Lecture18_notes.pdf

Motor Control We will be discussing two main topics in motor control: 1. Mechanisms producing locomotion (brainstem and spinal cord). 2. Mechanisms producing more complex movements such as writing, typing, speaking (motor cortex). Themes across both categories. 1. There is a modular organization (th...

Motor Control We will be discussing two main topics in motor control: 1. Mechanisms producing locomotion (brainstem and spinal cord). 2. Mechanisms producing more complex movements such as writing, typing, speaking (motor cortex). Themes across both categories. 1. There is a modular organization (this is true of sensory systems also). 2. There is topographical organization (this is true of sensory systems also). Specifically, control systems are mapped somatotopically, that is across the body surface. Spinal cord organization Below is a diagram of a cross section through the spinal cord: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Sensory neuron: cell body in dorsal root ganglion Dorsal Horn White matter: fiber tracts, axons Grey matter: cell bodies Ventral Horn Alpha motor neurons 1. There is an anatomical separation of sensory input (which enters the dorsal horn) and motor output (which exits the ventral horn). 2. There are many sensory organs in skin, muscle, tendons, ligaments etc. providing information about: heat, cold, light touch, pressure, pain, muscle length, muscle tension, joint angle, etc. (the somatosensory system). The cell bodies of the neurons carrying this sensory information from the sensory receptors are all in the dorsal root ganglion. (A ganglion is a collection of neuron cell bodies in the PNS. The same structure in the CNS is called a nucleus). 3. Some of this sensory input projects to spinal CPGs (and to various other areas in the brainstem) from where it is relayed from the thalamus to sensory cortex. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Central Pattern Generator (CPG). 1. The symbol for a CPG is... This represents 3 neurons (A, B, C) firing in a cycling pattern. A B C 2. A CPG is a network of neurons that produces rhythmic output by its anatomical and synaptic interactions added to the intrinsic properties of its individual neurons. (spontaneously active neuron(s)). A CPG can generate a complex patterned output even if the CPG is isolated from any patterned input. 3. Below is a diagrammatic example of the firing pattern produced by a CPG: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. A A B C B C Cycle 1 Cycle 2 Cycle 3 Note that B & C alternate, inducing alternating activity of the motor neurons and thus alternating contractions of the muscles. Command Neuron. 1. A command neuron is a single neuron that generates a complex behavior by its synaptic effects on the neurons to which it projects. 2. Below is an example of a command neuron: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Command Neuron Slow EPSP Fast EPSP Slow IPSP 3. The different synaptic effects produced by the command neuron can generate different patterns of activity in the -motor neurons (-MN), thus generating a patterned behavior. Note that even though this command neuron secretes one neurotransmitter, different effects are produced in the recipient cells due to different receptors for the neurotransmitter. Command Group 1. A command group is a set of neurons with similar properties that together generate a complex behavior. That is, a command group does that same thing as a command neuron but consists of multiple neurons. Locomotion A. What evidence suggests that mechanisms for locomotion reside in the brainstem and spinal cord and not in the forebrain? The figure below diagrams the main components of the neurocircuitry generating locomotion: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Spinal cut "Coordinating Fibers" Mesencephalic cut Alpha Motor neurons Sensory Mesencephalic Locomotor nucleus "Command Neurons" Treadmill 1. Evidence from "mesencephalic" animals suggests that mechanisms for locomotion reside in the brainstem and spinal cord. A mesencephalic animal is an animal with a cut at the anterior portion of the midbrain, which will separate the forebrain from the brainstem and spinal cord. These animals are paralyzed and can't consciously control their body. 2. The mesencephalic animal is put in a sling to support its body weight and its legs are placed on a treadmill. When the treadmill is turned on... a. at low treadmill speeds, the animal will walk with the appropriate coordination. b. at a higher speed, the animal will trot with the appropriate coordination. c. at an even higher speed, the animal will gallop with the appropriate coordination. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 3. Next the mesencephalic animal is put in a sling to support its body weight with its legs hanging in the air. If you electrically stimulate in an area in the midbrain called the mesencephalic locomotor nucleus (MLn)... a. With low intensity stimulation the mesencephalic animal's legs will move in a walking pattern. b. With medium intensity stimulation the animal's legs will move in a trotting pattern. c. With high intensity stimulation the animal's legs will move in a galloping pattern. 4. All of this evidence with mesencephalic animals suggests that the neural mechanisms sufficient to produce patterned locomotion reside in the brainstem and spinal cord and not in the forebrain. B. What are the mechanisms generating locomotion? 