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• Senses are the physiological capacities
within organisms that provide inputs for perception.
• Human beings have a multitude of senses, the five
major are the senses of sight (ophthalmoception),
hearing (audioception), taste (gustaoception), smell
(olfacoception or olfacception), and touch
(tactioception)
• other senses include temperature (thermoception),
kinesthetic sense (proprioception), pain
(nociception), balance (equilibrioception) and
acceleration (kinesthesioception).

• General senses are those with receptors
distributed over a large part of the body.
• The general senses are widely distributed
throughout the body and include the senses
of touch, pressure, pain, temperature,
vibration, itch, and kinesthetic sense
(proprioception).
• They are divided into 2 groups, the somatic
senses and the visceral senses.
• Many of the general senses are associated
with the skin and others are associated with
the deeper structures.

Somatic senses

• Somatic senses provide sensory
information about the body and the
environment.

Visceral senses

• Visceral senses provide information
about the various internal organs.
• They consist primarily of pain and
pressure.

• These are sensory nerve
endings or specialized cells
that are capable of
responding to stimuli by
developing action potentials.
• These are associated with
both special and general
senses.

• Mechanoreceptors- respond to
mechanical stimuli such as bending or
stretching of the receptors.
• Chemoreceptors- respond to chemicals
such as odor molecules.
• Photoreceptors- respond to light.
• Thermoreceptors- respond to changes
in temperature.
• Nociceptors- respond to stimuli that
result in the sensation of pain.

• Free nerve endings- relatively unspecialized
neuronal branches similar to dendrites. Free
nerve endings are distributed throughout the
body .
• There are different type
of free nerve endings
and each responds to
different stimuli such as
painful stimuli, itch,
temperature and
movement.

• Touch receptors- structurally more
complex then free nerve endings and
many of them are enclosed in capsules

• Merkel’s disks- superficial nerve
endings in detecting light touch
and superficial pressure.
• Hair follicle receptors- also
involved detecting light touch.
- Light touch receptors are very
sensitive although they are not
very discriminative making it
difficult to locate the point of
touch.

• Meissner’s corpuscles- located deep in the
epidermis, they are very specific in localizing
tactile sensations.
• Pacinian corpuscles –
the deepest receptors
associated with
tendons and joints,
relays information
concerning deep
vibration, pressure, and
position
(proprioception).

• Ruffini’s end organs-
deeper tactile receptors,
plays an important role in
detecting continuous
pressure in the skin

• Pain is a sensation characterized by a
group of unpleasant perceptual and
emotional experiences.
• The two type of pain are:
– Sharp, well localized, pricking or cutting
pain resulting from rapidly conducted
action potentials.
– Diffused, burning, or aching pain resulting
from action potentials that are propagated
more slowly.

• Superficial pain- are sensations in
the skin that are highly localized as
a result of simultaneous stimulation
of pain receptors and the tactile
receptors.
• Deep or visceral pain- sensations
not highly localized because of the
absence of tactile receptors in the
deeper structures so it is normally
perceived as diffused pain.
- Tactile receptors- helps localize the
source of pain stimuli.

• Local anesthesia- suppressing pain by
injecting chemical anesthetics near a
sensory receptor or nerve. This results
in reduced pain sensation.
• General anesthesia- suppressing pain
by producing a loss of consciousness
and can be accomplished by chemical
anesthetics affecting the reticular
formation.

• Referred pain is a painful sensation perceived
to originate in a region of the body that is not
the source of the pain.
• Referred pain is sensed in the skin or other
superficial structures when deeper structures
such as internal organs are damaged or
inflamed.
• This occurs because sensory neurons from the
superficial area to which pain is referred and
the neurons from the deeper visceral area
where the pain originates converge onto the
same ascending neurons in the spinal chord.

• The senses of smell, taste, sight,
hearing, and balance are associated
with very specialized local sensory
receptors
• They are localized to specific parts of
the body.

• Occurs in response to airborne
molecules called odorants that
enter the nasal cavity.
• Olfactory neurons are bipolar
neurons within the olfactory
epithelium lining the superior
part of the nasal cavity.
• The dendrites of the olfactory neurons extend
to the epithelial surface of the nasal cavity and
their ends are modified to bulbous
enlargements that posses long specialized
cilia.

• The cilia of the olfactory cells lie in a thin
mucus film on the epithelial surface. The
mucus plays an important role in olfaction.
• Mucus- keeps the
nasal epithelium
moist, traps and
dissolves airborne
molecules and
facilitates removal of
molecules and
particles from the
nasal epithelium.

• Airborne odorants come in contact with the
mucus on the surface of the epithelium, dissolve
and bind to the receptor molecules on the
membranes of the specialized. Once the
dissolved odorant comes in contact receptors, it
causes olfactory neurons to depolarize.

• The threshold for detection
of odors is very low so very
few odorants bound to an
olfactory neuron can
initiate an action potential.
• Once an odor molecule is
bound to the receptor, the
receptor does not respond
to another odor molecule
for some time.

• Odorants come in contact
with the cilia and pass
through the olfactory
receptor cells through the
cribriform plate, to
olfactory bulbs, then the
signal travels through
olfactory tracts and finally,
it terminates in the
olfactory cortex.

• It stated once a odorant molecule is
bound to a receptor cell, it does no
function so prolonged exposure to a
given odorant will disable the receptor
cells receiving stimuli from the smell
resulting in a person adapting to a
specific odor.
• Once a person adapts to an odor he or
she no longer smells the odor or the
effect of the odor is lessened

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