SALMON: Study And Learning Materials ONline
The Nerve Impulse: A Marriage of Chemical and Electrical Events

Nerve cells (neurons) communicate with other nerve cells using a process called neurotransmission.

In neurotransmission chemicals (neurotransmitters) travel across the synaptic cleft between nerve cells and bind onto receptors on the postsynaptic terminal.This can either excite or inhibit an electrical process in the nerve cell.

This electrical process involves ions passing in and out of the nerve cell to produce or inhibit an action potential.

When an action potential reaches the presynaptic terminal it releases neurotransmitter.

The process continues ... Neurotransmitter travels across the synaptic cleft and binds onto receptor on the postsynaptic terminal.This can either excite or inhibit the electrical process in the nerve cell....

Animation of chemical and electrical events involved in nerve impulses

  • The red cone represents an excitatory neurotransmitter
  • The blue cone represents an inhibitory neurotransmitter
  • The red plus + cones represent the combined effects of excitatory + inhibitory neurotransmitters In the model
  • represents chlorine ions (Cl -)
  • represents sodium ions (Na +)
  • represents a very shortsection of axon
  • Accessing the VRML 3D model incorporated in the video

    FreeWRL is an X3D/VRML open source viewer for Windows, Linux and Android.

    FreeWRL can be downloaded here

    The 3D VRML model can be downloaded here

  • Click on red cone to see effects of excitatory neurotransmitter
  • Click on blue cone to see effects of inhibitory neurotransmitter
  • Click on red + blue cones to see effects of excitatory + inhibitory neurotransmitters In the model
  • represents chlorine ions (Cl -)
  • represents sodium ions (Na +)
  • represents a very shortsection of axon
  • The resting potential

    In its resting state, the neuron is more negative inside compared to outside. The resting membrane potential is -70 millivolts (mV). The reason for this is because of the distribution of charged ions:

  • the inside of the neuron contains negatively charged organicions. These ions are so big that they cannot escape through pores (small holes) in the neuron's wall (membrane)
  • negatively charged chlorineions (Cl -) are found on both sides of the membrane (i.e. inside and outside the neuron), but there is a greater concentration outsidethe membrane
    Chlorine ions are colored yellow in the VRML model.
  • Sodium(Na +) and potassium(K +) are positive ions. They are found on both sides of the membrane. Na +ions are in greater concentration outsidethe neuron whereas K +ions are more concentrated insidethe neuron.
    Na +ions are colored pink in the VRML model. (K +) are not shown in the VRML model.

  • This table shows the relative concentration of positive +and negative -ions inside and outside the neuron during the resting state.

    Inside Outside
    organic - high absent
    Cl - low high
    Na + low high
    K + high low

    Neurotransmitters and membrane permeability

    Neurotransmitters affect the membrane's permeability to ions. In the VRML model neurotransmitters are represented by the red and blue cones (located in the bottom left hand corner of the model).

    Clicking a cone initiates two events in the model:

  • movement of Na +/ Cl -ions across the nerve membrane
  • a change in the electrical potential of the nerve membrane
  • Three types of neurotransmitter effect are illustrated in the model:

  • excitatory (colored red)
  • inhibitory (colored blue)
  • mixed (inhibitory and excitatory, colored red and blue)
  • Excitatory neurotransmitters

    The (red cone in VRML model ) open channels in the membrane that allow Na +to rush into the neuron. This causes the membrane potential to change. The increase in +ve ions inside the neuron results in the membrane becoming depolarized. This is illustrated by a decrease in membrane potential to a value less than -70mV.

    The axon of a neuron has a threshold of -60Mv. If the membrane is depolarized beyond this threshold, an action potential is generated. At the peak of the action potential the membrane potential has a value of +30mV. Then the membrane potential recovers and there is a short period of time called the refractory period during which the membrane potential increasesbeyond its resting value of -70 mV.

    During the refractory period the membrane is temporarily hyperpolarized

    Inhibitory neurotransmitters

    The (blue cone in VRML model ) open channels in the membrane that allow Cl- to rush into the neuron.

    This causes the membrane potential to change. The increase in -ve ions inside the neuron results in the membrane becoming hyperpolarized. This is illustrated by a increase in membrane potential to a value greater than -70mV.

    Of course this means that the nerve will not generate an action potential.

    Mixed neurotransmitter effects

    The (red and blue cone in VRML model ) occur when the nerve is simultaneously influenced by excitatory and inhibitory neurotransmitter molecules.

    In this situation both types of membrane ion channels are opened and Na +as well as Cl -rush into the neuron. The change in membrane potential is a reflection of both effects.

    In the model I have arranged things so that the hyperpolarizing effects of Cl -are only partially canceled out by the depolarizing effect of Na +. The membrane does not reach the critical threshold value (-60mV), and consequently an action potential is not generated

    Graded potential changes.

    The number of ion channels that are opened by excitatory and inhibitory neurotransmitters is a function of the amount of neurotransmitter that reaches post-synaptic receptor sites. The greater the amount, the more channels open. Consequently whether or not the axon reaches the critical threshold is a function of how much of each type of neurotransmitter interacts with receptor sites.

    How does the VRML model relate to diagrams in my textbook?

    You will find this diagram - or one very similar to it - in most textbooks. The curvy line in this diagram shows changes in membrane potential as a function of time after the neuron has been exposed to neurotransmitter molecules. In the VRML model the moving white squarecorresponds to this line. The line is not solid in the VRML model because the model shows what happens from moment to moment. One thing is very clear in the diagram, but may have escaped your attention in the VRML model. Notice how the neuron enters a brief refractory state just after the action potential reaches its peak value. During the refractory state the membrane is hyperpolarized(i.e. membrane potential momentarily drops below -70 mV before returning to the resting potential at -70 mV.
    I have added a sound track to the VRML model to emphasize the explosive nature of the action potential, and hyperpolarization of the membrane during the refractory period.

