Unit 5: Touch and the Chemical Senses

Proprioception

Perception of the body mediated by kinaesthetic and vestibular receptors

Somatosensation

A collective term used for all sensory signals from the body plus the vestibular system

Skin

Largest sensory organ

Touch Receptors

Embedded in the outer layer (epidermis) and underlying layer (dermis) of the skin

Types of Tactile Receptors

1. Meissner Corpuscles

FA I

Fast adapting type I, small receptive field

FA II

Fast adapting type II, large receptive field

SA I

Slow adapting type I, small receptive field

SA II

Slow adapting type II, large receptive field

Kinesthetic receptors

Play important role in sense of where limbs are, what kinds of movements are made

Spindles

Convey the rate at which the muscle fibres are changing in length

Type Ia sensory fibre

Adapts, signals change in stretch, connects to spindles

Type II sensory fiber

Does not adapt, signals amount of stretch (position of the muscle fibres)

Ian Waterman

Got a viral infection that destroyed the Kinesthetic mechanoreceptors

• unable to walk but learned to walk again, dependent of vision for the limb position

• Cutaneous nerves that connected his mechanoreceptors to brain was destroyed

Thermoreceptors

Sensory receptors that signal information about changes in skin temperature

Nociceptors

Sensory receptors that transmit information about noxious (painful) stimulation yay causes damage or potential damage to the skin

Analgesia

Decreasing pain sensation during conscious experience

Gate Control Theory

System that transmits pain that incorporated modulating signals from the brain

• feedback circuit located in substantial gelatinosa of the dorsal horn of the spinal cord

• Gate neurons that block pain transmission can be activated by extreme pressure, cold, other noxious stimulation applied to another site distant from source of pain

Hyperalgesia

Once damage has occurred, site can become more sensitive; increase in pain

Neuropathic Pain

Pain as a result of damage to or dysfunction of the nervous system

“Nociceptive” Pain

Nociceptors provide signal when there is impending or ongoing damage to body’s tissue

Anterior Cingulate Cortex

Generally carries the signal if something wrong, can be pain or error in a cognitive task

Secondary pain effect

Emotional response associated with long-term suffering (associated with the prefrontal cortex)

Discriminative Touch

The classic touch sensations of tactile, thermal, pain, and itch experiences

Social Touch

• mediated by unmyelinated peripheral C fibers — C tactile afferents

• Respond best to slowly moving, lightly applied forced

• Processed in the orbitofrontal cortex

Spinothalamic

• slower, evolutionary older

• Heat, pain, temperature, crude touch

• Multiple synapses

Dorsal-column-medial-lemniscal

• faster

• Tactile and proprioceptive information

• Fewer synapses- fast transmission

Spinothamic Pathway

Dorsal Horn of spinal cord —> lateral spinothalamic tract —> connection in ventral posterior nucleus of the thalamius —> travels to cerebral cortex

• several synapses

• Provides mechanisms for pain inhibition

DCML Pathway

Dorsal Horn of the spinal cord —> connection in the gracile nucleus (on the cuneate nucleus) —> travels to medial leminscus —> thalamus —> somatosensory cortex

• wider axons

• Fewer synapses

• Used for planning and execution of fast movements

Primary Somatosensory Cortex (S1)

• on the post central sulcus

• Adjacent on the pre central sulcus are the corresponding motor areas

Phantom Limb

Perceived sensation from a physically amputated limb of the body

• real pain might be felt if they perceived their phantom limbs to be in uncomfortable positions

• Can be felt in area adjacent to the amputated limb (eg. Sensations of hand felt on arm)

Max von Frey

Developed an elegant way to measure mechanical pressure

• used calibrated stimuli: horse and human hairs

Modern Research of Mechanical Pressure

Use nylon monofilaments of varying diameters

• use hair to poke on your lips vs thighs or upper arm or sole of feet

Two-point touch threshold

The minimum distance at which two stimuli are just perceived as separate

Grating Orientation

Participants must tell if the grating orientation when pressed on a certain body area

Deciding Temporal Details

• method to decide whether two events are simultaneous or sequential

• Typically 5ms (if they are both apart)

• Better than vision (25 ms) but worse than audition (0.01 ms)

Haptic Perception

Knowledge of the world that is derived from sensory receptors in skin, muscles, tendons and joints, usually interaction with eh environment

