Introduction to Physiological Psychology Psych 260
Kim Sweeney [email protected] cogsci.ucsd.edu/~ksweeney/psy260eve.html
Today… n Gustation and Olfaction n Somatosensation
1 Gustation
Different modalities are encoded by different receptor types
Somatosensation
Proprioception
Olfaction
Gustation
Audition
Vision
2 Gustation n Taste receptors are scattered around surface of the tongue in clusters (taste buds) – NB- this means high convergence at very lowest level!
Taste buds and papillae
n Average person has ~5,000 taste buds, but exceptional individuals may have 500… or 20,000! (supertasters) n An individual taste receptor w/in a taste bud lives ~2 weeks
3 Gustation n 4 primary tastes (sweet, sour, salty, bitter) n 5th is umami, meat or savory (MSG)
?
Gustation
n Transduction of taste is similar to the chemical transmission that takes place at synapses.
n The tasted molecule binds with the receptor and produces changes in membrane permeability that cause receptor potentials.
n Different substances bind with different types of receptors, producing different taste sensations.
4 Receptors n Two different receptors are responsible for detection of sweet tastes. n Bitterness is detected by members of a family of about thirty different receptors. n The existence of so many different bitterness receptors suggests that although different bitter compounds share a common taste quality, they are detected by different means.
Gustation salt n Most taste
receptors (~90%) sweet respond to at least 2 basic tastes… sour
bitter
Ogawa et al., 1968
5 Gustation n If a given receptor does not respond exclusively to one kind of taste… n A given gustatory axon doesn’t either.
Sucrose Salt Sour Bitter
Gustation n We respond to many tastes that can not be created by combining primaries … – So how do we distinguish between chocolate and banana… and cilantro?
6 Population coding!
The neural pathways of taste
7 The neural pathways of taste
/(Solitary Nucleaus)
VPM nucleus (thalamus) GUSTATORY,
Top-down processing matters n Culture n Current experience – Information from other sensory modalities n Past experience
8 Top-down processing matters
Peter Menzel, photography
Taste and Smell work together n Try eating a jelly bean while holding your nose!
9 Smell and Vision work together
Brochet, 2001
Taste and Sound work together!
Spence et al., 2009
10 What could possibly go wrong? n Ageusia – inability to taste – Rare, because multiple pathways carrying taste information… but can occur after stroke or tumor damage to VPM thalamus or gustatory cortex
The Chemical Senses: Smell and Taste
n Gustation (taste) – Responds to chemicals in the mouth n Olfaction (smell) – Detects airborne chemicals n Food acts on both systems to produce flavor!
11 Olfaction- Broadly Speaking
n Odorants enter through the nose, hit olfactory receptors, which have axons that enter the olfactory bulbs. n From the olfactory bulb, the olfactory tract projects to many, many brain areas, including amygdala and piriform cortex.
Remember: Olfaction is the only one of our senses to have direct access to the brain, without going through the thalamus first!
But… it makes it to the thalamus eventually.
12 Olfactory bulb
n Receives sensory information from olfactory receptor cells n Sends information to olfactory (piriform) cortex (among other places) n Also receives top-down information from cortex, amygdala, hippocampus
Olfaction n Receptors are embedded in the olfactory mucosa of the nose – ~40,000,000 receptors in humans, – ~2,000,000,000 in a German Shephard!
13 Olfactory receptor cells n Like auditory receptor cells, they terminate in cilia
n Transduction occurs when an odorant binds to the cilia
n 6- Olfactory receptor cells n 5- Glomeruli n 4- Olfactory mucosa n 3- Cribriform plate n 2- (mitral cells) n 1- Olfactory Bulb
n Glomeruli each contain many axons (~2,000!) from olfactory receptors… but any given glomerulus receives input from only one kind of receptor!
14 Another way of looking at it n There is high convergence: – Many receptor neurons converge onto few glomeruli (~150:1) – Many glomeruli converge onto a single neuron of the olfactory tract (~25:1) n This convergence increases the sensitivity of the olfactory signal sent to the brain!
Olfaction n In humans there are (only!) several hundred different olfactory receptors n How can a (relatively) small amount of receptors lead to such a vast array of smells? – A particular odorant binds to more than one receptor, thus different odorants produce different patterns of activity in different glomeruli
15 The human olfactory system
Vomeronasal organ
Pheromones n Chemicals that influence that behavior of conspecifics (members of the same species) n Evidence of human pheromones – Changes in olfactory sensitivity across and menstrual cycle – Synchronization of menstrual cycles – Sex identification by smell (especially by women… and healthy mixes preferred) – Men can identify menstrual stage by smell
16 What can possibly go wrong? n Anosmia- the inability to perceive smells – A strong blow to the head can sever those axons that pass through the cribriform plate! – Old age is also often accompanied by a decreased ability to smell n Olfactory agnosia- inability to identify smells n Olfactory hallucinations n … among other things!