1. There are multiple CPGs in the spinal cord and the neurons of the CPGs project to -MNs. 2. The fundamental rhythm of locomotion is produced by these spinal CPGs. C. What turns the CPGs on in mesencephalic animals? 1. There are command neurons (command group) projection from the mesencephalic locomotor nucleus that synapses onto CPGs in the spinal cord. a. You can remove sensory feedback information to the spinal cord and stimulate in the mesencephalic locomotor nucleus and you can still get coordinated movement (although not as well coordinated as when there is sensory feedback information involved). © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. b. In the "normal" animal there are neurons in the forebrain that project to command neurons in the mesencephalic locomotor nucleus and this is the presumed mechanism for the voluntary initiation of locomotion. 2. When the treadmill is turned on sensory feedback from the moving legs turns on the CPGs. a. Proprioception is sensory information that provides information about the location of parts of your body. One of the several types of sensory receptors that provide different types of proprioceptive information is the "muscle spindles." These are embedded in muscles and provide information about muscle length. Skeletal muscle 1A Afferent: a sensory nerve projecting back to the spinal cord. Muscle Spindle: a sensory organ that detects muscle length b. There are also different sensory organs embedded in ligaments that tell about the angle of rotation of joints and other sensory organs embedded in tendons that provided information about muscle tension. All are proprioceptors and their combined information is integrated, largely without awareness, to provide information about the location of our body parts. c. So, when the treadmill is turned on, the legs are dragged backwards which changes the sensory feedback information from the muscle spindles and other proprioceptors and this information is sent to the spinal cord and this switches © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. on the CPGs, and patterned movement is generated. Proprioceptive information can also regulate the rate of CPG activity. D. So there are several different mechanisms to initiate locomotion. 1. You can consciously decide to locomote (forebrain pathway). 2. Sensory information allows you to react to the environment. E. What evidence suggests that there are multiple CPGs? 1. If you take a mesencephalic animal and transect (cut) the spinal cord (between the lumbar and cervical enlargements) as shown in the diagram and then put the animal on the treadmill the animal will still generate a locomotion pattern. 2. The front legs will move in alternation with each other and the back legs will move in alternation with each other, but forelimbs and hindlimbs no longer move in correct coordination with each other. This suggests that there is at least one CPG that generates the rhythm produced by the front legs and another CPG that generates the movement produced by the back legs. F. How do you get coordinated activity between CPGs? There are "coordinating fibers" that connect the CPGs together. G. How many CPGs are there? Are there two CPGs or many CPGs? 1. There are many CPGs and they are distributed throughout the spinal cord, particularly in the cervical and lumbar enlargements. 2. The evidence for this is if you take a thin section from anywhere in the spinal cord and put it in a dish, patterned output from the alpha motor neurons can be generated. 3. Below is a diagram depicting a thin slice of spinal cord in a dish. We are recording from the left and right ventral roots and stimulating one of the dorsal roots (or adding glutamate to the bath). © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Alpha motor neurons form ventral root AP left side ventral root AP right side ventral root 4. So, you can get patterned motor output which suggests that the mechanisms for producing patterned output exist within even a thin slice of spinal cord. This can be done with sections throughout the spinal cord. Therefore, there are multiple CPGs in the spinal cord and their activity can be initiated by sensory stimulation (dorsal root stimulation). © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 5. The CPGs can also be turned on by adding glutamate into the bath. That is, glutamate will generate rhythmic ventral root activity. 6. Now, as shown in the figure below, bisect the thin section of spinal cord. Afterwards, you can still get rhythmic motor output from the left and the right sides but now the output from the two sides is no longer coordinated with each other. Knife cut Alpha motor neurons form ventral root AP left side ventral root AP right side ventral root © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. a. Therefore, there are CPGs in both the left and right sides of the spinal cord. b. Why was the activity from the left and right sides coordinated before the bisection? Answer: because of "coordinating fibers" connecting the CPGs together. Cortical mechanisms of motor control. A. The types of motor control that we will discuss are motor acts that particularly involve the hands and face. These motor acts are complex, learned acts such as writing, typing, playing the piano (hands), and the spoken language (face). B. Not all parts of the cortex are involved in these types of motor acts. C. Below is a diagram of a lateral view (side view) of a human brain depicting in simplified form the areas of the cortex involved in these types of motor acts. