Physio C15 - Cortical Control of Movement PDF

PHYSIO C15 – Cortical control of movement, the use of the hand The system organizing voluntary movements is a hierarchical system with the cortex (1) , the brainstem (2) and the spinal cord (3). The hierarchical aspect of the NS does not define of a part if more essential than another. For example...

PHYSIO C15 – Cortical control of movement, the use of the hand The system organizing voluntary movements is a hierarchical system with the cortex (1) , the brainstem (2) and the spinal cord (3). The hierarchical aspect of the NS does not define of a part if more essential than another. For example, the Lou Gehrig’s disease shows how essential motor neurons are: if there aren’t motor neurons, the spinal cord can’t organize movements, nor with reflexes nor with other ways as they are the final common path to the skeletal muscles. The cortex is the leader not because is the highest in position but because dominates all the other stations below, the brainstem and the spinal cord. 1.​ Organization of the spinal cord The spinal cord functions as part of the CNS, the nerves only are the periphery. Acting on the motor neurons, the brain has parameters that it can control directly, to modulate the force exerted by the muscle fibers by: ​ the frequency of discharge ​ the amount of motor units recruited with the size principle. Moreover, the brain is able to recruit force with a higher effect than the program, which means controlling the velocity of recruitment that is the final intensity of the force recruited. The muscle has also its own properties: it does not behave the same way at different lengths: for a same frequency of disacharge, the muscle length can change the amount of force produced. Some paths do not include the higher levels, such as the stretch reflex: the Ia fibers directly adjust the excitability of the motoneurons to the actual length of the muscle. The spinal cord automatically tunes the excitability of the motor neurons. An excess of force is also tuned by the Ib fibers, originating from the tendon organs: a too high foce causes the muscle to relax. Motor neurons synapse in the lamina IX of the anterior horn of the spinal cord. They are organized in motoneuron pools in a with a specific somatotopic organization and normally involve more than one segment. The sensor and motor system talk the same language and they organize the same way, if not it would be dysfunctional because they could never work together.  a.​ Motor neurons Remember the position: the ventral horn of the spinal cord has a dorsolateral portion (green in the slide) and a ventromedial portion (blue in the slide). -​ In the ventromedial portion are represented the motoneurons that are subserving the axial proximal muscles, therefore the epaxial and proximal muscles that keep in position the spine and the proximal muscles; -​ the dorsolateral portion is enlarged at the level of the cervical and lumbar seglents of the spinal cord, to supply the upper and lower limbs The spinal cord is not only where the efferent branches (motor neurons). The laminae I-VI contain the sensory afferents. The lamina VI-VII and part of the VIII belong to the autonomic system and then anteriorly there are the motoneurons. In lamina VIII there are also the interneurons of the spinal cord. b.​ interneurons Among the interneurons, the Ia inhibitory interneuron involved the stretch reflex reciprocal inhibition. There are also other interneurons, for example in the withdrawal reflex to withdraw three joints, through 3 flexor groups of muscles, in either lower or upper limb. These interneurons are called propriospinal or segmental neurons and are connected to the spinal cord neurons to build up chains, Example of two propriospinal neurons, short and long propriospinal interneurons: ​ the short propriospinal interneurons connect motor neurons in adjacent or near segments ​ the long propriospinal neurons connect from the cervical to the sacral segment. They are essential for posture and anticipatory postural reactions, more specifically for the synergies of muscle used by the nervous system to prepare the entire body to produce a movement without losing the center of mass, basically stabilizing the body. Without stabilizing the body, if a chain of activation goes down to the leg, the center of mass will be moved. The spinal cord is equipped with drivers, the neurons, that are computing the information from the sensory system, and with interneurons that builds up local circuits. They are like functional units that the brain can use to build up complex movements: interneurons are modulating, modulating is the intrinsic relevant part of the brain work, modulating who, how and who is controlling the modulator. There are circuits able to produce simple movements