Lateral Motion

Back and forth motion, find out the texture of the object

Pressure

Press finger into an object to finger out the hardness

Static Contact

Finger on object to determine the temperature

Unsupported Holding

Hold the object in our hand to discover the weight

Enclosure

Enclosing the object with our hands to discover shape, volume

Contour Following

Moving hands around the object to determine the contour and its global shape/exact shape

What system (ventral)

• material properties of objects are crucial for haptic recognition

• 2D pictures or objects are recognized easily visually but poorly haptically

Braille Alphabet

• dot optimized for touch perception

• Discrimination between letters in the dot patterns

Jack Loomis

Touch acts like blurred vision when the fingertips explore a raised pattern

• visual stimuli blurred to match the acuity of fingertip skin

• Visual stimuli and haptic stimuli showed the same confusion errors

• The accuracy of touch feels like blurry vision

Tactile Agnosia

Inability to identify objects by touch

• caused by lesion to the parietal lobe (where S1 and S2 are)

• Patient by Reed and Caselli

  • Could not recognize weight and roughness of either hand

  • Lacked a connection b tween sensation and recognition systems on the impaired side: unable to integrate perceived properties about objects

Where system (parietal)

• knowing where objects are in the environment when only using touch perception

• Frame of reference

• Egocenter

Frame of reference

The coordinate system used to define locations in space

Egocenter

The centre of a reference frame used to represent all locations relative to the body (where you are and how you encode locations of objects relative to your body)

Body Image

The impression or our body in space

• our body is highly changeable

• Possible to induce an out-of-body experience

Tacoma Method

Speech perception for deaf and blind people

• thumb on speaker’s lips, fingers along jawline, littler finger feels vibrations of the throat

• Invented by Sophie Alcorn

Olfaction

The sense of smell

Gustation

The sense of taste

Odorant

Any specific aromantic chemical

• chemical compounds

• Must be volatile, small, and hydrophobic

Olfactory Cleft

A narrow space at the back of the noise into which air flows, where the main olfactory epithelium is located

Olfactory Epithelium

A secretory mucosa in the humans nose whose primary function is to detect odourants in the inspired air; the “retina” of the nose

Supporting cells

Provide metabolic and physical support from the olfactory sensory neurons

Basal Cells

Precursor cells to olfactory sensory neurons

Olfactory Sensory neurons (OSNs)

The main cell type in the olfactory epithelium

• located beneath a watery mucous layer in ther olfactory epithelium

• Have 5-10 million

Cribriform Plate

Bony structure riddled with tiny holes, at the level of the eyebrows, that separates the nose from the brain. Axons of OSNs pass through the tiny holes to enter the brain

Cilia

Hairlike protrusions on the dendrites of OSNs, contain receptor sites for odourant molecules

Olfactory Receptor (OR)

Region on the cilia of OSNs where odorant molecules bind

• takes 7-8 odorous molecules binding to a receptor to initiate an action potential

• Takes about 40 events to perceive an odor

Olfactory Sensory Neuron Measured

• pipette inside the cell

• Records the membrane potential of the cell

• Reliever an odourant through a pipette to the cilia of the olfactory sensory neurons

• Record action potential and realization when it occurs

Olfactory Nerves

First pair of cranial nerves, formed by the axons of the olfactory sensory neurons that bundle together after passing from the cribiform plate to form the olfactory nerve

Olfactory Bulb

Blueberry-sized extension of the brain just above the nose, where olfactory info is first processes

• two olfactory bulbs one in each brain hemisphere

• Connections are ipsilateral unlike vision, hearing, or touch

Mitral cells

Main projective output neurons in the olfactory bulbs

• axons in these cells carry the information further into the brain into the primary olfactory cortex and other brain structures

Glomeruli

Spherical conglomerates containing the incoming axons of the OSNs

• axons for the olfactory sensory neurons make first synapse here

• Many of them

Primary Olfactory Cortex

Neural area where olfactory information is first processed, which includes the amygdala, parahippocampal gyrus and interconnected areas, and also entorhinal cortex

Entorhinal Cortex

Phylogenetically old cortical region that provides the major sensory association input into the hippocampus. Also receives direct projections from olfactory regions

