Sensory Biology Flashcards
what is the mind-body problem?
how do mental properties (thoughts and feelings), arise from physical properties (biochemical and electric signals)
In order for an organism to detect and respond to these physical stimuli, what must they be?
Collected (at the boundary between the external environment and the internal environment (the cell membrane)
Transduced into messages within the organism, which it can respond to (chemical and electrical signals)
why is Fluidity is of significance in sensory systems?
because proteins are shuttled in the plane of the membrane to relay signals (e.g. G-protein signal transduction)
what do Most membrane-bound proteins?
project all the way through the bilayer from the extracellular to the intracellular space (these are called integral proteins)
what does the intramembranous domain constist?
alpha-helical segments, of mostly hydrophobic amino acid residues
what is fundamental to sensory systems?
Conformational changes in membrane proteins.
what does a stimulus at a biomembrane result in?
a protein changing shape (initial detection of the stimulus), which results in a membrane potential (transduction of the signal)
o This process allows the organism to sense and respond to the stimulus
What is an overview of how receptor proteins allow cellular responses to occur?
o Receptor protein í activation signal í effector molecule
o Release of second messenger í cellular response
What is the most important type of receptor molecule? What is it
the 7TM (seven transmembrane) receptor. They make seven passes through the membrane, therefore have seven transmembrane domains. - Large superfamily of proteins: 2% of the mammalian genome codes for them
What do the intracellular loops of GPCRs provide?
recognition surfaces for specific G-proteins.
what is desensitisation and what part of GPCRs are involved?
occurs when a receptor is overexposed to its agonist and becomes unresponsive. phosphorylation sites on the c-terminal are involved in desensitisation
when is sensitivity of a GPCR restored after sensory adaptation?
o Sensitivity is restored when the tail is dephosphorylated again by phosphatase enzymes in the cytosol.
what type of activity does the α subunit of GPCRs have? What does this mean?
GTPase activity - its attached GTP is soon hydrolysed to GDP, switching the mechanism off again by changing its conformation back to the deactivated form.
What happens after the effector molecule is activated?
Classes of effector molecules include cyclase enzymes, phospholipases, phosphodiesterases and membrane channels. o They give rise to the second messengers including cyclic-AMP, inositol triphosphate (IP3), diacyglycerol (DAG) and the Ca2+ ion. Adenylyl cyclases (ACs) catalyse the formation of the second messenger cAMP.
Activation of the effector protein phospholipase C-β, results in the formation of two second messengers
what does A stimulated GPCR results in the activation OF?
membrane-embedded PLC-β (which is the effector molecule), which reacts with PIP2, to produce the second messengers IP3 and DAG
WHAT DOES DAG DO?
DAG activates PKC when the Ca2+ concentration of the cytosol rises
what are the different types of channels in a membrane and how are they activated?
Some channels are activated directly by environmental change, others are activated by binding of ligands (ligand-gated ion channels LGICs), others by a change in voltage across the membrane (voltage-gated ion channels VGICs).
…result in a receptor potential
what is the difference between the tertiary + quaternary structure of TRP channels?
The tertiary structure involves six transmembrane domains (s1-s4 and s5 and s6)
The quaternary structure consists of four of these 6TM subunits, grouped around a central pore
what happens When a ligand attaches to binding sites on the channel?
the channel opens and cations flow along their gradients. Selectivity filters (charged amino acids lining the pore of the channel) can select the ions that pass.
How are voltage gated ion channels activated?
by changes in membrane potential, and they differ in the ions that they allow to pass (K+, Cl-, Ca2+ and Na+). o Any change in potential gradient, results in conformational change in membrane bound voltage-sensitive proteins.
What causes the inactivation of the voltage gated ion channels; NA+ channels?
o The inactivation is due to the intracellular segment between domains III and IV blocking the channel.
o The three conformations of this channel are: closed, open and inactivated
why does Electric current flows between these two points. in the membrane?
