Basal Ganglia, Brainstem, and Spinal Cord in Motor Control

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.