Comp.Neuro1 (1).pdf

RECAP ❖ A cell at rest has more potassium ions intracellular than extracellular and more sodium ions extracellular than intracellular. There is a negative net charge within the cell being maintained by the voltage gradient. ❖ When the cell becomes depolarized sodium ions enter the...

RECAP ❖ A cell at rest has more potassium ions intracellular than extracellular and more sodium ions extracellular than intracellular. There is a negative net charge within the cell being maintained by the voltage gradient. ❖ When the cell becomes depolarized sodium ions enter the cell. The charge within the cell becomes more positive. ❖ When the cell repolarizes potassium ions leave the cell. The charge within the cell go from positive to negative as it goes back to the resting state. ❖ When the cell is appropriately negative, there are still potassium ions flowing out. This causes the voltage to fall slightly below the resting potential, a feature which we call the undershoot or hyperpolarization. The continuous function of sodium-potassium pumps and the leakage channels ultimately compensate for this slight undershoot, and re - establishes the initial resting potential. An ion is an atom or group of atoms that has an electric charge(has no of electrons doesnot match with no of protons). Ions with a positive charge are called cations. Ions with a negative charge are called anions. Many normal substances exist in the body as ions. Common examples include sodium, potassium, calcium, chloride, and bicarbonate. 1. The difference in charge across the membrane induces a voltage difference and is called the _______________ 2. Neurons can produce a spike of electrical activity called_____________ 3. The electrical burst in neuron travels along the neuron’s ____________ to its ________,Where it passes the signals to other neurons Nernst equilibrium potential The electrical potential generated in a neuron is the result of ions flowing across the neuron’s membrane which is caused by the following two principles: Opposite charges attract, and Concentration gradients seek to equalize. In order for ions to flow, a concentration gradient must be established because the difference in ion distribution across the membrane leads the ions to either pass into or out of the neuron in an attempt to equalize sodium-potassium pump. This pump, which is suggested to use roughly 50% of total brain energy, pumps three sodium ions out of the neuron for every two potassium ions pumped in, thus forming the two respective concentration gradients Diffusion Potential. The resting potential of the neuronal membrane reflects the combined effects of the concentration gradients of different ions. We can use the Nernst equation to estimate the resting potential. Assume the cell has only potassium channel and the concentration of potassium inside the cell is 140mEq/L and the concentration inside is 4 mEq/L Therefore, the concentration gradient favors the potassium ions to exit / diffuse out of the cell. As they do so they carry positive charge with them and thereby the outside of the cell becomes more positive and inside of the cell becomes more negative As more ions diffuse, the Positivity outside the cell tends to repel the positively charged potassium ions back into the cell and the negativity inside the cell tends to keep the potassium inside. This electrical gradient is called diffused potential. As more potassium ions keep diffusing out, this potential gets stronger and stronger.At some point this potential block the further exit of potassium inspite of the existing concentration gradient (in other words concentration gradient balances the electrical gradient) This is Equilibrium position if the cell is permeable to potassium ion and this happens at -94mV inside the cell.and for sodium it is +61mV. Example : Given Intracellular concentration (mM) of Na+ ion Extracellular concentration (mM) of Na+ ion What is the Nernst potential for sodium? Ans: Assume if there are multiple channels and allows the diffusion of ions at the respective channels then we can find the resting potential by using Goldman Hodgkin Katz (GHK) equation. Example: Summary Hodgkin and Huxley Model A model of an action potential (spike).The model idea is based on the nonlinear interaction between membrane potential (Voltage) and the opening and closing of Na+ and K+ ion channels. Both Na+ and K+ ion channels are voltage-dependent, so their opening and closing changes with the membrane potential. The Hodgkin-Huxley Model acknowledges the role of ionic sodium and potassium currents in addition to leak currents through the use of conductance terms, known as gating variables. For the purposes of this model, voltage-gated potassium and sodium channels consist of four independent subunits, and each of these subunits are comprised of a gating variable, n, m, or h. The value of these gating variables represents the probability that an ion channel is open at a given voltage.