Q1: Name 2 classes of neurons located at this level on which axons of the lateral corticospinal tract synapse? ANS: 1. Interneurons 2. Motor neurons Explanation To control limb and trunk movements, axons of the lateral corticospinal tract synapse directly on motor neurons. Because of this connection, corticospinal tract neurons are often called upper motor neurons. This monosynaptic connection is thought to be important for the most precice kinds of movements we make, whether the movements are made with intrinsic hand muscles, such as in playing a violin, or with more proximal muscles, such as when a violinist controls the violin bow. Corticospinal tract neurons also contact interneurons that, in turn, synapse on motor neurons. There are actually more of these disynaptic connections than monosynaptic connections. This disynaptic circuit is thought to activate groups of muscles that are recruited during movements. For example, when a violinist’s fingers of the left hand are used to produce particular notes other arm muscles are contracted to stabilize the instrument. The more coarse stabilizing movement may be mediated by the disynaptic circuit. Q2: Where are the cell bodies of axons at the tip of the pointer? ANS: Facial motor nucleus Explanation These are cranial nerve 7 motor axons, which innervate the muscles of facial expression. Their cell bodies are located in the facial motor nucleus. The nerve contains axons that innervate all of these muscles, so damage to the nerve will paralyze or weaken all ipsilateral facial muscles. By contrast, unilateral damage to the corticobulbar tract characteristically produces lower facial motor weakness or paralysis, sparing the upper muscles. This is due to the pattern of corticobulbar connections to the nucleus. Briefly, there is some degree of bilateral control of the muscles supplying the upper face, so when one path is damaged the path from the other side can take over. By contrast, control of the lower facial muscles is strictly contralateral. Q3: In which of the marked regions do axons of corticospinal tract neurons from the right motor cortex descend? ANS: E Explanation Axons in the genu and posterior limb descend in the middle part of the basis peduncli. These are the regions of the internal capsule where the corticobulbar and corticospinal tracts descend. Because the axons in the basis pedunculi have not yet decussated, corticospinal (and corticobulbar) axons will be in the right side. Q4: Identify nucleus A and indicate where neurons in this nucleus synapse. ANS: Ventral lateral nucleus of the thalamus Explanation The ventral lateral nucleus, or VL, is one of the key motor control relay nuclei of the thalamus. It transmits information from the cerebellum to the primary and premotor cortex for controlling movements. We will learn of the other major motor relay, the ventral anterior nucleus, in the basal ganglia lab. Because VL and VA are hard to distinguish and they have similar functions, we usually call them by their combined names--VA-VL. So, the thalamus is not just for sensory processing. Virtually all information that the cortex receives from subcortical structures--sensory, motor, emotional, cognitive, etc--is transmitted by thalamic nuclei. Q5: What is the function of the tract at the tip of the pointer? ANS: Voluntary movement control Explanation The pointer marks the pyramid, which contains corticobulbar and coticospinal tracts. These paths are essential for voluntary control of body muscles. At the most caudal level of the pyramid, there are only corticospinal axons because all of the corticobulbar axons have left the tract and terminated in the cranial motor nuclei. Q6: Name an artery that supplies the territory marked by the double arrow, at this and more inferior levels. ANS: Anterior choroidal artery Explanation The anterior choroidal artery is a direct branch of the internal carotid. It supplies the more inferior parts of the posterior limb. The middle cerebral artery supplies more superior levels. Other internal capsule branches, which are small and without particular names, supply the inferior parts of the internal capsule. Part of the genu is supplied by a deep branch of the anteior cerebral artery. Q7: The arrow points to the central sulcus. From which labeled region of motor cortex does the corticobulbar tract originate? ANS: C Explanation The corticobulbar tract controls cranial nerve motor functions. It originates from the cranial/face representation of the motor cortex, which is located laterally in the precentral gyrus. The lateral corticospinal tract originates from the arm and leg regions; the ventral corticospinal tract originates from the axial representations (neck and trunk). Q8: Will axons at the tip of the pointer control ipsilateral or contralateal limb muscle? ANS: Contralateral Explanation Axons in the pyramid controlling limb muscle cross in the pyramidal decussation. Since the marked level is rostral to the decussation, the axons will control contralateral muscles. Axial muscle control is mostly bilateral. Lesion of the pyramid/corticospinal tract on one side ususally does not weaken proximal/axial muscles. Q9: What limb of the internal capsule is cut in this picture? ANS: Anterior limb Explanation This is a section drawn through the brain rostral to the thalamus.The anterior limb is rostral to the thalamus. Q10: Name two tracts that originate from this level of the midbrain. ANS: Rubrospinal; tectospinal Explanation This MRI is cut through the rostral midbrain. The red nuclei are the circular structures in the middle. The magnocellular part gives rise to the rubrospinal tract. The superior colliculi are also at this level. The tectospinal tract originates from the superior colliculi. The function of the tectospinal tract is to coordinate head and eye movements. It is part of the medial descending pathways. The rubrospinal tract is involved in limb control, although its precise function is not know. It is part of the lateral descending motor pathways.