The basal ganglia are responsible for voluntary motor control, procedural learning, and eye movement, as well as cognitive and emotional functions. Trigeminal nerve (CN V): the motor component of trigeminal nerves supplies the muscles of mastication. Feedforward processing: Refers to the unidirectional movement of signals through the system from input to output, with very little recurrent internal transmission. All the neurons contributing to the pyramidal and extrapyramidal systems should be called upper motor neurons (UMN). Because of this fine-tuning function, damage to the cerebellum does not cause paralysis, but instead produces disorders in fine movement, equilibrium, posture, and motor learning. Describe the role of the basal ganglia in movement. It consists of the pyramidal and extrapyramidal system. Axons of upper motor neurons decussate before synapsing with lower motor neurons, so the right motor cortex controls the left side of the body, and vice versa – contralateral control. CC licensed content, Specific attribution, http://en.wikipedia.org/wiki/Spinal_tracts%23Motor_organization, http://en.wikipedia.org/wiki/Motor_system, http://en.wikipedia.org/wiki/Upper_motor_neuron%23Pathways, http://en.wikipedia.org/wiki/motor%20system, http://en.wikipedia.org/wiki/corticospinal%20tract, https://en.wikipedia.org/wiki/Pyramidal_tracts, http://en.wiktionary.org/wiki/cerebral_cortex, http://en.wikipedia.org/wiki/File:Spinal_cord_tracts_-_English.svg, http://en.wikipedia.org/wiki/File:Gray684.png, http://en.wikipedia.org/wiki/File:Brodmann-areas.png, http://en.wikipedia.org/wiki/Basal_ganglia%23Function, http://en.wiktionary.org/wiki/hemiballismus, http://en.wikipedia.org/wiki/voluntary%20motor%20control, https://en.wikipedia.org/wiki/Nucleus_accumbens, http://en.wikipedia.org/wiki/File:Basal_ganglia_circuits.svg, https://en.wikipedia.org/wiki/Basal_ganglia#/media/File:Blausen_0076_BasalGanglia.png, http://en.wikipedia.org/wiki/Cerebellum%23Principles, https://en.wikipedia.org/wiki/Granule_cell, https://en.wikipedia.org/wiki/Purkinje_cell, https://en.wikipedia.org/wiki/Mossy_fiber_(cerebellum), https://commons.wikimedia.org/wiki/File:Sobo_1909_653.png, https://commons.wikimedia.org/wiki/File:Gray706.png, http://en.wikipedia.org/wiki/feedforward%20processing, http://en.wikipedia.org/wiki/File:Gray706.png, http://en.wikipedia.org/wiki/File:Microzone.svg. The basic concept of the Marr-Albus theory is that the climbing fiber serves as a teaching signal, which induces a long-lasting change in the strength of synchronously activated parallel fiber inputs. The difference between upper and lower motor neuron lesion is such that an upper motor neuron lesion is the lesion that occurs in the neural pathway above the anterior horn of the spinal cord or cranial nerves motor nuclei; whereas a lower motor neuron lesion affects the nerve fibers that travel from the anterior horn of the spinal cord to the associated muscle. Pathways between motor cortex and muscles may be thought of as being arranged in two neuronal groups: upper motor neurons and lower motor neurons. It is a component of the nervous system. Everything NICE has said on assessing and managing motor neurone disease in adults in an interactive flowchart UPPER MOTOR NEURON• Upper motor neurons (UMN) are responsible for conveying impulses for voluntary motor activity through descending motor pathways that make up the upper motor neurons.• UMN send fibers to the LMN, and that exert direct or indirect supranuclear control over the LMN of the cranial and spinal nerves.. In most regions of the brain, the predominant classes of neurons use glutamate as the neurotransmitter and have excitatory effects on their targets. The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and either synapse on ventromedial lower motor neurons in the ventral horn ipsilaterally or descussate at the anterior white commissure where they synapse on ventromedial lower motor neurons contralaterally. Here in this article, let us look at the various difference between upper and lower neurons. Upper motor neurons of the pyramidal tract have the majority of their cell bodies located in the precentral motor cortex (Brodmann area 4) and the premotor area (Brodmann area 6). They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus, and other brain areas. It is believed that upper motor neurons in the motor cortex control multiple lower motor neurons in the spinal cord that innervate multiple muscles. Because of the loss of inhibitory modulation from descending pathways, the myotatic (stretch) reflex is exaggerated in upper motor neuron disorders. Upper motor neuron (UMN) pathways responsible for motor speech and swallowing originate in the motor cortex in each cerebral hemisphere and descend through the genu and posterior limb of the internal capsule, via the cerebral peduncle, to the pons and medulla (and upper cervical cord for