
• If the brain perceives the threat as
credible, it creates the sensation of
pain to direct attention to the body
part, so the threat can hopefully be
mitigated; this process is
called nociception.
16. VISION IN INVERTEBRATES
Most invertebrates
• Have some sort of light-detecting organ
One of the simplest is the eye cup of
planarians
• Which provides information about light
intensity and direction but does not form
images
Light
Light shining from
the front is detected
Photoreceptor
Visual pigment
Ocellus
Nerve to
brain
Screening
pigment
Light shining from
behind is blocked
by the screening pigment
17. • Two major types of image-forming eyes have evolved in invertebrates
• Compound eyes are found in insects and crustaceans and consist of up to
several thousand light detectors called ommatidia
• Single-lens eyes are found in some jellies, polychaetas, spiders, and many
mollusks
• It works on a camera-like principle.
18. THE VERTEBRATE VISUAL
SYSTEM
In vertebrates the human eye is
able to:
• Detect a vast variety of colors,
• Form images of objects that are
far away, and
• Respond to as little as one photo
of light.
19. STRUCTURE
OF THE EYE
• Conjunctiva, covers the surface of the
eye and lines the inner parts of the
eyelids. It functions in keeping the
eyes moist.
• Sclera, the white of the eye. It forms a
wall and maintains the shape of the
eyeball.
• Cornea, a transparent that works like
a camera lens and focuses incoming
light.
Ciliary body
Iris
Suspensory
ligament
Cornea
Pupil
Aqueous
humor
Lens
Vitreous humor
Optic disk
(blind spot)
Central artery and
vein of the retina
Optic
nerve
Fovea (center
of visual field)
Retina
Choroid
Sclera
20. STRUCTURE
OF THE EYE
• Choroid, a thin pigmented layer, that
contains major blood vessels and
functions to provide oxygen and
nutrients to the eye.
• Iris, formed from the anterior choroid
and gives the eye its color. It controls the
amount of light that enters the pupil.
• Pupil, an opening in the center of the
iris.
• Retina, inside the choroid and forms the
innermost layer of the eyeball and
contains photoreceptor cells.
Ciliary body
Iris
Suspensory
ligament
Cornea
Pupil
Aqueous
humor
Lens
Vitreous humor
Optic disk
(blind spot)
Central artery and
vein of the retina
Optic
nerve
Fovea (center
of visual field)
Retina
Choroid
Sclera
21. THE HUMAN EAR
Ear, organ of hearing and equilibrium th
at detects and
analyzes sound by transduction (or the
conversion of sound waves into
electrochemical impulses) and maintains
the sense of balance (equilibrium).
22. OUTER EAR
The outer part of the ear collects sound.
Sound travels through the auricle and the
auditory canal, a short tube that ends at the
eardrum.
• auricle (cartilage covered by skin placed on
opposite sides of the head)
• auditory canal (also called the ear canal)
• eardrum outer layer (also called the
tympanic membrane)
23. MIDDLE EAR
The primary function of the middle ear is to
efficiently transfer acoustic energy
from compression waves in air to fluid–
membrane waves within the cochlea.
• eardrum
• cavity (also called the tympanic cavity)
• ossicles (3 tiny bones that are attached)
• malleus (or hammer) – long handle
attached to the eardrum
• incus (or anvil) – the bridge bone between
the malleus and the stapes
• stapes (or stirrup) – the footplate; the
smallest bone in the body
24. INNER EAR
The inner ear has two special jobs. It changes
sound waves to electrical signals (nerve
impulses). This allows the brain to hear and
understand sounds. The inner ear is also
important for balance.
• oval window – connects the middle ear with
the inner ear
• semicircular ducts – filled with fluid;
attached to cochlea and nerves; send
information on balance and head position to
the brain
• cochlea – spiral-shaped organ of hearing;
transforms sound into signals that get sent to
the brain
• auditory tube – drains fluid from the middle
ear into the throat behind the nose
25. TASTE:
THE GUSTATORY SYSTEM
• The primary tastes detected by humans
are sweet, sour, bitter, salty and umami
(savoriness, which tends to indicate that a
food is high in protein).
• Detecting a taste relies on activation of
specific chemical receptors in taste
receptor cells (gustatory receptors).
26. TASTE BUDS
• The primary organ of taste is the taste bud.
• A taste bud is a cluster of gustatory receptors
(taste receptor cells) that are located within the
bumps on the tongue called papillae (singular:
papilla).
