Overview: Neuronal Coding




This page picks out one specific neuron, a Meissner's corpuscle touch neuron in the fingertip, and carefully follows the sequence of events through which it codes and transmits information.

Use this page to stay organized as we elaborate in lecture on the sequence of events. It is written to be read sentence by sentence. Refer back to this page as we work on these topics in lecture. Each concept is reliably defined, with consistent, accurate terminology.

The same text is also found on the lecture outline.

As you become familiar with the sequence of events, be sure you understand each of the following:



OVERVIEW:

Before studying each of the various processes involved in information coding and transmission in neurons, let us first step back and look at a typical sequence of events in one specific neuron. This overview presents only a common sequence of events; not every exception is mentioned. As we have discussed, various factors may change a cell's membrane potential, and this in turn may trigger certain cellular processes. In neurons, above all cells, changes in the membrane potential serve key coordinating functions.

The specific neuron we will examine is an afferent neuron responding to touch at a fingertip (See figure on right). The dendrites of this neuron are embedded in the skin at the fingertip, perhaps in a Meissner's corpuscle. From this point, the axon projects in a nerve to the spinal cord. Within the spinal cord the axon branches. Some of the branches form synapses with other neurons in the spinal cord; others extend to the brain.

As we all know from normal perception, this afferent neuron can influence neurons in the brain almost instantaneously following the touch of an object on the fingertip. We now ask how such speed is possible.

In the first step, deformation of the skin by the touch stimulus causes mechanically gated ion channels in the dendrites to open, allowing a net flow of positive ions into the dendrites. The important ion here is Na+, which is far from electrochemical equilibrium, and thus flows readily into a neuron when the ion channels open.

The depolarization caused by the opening of the gated ion channels is called a receptor potential. This term is used only with afferent neurons, which act as sensors of the external or internal environments. Since a touch neuron senses a mechanical stimulus, the ion channels in our example are mechanically gated. But the factor gating the channels is different for each type of afferent neuron. In each case, the ion channels open in response to the specific stimulus detected by the particular afferent neuron.

The receptor potential varies in amplitude, depending on the size of the sensory stimulus. A light touch opens only a few ion channels and thus produces a small receptor potential; a heavy touch opens many ion channels and results in a large receptor potential.

A receptor potential affects less than a few millimeters of a neuron and does not travel. It occurs only in the local region where the sensory stimulus opens the corresponding ion channels. The depolarization does not spread very far because electric current can not flow very far longitudinally down an axon. The fairly high permeability of the plasma membrane to several ions makes an axon a "leaky" conductor of electricity.

But how then is the depolarization in the sensory dendrites transferred to the central nervous system? Only one type of depolarization travels, and that is a phenomenon called an action potential (nerve impulse). An action potential begins when the depolarization of a receptor potential is large enough to surpass the threshold. Once the threshold is surpassed, the depolarization of an action potential rapidly increases until it reaches its full, maximum amplitude. In other words, an action potential is not graded, but rather is a pulse of only one size. For this reason, an action potential is referred to as an all-or-nothing depolarization. The duration of an action potential is brief, lasting roughly one millisecond at any one location in a neuron.

A burning fuse is the best analogy for grasping the mechanism by which an action potential travels. When a burning match raises the end of a fuse beyond a certain minimum temperature (its "threshold"), the end of the fuse suddenly flares. Heat from this region then ignites the next region to beyond its threshold, and this next region flares. The process continues until the all-or-nothing pulse of fire travels to the end of the fuse. Thus, in both a fuse and an axon, an all-or-nothing response in one region triggers an all-or-nothing response in the next adjacent region. By this process the disturbance travels to the end of the structure.

Notice that a fuse does not conduct heat readily. Nonetheless, if a fuse is heated above its threshold, a pulse of fire travels to its end. This can occur because stored chemical energy is released sequentially as the fuse burns.

Something similar happens with an action potential. While an axon will not passively conduct electricity very far, the axon has potential energy poised for sudden release all along its length. This potential energy results from the effects of the Na+/K+ pump on the distribution of ions on the two sides of the plasma membrane. The steady expenditure of energy by the Na+/K+ pump keeps the concentration of Na+ well away from equilibrium. During an action potential, some of this potential energy is released.

Unlike a fuse, which burns only once, a single action potential consumes only a small fraction of the energy stored. For this reason, numerous sequential action potentials can occur, even without intervening active transport of Na+ and K+. To emphasize: the Na+/K+ pump works steadily, storing potential energy by maintaining concentration differences. An action potential is a brief pulse powered by a small fraction of this potential energy.

Upon reaching a presynaptic terminal (axonal terminal in Vander), an action potential triggers the release of neurotransmitter. The neurotransmitter diffuses nearly instantaneously across the narrow synaptic cleft to the postsynaptic neuron. The neurotransmitter then binds to postsynaptic receptors, which in turn leads to the opening of ion channels in this second neuron. Ions flowing through these ion channels cause a change in the membrane potential termed a postsynaptic potential.

A postsynaptic potential is similar to a receptor potential in two ways. First, it varies in amplitude . But here the amplitude depends on the amount of neurotransmitter released (rather than the size of a sensory stimulus). Second, a postsynaptic potential does not travel. The ion channels responsible are found only in the membrane immediately under the presynaptic terminal.

A single postsynaptic potential is usually too small to depolarize a neuron above threshold. But multiple postsynaptic potentials occuring in a short period of time can summate to depolarize the membrane above threshold. If this happens, one or more action potentials occur in the postsynaptic neuron. Each of these action potentials conducts along the axon to the next presynaptic terminals. Here neurotransmitter is released to act on the next cell.

To summarize: In the case of an afferent neuron, a sensory stimulus causes a receptor potential, which occurs only in the sensory dendrites. If the receptor potential surpasses threshold, action potentials are triggered, which travel to the presynaptic terminals and release neurotransmitter. In the case of all other neurons, neurotransmitter causes a postsynaptic potential. If a number of these occur around the same time, they summate to above threshold. Now action potentials are triggered, which travel to the next presynaptic terminals and release neurotransmitter.

receptor potential amplitude depends on size of
sensory stimulus
does not travel depolarizing or
hyperpolarizing
small
(0.1 to 10 mV)
postsynaptic potential amplitude depends on amount of
neurotransmitter released
does not travel depolarizing or
hyperpolarizing
small
(0.1 to 10 mV)
action potential always all-or-nothing travels depolarizing large
(70 to 110 mV)


Quick Quiz

Fill in Answer Correct False Correct Answer
What factor gates the ion channels in the sensory dendrites of an afferent neuron of a Meissner's corpuscle?
What factor gates the ion channels found in the axon of an afferent neuron?
Name the only kind of change in the membrane potential that is all-or-nothing.
Name a type of change in the membrane potential that varies in amplitude (graded).
What factor gates the ion channels that cause a postsynaptic potential?
What type of neuron is the only type that has a receptor potential?
What part of a neuron is the only location where a receptor potential is found?
Name a type of change in the membrane potential that does not travel (local).

(Spelling must be correct)
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