Re: What are neuronal cicruits? How do they work?

What is a Neural Circuit?

In my mind, there are at least three ways you can think about circuits in the brain.

One type of circuit is an equivalent electrical circuit of a particular neuron. This means you make an model of the neuron to describe its electrical properties in a measurable, mathematical way. In this sort of model, parts of the neuron represent different parts of an electrical circuit, such as batteries or capacitors. These sorts of electrical circuit models are critical to understanding how neurons use electrical signals to communicate with one another. An ambitious person might try constructing an equivalent electrical circuit for a group of interconnected neurons to see how they might interact.

A second type of circuit is an anatomical circuit. This type of circuit really has nothing to do with electronics, but is a convenient way to talk about groups of interconnected neurons. An example of this type of circuit is a basal ganglio-thalamo-cortical circuit, which is just a way of describing a group of neurons which send information from the basal ganglia to the thalamus, and then from the thalamus to the cerebral cortex. Use of this meaning of the term circuit is common among neuroscientists who study systems, such as the motor system or the visual system. This way of talking about circuits is also used by neuroscientists studying the structure of smaller anatomical units in the brain, such as a cortical column.

A third type of circuit is a functional circuit. When discussing functional circuits, the speaker may or may not be referring the electrical properties of the cells, depending on the context. For example, one type of functional circuit is a "working memory" circuit. One way of talking about a working memory circuit is to try to describe how "memory" information is coded and transmitted by neurons. A second way of talking about a working memory circuit is to describe the anatomical connections between different groups of neurons which are thought to process information about working memory. The former requires information about the electrical properties of the neurons (discharge rate, whether or not they burst or are spontaneously active, etc., ) the latter does not. Anatomical and functional circuits are related to the extent that particular areas of the brain contribute to specific functions.

How does a neuronal circuit work?

Entire books can and have been written trying to answer this question. I think one way to try to answer this question is to briefly describe to you what happens in a particular circuit. The circuit I will describe is the retino-geniculo-cortical circuit (anatomical name). This circuit starts in the retina, and synapses first at the lateral geniculate nucleus of the thalamus and then at the primary visual cortex. Therefore, functionally it is a visual circuit.

Transforming Circular Receptive Fields into Linear Receptive Fields

Visual information enters the eye and is picked up by cells in the retina. Each retinal cell responds to light from a particular portion of the field of view. This is called its visual receptive field. Retinal cells have receptive fields which are circular and have a center- surround organization. This means that the cell responds strongly to stimuli in the center of its receptive field and is inhibited by stimuli in the periphery of its receptive field. The best kind of stimulus to cause these cells to fire is a small dot of light or darkness.

Visual information from retinal cells is transmitted to cells in the lateral geniculate nucleus of the thalamus, which also have circular receptive fields. Then the information is sent to the primary visual cortex. Most of the cells in the primary visual cortex do not have circular, center-surround receptive fields. Instead, their receptive fields are rectangular and oriented in a particular direction. If you shine a dot of light in the receptive field of a visual cortical cell, it will not fire, but if you use a lighted bar, it will. These cells are called simple cells.

In their experiments on the visual cortex, David Hubel and Torsten Wiesel hypothesized that these liner receptive fields are made by lining up several circular receptive fields. This can be done if the lateral geniculate cells are connected to the visual cortical cells in the right way. So, in this example, the circuit "works" by funneling the information of several lateral geniculate cells onto one visual cortical cell. The transformation is important because it allows the visual system to "see" lines, such as the edge of an object, instead of just dots.

The transformation of circular receptive fields to linear receptive fields is just one example of how a neuronal circuit can transform information. This particular example used primarily anatomical principles to achieve the transformation. Information can also transformed by a variety of physiological methods as well. One example of a physiological transformation is lateral inhibition, which can make stimuli stand out better from the background. Most of this stuff can be found in any good neuroscience textbook.

Neuromodulation

To get to your second question, neuromodulation can be simply and generally defined as changing the electrical properties of a neuron through chemical means. Generally, neuromodulation occurs through the effects of neurotransmitters or hormones, but other substances, such as prescription or non-prescription drugs, have the same effect. I suppose you might say that synaptic neuromodulation is intrinsic and hormonal or drug induced modulation is extrinsic if it helped to clarify these issues in your mind.

What exactly does neuromodulation do?

A brief answer would be that by changing the electrical properties of a neuron, you change the way it processes and transmits information. One example of this is the phenomenon of long term potentiation. To cause long term potentiation, a brief train of high frequency pulses is delivered to a neuron. Later, that neuron will give a stronger response to other pulses of the same strength. Long term potentiation is an example of synaptic neuromodulation, and many scientists believe it is the basis of some forms of memory. Another example is the effect of certain opioids on dorsal root ganglion (DRG) neurons. Opioids can reduce the duration of the action potential in DRG neurons. A shorter action potential depolarizes the synaptic terminal for a shorter time, and so releases less neurotransmitter.

If you have any questions or comments, or would like a list of the references I used to construct this answer, please send me an email.