BASAL GANGLIA, BRAINSTEM, AND SPINAL CORD IN MOTOR CONTROL I. Introduction This lecture delves into the role of the basal ganglia, brainstem, and spinal cordin motor control. Understanding these structures is crucial for comprehending how movements areinitiated, regulated, and adapted. II. Basal Ganglia A. Overview The basal ganglia form a network of interconnected nuclei within the cerebrum,thalamus, and brainstem. The striatum, consisting of the caudate nucleus and putamen, serves as the entrypoint, receiving input from the entire cortex. Key exit structures include the globus pallidus internus (GPi) and substantia nigrareticularis (SNr). Other components are the globus pallidus externus (GPe), nucleussubthalamicus (STN), and substantia nigra compacta (SNc). This network can inhibit or disinhibit cortical areas and brainstem nuclei,influencing specific actions. B. Functional Groups The basal ganglia can be categorized into two functional groups: the directpathway and the indirect pathway. The direct pathway (red arrows) enhances movement by promotingthalamocortical projections. The indirect pathway (blue arrows) dampens movement by reducing thalamicexcitation. Dopamine from the SNc (violet arrows) modulates these pathways, promotingmovement with dopamine release and inhibiting it with dopamine deficiency (e.g.,Parkinson's disease). A hyperdirect pathway (green arrows) from the cortex to the STN facilitates actionselection.
III. Brainstem and Spinal Cord A. Brainstem The brainstem controls posture and enables targeted movements. It integrates sensory information from various sources, including muscle length,tension, joint angles, vestibular input, and vision. Reflexes such as the vestibulospinal reflex, tonic labyrinthine reflex, and rightingreflex help maintain upright posture and gaze stability. The brainstem also plays a role in visceromotor tasks like respiratory andcardiovascular control, adapting them to motor demands. It offers a basic repertoire of movements that can be combined into complextasks. B. Spinal Cord The spinal cord is the lowest level of skeletal motor control. It adjusts limb positions and muscle contractions through reflex arcs, maintainingstability against external perturbations. Reflex arcs enable the spinal cord to generate complex movement patterns inresponse to stimuli, such as the flexor reflex during kicking. IV. Brain Anatomy and Functions Across the Lifespan The brain undergoes developmental changes and adaptations throughout life. Maximum brain weight, around 1400 g, is reached at approximately 20 years old. Brain weight gradually decreases by about 20% between ages 25 and 80. Neuronal plasticity allows for continuous learning and adaptation across thelifespan, with the brain constantly forming, modifying, and degrading neuralnetworks in response to stimuli.