Orchestration of posture and movement

The simplest motor task is the contraction of a muscle. The simplest stimulus driving this contraction is stretching of the muscle as detected by muscle spindles. This myotatic reflex is the simplest reflex and involves stimulation and contraction in the same muscle.

Increasing complexity of motor function requires integration in the nervous system, using interneurons (junior management) to recruit other LMN. Recruitment will lead to other reflex effects such as (a) relaxation of the muscles acting around the same joint that antagonise the first muscle, (b) stimulation of muscles with similar or complimentary activity to the first muscle in the same limb, (c) reflex muscle contraction or relaxation in the opposite limb and (d) reflex activity in muscles of the limbs in the other girdle.

Repetitive activities, such as locomotion, scratching or chewing, use central pattern generators to stimulate local reflex circuits for alternating contraction and relaxation of extensor and flexor muscles.

Movement stimulates muscle spindles and proprioceptive input from muscles; this input makes an important contribution to locomotion. The proprioceptive input reflexively induces muscle contraction, but is also sent cranially to the brain to inform coordinating and motor centres about the position and movement of body parts.

Superimposed on spinospinal circuitry are caudally directed outputs from the brain’s UMN centres (senior management). These supraspinal inputs initiate, modify, regulate, coordinate and terminate the activity generated in the spinal cord. Certain UMN tracts facilitate flexor activity, others facilitate extensor activity. The extrapyramidal system is especially important for posture and locomotion, working with intraspinal circuitry and muscle reflexes.

The cerebellum acts to keep the animal’s centre of gravity located over its supports (limbs) both at rest and during locomotion. The cerebellum coordinates agonist-antagonist muscle function in the limbs, trunk and neck muscle using continuous proprioceptive input from these muscles (subconscious proprioception). With that input it modulates output from the UMN centres for posture, locomotion and voluntary movement. It also receives input from the executive and UMN motor centres about planned motor activity and compares that input with proprioceptive input from the body, limbs and head. By comparing planned movement with where the body parts are located and how they are moving, it can modulate the activity of the senior management UMN centres. It exerts both precontrol over movement which is about to occur (e.g. establishing the postural platform) and regulates movements as they occur.

At the top of this hierarchical spectrum are the executive centres, primarily located in the forebrain. Executive motor management plans motor activity ranging from purposeful gait to voluntary learned movements. The executive uses inputs from sensory areas (e.g. touch, kinaesthesia, thermoreception, nociception, vision, audition, olfaction), and motivation, memory and behaviour areas of the forebrain, including the limbic system (Chapter 12). It also relies on information from integration centres of the cerebellum, forebrain (basal nuclei) and reticular formation. The executive motor planning centres then direct UMN centres of the motor cortex and brainstem, which then utilise LMNs to perform consciously directed, voluntary movement.

Thus, while a significant portion of posture and locomotion is based on local spinal reflexes, coordinated, balanced movement, that starts and stops at the right moment and has the correct rate range and force of movement, requires supraspinal inputs.