Neurobiology Flashcards
What are the basic properties of neurons?
Sensory neurons detect incoming sensory info.
Interneurons = neurons in CNS
Axons allow sensory neurons to contact the interneurons
Neurons interact at synapses
CNS interneurons synapse with motor neurons which synapse with muscles.
What type of signals do neurons need?
- Electrical signals for speed over distance + also chemicals over short distances (e.g. in synapses
What maintains the Resting potential of neurons?
inside negative relative to outside: . Lipid bilayer bridged by protein pores (ion channels)
o most let K+ through
o too small for Cl-, Ca+ (or Na2+) (more Cl- ions outside than inside)
- Salts exist as ions in aqueous solution
- Charged ions move from high to low concentration (move down conc gradient)
- Therefore K+ leaves cell through pores í K+ leaving makes inside -ve (anions remain inside).
What does the active pumping across a neurone do?
IT DOES NOT maintain resting potential
Separation of ions between inside + outside: All cells have high [K+] (400mMi, 20mM o) inside & high [Na+]outside (450Mm O; 50 mM i)
3. Active pumping maintains concentrations (2k+ into cell for each 3Na+ it pumps out í imbalance in conc + electrical charge across the membrane; also other effects that change in the LT (ATPí ADP).
What is the equilibrium potential?
where the concentration + charge of K+ or Na+ are equal so there is no net movement of ions in our out of the cell
What is the Soma?
the cell body of the neuron
What did Hodgkin + Huxley win the Nobel prize in the 1963?
They worked worked on large axons in squid + squeezed out cytoplasm and measured ion concentrations as well as making electrical recodtings
How can the electrical potential be measured across an axon?
- Axon isolated + out in physiological saline to mimic external environment of neuron
- Electrode pushed into axon which contains KCl to conduct electricity
- Electrode connected to amplifier
- This measures electrical charge on inside compared to outside
- There is a reference electrode on the outside (in the saline solution)
How are Na+ ions invlolved in the resting potential?
Na+]o > [Na+]i so Na+ moves in by concentration and charge
o Na+ would enter until inside is sufficiently (+) to balance concentration (equilibrium, can calculate with Nernst equation)
when reduce Na+ conc outside of cell, from (460 to 230) o í Resting potential becomes more negative by ~ 1 mV (sodium does contribute much to RP, so halving Na+ outside moves RP closer to Ek)
How many mechanoreceptors in a crab?
~30 000
What is a Proprioceptor?
sensory receptor which receives stimuli from within the body, especially one that responds to position and movement eg. stretch under cuticle (crab)
What is sensory adaptation?
Neural adaptation or sensory adaptation is a change over time in the responsiveness of the sensory system to a constant stimulus
How is a receptor potential created?
A receptor potential is often produced by sensory transduction. It is generally a depolarising event resulting from inward current flow (Na ions into the cell through now open ion channels)
Experimental evidence for the involvement of Na ions when generating a receptor potential
o Reduce outside concentration of Na ions: should reduce the amplitude of receptor potential (directly involves Na ions)
o Show that receptor potential is not blocked by TTX (toxin that blocks Na channels): doesn’t have effect on Receptor potential
What are ommatidia?
optical units that make up the compound eye of an insect. (eg. honey bee).
What is a rhabdom?
a translucent cylinder forming part of the light-sensitive receptor in the eye of an arthropod
What causes the release of Na ions into the reticular cells (depolarisation)?
photons are absorbed by photopigments: activates cell signalling pathway which results in opening of Na ion channels
how does the escape system work in the squid?
If a stimulus comes into animal, goes to brain + the signal travels to stellate ganglia. These signals excite impulses in giant axons that run down muscles in the mantle, this makes the muscles contract rapidly + squirt water from the mantle cavity through the siphon which propels the animal away. These giant axons are part of the escape response.
how can you measure the current across the membrane?
Voltage clamp
how did H+H see that sodium was involved in the squid action potentials ?
It was already known that if you replaced sodium outside of the cell with coline (won’t go through NA+ channels) , you reduce / block the AP. í start of AP carried by Na+. with this replacement (wne no NA+ outside): no inward current but left the outward current.
what are the 4 terms of basic H+H equations?
the leak of K+ (to a lesser extent Na+); the opening of Na+ channels; opening of K+ channels; inactivation of the Na+ channels.
- At the peak of Na current: NA+ channels stay open but are inactivated. - this inactivation has to be removed before another AP can be generated.
- Opening of channels are voltage dependent
what does TEA block?
K+ channels
What do action potentials?
APs carry signals over distance in axons without getting smaller: continually regenerated as they pass along the axon
Where are action potentials found?
- Cardiac muscle cells
- Cynops pyrrhogaster
- Mimosa pudica - some plants.
- In the skin of amphibian embryos, carrying info through the skin
How do electric organs work?
- Stacks of modified muscle cells called ELECTROPLAQUEs.
- The masses of these electroplaques are surrounded by lipid to insulate them. There are openings to the surface of the body through pores.
- The electric signals come in pulses, each lasting 2-3ms means they are like impulse activity.
- Electroplaque cells have a RP (-ve on the inside with respect to the outside), as well as voltage gated ion channels (needed for impulses), but only in the front face of each cell.
What happens when the electroplaque cells become active? (in electric organs of fish)
the front face generates an AP (inside +ve with respect to outside), but back face doesn’t so there is a potential difference across the whole cell
- RP relatively high (-84mV)
- Across the front face on the inside: +67mV
- í across the whole cell there is a potential difference of ~150mV.
- These cells are found in stacks of 5k-10k: combining them means the electroplaques can briefly generate pulses of 4-700v.
- The stacks are in parallel so the current generated is much higher than you’d get across single stacks.
How do electroplaque cells all fire the impulses simultaneously?
- For this to work, all the electroplaque cells have to fire the impulses simultaneously.
- Like other motorcells they are controlled by motorneurons, and the motorneuron input is highly synchronised and muscles generate the impulses at the same time.
what is the structure of a typical synapse?
cleft of around 20-30 nm between two neurons (pre-synaptic + post -synaptic
- In the pre-synaptic we have vesicles containing transmitter (30-100nm).
What are the evidence for synaptic transmission?
- Delay between impulse in MN + PSP (~0.7ms) í no direct electrical current flow between pre + post synaptic cell: something else was happening.
- Drug - curare í made PSP smaller + you could see the shape - therefore a chemical is involved (the drug had weakened the PSP). í now know it is Ach. (curare blocks transmission involving Ach)
- Calcium - ↓ [Ca2+] in saline around the recording í↓ PSP, therefore Ca2+ needed
- Miniature PSPs seen quite often even if there was no impulses (spontaneous): either the same size or whole number multiples of the same size: quanta release- vesicles contain packets of transmitter (quanta)
- E.M Heuser: used electron microscopy - known that you could see vesicles fusing with the pre-synaptic membrane + exocytosing to release content.
- Record pre and post - clear delay
- Calcium - Ca2+-sensitive dye or Ca2+ injection
How can you see if the vesicles fuse with pre synaptic membrane is in response to an AP?
Freeze slamming to freeze at time of impulse + FREEZE FRACTURE: split the s
split the synapse along the plane of the cleft of the synapse. í evidence of vesicles opening in the membrane at exactly the time the impulse occurred.
How can you prove calcium is involved in an AP?
a. Calcium sensitive dye into the presynaptic terminal to show there is an ↑ in calcium at the time of transmitter release + Ca2+ injection to stimulate transmitter release.
What is the function of a synapse?
Impulse arrives at nerve terminal and initiates the process of transmitter release
What is a PSP?
movement of ions through ion channels in the post synaptic membrane
steps in mechanoreception?
external mechanical energy mechanical coupling trandsuction in the sensory cells (or impulses) receptor potential depolarisation at synapse Ca2+ Transmitter release signal passed on to next neuron
What is the purpose of sensory adaptation?
Allows NS to focus on changing stimuli + avoid being swamped by unnecessary info
what are the 2 different possible ways that adaptation can be produced?
in impulse generation e.g. in sensillum campaniform
mechanical nature: - The end of the axon of this kind of cell is wrapped in layers of membrane
- And as long as that wrapping is intact you see that there is a rapidly adapting response to a sustained stimulus
what are some problems with animal sensory systems?
o Huge numbers of sensory cells
o How to get specificity in sensory signals?
o How do NS ignore irrelevant information + concentrate on important info?
what types of sensory nerve endings does mammalian skin have?