Lambic System

• Direct and intimate connection with the olfaction system

• Involved many aspects of emotion and memory

• Why sense tends to have a strong emotional sensation association

Sensation vs Perception

• Sensation occurs when scent is neurally registered

• Perception occurred when becoming aware of sensations several 100 ms later (lot of time)

Trigeminal Nerve

Cranial nerve V, sensations of the scent mediated

Shape-pattern Theory

Different scents activate different arrays of olfactory receptors in the olfactory epithelium as a function of odorant-shape to OR-shape fit

Combinatorial Neural Code

Detect the pattern of activity across various receptor toes

• intensity of odorant also a changes which receptors will be activities

• Specific time order of activations of the activation of the olfactory receptors is also important

Binaral Rivalry

Competition between the two nostrils for odor perception

• when a diff scent is presented to each nostril, we experience one scent at a time, not a combination

• Similar to binocular rivalry

Zhou et al.

Binocular rivalry stimulus of markers in one eye and and a rose in the other eye

• when exposed to smell of marker of rose

• timing was now bias to the binocular rivalry

• Subjects saw corresponding stimulus more often

Odor Imagery

Humans have little to no ability to conjure odoro “images”

• cannot think in smells well

• Do not imagine smells well— dreams with it are very rare

• Other animals whom smell plays a more central role may do better than humans

Amosmia

Totally inability to smell, most often resulting from sinus illness or head trauma

• lead to depression, loss of libido

• Early symptom of Covid-19 (55%)

Congenital anosmia

Children often pretend to be able to smell

Specific Anosmia

Inability to smell one specific compound amid otherwise normal smell perception

Aromatherapy

Contention that doors can influence, improve, and alter mood, performance, and well-being, as well as the physiological correlates of emotion such as heart rate, blood pressure, and sleep.

• can make you feel energized

• No evidence for pharmacological effect of odors in humans

Vomeronasal organ (VNO)

Chemical sensing organ at the base of the nasal cavity of many animals with a curved tubular shape.

• evolved to detect chemicals that cannot be processed by the olfactory epithelium such as large and/or aqueous molecules, types of molecules that constitutes pheromones

Pheromones

Signals for chemical communication between members of the same species that do not need to have any smell

Retronasal olfactory sensation

Sensation of an odor that is perceived when chewing and swallowing force an odorant in the mounts up behind the palate into the nose

• odor sensations are perceived as originating from the mount, even though the actual contact of the odorant and receptor occurs at the olfactory mucosa

Flavor

The combo of true taste (sweet, slaty, dour, bitter, umami) and retronasal olfaction

Chords tympani

Blanch of the cranial nerve VII (7th, facial nerve) that carries taste information from the anterior, mobile tongue

Tastant

Any stimulus that can be tasted

• some can be made up of small, charged particles of salty or sour

• Other tastants are perceived via G protein-couples receptors (GPCRs) similar to that in the olfactory system, molecules taste sweet, bitter or savoury

Taste buds

Create neural signals conveyed to brain by taste nerves

• embedded in structures of papillae

• Arch taste bud contains taste receptor cells

• Info is sent to brain via cranial nerves

Microvilli

Slender projections on the tips of some taste bud cells tat extend into the taste pore

• contain the sites that bind to taste substances

• Not tiny hairs, extensions of the cell membrane

• Taste receptors cells

Insular cortex

Primary cortical processing area for taste, the part of the cortex that first receives taste information

Orbitofrontal cortex

The part of the frontal lobe of the cortex that lies above the bone (orbit) containing the eyes

• receives projections from insular cortex

• Involved in processing of temperature, touch, smell, and taste, suggesting it may be an integration area

Arthur Fox

Discovered that phenylthiocarbamide (PTC) tastes dramatically different to different people

• bitter taste to some but not others

• Started using PROP (propylthioracil) bc its safer

  • individuals with two recessive genes are non tasters of PTC/PROP

  • Individuals with one or more of the genes are tasters of PTC/PROP

Supertaster

Individual who is a taster of PTC/PROP and has a high density of fungiform papillae

Specific hungers theory

Idea that deficiency of a given nutrient produces craving (a specific hunger) for that nutrient

• cravings for salty or sweet are associated with deficiencies in those substances

• Theory has not been supported for other nutrients

• Only holds for sweet and salty foods

Orthonasal olfaction

Olfaction through the nostrils