wire coated with combustile or explosive material similar to gun powder

action potential is analogous to explosion at any point(action potential = explosion at any point of caoted wire) properties of action potential are all or none law threshold potential positive feedback

bare wire = neuron without nodes of ranveir wire coated with combustile or explosive material = neuron with nodes of ranveir for continuous propagation without stop heat sensitive explosive = voltage gated(depolarization sensitive) sodium ion channels explosives or combustile powder along the wire = voltage gated ions channels all along the axons membrane heat = depolarization cycle that generates explosion = explosion (large amount of heat) = action potential (large depolarization)

with an explosive material like gun powder once heat reaches threshold ,explosion — all or none with an resting potential like ions once stimulus reaches treshold potential,action potential — all or none rule

THE SYNAPSE

Cell body(receiving region of neuron) = iron frying pan axon hillock(action potential generated near axon hilock) = iron handle of pan

one can add a large long sparkler or fuse extending from the handle = axon heating = depolarization at an excitatory synapse cooling = hyperpolarization at an inhibitory synapse (excitatory synapse )= sources of heat like little torches or candles) that can (heat = depolarize) the (frying pan = cell body) inputs that cool the pan = wind

the iron pan integrates(sums) all the excitatory inputs and gets hot . if there is enough heat added to the pan,the handle gets hot and if it reaches threshold and would ignite the fuse

The Analogy The axon of a neuron is represented by a sparkler, a wire coated with explosive material, with hundreds of explo-sive particles similar to gunpowder at any point. An action potential is analogous to the explosion at any point, and such an explosion can illustrate positive feedback, thresh-old, and the all-or-none property of an action potential. In this analogy, the heat-sensitive explosive is analogous to the voltage-gated (depolarization-sensitive) sodium channels, and heat is analogous to a depolarization. An explosive particle will ignite if it is heated to threshold, the initial heat usually supplied by a match. The ignition of one particle produces additional heat which causes additional particles of explosive to ignite. Thus, an explosion is an example of positive-feedback in which heat produces more heat to produce a large amount of heat, an explosion. The cycle that generates such as explosion is illustrated in the magnified inset of Figure 2. In the case of an action potential, via voltage-gated sodium channels, an initial threshold depolarization produces more depolarization to produce a very large depolarization, an action potential. An explosion is a large amount of heat; an action potential is a large depolarization. With an explosive material like gunpowder, once the heat reaches threshold, an explosion.

will occur; thus an explosion is all-or-none. In the axon, once the depolarization reaches threshold, an action potential will occur. Thus, all of the characteristics of an action potential—threshold, positive-feedback, and the all-or-none property—can he illustrated with the explosive used in a sparkler. Continuous Propagation of the Action Potential Once generated, the action potential travels down the axon to the axon terminals (e.g., Fumes et al., 2001). Since the action potential must travel over relatively long distanc-es, an active mechanism is needed; passive spread would not produce a large depolarization at the end of the axon. Passive spread is easily explained using a heat analogy. If you heated one end of a long piece of bare wire, the heat would spread passively in the wire so that the wire right next to the place you heated would get hot, but the other end would not get very hot. If neurons used only passive spread, the signal (the change in membrane potential) would become very small as it spread passively down the axon (e.g., Kandel et al., 1995). Neurons avoid this problem by producing a new action potential in each patch of membrane along the axon. The action potential does not passively spread from one end of the axon to the other; rather a new action potential is generated at each point along the axon. This mechanism is fittingly called regeneration or propagation. The action potential that occurs in the axon of the sensory neuron near the toe, the action potential in the axon farther up the leg, and the action potential that occurs in the axon as it enters the spinal cord are each full, all-or-none, newly-made action potentials, and the signal therefore maintains its amplitude and fidelity.

The sparkler can be used to illustrate the propagation of the action potential along the axon's membrane. In some axons (called unmyelinated axons) a new action potential is generated at each point along the axon because there are voltage-gated sodium channels all along the axon. The action potential at one point, itself a huge depolarization, easily depolarizes the adjacent membrane so that this adja-cent region reaches threshold and generates a new action potential. Propagation that occurs all along the axon is called continuous propagation. Another form of propagation (found in myelinated axons) is called saltatory conduction, but is not easily demonstrated with the sparkler. The Sparkler Demonstration A sparkler consists of explosive material spread along a metal wire and is a good model to demonstrate continuous propagation. The presence of explosive all along the spar-kler is analogous to the presence of voltage-gated channels all along the axon's membrane. When the sparkler is lit and reaches threshold to produce an initial explosion, the heat from the initial explosion easily spreads (passively) to the next region of the sparkler, bringing it to threshold and caus-ing a new explosion there. Thus, a new explosion is made at each point along the sparkler, and the size of explosion at the end

is the same magnitude as the initial explosion. An action potential is similar. 