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Central Sulcus Primary Motor Cortex Primary Sensory Cortex Premotor Cortex Posterior Parietal Cortex Brainstem Cortical spinal tract (aka pyramidal tract) Spinal Cord Hand © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 1. The central sulcus divides the frontal and parietal lobes. 2. The primary motor cortex (M1) contains neurons that project directly to -motor neurons in the brainstem and spinal cord. The axons of these cortical neurons form what is known as the pyramidal tract, until they reach the junction of the medulla oblongata and spinal cord. At that point, the axons cross the midline and form what is called the pyramidal decussation and then form the lateral corticospinal tract after crossing (same axons with different names at different points) before synapsing on the -motor neurons of the ventral horn. 3. The primary somatosensory cortex (S1) is, by definition, the first cortical area receiving synaptic input conveying somatosensory information. (Note that primary visual cortex is the first cortical area receiving visual information). 4. The M1and S1 cortex are both somatotopically mapped, and these maps are in parallel. There are projections from neurons in S1 to neurons in corresponding regions of M1. 5. Neurons in the premotor cortex project to neurons in M1. 6. Neurons in the posterior parietal cortex project to premotor cortex. D. There are three cortical areas that perform different stages in the generation of complex movements... The first is posterior parietal cortex, which projects to the second, which is premotor cortex, which projects to the 3rd, which is M1. 1. The neurons in the posterior parietal cortex act as a spatial map, which provides you with information about where you are with respect to a goal object. 2. The premotor cortex in involved in planning a complex motor act. 3. The M1 is more directly involved in directing the execution of the task. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. E. Evidence that posterior parietal cortex acts as a spatial map comes from humans with brain damage of this area. 1. Patients with brain damage in the posterior parietal cortex neglect the contralateral half of the body and the contralateral side of objects. 2. Patients with brain damage in the posterior parietal cortex also have a dramatic loss of the ability to use spatial maps. a. They cannot use a road map. b. They can't navigate. Even in a bathroom they can get lost (they can't map where they are relative to the environment they are in). F. How do we know that M1 is somatotopically organized? 1. If you electrically stimulate different areas of cortex you will find that M1 is the region with the lowest threshold for producing movement. 2. When you stimulate, you get movement on the contralateral side of the body. 3. Somatotopic organization... a. As you move the stimulating electrode across adjacent regions M1, you will produce movement in more or less adjacent regions of the body. e.g. the toes, then the foot, the legs......then each of the fingers of a hand, then the face. b. You will also find that the hands and face have a disproportionally large representation in M1, as compared to other body areas that are larger, such as the trunk. c. When you stimulate in different areas of M1, you get movements in different areas of the body on the contralateral side. G. How does this map in M1 come about? © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 1. It is the anatomy, what is connected to what, that creates the map in M1. 2. Below is a diagram of a coronal section of a human brain and spinal cord, depicting the anatomical connections that create the somatotopic map. Foot Area Hand Area Primary Motor Cortex Face Area Pyramidal tract Cut 1 Axons cross midline at pyramidal decussation to form lateral corticospinal tract Cut 2 Cut 3 Lateral Corticospinal tracts © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. a. If there was a cut at location 1, the person would have paralysis of the face, hand, and foot on the side of the body contralateral to the cut. b. If there was a cut at 2, the person would have paralysis of the hand and foot on the side of the body ipsilateral to the cut. c. If there was a cut at 3, the person would have paralysis of the foot on the side of the body ipsilateral to the cut. d. If there was brain damage to M1 in the region of the hand, the person would have paralysis of the hand on the side of the body contralateral to the brain damage. e. These cuts and lesions do not produce total paralysis. The person cannot do complex tasks such as writing, etc. But they can still do thing like walk or climb a tree (talking about monkeys here). Why is this? Because the mesencephalic locomotor nucleus and spinal CPGs are still intact. They are parts of a different neural motor control system, one producing locomotion. So, there are different motor control systems with different functions and they may control the same part of the body [hands write and are involved in locomotion, in the same way the mouths are involved in speaking (one control system) and chewing (generated by a different control system)]. Different control systems may have inputs to the same motor neurons. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed.