07 - Basal Ganglia

Basal Ganglia

© SPMM Course  In right-handed people, the left hemisphere is mostly dominant. In 10% of right-handed people, the right hemisphere is dominant. Among left-handed people only about 20% are right hemisphere dominant, with 64% left hemisphere dominant and 16% showing bilateral dominance.  Size asymmetry: The planum temporale is a triangular region on the upper surface of the superior temporal gyrus. It is important for language processing and is larger on the left than the right hemisphere in 65% brains. It is probably the most asymmetrical structure in the human brain, with some individuals having a five times larger planum temporale on the left than on the right. This asymmetry is reportedly reduced or reversed (right>left) in schizophrenia.

B. Subcortical structures Limbic system/ Papez circuit  Broca first described the limbic lobe. Papez and later Maclean assigned the function of emotional processing to limbic structures though this view is challenged in recent times.  The Papez circuit consists of the hippocampus → fornix → mammillary bodies → mammillothalamic tract → anterior thalamic nucleus → genu of the internal capsule → cingulate gyrus → parahippocampal gyrus → entorhinal cortex → perforant pathway → back to hippocampus  The boundaries of the limbic system were subsequently expanded outside of the Papez circuit to include the amygdala, septum, basal forebrain, nucleus accumbens, and orbitofrontal cortex.  The limbic system is thought to be involved in various functions such as mediation of emotional responses (through amygdala), influencing neuroendocrine responses (via hypothalamus) and reward system regulation (via nucleus accumbens).  The limbic system is often considered to be evolutionarily older than the higher cortical centres. Medial temporal structures  Include the hippocampus, amygdala, entorhinal and parahippocampal cortex.  Hippocampus appears to play an important role in memory processes. It is one of the few brain regions where the continuous production of new neurons is noted even in adult life.  Amygdala appears crucial for fear conditioning and emotional regulation Basal Ganglia  The basal ganglia are a group of gray matter nuclei forming the largest subcortical structure in the brain. They are involved in the planning and programming of movement, and also have a role in the processes by which an abstract thought is converted into voluntary action Left Hemisphere lesions Right Hemisphere lesions Aphasia Visuospatial deficits Right-left disorientation Anosognosia Finger agnosia Neglect Dysgraphia (aphasic) Dysgraphia (spatial, neglect) Dyscalculia (number alexia) Dyscalculia (spatial) Limb apraxia Constructional apraxia Dressing apraxia

Face recognition (bilateral)