Central Pattern Generators
Motor programs are neural circuits set in place before movement begins that can be sent to muscles with appropriate timing. This removes the need for sensory feedback during a sequence of movements.
The fact that a chicken with its head cut off will run around shows that central pattern generators are present in the spinal cord.
Central pattern generators mediate behaviours that include walking, running, breathing, chewing, certain eye movements, and even scratching. Central pattern generators for breathing reside in the brain stem, while those for locomotion are present in the spinal cord. Initiation and speed are controlled by the brainstem.
How are Central Pattern Generators Built?
At the core of CPGs is a set of cyclic, coordinated timing signals generated by a cluster of interconnected neurons. These affect as many as several hundred muscles that precisely contract or relax during a movement.
To achieve coordination among the limbs, sets of CPGs are interconnected. They are also very flexible, turning on or off as needed or adapting to variables such as terrain.
There is no single way to generate rhythmic patterns, and different mechanisms are used. The simplest are single neurons with membrane properties making them pacemaker cells, similar to cardiac nodal cells. These cells are embedded within interconnected circuits.
Other, more complex models of pattern generation include the half-centre model, which suggests two excitatory interneurons are connected to reciprocally inhibit one another. This leads to rhythmic cycling from one center to another.
Feedback and central pattern generators
Sensory feedback, in the form of stretch receptors, acts to terminate activity on one side and initiate it on the other. Spinal networks also reciprocally communicate with the brainstem to maintain control capacity in higher centres.