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Neurons never function in isolation, being organized into circuits that process specific kinds of information. A neural circuit is a population of neurons interconnected by synapses to carry out a specific function when activated. What makes our nervous system such a fantastic device is not that it has 100 billion neurons, but that these nerve cells are capable of communicating with each other in a highly interactive set of neuronal networks. [1]
Local neural circuit
Three classes of neurons are the basic constituents of all neural circuits.
Efferent neurons: Nerve cells that carry information away from the brain or spinal cord (or away from the circuit in question).
Interneurons (IN): Nerve cells that only participate in the local aspects of a circuit.
The synaptic connections in a circuit are typically made in a dense tangle of dendrites, axons terminals, and glial cell processes that together constitute neuropil[2][3].
Neural Circuits for Movement
Anatomy of a multipolar neuron
The neural circuits responsible for the control of movement can be divided into four distinct, highly interactive subsystems
Local circuitry within the gray matter of the spinal cord and the comparable circuitry in the brainstem: Includes the lower motor neurons and the local circuit neurons. All commands for movement are eventually conveyed to the muscles by the activity of the lower motor neurons, the final common path for movement. The local circuit neurons receive sensory inputs and descending projections from higher centres, allowing for coordinated essential, organised movement. Even after the spinal cord is disconnected from the brain animal studies have shown that appropriate stimulation of local spinal circuits elicits involuntary but highly coordinated movements of the four limbs that resemble walking.
Neural networks in the brain
2. Neurons whose cell bodies lie in the brainstem or cerebral cortex: The axons of these upper motor neurons descend to synapse with the local circuit neurons. The upper motor neuron pathways that arise in the cortex are essential for the initiation of voluntary movements and for complex movement. In particular, descending projections from cortical areas in the frontal lobe (eg the primary motor cortex and premotor cortex) are essential for planning, initiating, and directing temporal sequences of voluntary movements. Upper motor neurons originating in the brainstem are responsible for regulating muscle tone and for orienting the eyes, head, and body with respect to vestibular, somatic, auditory, and visual sensory information.
Model of a neural circuit in the cerebellum
The third and fourth subsystems are structures (or groups of structures) that have no direct access to either the local circuit neurons or the lower motor neurons; instead, they control movement by regulating the activity of the upper motor neurons.
3. The Cerebellum: acts via its efferent pathways to the upper motor neurons, detecting the difference, or “motor error,” between an intended movement and the movement actually performed. The cerebellum uses this information about discrepancies to mediate both real-time and long-term reductions in these motor errors (the latter being a form of motor learning). Patients with cerebellar damage exhibit persistent errors in movement.
Basal ganglia networks
4. Basal Ganglia: embedded in the depths of the forebrain. The basal ganglia suppress unwanted movements and prime upper motor neuron circuits for the initiation of movements. The problems associated with disorders of basal ganglia eg Parkinson's disease and Huntington's disease, attest to the importance of this complex in the initiation of voluntary movements[3]
Examples
Baroreceptor reflex block diagram
A simple example is the circuit that subserves the myotatic (or “knee-jerk”) spinal reflex.[2]
More complex examples are given below
Having a conversation requires a high level of coordination between people. Speakers take turns and need to anticipate when others will finish talking. Replies are rapid. The gap between speakers is typically 200 milliseconds, or about the blink of an eye. This means that people are often planning their responses while listening. Researchers identified brain networks involved in planning responses during a conversation. The findings point to neural circuits critical for holding conversations and may help uncover the basis for certain communication disorders[4].
The neural circuit in the baroreceptor reflex is responsible for short-term regulation of arterial blood pressure[5]
The neural circuitry in the vestibular nuclei of the brainstem controls our sense of balance and spatial orientation and it is involved in coordinating movement with balance. All of these tasks function autonomously without the need of our conscious intervention.[6]
Automaticity of walking is made possible by specialized circuits in the central nervous system (CNS) that are capable of coordinating complex patterns of neuromuscular activation. The circuits have been fine-tuned over millions of years of evolution to allow for a stable yet flexible locomotor control strategy that does not require continuous attentional control. The most well-described circuits (primarily revealed by animal studies of locomotor control) are located in the spinal cord, brainstem and cerebellum[7].
Distinct interneuron populations organized into functionally hierarchical modular circuits producing complex locomotor schemes eg chewing, scratching, swimming, and walking[8]
How to Rewire Your Brain
Watch this great 6 minute video to get an idea of the formation of networks.