Once threshold is reached, the large depolarization from the initial action potential brings the next region of the axon to threshold and generates a new action potential there. Thus, a new action potential is made at each point along the axon.

I demonstrate the above concepts by first reviewing the parts of a neuron and the generation and propagation of an action potential. I then explain the analogy, making a simple chart indicating what properties are analogous: heat - depolarization explosion • action potential explosion is a large amount of heat action potential is a large depolarization I then light a match (or a candle) and briefly heat the sparkler saying something like, This is not a large enough depolarization; we have not yet reached threshold; therefore there is no action potential. I then heat the sparkler longer (or better yet, use a larger candle), and when it still doesn't light, I repeat the above and heat longer. When the sparkler finally does light, I wait a few seconds for the ohs and ahs to end and then say, Now we've reached threshold and an 'all or none' action potential occurs? As the sparkler burns, I narrate continuous propagation. Figure 2 provides a graphic to compare the axon and the sparkler.

It is useful to perform the demonstration again, possibly having students help narrate the sequence of steps involved in generating and propagating an action potential. The goal is for the students to see the sparkler and yet talk about the neuron in terms of depolarization, threshold, action poten-tials, propagation, and refractory periods. Students should be warned not to repeat the demonstration unsupervised. One also needs to be aware of smoke detectors, and give appropriate notification that you will be lighting a sparkler in your classroom.

The Synapse

A related analogy, heating and/or cooling a frying pan, is useful to explain how information is transferred from one neuron to another. When the action potential in one neuron reaches the axon terminal, it triggers the release of neurotransmitter molecules which diffuse to a target, often the dendrites or cell body of another neuron. The special-ized structures involved in the release of neurotransmitter and the structures involved in its action on the target neuron occur at a region called a chemical synapse. In the example above, the sensory neuron from the toe makes chemical synapses with second-order neurons in the spinal cord and brain. At each target neuron, the chemical neurotransmitter binds to receptor proteins causing ion channels in the target neuron's membrane to open or close. These chemically-gated channels produce small changes in the membrane potential of the target cell called post-synaptic potentials. Post-synaptic potentials determine whether the target cell reaches a threshold depolarization and generates an action potential or not. The post-synaptic potential in the second-order neuron plays the same role as the potential in the receptor end (receptor potential) in the sensory neuron, such as the response in the toe. If the second or subsequent neurons do not generate an action potential, perception of a sensory stimulus may not occur. In fact, animals do not perceive most of the sensory information that impinges on them.

Depending on the specific synapse, post-synaptic poten-tials can be either excitatory or inhibitory (e.g., Kandel et al., 1995). Excitatory post-synaptic potentials depolarize the target and help it reach threshold. Inhibitory post-synap-tic potentials make it more difficult for the target to reach threshold (one mechanism of inhibition is a hyperpolariza-tion, making the membrane more polarized or more negative inside). The change in potential at each synapse is usually small, so that many excitatory post-synaptic potentials are needed to reach threshold (or many inhibitory post-synaptic potentials to prevent the target from reaching threshold). A typical neuron in the central nervous system receives more than 1,000 synapses (e.g., Kandel et al., 1995). The oppos-ing mechanisms—depolarization from excitation and hyper-polarization from inhibition—are combined or integrated in the membrane potential of each target cell. This integration or summing of excitation and inhibition determines wheth-er the target cell will reach threshold or not.

An Analogy of Synaptic Integration

In most texts, chemical synapses are shown impinging onto the cell body or dendrites, and the action potential is generated near the axon hillock, a region where the axon protrudes from the cell body. An iron frying pan can be used as a model of the cell body (or more generally, the receiving region of a neuron) with synapses occurring onto the pan, and with the iron handle of the pan analogous to the axon hillock. One can add a long, large sparkler or fuse extending from the handle, analogous to the axon (see Figure 3). For a neuron with an axon 10 mm in length, the fuse or spar-kler representing the axon would need to be about 20 m in length to be at the same scale as the frying pan.

Heating is again analogous to a depolarization at an excitatory synapse, while cooling signifies a hyperpolariza-tion at an inhibitory synapse. At an analogous excitatory synapse onto the frying pan, heat is generated; thus, excit-atory synapses can be represented as sources of heat such as little torches or candles that can heat (depolarize) the frying pan (cell body). The iron pan integrates (sums) all these excitatory inputs and gets hot. If there is enough heat added to the pan, the handle gets hot and, if it reaches threshold, would ignite the fuse. By analogy, in a neuron, excitatory synapses cause depolarizing post-synaptic potentials that sum together toward threshold. From the synapse to the axon hillock, the spread of depolarization is passive, allow-ing depolarization from many synapses to sum together. These potentials are not 'all or none' but rather are graded in amplitude. If threshold is reached at the axon hillock, an action potential is generated and actively propagated down the axon to the next synapse. In Figure 1, this passive spread is indicated by the open arrows; the active propagation is indicated by the filled arrows.