- Info carried in Unmyelinated axons e.g. free nerve endings
- some sensory endings are associated with myelinated axons (messier corpuscle)
how are three ways in which free nerve endings are excited?
” hairs in the skin, wrapped around the base- response will be amplified by movement of the hair;
“ others excited by warming of skin: max rate of impulses comes with ↑ in temp;
“ others respond to cooling: max firing rate comes with ↓ in temp. - between these two you can get good temperature discrimination in the skin)
how can human sensory nerve activity be measured?
- Tungsten electrodes inserted into the skin into nerve bundles in the arm to record responses in these sensory cells (from sensory axons)
- Stimuli applied to various parts of the periphery of the hand
What is the receptive field?
the region of the skin with that sensory cell response
why is pacini fast adapting?
only responds (fires impulses) as the stimulus is applied: this means it is fast adapting -
What are two characteristics that are used to distinguish receptor types?
adaptation rate and the receptive field
What types of coding are there?
a frequency code (stronger stimulus í higher freq of impulses) AND a population code (stronger stimulus likely to produce a response in more sensory cells).
Strength of coding: few cells responding, few impulses to many cells responding with many impulses.
why is sensation in mammalian skin is partly based on labelled lines
We can conclude for the mammalian skin that there are diff anatomical types of sensory receptors +each of the different anatomical types responds differently: carry info about diff kinds of mechanical stimuli + the sensation that results from that depends on which kind of receptor has been stimulated ( so dependent on where the stimulus has come from)
í sensation in mammalian skin is partly based on labelled lines.
what are the sensory receptors in arthropods?
they have sensilla: base with sensory neuron with axon, + dendrite which pokes into little extension of the cuticle, where you get a little hair at the surface.
- There are lots of these over the surface of insects, crustaceans etc
Why do arthropods need sensilla?
- Arthropods have a hard exoskeleton and so struggle to detect sensory stimuli
How do sensilla respond to a stimulus?
- Hair of sensillum bent í distort dendrite í opens STRETCH sensitive ion channels in dendrite membrane í Na+ into receptor cell í depolarisation í RP (lecture 2) í travels through cell to axon + if big enough will generate impulses in the axon í impulses carry info up to the CNS
- These sensilla can respond to touch + carry info on touch to the CNS
what are the different variants on the basic pattern of sensiillum?
- hair plates - groups of sensilla
- Filiform sensilla on cricket cerci
- chemoreceptor sensilla on moth antenna
- Cockroach campaniform sensillum
What are hair plates and how do they work?
on arthropods
Hair plates. - groups of sensilla
o Field of hairs that are found at joints
o signal position of body parts
o as joint moves, hairs are bent which signals the movement.
o Hairs between head and neck give sense of gravity to dancing worker bee
o Facial hair beds in locusts detect wind direction during flight
o Wing base in cockroach
What are Cockroach campaniform sensillum?
Hair has disappeared, just a dome of cuticle at the surface
Respond to stresses in the cuticle - e.g in limb when the insect puts the limb down it takes up load + these kind of receptors can detect this.
What does the escape mechanism of the cockroach from the toad involve?
specialised use of sensilla & mapping of connections
What do cockroach cerci have along the length?
long filifiorm hairs (more than 2000 sensilla on each cercus)
How are long aliform hairs involved in the escape of the cockroach?
detect the air movement produced by the toad shooting out its tongue.
o The air flow when the toad shoots its tongue has a particular pattern of air flow: the acceleration is very high and this is important. (velocity 12mms-1; acceleration 600mms-2)
How do filiform sensilla produce directional responses in the cockroach?
Freq of impulses depends on directions: strength of respond indicated by polygon.
- - the individual sensilla are responsive to air coming from particular directions.
on each segment, there are 9 sensual each of which are max responsive to air from a particular direction.
