Question

1) lon Flow (15 points): Consider a neuron (at body temperature) with a chloride concentration of 15 mM inside the neuron and 150 mM outside the neuron. Assume that the neuron has a resting membrane potential of-50mV. Which direction will the chloride ions move if all chloride channels open? Does opening of chloride channels have an excitatory or inhibitory effect on the neuron? (5 points) a) -100mV. Which direction b) Assume that the neuron has a resting membrane potential of the chloride ions move if all chloride channels open? Does opening of chloride will channels have an excitatory or inhibitory effect on the neuron? (5 points) What should be the resting membrane potential so that there will be no flow of chloride ions across the membrane after the chloride channels open? What is the role of chloride channels in this case? (5 points) c)

Answer 1

a)  Chloride ions (Cl-) move freely across the neuronal membrane at rest. ... The high concentration of (Na+) outside the cell pushes this ion into the cell down the concentration gradient.

Chloride ions (Cl-) move freely across the neuronal membrane at rest. The negative charge within the neurons readily pushes Cl- outside the neuron via electrostatic pressure (similar charges repel). As Cl- ions accumulate outside the neuron, there is an increased tendency for them to move back into the neuron down the concentration gradient. The point at which the electrostatic pressure pushing Cl- outside the neuron reaches equilibrium with the concentration gradient drawing Cl- inside the neuron, is equal to the resting membrane potential of -70 mV.

Inhibitory neurotransmitters, by contrast, open either Cl^-channels or K⁺channels, and this suppresses firing by making it harder for excitatory influences to depolarize the postsynaptic membrane. ... In fact, for many neurons, the equilibrium potential for Cl^- is close to the resting potential—or even more negative.

With the ions distributed across the membrane at these concentrations, the difference in charge is measured at -70 mV, the value described as the resting membrane potential. The exact value measured for the resting membrane potential varies between cells, but -70 mV is most commonly used as this value. This voltage would actually be much lower except for the contributions of some important proteins in the membrane. Leakage channels allow Na⁺ to slowly move into the cell or K⁺ to slowly move out, and the Na⁺/K⁺ pump restores them. This may appear to be a waste of energy, but each has a role in maintaining the membrane potential.

+30 Depolarization Repolarization 0 一一55 2 Hyperpolarization 70 Time

c) To stop the flow of ions the Typical values of membrane potential are in the range –40 mV to –70 mV. Many ions have a concentration gradient across themembrane, including potassium (K⁺), which is at a high concentration inside and a low concentration outside themembrane.