Chapters 8 and11: Nonvisual Sensory Systems

Sensory Systems

• The brain detects events in the external environment and directs the corresponding behavior– Afferent neurons carry sensory messages to brain– Efferent neurons carry motor messages to the muscles

• Stimulus: any energy capable of exciting a receptor. This defines broad categories of sensory systems

– Mechanical– Chemical– Thermal– Photic

Introduction

• Sensory Systems– What is the energy and how is it

transduced?– What is the pathway to brain?– What common features does it share with

other sensory systems?

Introduction: Chemoreceptors (gustatory and olfaction)

• Animals depend on the chemical senses to identify nourishment

• Chemical sensation– Oldest and most common sensory system

• Chemical senses– Gustation– Olfaction– Chemoreceptors

Taste

• The Basics Tastes– Saltiness, sourness, sweetness, bitterness, and

umami– Examples of correspondence between chemistry

• Sweet—sugars like fructose, sucrose, artificial sweeteners (saccharin and aspartame)

• Bitter—ions like K+ and Mg2+, quinine, and caffeine– Advantage – Survival

• Poisonous substances - often bitter

Taste

• The Basic Tastes– Steps to distinguish the countless unique

flavors of a food• Each food activates a different combination of

taste receptors• Distinctive smell• Other sensory modalities

Taste• The Organs of Taste

– Tongue, mouth, palate, pharynx, and epiglottis

Taste

• The Organs of Taste– Areas of sensitivity on the tongue

• Tip of the tongue– Sweetness

• Back of the tongue– Bitterness

• Sides of tongues– Saltiness and sourness

Taste

• The Organs of Tastes– Taste receptors

– Threshold concentration• Just enough exposure of single papilla to

detect taste

Taste• Tastes Receptor Cells

– Apical ends Microvilli Taste pore– Receptor potential: Voltage shift

Taste• Taste Receptor Cells

Taste

• Mechanisms of Taste Transduction– Transduction process

• Taste stimuli (tastants)– Pass directly through ion channels– Bind to and block ion channels– Bind to G-protein-coupled receptors

Taste

• Mechanisms of Taste Transduction– Saltiness

• Salt-sensitive taste cells– Special Na+ selective

channel

• Blocked by the drug amiloride

Taste

• Mechanisms of Taste Transduction– Sourness

• Sourness- acidity – low pH• Protons causative agents of

acidity and sourness

Taste

• Mechanisms of Taste Transduction– Bitterness

• Families of taste receptor genes - TIR and T2R

Taste

• Mechanisms of Taste Transduction– Sweetness

• Sweet tastants natural and artificial• Sweet receptors

– T1R2+T1R3– Expressed in different taste cells

Taste

• Mechanisms of Taste Transduction– Umami

• Umami receptors:– Detect amino acids– T1R1+T1R3

Taste

• Central Taste Pathways– Gustatory nucleus

• Point where taste axons bundle and synapse

– Ventral posterior medial nucleus (VPM)• Deals with sensory information from the head

– Primary gustatory cortex• Receives axons from VPM taste neurons

Taste

• Central Taste Pathways (Cont’d)– Localized lesions

• Ageusia- the loss of taste perception

– Gustation• Important to the control of feeding and

digestion– Hypothalamus – Basal telencephalon

Taste

• The Neural Coding of Taste– Labeled line hypothesis

• Individual taste receptor cells for each stimuli

• In reality, neurons broadly tuned• Population coding

– Roughly labeled lines– Temperature– Textural features of food

Difference between smell and pheromones?

Smell (Olfactory)— a mode of communication–Important signals

• Reproductive behavior• Mate Selection• Territorial boundaries• Identification• Aggression

–Pheromones• Mate Selection• Territorial boundaries• Identification• Aggression

–Role of human pheromones

Pheromone Actions in Animals

• Pheromones are chemicals that transmit a message from one animal to another– Pheromones can alter reproduction

• Lee-Boot effect: the estrous cycle stops when groups of female mice are housed together

• Whitten effect: the estrous cycle restarts in synchrony when a group of female mice are exposed to the urine of a male mouse

• Bruce effect: involves the failure of pregnancy when a recently impregnated mouse is exposed to a normal male mouse (other than the one with which she mated)

