Scientific Understanding of Consciousness
Central Pattern Generators
Neuron, Volume 52, Issue 5, 7 December 2006, Pages 751-766
Biological Pattern Generation: The Cellular and Computational Logic of Networks in Motion
1Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institute, SE 171 77 Stockholm, Sweden
In vertebrates, the generation of rhythmic activity in hindlimb muscles, locomotor activity, does not require sensory input but is generated by central pattern generator networks (CPGs). In all animals, vertebrates or invertebrates, movements are controlled by CPG networks that determine appropriate sequences of muscle activation. Each animal is endowed with a broad repertoire of CPGs, located in different regions of the central nervous system and available for differential activation, thus providing animals with a distinctive set of solutions to accommodate their widely divergent patterns of behavior. For example, the recruitment of different CPGs enables a newborn chicken to perform appropriate hatching movements—to break the eggshell and to stand, walk on two legs, breathe, and to perform the appropriate neck and eye movements to identify and peck grains on the ground, and finally to swallow them.
Along the neuraxis, different motor programs/CPGs are located that can be recruited when needed, from protective reflexes and locomotor CPGs in the spinal cord to respiration and saccadic eye movements at the brainstem level. These different motor programs/networks form together a motor infrastructure. Each motor program can be recruited into action by neural mechanisms that determine when a given motor program should be selected.
Although some CPGs, like those involved in breathing, are active continuously throughout life, most are quiescent under resting conditions and become recruited only when driven by neurons with command functions. A clear-cut example of such top-down control is the command center for locomotion, which is conserved throughout vertebrate phylogeny. This center is located in the midbrain and determines when locomotor CPGs are to be activated and also the level of activity (e.g., fast or slow locomotion).
The Network Logic of Central Pattern Generators: A Repertoire of CPGs Forms a Species-Specific Motor Infrastructure
The term CPG in a broad sense, denotes any network within the CNS that coordinates a motor behavior or a part thereof. Viewed from this perspective, it is the entire inventory of CPGs that forms the motor infrastructure of a species or an individual. The simplest case can be represented by the family of withdrawal reflexes that removes the body surface from an irritant. Such reflex responses, finely tuned by sensory experience during the neonatal period, are determined by a set of spinal interneurons. More complex CPGs include those that coordinate swallowing, coughing, or sneezing — they generate a standard pattern that requires timing at the millisecond level, the activation of different muscles in a precise and sequential manner.
CPGs that control rhythmic behaviors like breathing, chewing, and locomotion generate rhythmic neural activity over an extended period. In locomotion, for example, hundreds of muscles are coordinated with precise timing. In all vertebrates, the CPGs for locomotion are located in the spinal cord and are controlled by descending inputs from specific locomotor command regions in the brainstem. A somewhat more complex neural organization is that formed by the motor representation underlying saccadic eye movements. This representation is located in the superior colliculus (tectum), and a brief intense burst of activity in different locations within the collicular map releases a saccadic eye movement in a given direction and amplitude.
Motor programs also underlie the expression of emotions, as first pointed out by Darwin (1872). Humans express several specific forms of emotions. Some, for example smiling, involve only facial muscles, whereas others, crying and laughing, also recruit the respiratory system. Some of these motor programs, such as that underlying crying, are known to be generated by brain stem circuits, and it is likely that other emotional behaviors are similarly activated—stimulation of the central nucleus of the amygdala, for instance, elicits the expression of fear. In mammals and birds, warning calls and other innate signals can be elicited by stimulation of the periaqueductal gray region of the midbrain. Finally, different patterns of goal-directed behavior can be triggered by stimulation of hypothalamic structures. These behaviors include attack (sham rage), sexual behavior, and the search for water. In these cases, a sequence of motor programs becomes activated, and the resulting motor acts are adapted well to the surrounding world.
Strikingly, a large part of the standard motor repertoire can be generated spontaneously in animals that lack their cerebral cortex but have the basal ganglia and other parts of the forebrain intact. Decorticate cats walk around, display sham rage, and may even attack other cats. Seemingly, they get hungry, search for food and eat, and can even recall from previous experience the location of the food. These classic experiments tell us that different patterns of behavior can be initiated and coordinated with the help of the remaining forebrain structures, among which the basal ganglia are particularly important for the control and coordination of the different CPGs. But, if the lesions include the basal ganglia, the behavior is fundamentally different, lacking the adaptive component, although individual CPGs can still be activated by appropriate stimuli.
Modular Design of CPGs
Mammals can walk, trot, and gallop and sometimes employ a variety of other gaits, revealing several different strategies of coordination. Each limb is controlled by a separate spinal CPG network that provides a standard pattern of activation of the muscles of that limb. Each limb CPG is thus part of a system of interacting limb CPGs that can be coupled in a few stable modes, alternation or in-phase coordination, as in a walk or a gallop, respectively. Each limb CPG can also be used in isolation or in combination, as in a circus dog walking on its hindlimbs or a human walking or running.
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