Electrical Properties of Nerve Cells

The resting membrane potential

The rapid opening of voltage-gated Na + channels allows rapid entry of Na +, moving membrane potential closer to the sodium equilibrium potential (+40 mv) The slower opening of voltage-gated K + channels allows K + exit, moving membrane potential closer to the potassium equilibrium potential (-90 mv) Action potential mechanism A cell is “polarized” because its interior is more negative than its exterior. Repolarization is movement back toward the resting potential.

The rapid opening of voltage-gated Na + channels explains the rapid-depolarization phase at the beginning of the action potential. The slower opening of voltage-gated K + channels explains the repolarization and after hyperpolarization phases that complete the action potential.

Important Potentials Resting membrane potential is -70mV Depolarization peak is at +40mV Hyperpolarization peak is at -90mV Threshold potential is about -55mV +40mV is the Na+ equilibrium potential -90mV is the K+ equilibrium potential

K + in Compartment 2, Na + in Compartment 1; BUT only Na + can move. Ion movement: Na + crosses into Compartment 2; but K + stays in Compartment 2. buildup of positive charge in Compartment 2 produces an electrical potential that exactly offsets the Na + chemical concentration gradient. At the sodium equilibrium potential: Na + equilibrium potential:

K + in Compartment 2, Na + in Compartment 1; BUT only K + can move. Ion movement: K + crosses into Compartment 1; Na + stays in Compartment 1. buildup of positive charge in Compartment 1 produces an electrical potential that exactly offsets the K + chemical concentration gradient. At the potassium equilibrium potential: K + equilibrium potential:

Graded Potential A weak stimulus can “depolarize” or “hyperpolarize” the membrane generating a membrane potential which is not enough to generate an action potential. This is known as graded potential Graded potential causes potential change in limited areas The graded potential spreads along the membrane by changing the charge on the membrane capacitance and by flowing through opened channels

Graded Potential As the current flows along the membrane, some of the current leaks through open channels in the neighboring areas. As a result the membrane potential progressively decreases with increasing distance from the source point This spatial pattern is exponential and the distance where the voltage changes to 37% of its original value is the “ length constant”

The size of a graded potential (here, graded depolarizations) is proportionate to the intensity of the stimulus.

Graded potentials can be:EXCITATORYorINHIBITORY (action potential(action potential is more likely)is less likely) The size of a graded potential is proportional to the size of the stimulus. Graded potentials decay as they move over distance.

Remember: 1.Membrane potential changes due to change in stored charge on membrane capacitor 2.Membrane conductance changes due to flow of ions through gated channels during graded and action potentials

Excitable cells  As most neurons and muscle cells are much longer than their length constants, the graded impulses disappear when flowing along the cell, thus the responses cannot deliver signals from one end to the other in the cell  Excitable cells are distinguished by their ability to generate active potentials that can propagate without losing their amplitude

Excitable nerve cells A typical neuron has a dendritic region and an axonal region. The dendritic region is specialized to receive information whereas the axonal region is specialized to deliver information. Nerve cells have a low threshold for excitation. The stimulus may be electrical, chemical or mechanical

Dendrites: receive information and undergo graded potentials. Axons: undergo action potentials to deliver information, typically neurotransmitters, from the axon terminals. Neuron

Two types of physicochemical disturbances 1.Local, non-propagated potential (Graded potentials) 2.Propagated potentials, Action potentials or nerve impulse

All-or-None Principle The all or none feature of action potential implies that stimulus less than certain threshold level of depolarization results in a graded response which would not be transferred. However a stimulus big enough to move the membrane potential beyond the threshold will generate action potential that can propagate to distant regions of the cells Threshold potential of-55mV corresponds to the potential to which an exccitable membrane must be depolarized in order to initiate an action potential

Throughout depolarisation, the Na+ continues to rush inside until the action potential reaches its peak and the sodium gates close. If the depolarisation is not great enough to reach threshold, then an action potential and hence an impulse are not produced. This is called the All-or-None Principle.

Spatial or temporal summation Graded responses can interact with each other and can be spatially or temporally summed If two graded potentials occur at the same time in close enough /same places, their effects add up. This is called “spatial summation” If two graded potentials occur at the same place in succession, their effects add up. This is called temporal summation As an analogy, spatial summation is like using many shovels to fill up a hole all at once. Temporal summation is like using a single shovel to fill up a hole over time. Both methods work to fill up the hole

Graded and action potential in neurons In neurons, the axon hillock (initial point of axon) has the lower threshold with relatively high densities of Na+ channels and is thought to be the principal trigger zone The graded responses produced throughout the dendrites or cell body is summed spatially and temporally, and if the summed response is large enough to pass the threshold, an action potential will be generated at axon hillock.

The propagation of the action potential from the dendritic to the axon-terminal end is typically one-way because the absolute refractory period follows along in the “wake” of the moving action potential.

One–way propagation of the AP