the spinal nucleus of cranial nerve XI). The lateral tract contains upper motor neuronal axons that synapse on the dorsal lateral lower motor neurons, which are involved in distal limb control. The main difference between upper and lower motor neuron is that upper motor neuron is the motor component of the central nervous system that transmits impulses from the brain to the synapses of the lower motor neurons whereas lower motor neuron is the motor component that connects with the muscles. They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles to contract or relax. Upper motor neurons include all of the connections between the cerebral cortex (both direct and indirect) to the spinal cord. UPPER MOTOR NEURON• Upper motor neurons (UMN) are responsible for conveying impulses for voluntary motor activity through descending motor pathways that make up the upper motor neurons.• UMN send fibers to the LMN, and that exert direct or indirect supranuclear control over the LMN of the cranial and spinal nerves.. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Four principles of cerebellum function have been identified. The six pathways of the upper motor tract are the corticospinal tract, corticobulbar tract, colliculospinal tract, rubrospinal tract, vestibulospinal tract, and the reticulospinal tract. Feedforward processing means signals move in one direction through the cerebellum, from input to output. Axon(away from the cell body) 3. The strongest clues to the function of the cerebellum have come from examining the consequences of damage to it. A neuron is made up of: 1. These are the rubrospinal tract, the vestibulospinal tract, the tectospinal tract, and the reticulospinal tract. Upper motor neurones make up one half of the body’s somatic nervous system – the other half being made of lower motor neurones. Dopamine is used by the projection from the substantia nigra pars compacta to the dorsal striatum and also in the analogous projection from the ventral tegmental area to the ventral striatum (nucleus accumbens). Neurons are broadly tuned, discharge from an individual upper motor neuron cannot specify direction of arm movement because too broadly tuned, need net action from population of neurons Contributions of cells that fire during movement in a particular direction added vectorally to produce population vector. direction of vector indicates direction of movement. Legal. Describe the role of the cerebellum in movement modulation. The corticospinal tract is a motor pathway that carries efferent information from the cerebral cortex to the spinal cord. In terms of anatomy, the cerebellum has the appearance of a separate structure attached to the bottom of the brain, tucked underneath the cerebral hemispheres. The pyramidal tract is visible in red, and pyramidal decussation is labeled at lower right. The pyramidal tract divides further into the corticospinal tract and the corticobulbar tract. Signals enter the circuit, are processed by each stage in sequential order, and then leave. Describe the organization of motor neuron pathways. Modularity: The cerebellar system is functionally divided into independent modules. A neuron is a brain cell that processes and transmits information by chemical and electrical signalling. Within this thin layer are several types of neurons with a highly regular arrangement, the most important being Purkinje cells and granule cells. Upper motor neurones for the facial nerve (CN VII) have a contralateral innervation. Although the role of the basal ganglia in motor control is clear, there are also many indications that it is involved in the control of behavior in a more fundamental way, at the level of motivation. The function of the cerebellum can be described by the principles of feedforward processing and modularity. A bump of neural activity in the deep layers of the SC drives eye movement toward the corresponding point in space. Action selection is the decision of which of several possible behaviors to execute at a given time. Upper motor neurons usually belong to the corticobulbar (neuron 3), corticospinal (neuron 4), or descending brainstem motor (also called bulbospinal, neuron 5), pathways. The primary motor neuron components and lipidome pathways in MND/HSP. One of the most intensively studied functions of the basal ganglia is their role in controlling eye movements. Neurons are connected by a single axon, or by a bundle of axons known as a … Brodmann areas of the brain: This drawing shows the regions of the human cerebral cortex as delineated by Korvinian Brodmann on the basis of cytoarchitecture. Recall that somatic motor pathways involve at least two motor neurons: an upper motor neuron, whose cell body lies in a central nervous system processing center, and a lower motor neuron, depicted here in blue, whose cell body lies in a nucleus of the brain stem as it relates to cranial nerves or in the spinal