Introduction
Neuromuscular control is the necessary aspect that is responsible for moving and studying new motions. This process is active when a kid learns walking, and when an extreme sport admirer learns how to perform new tricks. In fact, the neuromuscular control is the process that makes us able to walk, eat, drive a car, ride a bike, be involved into sport activity etc. Neuromuscular control processes are active even when we are picking our nose, hence, the mechanism of this process is comprehensive, and needs to be studied in detail.
The aim of this paper is to explain the aspects of neuromuscular control and explain the importance of this theory for studying the processes of Central Neural System. In general, this theory is based on the premise that all the motions are not simply controlled by motor cortex, but the dynamic stereotype is formed in accordance with the life experience (auditory and visual memory), plan of movements created by prefrontal cortex part, and the perception of space and location. Moreover, spinal cord and Peripheral Nervous System are involved in controlling muscles. Hence, the control over muscles is performed by the synchronized and united system.
Motor Control Theory
Movements of limbs, corps, head, neck eyes, tongue etc are the inevitable parts of our life. However, in spite of the fact that these are the basic requirements of a full-fledged life, scientists still are not able to give the definite answer why people do move, and what is the source of signal, that makes us move. As it is stated by Albus (2001, p. 61):
The biological motor system is in some sense an ideal realization of control. It consists of actuators, sensors and controllers, like usual control systems do. Unlike artificial control systems, however, it exhibits much higher performance with great flexibility and versatility in spite of the nonlinearity, uncertainties and large degrees of freedom of animal bodies. Actually, motor control has been a main focus of neuroscience research for a long time, but what is strange is that control theorists have rarely tried to seriously investigate a theoretical basis of biological motor control.
Hence, it should be emphasized that that the motor control study progress strongly depends in the computational neuroscience provides the required theoretic issues in essentially clearer ways. In general, it should be emphasized that the movement system may be theoretically divided into two integrated parts: sensory and movement. This division may help realize deeper the nature of movement, however, senses are closely interconnected with motions, as it is impossible to feel the motion without motion itself (if only a person practices studying of feelings origin), and motions for motions are available to sportsmen or dancers only.
The basics of motor control theory are relied on three aspects of movement, and biological functions of the movement system elements. These are as follows:
- The adaptive control performed by cerebellum and its components
- Basal ganglia, and its activity from the perspective of movement control
- Modular control and involvement of cortex
The activity of the regarded components shapes the movement system, while the details of this activity requires in-depth study and analysis. The basic knowledge that is available give the review explanation of motion functions, however, the nature and origins of these activities, as well as details of functioning are not known. In fact, the regarded components is not a firm structure, as it may be supplemented by other elements of central neural system. Hence, auditory, visual and motor memory parts “help” motor cortex perform the required tasks. When a movement is new, brain activates attention lobes. This premise is explained by Barto (2006, p. 225):
In neural motor control, the situation is totally different. The brain must construct models of an object and the environment only through learning by experience and memorize them in its neural network in a format usable for motor control. Lately, the problem of how to acquire and store models of controlled objects in the neural network has come to be a central issue.
In the light of this statement, it should be claimed that the images and models that are formed jointly by various parts of cortex help movement cortex and cerebellum perform the required motions basing on the initial position of limbs and body in space.
The Nature of Motion
Depending on the amount of joints, muscles and ligaments involved into a motion process, various brain lobes, and parts of peripheral NS are involved into the control process. If a movement is simple like raising a hand, primary motor cortex lobe is sending a signal to cerebellum, and then it goes through thalamus and spinal cord to motional control nerves, which make our muscles work. However, if we are studying a new motion (dancing, running, riding, painting, etc), associative lobes are involved in order to find out whether we have already performed similar motions.
Additionally, auditory and visual memory is connected, as the first attempts would require systematic repetition of the heard and seen information for better involvement of movement memory lobe. Additionally, when a movement is new for a brain, attention part is involved into the motion process in order to control every aspect required for performing the required motion. Then, attention is switched to analyzing the surrounding space and location, while a movement is performed in automated regime. So, what is happening inside a body when we wish to make some motion?
