lecture 1 Flashcards
Sensory Systems
•Senses detect and convey physical information about external and internal environment
•Leads to conscious
awareness of
environment and
environmental events
•Maintain contact
with environment
•Respond to different stimuli (eg. light, heat, sound, movement, chemicals), through specific receptors
•Provide information on amount of stimulus, duration and location
•Information coded in nerves and conveyed to brain centres
•Processing centres in the spinal cord, brainstem, thalamus and cerebral cortex
Common features of sensory systems
Senses may differ in modes of reception but all share common features:
–
Require a physical stimulus
–
Events transform stimulus into nerve impulses
–
Response of signal gives rise to perception or experience of sensation
Sensation and Perception
•Sensation Detection of physical stimulus
•Perception Interpretation modified by experience
–Detect light of different wavelengths and perceive as colour
Detect chemicals in air or water but experience smells and tastes
Detect vibrations of different amplitudes and frequencies but hear sounds, speech
Sensory System Attributes
Sensory System Attributes
•Modality (type): class of stimulus
•Location: position of stimulus in space
•Intensity: a measure of the severity or amount of the stimulus
•Timing: Onset, duration and offset of stimulus
All convey common information that can be quantitatively correlated to sensation
Modality
Modality Sensory modality determined by stimulus energy • Five major classic modalities: –Vision –Hearing –Taste –Smell –Touch • Add: –Vestibular (balance) –Additional somatosensory (pain, temperature, itch and proprioception) –Electrosensory –Magnetic
Modality process
Modality encoded in specific neural response which involves:
•Sensory receptors transform signal into electrical energy (transduction)
•Excitation leads to electrical response or receptor potential
•Receptors morphologically specialised for specific form of energy (receptor specificity)
•Specificity of response underlies labelled line code meaning that neurons from specific receptors are modality specific
Receptor class for modality
Receptor Class
•
Four classes of receptor (in humans)
–Mechanoreceptors: detect mechanical deformation
–Chemoreceptors: detect chemicals
–Thermoreceptors: detect changes in temperature
–Photoreceptors: detect light
Modality- Transduction mechanisms
•Vary according to the physical stimulus
–Mechanical stimulus acts directly on cell structures to cause receptor potential
–Chemical utilise intracellular second messenger systems or sense chemical directly (eg salt on tongue)
–Photoreceptors utilise intracellular second messenger systems
–Each responds to narrow range of stimuli to give specificity
Spatial (Location)
Spatial (Location)
•Spatial arrangement of receptors :
–provides information on location of stimulus source on body or in space
–enables discrimination of size and shape of object
–enables resolution of fine detail of stimulus or environment
•Spatial arrangement is receptive field
Intensity
Intensity
Intensity (amount) of stimulus (sensation) is determined by stimulus amplitude
•Amplitude coded by number of neurons activated and frequency (rate) of neural impulses
Timing
Timing
•
Onset, duration and offset of stimulus coded by frequency of the neural activity
Neural Pathways in Sensory Systems
Neural Pathways in Sensory Systems
•Sensory, afferent or ascending pathways carry information about a single type of stimulus
•Multisynaptic pathways to the cerebral cortex which provides complex analysis of incoming information
•Multisensory integration also occurs and improves detection and localisation
Chemical senses
- Animals depend on chemical senses: to identify food, noxious substances, suitability of mate.
- Oldest and most common sensory system (even brainless bacteria can detect and react to chemicals in the environment)
- In humans evoke very strong emotional responses
- Taste (gustation) and Smell (olfaction): sensations from interaction of molecules in air and fluid with CHEMORECEPTORS
- Smell and taste COMPLEMENT each other but systems are separate except at highest levels in the brain
Taste receptors
Taste receptors
•50-150 taste receptor cells per taste bud
•Total of 2000-5000 taste buds in tongue
•At low concentrations most papillae very selective to specific stimulus
•At high concentrations less selective
•Taste pore, sensitive part at apex
•Synapse with gustatory afferent axons at basal end
•Constant turnover of taste cells (~2 week lifespan)
Activation of taste receptors
- Activation of receptor with stimulus leads to depolarisation or hyperpolarisation
- Activation leads to release of neurotransmitter or reduction in activity
- Most taste receptor cells respond to two or more taste stimuli
Transduction mechanisms for bitterness, umami and sweetness
Bitterness, Sweetness, Umami:
•Specific G-protein receptors.
•Activate Phospholipase C and second messenger IP3
•Na channels open, depolarisation
•Voltage-gated calcium channels (internal calcium stores)
•Release neurotransmitter (serotonin, ATP?, glutamate?)
Bitterness
Bitterness:
•T2R receptors, 30 different types of T2R receptor; not possible to distinguish bitter compounds only that they are “bad”
Sweetness
Sweetness:
•T1R receptors
•Need two T1R (T1R2 and T1R3) receptors together to detect sweet compounds
Umami
Amino Acids (Umami):
•Detect some amino acids (protein eg. glutamate and aspartate
•Requires two TR, T1R1 and T1R3,
Sense of Smell (Olfaction)
Sense of Smell (Olfaction)
•Olfactory organ in upper part of nose
•Can detect about 200,000 different substances
•Smell evokes pleasant and unpleasant responses
•Also involved in communication: pheromones. Important for signalling reproductive behaviours, marking territory identification, signalling aggression, fear
Sniffing Out Social and Sexual Signals
Sniffing Out Social and Sexual Signals
•Odors can arouse emotions and affect behaviour
•Accessory olfactory system detect pheromones; mediate social and sexual behaviours.
–eg. Odors from female mouse cause male mouse to mate, but odors from other males may make him aggressive. Important for bonding mother and baby.
•Involves separate parallel olfactory system, vomeronasal organ in base of nose.
Olfactory receptors
- Each receptor specialised to one or few specific odorant molecule
- Odorant receptor genes (ORgenes) in rodents, largest mammalian gene family (900 genes in humans, 1500 in mouse)
- Olfactory organ organised into zones which express a particular subset of receptor genes
- One gene, one receptor hypothesis
- Vomeronasal organ has different gene subset (V1R, V2R genes)
