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NEUROANATOMY 3 – The neuroanatomy of movement Aims and objectives: • Recognize the appearance of the spinal cord at the four main subdivisions (session one) • Recognize the internal and surface markings of the four main motor pathways fromt he brain through to the spinal cord • Describe the topography and basic connections of the basal ganglia – be able to correctly label diagrams of horizontal and coronal sections through the basal ganglia • Explain what is meant by the term ‘cortical column’ • Describe the six main motor pathways • Understand, to the extent outlined in the workbook, the connections of the cerebellum Medulla Identify: • In the ventral midline, the anterior median fissure • The medullary pyramid, marking the position of underlying fibres of the corticospinal (pyramidal) tract • At the caudal end, the decussation of the pyramids (80% of corticospinal fibres cross here) • Lateral to the pyramid, the olive The Cerebellum • • • • • • Develops from the metencephlon Comprises of two ovoid hemispheres joined in the midline by a narrow median vermis Consists of an outer layer of grey matter and an inner core of white matter White matter surrounds centrally placed aggregations of nerve cells, the deep nuclei Ridges covering the surface are known as folia The anterior part of the superior vermis extends onto the superior medullary velum as a single lamella of cortical tissue, the lingual Identify on the cerebellum: • Horizontal fissure • Primary fissure (marks the division between the anterior and middle lobes) • Vallecula (deep hollow on the inferior surface, in which lies the inferior vermis) • Tonsil (a round swelling of the cortex on either side of the vermis) • The inferior vermis is divided into three sections: o Uvula (lies between the two tonsils) o Nodule (ventral to the uvula) o Pyramid (dorsal to the uvula) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Lobulus centralis Vellum medullare anterius Nodule Uvula Pyramid Vallecula Tonsils Hemisphere Pedunculus cerebelli Flocculus Horizontal fissure Immediately caudal to the entry point of the facial nerve, and crossed anteriorly by the emerging filaments of the vagus and hypoglossal nerves, lies the flocculus: th • It lies immediately posterior to the lateral foramen of the 4 ventricle • It is continuous with the nodule of the inferior vermis • Together they constitute the posterior or flocculo-nodular lobe (concerned with vestibular information) Midbrain The midbrain is divided into dorsal and ventral portions at the level of the cerebral aqueduct: • The dorsal portion is called the tectum (2 superior and 2 inferior colliculi) • The ventral portion is known as the tegmentum Identify: • Superior cerebral penduncle (passes from the cerebellum) • • • • Lateral lemniscus (emerges from lateral sulcus of cerebral peduncle and crosses obliquely to the inferior colliculus – the fibres form part of the auditory pathway) Inferior brachium (ridge of white matter passing anteriorly from each inferior colliculus to the medial geniculate body of the thalamus) Superior brachium (a similar ridge carrying visual information to the superior cooliculus from the optic tract) Superior medullary velum (bridges the midline gap between the superior peduncles) Superior meduallry velum Superior brachium Inferior brachium Lateral lemniscus Basal Ganglia When passing from the motor cortex to the spinal cord, motor fibres must pass the deep brain nuclei and the diencephlon. Identify: • Extreme, external and internal capsules • Lentiform nucleus o Globus pallidus (paler medial part) o Putamen (darker more lateral) o Lateral medullary lamina (separates the globus pallidus and the putamen) o Medial medullary lamina (separates medial and lateral segments of the globus pallidus) Putamen Insula Identify: • Caudate nucleus (separated form the lentiform nucleus by the internal capsule) o On anterior coronal slices, the head of the caudate is continuous with the putamen below the internal capsule • Nucleus accumbens (below the anterior horn of the lateral ventricle) • Septal nuclei (superior and medial to the nucleus accumbens) • Anterior commisure (horizontal mass of white fibres) • Anterior perforated substance • Amygdala Identify: • Unicate fasciculus (bundle of fibres interconnecting the frontal and temporal lobes) • Superior longitudinal fasciculus (Found superiorly around the external capsule, runs in the long axis) Both these fascicule carry association fibres, axons the interconnect different areas of the cerebral cortex Two structures which are not part of the basal ganglia anatomically, but which are closely linked to them functionally and in terms of connections are the substantia nigra and the subthalamic nucleus. Cerebellar Nuclei The dentate nucleus is the largest and most lateral of the nuclei, and the only one that can be seen on an unstained section. Others include: • Emboliform nucleus • Globose nucleus • Fastigial nucleus Midbrain Nuclei Identify: rd th • Cerebral aqueduct (interconnects the 3 and 4 ventricles) • Red nucleus • Substantia nigra (black band behind the crus cerebri) Connections of the Basal Ganglia The Striatum (caudate, putamen and globus pallidus) Afferents: 1. The whole cortex projects in an organised manner upon the caudate and putamen (probably using glutamate as a transmitter) 2. The intralaminar nuclei of the thalamus send organised fibres to the striatum 3. The pars compacts of the substantia nigra sends a strong dopaminergic projection Also receives serotoninergic fibres from the dorsal raphe nucleus of the brain stem and a projection from the amygdala Efferents: 1. To both segments of the globus pallidus 2. To the pars reticulate of the substantia nigra The striatum contains many cholinergic interneurons, which are severely affected in Huntington’s disease The Globus Pallidus Represents the major source of efferent fibres leaving the basal ganglia • Receives GABAergic fibres from the striatum • Projects to VA, VL and CM of the thalamus • In turn projects to the motor and premotor areas of the frontal neocortex • There is also a much smaller efferent projection to parts of the reticular formation • The external segment projects to the subthalamic nucleaus which in turn projects backto the internal segment • There are fibres from the pallidum to the substantia nigra The Subthalamic Nucleus Major connections are with the globus pallidus • Receives fibres from the external segment and projects back to the internal segment Also receives fibres from: • Pars compacta of the substantia nigra (dopamine) • Dorsal raphe nuclei (serotonin) • Locus coeruleus (NA) Projects to the: • Pars reticular of the substantia nigra Substantia Nigra Divisible into: • Pars compacta (small, closely packed dopaminergic cells) • Pars reticular (more ventral with larger more widely spaced cells) o Arguably part of the globus pallidus, which has been separated by the internal capsule and fibres passing onto the crus cerebri o Indistinguishable from the pallidum and its connections are almost entirely the same Receives from the stritum Projects to the medial part of the ventrolateral nucleus of the thalamus Also projects to the superior colliculus Sends dopaminergic fibres to the whole striatum Other afferents include: • Fibres from the amygdale • Fibres from the dorsal raphe nucleus • Fibres from the globus pallidus Connections of the Cerebellum The cerebellar cortex receives proprioceptive input via the olive and the spinocerebellar tracts. The cerebellar cortex then sends fibres to the deep cerebellar nuclei: the dentate, eboliform and globose nuclei all send fibres to the red nucleus (giving rise to the rubrospinal tract, influencing activity in the contralateral ventral horn) Fibres are also sent to the contralateral olive (giving rise to the olivospinal tract) The three cerebellar nuclei also give rise to: • an ascending projection to the interposed nuclei of the thalamus • an ascending projection to the ventrolateral nucleus of the thalamus (then projecting to the motor cortex) The fastigal nucleus sends fibres to the vestibular nuclei on both sides (giving rise to the vestibulospinal pathway, influencing activity in the ipsilateral ventral horn)
