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42a - March 21 2013 .Basal Ganglia 2. Normal Structure and Function.doc

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University of Toronto St. George
Mac Burnham

BASAL GANGLIA 2. NORMAL STRUCTURE AND FUNCTION OUTLINE Anatomy Physiology Neurochemistry Throughput Pathway Modulation Pathway Schematic – Penney and Young 1.ANATOMY The basal ganglia per se: Caudate-putamen (“striatum”) and globus pallidus (“pallidum”) (all basal forebrain / telencephaplic structures) • Basal ganglia, strictly defined, is caudate, putamen, and GP. All in the basal • The caudate putamen is a single structure in lower animals. In higher animals, the internal capsule (white matter tract) goes down and splits it into caudate AND putamen. When you’re looking at it as one structure will be called striatum. • Corpus striatum: old name for basal ganglia • Neostriatum: may also be applied to caudate-putamen. In justaposition to ventral striatum, which is nucleus accumbens and parts of olfactory tubercule. • Globus pallidus is also called the pallidum. Related structures: Substantia nigra (mesencephalon) and subthalamic nucleus (diencephalon). • There are related structures, one in floor of midbrain (substantia nigra) and one in the diaencephalon (subthalamic nucleus) • SN: group of dopaminergic cell bodies lie here. Cell bodies here mostly go to caudate-putamen. Its right next to the VTA which projects to the nucleus accumbens and PFC. • Substantia nigra pars compacta to striatum, and this connection is called the nigrostriatal pathwayaffected in parkinson’s disease. • Subthalamic nucleus: floor of diencephalon. This is also going to be part of basal ganglia. Some books include both these structures as part of the BG, others consider it as related structures (like the prof) Figure: Carlson 8:24 • Globus pallidus has two divisions. External and internal division. • Basal ganglia next to the thalamus. Note: there is a connection to the motor centers of the brain stem that give rise to the ventromedial pathway • There is a connection from the GP down to the brain stem. What we’re dealing here is old motor structure. Later as neocortex developed, it was rewired so that it heavily interacts with frontal neocortex specifically with the motor areas of frontal neocortex, but it still has old connections down to brainstem, which are probably important in some aspects. • These old connections of BG down to brainstem innervate parts of brainstem that give rise to ventromedial pathway, which would be from reticular core of brainstem, reticulospinal being most important onehas to do with posture and repetitive movements. 2. PHYSIOLOGY / FUNCTION • The motor cortex is up here. It’s connected to the thalamus, and two thalamic nuclei important are va and vl. Ventro anterior and ventro lateral. • This connection is essentially an excitatory feedback loop. • Thalamus excites cortex, cortex reexcites thalamus. • Up in motor cortex, you’re going to have movement programs. When you want to move voluntarily, it will be generated here with support from the thalamus, and this support is crucial because it can’t function without it. • Here we have corticothalamic loop which creates voluntary behaviour. • Next to it, you have GP internal, which tonically inhibits VA/VL. If it weren’t doing that, you would move constantly. In other words, thalamocortical is excitatory, if functioned unchecked, would move all the time, like in huntington’s chorea. • But when does GP release its inhibition? Have two pathways, the direct pathway and indirect pathway. • So thalamus wants to move, GP is holding it down, and when movement isamus). appropriate, caudate-putamen will inhibit proper part of GP which allows movement to occur in corticothalamic pathway. (inhibition of inhibition) • The direct pathway is the GO pathway, will cause voluntary movement to occur. i) Direct path - Structure: Whole cortex to striatum (excitatory); striatum to internal globus pallidus (inhibitory); internal globus pallidus to VA/VL thalamus (inhibitory), VA/VA thalamus to cortex and frontal cortical areas (excitatory). (two inhibitory neurons makes the whole pathway excitatory – when they’re all active) - This is the “go “ pathway. It promotes movement. Function: Internal globus pallidus tonically inhibits thalamocortical circuits. Striatum phasically disinhibits thalamocortical circuits by inhibiting internal globus pallidus. Cortex tells the striatum when to act. • The indirect pathway is the stop pathway and its longer. • Instead of reducing inhibition of thalamocortical circuits, it increases it. • It starts off in caudate-putamen, which projects to GP EXTERIOR, so goes to GPe and inhibits it. GPe goes to subthalamic nucleus and inhibits that. And the subthalamic nucleus excites GPI. • Essentially GPi is the stop structure, direct pathway removes activity of the stop structure, and indirect pathway reinforces it. • it removes inhibition to subthalamic nucleus which then excites GPi.bits GPe, ii) Indirect path - Structure: Whole cortex to striatum (excitatory); striatum to external globus pallidus (inhibitory); external globus pallidus to subthalamic nucleus (inhibitory); subthalamic nucleus to internal globus pallidus (excitatory); internal globus pallidus to VA/VL thalamus (inhibitory), VA/VA thalamus to cortex and frontal cortical areas (excitatory). (three inhibitory neurons makes the whole pathway inhibitory – when they’re all active) - This is the “stop” pathway. It suppresses movement. Function Internal globus pallidus tonically inhibits thalamocortical circuits. Subthalamic nucleus phasically reinforces the inhibition of the thalamocortical circuits. External globus pallidus tonically (?) inhibits the subthalamic nucleus. Striatum phasically inhibits the external globus pallidus (releasing the subthalamus, and therefore inhibiting the thalamocortical circuits). Cortex tells the striatum when to act. • What governs caudate-putamen? The whole neocortex, although most heavily innervated from pre or post central gyrus. • Neocor
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