m echanisms of p erception : h earing, t ouch, s mell, t aste & a ttention
TRANSCRIPT
5 exteroceptive sensory systems1. Visual2. Auditory (hearing)3. Somatosensory (touch)4. Olfactory (smell)5. Gustatory (taste)
Somatosensations: sensations from your body
3 separate but interacting systems1. Exteroceptive system
Senses external stimuli interacting with the skin
2. Proprioceptive system Monitors body position Receptors in the muscles, joints & organs of
balance
3. Interoceptive system General info on the internal body conditions
Ex: temp, BP
EXTEROCEPTIVE SYSTEM
3 distinct divisions for perceiving different types of stimuli1. Mechanical (touch)2. Thermal (temperature)3. Nociceptive (pain)
CUTANEOUS RECEPTORS Receptors in the skin; many types Free nerve endings
Simplest; neuron endings with no specialized structures Sensitive to temperature change & pain
Pacinian corpuscles Largest & deepest Adapt rapidly Respond to sudden displacements of skin, not constant
pressure Merkel’s disks
Adapt slowly Respond to gradual skin indentation
Ruffini endings 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
DERMATOMES
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
Info about touch & proprioception
2. Anterolateral system Info about pain & temperature
However, there is overlap in the type of info each pathway carries
1. Dorsal-column medial-lemniscus system
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!
2. Anterolateral system 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 The primary somatosensory cortex is
located on the postcentral gyrus 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
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 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
These senses respond to chemicals in our environment Smell for airborne chemicals Taste for those that dissolve in our oral cavity
Smell & taste are highly integrated 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 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 The 5 traditional tastes1. Sweet2. Salty3. Sour4. Bitter5. 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
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