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Monday, Jan. 23, 2006 03-55-485 Pathways through vestibular nuclei • Cranial nerve VIII --> vestibular nuclei --> descending axons Lateral vestibular tract OR Medial vestibular tract Monday, Jan. 23, 2006 03-55-485 Pathways through vestibular nuclei • Also, pathways from upper motor neurons to lower motor neurons that control eye movements. Monday, Jan. 23, 2006 03-55-485 Reticular Formation • Network of circuits • Located at the brainstem core, from midbrain to medulla. • Descending motor control pathways to the spinal cord - terminate primarily in the medial parts of the gray matter where they influence the local circuit neurons that co-ordinate axial and proximal limb muscles. Monday, Jan. 23, 2006 03-55-485 Reticular formation: functions • • • • • cardiovascular & respiratory control. sensory motor reflexes org. of eye movements regulate sleep and wakefullness * temporal and spatial co-ordination of movement Monday, Jan. 23, 2006 03-55-485 Posture • In response to disturbances in body position (env or self) – -->Vestibular nuclei and reticular formation – --> information to the spinal cord Monday, Jan. 23, 2006 03-55-485 Vestibular nuclei response from inner ear to change in balance Inner ear Vestibular nuclei Direct projections to the spinal cord Compensation in stability Monday, Jan. 23, 2006 03-55-485 Adjustments to stabilize posture during ongoing movements • Motor centers in cortex or brain stem reticular formation Appropriate neurons are activated Monday, Jan. 23, 2006 03-55-485 How do upper motor neurons of the reticular formation maintain posture? • Look at voluntary movements. • Fig. 16.5. A subject uses his arm to pull a handle in response to an auditory tone Monday, Jan. 23, 2006 03-55-485 Feedforeward • “predicts” body stability disturbance and body can generate appropriate stabilizing response Monday, Jan. 23, 2006 03-55-485 Hear the auditory tone activity in the biceps begins about 200 ms after the tone. Monday, Jan. 23, 2006 03-55-485 This shows that posture control entails an anticipation effect (feed forward). • Effect of contracting the biceps will move the body forward, and you need the gastroc to be contracted to hold the body weight. Monday, Jan. 23, 2006 03-55-485 Animal models (cats): • Forepaw movement feedforward postural adjustment to other legs Expt: electrical stimulation of motor cortex forepaw lifted and postural adjustment. Monday, Jan. 23, 2006 03-55-485 Pharmacological blockade of the reticular formation electrical stimulation of motor cortex forepaw lifted and NO postural adjustment. Monday, Jan. 23, 2006 03-55-485 Cortical upper motor neurons initiate both forepaw movement and posture adustment. • Direct – Upper motor neurons from motor cortex project directly to spinal cord Indirect Upper motor neurons from the motor cortex project to brain stem areas (including reticular formation) Activate neurons that project to the spinal cord Monday, Jan. 23, 2006 03-55-485 Damage to direct corticospinal pathway at medulla indirect projections from cortex to brainstem and brainstem itself can sustain motor behaviour involving proximal muscles. Direct projections for the motor cortex to the spinal cord provide the speed and agility of movements, these enable precision of finger movement. Eg. After motor cortex damage in humans, get partial recovery of voluntary movements. These may be the indirect projections through the brainstem centers. Monday, Jan. 23, 2006 03-55-485