as reflex, complex movements like locomotion and then the interneurons are able to connect the different portions of the muscles. Classification of interneurons: ​ Ia interneurons ​ Ib interneurons, Ib fibers come from the GTOs and inhibit the α motor neuron innervating the contracting muscle. The Ib interneuron is in-between the Ib fibers and the motorneurons. o​ This autogenic inhibition can be appreciated as a circuit in its over expression in some pathologies, normally it happens that the autogenic inhibition is working progressively the more the higher is the level of the force that is exerted in order not to let the muscle over-perform, breaking the tendon or stretching too much. ​ Renshaw cells, are inhibitory interneurons activated by a branch of the axon of the α motoneuron. On their turn, Renshaw cells inhibit the motoneuron. It seems similar to Ib because is a negative feedback loop acting on the motor neuron, it is an inhibition, the neuron is firing and as soon as the potential goes, goes to the Renshaw cells which inhibit the neuron. These cells are clearly modulators of the excitability of the motor neurons. ​ short propriospinal neurons ​ long propriospinal neurons Hence, the motoneurons are talking with a lot of interneurons in the spinal cord. Then they are also talking with neurons coming from upstairs. Who is modulating and organizing voluntary movements? The answer is the descending systems. 2.​ Overview of the descending system The locomotion’s circuit is present in the spinal cord but is modulated by the descending systems. Humans are better than other mammals in voluntary movements is the dexterity, the ability to use the hand and in general to perform voluntary movements with a very high degree of precision. Humans are good in using the hand or distal segments with very refined voluntary movements. Independent movements of the fingers are not granted, humans have common muscles and some intrinsic muscles of the hand. With a patient having a stroke running the first thing to ask is to do independent movements of the fingers, because there is not any independent muscle to be moved but it is a complex work. In order to move independently one finger, the cortex has to organize a synergy of moving muscles and then organize the intrinsic movement, most of the monkeys cannot do that, they grasp with the whole, the precision grip is depending on the ability to move the fingers independently and to move the first finger in all positions with respect to the others. This is not something that relays on the anatomy. Hence, there is a system and circuits in our brain, different from the others, that allow us to move hands as a perfect tool. The structure at the base of the voluntary movements is the descending system acting on the spinal cord machine and on its motoneurons. Dissecting all the different descending pathways and analyzing who is coming down, it’s needed to pay attention on: ○​ the origin: location of cells of origin of the pathway ○​ Synaptic input: the nature of the input of the cell of origin. Not the one of destination. In fact, in order to understand the function it’s useful to look at the company that is talking with the driver, the circuit, because neural functions are based on circuits. It is very important to understand who is talking with the cells that go down to the spinal cord machine ○​ Fibers number and size, ○​ Course, trajectory of the fibers belonging to the pathway. ○​ Target/termination: location and type of interneurons and motoneurons receiving terminations of the pathway. They are defined by the their level in the spinal cord and the lamina in the grey matter. ○​ Collaterals: other supraspinal targets innervated by the axon collaterals from the same pathway. ○​ Molecular features, features that are characterizing the descending system ○​ transmitters and neuromodulators: transmitters employed and presynaptic targets of the descending pathways. ○​ Activity/information transmission: timing, pattern and type of activity exhibited by neurons contributing to the pathway. ○​ effect on the behavior, can be assessed on lesion study, see what happens if the structure is cut  3.