General introduction
Normal posture, gait and voluntary movement require input from sensory systems, planning and coordination centres, storage (memory) areas, and output through motor systems using UMNs and LMNs.
Upper motor neurons are the ‘managers’ and are confined to the CNS. Lower motor neurons are the ‘workers’ and have their nerve cell body in the CNS, but the axon, which forms the majority of the cell, travels via spinal or cranial nerves in the PNS to synapse at the neuromuscular junction. Thus we suggest that motor neurons could be described as either UMNs / central motor neurons, which are managers; and LMNs / peripheral motor neurons, which are workers. In the case of the autonomic efferent fibres, there are a pre-synaptic and a post-synaptic LMN (peripheral motor neuron). The terms central and peripheral motor neurons are more descriptive than upper motor neuron and lower motor neuron, however UMN and LMN are widely used, despite their propensity to cause significant confusion.The strength of muscle contraction is directly proportional to the frequency of action potentials in the nerve supplying the muscle. Conversely, the precision of muscle function is inversely proportional to the size of the motor unit, where a motor unit is defined as a single α-LMN axon and the muscle fibres with which it synapses. Muscles with large motor units, such as proximal limb muscles, are used for imprecise movements. Small motor units, such as found in the extraocular muscles, enable fine, specific movement of the target organ, such as the eye.
Muscle activity for maintaining posture (at rest or during motion) arises largely at a subconscious/subcortical level, whereas voluntary movement arises primarily from a conscious/cortical level. The subconscious level utilises reflex arcs linking function within, and between, limbs. Subcortical control results in postural changes (sitting, standing, etc.) and repetitive movement, such as breathing, basic locomotion, scratching and chewing.
Cerebrocortical control is used for voluntary, complex and learned movements such as hunting or the pet offering its paw to be shaken. Primates and humans have a much greater dependence on cortical motor centres of the forebrain for all movement including gait. In comparison, the spinal reflexes in domestic mammals, are the basic functional unit that underpins all posture and locomotion on which is superimposed, supraspinal input originating largely in the brainstem. As such, forebrain lesions can result in hemiparesis/plegia in humans, but do not compromise significantly locomotion in domestic mammals.