Basic stepping movements and control systems

Stepping, and the oscillation between extension and flexion of limbs and body, weight bearing and non-weight bearing, is based primarily on reflex circuitry within the spinal cord; it also involves central pattern generators. The flexion of one limb can induce reflex extension in the other limbs, both of the same limb girdle and the other limb girdle (see Fig. 9.1); this utilises the crossed extension reflex. Note that in the normal animal, when recumbent, the crossed extensor reflex is inhibited. In recumbent animals with UMN lesions, active flexion of one limb (e.g. withdrawal reflex) can result in the contralateral limb extending. This indicates loss of descending UMN pathways that inhibit crossed extension reflexes.

Fig. 9.1 Extension and flexion of alternate limbs is mediated largely by reflex circuitry within the spinal cord, but supraspinal input is required for locomotion.

The extensor postural thrust reflex is utilised when the foot makes contact with the ground, and the limb joints begin to flex under the effect of gravity. This activates the muscle spindles in extensor muscles, inducing reflex extension and thrust is generated against the ground, resulting in support for the body. This utilises the same circuitry as the mytotatic reflex.

Input, integration and output of the reflexes: Sensory input for the reflexes comes from muscle spindles, Golgi tendon organs, joint and tactile receptors. Integration of that input in the spinal cord causes inhibition or excitation of LMNs, as appropriate, in that same limb, in the opposite limb or the limbs of the other girdle. The activity of the epaxial and hypaxial muscles of the trunk, neck and tail are also interlinked. For example, look at how a cat uses its tail to maintain its balance by offsetting muscle activity in the trunk when it is ‘tight-rope walking’ along the top of a narrow fence. Integration of neural activity between the muscles of the limbs and body is done primarily using spinospinal tracts such as the propriospinal tract. This tract connects between spinal cord segments and is located in all funiculi immediately surrounding the grey matter (see Fig. 4.5). Constant proprioceptive input from all body and limb muscles both activates the reflexes and sends sensory information to the cerebellum for use in coordination of motor activity. Proprioceptive input to the forebrain gives rise to conscious awareness of posture and movement; this is called kinaesthesia (see Chapter 6).

The LMNs are stimulated or inhibited by both reflex connections and the input from the UMN system. The UMN tracts stimulate, or inhibit, muscle activity of the limbs and body. In quadrupeds, UMN nuclei of the brain stem are the nuclei primarily responsible for locomotion; there is minimal input from the motor cortex of the cerebrum. Via spinal UMN tracts, UMN nuclei recruit spinal reflex circuitry for locomotion. Their input initiates, regulates, modulates, coordinates and terminates activity in the reflex circuits and in specific LMNs. There are specific locomotive trigger centres in the brainstem. When stimulated, ambulation is triggered. When inhibited, ambulation is terminated.

The UMN tracts primarily responsible for the protraction and support phase are the vestibulospinal and pontine reticulospinal tracts facilitating extensor muscles. For the retraction/flexor phase, extensor muscle activity must be inhibited and flexor muscle LMNs must be stimulated utilising the medullary reticulospinal and the rubrospinal tracts (Table 4.3). At the end of retraction, extension and subsequently, the support phase, occurs again.

Coordination of locomotion

The overall function of the cerebellum is to coordinate agonistic/antagonistic muscle activity to permit posture and to create movement that occurs at the correct rate, range and force (see Chapter 7).

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