​ Descending systems of the cortex and brainstem The descding systems from the cortex has two ways, the pyramidal system is involved in the corticospinal and corticobulbar system going down to the spinal cord in the homolateral and contralateral way. ​ The ventral corticospinal tract is homolateral but by means of the commissure of the spinal cord is controlling bilaterally, its termination is ventromedial so it’s controlling epaxial and proximal muscles, it makes sense that there is a bilateral control as the spine must be kept inside and is dysfunctional to think at completely different activities from one side to the other. ​ The lateral corticospinal tract is the branch of the corticospinal tract that is decussating, vast majority in humans as it represents the 80%, it decussates at the level of the pyramids and then goes on the other side. It ends up everywhere so the crossed portion of the corticospinal tract innervates the whole anterior horn but not only. The descending systems from the brainstem are divided into medial and lateral pathways. ​ The medial pathway comes from the tectum, the reticular formation and vestibular nuclei, the so called reticulospinal, tectospinal and vestibulospinal. The number of the tracts, origin and target are very simple. They go down bilaterally and mainly target the ventromedial portion, therefore proximal and epaxial muscles. ​ The lateral pathway is formed by only one proper nucleus of the brainstem called the red nucleus, originates the rubrospinal tract that decussates and goes on the lamina 8, the one which contains mainly interneurons. OVERALL this means that corticospinal and brainstem descending systems are subdivided into two families: ○​ The medial system targets the ventromedial portion of the anterior lamina which means epaxials and proximal muscles, generally speaking is bilateral, not matter if decussates before or into the spinal cord, in any case it goes on both sides. This involves the: o​ homolateral portion of the corticospinal tract (ventral corticospinal tract) o​ the medial descending system of the brainstem formed by: ▪​ tectospinal tract ▪​ vestibulospinal tract ▪​ reticulospinal tract ○​ The lateral system overall involves the:  o​ crossed corticospinal tract (lateral corticospinal tract) that goes everywhere, also to the medial zone of the spinal cord, it means that controls the dorsolateral, ventromedial and intermediate zone, therefore the whole anterior horn o​ the rubrospinal tract as contribution of the brainstem. Thus, the crossed corticospinal and crossed rubrospinal are forming the lateral system. The homolateral corticospinal and tecto- vestibulo- and reticulospinal tracts are forming the medial system going to epaxial and proximal muscles. So there are two systems, the problem is who is the leader of the two systems that are targeting the same areas. The brainstem is number 2 because the cortex has a domain also on systems of the brainstem. There is a redundancy: so many descending systems from the brainstem but also the cortex controls them. This is because the cortex is really smart and if can delegate something to others, it does. The impulses arriving to the vestibular nuclei especially, can be computed between the brainstem and then go down and organize activities without bothering the cortex, so the cortex is free to work at other functions. The cortex uses the complex of systems from the brainstem to organize the activities of the spinal cord but all of them are working fed by impulses, each one is somehow involved when a sensory system starts, with the exception of the rubrospinal. In any case the cortex has a domain. 4.​ Descedning pathways of the brainstem The brainstem descending system is formed by the medial and lateral brainstem pathways. The tectal nucleus, the medial reticular formation, the medial and lateral vestibular nuclei will give rise to homolateral projections, apart from the tectum, but in the spinal cord they do communicate at the level of the anterior commissure. All these tracts never terminate into motoneurons but on the interneurons that eventually talk with the motor neurons. Normally, depending on which is the system they terminate on local interneurons.  ○​ Given that they are talking with epaxial and proximal muscles that stabilize the position of the body, the targets are propriospinal short or long neurons organizing the whole body set up to maintain the posture when undergoing a perturbation or to set up the posture to perform a movement. The rubrospinal goes into the dorsolateral portion which means extremities, here is divided but does not terminate on motoneurons directly, always on intermediate interneurons. This means that the descending systems coming from the brainstem are talking with the motoneurons and organizing a complex system of synergies in the spinal cord machine, but first they are normally fed by a sensory impulse (the most evident is the vestibular nucleus) and they do not talk to the driver directly, they need interneurons to talk to the drivers. Dissecting and studying all the systems of the motoneurons, a classification in group A and group B fibers can be done based on the muscles innervated. -​ group A: o​ Formed by: reticulospinal, tectospinal and vestibulospinal tract o​ The course is in the ventral and ventrolateral funiculus o​ The termination is in the ventromedial part of the intermediate zone on long propriospinal neurons