topic 8 Flashcards
resting potential
1) Na+/K+ pump creates concentration gradient across the membrane
2) K+ diffuses out the cell down the K+ concentration gradient, outside the membrane = positive, inside the membrane = negative
3) potential difference will pull K+ back into the cell
4) -70mV potential difference, they counteract each other & there is no net movement
depolarisation
- neurone stimulated
- change in PD causes a change in of the Na+ gate, opening some VDGC
- sodium ions flow in due to Na+ concentration gradient = depolarisation of the membrane, build up of positive charges
- positive feedback as more depolarisation causes more channels to open
- PD of membrane reaches +40mV
repolarisation
- after about 0.5ms the Na+ VDGCs spontaneously close, Na+ permeability returns to normal
- depolarisation causes K+ channels to open, potassium moves out of the axon down the EC gradient
- cell becomes more negative inside than outside
hyperpolarisation
- membrane highly permeable to potassium ions, move move out of the cell then at resting potential
- more negative than resting
re-establishment of resting potential
closing of the K+ VD channels & potassium ion diffusion into the axon
how is the impulse passed along the axon?
as part of the membrane becomes depolarised, a local electric current os created as the charged sodium ions flow between the depolarised region and the adjacent resting region.
-> action potential triggered in adjacent region
=wave of depolarisation
what is the refractory period?
a new action potential cannot be generated in the same section of membrane fir about 5 milliseconds
-> ensures impulses only travel in one direction (until all sodium and potassium channels have closed & resting potential is restored)
what does the size of stimulus affect?
- frequency of impulses
- number of neurones in a nerve that are conducting impulses
the … the diameter, the … the impulse travels
wider
faster
what does the myelin sheath do?
acts as an electrical insulator along most of the axon, preventing ion flow across the membrane
what are Nodes of Ranvier?
gaps in the myelin sheath at regular intervals & are the only place that depolarisation can occur
-> ions flow across the membrane at one node during depolarisation, a circuit is set up which reduces the PD of the membrane at the next node, triggering an action potential
what is saltatory conduction?
impulse jumping from one nod to the next
-> has a higher impulse velocity
what is a synapse
where 2 neurones meet
what is the synaptic cleft
gap between the cells
what was the first neurotransmitter to be discovered?
acetylcholine
what happens at a synapse?
1) action potential arrives and depolarisation occurs
2) calcium ion channels open, increased permeability to calcium, calcium ions enter the neurone
3) increased calcium concentration causes synaptic vesicles containing the neurotransmitter fuse with the presynaptic membrane, releasing it into the cleft via exocytosis
4) neurotransmitter binds to specific receptor proteins w/ specific binding site to the neurotransmitter
5) neurotransmitter binds, changing shape of the protein, opening cation channels & making the membrane permeable to Na+
6) causes depolarisation & an action potential in the post synaptic neurone
What happens at the post synaptic neurone
- The neurotransmitter binds to a specific receptor protein that has a complementary site to the neurotransmitter
- Neurotransmitter binds, changing the shape of the receptor, opening cation channels
- membrane permeable to sodium ions, the flow in and depolarisation occurs
- action potential produced
What happens to neurotransmitters afterwards?
Reuptaken or broken down
What are the roles of synapses?
To control nerve pathways
Integration of information for a coordinated response
What factors affect whether the post synaptic membrane will depolarise?
- type of synapse
- number of impulses received
What do excitatory synapses do?
- Make the post synaptic membrane more permeable to sodium ions
- One isn’t enough to trigger depolarisation
- each impulse adds to the effect of the other (summation)
What is spatial summation?
Impulses from several different neurones produce an action potential in the post synaptic neurone
What is temporal summation?
Several impulse s along one neurone produce an action potential in the postsynaptic neurone
What do inhibitory synapses do?
Make it less likely that an action potential will result in the postsynaptic cell
-> the neurotransmitter from these synapses opens channels for chloride and potassium ions, which move down their diffusion gradients.
-> chloride move in (-charge), potassium moves out (+charge)
= greater PD across the membrane (-90mV)
=subsequent depolarisation is less likely, more excitatory synapses required to depolarise the membrane
What is nervous control like?
electrical transmission fast short term changes action potentials to neurones to specific cells local
Hormonal control
chemicals in the blood slower long term hormone to all cells, only target cells respond widespread
What are chemoreceptors stimulated by?
Chemicals
What are mechanoreceptors stimulated by?
Forces that move the sensor
What are photoreceptors stimulated by?
Light
What are thermoreceptors stimulated by?
Heat or cold
What does the cornea do?
Bends light
What does the lens do?
Focus light on the retina
What does the iris do?
Control the amount of light entering the eye
What does the sclera do?
Protective layer
What does the conjunctiva do?
Protects cornea
What do the ciliary muscles do?
Alters the thickness of lens for focusing
What does the choroid do?
Prevents the internal reflection of light
What is the vitreous humour
Transparent jelly
What does the retina contain?
