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An Action Potential in a Neuron

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This essay will describe the electrochemical processes that allow an Action potential to occur in a neuron. This will be achieved by firstly, defining the purpose of neurons in the body along with a description of the components within a neuron and how they enable information to be passed through the cell membrane and on to other neurons. Secondly, the resting potential of a neuron will be explored with relation to the concept of selective permeability and the purpose of the Sodium - Potassium pump. Thirdly, the molecular basis of the Action Potential will be explained including a description of hyper polarisation, depolarisation and the purpose of the refractory period. Fourthly, a description of how a signal moves through the components of the neuron will be given as well

as an overview of how the information is then transferred to other cells. Lastly, an account of specific problems that can interfere with the production of an Action Potential and the transmitting of information between neurons will be highlighted.

Neurons are regarded as being the most important 'building blocks' within the entire nervous system(Rozenweig 1996). Neurons are responsible for receiving and transmitting information to other cells and it is thought that there are 100 to 150 billion neurons in the human body which underlie the simplest to the most complex abilities and tasks (Kandel, Schwartz &Jessell 1991). The production of an action potential is involved in this information transmission. However, in order to give a clear explanation of the process occurring before, during and after the action potential it is first necessary to define the specific components of a neuron which are involved in the process.

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From the diagram above the three main components of a neuron; the cell body, the axon and the dendrites can be identified. The dendrites which extend from the cell body are responsible for receiving signals and then sending them on to the cell body. The cell body contains the information for keeping the cell alive e.g. replication, protein production etc. At the end of the cell body is the axon hillock which is important as it is where the electrical charge of the neuron builds in order to produce an action potential. The axon then extends down from the cell body and is responsible for sending signals out from the cell towards other neurons. The axon length which extends down towards the axon terminals is typically coated in myelin which small sections of about 1mm that are unmylenated. These 'nodes of Ranvier' allow the signal to move down the axon more quickly.(Kalat 2006) . At the axon terminals the signal is released. The small gap between this and another neurons dendrite or cell body is referred to as the synapse - synapses are vital in passing the information on from neuron to neuron. (Mader 2002).

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The diagram above shows the components that are necessary for the information to be passed across neurons. The neuron sending the signal is called the presynaptic neuron and the one receiving the signal is referred to as the post synaptic neuron. The area in between is the synaptic cleft which is important in passing the signals on to the receptors on the postsynaptic neuron. Johnson (2001) explains that the signal is able to transfer across to the receiving neuron through synaptic transmission i.e. a chemical process that allows the information to be passed on through neurotransmitters e.g. in order to be able to wiggle toes a signal has to be passed down from the neurons in the spinal cord to the muscles and in the feet. The production of action potentials are fundamental to information being processed across the body and form the initial stage in this transmission.

Before an action potential occurs a neuron is stable and is not receiving any new signals or information. This period is referred to as the 'resting potential'. Kalat (2006) describes an important feature of this stage as being the presence of sodium and potassium ions both inside and outside the neuron and that they are unequally distributed across the membrane i.e. more potassium ions present inside the cell membrane than outside, conversely more sodium ions are concentrated outside the neuron than inside. As there are more positively charged potassium ions inside the membrane and lots of positively charged sodium ions on the outside along with some positively charged potassium ions this results in there being a more positive charge outside than in - the charge inside the membrane is therefore negative at -70mV (Johnson 2001). This unequal distribution is made possible in the first instance through the 'selective permeability' of channels which control the rate at which certain ions pass (Mader 2002). During the resting potential some of the gates that control the rate of Potassium ions movement across the membrane are open which allows these ions to pass across the membrane at a moderate rate. At the same time the sodium gates are closed restricting sodium ions from crossing the membrane, hence why there is more Potassium than Sodium concentrated inside the neuron than out (Johnson 2001). Another factor which contributes is the Sodium - Pottasium pump. The pump transports 3 sodium ions out of the cell for every 2 Pottasium ions brought in. This along with the restrictive nature of the channels further maintains the electrical charge of the cell negative inside and positive outside. A neuron will remain in the resting potential for as long as it is not receiving any new information.

For the Action Potential to occur the neruon has to be stimulated at the channels on the dendrites - this signal passes down to the axon hillock where the strengh of the signal builds up. Mader (2002) explains that the signals ability to trigger an action potential depends on its strength to reach and pass through the threshold allowing the sodium gates to open fully. Furthermore Mader (2002) goes on to explain that sometimes individual signals simply result in graded potentials - i.e. they move the voltage of the neuron closer to zero or move the voltage even more negative and once the signal ceases the cells charge returns to the original level. In order for an Action Potential to be triggered the threshold must be passed - for that to occur the impulse must initiate the resting potential of the membrane to change from -70mV to about 5-15mV less negative (Kalat 2006).

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