o​ It controls bilaterally the proximal and epaxial muscles, involved in respiration because controls the trunk muscles so the chest. ​ group B: ○​ Formed by the rubrospinal tract ○​ The course is in the ventral and ventrolateral funiculus ○​ The termination is in the dorsolateral part of intermediate zone on short propriospinal neurons ○​ It controls contralaterally the distal segments. At the end, the position matters. There are tracts dedicated to the posture of the overall setting and tracts dedicated to the extremities. The hand is an extremity. This discussion applies with the same rule talking about the muscle of the head: the corticobulbar system follows the same rules, same discussion, motor nuclei of the 5th and 7th nerve controlling the phono-articulator apparatus for the 5th and the mimic muscles for the 7th, is the same rational as the hand is considered distal. 5.​ Vestibulospinal tract The labyrinth involves the vestibular nuclei. The vestibulospinal tract has four vestibular nuclei. Their projections go: -​ up into the oculomotor through the medial longitudinal funiculus and -​ down with the other part of the medial longitudinal funiculus divided into medial and lateral tracts: o​ Medial tract, controlling head and neck muscles, terminates at the level of the upper segments of the thoracic spinal cord. It plays with the muscles of the head and neck working with the short propriospinal segmental interneurons o​ Lateral tract goes down until the lumbar enlargement, down until the end. It goes into the interneurons, controls the long propriospinal nucleus, Both medial and lateral tract talk also with the nuclei of the reticular formation that are going down into the spinal cord. Do not consider these two separated, they work together. The descending system of the spinal cord is not only dedicated to movement because for example there’s also the efferent part of the vegetative system, which is not secondary, the VII lamina which is also in the thoracic segments. There’re different types which are not actually immediately related to the ventral horn so the motor system. 6.​ Rubrospinal tract The rubrospinal tract starts from the red nucleus, it crosses and mimics very much what the lateral corticospinal tract does. 7.​ Other brainstem pathways: the emotional motor The descending systems to the spinal cord are not only dedicated to movement, but the spinal cord also has the efferent part of the vegetative system. In addition, the VII lamina, located in the lateral horn of the thoracic segments, hosts the pre-ganglionic fibers of the orthosympathetic nervous system. Different other descending systems are present that are not immediately related to the ventral horn so to the motor system. Running aside the medial compoenet are tracts that terminate at all levels of the spinal cord: - The coerulo-spinal tract coming from the locus coeruleus, - the Raphae-spinal system that is the raphae Magnus tract, involved in the modulation of pain perception and modulation, - the endorphin descending system (dopaminergic system): it is activated by endorphin and is important as an antalgic system. Morphine generates a descending system that is switching on the coeruleus nucleus and the Raphae-Magnus nucleus, going down to interfere with the projection of pain at the level of the second neuron projecting in the nociceptive system. Modulation is a top-down modulation fed by endorphin, another way to modulate pain is the gate theory (For example when someone hurts its hand and start to shake it very fast). In this way, there’s an activation of the cutaneous fibers that are converging on lamina V on the projecting neurons specific for pain going up. The Coeruleus and Raphae-spinal tracts use ≠ NT: respectfully the serotonin and norepinephrine. These are going down to the spinal cord and modulate the power supply of other spinal cord pathways. They have an action of reflex gain, affect membrane properties and the excitability of ≠ motoneurons. After an accident the system shuts down, completely black out, the problem is not only to cut the connection between the higher center and the spinal cord, but the problem is also that there’s a spinal shock as nobody is talking with the ones under the lesion, complete black out, it needs time to rehearse a bit. In the regeneration that is very poor, some of these tiny fibers try to reconnect with the region below the lesion to keep up the supply. Diencephalon hypothalamus and telencephalon are talking with the spinal cord, central supra-axial portion of the autonomic nervous system. The central supra-axial portion of the vegetative autonomic system formed by the hypothalamus but also part of the cortex, especially the extended limbic system, is able to drive human’s behaviours that have to do something not only with survival. Running aside the lateral components are the diencephalic and telencephalic tracts. They have a role in autonomic actions such as defensive acyion, pupillary reflexes, sexual behavior, vocalization, CV control, … They go to the spinal cord via the brain stem, via the preacqueductal grey. 8.​ The cortical descending pathways The corticospinal system or pyramidal system originates from the frontal lobe: -​ primary motor cortex area 4 -​ non-primary premotor cortex that is area 6, -​ supplementary motor area and areas from 1 to 3, -​ part of the somatosensory cortex. The question is: Isn’t the somatosensory area devoted to receive? Here there is something that is projecting down to the anterior horn of the spinal cord, not only but for sure is a descending system. We could have expected everything but a sensory area that projects down. But this is not the case. The ventral corticospinal tract, which is homolateral, accounts for 20%. The lateral corticospinal tract runs into the internal capsule, runs into the brainstem that goes anterior, forms the pyramids, decussates and goes on the other side of the spinal cord. This is the brain of a macaque which is the model that have been used for the study of the corticospinal system since macaques are the animal models proximal to humans in terms of abilities. The challenge is to study on humans directly without the animal models because not only it’s not pleasant but also because they are models. In the picture is possible to see the frontal lobe. The regions in grey are all the areas projecting down to the spinal cord from the central sulcus, the corticospinal tract is not only from the primary motor cortex but also from dorsal and ventral premotor, cingulate cortex and the parietal lobe. In the macaques it’s possible to see very well the division between ventral and dorsal which is not present in humans. In the medial portion there’s the supplementary but also the cingulate cortex. Every dot is a neuron that has been retrogradely labelled by injection into the corticospinal tract. Here is a flat vision of the central sulcus, there’re both the motor and the sensory neurons. The corticospinal system originates from the motor, premotor, primary sensory and posterior parietal cortex. -​ 60% of the action of the corticospinal tract origins on the frontal lobe -​ 40% origins on the parietal. This means that the sensory areas are a relevant component of the descending system. a.​ Action of fibers coming from the corticospinal tract The primary motor cortex acts on the lamina IX directly. The dorsal more than the ventral communicates with the interneuron and motor neurons of the anterior laminae. The cingulate goes almost everywhere, the parietal cortex doesn’t talk to the anterior horn but to the posterior. Direct action of the corticospinal fibers on the α motor neurons: the corticospinal system is the only descending system that directly talks with the drivers, direct cortico-motor neuronal connection. The pyramidal cell in the motor cortex is talking to the motoneuron which is driving the flexor carpal radialis without anything in between. The presence of a muscular field formed of the collaterals of one fiber within the motor pool allows an excitatory effect within the motor pool as the inhibitory effect is mediated by interneurons. The result allow the convergence of multiple corticospinal neurons onto one single motor neurons. Muscular field: the collaterals of one fiber within the motor pool. By looking at the motor neuronal pool it’s possible to see that the corticospinal fiber branches at the end and takes synapse on a group of drivers of the same motoneuronal pool. The amplitude of the inhibitory postsynaptic potentials which are always excitatory, are very small. This means that control is very refined. Considering a column of cortical cells that are projecting on the same motoneuronal pool, what happens is that pyramidal cell A will talk with a group of cars, then pyramidal cell B will talk with a group of cars partly overlapped and so on until it’ll finish the group of fibers. This means that each one will give a very tiny contribution. According to the recruitment of the motor units based on the side principle, the control of the cortex on the motoneuronal pool is highly refined because there’s never someone that goes down with a very huge motor command, but the hardware of the system is structured such as the refined control is always allowed. This allows an excitatory effect: Because everyone gives a tiny contribution, it means that if given a tiny inhibitory postsynaptic potential and go down on a motoneuronal pool touching small and big neurons, with the drive of a single or a small number of pyramidal, the summation will allow the firing of the small ones but will not be enough for the big ones. In order to switch on the big ones, more pyramidal fibers need to be switched on or asked to fire with a higher frequency to discharge. This means that the control of the force up there is so refined and the cortex is always able to do that because it knows perfectly that the stretch reflex has already tuned the system for the length. So, it’s a perfect system, direct action on first action of the corticospinal. b.