3.6 Organisms respond to changes in their internal and external environments Flashcards
action potential
An A.P. is a depolarisation of the cell membrane, where it reaches a potential difference of +40mV compared to the outside
action potential- rest
the membrane is polarised at -60mV with some K+ channels open and all Na+ channels shut
action potential- slight increase in potential difference
some Na+ channels open due to energy changes in the environemnt allowing some ions to diffuse down the conc gradient. membrane starts to depolarise
action potential- great increase
once membrane reaches a threshold potential of around -50mV, voltage gated Na+ channels open to allow even more Na+ to flood in
causes membrane potential to rise sharply and become positive charged compared to outside
eventually reaches a peak of around +50mV
action potential- decrease
all Na+ channels close and all K+ channels open
as K+ diffuse back into the cell, the membrane potential becomes more negative (repolarisation)
action potential- lowest dip
as slightly more K+ channels are open than usual the potential difference overshoots slightly and becomes hyperpolarised
action potential- refractory period
takes time for pumps to restore the ion concentrations for the next action potential so there is a period of time where the action potential cannot be stimulated
prevents depolarisation
all or nothing principle
all action potentials are the same size all impulses are of the same amplitude
more intense stimulus=greater frequency of impulses
speed of transmission
myelination and salutatory conduction
distance involved
axon diameter
temperature
synapses
gaps between neurones
information is sent between neurones by chemical transmission
synaptic transmission steps
step 1- calcium channels open step 2- neurotransmitter release step 3- sodium channels step 4- new action potential step 5- acetylcholinesterase step 6- remaking acetylcholine
synaptic transmission- step 1
calcium channels open
incoming A.P causes depolarisation in synaptic knob
causes calcium channels to open and ions flood into the synaptic knob
synaptic transmission- step 2
neurotransmitter release
influx of calcium ions causes synaptic vesicles to fuse with presynaptic membrane
releases neurotransmitter into a cleft
synaptic transmission- step 3
neurotransmitter is released into the synaptic cleft
binds to the receptor sites on sodium ion channels
sodium ion channels open\
synaptic transmission- step 4
new action potential
depolarisation inside postsynaptic neurone must be above a threshold value
if threshold is reached a new action potential is sent along the axon of the post synaptic neurone
synaptic transmission- step 5
acetylcholinesterase
hydrolytic enzyme
breaks up acetylcholine into acetyl and choline
synaptic transmission- step 6
ATP is released by mitochondria is used to recombine acetyl and choline
stored in synaptic vesicles
more acetylcholine can be made at the SER
sensory receptors
These are specialised cells that detect a change in the surroundings to trigger a nerve impulse
They are energy transducers, as they convert a change in energy (a stimulus) into an electrical impulse
pacinian corpuscles location
Abundant deep in skin – Fingers, soles of the feet, external genitalia. Occur in joints, ligaments and tendons to show a change of direction
pacinian corpuscles and producing a generator potential
When pressure is applied the membrane becomes distorted and stretched
Sodium channels are then open and sodium ions diffuse into the neurone
The sodium ions change the potential of the membrane and it becomes depolarised producing a generator potential
The generator potential creates a nerve impulse (action potential) along the sensory nerve to the CNS
transducers
Both rod and cone cells act as transducers by converting light energy into the electrical energy of a nerve impulse.
rod cells
Cannot distinguish different wavelengths of light and therefore produce images only in black and white.
Rod cells are more numerous than cones.
share a single sensory neurone. Rod cells can therefore respond to light of very low intensity because a certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are attached.
rod cells and bipolar cells
A number of rod cells are attached to a single bipolar cell (= retinal convergence), there is a much greater chance that the threshold value will be exceeded than if only a single rod cell were attached to each bipolar cell.
As a result, rod cells allow us to see in low light intensity (i.e. at night), although only in black and white.
cone cells
Cone cells are of three different types, each responding to a different wavelength of light. Depending on the proportion of each type that is stimulated, we can perceive images in full colour.
cone cells and bipolar cells
Each cone cell usually has its own bipolar cell connected to a sensory neurone. This means that often the generator potential is not exceeded. As a result, cone cells only respond to high light intensity and not to low light intensity.
cone cells and fovea
Light is focussed by the lens on a point known as the fovea. The fovea therefore receives the highest intensity of light.
Therefore cone cells, but not rod cells, are found at the fovea. The concentration of cone cells diminishes further away from the fovea. At the peripheries of the retina, where light intensity is at its lowest, only the rod cells are found.
cone cells and pigments
Cone cells contain a different pigment to rod cells (iodopsin). This requires a higher light intensity to be broken down and create a generator potential.
As cone cells are attached to their own bipolar cell, if 2 adjacent cells are stimulated, the brain receives 2 separate impulses.
Cone cells give very accurate vision, they have good visual acuity.
