Chapter 11 - Fundamentals of the Nervous System and Nervous Tissue - Review Questions - Page 426: 20a

EPSP EPSP: This stands for excitatory post-synaptic potential. When a potential is applied to a polarized membrane, the membrane may be depolarized or hyperpolarized . A potential that depolarizes and excites a post synaptic cell, or neuron, is called a excitatory post-synaptic potential. IPSP. This stands for inhibitory post-synaptic potential. A potential that hyperpolarizes a polarized membrane, moves the depolarized membrane farther from threshold and makes it harder for the membrane to be excited and to fire., A potential that hyperpolarizes a membrane is an inhibitory post-synaptic potential.

EPSP and IPSP EPSPs are local potentials generated by the binding of neurotransmitters to post-synaptic neurons. The binding causes ion channels to open; sodium ions and potassium ions then simultaneously pass through the post-synaptic membrane--in opposite directions.The number of Na+ ions that pass in is greater than the number of K+ ions that pass out.This is so because the Na+ electrochemical gradient is greater than the K+ electrochemical gradient. However, the K+ ions do prevent an axon potential from being generated. Instead, only local graded potentials are generated by this excitation. Some chemical synapses are inhibitory (IPSPs). Neurotransmitter binding of post-synaptic membranes of such synapses results in hyperpolarization. This means that the chemical binding increases the negativity of the inside of the post synaptic membrane .In such a case, if the polarization voltage is 70 mV, after neurotransmitter binding the hyperpolarization voltage may reach 100mV. This effect is caused by the opening of channels for either K+ ions or Cl- ions --not both at the same time--sodium permeability is not affected. If K+ channels open, potassium ions move out of the cell. If chloride channels open, chloride ions move into the cell. In either case, the inside of the cell membrane becomes more negative. As the membrane hyperpolarizes, its polarity increases and the potential is driven farther from threshold. It will take a larger depolarizing current to create a potential in this inhibited neuron/cell. A potential that hyperpolarizes a membrane is an inhibitory post-synaptic potential—an IPSP.