Falter

Please note that this page is “under construction”!

Overview:

Falter- a : to walk unsteadily : stumble b : to give way : totter <could feel my legs faltering> c : to move waveringly or hesitatingly.

This article will continue with a top-down description specifically of locomotor pattern like running and cycling. It will focus on recent advances in neuroscience of the control and support of gait. First current knowledge of cortical structures regulating initiation and maintenance of gait will briefly summarized, then information about the supraspinal mechanisms regulating changes in gait and medullary/spinal motor pattern generating gait will be described.[1]

For example, running a marathon is a behavior of the organism itself and is a holistic state of mind. “A ‘state of mind’”, wrote Siegel (1999), “can be defined as the total pattern of activations in the brain at a particular moment of time.” The running state is built around the activations associated with the regulating the gait required for running. Since gait is ongoing cyclic physical activity, then the focus on generating and maintaining a rhythmic locomotor pattern that oscillates through the stance and swing phases.

A motor task is classified by its goal (Stein & Smith, 1997), and for locomotion, the goal is to move the body from location to another. One form of locomotion is marathon running, and in this case, the form’s goal is a smooth running gait. Specifically, this running form consists of a locomotor rhythm that is a “distinct set of relative timings of [its] kinematic variables” (Stein & Smith, 1997) and a locomotor pattern that is “the temporal sequence of motoneuron activations” (Stein & Smith, 1997). Also, this form can be learned so well that it recalled automatically and without effort as defined by Hebb’s Axon (Hebbs 1947 – cited by Siegel, 1999, p 26). This ‘law’ states that, “Neurons which fire together at one time will tend to fire together in the future.” Or as Siegel (1999, p 26) paraphrases it: “Neurons that fire together wire together.”

From above, factors associated with limiting endurance performance are:

  • a greatly increased muscular drive (i.e., effort) and
  • weak wobbly legs

(see more)

Cortical/Subcortical Organization and Regulation

Whole body motor processes, i.e., circuits

  • Anticipation machine
  • Representations, motor learning, and motor patterns
  • Action hierarchy
  • Forebrain motor control and conscious awareness
  • Execution circuit and feedforward command (motor drive)
  • Planning circuit

Brainstem Organization and Regulation

Segmental motor processes

locomotor regions in midbrain and in lateral hypothalamus of the diencephalon  (Enoka p. 282 )
MLR and DiLR reticular formation drive

Segmental movement regulation via proprioceptive reflexes that includes cerebellum.
Posture maintenance and adjustments
cerebellar predictive feedforward adjustment of locomotor rhythm p 284
movement coordination.
matching of motor pattern mismatching as error signals

activation of specific motor programs utilizing specific muscles (ref)
initiation: disinhibition, i.e., release of inhibition, to allow motor unit activation
basal ganglia and subthalamic nucleus p. 283

Modification of motor activity magnitude p. 281
Descending brainstem-midbrain monitoring and adjusting (see Rossignol et al., 2006)Sensory modulation during initiation, maintenance, and modification of rhythm plus phase transitions p. 280.
Other proprioceptive visual, auditory, and vestibular guidance p 285

Plasticity (Rossignol, 2006).

Spinal Organization and Regulation

Gait and movement modification processes

The nearly automatic generation of running gait appears to be the role of the spinal cord and the lower brain (SC-LB). That is, humans and lower vertebrates can execute postural and locomotor tasks like running largely without control from higher neural structures (Stein & Smith, 1997; Edgerton & Roy, 2006). Locomotor automaticity is due to the ability of the SC-LB’s neural circuitry “to interpret complex sensory information and to make appropriate decisions to generate successful postural and locomotor tasks” (Edgerton et al., 2004). A decerebrate animal can walk, run, speed up,  slow down, and move over obstacles. Edgerton and Roy (2006) state that these abilities have “lead to the concept that the [SC-LB] is smart.”

For simplicity, the wiring of the SC-LB that controls the running form’s step cycle mechanisms will be conceptually grouped into its two aspects: rhythm generation and motor pattern selection.