​ Action on segmental interneurons The Ia interneuron is stimulated in the stretch reflex by the Ia afferents (who mostly stimulate the agonist motoneurons. The Ia inhibitory interneuron acts on antagonist motorneuron. The agonist motorneuron exerts an inhibitory effect via Renshaw cells. During the stretch reflex of the antagonist muscle, the Ia fibers of the antagonist muscle inhibit the Ia interneuron. The Ia is receiving from many cutaneous afferents, the ipsilateral vestibulospinal tract, the corticospinal tract and the rubrospinal tract. For the Ib is the same story, it is receiving a lot of fibers as the ones coming from reticulospinal, corticospinal, interneurons and by the way reflex arch. It can also receive the propriospinal neurons that received rubrospinal, tectospinal, corticospinal, also fed by afferents. From the Renshaw cells these interneurons can receive inhibition on alpha motor neurons and descending system. Relying on interneurons is an intermediate step, it is always risky for the leader because it’s not always possible to know what happens in between, for example someone can shut up an inhibitor and then the initial command won’t exist anymore. This is the input and output of the motoneuron. given the current to frequency relationship is a direct relationship in the motor neuron, the higher the intensity of the current, the higher the frequency of discharge. But the slope depends on the Renshaw cell set up which means that if someone shuts down or increases the excitability of this cell, the slope changes. So, for every amount of force, a specific intensity is needed depending on the state of the Renshaw cell. This means that the pool of the motor neuron can be increased or decreased in excitability acting on the Renshaw cell rather than directly on the motor neuron. The common feature is that whoever is in the family, there’s always the corticospinal. Then the corticospinal adds also on the gamma motor neurons; they tune the sensitivity on the muscle spindles which are the stretch reflex receptors and then the cortex determines for example if the muscle spindle of the quadriceps has to be tuned up for the dynamic sensitivity and turned down for the static or vice versa. 9.​ Action on incoming sensory afferents from S1 The cortical spinal system can act on all the incoming fibers that are entering the spinal cord. The sensory cortex going down from S1 terminates posteriorly in the spinal cord. For example If i want to make a movement where the touch is very important as playing piano, fast movements, learning a movement is not only about the muscles that need to be used but also the relevance stimuli that control these movements so which is the expectance in terms of sensory feedback. Which are the sensors that need to wake up and monitor. Which are the reflexes that can be dysfunctional and need to be shut down. Increase the volume of the stretch reflex, dynamic on the prime movers. Motor program means to be able to control the whole spinal cord machine in order to execute perfectly the movement. The reason why the cortex is the leader is because no one escapes its control. The corticospinal tract is powerful because it’s able to go on the drivers without any intermediate and all the others are under its control. The same is true for the descending systems of the brainstem. The motor program is the activity of the cortical areas talking together generating an output played by the different areas of the corticospinal tract that act on the spinal cord and then the different part of the corticospinal tract that are talking all together. Imaging the spinal cord like a piano, the cortex is playing the piano. It takes years to learn voluntary movements. 10.