How are the filiform hairs (sensilla) arranged on the cerci of the cockroach?
each of the cerci is segmented
- -if you look at a single segment, it has 9 sensilla, each of which is max. responsive to air from a particular direction.
what are the directional sensitivities of the giant interneurons of the cockroach?
on each side 2 of them don’t show directional sensitivity: they respond generally to the stimulus
- The other five show clear directional sensitivity.
What has given the GIs of a cockroach good directional sensitivity?
Very precise mapping of sensilla onto the GI has given the interneurons a very good directional sensitivity.
The specificity of response can be produced by the accurate mapping of sensory neurons onto neurons in the CNS.
How are giant interneurons involved in controlling the escape of the cockroach?
- Stimulating these GIs individually excited leg-motor-nerve activity
- This comes about because the axons of the GIs go up the ventral nerve cord to the thoracic ganglia which control the working legs (they excite neurons in the thoracic ganglia)
- The way that the neurons in the thoracic ganglia use this info is complex but generally: if more impulses travelling up one side of the animal than the other, the animal will turn in that direction - it is the balance between impulses travelling up the two sides.
Give a summary of how the sensory receptors _ other mechanisms in the cockroach produce a response?
Specialised sensory receptors: filiform sensilla which respond to a specific stimulus (acceleration of air produced by toad’s tongue, which gives array of sensilla a directional responsiveness), Then organised, carefully mapped central connections in these neurons and those GIs distribute info to the motor system that actually produces the escape response.
what is important about the cockroach escape response?
This response doesn’t involve the brain: the signal will grow to the brain but it isn’t necessary for this response - a complex BHV being organised w/out the brain.
what is the structure of the spinal cord?
- grey matter on inside (cell bodies), dendrites of neurons + outside the white matter with axons running the length of the spinal cord.
- Dorsal (upper) = sensory; ventral= motor í dorsal horn is sensory in function; ventral mostly motor.
- Along the length of the spinal cord, segmentally there are dorsal roots in, dorsal ganglia + ventral roots out.
- The dorsal root ganglia are where there are sensory neurons coming into the SC (from skin receptors).
- At the bottom there are motor neurons with axons running out into the muscles.
what are the two types of projection neurons?
one group of which has axons that cross to the opposite side + then travel up to the brain, and the other group have axons on the same side that travel up to the brain.
What does the projection of a neuron mean?
the places of the axons + where they run
Which sensory pathway has info from sensory neurons with unmyelinated axons?
Spinothalamic pathway
what are the steps of the Spinothalamic pathway?
- Tend to carry information of noxious stimuli (pain, temp + some touch)
- These neurons synapse with the projection neurons that cross to the opposite side. + then travel up the brain í these groups of axons form the Lateral spino-thalamic tract
- Takes info up to forebrain (thalamus) + from the thalamus the info goes out to neurons into the sensory cortex (outer part of the front of the forebrain)
which sensory pathway carries info from sensory neurons with myelinated axons?
Lemniscal pathway
what are the steps / features of the Lemniscal pathway?
- Deep touch, pressure, vibration, proprioreception.
- This info goes through a pathway on the same side of the SC: these neurons connect to projection neurons with axons on the same side.
- This set of PNs runs up to structures/ ganglia/ nuclei in the brainstem called the Dorsal column nuclei. E.g. the cuneate + gracile nucleus
- At the DCN there are then synapses to neurons that again cross to the opp side + travel up to the thalamus + form a structure: the medial lemniscus.
- (eventually the pathway crosses + again the info goes to the cortex - outer part of the forebrain).
what can a shows a mapping of where the connections go in the cortex show us?
- Can see that regions of the body surface that are close together map onto regions close together in the cortex
- What we can also see is the density of connections: a lot of sensory info in humans is associated with hands + fingers; face, lips + tongue + things associated with vocalisations.
- The key message: there is huge speciity in the connections as you move from the sensory neurons to where they end up in the sensory cortex of the forebrain - think in terms of labelled lines (lecture 5).
What are the steps of parallel pathways in birds + mammals?
Info is going up to the cortex that can then control motor responses.
in a similar way to the cr, the are important connections at all the levels between sensory info coming in + the motor responses: doesn’t all happen at the cortex.