The cytoplasm and the extracellular fluid are ionic solutions ( they conduct electric currents).
o Depolarising the membrane at a certain point creates a voltage difference between that point and the membrane a small distance away.
what happens when the sensitive ending of the neurosensory cell is stimulated?
depolarisation of the membrane occurs, which spreads by a local circuit, until it reaches a region of the cell membrane populated by the voltage-gated Na+ channels
- If the depolarisation reaches a threshold value, the Na+ channels are opened, and an action potential is initiated
- The action potential propagates to the central nervous system (CNS).
- Because the initial depolarisation does not occur in a separate cell, it is sometimes called a generator potential.
what are the are different biophysical causes for adaptation in different systems?
o E.g. it is a result of the methylation of receptor transducer proteins in bacterial chemosensitivity
o In animal G-protein systems it is a result of phosphorylation which occurs on the cytosolic C terminus
Why is the The refractory period of great importance in sensory signalling?
because it restricts the frequency at which impulses can travel down a sensory fibre - there can be no further action potentials while the Na+ gates remain closed.
How does the diameter of the nerve fibre determine the rate of impulse propagation?
- As the diameter increases, the local circuits can spread further along the axoplasm, and open Na+ gates further away from the active region.
- Some invertebrates such as annelid worms and cephalopod molluscs have developed giant fibres, which allow rapid conduction in emergency reactions.
What do taste buds consist of?
consist of several receptor cells with
specialized microvilli located in
taste pores
Microvilli detect dissolved chemicals, leading to the activation of receptor cells.
Taste buds are arranged in papillae
why is taste a gate-keeper?
Function: test food before ingestion
Perceived through taste buds in oral cavity
Chemical has to be water soluble
Basic ‘primary’ tastes in humans
What is a chemical stimulus?
Every chemical compound - extremely diverse Inorganic salts Other ions, including metal contact receptors - taste Organic based on carbon extremely diverse communication
What are the Mechanisms of primary tastes?
Each taste cell has specific receptors in its microvilli
Bitter, sweet and umami have specific receptor proteins (GPCRS)
sour H+ ions block ion channels
salty Na+ ions through Na+ gated ion channels
what are Tasters, nontasters and supertasters?
Tasters and nontasters - inherited ability to taste certain bitter compounds
Percentage differs between ethnic groups and sexes
Supertasters - more sensitive to a wide range of oral stimuli
Higher density of fungiform papillae
What are the two different olfactory systems in mammals?
All mammals have the standard main olfactory epithelium (MOE)
Receptor cells in the MOE connect to glomeruli in the main olfactory bulb (MOB)
Vomeronasal (Jacobson’s) organ is a second system
Mainly for pheromones
Oral cavity
Links to accessory olfactory bulb
Flehming
Ungulates, felids and other mammals
How is olfaction detected?
Fluid covering olfactory epithelium ‘catches’ volatile molecules
Interact with receptor proteins in ciliary segment of receptor cells
Generate electric response in receptor cells
Receptor cells project to olfactory bulb
Glomeruli collect/sort responses from similar receptors
Axel 2005 (Nobel lecture)
What is Olfactory adaptation
Sense of smell (like other senses) is a change detector
Receptor adaptation: after continuous exposure to an odorant the receptors stop responding to the odorant and detection ceases (biochemical)
Example: Walking into a bakery and can only smell fresh bread for a few minutes.
Cognitive habituation: The psychological/neuronal process by which, after long-term exposure to an odorant, one is no longer able to detect that odorant
Example: Going away, coming back and noticing how your house smells
What are Qualities of chemical communication?
Directionality: In contrast to light and sound, direction of particle ≠ direction to source
Speed: very slow, milliseconds for sound, minutes to days for chemical stimuli
Temporal pattern: lost within a short distance from sender
Summary: not great, hence evolution of other senses
what is flavour?
combination of the sense of taste (mediated by the specialised taste receptor cells in the mouth), olfaction (olfactory input through the back way into your nose, via the mouth), and the somatosensory perception of the food or drink - what it feels like, or its consistency
what are the basic primary tastes in humans?
o Salty (organic salts, e.g. NaCl table salt) o Sour (acids, e.g. vinegar) o Sweet (carbohydrates and amino acids, e.g. glucose) o Bitter (alkaloids, often poisonous, e.g. quinine) o Umami "delicious" (amino acids, L-glutamate and aspartate) o "Metallic" (status unclear)
What is electroreception?