Locomotor rhythm generation can be described perhaps too simply by using Brown’s half-center model (1911, 1914) of the cyclic neuronal circuits in the SC-LB. The pace for the running form can stay steady, speed up, slow down, or stop. And pace is generated by an oscillating, rhythmic motor pattern belonging to a collection of the ‘half-center’ neural networks. Therefore, these networks function as a central pattern generator (CPG; Grillner and Zangger, 1979 – cited by Rossignol, 1996) and are distributed along the spinal cord (Grillner, 1981; Rossignol, 1996; Stein and Smith, 1997; Enoka, 2008). Each ‘half-center’ consists of one motoneuron and the interneuron networks associated with it, and the spinal CPG can execute locomotor “tasks largely without supraspinal control” (Edgerton & Roy, 2006). The locomotor motor rhythm for motoneurons and their segmental interneurons seems to be driven by a common oscillatory core (Zehr, 2005; Zehr et al., 2009) These core units are connected by reciprocal innervation to antagonistic core units in the ipsilateral and contralateral motor areas of each spinal segment. For lower (hind) limb movements, the focus of rhythm generation is in the L3-L4 spinal segments (Jankowska et al., 2003; Rossignol et al., 2008). For upper (fore) limb movements, the focus of rhythm generation is in the cervical region at the segments C3-C4 (see Alstermark et al., 2007 – cited by Delivet-Mongrain et al., 2008).

Furthermore, animal research suggests that long propriospinal tracts connect the CPG neural networks couple the hindlimbs and forelimbs in both directions (Zehr et al., 2009). Comparable observations of coordinated coupling of the arms and legs during walking, creeping, and swimming are found in humans (Dietz, 2002 – cited by Zehr, 2005). These forms are found to have the lower limb rhythmic activity dominant within the coupling to upper limb actions, except the upper limb can stabilize, correct, or enhance lower limb gait rhythm (see Zehr et al., 2009).

All the basic spinal neural networks for locomotor rhythm are genetically determined and present at birth (Grillner, 1973 and Forssberg et al., 1980a,b – both cited by Rossignol, 2006). Various rhythms are innate – mice at birth (refs).

As discussed below, in the SC-LB, both somatosensory reflexes feedback and direct input from descending pathways are essential to generating and modifying the rhythmic locomotor pattern.

For Module Two, the focus is on selecting the pattern used for locomotion. In general, the patterns for locomotion are walking, running, galloping, stopping, stepping sideways, veering, and turning. Also patterns adjustments must be made for going uphill- and down-hill, travelling on a side hill, and for moving over uneven terrain.

A locomotor pattern is defined to be a sequence of muscle or motoneuron activations (see Stein & Smith, 1997), and it has also been termed a motor program (see Grillner et al., 2005; Enoka, 2008), or an action representation (see Jeannerod, 1994; Doyon et al., 2008).
Neonatal mice body maps closely related to motor patterns emerges through a self-organizing process, termed motor-directed somatosensory imprinting (Petersson et al., 2003), that is later consolidated during sleep (Schouenborg, 2005; Granno et al., 2008). For some newborn vertebrates, and later for human infants, innate patterns are found for posture and locomotor activities of walking, scratching, hand (paw) shaking, or pain withdrawal (see Grillner et al., 2005; Rossignol, 2006). Various and “smoothing” of these motor patterns are learned quickly via, and after only several repetitions become a memory trace (see Shutoh et al., 2006; ????). In addition, basic locomotion for walking and running consists of five patterns common to walking and running pp 286-8.
Also called synergies (ref).
Modules
Combination of alternating left and right upper and lower creates gait.

Again, SC-LB contains the essential mechanisms for locomotor patterns with the location of the spinal motor programs or pattern representations for lower (hind) limbs appears to be mid-lumbar, focused on L3-L4, and the commissural interneurons found in these segment might contain the basic and learned action representations (Jankowska et al., 2003; Delivet-Mongrain et al., 2008). A similar one focus in thoracic spinal segments for upper (fore) limbs (see ref).

Spinal cord locomotor patterning requires brainstem sensorimotor afferent feedback activity and descending feedforward drive (see Rossignol et al., 2006).

 

Critical Elements:

  • Weakened force production and increased effort to maintain pace
  • Impaired coordination
  • Metabolic Depletion
    • Fatty acid mobilization and oxidation
    • Glycogen mobilization and depletion
  • Electrolyte imbalance

 

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