This year’s special lecture on motor systems was given by Dr. Ole Kiehn, from Karolinska Institutet and the University of Copenhagen.
Movements are essential for animals. There is rarely anything an animal can do that does not imply or is expressed by a movement –a proof of it is that most sensitive, behavioral and cognitive tests imply the presence or absence of a movement-.
One of the movements that characterize animals is locomotion. Although it may seem effortless, locomotion is a type of movement that is hard to control (think about the recovery of walking on patients after a stroke, for example).
In his lecture, Dr. Kiehn dissected the last discoveries from his lab on the spinal and brain circuits that control locomotor behaviors.
The locomotor system is organized in a way where the brain is on charge of the planning of the movement, and the spinal cord executes the movement. Dr. Kiehn addressed these two functions separately:
By a first part of the talk, dedicated to executive spinal circuits, responsible for the timing and coordination of locomotion
And a second part, dedicated to the circuits on the midbrain and brainstem, responsible for signaling the start, the continuation, the end and the regulation of a movement. This regulation depends on the context where the movement takes place
First part of Dr. Kiehn’s talk: executive neuronal circuits in the spinal cord.
Executive neuronal circuits in the spinal cord, control the timing and patterning of locomotor movements.
In Dr. Kiehn’s lab, they use mice as a model. The advantage of mice is that their genetics, anatomy and function are well known and therefore easier to study. Using this model, his team studies the Central Pattern Generators (CPGs), a specialized network in the spinal cord that generates both the pattern of locomotion -phasic activation of motoneurons to produce the different types of gait- and rhythm generation –cycle frequency, the speed of locomotion-.
The correct regulation of locomotion by the CPGs implies a left-right coordination, an alternation between flexor and extensor muscles, the production of different types of gait, and the capacity to generate rhythm (increase or decrease speed within the same type of gait).
It is well known that the base for left-right coordination circuits are commissural interneurons. These are spinal interneurons, therefore they are located in the spinal cord and project to other neurons in the spinal cord, and they are called commissural because their axons cross to the other side of the spinal cord (through the ventral commissure).
Spinal interneurons are identified by their gene expression. One type of interneurons, V0 neurons, express Dbx. There are at least two subtypes of V0 interneurons: V0V interneurons express Dbx and Evx1, and are excitatory, while V0D interneurons express Dbx and Pax7, and are inhibitory.
It is known that both excitatory and inhibitory commissural interneurons are implicated in left-right coordination. The group of Dr. Kiehn wondered: how is this modulated? And, how is this modulation related to the locomotor speed?
They performed a genetic ablation of all V0 interneurons, which lead to a disparition of the left right alternation at all frequencies of locomotion; mice cannot walk, and the gait they use for locomotion is quadrupedal hopping. Dr. Kiehn’s group determined that V0 interneurons are necessary for left-right alternation.
But this is not the only function of V0 interneurons. Dr. Kiehn’s team found also that V0V and V0D commissural pathways control left-right alternation in a speed-dependent way: the genetic ablation of V0D interneurons leads to a loss of left-right alternation at low and medium frequencies, while alternation is preserved at high frequencies; on the contrary, the ablation of V0V interneurons leads to a preservation of left-right alternation at low frequencies, but produces hopping at medium and high locomotor frequencies.
Mice locomotion is expressed at distinct gaits at different speeds; V0 neurons control the expression of specific gait modules
If wild-type mice are freely moving, they walk –type of locomotion where they have at least three feet in the ground all the time-, trot –diagonal pairs of limbs move forward simultaneously, in alternation with their homologous pairs- or bound –forelimbs and hindlimbs move in synchrony: they all lie in the ground at the same time, and they are all in the air at the same time-.
Genetic ablation of commissural V0V neurons impaired the expression of trot, but left intact walk and bound. Ablation of commissural V0V and V0D neurons led to a loss of walk and trot, but bound was preserved. This study provided evidence that gait expression depends on the selection of different neuronal networks-as if one network is one module that produces one type of gait-, and that V0 interneurons are implicated in this selective activation.
The presence of inhibitory V0D and glutamatergic V0V has also been found in larval zebrafish, and inhibitory commissural interneurons are implicated in locomotion in lampreys and Xenopus tadpoles. This is a phylogenetically preserved system, present in most animals.
Coordination of flexor and extensor muscles
Dr. Kiehn’s team found that the minimum circuit for flexor-extensor alternation during locomotion were Ia inhibitory interneurons. Genetic ablation of glutamatergic neurons in a mouse did not avoid the alternation between flexor and extensor muscles underlying the generation of a locomotor rhythm, meaning that reciprocal inhibition by Ia interneurons is enough to provide this alternation.
There are neurons at the spinal cord capable to set the pace of the locomotion; which are these rhythm generators?
Rhythm-generating circuits set the pace of the locomotion. Excitatory V2a interneurons from the spinal cord or hindbrain (excitatory, ipsilateral projections), they are essential and sufficient to start and maintain the rhythm.
Dr. Kiehn’s team found that excitatory commissural interneurons at the spinal cord are essential for left-right alternation. Also, they found that there is a variety of excitatory subpopulations that contribute to the locomotor rhythm. This shows that CPGs are actually composed by different modules, from which the gait and frequency are chosen.
