The brain is made up of a network of cells called neurons, which are primarily the building blocks of the brain. The brain is an extremely complex organ, so it is preferable to focus on a small area.

We will identify three regions of the cell: the cell body (soma), the dendrites and the axon.

The cell body contains the basis of the neuron and is surrounded by a number of off-shooting branches called the dendrites. The axon extends away from the cell body and provides the pathway over which signals can travel from the cell body for long distances to other parts of the brain and the nervous system. The functioning of the brain depends on the flow of information through the networks of neurons.

Information is transferred from one cell to another at specialised points of contact: referred to as the synapse. Each synapse has a certain conductance (ease with which a current flows through a substance) which affects the intensity of the signal passing through it. This conductance is represented as the weight of the synapse. Information is relayed from one neuron to another by means of a Neurotransmitter, which transmits a signal. As the synapse is activated, this results in the “firing” of a neuron (the generation of nerve pulses). Some synapses are excitatory (the synaptic weights are positive) in that they promote firing, whereas other are inhibitory (the synaptic weights are negative) and so are capable of cancelling signal that otherwise would excite a neuron to fire.

To understand this communication along a neurons axon, note that the cell is enclosed by a membrane (protective layer), across which there is a difference in electrical charge. This potential difference between the inside and outside can be changed. If the change in potential difference is large enough, it exceeds a threshold and a pulse (an action potential) is generated which will be transmitted at full strength.

Astrocyte

There is another class of cells in the central nervous system called astrocytes or glial cells. They occupy all the space in the nervous system that are not taken up by the neurons themselves.Astrocytes are very sensitive to the level of neuronal activity because of their position and sensitivity to activity dependent changes in the environment shared by neurons. The astrocytes are affected by the calcium signals (calcium waves that spread across the network), which make an astrocyte active. 

Each astrocyte is associated to two nearby neurons. If the input signal of an astrocyte exceeds a certain threshold, the astrocyte will become active. The neurons input will be increased when an astroctye is activated (proportional to its state) but if the astrocyte is over activated then it will decrease the input of the neuron proportionally.

Capillary

The capillary is the smallest of the body’s blood vessels. A capillary is associated with a number of neurons. If the number of firing neurons (associated with the capillary) exceeds a certain threshold, then the capillary will dilate at that time. The dilation will be indicated by a proportional increase in size of the capillary.

Neuron Vascular Coupling

The explanations of the neuron, astroctye and capillaries constitute to the goal of the system, which is to demonstrate the Neural Vascular Coupling. The system shows the results of the coupling of neuronal, astrocyte and cerebrovascular (capillary) activity. In the neuron vascular coupling, distribution of synapse strengths affects neuronal behaviour and the balance of blood flow (the capillaries).

Running the applet

This Java applet requires Java 1.4 Runtime Environment or a more recent version.
You can verify if the Java Runtime Environment is installed here
 

Representation

This represents the network of neurons.

If a neuron is active then it is represented by a blue colour and transmits a slow (blue) signal to the following neuron.

As a neuron is active and its level of activity is increasing, its speed increases proportionally. As its firing level exceeds beyond a threshold, it will transmit a fast (red) signal to the following neuron.

If a neuron is not active then it is represented by a green colour and will not send a signal to the following neuron.

This represents the network of astrocytes

If the astrocyte is active, this is indicated by an increase in size.

If the astrocyte is over activated, this is shown by a further increase in size.

If the astrocyte is not active, it remains normal size.

This represents the capillaries

If the capillary is dilated, its size is expanded.

If the capillary is not dilated, it remains at normal size.

How to use

The User Adjusts the Neuronal Scrollbar

Enables the user to adjust (Increase/decrease) the level of neuronal activity.

As the value of the scrollbar is increased, it affects the synaptic weight of the neurons.

Initially, there may be only few neurons active.

As the scrollbar is increased, more neurons will become activated and starting sending signals to the following neurons.

As the value of the scrollbar increases further, the speed of signals will increase. As the neurons increase in speed, this will cause the capillary (associated with the number of neurons) to dilate. This occurs as the number of firing neurons associated with the capillary exceeds the set threshold.

The User Adjusts the Astrocytes Scrollbar

Enables the user to adjust (Increase/decrease) the level of astrocyte activity.

As the value of the scrollbar is increased, this affects the calcium signal of the astrocytes.

Initially the astrocytes are not active.

As the scrollbar is increased, and astrocyte level exceeds the threshold, then the astrocyte is active and this is shown by an increase in size of the astrocyte.

As a result of this state of the astrocyte, the activity of the neurons will be affected. The level of neuronal activity will increase.

As the value of the scrollbar increases further, the astrocyte level will over exceed the threshold, then the astrocyte will become over activated and this is shown by a further increase in size.

As a result of this state of the astrocyte, the activity of the neurons will be affected. The level of neuronal activity will decrease.

Due to the interaction between the neurons and astrocytes, the astrocytes affect the activity of the neurons.

Therefore, the dilation of the capillary is affected by the state of the associated neurons.