During the course of evo: progressively higher layers have been added in. this has meant that veterbates like us have conscious perception from stimuli
Why are higher centres not always the most important thing in a NS?
with the brain removed frogs can still wipe off a piece of filter paper soaked in vinegar –> o í lower centres in the NS can still control sophisticated behavioural responses.
- Now it is known that other vertebrates like turtles can do the same kind of thing.
what are the receptor cells in the lateral line system and what are their structure?
The receptor hair cells have cilia at the surface embedded in a jelly structure = the cupula which acts like a lever. There is two types of cilia at the top of the hair cells: shorter, stereocilia + a single large kinocilium
how do the cilia in the lateral line system detect movement?
Movement of the cupula + cilia by currents /vibrations sets of the transduction process (this is the receptor organ of the LLS): Movement of these cilia in different directions excites or inhibits activity of the hair cell and ↑ or ↓ release of transmitter from the synapse at the base.
This transmitter release is onto a sensory neuron: an afferent neuron (taking signals to the brain)
How do the cilia in the lateral line system excite the afferent neurons?
If something bends the cupula towards the kinocilium, this ↑ transmitter from the hair cell which excites impulse activity in the sensory neuron so see an ↑ in freq.
If the cupula moves away from the kinocilium, the transmitter release ↓ which ↓ / stops firing in the sensory nerve.
This is an important basic principle: stimuli of different directions can ↑ OR ↓ the response in the sensory nerve. This is because it has a resting rate of firing.
what are the different structures of the lateral line system?
free neuromast organ: simplest; hair cells + jelly cupula are at the skin surface and the skin is slightly raised.
Pit organ - some teleost fish: cupula structure is sunk down into a pit in the body surface
Canal organs - most extreme: hair cells in groups of canals below the surface that connect to the surface by holes (openings). - these are found in cartilaginous fish (sharks, rays, skates).
How do animals with a lateral line system prevent self-stimulation of the nervous system?
efferent nerves are inhibitory: inhibit activity of the hair cells, and become active when the animal is very active so when the animals move they don’t respond to their own movement.
what are some of the roles of the lateral line system?
- Allow aquatic vertebrates (fish, some amphibians) to detect objects in water, to orient their bodies in weak currents, helps with schooling behaviour
- Allows small fish + amphibian tadpoles to detect predators + avoid predation
- Can help some detect prey (e.g. insects at the water surface which produce ripples)
maybe cichlids can asses status of others with the LLS
How was it found that dog fish use electro-receptors to detect fish?
when a flatfish was placed in sand the dog fish could find it, and still in a jelly box so no signals from movement.
but with a polythene sheet to insulate electrical activity, the dogfish couldn’t detect.
dogfish found electrodes mimicking the flatfish electrical signals –> use electrical signals to locate prey
what is he structure of the ampullary system?
there is an array of canals contains under the skin with openings at the surface + at the ends they connect to nerves leading to the brain. the nerves lead to the ampullae of lorenzini at the base of the structures.
how does the ampullary system detect electrical activity?
- Canals contain jelly that is conductive to electrical signals like sea water: electrical signals at the surface can travel through this jelly medium
- The wall is resistive (insulator) í electrical current has to flow along the canal
- í electrical signals go through the canals
- In the wall of the ampulla, you find modified hair cells (this system has same origin of LLS); hair cells are the receptors and have a single cilium + their release of transmitter depends on electrical signals being detected by the hair cell (they detect the electrical current flowing through the canal).
- The detection of electrical signals controls release of transmitter from the base of the hair cell onto an afferent nerve (a sensory neuron)
How do impulses from the electro-receptor change depending on the stimulus
there is a resting discharge in the afferent nerve (when there are no volts there is a firing rate of about 40Hz - 40 impulses /s), this ↑ or ↓ in size depending on the size and the polarity of the electrical current detected (the electrical stimulus)
How do weakly electric fish use electrical signals?
- they generate long lasting, continuous trains of pulses (~500Hz), the timing of the pulses is controlled by pacemaker cells: control firing in motor neurons that drive high freq of pulses in muscles to generate high freq pulses of electrical signals in the electric organs.
- have pores at the front + back that act like the poles of a battery: these high freq electrical signals can get out of the body + generate an electric field around the fish.
- Then they detect this electrical field using electroreceptors