– The vomeronasal organ detects nonvolatile chemicals in urine• The vomeronasal organ projects to the accessory olfactory bulb which in turn

projects to the amygdala which has connections with the hypothalamus• Lesions of the accessory olfactory bulb disrupt the Lee-Boot, Whitten and Bruce

effects

Pheromone Actions in Humans

• Humans possess a vomeronasal organ• Exposure to chemical present in sweat can alter human

behavior– McClintock studied the menstrual cycles of women who attended

an all-female college• Women who spent time together showed synchronization of their menstrual

cycles• Women who spent time with men showed shorter cycles• Exposure to underarm sweat elicited synchronization

– Pheromones present in human sweat can alter social behavior• Androstenol placed on a necklace had no effect on the social interactions of

men, but women exposed to androstenol showed more interactions with me

Smell• The Organs of Smell

– Olfactory epithelium• Olfactory receptor cells, supporting cells, and

basal cells

Smell

• The Organs of Smell– Odorants: Activate transduction processes in

neurons– Olfactory axons constitute olfactory nerve– Cribriform plate: A thin sheet of bone through

which small clusters of axons penetrate, coursing to the olfactory bulb

– Anosmia: Inability to smell– Humans: Weak smellers

• Due to small surface area of olfactory epithelium

Smell• Olfactory Receptor Neurons

– Olfactory Transduction

• Olfactory Signal– Adaptation: Decreased

response despite continuous stimulus

Smell

Central Olfactory Pathways

Smell

Central Olfactory Pathways (Cont’d)

Smell

• Central Olfactory Pathways (Cont’d) – Axons of the olfactory tract: Branch and

enter the forebrain– Neocortex: Reached by a pathway that

synapses in the medial dorsal nucleus

Smell

• Spatial and Temporal Representations of Olfactory Information– Olfactory Population

Coding– Olfactory Maps

(sensory maps)– Temporal Coding in

the Olfactory System

• Olfaction paradox

Smell

Concluding Remarks• Transduction mechanisms

– Gustation and olfaction

• Similar to the signaling systems used in every cell of the body

• Common sensory principles - broadly tuned cells– Population coding – Sensory maps in brain

• Timing of action potentials– May represent sensory information in ways not yet

understood

The Nature of Sound

• Sound– Audible variations in air pressure– Sound frequency: Number of cycles per second

expressed in units called Hertz (Hz)– Cycle: Distance between successive compressed

patches– Range: 20 Hz to 20,000 Hz– Pitch: High and Low– Intensity: Difference in pressure between

compressed and rarefied patches of air

The Structure of the Auditory System

• Auditory System

The Structure of the Auditory System

• Auditory pathway stages – Sound waves– Tympanic membrane– Ossicles– Oval window– Cochlea fluid– Sensory neuron response

• Brain stem nuclei output– Thalamus to MGN

• Components of the Middle Ear

The Middle Ear

Tonotopic Mapping

• Sound Force Amplification by the Ossicles– Pressure: Force by surface area– Greater pressure at oval window than tympanic

membrane, moves fluids

• The Attenuation Reflex– Response where onset of loud sound causes

tensor tympani and stapedius muscle contraction– Function: Adapt ear to loud sounds, understand

speech better

The Middle Ear

• Physiology of the Cochlea– Pressure at oval window, pushes perilymph into

scala vestibuli, round window membrane bulges out

• The Response of Basilar Membrane to Sound– Structural properties: Wider at apex, stiffness

decreases from base to apex

• Research: Georg von Békésy– Endolymph movement bends basilar membrane

near base, wave moves towards apex

The Inner Ear

• The Organ of Corti and Associated Structures

The Inner Ear

• Transduction by Hair Cells– Research: A.J.

Hudspeth.– Sound: Basilar

membrane upward, reticular lamina up and stereocilia bends outward

The Inner Ear

Central Auditory Processes

Auditory Pathway

• Primary pathway: Ventral cochlear nucleus to superior olive to inferior colliculus to MGN to auditory cortex

• Principles in Study of Auditory Cortex– Tonotopy, columnar organization of cells with

similar binaural interaction

• The Effects of Auditory Cortical Lesions and Ablation– Lesion in auditory cortex: Normal auditory function

(like vision?)– Lesion in striate cortex: Complete blindness in one

visual hemifield– Different frequency band information: Parallel

processing, localization deficit

Auditory Cortex

Concluding Remarks

• Hearing and Balance – Auditory Parallels Visual System

• Tonotopy (auditory) and Retinotopy (visual) preserved from sensory cells to cortex code

– Convergence of inputs from lower levels Neurons at higher levels have more complex responses

Chapter 12: The Somatic Sensory System

Introduction

• Somatic Sensation– Enables body to feel, ache, chill– Sensitive to stimuli– Responsible for feeling of touch and pain– Somatic sensory system: Different from

other systems• Receptors: Distributed throughout• Responds to different kinds of stimuli

