These Circuits Were Made for Walking

July 21, 2020

Written by: Yarden Wiesenfeld

 

This is the second post from summer guest author Yarden Wiesenfeld.  Yarden is a Penn undergraduate student in Dr. Michael Granato’s lab who studies the molecular mechanisms that drive learning in zebrafish. She is a dancer in Penn’s swing dancing troupe and also loves art and cooking.

 

 

Watching a baby take its first steps is an important milestone for many parents. From the ages of 4 to 9 months, babies progress from sitting with support, to crawling, to cruising while gripping the edges of low furniture. Waddling alone on their own two feet marks an exciting development. A baby’s first steps also represent a new horizon of exploratory behavior. Items that were out of harm’s way before now become reachable. Babies quickly learn more about how their 3D world works by shaking, throwing, hitting and mouthing objects around them. Many aspects of psychological development, including rapid changes in spatial search, memory, goal-directed behaviors, and perception of one’s actions and those of people around them, are associated with walking independently1.

 

But how does locomotion develop, and how do we learn to walk without even being taught how?

 

The key to walking is rhythm. The alternating movement of our limbs, order of muscle contractions, and timing of our strides is highly rhythmic, even if we might not think about it that way. These patterns inherent to walking are guided by neural circuits called central pattern generators2.

 

Central pattern generators, or CPGs, are neural circuits that create periodic activity without receiving rhythmic input. CPGs are involved in all sorts of behaviors, from how we breath to the way we swallow. A central pattern generator in the lower part of our spinal cord modulates walking. This network activates motor neurons that in turn drive our muscles.

 

CPG circuits in the spinal cord seem to begin developing before a baby is even born. A “newborn stepping reflex” is observed in babies of less than 2 months old3. Newborn stepping seems to be a continuation of rhythmic movements that occur in the womb, where an infant is surrounded by fluid and doesn’t have to support its own weight. Babies will spontaneously step in alternating movements when they are in water, supported by water buoyancy, or held on a motorized treadmill where they are sufficiently supported (~70% of their body weight)4. Do they “think” about walking? The answer, surprisingly, is no.

 

In fact, it has become clear that cortical control is not required for locomotion. A pathway originating in the brainstem called the reticulospinal tract receives inputs from the cerebral cortices and projects to the spinal cord, where it activates the CPG to elicit motor behaviors5. In animals whose cerebral hemispheres have been physically removed, locomotion can still occur spontaneously, showing that only brainstem structures are essential to initiate movement5.

 

What, then, makes a newborn’s stepping different from walking?

 

A research group in Italy4 aimed to answer this question by studying how muscle activation during walking develops with age. They compared the gaits of newborns, toddlers, preschoolers, and adults, focusing on critical timepoints including touchdown, midstance, lift off, and midswing (Figure 1). To measure muscle activity, the researchers applied EMG electrode stickers to different areas of the hips and legs.

Ivanenko_baby_walking
Figure 1: The step cycle involves 4 key timepoints: touchdown, midstance, lift off, and midswing. EMG electrode stickers were attached to the body to monitor muscle activation throughout the stride cycle. Image reproduced from Ivanenko et al., 2013.

They found that in newborns, lower segments of the body were more highly activated than higher segments. Compared to the older groups, their steps were more irregular, stride cycles took longer, and step length was shorter relative to their limb length. Their peak muscle activation occurred midstance or midswing. This was unlike toddler and adult walking, which showed highest activation in response to foot-ground contact—at touchdown and lift-off. Newborns took a hip-flexed posture, with bent knees and no heel strike. This stance is in keeping with a baby’s central goal when beginning to walk—not falling! A shorter step and lower bearing helps to minimize this risk6.

 

In toddlers and preschoolers, activation of the lower and higher muscles was more segregated. Lower muscles were activated at touchdown, while higher segments were most active at midstance. This reflected a halfway point to adult walking patterns, in which top and bottom segments were further distinguished and had much shorter activation times.

