User:Aryberg89/Premovement Neuronal Activity

Premovement neuronal activity in neurophysiological literature refers to neuronal modulations that occur in neuronal firing rates before a subject produces movements. Through experimentation with multiple animals, predominantly monkeys, several regions of the brain are particularly active and involved in initiation and preparation of movement. Also, a particular membrane potential, named the Bereitschaftspotential, or the BP and contingent negative variation has been shown to be directly involved in planning and initiating movement.

Regions of the Brain Involved in Pre-movement

Pre-frontal area

Functions in:

•Decision making

•Response selection with move

•Timing of movement

•Initiation/suppression of action

Pre-SMA and the Lateral pre-motor cortex

Functions in:

•Preparatory processes

SMA proper and the Motor Cortex

Functions in:

•Initiation of movement

•Execution of movement

Bereitschaftspotential

In 1964, the two movement related cortical potentials were discovered. These potentials are the Bereitschaftspotential (BP, readiness potential) and the Contingent Negative Variation (CNV). The difference between these two potentials is that the PB is involved in self paced, or voluntary movements, whereas the CNV is involved with cued movements. The Bereitschaftspotential is a movement related potential. The initiation of the BP occurs prior to movement by 1.5s to 1s. The BP is an index of motor preparation and is therefore called the “readiness potential” as it is the potential for movement to occurs. The initial stage of the BP, or readiness potential, is an unconscious intention of, and preparation for movement. After this initial stage, the preparation of movement becomes a conscious thought. The BP, more specifically, is composed of movement related cortical potentials (MRCPs) the peak being the MP or Motor potential. MRCPs tend to resemble a “set of plans” used by the cortex for the generation and control of movement. The BP is activated by voluntary movements involving the SMA and the somatosensory cortex in movement preparation and initiation. The three main hypotheses for the most likely generators of the BP 1.The early BP is generated by the SMA, whereas the late BP is generated by later activation of the contralateral motor cortex 2.Both the early and late BP are generated by bilateral activation of the motor cortex with no or minimal contribution from the SMA 3.Both the early and late BP are generated by bilateral activity in the motor cortex as well as the SMA.

Organization of Primary Motor Cortex

Wilder Penfield’s was a neurosurgeon in Montreal in the 1950’s. His experiment began with the knowledge that his epileptic patients experience a warning sign before the seizures would occur. This knowledge started the beginning of the stimulation experimentations, where Penfield tried to induce this warning and specifically pinpoint the source of epilepsy. Penfield confirmed the presence of a spatial map of the contra lateral body of the brain. Penfield noted the location of muscle contractions with the site of electro-stimulation on the surface of the motor cortex and subsequently mapped the motor representation in the pre-central gyrus. This follows the same trends and disproportions in the somatic sensory maps in the post central gyrus. Intra-cortical micro-stimulation enabled a more detailed understanding of motor maps. By injecting current via a sharpened tip of a microelectrode into the cortex, the upper motor neurons in layer 5, which project to motor lower neurons, can be stimulated. Neurons at different locations on the motor map are connected for the purpose of generating specific movements rather than generating specific muscle movements or contractions. These neurons are associated with the neurons in the spinal cord, and thus stimulating specific movements which occur in specified muscular regions rather than stimulating specific muscles which produce those movements.

Experimentation with monkeys

Experimentation was done by using implanted microelectrodes to record electrical activity of individual motor neurons in awake and behaving monkeys. This provided a way to figure out the correlation between the neuronal activity and voluntary movement. It was found that the force generated by contracting muscles changed as a function of the firing rate of upper motor neurons. These firing rates of the active neurons often change prior to movements involving very small forces. This suggested that, the primary motor cortex contributes to the initial phase of the recruitment of lower motor neurons, involved in the generation of finely controlled movements. Spike Triggered averaging allowed a way to measure the activity of one cortical motor neuron, on a group of lower motor neurons in the spinal cord. Experiment all confirmed that single upper motor neurons connected to multiple lower motor neurons. This supported the general conclusion that movements and not individual muscles are controlled by the activity of upper motor neurons.

