Projection Systems

Abstract

Sensory and motor information travels through a complex system of ascending and descending projection fibres that functionally connect the peripheral and central nervous systems. Ascending sensory pathways are comprised of two major somatosensory routes: the spinothalamic tract runs within the dorsal column of the spinal cord and the medial lemniscus of the brain stem and conveys somatosensory information from the body; the trigeminal-thalamic pathway carries somatosensory information mainly from the head (face, oral and nasal cavities, etc.). Other incoming sensory projections are formed by the auditory, visual and olfactory pathways. The three major descending cortico-subcortical projection systems are concerned mainly with motor functions, and include the corticospinal and corticobulbar tracts, and the cortical efferents to the basal ganglia, and the projections to the cerebellum via the pontine nuclei. The projections to the basal ganglia and cerebellum are indirectly reciprocal, so that the cortex also receives projections from these centres via the thalamus, to create complex cortico-basal ganglion and cortico-cerebellar circuits. These projection systems are only partially segregated anatomically, as they share several subcortical relay stations (e.g. the thalamus). Figure 10.1 illustrates the trajectories of the major projection pathways reconstructed with diffusion tensor tractography.The anatomical nomenclature of these projection pathways changes along their course through the cerebral hemispheres and brainstem, and names derived from descriptive anatomy have often been adopted. For example, the fan-like array of fibres between the cortical surface, basal ganglia, and thalamus is called the corona radiata because of its crown-like shape, formed partly by the ascending thalamic fibres radiating out to various cortical destinations, and partly by descending motor fibres converging towards the base of the cerebrum.More inferiorly, the fibres of the corona radiata enter the internal capsule, a space confined between the thalamus and caudate nucleus medially and the lentiform nucleus laterally. In horizontal section, the internal capsule has the appearance of a shallow letter V, divided into an anterior limb, a middle genu, and posterior limb. Within the internal capsule the fibres have an orderly arrangement from front to back according to their destination or origin, and in the case of the somatosensory and motor fibres, the parts of the body they serve (Figures 10.1 and 10.2) (Beevor and Horsely, 1890; Dejerine 1901; Crosby et al., 1962; Behrens et al., 2003; Newton et al., 2006). The anterior limb of the internal capsule carries ascending thalamic projections concerned with cognitive, limbic, basal ganglionic, cerebellar, and other complex functions of the frontal lobe. The anterior limb also contains descending fronto-striatal fibres to the basal ganglia, and fronto-pontine fibres to the pontine nuclei, which in turn project to the cerebellum. More posteriorly, the genu contains ascending somatosensory thalamic projections from the trigeminal pathway to the postcentral gyrus, and descending corticobulbar motor fibres serving cranial nerve somatomotor nuclei. The posterior limb contains ascending thalamic fibres to the parietal, occipital, and temporal lobe, and descending corticopontine and corticospinal motor fibres (Figure 10.2). At the most posterior end of the internal capsule, visual projection fibres from the lateral geniculate nucleus of the thalamus run towards the primary visual cortex in the occipital lobe, via the capsule's retrolenticular part, whilst auditory projection fibres transit its sublenticular region en route to the primary auditory cortex in the superior temporal gyrus.In the midbrain, the descending motor (corticospinal, corticobulbar and corticopontine) fibres collect inferiorly and posteriorly at the base of the internal capsule into compact bundles within the most anterior region of the cerebral peduncles. Ascending in the midbrain, posterior to the cerebral peduncles, are three major sets of somatosensory afferents to the thalamus. Dorsolaterally, the fibres of the spinothalamic (anterolateral) tract convey pain, temperature, and coarse touch information from the contralateral side of the body. More ventromedially, the medial lemniscus carries conscious proprioceptive, fine discriminative touch, and vibration sensory information from the contralateral side of the body. Finally, the trigeminal lemniscus is formed by trigeminothalamic fibres that carry somatosensory information from the opposite trigeminal sensory nucleus to the thalamus.