Dr. Kiehn’s team characterized the contribution to CPGs of a particular set of interneurons: Shox2+ interneurons. Many Shox2+ neurons are also Chx10+. Shox2 interneurons are rhythmically active in locomotion. Acute spinal cord specific inhibition of all Shox2+ interneurons slows locomotion in vivo, without affecting pattern generation (maintaining right-left and flexor-extensor alternation). However, ablation of Shox2+ neurons that receive V2a afferents, did not have this effect. Most Shox2 interneurons are implicated in the generation of locomotor rhythm.
To conclude this first part of the presentation, Dr. Kiehn highlighted that at the level of the spinal cord, there is a complex network organization, composed of different modules, which can be reconfigured to adjust to different gaits and behaviors.
And from which regions does this network receive commands?
Second part of Dr. Kiehn’s talk. Activation and stop of locomotion at the brainstem centers.
The second part of Dr. Kiehn’s talk was dedicated to the centers in midbrain and brainstem that regulate locomotion.
There are circuits on the midbrain and brainstem that indicate the CPGs to start, continue, end locomotion, to change speed or gait, all of this, according to the context (i.e.: stop walking when there is a danger ahead).
It has traditionally been considered that the brainstem input activates the CPG, animals move, and then in absence of this input, the CPG would eventually stop working. However, the process by which locomotion stops, had not been studied in detail.
So Dr. Kiehn’s team wondered: how is locomotion stopped?
There are two possible ways of doing this: either there is a mechanism to let go of speed –in line with the traditional views-, or there is a mechanism of active stopping, as “pressing a break”.
V2a neurons from the brainstem, which are Chx10+, project to the spinal cord. Optogenetic activation of these neurons, stops ongoing locomotion: this is a new pathway for active stop. They did experiments to locate these neurons: the V2a stop neurons are located in the upper medulla.
The stop effect, the brake, may also act at two different levels: by inhibiting motoneurons directly, or by inhibiting the CPG. Dr. Kiehn’s lab showed it is by inhibiting CPG: V2a stop neurons terminate in the CPG area and contact inhibitory and excitatory interneurons.
Activation of V2a stop neurons in vivo, halts ongoing locomotion. On the contrary, blocking the synaptic output of this neurons with a TeLC virus, increases locomotion -the mice do not stop walking, even if there is an obstacle-.
V2a stop neurons project to the spinal cord and provide an active stop mechanism.
To conclude with the talk, Dr. Kiehn exposed part of his last, unpublished work, where his team has studied the role of other glutamatergic neurons from the brainstem in the start of locomotion and speed control.
However, the team keeps in mind V2a stop neurons, and they would be interested in knowing the activity of these neurons in certain pathological conditions, like Parkinson’s disease.
This is a future direction of their studies, but not the only one. The circuits for locomotor control are better known now, but they are still not fully determined. This is a good moment in science history to look for the missing pieces.
Dr. Kiehn’s group will continue to work on their studies. Moving towards their ultimate goal: understanding the circuits that underlie motor control.
Be attentive to catch up with their next discoveries!
Bellardita C, Kiehn O (2015). Phenotypic characterization of speed-associated gait changes in mice reveals modular organization of locomotor networks. Curr Biol 25(11):1426-1436.
Bouvier J, Caggiano V, Leiras R, Caldeira V, Bellardita C, Balueva K, Fuchs A, Kiehn O (2015). Descending Command Neurons in the Brainstem that Halt Locomotion. Cell 163(5):1191-1203.
Caldeira V, Dougherty KJ, Borgius L, Kiehn O (2017). Spinal Hb9::Cre-derived excitatory interneurons contribute to rhythm generation in the mouse. Sci Rep 7:41369.
Dougherty KJ, Zagoraiou L, Satoh D, Rozani I, Doobar S, Arber S, Jessell TM, Kiehn O (2013). Locomotor rhythm generation linked to the output of spinal shox2 excitatory interneurons. Neuron 80(4):920-933.
Hägglund M, Borgius L, Dougherty KJ, Kiehn O (2010). Activation of groups of excitatory neurons in the mammalian spinal cord or hindbrain evokes locomotion. Nat Neurosci 13(2):246-252.
Hägglund M, Dougherty KJ, Borgius L, Itohara S, Iwasato T, Kiehn O (2013). Optogenetic dissection reveals multiple rhythmogenic modules underlying locomotion. Proc Natl Acad Sci U S A 110(28):11589-11594.
Harris-Warrick R (2013). Locomotor pattern generation in the rodent spinal cord. Encyclopedia of computational neuroscience. 1601-1614.
Kiehn O (2016). Decoding the organization of spinal circuits that control locomotion. Nat Rev Neurosci 17(4):224-238.
Talpalar AE, Bouvier J, Borgius L, Fortin G, Pierani A, Kiehn O (2013). Dual-mode operation of neuronal networks involved in left-right alternation. Nature 500(7460):85-88.
Talpalar AE, Endo T, Löw P, Borgius L, Hägglund M, Dougherty KJ, Ryge J, Hnasko TS, Kiehn O (2011). Identification of minimal neuronal networks involved in flexor-extensor alternation in the mammalian spinal cord. Neuron 71(6):1071-1084.
Aix-Marseille Université, France
Universitat Autònoma de Barcelona, Spain
ICN PhD Program in Neuroscience: http://neuro-marseille.org/en/phd-program-en/le-phd-program/