Light sensitive cells
What is the blind spot?
No retinal cells, where optic nerve leaves the eye
What is the yellow spot
Fovea
-> most sensitive part of the retina, located within the macula ( central area of the retina )
Why does the retina contain
Rods and cones
What do cones allow
Colour vision in bright light
What rods do
Give only black and white vision, but work in dim as well as bright light
In the centre of the retina, what is there?
small area of only cones
-> allows us to accurately pinpoint the source and detail of what we’re looking at
How is the retina made up?
Rods & cones synapse with bipolar neurones, which in turn synapse with ganglion neurones
In rods, what absorbs the light?
Rhodopsin (photochemical pigment)
What happens to rods in the dark?
- Sodium ions flow into the outer segment through non specific action channels
- move down concentration gradient into inner segment
- pumps in the inner segment transport the sodium ions out of the cell
- influx of Na+ creates a slight depolarisation of the cell, it’s at about -40mV
- the slight depolarisation triggers the release of a neurotransmitter (glutamate) from the rod cells, through an inhibitory synapse, the neurotransmitter binds to the bipolar cell, stopping it from depolarising
- > in the dark this neurotransmitter is released continuously
What happens to rods in the light?
Light falls onto the rhodopsin molecule, it breaks down into retinal and opsin
->opsin activates a series of membrane bound reactions, ending in the hydrolysis of a cyclic nucleotide. This is attached to the action channels in the outer segment, the breakdown of the nucleotide causes the cation channels to close
->influx of Na+ decreases, but is still being pumped continuously out of the inner segment
-> inside of the cell becomes hyperpolarised & glutamate is not released
=depolarisation of the bipolar cell, action potential in the optic nerve
Why is it essential that rhodopsin is rapidly converted back to normal?
Needs to be used in the perceiving of other stimuli
Higher light intensity means what for rhodopsin?
The more of it is broken down, and the longer it can take for all the rhodopsin to reform
What is dark adaptation?
The reforming of rhodopsin
What do photoreceptors detect?
The quantity, direction and wavelength of light
What does a phytochrome consist of ?
A protein component bonded to a non-protein light absorbing pigment molecule
What are the diffent types of non protein component?
Phytochrome red ( absorbs red light, 660nm) Phytochrome far red ( absorbs far red light, 730nm)
What is special about these isomers?
They are photoreversible
Absorption of red light causes what ?
Converts Pr into Pfr
Absorption of far red light causes what.
Conversion of Pfr to Pr
Why does Pr to Pfr dominate in sunlight?
More red light is absorbed than far red light
What accumulates in the light?
Pfr
What happens to Pfr in the dark?
Slowly converts back to Pr
What does Pfr do?
Stimulate developmental processes
Red light … germination
Far red light … germination
Triggers
Inhibits
What is a photoperiod
The relative length of day and night
Determines time of flowering
When do long day plants flower?
When the day length exceeds a critical value, uninterrupted darkness = less than 12hrs
-> Pfr is needed to stimulate flowering
When do short day plants flower
When the period of uninterrupted darkness is 12+hrs
- > need to convert all the Pfr to Pr
- > Pfr inhibits flowering in short day plants
What is greening?
Profound changes in the form and biochemistry of a plant
What can phytochromes do?
Promote the development of primary leaves, leaf unrolling, production of pigments
Inhibits the elongation of internodes
What does the changing of one form of phytochrome to another do?
light exposure brings about a change in shape (light activates phytochrome)
Each activated phytochrome interacts with each other’s proteins ( may bind to/ disrupt the binding of a protein complex)
(activated proteins in a signal pathway)
These signal proteins may act as/activate transcription factors that bind to DNA to allow transcription of light regulated genes. (activate transcription factors)
= result in the plant’s response to light
How does gravity affect plants?
Stimulates roots to grow downwards & shoots to grow upwards
What does a mechanical stimulus do to plants?
Activates signal molecules whose end result is the activation of genes that control growth
What does the thalamus do?
Routing the incoming sensory info to the correct part of the brain
What does the hypothalamus do?
Contains the thermoregulatory centre
Acts as an endocrine gland
What is the hippocampus involved in
Laying down long term memory
What do the basal ganglia do?
Selecting and initiating stored programmes for movement
What does the corpus callosum do?
Produces connections between the two hemispheres & between the cortex and the brain structures below
What is the role of the cerebellum?
Responsible for balance
Coordinates movement
What does the midbrain do?
Relays info to the cerebral hemispheres
What does the medulla oblongata do?
Regulates unconscious body processes
What is the frontal lobe responsible for?
Decision making , reasoning, planning, conciousness of emotions
What does the parietal lobe do?
Orientation, movement, sensation, calculation &some types of recognition and memory
What is the role of the occipital lobe?
Processing info from the eyes
What is the role of the temporal lobe?
Processing auditory info
what happens to the neurotransmitter after ?
- some are actively taken up by the presynaptic membrane and are taken up to be used again
