Motorcortex

• Primary motoric area
• Pre / Supplementär - motor areas
• Primary sensory areas
• Sensible association areas
• Hörfelder
• Fields of view

The Moto (r ) cortex ( from Latin motor, " mover ", from the Latin cortex " bark " ), also motor and somatomotor cortex, is a histologically definable area of the cerebral cortex ( neocortex ) and the functional system, controlled by the voluntary movements of from simple and complex patterns of movement sequences are compiled. It forms the superordinate control unit of the pyramidal system and is in the rear ( posterior ) areas of the frontal lobe.

Illustrative reflex movements ( reflexes and polysynaptic reflexes ) occur, however, at low levels ( in the spinal cord or in the brain stem ) and are therefore not deliberately influenced. Other functional systems are participating in exercise performance: For the control of muscle tone, the basal ganglia are important. For spatial design, estimating the necessary strength and speed and smoothing the movements, the employees of the cerebellum is necessary. These are like the olive and red nucleus elements of the extrapyramidal system. This categorical classification ( pyramidal - extrapyramidal ) but has been abandoned because the cortical and subcortical systems largely overlap, and also non-motor brain areas such as the back ( posterior ) parietal cortex, a crucial role in the planning and execution of simple and complex motor acts (eg, goal-directed grasping objects ) has.

• 4.1 afferents
• 4.2 connections within the motor cortex
• 4.3 Efferenzen

• 5.1 Follow a lesion of the motor cortex
• 5.2 Important diseases

Definition and anatomical localization

Formerly the whole of the excitable cortex was considered motor cortex. By this is understood the sum of the cerebral areas in which external electrical stimulation visible movements can be caused. Since this is possible at sufficiently high stimulus voltages also to virtually all associative and number of sensitive areas, there is a tendency today to be among motor cortex just before the central sulcus ( sulcus centralis ) lying areas, which have a typical cytoarchitektonischen construction, to be taken.

These include the precentral gyrus and the rear ( posterior ) portions of the superior frontal gyri, frontal medius and inferior frontal and the front (anterior ) portion of the paracentral lobule.

Histology

Histologically, the motor cortex is part of the isocortex. This means that he has a defined six-layer basic structure, which he shares with all phylogenetically young areas of the cerebral cortex. The inner granule cell layer, which is very pronounced in sensory areas and also occurs in the prefrontal region, or lacking here is not delimited by the outer pyramidal cell layer. One therefore speaks of agranulärem cortex. Due to the occurring only here giant neurons in the fifth layer ( Betz giant cells ), the primary motor cortex is also referred to as Area gigantocellularis. The frontal adjacent fields are similar, but do not have giant neurons, they are therefore also described as Area paragigantocellularis. The primary motor cortex reached in some areas ( paracentral lobule ) with 3 to 5 mm, the maximum width of the cortex at all.

The layers are called as follows:

• I. Lamina molecularis (molecular layer)
• Lamina II granularis externa ( external granule cell layer)
• III. Lamina pyramidalis externa ( outer pyramidal cell layer)
• ( IV lamina interna granularis; inner granule cell layer)

• V. Lamina pyramidalis interna or lamina ganglionaris (internal pyramidal cell layer)

• VI. Lamina multiformis ( vielformige layer)

Functional and histological classification

Functionally, the primary motor cortex (in Anglo-Saxon literature: M1) distinguished from the supplementary motor cortex ( supplementary motor area, SMA ) and the premotor cortex ( premotor area, PMA, or even PM). The latter are used by today's creating certain motion sequences from a pool of learned movements and the preparation of arbitrary ( both conscious and unconscious ) movements. The motor neurons located in the primary motor cortex are the main common output of the motor cortex, mainly because their axons reach the spinal cord and the motor cranial nerve nuclei. For that places local switching on the peripheral motor neuron ( anterior horn cell) the commands arrive finally to the arbitrariness muscles.

After histological brain atlas of Korbinian Brodmann (see Brodmann 's area ) corresponds to the area 4 of the primary motor cortex; the supplementär - motor cortex and the premotor cortex are formed from complex 6. Modern subdivisions, which are based more on the function, distinguish seven to nine fields, these subdivisions are only on the basis of studies on non-human primates, such as rhesus monkeys, takes place (eg after G. Rizzolatti F1 [ = M1] to F7).

