Comparative anatomy of the corticospinal system

The comparative anatomy of the long, corticofugal pathways from cortex to subcortical structures is important because we can interpret the key features of these pathways in humans on the basis of their similarities and differences when compared with other animals. This is particularly true of the corticospinal system. All mammals have a corticospinal tract, which originates from a number of parietal and frontal cortical areas, and which projects to different regions of the spinal cord. However, in functional terms, the different components of the corticospinal projection show large species-specific contrasts. There are also marked variations in other anatomical features, such as range of fibre sizes, spinal trajectory, pattern of termination within the spinal grey matter, and extent of the tract along the length of the spinal cord.

Cortical Origin

There is a basic pattern across all mammals for the cortical areas that give rise to the corticospinal tract. There are more areas in primates than in non-primates. In the former they include primary motor (M1) and somatosensory (S1) cortex, parietal area 5, S2, and frontal areas including dorsal and ventral premotor cortex and the supplementary motor area (all area 6) and a family of cingulate motor areas in the cingulate cortex on the medial surface of the hemisphere. In all of these areas, corticospinal fibres arise only from lamina V, which of course is very thick in motor cortex. In humans, all of these areas probably give rise to corticospinal fibres. Classical studies suggested that 60% of these fibres come from the frontal areas and 40% from parietal.

Number and size of axons

The number of fibres in the mammalian corticospinal (pyramidal) tract varies from around 30,000 from each cortical hemisphere in the mouse, up to around 1 million fibres in humans. There is a close relationship between fibre number, overall body weight and brain size. In all mammals the great majority of the fibres are of small diameter (<3 μm). In some large-bodied primates, there are larger fibres (>8 μm) and the largest are in humans (22 μm), but large fibres make up only a few per cent of all fibres in the tract.

Spinal trajectory

The course taken by corticospinal fibres within the spinal cord shows great variation. In rodents, rabbits and opossums most of the fibres are located at the base of the dorsal columns, whereas in elephants and hedgehogs, they are exclusively in the ventral funiculus. In carnivores and primates, most of the fibres are in the dorsolateral funiculus, with a smaller number of uncrossed fibres travelling in the ventral funiculus. A few fibres may also be scattered in the dorsal columns. In humans, fibres making up the lateral corticospinal tract are found throughout the dorsolateral funiculus ( Fig. 3.1.1 ), although they are considered to lie slightly medial to the ascending dorsocerebellar tract fibres, which occupy the most lateral part of the dorsolateral funiculus. There is no obvious somatotopic organization within the various corticospinal tracts: fibres from different subdivisions (arm, leg) of M1, for example, are intermingled, as are projections from different cortical areas.

Fig. 3.1.1, Pattern of corticospinal projections from motor cortex in five different species. The black dots in the white matter funiculi indicate the trajectory of crossed corticospinal fibres, which are found in the dorsal columns in the opossum, but mostly in the dorsolateral funiculus in the other species shown. Uncrossed fibres are shown travelling in the ventral funiculus. The black dots in the spinal grey matter indicate the termination of corticospinal fibres, as determined from visualization of degenerating terminals. The diagrams illustrate the ventral shift of terminations from opossum, where most fibres terminate in the contralateral dorsal horn; to cat, where fibres terminate in the contralateral intermediate zone; to the rhesus monkey, chimpanzee and human, which show progressively greater numbers of terminals in the lateral motor neurone pools of the ventral horn. Terminations in ipsilateral lamina VIII are also shown.

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