Muscle movements in animals are highly complex. Controlled by an impressive array of neural networks, they co-ordinate movements and keep us trundling along. But many of these movements are only possible because animals are capable of maintaining an erect posture, preventing themselves from collapsing in a heap on the floor. Morgane Le Bon-Jego, working with colleagues in Bordeaux and Marseille, France, is interested in a reflex mechanism called the resistance reflex, which helps to maintain a creature's posture. Knowing that the hormone serotonin can modify posture, they decided to find out exactly how it affects the resistance reflex. To do this, they carried out a series of experiments on the crayfish Procambarus clarkii to see how this reflex is affected by serotonin.
Le Bon-Jego already knew that the resistance reflex relies on the chordotonal organs in the legs. Arthropods' chorodontal organs are located in the limb joints and code the joints' movement and position, depending on how stretched or relaxed they are. When a leg is moved, the chordotonal organ activates the motor neurons to the opposing muscle in the limb, which contracts and maintains the limb's status quo, keeping the creature upright. Curious to know how serotonin affects the resistance reflex, the team made recordings from the crayfish's leg motor neurons whilst they stretched and relaxed the chordotonal organ to simulate leg movements. They compared neural activity in the motor neurons with the opposing muscle before and after they applied serotonin to the crustacean's nervous system.
When the team simulated an upward movement of the leg by stretching the chordotonal organ in the presence of serotonin, the size and frequency of spikes firing in the motor neurons to the opposing muscle increased. This was caused by serotonin activating pathways with many connections onto the motor neurons. Serotonin appears to boost the signal in the motor neurons, enhancing the resistance reflex.
Secondly, the team looked at the motor neurons' properties and found that the addition of serotonin makes the neurons more likely to fire, which means they respond more readily to inputs from the pathways activated by serotonin in the first experiment. The team have produced an elegant mathematical model to show how the motor neurons combine and compute all the small effects of serotonin on the nervous system. The overall effect of adding serotonin is an increase in motor neuron activity, which therefore enhances the resistance reflex.
There is, however, a final twist to the tale. There was quite a bit of variation in the response of the crayfishes' resistance reflex to serotonin,which the team speculate could be explained by the social status of the animals. Dominant animals reign supreme, holding themselves in a tall upright posture to emphasise their status. To maintain this posture, it could be that their reflexes are highly sensitive to higher levels of serotonin, whilst subordinate animals are less sensitive and have a tendency to slouch. Although the current experiments cannot prove this hypothesis, they do show that standing still can be as finely controlled as highly co-ordinated movements of many pairs of legs.