Muscles and Movement Units
In general, we may imagine a simplified form of neuromuscular control. There are two types of neuromuscular control: sensory, which starts from peripheral components of movement system, and comes up to brain, and motor, which starts from motion cortex to periphery, where muscular structures are located, that perform motion. Motion system also involves several hierarchical levels and ways of parallel information processing. The effectiveness of peripheral components is also defined by the sensory projections of the body in brain.
Muscular system is based on the premise that a muscle goes from one bone to another, and when a muscle is contracted, this contraction makes one of the bones move. There are only two exception from “two-bones” rule: eye muscles that make eyeball move, and tongue muscles. Another principle is that any muscle that makes any movement is opposed by another, which causes the opposite effect. These are called antagonist muscles.
When a motor nerve (movement unit) is activated, a neuromuscular synapse is producing a chemical mediator acetylcholine which makes a muscle contract. This mediator is intended to link the special receptors on the membranes of muscle fiber. The nature of motions is common for all muscles, independently, whether we are able to contract it intentionally, or it works in accordance with reflexes. However, independently on the nature and the exterior reason of motion, all the movements are performed after the “command” of a motor nerve. Hence, we are talking about motor axon and motoneuron as a final destination point of movement control.
In general, any muscle fiber is controlled by a single motoneuron, however, one neuron may control several fibers by the ramified system of axons. Everything depends on the fineness of the required movement. Hence, in eye muscles one neuron controls up to three fibers in average, while a hip muscle involves up to hundred fibers for a single neuron. Additionally, a power, which may be developed depends on the amount of muscle fibers. Motoneurons that control large muscles have extensively ramified axons.
Spinal Cord
Spinal cord is the following component of the movement control system. In general, it may be compared to retina, as it performs similar functions. Both consist of neural amassment which are located in depth – at a distance from periphery. Spinal cord is performing integration and filtration of signals by using local networks of neurons and axons. However, these independent integrations which are performed on this level are quite simple, while integrations that are performed on the spinal cord under movement centres of the large hemispheres’ cortex. Additionally, the cord perceives information from proprioceptors which inform the cord on the load and position of a muscle in space.
This helps to define the position of a joint, and this information is used for defining the requirements of the following movement. The proprioceptors are helpful for unconscious movements when a neural system reacts for an irritant. Hence, if any receptors experiences the action of any irritant, the muscle contracts instantly even before a pain is felt. However, if you need to point some object with your finger, reflexes are helpless, and movement commands are originated by a higher level “commander”.
Movement Cortex
Parts of the cortex that are responsible for movement were discovered while studying such deceases as paralysis and brain injuries caused by cerebrovascular accident. Both of large hemispheres involve stripes of cortex that are responsible for movement function. Both motor zones are joined with cortex parts with primary sensory projections of body surface that are also located in both hemispheres. Neurons in movement cortex are arranged in vertical columns that are forming a single functional movement column.
These neurons control a group of associated muscles. The most important function of cortex is to control and regulate the position of joints but not simply activate muscles. Depending on the initial position of a joint, the corresponding column should either of the antagonistic muscles. In general, it should be stated that cortex coordinates all our muscles by commands which define the particular position of joints, but not commands that contract muscles.
Neurons of the cortex, that are directly linked with motoneurons of spinal cord are called Betz cells, or large pyramidal cells. Their axons are joint into a single large bundle of neuron fibres that are called pyramidal tract. Reaching the spinal cord, axons of Betz cells are crossed – a bundle, coming from right hemisphere goes to left side, and vice versa.
The stimulation source that controls neurons of movement cortex is probably located in neurons of sensory cortex on the latest stage of sensory information processing. This stage is featured with appearing the sensory of high abstraction level which define the location of limbs, and the necessity of quick performance of any particular actions. This information, including the full data on the joint positions, supplemented with muscle tension information forms the basis which is used by movement cortex for initiating definite movements.