"Closed-loop" motor tasks vs. "Open-loop" motor tasks

Conditional “closed-Loop” motor tasks are the primary function of approximately 65% of the neurons in the pre motor cortex. In experimentation using monkeys, when they were trained to reach in different directions, depending on the specified visual cue, the appropriately coordinated lateral pre motor neurons begin to fire at the appearance of that specified cue, but way before the actual signal to perform the movement. As learning takes place, to associate a new visual cue with a particular movement, the appropriately coordinated neurons increase their rate of release in the time between the initial specified cue and the actual signal for the initiation of the movement. It now seems that these specific neurons do not command the initiation of the movements but the intention to perform the movements. Thus these pre-motor neurons are especially involved in the selection of movements based on external events. More evidence that the lateral pre motor area is involved in movement selection comes from observations of the effects of cortical damage on motor behavior. Lesions in the area severely impair the ability of monkeys to perform visually cued conditional tasks. Meaning that on command, it becomes extremely difficult for the monkey to perform the trained movement. But, when placed in another setting, the monkey is perfectly capable to perform that movement in a spontaneous, self initiated manner, as a response to the same visual stimulus. The medial pre motor cortex seems to be specialized for initiating movements specified by internal rather than external cues, which are called “open-loop conditions.” In contrast to lesions in the lateral pre motor area, removal of this medial pre motor area reduces the number of self initiated or spontaneous movements that the animal makes. Ability to move in response to an external cue is largely intact.

Parietal Area 5

The parietal cortex plays a role in the internal command of actions. Most specifically, Parietal Area 5 is responsible for the actions which precede movement. Area 5 neurons exhibit pre-movement activity in response to self initiated movements. The neurons in Area 5 were found to play a role in the initiation of movement and respond at enormously quick speeds. An EMG (electromyogram) is a test of electrical activity in muscles. The Neurons in Area 5 respond at least 100ms faster than EMG detectable activity allows. The cerebral cortex forms a series of loops with the basal ganglia and the cerebellum which drive the initiation of movements, via these positive feedback loops. The neurons on the parietal associative cortex are most strongly involved in programming and execution of voluntary movements. The learned act is the movement which is produced when the starting sensory signal launches the programmed execution. This action requires the neurons of the parietal associative cortex. “The readiness potential indicates that there are 2 processes corresponding to the early and late phases of pre movement: planning of programmed movement and stimulation of the movement’s direct implementation.” The early phase of the readiness potential occurs in the supplementary motor region and is involved in voluntary movement generation. The late phase of pre movement occurs in the cortical regions and is responsible for definite voluntary movements. The two formal stages of pre movement are planning and initiation.

Mirror Motor Neurons

A subset of this response not just for the preparation of the execution of movement but response when the same action is observed mirror motor neurons respond less well when the same action is pantomimed without the presence of a behavioral goal. Summary these findings suggest that the mirror motor system is involved in encoding in the intentions and the behaviorally relevant actions of others and may participate in an extended network of parietal and frontal regions that sub serve imitation learning. The function of the mirror motor system is the most actively studied and debated domains of motor and cognitive neuroscience.

Preparatory changes in neuronal activity

Preparatory changes may occur in neuronal activity before the execution of a movement. During such delay periods there is an interval of time between the instruction movement cue and the subsequently triggered movement. The Primary motor cortex, the pre-motor cortex, the supplementary motor area and the basal ganglia all may experience these preparatory delay periods. These activities coordinated during the delayed periods reflect movement planning in accordance with the instructional cue and the subsequent movement but occur prior to muscle activity. The movement planning may be anything from direction to the extent of the movement.

Short Lead Changes vs. Long Lead Changes

Premovement neuronal activity has been widely experimented upon in three major motor fields of the frontal cortex with the goal of comparing the neuronal activity which comes from visual signals, versus neuronal activity which comes form nontriggered or self paced movements. From these comparisons they found that there were 2 changes that occurred in different time spans in relationship to the onset of the movement. There was the short lead changes which were observed about 480ms before the movement and the long lead changes which occurred earlier by about 1-2 seconds. The short lead changes were exhibited in the SMA (supplementary motor area) and the PM (premotor area) during both the visual signal trials and the nontriggered/self paced trials. The precentral motor cortex was also identified in this study as having similar neuronal activities as those in the PM and SMA. It was found that approximately 61% of the neurons in the PM were preferentially related to the movements during the triggered (visual) trials. The long lead neuronal changes were more frequently active during the self paced stimuli than before the triggered movements. These long lead changes were particularly abundant among the SMA neurons. In summation these experiments challenge the idea that the SMA primarily takes part in self paced movements and the PM is only involved in visually triggered movements. Although the PM neurons showed more preference for the visual trigger signals and the SMA neurons are intimately related to a long lasting process leading to the initiation of the self paced movements. Both are involved in the both types of stimuli.