Question

For the following question, please answer in essay format( a page and half long minimum please print legibly and be clear about which part of the question are answering, in complete sentences, with as much detail as possible. Thanks!

How is a nervous impulse transmitted in a neuron? Which ionic currents must be allowed in order for a neuron to transmit this impulse? How does myelination facilitate this transmission? What happens to this transmission when it reaches the end of the neuron (the terminus), and what role does signal transduction play (which chemicals are involved)?

Answer 1

The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of electrical changes across the membrane of the neuron. The membrane of an unstimulated neuron is polarized—that is, there is a difference in electrical charge between the outside and inside of the membrane. The inside is negative with respect to the outside.
Ions responsible:
Polarization is established by maintaining an excess of sodium ions (Na +) on the outside and an excess of potassium ions (K +) on the inside. A certain amount of Na + and K + is always leaking across the membrane through leakage channels, but Na +/K + pumps in the membrane actively restore the ions to the appropriate side.

Other ions, such as large, negatively charged proteins and nucleic acids, reside within the cell. It is these large, negatively charged ions that contribute to the overall negative charge on the inside of the cell membrane as compared to the outside.

In addition to crossing the membrane through leakage channels, ions may cross through gated channels. Gated channels open in response to neurotransmitters, changes in membrane potential, or other stimuli.

Myelination of axons during development may enhance neural processing because this lipid-rich coating provides insulation that facilitates the propagation of action potentials along the axons via saltatory conduction. Indeed, increases in indices of myelination like fractional anisotropy (FA) and myelin volume fraction during development have been associated with higher processing speed, as measured by apparent (“residual”) conduction speed and inspection time.

It should be noted, however, that FA is a measurement sensitive to many tissue properties aside from myelination, including axonal orientation and density. Myelination of axons within this pathway could potentially lead to faster neurotransmission, but this is not known because electroencephalograms are only able to assess the amount of time it takes for a sensory stimulus to elicit a neural response. Thus, faster processing of the stimulus could be due either to increased conduction velocity along individual axons or to other factors changing in the multisynaptic pathway during development. Few studies have measured how fast electrical signals travel along individual axons to directly test how neurotransmission changes when axons are myelinated in the developing prefrontal cortex.

Signal Transduction:

Chemical synapses have two important advantages over electric ones in the transmission of impulses from a presynaptic cell. The first is signal amplification, which is common at nerve- muscle synapses. An action potential in a single presynaptic motor neuron can cause contraction of multiple muscle cells because release of relatively few signaling molecules at a synapse is all that is required to stimulate contraction.

The second advantage is signal computation, which is common at synapses involving interneurons, especially in the central nervous system. A single neuron can be affected simultaneously by signals received at multiple excitatory and inhibitory synapse. The neuron continuously averages these signals and determines whether or not to generate an action potential. In this process, the various depolarizations and hyperpolarizations generated at synapses move by passive spread along the plasma membrane from the dendrites to the cell body and then to the axon hillock, where they are summed together. An action potential is generated whenever the membrane at the axon hillock becomes depolarized to a certain voltage called the threshold potential. Thus an action potential is generated in an all-or-nothing fashion: Depolarization to the threshold always leads to an action potential, whereas any depolarization that does not reach the threshold potential never induces it.

The threshold potential for generation of an action potential in a postsynaptic cell. In this example, the presynaptic neuron is generating about one action potential every 4 milliseconds. Arrival of each action potential at the synapse causes a small.

Whether a neuron generates an action potential in the axon hillock depends on the balance of the timing, amplitude, and localization of all the various inputs it receives; this signal computation differs for each type of interneuron. In a sense, each neuron is a tiny computer that averages all the receptor activations and electric disturbances on its membrane and makes a decision whether to trigger an action potential and conduct it down the axon. An action potential will always have the same magnitude in any particular neuron. The frequency with which action potentials are generated in a particular neuron is the important parameter in its ability to signal other cells.
The terminal buttons are located at the end of the neuron and are responsible for sending the signal on to other neurons. At the end of the terminal button is a gap known as a synapse. Neurotransmitters are used to carry the signal across the synapse to other neurons.

The terminal buttons contain vesicles holding the neurotransmitters. When an electrical signal reaches the terminal buttons, neurotransmitters are then released into the synaptic gap. The terminal buttons essentially convert the electrical impulses into chemical signals. The neurotransmitters than cross the synapse where they are then received by other nerve cells.

The terminal buttons are also responsible for the reuptake of any excessive neurotransmitters released during this process.