Actually, the review of movement system may be ended, as the general picture is already given, however, there are some other components, which play rather important role in movement. These are basal ganglia and cerebellum.
Basal Ganglia
This part is located in the depth of the brain, and hidden with large hemispheres. It consists of four components, each with its particular function. These are striatum, pallidum, sustantia nigra, and subthalamic nucleus.
Striatum gets all the types of sensory information from all the lobes of cortex, as well as unprocessed sensory information from thalamic nucleus before it goes for processing to cortex. The third source of information is divergent links from substantia nigra. This component influences the movement system by dopamine mediator. Registration of neuron activity in striatum has shown that the signals are originated right before the motions of particular type – slow movements of a limb from one location to another. Hence, when you are trying to touch your nose when your eyes are closed, the main part of this movement is controlled by basal ganglia. The final phase – the moment of touch is controlled by cerebellum.
Cerebellum
Information comes to cerebellum from the cortex of hemispheres, brain stem and spinal cord. The letter transfers information of limbs, corpse, head, neck, eyes positions. All this information is integrated by Purkinje cells. They are constantly active, which signifies that they control the body, limbs and eyes positions. Purkinje neurons, in their turn, send information to larger neurons of deeper nucleus of cerebellum. Information which is originated by these nuclei changes the activity of movement cortex neurons. In spite of the refined regular structure of cerebellum, and properly studied neuron networks, the particular role of cerebellum in motion is not clear.
In accordance with some observations that are made while studying cerebellum injuries or irritations, it should be pointed out an important role of this organ in maintaining and regulating muscular tonus that is needed for maintaining the required pose. This feature is used for checking the alcohol intoxication level, when a person is asked to walk along a straight line, or stand straight with the eyes closed. Additionally, cerebellum defines the location of body parts in every particular moment while some fine motions are performed. It seems that cerebellum entails a copy of movement program that is used by movement cortex neurons.
Cerebellum coordinates the activity of movement cortex and spinal cord, thus providing more “polished” performance of the controlled tiny and refined movements. It also plays an important role while performing quick consecutive and simultaneous actions, like hand movements of an experienced musician, or finger moves by a student who types his / her custom paper.
Literature Review
Performing voluntary movements with the body, people are able to interact with the surrounding environment. It is essential for performing the tasks of life importance. In fact, the movement control system has not been studied properly, however, the separate researches emphasize the importance of the following parameters, required for movement: muscle properties, limb geometry, and neural control. Though it may seem that the result is strictly defined by the functional capabilities of the limbs, the relative importance of other components is rather high, as all the enlisted components are closely interconnected while moving.
It is stated that the main function that brain performs is assessing the needs of the organism, and performing the required actions for the satisfaction of these needs. Consequently, movement is closely integrated with memory, attention, analysis and other parts.
Movement system performs the processing of neural signals. It acts in accordance with the particular and ordered plan from movement initiation by motor cortex to contraction of muscle which control position and stability of joints. These signals are transferred through spinal motoneurons. Parallel modifying systems of cerebellum and basal ganglia provide the coordination and smooth performance of the movement program.
The integrated movement system involves such components as movement units, spinal cord, movement cortex, cerebellum and basal ganglia. While the functions of units, cord and cortex are known – processing and transmission of the information, the functions of ganglia and cerebellum are not studied properly at the moment. However, it is known that people with cerebellum injuries (Parkinson’s decease) experience essential difficulties for multi-joint movements, while single-joint movements are performed easily. Hence, cerebellum plays a synchronizing role for muscles and joints. Considering the fact that neural movement system controls joint positions making muscles contract. As it is stated by Astrom and Wittenmark (2001, p. 345):
The cerebellar circuit has a roughly feed forward organization with two major input pathways: mossy fibre inputs and the climbing fibre inputs. Mossy fibres send various sensory signals as well as motor commands to a massive number of granule cells, which in turn send parallel fibres to the Purkinje cells. Each Purkinje cell receives many thousands of parallel fibre inputs and only one climbing fibre input.