cord as it relates to peripheral nerves. Plasticity: The synapses between parallel fibers and Purkinje cells, and the synapses between mossy fibers and deep nuclear cells, are both susceptible to modification of their strength. The pyramidal tract, which includes both the corticospinal and corticobulbar tracts, serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei. The surface of the cerebellum is covered with finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral cortex. An upper motor neuron in an extrapyramidal tract, or any pathway outside of the pyramid tract, will probably be involved in a subconscious motor process such as balance or posture. An upper motor neuron lesion, also called pyramidal insufficiency, refers to damage to the motor neurons of the brain or brain stem that travel to the spinal cord. (adsbygoogle = window.adsbygoogle || []).push({}); The motor system is the part of the central nervous system that is involved with movement. The cerebellum adjusts to changes in sensorimotor relationships, possibly functioning as in the Marr-Albus theory: Strong inputs from a single climbing fiber serve as a teaching signal to change the strength of impulses from the corresponding group of parallel fibers. The impulses for movement are carried by nerves known as upper motor neurons (UMN). Videos you watch may be added to the TV's watch history and influence TV recommendations. The synapses between parallel fibers and Purkinje cells, and the synapses between mossy fibers and deep nuclear cells, are both susceptible to modification of their strength. Most of them derive from early models formulated by David Marr and James Albus, which were motivated by the observation that each cerebellar Purkinje cell receives two dramatically different types of input. Activation of the lower motor neuron triggers a contraction in the innervated muscle. Although a full understanding of cerebellar function remains elusive, at least four principles are identified as important: 1) feedforward processing, 2) divergence and convergence, 3) modularity, and 4) plasticity. These axons synapse with lower motor neurons in the ventral horns of all levels of the spinal cord. 12.9A: Organization of Motor Neuron Pathways, [ "article:topic", "license:ccbysa", "showtoc:no" ], https://med.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fmed.libretexts.org%2FBookshelves%2FAnatomy_and_Physiology%2FBook%253A_Anatomy_and_Physiology_(Boundless)%2F12%253A_Peripheral_Nervous_System%2F12.9%253A_Motor_Pathways%2F12.9A%253A_Organization_of_Motor_Neuron_Pathways, 12.9B: The Role of the Basal Ganglia in Movement, information contact us at info@libretexts.org, status page at https://status.libretexts.org, Describe the organization of motor neuron pathways. The ventromedial lower motor neurons control the large, postural muscles of the axial skeleton. The basal ganglia are studied extensively in the context of two disorders of the basal ganglia: Parksinson’s disease and Huntington’s disease. At these levels, they synapse with the various lower motor nuclei … For both of these disorders, the nature of the neural damage is well-understood and can be correlated with the resulting symptoms. Green arrows refer to excitatory glutamatergic pathways, red arrows refer to inhibitory GABAergic pathways and turquoise arrows refer to dopaminergic pathways that are excitatory on the direct pathway and inhibitory on the indirect pathway. This results in activation of an upper motor neuron causing excitation or inhibition in different neurons at once, indicating that the primary motor cortex is responsible for movements and not simply activation of one muscle. This feedforward mode of operation means that the cerebellum cannot generate self-sustaining patterns of neural activity, in contrast to the cerebral cortex. Deficits in non-motor functions are more difficult to detect. OBJECTIVE: This study describes a case of lesions of the upper motor neuronal pathway with locked-in features after lightning strike and cardiac arrest. In Parkinson’s disease, the ability to execute the components of movement is not greatly affected, but motivational factors such as hunger fail to cause movements to be initiated or switched at the proper times. Cells of the Cerebellum: Transverse section of a cerebellar folium, showing its principal cell types and connections. METHODS: In a 29-year-old male who was hit by a lightning strike during farming activities, cardiopulmonary resuscitation was provided first by co-workers and continued with success by the medical rescue service. Basal ganglia are also thought to play a role in motivation. The cerebellum uses feedforward processing and modularity to process information. On the left is a simplified illustration of what the cerebellar cortex would look like if all the folds were straightened out—the vertical dimension is the rostro-caudal axis of the