Descending motor control tracts Figure 23-1. The tracts that mediate orienting movements and postural control lie medial to those responsible for voluntary movements. The corticobulbar tract terminates in the caudal medulla and therefore is not present in this section through the cervical enlargement. The lateral corticospinal (lcst) and rubrospinal (rst) tracts are the primary tracts used for voluntary praxis using appendicular or limb muscles. The corticobulbar tract (not present here) is used for voluntary praxis using facial, oral and upper airway musculature. Orienting movements of the body are coordinated primarily by the medial vestibulospinal (mvst) and tectospinal (tst) tracts. The tectospinal tract runs the length of the spinal cord and reaches motor circuits controlling proximal leg and trunk movements. In contrast, the medial vestibulospinal tract ends at cervical levels and primarily influences shoulder and neck movements. The medial longitudinal fasciculus (not present here) runs the length of the brainstem and coordinates eye movements important for orienting movements. The anterior corticospinal (acst), reticulospinal (lateral, lrst; and medial, mrst) and lateral vestibulospinal tracts are the primary tracts used for ensuring postural stability. Proximal musculature both in the trunk and limbs is used for most postural adjustments.

Indirect vs. direct control/influence Figure 23-2. Neurons in motor control centers project to motor interneurons, motoneurons, or both. Pathways with at least some direct projections to motoneurons support fine fractionated movements of the distal limbs and lower face. Projections from motor control centers to interneurons including neurons that participate in central pattern generator circuits support axial, proximal, and bilaterally symmetrical movements.

With 100 engineers and lots of $$$... Figure 23-3. PETMAN is a robot with a bipedal gait modeled after our own. PETMAN has ankle-, knee-, and hip-like joints and walks using a typically human swing and stance gait. PETMAN was designed to walk with the heel striking the ground first and the toe leaving the ground last (B). PETMAN images courtesy of Boston Dynamics ©2010. …you can build a robot that is not nearly as flexible and capable as the body we have for free.

Center of mass and support surface Figure 23-4. When standing, the center of mass (red dot) for an average human is just below the belly button. A: Postural stability is easiest if the center of mass lies above the support surface (blue shape). B: Leaning forward displaces the center of mass (red arrow) to a point in front of the support surface. Maintaining stability for a posture in which the center of mass is outside of the support surface requires muscle force (black arrow) that opposes the displacement of the center of mass.

Anticipatory postural adjustments Figure 23-5 </FGN><FGC>In preparation for rising from a sitting position, people lean forward to advance their center of mass before rising. Leaning forward is absolutely necessary, as you will learn if you try to rise straight up from a seated position. In this cartoon of a man rising from a chair, the projection of the body’s center of mass is marked by a red x. The position of the support surface after standing is marked by a blue rectangle. Note that in the seated position, the support surface includes the region of contact between the body and the chair. To rise from a sitting position, the center of mass is first advanced by leaning forward. Yet, even by leaning forward does not place the center of mass within the standing support surface. To move the center of force over the support surface, force from the contraction of leg muscles shifts the center of force both forward and up.</FGC> <FGS> Modified from Muybreadge, E. The human figure in motion: an electro-photographic investigation of consecutive phases of muscular actions. London: Chapman & Hall, 1904.

Motor control of top and bottom face Figure 23-6. Virtually everyone can pull their mouth back to each side (A) but few people can raise each eyebrow laterally (B). C: Primary motor cortex (M1) projects strongly to facial motoneurons that innervate the muscles of the contralateral lower face and does not project directly to facial motoneurons that innervate upper facial muscles. D: The motor area in the anterior cingulate gyrus (M3) projects strongly to facial motoneurons that innervate the muscles of the upper face on both sides.

Disorders of facial expression Figure 23-7. Volitional facial expressions (frown, smile) are compared to an emotional facial expression (laugh at a joke) in a healthy adult (normal), an adult with Bell’s palsy, an adult with supranuclear facial paresis, and an adult with amimia. Damage to the facial nerve or facial motoneurons (Bell’s palsy) impairs movement of the entire ipsilateral face. Damage to the corticobulbar tract carrying information from M1 to the facial nucleus impairs volitional movements of the bottom face only. The side affected is ipsilateral to the facial nucleus and contralateral to the motor cortex. Projections from the anterior cingulate gyrus support emotional movements, so that smiling in response to a joke is preserved in people with corticobulbar lesions. Damage to the pathway from anterior cingulate gyrus to the facial nucleus impairs emotional facial expressions of the contralateral face but not volitional facial expressions.