In the light of this statement, it should be emphasized that cerebullar control plays an important role for coordinating movements, and making the performance of movement program smoother. The same should be stated on the matters of basal ganglia role in movement. This part requires strong and stable dopamine stimulation, as this is the key element for proper basal ganglia functioning.
The dopaminergic inputs are required for motor control functioning, while movement violation deceases (such as Parkinson’s and Huntington’s decease) are featured with dopamine lack. Though, the role of dopamine and basal ganglia in brain functioning is mainly unknown. It is presupposed that basal ganglia involves the required functional movement models that originate the performance of a particular movement program. A detailed description of basal ganglia functioning is given by Imamizu and Miyauchi (2003, p. 598):
The activity of the caudate neurons in basal ganglia does not simply encode the movement command, but seems to encode the reward value associated with the movement. This hypothesis also suggests that a form of stochastic sampling of actual actions based on the predicted reward, such as, is implemented in the down-stream of the caudate nucleus. This model of action selection based on action value functions can provide a new working hypothesis about the role of multiple feed-forward connections in the basal ganglia circuit.
Though, basic functions of basal ganglia and cerebellum are defined, their interaction still arises numerous questions. On the one hand, it is stated that their function is to smooth movements and control multi-joint movements: while basal ganglia is responsible for large movements, cerebellum controls fine movements of fingers and other joints. Hence, Ito (2005) presupposes that both these components entail the movement models that are compared and synchronized when movement is needed.
In accordance with other theory, these components help hemispheres create the correct image of a movement, and both send the required model, while hemispheres perform the following processing of the information (Wolpert and Kawato, 2003). Hence, it should be emphasized that the hypothesis of global cooperation of brain component is the least confirmed, nevertheless, tests show that it is quite reasonable. In particular, most tests show that cerebellum is actively involved in skill-learning procedures, as rostral pre-motor areas of cerebellum are active. This confirms that movement models are included into the cerebellum, as the serial search strategy of movement when a new task is learnt. (Astrom and Wittenmark, 2001).
In accordance with the computational theory stated by Barto (2006) the reinforcement learning is closely associated with the aspects of application modelling within the brain for goal-directed modelling. These modelling techniques appear to be helpful for studying the neural circuit in basal ganglia and cerebellum, however, it does not give any definite answer as for the matters of the functional role of these components. Anyway, the neural activity of these elements is closely integrated into the activity of the entire movement system, while the actual importance of integration is not doubted, though, it is not studied properly at the moment.
Conclusion
Finally, it should be stated that the brain maintains organism by realizing its necessities and requirements, and by impelling it to actions that are required for satisfying these needs. However, the human brain is capable to compare the present with the past and define which actions are necessary, and which are not. Numerous parallel information processing systems give us whales of interpretations which are required for filling the gap in movement experience.
Reference List
Albus, J. A theory of cerebellar function. Math. Biosci., Vol.10, pp.25–61. 2001.
Astrom, K. B Wittenmark. Adaptive Control. Massachusetts: Addison Wesley. 2001.
Barto, A. Adaptive critics and the basal ganglia, In: Models of Information Processing in the Basal Ganglia. Cambridge, MA: MIT Press, pp.215–232. 2006.
Imamizu, H., Miyauchi, S. Separated modules for visiomotor control and learning in the cerebellum: A functional MRI study. NeuroImage: Third International Conference on Functional Mapping of the Human Brain — Copenhagen, Denmark: Academic Press, Vol.5, p. 598. 2003.
Ito, M. Movement and thought: identical control mechanisms by the cerebellum. Trends in Neurosci., Vol.16, No.11, pp.448–450. 2005.
Wolpert, D., Kawato, M. Multiple paired forward and inverse models for motor control. Neural Netw., Vol.11, pp.1317–1329. 2003.