Primary motor cortex (M1 )

The primary motor cortex lies for the most part on the cow in front of the central groove camber ( dentate gyrus ). Of note is the so-called somatotopic, that is that neighboring areas of the body are in their representation of the primary motor cortex adjacent. The body is thus reduced and imaged upside down as a " homunculus " in the cerebral cortex. However, the proportions of the Homunculus are distorted because certain areas of the body have a very finely tuned motor skills, this is true in humans, especially for the hand and the speech muscles. Other regions, however, can only be comparatively coarse moved (back) or have a higher percentage of automatic regulation ( holding and supporting muscles). The respective cortical areas are correspondingly larger or smaller. However, in the somatotopic M1 is still significantly coarser pronounced than that of the primary sensory cortex (S1, area 3b), which has a precise representation of the body surface.

The descending ( efferent ) pathways that leave the cortex, which together form the optic tract corticonuclear that supplies the motor cranial nerve nuclei, and the corticospinal tract, ie, the pyramidal tract.

Premotor cortex ( PMA / PM / PMC)

This rather extensive cattle area ( Brodmann areae 6 and 8, = areae extrapyramidal ) is located in front of the primary motor cortex and is located slightly lateral (lateral ) on the convexity of the brain surface, while the supplementär - motor cortex to the center of the head to ( medial) and predominantly is, as it were located beyond the sheath edge to the opposite surfaces of the cerebral hemispheres. The task of this field is to create movement patterns and coordinated with the cerebellum and the basal ganglia. It also flow sensory information, such as defining the necessary amount of movement, a. An important subdivision of the Prämotorcortex is in the dorsal Prämotorcortex ( DPMC or PMd ) and the slightly deeper to the Sylvian fissure situated towards the ventral Prämotorcortex ( VPMC or PMv ). Both cortices are particularly involved in the transformation of the visual information (e.g., position and shape of an object ) into motor programs. Here, neurons of the DPMC encode preferred for the position of an object in space, whereas the VPMC the hand and finger opening controls for gripping an object. Both areas have intense connections to the parietal cortex, where more DPmc the superior parietal lobule (SPL ) and inferior parietal lobule from VPMC (IPL ) and in particular from the intraparietal sulcus ( IPS) receives projections. The parietal areas, however, are strongly associated with the visual Beef areas. Based on these anatomical events Rizzolatti has designed a theory for cortical control of reaching movements, namely that visual information from the primary visual cortex ( V1) through higher visual areas (V2 - V8 ) reaches the parietal cortex, in which analyzes the information on the location and shape is, which is forwarded to the corresponding premotor areas that drive ultimately the primary motor cortex. The test of this concept is the subject of current brain research.

Some neurons in the premotor area are both in the planning and execution as well as the passive observation of the same movement in another individual active ( so-called mirror neurons ). One suspects their importance in imitative learning processes (see also: model learning ). Newer hypotheses rather go on the assumption that the mirror neurons ( which can be found inferior parietal in other brain areas such as the lobule ) rather the " understanding " of an action code: In macaques, some mirror neurons fire only when the monkey for an apple means, but not when the movement is performed without apple. It is believed that disturbances of this " understanding network" could be a pathophysiological basis for autism.

The important for speech production Broca's area ( motor-driven language center, area 44 and area 45) and the so-called frontal eye field ( area 8) functionally belong to PMA, although they possess structurally more of a " prefrontal " pattern ( granular cortex). Here, however, shows the marked functional heterogeneity of Prämotorcortex.