• Each taste bud contains all five types of
gustatory receptors, which are elongated cells
with hair-like processes called microvilli at the
tips that extend into the taste bud pore.
• Tastants must be dissolved in saliva to bind
with and stimulate the receptors on the
microvilli, which is why the sense of taste isn’t
as strong when your mouth is dry.
27. SMELL:
THE OLFACTORY SYSTEM
• Humans have about 350 olfactory receptor
subtypes that work in various
combinations to allow us to sense about
10,000 different odors.
• Olfactory receptors are responsible for the
flavor of a food, via odorants detected in
the olfactory epithelium during chewing,
through a process called retronasal
olfaction (the flow of air from the back of
the throat up to the olfactory epithelium
via the back of the nose)
28. Why do we loss our senses?
•Our brains typically organize themselves based upon
function: we have the auditory cortex for sound, the
visual cortex for sight, the olfactory bulbs for smell, and
so on. For the vast majority of us, the sensory inputs we
receive from our environment travel through the nervous
system to their respective areas in the brain. Many
people, though, are born without the ability to do things
such as see or hear.
29. But what if you lost just one sense at a
time? How would the remaining ones
respond? Would you be able to survive?
•As each sense left your body, the remaining
ones would start working to compensate for
the loss. If you lost your sight, your brain
would re-wire itself to understand your
surroundings using auditory techniques like
echolocation.
30. Aging changes in the senses
• As you age, the way your senses (hearing, vision, taste, smell, touch) give
you information about the world changes. Your senses become less sharp,
and this can make it harder for you to notice details.
• HEARING-As you age, structures inside the ear start to change and their
functions decline. Your ability to pick up sounds decreases. You may also
have problems maintaining your balance as you sit, stand, and walk.
• VISION- All of the eye structures change with aging. The cornea becomes
less sensitive, so you might not notice eye injuries. By the time you turn 60,
your pupils may decrease to about one third of the size they were when you
were 20. The pupils may react more slowly in response to darkness or bright
light. The lens becomes yellowed, less flexible, and slightly cloudy. The fat
pads supporting the eyes decrease and the eyes sink into their sockets. The
eye muscles become less able to fully rotate the eye.
31. • TASTE AND SMELL-The senses of taste and smell work together. Most
tastes are linked with odors. The sense of smell begins at the nerve endings
high in the lining of the nose.
• You have about 10,000 taste buds. Your taste buds sense sweet, salty, sour,
bitter, and umami flavors. Umami is a taste linked with foods that contain
glutamate, such as the seasoning monosodium glutamate (MSG).
• The number of taste buds decreases as you age. Each remaining taste bud
also begins to shrink. Sensitivity to the five tastes often declines after age
60. In addition, your mouth produces less saliva as you age. This can cause
dry mouth, which can affect your sense of taste.
• Your sense of smell can also diminish, especially after age 70. This may be
related to a loss of nerve endings and less mucus production in the nose.
Mucus helps odors stay in the nose long enough to be detected by the
nerve endings. It also helps clear odors from the nerve endings.
• Certain things can speed up the loss of taste and smell. These include
diseases, smoking, and exposure to harmful particles in the air.
Editor's Notes
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception.
The main function of the sensory nervous system is to inform the central nervous system about stimuli impinging on us from the outside or within us. By doing so, it informs us about any changes in the internal and external environment.
Sensory receptors are specialized epidermal cells that respond to environmental stimuli and consist of structural and support cells that produce the outward form of the receptor, and the internal neural dendrites that respond to specific stimuli.
Mechanoreceptors are a type of somatosensory receptors which relay extracellular stimulus to intracellular signal transduction through mechanically gated ion channels. The external stimuli are usually in the form of touch, pressure, stretching, sound waves, and motion.
The four major types of tactile mechanoreceptors include:
Merkel’s disks are found in the upper layers of skin near the base of the epidermis, Merkel’s disks are densely distributed in the fingertips and lips. They are slow-adapting, unencapsulated nerve endings, which respond to light touch.
Meissner’s corpuscles, also known as tactile corpuscles, are found in the upper dermis, They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter.
Deeper in the dermis, near the base, are Ruffini endings, which are also known as bulbous corpuscles. These are slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints;
Pacinian corpuscles, located deep in the dermis of both glabrous and hairy skin, They are rapidly-adapting mechanoreceptors that sense deep, transient (not prolonged) pressure, and high-frequency vibration.
a chemoreceptor detects changes in the normal environment, such as an increase in blood levels of carbon dioxide (hypercapnia) or a decrease in blood levels of oxygen (hypoxia), and transmits that information to the central nervous system which engages body responses to restore homeostasis.
