MECHANISMS OF PERCEPTION:
HEARING, TOUCH, SMELL,
TASTE & ATTENTION
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1.
2.
3.
4.
5.
5 exteroceptive sensory systems
Visual
Auditory (hearing)
Somatosensory (touch)
Olfactory (smell)
Gustatory (taste)
SOMATOSENSORY SYSTEM
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
1.
Somatosensations: sensations from your body
3 separate but interacting systems
Exteroceptive system
Senses external stimuli interacting with the skin
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2.
Proprioceptive system
Monitors body position
Receptors in the muscles, joints & organs of balance
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3.
Interoceptive system
General info on the internal body conditions
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Ex: temp, BP
EXTEROCEPTIVE SYSTEM
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3 distinct divisions for perceiving different types
of stimuli
1.
2.
3.
Mechanical (touch)
Thermal (temperature)
Nociceptive (pain)
CUTANEOUS RECEPTORS
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
Receptors in the skin; many types
Free nerve endings
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Pacinian corpuscles
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Largest & deepest
Adapt rapidly
Respond to sudden displacements of skin, not constant
pressure
Merkel’s disks
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
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Simplest; neuron endings with no specialized structures
Sensitive to temperature change & pain
Adapt slowly
Respond to gradual skin indentation
Ruffini endings
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
Adapt slowly
Respond to gradual skin stretch
CUTANEOUS RECEPTORS

When constant pressure is applied to the skin,
there is a burst of firing in all of the receptors,
corresponding to the sensation of touch.
But after a bit, only the slowly adapting receptors
stay active & the sensation changes (often becoming
unnoticeable)
 So to maintain constant input, you move &
manipulate objects in your hands
 Stereognosis: identification of objects by touch

CUTANEOUS RECEPTORS
Each type has its own unique structure, but they
all basically work the same way
 Stimuli to the skin changes the chemistry of the
receptor, which changes the permeability of the
receptor cell membrane to ions, which sends a
neural signal
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DERMATOMES
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
Nerve fibers from cutaneous receptors come
together and enter the spinal cord at the doral
root
The area of the body
innervated by the
left & right dorsal
root at a given
spinal segment is a
dermatome
2 MAJOR SOMATOSENSORY PATHWAYS
1.
Dorsal-column medial-lemniscus system

2.
Anterolateral system
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
Info about touch & proprioception
Info about pain & temperature
However, there is overlap in the type of info
each pathway carries
PG. 176 & 177
1.
Dorsal-column medial-lemniscus system


2.
Anterolateral system
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
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Ipsilateral, decussates at the dorsal column nuclei,
contralateral
Neurons of this path that start in the toes are the
longest neurons in the human body!
Spinothalamic tract
Neurons decussate immediately upon entering the
spinal cord & travel up contralaterally
If both paths are cut by a spinal cord injury,
there will be no sensation from below that point.
CORTICAL AREAS OF SOMATOSENSATION
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The primary somatosensory cortex is located on
the postcentral gyrus
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Most input is contralateral
It is organized somatotopically; according to a
map of the body surface

Referred to as the homunculus (“little man”)
*my fave!*
CORTICAL AREAS OF SOMATOSENSATION
Secondary somatosensory cortex is just ventral to
the primary
 Association cortex is in the posterior parietal lobe
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SOMATOSENSORY AGNOSIAS

Astereognosia
Inability to recognize objects by touch
 Rare


Asomatognosia
Inability to recognize parts of your own body
 Usually only affects the left side of the body after
damage to the right posterior parietal lobe

PERCEPTION OF PAIN
Pain is the response to any kind of harmful
stimulation
 Serves as a warning system
 There is no clear cortical area involved in pain


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Although the anterior cingulate cortex is
activated during the emotional reaction to physical
pain
Amazingly, we can exhibit a lot of control over
our perception of pain

Gate-control theory: descending cognitive signals
from the brain can activate neural gate circuits in the
spinal cord to block incoming pain signals
DESCENDING PAIN-CONTROL CIRCUIT

Activity in the periaqueductal gray has
analgesic (pain blocking) effects
Also has specialized receptors for opioids, including
endorphins
 Potentially involves stimulation of serotonergic
neurons

NEUROPATHIC PAIN
Severe chronic pain in the absence of a
recognizable pain stimulus
 Often after an injury has healed & there should
be no more reason for pain
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THE CHEMICAL SENSES: SMELL
& TASTE
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These senses respond to chemicals in our
environment
Smell for airborne chemicals
 Taste for those that dissolve in our oral cavity
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Smell & taste are highly integrated
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Together they produce what we know as flavor
We use these senses primarily to recognize
flavor, but many other species use it for
communication, via pheromones
OLFACTORY SYSTEM: SMELL
Receptor cells are in the upper part of your nose,
within the olfactory mucosa
 The axons of these neurons actually project
through the cribriform plate in your skull &
enter the olfactory bulbs, which go via the
olfactory tracts to the brain
 Your olfactory receptor neurons can be
regenerated throughout your life
 Primary olfactory cortex: piriform cortex


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Medial temporal cortex next to the amygdala
Only sensory system that does not first go
through the thalamus!!
GUSTATORY SYSTEM: TASTE

Taste receptors are on the tongue & elsewhere in
the oral cavity
Occur in clusters of 50 called taste buds
 So each taste bud sends out many axons and many
individual neural signals


1.
2.
3.
4.
5.
The 5 traditional tastes
Sweet
Salty
Sour
Bitter
Umami

But not every taste we experience can be made from
any combo of those 5…
GUSTATORY PATHWAY

Afferent neurons leave the mouth as the facial,
glossopharyngeal & vagus cranial nerves; which
terminate in the solitary nucleus of the medulla,
to the ventral posterior nucleus of the thalamus,
to the primary gustatory cortex

Primary cortex: near the face area of the
somatosensory homunculus
DAMAGE TO THE CHEMICAL SENSES

Anosmia: inability to smell
Caused by blows to the head that rip the olfactory
nerves as they pass through the cribriform plate
 Symptom along with several other neurological
disorders


Ageusia: inability to taste

Rare
SELECTIVE ATTENTION
SELECTIVE ATTENTION
At any instance, we are receiving a LOT more
sensory input than we consciously perceive
 Selective attention is the process by which we
are able to “ignore” the rest
 Works to essentially enhance the signals that we
are focusing on & get rid of interference caused
by the signals we’re ignoring
 Cocktail-party phenomenon
 Change blindness
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