“Electroreception is the biological ability to perceive natural electrical stimuli.”
What is a dipole and do humans have one?
Equal but opposing charges e.g. a positive charge, with an opposing negative charge, separated in space.
Yes: Cell membranes are fantastic insulators : They separate charges on either side of the membrane, allowing electropotential gradients to form - which is the whole basis of receptor potentials, action potentials, and all neuronal activity.
What is electroreception used for ?
o Absence of sufficient light - can replace vision: if light absent animals have diff sensory modalities to find out (Animals often rely on electroreception when there is not enough light - it helps to replace vision)
Electroreception can be used for:
o Electrolocation (detecting, identifying and localising objects)
o Electrocommunication
What animals is electroreception found in?
Find electrolreception in:
o Nocturnal animals
o animals living in conditions with low visibility like murky waters and deep sea,
o can be used to penetrate into a substrate to find buried prey
o (find hidden prey buried under a substrate) E.g. hammerhead shark finds prey buried in the sand
What are the 2 types of electroreception?
a) Passive electroreception - e.g. in hammerhead sharks
o Pick up existing natural electrical stimuli
o Just electroreceptive sensors needed - The sensory equipment is all that is needed- they are only sensing electric fields from other organisms
b) Active electroreception. - generate a weak electrical signal: electrogenesis (creating your own electric field)
o Electroreceptive sensing organs are still needed to sense the electric fields
o But they also need an organ to create an electric signal
o These organisms generate, and then pick back up their electric signals with electroreceptive sensors
o They pick up changes in their self-generated electric fields, which occur by the presence or absence of objects in the field -
o E.g. Electric eel
o (This is a similar concept to echolocation, but with electric fields)
Who uses electroreception?
o Most non-teleosts
o Agnatha, Elasmobranchii, Holocephali, Chondrostei, Polypteri, Dipnoi
o Some teleosts:
o Siluriformes (catfishes)
o Gymnotiformes (knifefishes)
o Mormyriforms (elephant nose fishes)
o Xenomystinae (african knifefishes)
o Amphibian larvae
o Platypus & Echidnas (Pettigrew 1999)
o Guiana dolphin (Czech-Damal et al. 2012)
Receptor physiology - how the receptors work?
o Current flows into cell body following field lines
o Electroreceptor cells do not have their own axon, they make a synapse with an underlying sensory neuron
o If the membrane potential of the cell changes, it will be transmitted through the synapse, and be turned into action potentials in the afferent sensory fibre
o In electroreceptors, a change in the external field causes a change in the membrane potential
o There are leak channels in the bottom and the top of the cell, so any external charge following an electric field line can move freely through the cell
o By entering the cell, a charge causes the polarity of the cell inside to change E.g. if a negative charge is following an external field line, the cell becomes more negatively charged when it is inside, or less negatively (more positively) charged when it moves out of the cell again
o The receptor potential of the cell can therefore change (in response to current) because of this charges that are following external field lines
o So an external field induces a current through the cell í This changes the receptor potential of the cell –> Receptor connected with synapse to afferent fibre –> turned into action potentials by the afferent nerve fibre –> postsynaptic action potentials in afferent fibre
Electroreceptors in lampreys?
o Electroreceptive organs in the lamprey are grouped in epidermal end buds (3-30 sensor cells per end bud)
o Microvilli at the top of the receptor cells contain the leak channels, which allow the electric current to pass through
o Leak channels are essential for this mechanism to work, because the charges need to be able to flow through the cells
o Afferent fibres make synapses with the receptor cells underwater
Electroreceptor in Cartilaginous fishes (Chondrichthyes)?
The electroreceptive organs in a skate are in the form of lines of dark pores on the ventral side near the mouth
o Ampullae of Lorenzini
o mucus filled ducts, with an opening or pore to the external and a sensory receptor at the base
