Animal Behavior/Motor Systems

Neural Control of Motor Systems

I am coming more and more to the conviction that the rudiments of every human behavioral mechanism will be found far down in the evolutionary scale and also represented even in primitive activities of the nervous system—Karl Lashley, 1951

The neural control of behavior will be most obvious in cases where the brain performs only a small amount of processing. Alternatively we can explore neural systems where functions are limited to a primary goal and which are highly optimized to achieve it. The latter is particularly true for sensory systems which encode a single stimulus parameter, or motor systems in escape behaviors. We can ask how a particular behavior is produced, with characterizing muscle activities in the behavior, exploring the neural motor centers that control them, working our way upstream in the flow of information processing, until we reach the mechanisms responsible for sensory input.

Executive control

A set of cognitive processes serve as a supervisory attentional system in behavior. These system impact behavior via control of attention, inhibition, working memory, cognitive flexibility, reasoning, problem solving, and planning. In humans, the prefrontal cortex, the caudate nucleus and the subthalamic nucleus are thought to play central roles in the inhibitory control of behavior. Addiction and attention deficit hyperactivity disorder may result from an impairment in executive control.

Motor Patterns

Complex and coordinated motor patterns can often be centrally generated without requiring sensory input. Such fictive motor patterns may often even be elicited in reduced preparations and studied in vitro. Such fictive motor patterns arise from bursts of activity in isolation, or as a consequence of circuit interactions.

Command Neurons

Command Neurons are cells, when activated, are capable of producing complex behavior patterns in the absence of any meaningful external stimuli. Activation of these neurons is both necessary and sufficient. Examples include the production centers for mating song in crickets, the giant fibers in crayfish escape circuitry, or the Mauthner neurons in goldfish. The concept in which single spikes are viewed as sufficient for behavior may be somewhat dated. There is little evidence that any single neuron is able to exert control over a complex behavior and its motor programs. Neurons cannot be viewed as single identities, they are always part of larger circuits, they control as well as receive input, and their activity thus only makes sense within the context into which they are embedded.

Central Pattern Generator

Central Pattern Generators (CPGs) are circuits that organize repetitive motor patterns, such as those underlying feeding, locomotion or breathing. The production of rhythmic motor patterns and its control by higher-order command and modulatory interneurons can be explained by the intrinsic membrane properties and connections of these neurons. Studies concern the specific timing of activation of the component neurons, how sensory neurons alter or gate CPG output, or how motor patterns are activated, inhibited, or modified by modulatory drive (Marder et al., 2005). Reciprocal Inhibition of functional antagonists can produce alternating discharges in neurons and its associated motor output. Complex, multidimensional oscillations can emerge from multiple, overlapping mechanisms.

Examples

Stomatogastric System of Crustaceans

The crustacean stomatogastric nervous system (STNS) generates rhythmic motor patterns in the stomach and other regions of the foregut. The patterns, controlling more than 40 pairs of striated muscles, serve to grind and filter food using multiple and variable cycles. Over the past 40 years the small section of the nervous system that controls it, has been used to gain an understanding how neural circuit dynamics arise from the properties of its individual neurons and their connections. The STNS consists of a group of four linked ganglia, the paired commissural ganglia (CoGs), the unpaired esophageal ganglion (OG), and the STG. The STG consists of ∼30 motor neurons that move the muscles of the gastric mill and pyloric regions of the stomach. The CoGs and OG control STG activity via descending modulatory input. Accessible to simultaneous recordings of all relevant circuit neurons, the electrical, chemical and molecular properties of individual nerve cells and the direct and modulatory connections between them, can be studied along with the various emerging network properties.

Crayfish Escape

A tailflip consists of a rapid, ventral flexion of the abdomen. It is produced in response to stimulation from a potential predator and results in a backward or upward propulsion that causes the animal to escape from it. The control system for this behavior is formed by 2 pairs of giant neurons - the lateral giant neurons (LGN) and the medial giant neurons (MGN). The concept of command neurons was used to explain the causation of that behavior. The actual components of the system are:

• Bipolar sensory neurons: tuned to touch or high frequency water movements (>80 Hz) - sensory filtering. Sensory neurons produce chemical synapses onto non-giant interneurons, and electrical synapses to giant interneurons subthreshold stimulation triggers non-giant escape swimming, produced by a pattern generator consisting of interneuron networks
• Medial Giant Interneurons: brain to last abdominal ganglion, contraction of fast flexor muscles in all abdominal segments, direction of escape: straight back
• Lateral Giant Interneurons: segmentally repeated; input in abdominal segments, flexor contraction in anterior segments of abdomen, direction of escape: upwards giant motor neurons and fast flexor motor neurons for flexion of abdomen via giant fibers. Habituation only occurs if stimulation of sensory neurons is sufficiently intense to drive an action potential in the giant fiber system
• Non-Giant Interneurons: escape swimming
• Re-Extension of abdomen follows lateral giant tailflips within 100ms of stimulus, reflex of hair receptors and muscle receptor organs
• Excitation of extensor systems via excitatory interneurons with onset of tailflip to prepare for re-extension
• Inhibition of extensor systems via inhibitory interneurons while tailflip is happening then Inhibition of further tailflips while re-extension is happening: inhibition of sensory neurons driving giant fibers, inhibition of flexor muscles

References

• Marder E, Bucher D , Schulz DJ, Taylo AL. 2005. Invertebrate Central Pattern Generation Moves along. Current Biology: 15: R685–R699