Supplementär - motor cortex ( SMA)

The supplementär - motor cortex plays a role in the learning of action sequences and in preparing complex movement patterns. Tests on monkeys showed that temporary blockade of the SMA results in the inability to initiate movements. Regard to the preparatory and movement- initiating function of the supplementary motor cortex is also an increased electrophysiological activity, there is already more than a second before the visible onset of a movement can be detected: the so-called readiness potential (see also Libet experiment). The SMA also has an important function for the control of bimanual, that is, two-handed movements: Professor Tanji described in the 80s in macaques for the first time the presence of neurons that fired in both single-handed as well as two-handed gestures. The exclusive role of the SMA for bimanual actions but has been now revised, as have also other regions - such as M1 bimanual neurons. Functional imaging studies in humans have shown that the networks, that is, the regions involved, do not differ greatly between uni- and bimanual movements. In contrast, connectivity studies - ie, investigations, how - areas shown to interact with each other that at two-handed movements intensive coupling of the two cerebral hemispheres occurs, and that here also the SMA seems to play an important role in integration. Interestingly, the left SMA has a predominance over the right SMA, so some researchers see here a biological equivalent of handedness.

Pyramidal tract

The pyramidal tract ( corticospinal tract ) is the summary of all axons, which originate from the primary motor cortex and pass commands to the spinal cord or cranial nerve nuclei. They have a common and in turn somatotopically structured course. Pure functional terms, the pyramidal tract is a direct component of the motor cortex.

The descending axons originate approximately 25% of small pyramidal cells of the primary motor cortex, and another 30 % come from the secondary - and supplemental areas and 40% have their soma even in the somatosensory cortex ( areae 1, 2, 3, 5 and 7 after Brodmann ). Only about five percent anticipate the large Betz giant cells of the primary motor cortex. The somatosensory fibers appear to be functional but less significant because she does not have monosynaptic connections to the motor anterior horn cells. However, you might have an important function for the functional recovery after brain injury, such as after a stroke.

Neural connections

Afferents

The afferent pathways of the motor cortex are mainly from the thalamus, in particular from its ventral districts. Including information from the cerebellum and the basal ganglia, and sensory stimuli from the lemniskalen system are summarized. The webs of the basal ganglia (especially from the globus pallidus ) go mainly in the pre-and supplementär - motor cortex.

About association fibers, ie connections within the cortex of one hemisphere, get the premotor areas of extensive sensory and sensory information from the parietal lobe, the supplementary motor areas, however, are mainly fed by the prefrontal cortex, with higher cognitive functions (consciousness, intention, motivation) is associated. This is interpreted as an indication of the role of the SMA as " enabler " of a planned movement. Connections from the cingulate gyrus, which is attributed to the limbic system, are made to all parts of the motor cortex.

Connections within the motor cortex

Within the motor cortex, the tracks run primarily from the pre-and supplementary motor areas to primary motor cortex. The anterior portions of the PMA and SMA appear to control the function of the rear and to inhibit optionally but not send direct fibers in the M1.

Efferents

The giant pyramidal cells of the primary motor cortex send their axons almost exclusively in the pyramidal tract, where they make up about 5 % of the fibers. In addition, the axons of the small pyramidal cells präcentralen there radiate a ( 25-45 %) and fibers from the premotor and supplementary motor cortex (5-10% ) and from the somatosensory cortex ( 20-50 %). Collaterals, ie branches of the axons of motor neurons reach the red nucleus and the reticular nuclei of the medulla oblongata. A lot of the tracks also ends at the core regions of the pons and the olivary nucleus, from where they are forwarded to the cerebellum, or leaves already in the internal capsule, the corticospinal tract, to drive the thalamus and the corpus striatum. The proportion of axons that accompany the pyramidal tract to the spinal cord, is 15%. Overall, the axons exit from an estimated two to three million motor neurons, the cerebral cortex. With a total number of around ten billion nerve cells, this figure is surprisingly low.

The pre-and supplementary motor areas send regardless of their connections to the primary motor cortex and efferent fibers to the reticular formation of the brain stem. From there, the tonic active trunk muscles is controlled mainly.

