Layman's guide to synapses

Different synapse types regulate neuronal networks

This page illustrates how excitatory and inhibitory neurons can be used to co-ordinate movement through regulation of information flow in neuronal circuits. To learn more about nervous system wiring principles in this example, have a look here (>>>). It should become clear that any developmental error during synapse formation would impair the function of this circuit. Essential errors could be: a) formation of synapses onto the wrong postsynaptic neuron (so that information flows along the wrong path); b) failure to install functional synapses (so that information flow is interrupted); c) assembly of the wrong synapse type at a certain connection (so that the information is wrongly processed).

The image shows two antagonising muscles of the arm, one flexor muscles, which bends the arm, and one extensor muscle, which stretches the arm. The arrow points at a spindle organ which senses stretch of muscles. Sensation at this organ is transmitted via a sensory neuron (red) to the spinal cord. Here it forms an exciatatory synapse onto a motor neuron (blue) which in turn innervates the flexor muscle where the spindle organ is located. Sensory and motor neuron form a local circuit in form of a feedback loop which functions in a subconscious manner (see below for more detail).

Voluntary movements are mediated via activation of excitatory interneurons (light brown, inter-ex) in the primary motor cortex of the brain. These neurons send processes into the spinal cord where they form excitatory synapses onto local neurons. Here shown are synaptic connections with a local inhibitory interneuron (black, inter-in) and a motor neuron innervating the extensor muscle.

The image shows the same circuit as above, however, the colours of the neurons represent their voltage (see colour code in page 3).

A) If the arm is stretched passively due to an external force, the spindle organ of the flexor muscle senses the stretch and activates the sensory neuron. Via its synaptic contact in the spinal cord, the respective motor neuron is activated. It stimulates the flexor muscle thus mediating a compensatory muscle contraction which brings the arm back into its original position.

B) A voluntary stretch of the arm is mediated via activation of excitatory interneurons (inter-ex) in the primary motor cortex of the brain. Subsequent activation of the extensor muscle leads to the intended stretch of the arm. As in A) the spindle organ of the flexor muscle senses the stretch and the sensory neuron transmits this information to the spinal cord. However, activation of the appropriate motor neuron of the flexor muscle fails (red circle; same constellation as shown at higher resolution in page 3). This failure is due to the fact that the interneuron from the brain co-activates an inhibitory interneuron (inter-in) which suppresses activation of the flexor motor neuron. This inhibition prevents that antagonising flexor and extensor muscles are co-activated thus causing potential damage.

Understanding the regulatory mechanisms leading to this precise and reproducible arrangement of specific synaptic contacts during the developmental of neuronal circuits is a fascinating challenge for neurobiological research. The next pages address ideas and strategies useful for the investigation of mechanisms underlying synapse and circuit formation.

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Which components at synapses mediate their structure and function? >>>