    The Nerve Impulse: A Marriage of Chemical & Electrical Events

    Receptor sites

    The binding between neurotransmitters and receptor sites is often expressed as a lock-and-keyrelationship because neurotransmitters fit particular receptor sites in the same way that a key fits a specific lock.

    It is very important to appreciate that this chemicalevent (transmitter binding to receptor sites) initiates a series of electricalevents. When they bind to receptors, neurotransmitters open channels in the nerve membrane that allow ions to flow in and out of the neuron. These effects are responsible for the changes in membrane potential that you have just explored.

    Synapses

    Before they are released, neurotransmitters are stored in vesicles within the presynaptic terminal .

    When they are released neurotransmitters travel across the synapse and bind with receptors located on the postsynaptic terminal .

    Synapses are the very small gaps between the axon of one neuron and the dendrites of its neighbor. The synapse is so small that it can only be seen under an electron microscope. Electrical impulses cannot normally jump across the synapse. Neurotransmitters are the way in which information from one neuron is carried to neighboring neurons.

    These relationships are summarized in this table

    Presynaptic terminal Synapse Postsynaptic terminal
    neurotransmitters stored in vesicles gap between neurons contains receptor sites

    Neurotransmitters and the action potential

    So far we have studied the way in which neurotransmitters set in train a series of changes in the movement of ions across the nerve membrane that can trigger an action potential. The effect of an action potential passing down an axon is to cause the release of neurotransmitters from the axon's presynaptic terminal.

    It is important to appreciate that action potentials are not always produced when neurotransmitters bind to a neuron's receptor sites.

    An action potential is produced:

    1. when sufficientreceptor sites are bombarded with neurotransmitter molecules
    2. when more excitatorythan inhibitory receptor sites are activated - this effect is seen when you click on the mixed receptor icon in the VRML model
    Before we discuss these issues, review the following parts of the neuron:

  • dendrite (short structure with postsynaptic terminal)
  • cell body (circular bulge)
  • axon (long structure with presynaptic terminal)
  • the axon hillock lies at the junction between cell body and axon
  • Here is a picture of a neuron that appears in one of the recommended texts for this course (Green, Principles of Biopsychology, Lawrence Erlbaum, 1994).

    This tracing represents the action potential passing down the axon. Notice how its progress is rather 'jerky'. This represents the action potential jumping between the Nodes of Ranvier.

    This tracing represents the action potential passing down the axon. Notice how its progress is rather 'jerky'. This represents the action potential jumping between the Nodes of Ranvier.

    Notice the smaller peak in this tracing which represents the postsynaptic potential as it moves down the dendrite across the cell body to end at the axon hillock.

    Differences between action potentials and postsynaptic potentials

    1. the magnitude, measured in millivolts, of a postsynaptic potential is lessthan an action potential
    2. postsynaptic potential move smoothlyacross dendrites and the cell body because these structures do NOT have Nodes of Ranvier
    3. action potentials are 'all-or-none'
    4. in contrast, the magnitude of a postsynaptic potential is a function of how many neurotransmitter molecules have bound to postsynaptic receptor sites
    5. consequently the effects of two or more postsynaptic potentials that occur close together in time can summate
    6. this summation of postsynaptic potentials may be great enough to trigger an action potential

    Types of postsynaptic potentials

    There are two types of postsynaptic potential:

    1. Excitatory postsynaptic potentials (EPSP)are generated by excitatory neurotransmitters binding onto the neurons postsynaptic receptor sites. They cause the nerve membrane to become depolarized so the membrane potential is less than -70 millivolts and thus the neuron is more likely to generate an action potential
    2. Inhibitory postsynaptic potentials (IPSP)are generated by inhibitory neurotransmitters binding onto the neurons postsynaptic receptor sites. They cause the nerve membrane to become partially hyperpolarized so the membrane potential is greater than -70, and thus the neuron is less likely to generate an action potential millivolts.
  • EPSPs can summate to make it more likely that an action potential will be generated by the axon hillock
  • IPSPs can summate to make it less likely that an action potential will be generated by the axon hillock
  • EPSPs and IPSPs can interact with each other - this effect is seen when you click on the mixed receptor icon in the VRML model
  • The story behind SALMON (Study And Learning Materials ONline)

    Paul Kenyon

    Study And Learning Materials ONline (SALMON) was developed and is maintained by Dr Paul Kenyon who graduated from Queens University Belfast in 1969 with a B.Sc. (Hons) in Psychology. In 1976 he was awarded a Ph.D. by the University of Reading. He lectured on the Biological Bases of Behaviour, Behavioural Neuroscience and Evolutionary Psychology in the Department of Psychology, University of Plymouth from 1973 to 2006. He has published papers in psychopharmacology, psychoteratology, physiological psychology and laboratory computing.

    Paul started work on SALMON in 1994 to support his 1st, 2nd and 3rd year undergraduate students studying Evolutionary Psychology, Biological Bases of Behaviour and Psychobiology

    SALMON won the 1998 Universities and Colleges Information Systems Association (UCISA) Teaching and Learning Awardfor demonstrating innovative or exemplary use of the Web.

    SALMON was a finalist at the European Academic Software Awards (EASA)held in Oxford in 1998

    Paul retired from his university post in 2006 and devotes more time than is reasonable to maintaining this website and his lifelong love of all things associated with trout and rivers. He and his colleague Geoff Stephens now operate Fly Fishing Devonwhich offers fly fishing instruction and guiding on Dartmoor and in South Devon, UK.

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