Mechanisms of Perception: Hearing, Touch, Smell, Taste & Attention

MECHANISMS OF PERCEPTION: HEARING, TOUCH, SMELL, TASTE & ATTENTION

5 exteroceptive sensory systems1. Visual2. Auditory (hearing)3. Somatosensory (touch)4. Olfactory (smell)5. Gustatory (taste)

SOMATOSENSORY SYSTEM

Somatosensations: sensations from your body

3 separate but interacting systems1. Exteroceptive system

Senses external stimuli interacting with the skin2. Proprioceptive system

Monitors body position Receptors in the muscles, joints & organs of

balance3. 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 PATHWAYS1. 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

PG. 176 & 177

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

THE CHEMICAL SENSES: SMELL & TASTE

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

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