 

How did their walking rhythms, a function of their central pattern generators, differ? The researchers identified basic patterns underlying the muscle activations of each age group7. Newborns exhibited two patterns that both looked like sine curves, or smooth waves. One pattern corresponded to activation of muscles that hold the baby’s weight during stance and the other represented muscles that tensed to propel the limb during swing. Amazingly, at the toddler stage, two new basic patterns appeared. These new patterns were in sync with touchdown and lift off. By adult age, the four patterns had become tightly tuned to the critical moments of heel strike, weight shift, propulsion forward, and lift off.

 

How did these new basic patterns come about in the early months of growth? Its neural basis is not certain yet, but one convincing answer is that complex motor patterns evolve with stronger sensory influences on CPGs:

 

Balance. Many features of our sensory system are far from fully developed at birth. Take balance, for example. Thought of by many as our “sixth sense,” balance is critical to walking. Our bodies need to find a way to keep our center of gravity directly above our (only) two feet as we walk. Vestibular nerves that project to the spinal cord and control balance are not mature in newborns8. This is why beginner walkers tend to take a wide stance.

 

Pressure sensors. Immature pressure sensors in a baby’s feet could be the reason for their lack of foot-contact related muscle activity. Though sensory information from the feet travels through the spinal cord, it has to be processed in the brain. Connections between an infant’s brain and spinal cord are not yet fully formed8. While toddler and adult walking patterns reflected responses to touching the ground and pushing off, the continuous wave of infant rhythms displayed their lack of functionality.

 

Vision. Visual cues are essential for stability and perception of our 3D environment. Pathways from vision processing centers in the brain to the spinal cord are not mature in an infant8, which may help to explain their walking rhythm.

 

Learning is important for us to be able to integrate sensory input about our bodies relative to our surroundings. Pathways connecting our perception and sensory abilities with locomotor control may be at the core of differences between the gaits of infants and adults. While stepping might feel natural, and may even be programmed automatically from birth through spinal CPGs, nervous system growth in parallel with motor development is necessary for the more complex rhythms that characterize walking. A baby’s sensory processing systems will have to catch up before it can walk!

 

 

 

 

 

 

References

  1. Anderson, D. I. et al. The role of locomotion in psychological development. Front. Psychol. 4, 440 (2013).
  2. Dimitrijevic, M. R., Gerasimenko, Y. & Pinter, M. M. Evidence for a spinal central pattern generator in humans. Ann. N. Y. Acad. Sci. 860, 360-376 (1998).
  3. Adolph, K. E. & Franchak, J. M. The development of motor behavior. Wiley Interdiscip. Rev. Cogn. Sci. 8, 10.1002/wcs.1430. Epub 2016 Dec 1 (2017).
  4. Ivanenko, Y. P. et al. Changes in the spinal segmental motor output for stepping during development from infant to adult. J. Neurosci. 33, 3025-36a (2013).
  5. Jordan, L. M. & Sławińska, U. in Neuronal Networks in Brain Function, CNS Disorders, and Therapeutics (eds Faingold, C. L. & Blumenfeld, H.) 215-233 (Academic Press, San Diego, 2014).
  6. Vaughan, C. L. Theories of bipedal walking: an odyssey. Journal of Biomechanics 36, 513-523 (2003).
  7. Dominici, N. et al. Locomotor Primitives in Newborn Babies and Their Development. Science 334, 997-999 (2011).
  8. Kubie, J. A newborn infant can take steps. Why can’t she walk? BrainFacts/SfN (2013).

 

 

Images:

Cover Photo by coombesy from Pixabay https://pixabay.com/photos/children-kids-babies-walking-450925/

Figure 1: Image reproduced from Ivanenko, Y. P. et al. Changes in the spinal segmental motor output for stepping during development from infant to adult. J. Neurosci. 33, 3025-36a (2013). CC BY-NC-SA 3.0.

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