Touch

• Types and layers of skin– Hairy and glabrous (hairless)– Epidermis (outer) and dermis (inner)

• Functions of skin– Protective function– Prevents evaporation of body fluids– Provides direct contact with world

• Mechanoreceptors– Most somatosensory receptors are

mechanoreceptors

Touch• Mechanoreceptors (Cont’d)

– Pacinian corpuscles– Ruffini's endings– Meissner's corpuscles – Merkel's disks– Krause end bulbs

Touch

• Mechanoreceptors (Cont’d)– Receptive field size and adaptation rate

Touch• Mechanoreceptors (Cont’d)

– Two-point discrimination• Importance of fingertips over elbow

• Primary Afferent Axons– AC– C fibers mediate pain and temperature– Amediates touch sensations

The Four Classes of Sensory Axons Differ in Size and Speed

Touch

• The Spinal cord– Spinal segments (30)- spinal nerves within

4 divisions of spinal cord– Dermatomes- 1-to-1 correspondence with

segments• Shingles

Touch

• The Spinal cord (Cont’d)– Sensory

Organization of the spinal cord

• Divisions– Cervical (C)– Thoracic (T)– Lumbar (L)– Sacral (S)

Touch

• Spinal cord (Cont’d)– Sensory

Organization of the spinal cord

• Division of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn

– Myelinated A axons (touch-sensitive)

Touch

• Dorsal Column–Medial Lemniscal Pathway– Touch information ascends through dorsal

column, dorsal nuclei, medial lemniscus, and ventral posterior nucleus to primary somatosensory cortex

• The Trigeminal Touch Pathway– Trigeminal nerves– Cranial nerves

Dorsal Column- Medial Lemniscal Pathway

Touch

• Somatosensory Cortex– Primary– Other areas

• Postcentral gyrus• Posterior Parietal

Cortex

Touch

• Somatosensory Cortex (Cont’d)– Brodmann’s Area 3b (or S1): Primary

somatosensory cortex• Receives dense input from VP nucleus of the

thalamus• Neurons: Responsive to stimuli• Lesions impair somatic sensations• Electrical stimulation evokes sensory

experiences

Touch

• Somatosensory Cortex– Cortical Somatotopy

• Homunculus• Importance of mouth

– Tactile sensations: Important for speech– Lips and tongue: Last line of defense

Touch• Somatosensory Cortex (Cont’d)

– S1: Rat• Vibrissae• “Barrel cortex”

Touch• Somatosensory Cortex (Cont’d)

– S1 – Owl monkey

Touch

• Somatosensory Cortex (Cont’d)– Cortical Map Plasticity– Remove digits or overstimulate – examine

somatotopy before and after• Conclusions of experiments

– Reorganization of cortical maps» Dynamic» Adjust depending on the amount of sensory

experience

Touch

• Somatosensory Cortex (Cont’d) – The Posterior Parietal Cortex

• Involved in somatic sensation, visual stimuli, and movement planning

• Agnosia• Astereoagnosia • Neglect syndrome

Pain

• Nociceptors• Pain and nociception

– Pain - feeling of sore, aching, throbbing – Nociception - sensory process, provides signals

that trigger pain

• Nociceptors: Transduction of Pain – Bradykinin – Mast cell activation: Release of histamine

Pain

• Nociception and the Transduction of Painful Stimuli (Cont’d)– Types of Nociceptors

• Polymodal nocireceptors, Mechanical nocireceptors, Thermal nocireceptors

– Hyperalgeia• Primary and secondary hyperalgesia• Bradykinin, prostaglandins, and substance P

Pain

• Primary Afferents and Spinal mechanisms– First pain and second pain– Referred pain: Angina

Pain

• Ascending Pain Pathways– Differences between touch and pain

pathway• Nerve endings in the skin• Diameter of axons• Connections in spinal cord

– Touch – Ascends Ipsilaterally– Pain – Ascends Contralaterally

Pain• Ascending Pain Pathways (Cont’d)

Pain

• The Regulation of Pain– Afferent Regulation– Descending Regulation– The endogenuos opiates

• Opioids and endomorphins

Pain

• The Regulation of Pain (Cont’d)– Descending regulation

Concluding Remarks

• Sensory systems exhibit similar organization and function

• Sensory represented in cortex• Repeated theme

– Parallel processing of information

• Perception of object involves the seamless coordination of somatic sensory information

End of Presentation