​Spinal reflexes This is an example of a reflex that in one case is inhibited and in another case is facilitated. Reflexes are made by the spinal cord. Physiologists studied the reflexes on decerebrated animals because by freeing the spinal cord, the reflexes are all enhanced so it’s possible to study them better. For example, this is a thresh reflex. If the woman is in this position, and this is the kind of perturbation, the result will consist on a contraction that resists the perturbation stabilizing her position. If she has a cup in one hand, she’ll never use that one to stabilize because the target is to keep the cup in the hand. So, in one case she will exert a force on the table, in the other case she will never leave the cup which means that reflexes can be controlled by the cortex. So, when an action is performed, that cortex has to take all the elements. Performing a movement means how to control the whole machine, not only moving the muscle. Sherrington, one of the most famous physiologists of old times said “A number of evolutionary processes together resulted in a purposefully use of the hand and arm under the dominant control of the cerebral cortex” means that the feature that distinguish humans from animals purposefully use of the hand is the corticospinal system. By comparing the frontal and parietal lobe of cats and humans is possible to see that in the cat, 40% of the frontal is mainly from the primary motor and 20 % from the parietal while in humans the contribution of the parietal increases. In primates, the uncrossed fibers are very few compared to the crossed ones. Normally, the layer originating corticospinal fibers is the 5th layer. The fibers that do most of the job are not the fastest in the primary motor cortex but maybe the smallest. The termination is cervical 50%, thoracic 20% and lumbar 30%. This makes sense with respect of what humans use. 11.​Termination of the corticospinal fibers This shows the spinal cord of different animals. These are the levels of termination in the spinal cord and the different symbols indicate the different laminae. The corticospinal tract is present in the goat but terminates in the cervical region, mainly in the posterior laminae. In cats, the corticospinal tract goes down till the sacral but terminates in the interneurons. In the macaques it’s possible to see the “triangles” so it means direct to the motor neurons. In humans it’s expected to have direct interaction to all the motor neurons, not only their segments, also the axial. This means that there’s an invasion of the spinal cord. In the monkey it’s possible to see that the frontal penetrates completely and goes anterior while the parietal stops posteriorly. Putting together means that the cortex invades completely all the laminae and uses all the possible devices. In humans, the rubrospinal has been put in a corner, the corticospinal has developed with expenses and neglecting the rubrospinal even if it could be an alternative way. The rubrospinal was a system using interneurons, in cat is dominant on the corticospinal. Normally was a system which uses interneurons, dominant in cat but in humans corticospinal has taken the lead. The main evolutionary processes: 1.​ Enlargement of the brain because more cortex to be devoted to a descending system is needed, so the brain becomes bigger in terms of gyri, cortex and the corticospinal becomes bigger in terms of number of fibers. 2.​ The connection from the fibers to the spinal cord goes down such as the amplitude of the excitatory post synaptic potential in a very short latency, potential is stimulating the fiber, the latency of the EPSP tells if it is not synaptic or if there is a synaptic chain because if it is late it means that there is another neuron in between. Going from cat to human, the proportion of late increases, the proportion of early decreases so it is expected that the rubrospinal is decreased in terms of importance while the corticospinal is increased.  3.​ The dexterity, so our ability to move independently the finger, increases parallelly to the occurrence of the corticomotor neuronal connection. So, the factor that gives humans the ability to move the hand as they do is the appearance of the corticomotor neuronal connection. 4.​ Praxis, is the ability also to associate a semantic meaning to the object and to use it, is different from dexterity, the macaque can do it only being trained to this thing but it will never be able to transfer this ability to something else. By looking at the ability of these different types of monkeys, they’re very different. For example, the marmoset are tiny monkeys which grasp with the whole hand rage, but they cannot do the precision rage. One activity that they socially do is cleaning each other, grooming; the macaque can groom the others with a precision grip while the marmoset cannot and uses the mouth. The spider monkey that uses the tail as a hand (there are corticomotor connections to the motor neurons moving the tail). The anatomy doesn’t explain these types of movements. The dexterity means to be able to manipulate. Another story is praxis, the ability to associate a semantic content to the pool and use it properly. If a patient with a lesion in the portion of the brain is asked what to do with the mobile, maybe they grasp perfectly but they use the hand to grasp the air, so something wrong, not in the movement or in the dexterity but in the use of the tool.