The cutaneous senses n What kinds of things can we feel with our skin? n Pressure, vibration, heat, cold, pain
17 Transduction! n In the visual system, rods and cones transduce light to neural signals… n In the auditory system, hair cells transduce sound waves to neural signals… n In the olfactory and gustatory systems, receptors transduce chemicals into neural signals n In the somatosensory system, sensory neurons in skin, joints, organs transduce pressure (or heat, or location, or…) into neural signals…
Receptor cells transduce stimuli into neural activity
Stimulus
Receptor Cell
Response
18 Sensing and Moving/Responding
n The two most basic nervous system functions! – Even single cell organisms can sense nutrients and toxins, and respond by moving towards or away n Complex organisms have specialized cells and systems to carry out these functions – our simplest but most crucial behaviors involve reflexive responses to sensory stimuli § pain reflex to avoid damage § increased breathing stimulated by blood CO2
Sensation and Perception n Generally speaking, the brain is sensitive to to change – when there is no change, no sensation
19 Somatosensation: Touch and Pain ` n Somatosensory system is three separate and interacting systems:
– Exteroceptive – external stimuli
– Proprioceptive and kinesthetic – body position and motion
– Interoceptive – body conditions (e.g., temperature and blood pressure)
Somatosensation: Touch and Pain n Exteroceptive System- specialized receptors respond to various stimuli
– Touch (mechanical stimuli) – Temperature (thermal stimuli) – Pain (nociceptive stimuli)
20 A sensory stimulus presents different kinds of information n Modality – What kind? n Intensity – How much? n Duration – How long? n Location – Where?
Your skin n Skin is the largest, heaviest organ of the body n Humans have both hairy and glabrous (hairless) skin.
21 Hairy and Glabrous n Hairy skin – Free nerve endings –painful stimuli, changes in temperature – Ruffini corpuscles –indentation of skin – Pacinian corpuscles –rapid vibrations – (Hair Follicles) n Glabrous skin – Free nerve endings, Ruffini and Pacinian corpuscles – Meissner’s corpuscles – stroking, fluttering, small bumps – Merkel’s disk – compression
Touch and Pain
Five cutaneous receptors that occur in the human skin
22 Somatosensation: Touch and Pain n Cutaneous Receptors:
– Free nerve endings unmyelinated § temperature and pain – Merkel’s disks § gradual skin indentation – Meissner corpuscles § low frequency vibration – Pacinian corpuscles myelinated § sudden displacements of the skin – Ruffini endings § gradual skin stretch
23 Somatosensory Receptors n Deep receptors: large receptive fields (big patches of skin); - low spatial resolution. n Superficial receptors: small receptive fields (small patches of skin). – high spatial resolution: e.g. for reading Braille.
Deep Receptors n (subcutaneous) (large RF): – Pacinian corpuscles: § Rapidly adapting § responds to rapid indentation of skin; vibration; motion (of object across skin).
– Ruffini endings: § Slowly adapting § sense stretch of skin or bending of fingernails: this compresses the nerve endings. Perception of shape of objects.
24 Different receptors adapt at different rates
Rapid Slow Very Rapid Slow
Shallow receptors n (in superficial layers of skin) (small RF): – Meissner’s corpuscles: § Rapidly adapting § stroking, fluttering; detecting small bumps
– Merkel disks: § Slowly adapting § give sustained responses to skin compression; pressure
25 Different receptors adapt at different rates
Rapid Slow Very Rapid Slow
Different receptors adapt at different rates
Rapid
Slow
Slow
Very Rapid
26 Receptive fields and adaptation of mechanoreceptors
The intensity of a stimulus is transmitted by the frequency of the neural response Maximum response (Saturation)
Sigmoidal response function/curve Firing Rate Rate Firing
Threshold
Stimulus Intensity
27 Characteristics of sensory receptors in the skin
Receptor Stimulus Sensation Adaptation Location Steady Merkel's disk Pressure Slow Shallow indentation Meissner's Low frequency Gentle fluttering Rapid Shallow corpuscle vibration
Ruffini's Rapid Stretch Slow Deep corpuscle indentation
Pacinian Vibration Vibration Rapid Deep corpuscle
Hair receptor Hair deflection Brushing Rapid or Slow Shallow
In the human hand…
n Meissner’s - 40% of mechanoreceptors – Low frequency vibrations, movement of textures across skin n Pacinian - ~15% of mechanoreceptors – Sense of vibration, discrimination of textures n Ruffini’s - ~20% of mechanoreceptors – Cutaneous stretching n Merkel’s disks- 25% of mechanoreceptors – Abundant in finger tips, lips, genitalia – Feelings of light pressure
28 The Pacinian Corpuscle is a rapidly adapting receptor cell
Capsule
Rebound
on off
The capsule of the Pacinian corpuscle is responsible for its rapid adaptation
Rebound
Ruffini corpuscles and Merkel’s disks don’t have capsules, so are continuously responsive!
29 The Pacinian Corpuscle responds to vibrations on the skin
Two point discrimination
30 Lateral inhibition refines the receptive field of higher-order sensory cells
Lateral inhibition improves precision in higher-order sensory cells
31 Lateral inhibition improves discrimination
Neurons in the somatosensory cortex have center-on, surround-off receptive fields… of different sizes
32 How do the neural messages get to the brain?
Broadly Speaking…
Sensory information travels through the spinal cord and thalamus to the sensory cortex.
33 Somatosensory Cortex
Central Sulcus
Cortical Areas of Somatosensation n Primary somatosensory cortex (SI) – Postcentral gyrus – Somatotopic organization (somatosensory homunculus) – more sensitive, more cortex – Input largely contralateral n SII – mainly input from SI – Somatotopic; input from both sides of the body n Much of the output from SI and SII goes to association cortex in posterior parietal lobe
34 Sensory Homunclulus
n Receptive fields in the somatosensory cortex comprise a somatotopic map of the body
The “homunculus” reflects the density of receptor cells
35 Neural plasticity
Merzenich et al.
Neural plasticity
Merzenich et al.
36