cerebellum, the horizontal dimension is the medio-lateral axis. The role in motivation of the limbic part of the basal ganglia—the nucleus accumbens (NA), ventral pallidum, and ventral tegmental area (VTA)—is particularly well established. Most is known about the mechanisms of motor neurone degeneration in the subtype of … An area of the nervous system is connected to another area by neural pathways. In fact, within the classification of a “motor neuron,” there lies both upper and lower motor neurons, which are entirely different in terms of their origins, synapse points, pathways, neurotransmitters, and lesion characteristics. eg The rubrospinal tract is heavily involved in involuntary movements to improve and maintain the body's balance. Two coronal slices have been superimposed to include the involved basal ganglia structures. This arrangement gives tremendous flexibility for fine-tuning the relationships between the cerebellar inputs and outputs. The primary motor neuron components and lipidome pathways in MND/HSP. Signals enter the circuit, are processed by each stage in sequential order, and then leave. Cerebellum: View of the cerebellum from above and behind. Hereditary neuropathy with upper motor-neuron, visual pathway, and autonomic disorders. Numerous things that people find rewarding, including addictive drugs, good-tasting food, and sex, have been shown to elicit activation of the VTA dopamine system. These are the upper motor neurons of the corticospinal tract. Thus, the cerebellar network receives a modest number of inputs, processes them very extensively through its rigorously structured internal network, and sends out the results via a very limited number of output cells. Thus, following the initial excitation of the striatum, the internal dynamics of the basal ganglia are dominated by inhibition and disinhibition. A standard test of cerebellar function is to reach with the tip of the finger for a target at arm’s length. Pathways between motor cortex and muscles may be thought of as being arranged in two neuronal groups: upper motor neurons and lower motor neurons. The cerebellum is a parallel grooved structure at the bottom of the brain containing a highly regular cellular arrangement of Purkinje cells, granule cells, and other cell types. Divergence and convergence: The 1000 or so Purkinje cells belonging to a microzone may receive input from as many as 100 million parallel fibers, and focus their own output down to a group of less than 50 deep nuclear cells. Any motor command from the primary motor cortex is sent down the axons of the Betz cells to activate upper motor neurons in either the cranial motor nuclei or in the ventral horn of the spinal cord. Both can be damaged in various ways and both have distinct clinical presentations. In this article, we will consider the definition, function and clinical significance of the upper motor neurones. This results in activation of an upper motor neuron causing excitation or inhibition in different neurons at once, indicating that the primary motor cortex is responsible for movements and not simply activation of one muscle. The axons of these cells pass from the cerebral cortex to the midbrain and the medulla oblongata. Damage to UMNs results in characteristic clinical manifestations colloquially termed “upper motor neuron signs” or “upper motor neuron … Upper motor neurons are generally concentrated in the motor region of the brain stem or cerebral cortex. The axons of these cells pass from the cerebral cortex to the midbrain and the medulla oblongata. The anterior horn cells and the related neurons in the motor nuclei of some cranial nerves are called lower motor neurons (LMN).Axons of these cells give rise to the peripheral motor … The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. Have questions or comments? It receives input from sensory systems of the spinal cord and from other parts of the brain, including the cerebral cortex, and integrates these inputs to fine-tune motor activity. The motor impulses originate in the giant pyramidal cells (Betz cells) of the motor area, i.e., the precentral gyrus of the cerebral cortex. Such damage can occur as a result of a variety of disorders, including multiple sclerosis (MS), stroke, brain injury, or cerebral palsy. There are upper and lower motor neurons in the corticospinal tract. They continue to be able to generate motor activity, but it loses precision, producing erratic, uncoordinated, or incorrectly timed movements. The pyramidal tract is visible in red, and pyramidal decussation is labeled at lower right. Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. Several theoretical models have been developed to explain sensorimotor calibration in terms of synaptic plasticity within the cerebellum. Hemiballismus is characterized by violent and uncontrollable flinging movements of the arms and legs. It is comprised of a cell body, axon, and dendrites. describe the general pathway of upper motor neurons from the brainstem. These axons synapse with lower motor neurons in the ventral horns of all levels of the spinal cord. The motor impulses originate in the giant pyramidal cells (Betz cells) of the motor area, i.e., the precentral gyrus … What is Lower Motor Neuron The lower motor neuron is a motor neuron that transmits nerve impulses from the upper motor neurons to the effector muscles. In the human cerebellum, information from 200 million mossy fiber inputs is expanded to 40 billion granule cells, whose parallel fiber outputs then converge onto 15 million Purkinje cells. Zones and microzones in the cerebellum: This schematic illustration of the structure of zones and microzones in the cerebellum shows three levels of magnification. A zone is a longitudinally oriented strip of the cortex, and a microzone is a thin, longitudinally oriented portion of a zone. Modularity describes the modular nature of the cerebellar system, where modules with similar structures function relatively independently. Brodmann areas of the brain: This drawing shows the regions of the human cerebral cortex as delineated by Korvinian Brodmann on the basis of cytoarchitecture. Cortical upper motor neurons originate from Brodmann areas 1, 2, 3, 4, and 6, then descend into the posterior limb of the internal capsule, through the crus cerebri, down through the pons, and to the medullary pyramids, where about 90% of the axons cross to the contralateral side at the decussation of the pyramids. The SC is a layered structure whose layers form two-dimensional retinotopic maps of visual space. Upper motor neurons are cells in your brain and spinal cord that help you walk, talk, and eat. Acetylcholine binds its receptor and causes depolarization. There are upper and lower motor neurons in the corticospinal tract. What are upper motor neurons and lower motor neurons in descending pathways from BIO SCI D170 at University of California, Irvine Injury or lesions to UMN’s are common because of the vast areas covered by the motor neuron pathways. Thus, both are classified primarily as movement disorders. Adopted a LibreTexts for your class? An upper motor neuron lesion is a lesion of the neural pathway above the anterior horn of the spinal cord or motor nuclei of the cranial nerves. Any upper motor neuron lesion, also known as pyramidal insufficiency, occurs in the neural pathway above the anterior horn of the spinal cord.Such lesions can arise as a result of stroke, multiple sclerosis, spinal cord injury or other acquired brain injury.The resulting changes in muscle performance that can be wide and varied are described overall as upper motor neuron syndrome. The output of one module does not appear to significantly influence the activity of other modules. As Eccles, Ito, and Szentágothai wrote,”This elimination in the design of all possibility of reverberatory chains of neuronal excitation is undoubtedly a great advantage in the performance of the cerebellum as a computer, because what the rest of the nervous system requires from the cerebellum is presumably not some output expressing the operation of complex reverberatory circuits in the cerebellum, but rather a quick and clear response to the input of any particular set of information.”. The small amount of recurrence that does exist consists of mutual inhibition; there are no mutually excitatory circuits. (B) Classic and alternative pathways through which cholesterol can be converted into primary … Paralysis or weakness of movements of the affected side but gross movements may be produced. Modules consist of clusters of neurons with common inputs but distinct outputs. The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. (A) Simplified schematic of the core components of the primary motor pathway, including upper and lower motor neurons, for the transmission of voluntary commands from the primary motor cortex to skeletal muscles. Upper and lower motor neuron form the motor part of the somatic nervous … The cerebellum is important for motor control—specifically coordination, precision, and timing—as well as some forms of motor learning. Animals and humans with cerebellar dysfunction show, above all, problems with motor control. Green, Upper motor neurons typically originate in the brain and project to and control lower motor neurons. Huntington’s disease involves the massive loss of medium spiny neurons in the striatum. Spinal muscular atrophy pathology motor neuron diseases paradigm of spinal muscular atrophy lower motor neurone loss leads to. A neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable a signal to be sent from one region of the nervous system to another. Upper motor neurons have six pathways. Contents. These zones and microzones help explain the modular nature of the cerebellar function. Injury to UMNs in these tracts is common because