Photoreceptors are image forming cells.
Photoreceptors are specialized cells for detecting light.
Two types of photoreceptors reside in the retina: cones and rods. The cones are responsible for daytime vision, while the rods respond under dark conditions.
Rods are a type of photoreceptor cell in the retina. They are sensitive to light levels and help give us good vision in low light. They are concentrated in the outer areas of the retina and give us peripheral vision.
They respond differently to light of different wavelengths, and are thus responsible for color vision, and function best in relatively bright light, as opposed to rod cells, which work better in dim light.
Pain receptors, also called nociceptors, are a group of sensory neurons with specialized nerve endings widely distributed in the skin, deep tissues (including the muscles and joints), and most of visceral organs.
Mu: Mu opioid receptors are linked to mood, pain and reward triggers. Opioids that activate the mu receptor can cause pain relief, mood changes, physical dependence and respiratory changes. Most opioid drugs function primarily as mu agonists, meaning that they activate the mu receptor.
Delta: The delta opioid receptor seems to have a connection to mood. Previous research shows that blocking the delta receptor causes anxiety and depression in mice. This could mean that the delta receptor has a role to play in regulating a person’s mood.
Kappa: Some opioid drugs also activate the kappa opioid receptors. The kappa receptor seems to affect mood and reward responses. Opioids that activate the kappa receptor also have a history of causing pain relief, dysphoria and an increase in urination.
The outer covering of the eyeball consists of a relatively tough, white layer called the sclera (or white of the eye).
Near the front of the eye, in the area protected by the eyelids, the sclera is covered by a thin, transparent membrane (conjunctiva), which runs to the edge of the cornea. The conjunctiva also covers the moist back surface of the eyelids and eyeballs.
Light enters the eye through the cornea, the clear, curved layer in front of the iris and pupil. The cornea serves as a protective covering for the front of the eye and also helps focus light on the retina at the back of the eye.
After passing through the cornea, light travels through the pupil (the black dot in the middle of the eye).
The iris—the circular, colored area of the eye that surrounds the pupil—controls the amount of light that enters the eye. The iris allows more light into the eye (enlarging or dilating the pupil) when the environment is dark and allows less light into the eye (shrinking or constricting the pupil) when the environment is bright.
The outer ear is the external part of the ear, which collects sound waves and directs them into the ear.
Once the sound waves have passed the pinna, they move two to three centimetres into the auditory canal before hitting the eardrum, also known as the tympanic membrane. The function of the ear canal is to transmit sound from the pinna to the eardrum.
The eardrum (tympanic membrane), is a membrane at the end of the auditory canal and marks the beginning of the middle ear. The eardrum is extremely sensitive and pressure from sound waves makes the eardrum vibrate. In order to protect the eardrum, the auditory canal is slightly curved making it more difficult for insects, for example, to reach the eardrum. At the same time, earwax (cerumen) in the auditory canal also helps to keep unwanted materials like dirt, dust and insects out of the ear.
The bones of the middle ear
The eardrum is very thin, measures approximately 8-10 mm in diameter and is stretched by means of small muscles. The pressure from sound waves makes the eardrum vibrate.
The vibrations are transmitted further into the ear via three bones in the middle ear: the hammer (malleus), the anvil (incus) and the stirrup (stapes). These three bones form a kind of bridge, and the stirrup, which is the last bone that sounds reach, is connected to the oval window.
three small bones (ossicles) form a chain and conduct sound vibrations from the eardrum to the inner ear. Once in the fluid-filled inner ear, sounds are converted into nerve impulses and sent to the brain.
What is the oval window? The oval window is a membrane covering the entrance to the cochlea in the inner ear. When the eardrum vibrates, the sound waves travel via the hammer and anvil to the stirrup and then on to the oval window.
Umami, or savoriness, is one of the five basic tastes. It has been described as savory and is characteristic of broths and cooked meats. People taste umami through taste receptors that typically respond to glutamates and nucleotides, which are widely present in meat broths and fermented products.
Papillae are the little bumps on the top of your tongue that help grip food while your teeth are chewing. They also have another special job — they contain your taste buds, the things that help you taste everything
Tastants are taste-provoking chemical molecules that are dissolved in ingested liquids or saliva. Tastants stimulate the sense of taste. It can also be said that tastants elicit gustatory excitation. A tastant is the appropriate ligand for receptor proteins located on the plasma membrane of taste receptor cells.
