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Error bars are essential in the graph to validate for measuring standard deviation. The standard deviation observed in the error bars suggests that there is not much deviation in the work.
Describe the important features of the two current-voltage curves (answers should include potential at which currents start to activate, peak inward current, reversal potential).
The steady state (pA) currents starts to activate at an earlier time which is at -80pA before the peak current which starts at -60pA, however by the end it is swapped, as the steady-state starts to stay negative and goes in a straight line, while the peak goes from the negative into the positive when it reaches the end. The threshold potential of the steady state voltage current is at -20pA, whereas the threshold potential of Ipeak is noted to be at approximately -30pA. In terms of its peak, the steady state hits at -210pA for the peak inward current, which is lower than the Ipeak which is at -190pA, Ipeak's inward peak current, from then on Ipeak shoots up into the positive leaving the steady-state in negative and it is at a resting moment. The action potential of steady state curve is recorded at -20pA and lasts till -70pA whereas the action potential of I peak ranges from -30pA to -140pA. The depolarization of the steady state curve is recorded from -20mV to-210mV and the repolarization phase is recorded from -210 to -70 mV. On the other hand the depolarization of I peak is recorded to be ranging from -30mV to -190mV and repolarization state is recorded in between -190 to -70 mV. The refractory period of steady state current is recorded to be in between -100 mV to -70mV and simultaneously the refractory period for the I peak is recorded to be slightly for a shorter period of time within -140mV to -110 mV. There is only one reversal potential for the Ipeak as it passed the x-axis and entering the positive axis, while the steady-state does not get one. The more positive it gets for voltage until -24mV, the negative the current becomes and the more positive it gets after -24 mV, the more positive the current becomes. The curvature of the two I-E relations between -84 to -24 for steady state and -64 to -24 states could show the voltage-dependent of the hyperpolarisation's activation of the cochlea. None of them have a peak outward current, however, if looking ahead, the steady state would return to zero and the peak may go up to an outward current then return to zero, as when looking at the original graph, the peak state started later and may end later, while the steady-state started early and will end early. They converge at -4mV with -140pA.
The pharmacological manipulations shown in the middle and right-hand panels of the Figure show that the normal inward currents are carried by two different ionic conductances. Which conductances do you think they are and why? What might their functions be for the physiology of the cells?
In accordance with many evidences suggested by C.J. Kros (2007) and other researchers, the two main ionic conductances identified as sodium and potassium conductance. The ionic conductance observed in the middle hand side panel is the normal inward current taking place in presence of tetrodotoxin, which blocks the potassium conductance and the conductance observed in the right hand side of the panel is action potential in absence of sodium conductance. These two are comparable with the normal action potential of inward current (Kros, 2007).
The normal action potential observed and ionic conductances:
The cell membrane is a lipid bilayer and highly insulated which acts as a capacitor in separating charges across the interior and exterior surfaces. There are numerous ion-conducting channels present in the cell membrane and the conductance of cell membrane depends on density and types of ion channels (Aidley and Stanfield, 1996). Many of the ion channels are selective and allow the passage of only one kind of ion. The cell membrane also contains many selective pumps which work by spending energy to maintain difference between the concentration of ions inside and outside the cell (Hille, 2001).
In normal conditions, the potential of neuronal membrane ranges from about -90 to 5o mv. The smaller ions pass through the membrane by active transport however the larger ions are transported by signaling (Aidley and Stanfield, 1996).
Sodium and potassium conductances are observed in normal action potential with the ionotropic chloride ion flow. The Na+ and Ca2+ conductances possess positive reversal potential which causes depolarization of a neuron by making the membrane less negative. The potassium conductances have higher negative value of E which causes hyperpolarisation of a neuron by making the membrane potential more negative. The chloride ions have the quality of reversal potential at the resting membrane potential passing less amount of net current.
The primary impact of the ions is to change the resistance of the membrane of cell. These conductances are therefore described as shunting. The shunts performed by these ions are again termed as excitatory and inhibitory in accordance with the reversal potentials. Synapses with the reversal potential lesser than the action potential are termed as inhibitory and the synapses with the reversal potential greater than the reversal potential are termed as excitatory potentials (Anthony, 2006).
The figure showing voltage gated sodium ion channel of the membrane containing four subunits participating in conductance of cell.
The figure shows potassium gated ion channels long with the flow and conductance of potassium ions (Adapted from Anthony, 2006).
There are two types of receptors which are ionotropic receptors where the transmitter binds directly to the synaptic channel and then activates it. The other receptors are metabotropic where the transmitter binds to a distinctly placed receptor and activates it indirectly through a signaling pathway. Ionotropic receptors activate and deactivate the channels more rapidly when compared to the metabotropic receptors. G-protein coupled receptors and various second messengers participate in the pathways. Glutamate and GABA are the major examples of the transmitters which act both ionotropically and metabotropically (Aidley and Stanfield, 1996). The ionotropic receptor types for glutamate are AMPA and NMDA (Aidley and Stanfield, 1996).
The Cell conductance in the middle panel is slightly showing lesser action potential when compared to the normal conductance due to the allosteric blockage caused due to the tetradotoxin. This pharmacological blockade causes blockage of sodium channels to reveal potassium current and hence the figure in the middle panel shows the conductance as a result of potassium.
A figure which explains the role of tetradotoxin is as follows:
The conductance shown in figure 1 incorporates conductances of sodium and potassium. However the conductance seen in figure 2 is because of blockage to sodium channels caused by tetradotoxin.
Figure showing the effect of terodotoxin on the cell membrane conductance (Adapted from Noda, 1986).
The cell conductance seen in the right hand side panel is the normal conductance subtracting the conductance of sodium ion channel. The ionic conductance is thus studied in sodium free extra cellular matrix where the conductance is similar to that of the normal conductance but show smaller conductances (Aidley and Stanfield, 1996).
Conclusion: The ionic conductances decide the fate of flow of ions through the cell membrane. The potential uses of the conductances include the effect of synapses on axon, stoma, dendrites and pre synaptic or post synaptic targets (Hille, 1998). The voltage gated channels also play an important role in generation and propagation of nerve impulses in homeostasis of the cell. The advances in this field can be related to the innovations in molecular biology, spectroscopy and some other structural changes which alter voltage dependant channels (Hille, 1998).
Another major advancement in this field is to identify disease targets by estimating the nature of voltage gated channels. The molecular features of drugs are being studied to suit the drug targets on the basis of disease condition under study (Bezanilla, 2005). The main important function on regeneration of the inner hair cells in mouse. The action potential is essential for the development and survival of the inner hair cells in mature mammalian cochlea (Kros, 2007).
Further research can be done in drug discovery and development process where, the voltage gated channels and blockage of conductances can be used as molecular targets in treatment of disease.
The currents in sodium free extracellular solution appear as depicted below. These currents do not show the conductance of sodium because the extracellular solution does not contain sodium ions fro conductance. The action potential doesnâ€™t show the conducatance caused by sodium.
The voltage current graph could be depicted without any curves due to absence of sodium ions in the extracellular solution.