of the large areas covered by the motor neuron pathway. The lateral tract contains upper motor neuronal axons that synapse on the dorsal lateral lower motor neurons, which are involved in distal limb control. Thus, the general conclusion reached decades ago is that the basic function of the cerebellum is not to initiate movements, or to decide which movements to execute, but rather to calibrate the detailed form of a movement. While the term “motor neuron” evokes the idea that there is only one type of neuron that conducts movement, this is far from the truth. Learn how damage to these cells could affect your movement and what your doctor can do to treat it. Lower motor neurons are in your brain stem and spinal cord. The resulting changes in muscle performance that can be wide and varied are described overall as upper motor neuron syndrome. Lesions. So, neural pathways communicate information from one area to another area of the nervous system. It is responsible for the voluntary movements of the limbs and trunk. The motor system is the part of the central nervous system that is involved with movement. 2.Upper motor neurons are classified according to the pathways they travel in. Hemiballismus, a movement disorder arising from neuronal damage in the subthalamic nucleus, presents with violent movements of the arms and legs. Specifically, the corticobulbar tract carries upper motor neuron input to the motor nuclei of the trigeminal, facial, glossopharyngeal and accessory cranial nerves.This then supplies voluntary control over a number of head and neck functions;. The remaining 10% of axons descend on the ipsilateral side as the ventral corticospinal tract. Parkinson’s disease involves the major loss of dopaminergic cells in the substantia nigra. Obviously, this includes the corticospinal tract (a direct pathway), but also includes indirect pathways such as the cortico-reticular, reticulo-spinal pathway (among others). (A) Simplified schematic of the core components of the primary motor pathway, including upper and lower motor neurons, for the transmission of voluntary commands from the primary motor cortex to skeletal muscles. It consists of the pyramidal and extrapyramidal system. The motor pathway, also called the pyramidal tract or the corticospinal tract, serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei. All modules have a similar internal structure, but with different inputs and outputs. As the name indicates, Neural: related to nervous system Pathway: passage. Upper vs Lower Motor Neurons. The comparative simplicity and regularity of the cerebellar anatomy led to an early hope that it might imply a similar simplicity of computational function. Included in the diagram are the following motor pathways: corticospinal tracts (pyramidal tract), and extrapyramidal tracts (tectospinal tract not delineated). The immobility of patients with Parkingson’s disease has sometimes been described as a paralysis of the will. The axons of upper motor neurons descend from higher centers to influence the local circuits in the brainstem and spinal cord that organize movements by coordinating the activity of lower motor neurons (see Chapter 15). This means that the cerebellum, in contrast to the cerebral cortex, cannot generate self-sustaining patterns of neural activity. There are upper and lower motor neurons in the corticospinal tract. Start studying upper motor neuron pathways. Eye movement is influenced by an extensive network of brain regions that converge on a midbrain area called the superior colliculus (SC). These axons also synapse with lower motor neurons in the ventral horns. The function of lower motor neurons can be divided into two different groups: the lateral corticospinal tract and the anterior corticalspinal tract. The initial damage to the cerebral cortex causes flaccidity of … Cerebellum cells: View of the cerebellum from above and behind. Indirect Motor Pathway (Extrapyramidal) – From Brainstem – Unconscious Involuntary Movement Below the level of second lumbar vertebra there is a collection of nerve roots which runs down from the spinal cord resembling a horse's tail and hence is known as CAUDA EQUINA. Motor neurons that synapse above this level are called as UPPER MOTOR NEURONS and those that synapse at or below the level of the anterior horn cells are called LOWER MOTOR NEURONS. Direct Motor Pathway (Pyramidal) – From Cortex – Conscious Voluntary Movement – Corticospinal pathways – Corticobulbar pathways • 2. The motor system is the part of the central nervous system that is involved with movement. Upper motor neuron (UMN) pathways responsible for motor speech and swallowing originate in the motor cortex in each cerebral hemisphere and descend through the genu and posterior limb of the internal capsule, via the cerebral peduncle, to the pons and medulla (and upper cervical cord for the spinal nucleus of cranial nerve XI).