Pathology

Consequences of a lesion of the motor cortex

Damage to the upper motor neuron in the primary motor cortex leads regardless of the cause of damage to the characteristic movement disorders in the muscle groups that are controlled from the affected cattle district. Since most of the descending pathways (see pyramidal tract ) in the brain stem to the opposite side cross (so-called pyramidal decussation, decussation pyramidum ), the paralysis usually occurs mainly on the opposite side of the body in appearance ( hemiparesis ). Virtually always in control of the hull distant (distal) muscles is more pronounced than that of the restricted close to the fuselage ( proximal ) muscles. John Hughlings Jackson shared a movement disorders in positive and negative symptoms. Plus symptoms are:

• Increased muscular resistance to passive movement ( spastic tone increase )
• Enhanced reflexes
• The inducibility of pathological reflexes such as the Babinski sign
• Occurrence of mass movements and associated movements of the opposite side

To the negative symptoms include:

• Reduction of the developed muscle force ( paresis )
• Selective impairment of motion and loss of precision movements
• Impairment of the ability to rapidly alternating movements ( dysdiadochokinesia )
• Impairment of the ability to develop force rapidly, and longer time to keep constant (motor impersistence ).

Damage to the upstream cortical areas are rare and isolated lead to complex movement disorders. Similar problems can also lesions in the parietal association cortex and - occur in pathological processes of the basal ganglia and the cerebellum - at least partially:

• General coordinative clumsiness (ataxia), but may also occur with cerebellar damage or sensitive deficit
• Disturbed motion memory
• Inability to correctly implement exercise plans ( ideokinetische apraxia )
• Inability to create exercise plans and to combine individual actions make sense and in the correct order ( ideational apraxia )
• Impaired initiation of movement (start inhibition), which also occurs in Parkinson's disease

Important diseases

The most common cause of acute brain injury with motor impairment is cerebral infarction by vascular occlusion in the territory of the middle cerebral artery ( MCA ). If the dominant in most people left hemisphere is concerned, it is often in addition to language disorders (see Aphasia ). At the same time present apraxic and ataxic symptoms are often masked by the paralysis. Even the much rarer closure of the anterior cerebral artery (arteria cerebri anterior ) part of the motor cortex is involved, typically (according to the Homunculus ) for the control of the lower extremity. Other causes of damage to the motor cortex are brain haemorrhages, inflammations, tumors and injuries.

A rare disease that is associated with the degeneration of cortical motor neurons, the spastic paraplegia. Even with a lack of oxygen during birth may damage the delicate nerve cells of the motor cortex occur, the resulting disease is called infantile cerebral palsy. A neurodegenerative disease of the elderly, the cover, in addition to the anterior horn cells and central motor neurons is amyotrophic lateral sclerosis.

Epilepsy is a short duration, seizure-like dysfunctions of numerous nerve cells. When generalized tonic- clonic seizure ( grand mal ), the motor cortex of both sides massively excited. Result is - among other phenomena - the characteristic " spasmodic " twitch, which covers the entire body. In contrast, spread with another form of epilepsy, the focal Jackson seizures, the seizure potentials slowly over the primary motor cortex only one side and thus lead - on a " wandering" of the twitches ( march of convulsion ) - according to the somatotopy the muscle groups of the limb. Consciousness is retained. After the spasm potentials subsided, the motor function is almost always undisturbed. An exception to this is the Toddsche paralysis, which can mimic a stroke.

Evolutionary aspects

In the course of evolution a tendency to form an ever-increasing complexity of the brain structures and the increasing shift of control processes in the cerebral cortex ( corticalisation ) is observed. The motor cortex is a relatively recent development and is found only in mammals. The execution of movements is controlled in fish, amphibians, reptiles and birds also of a core area known as Archistriatum in the brain in mammals corresponds to the corpus striatum, which is also involved here of movement.

Especially primates have a keen motor-driven cattle area. They also have - also unlike all other mammals - many monosynaptic, so direct connections of the motor cortex to motor neurons in the brainstem and spinal cord. This suggests that the conscious, planned and fine-grained motion of individual muscles only to them is possible during run in most animals movement programs likely to " automatic " and without much arbitrary intervention. Ungulates have compared to a poorly developed pyramidal tract, which ends already in the throat swelling of the spinal cord ( cervical Intumescencia ) and especially for the facial expression plays a role. In dogs still reach about 30 % of the pyramidal fibers, the lumbar swelling of the spinal cord ( lumbar Intumescencia ), however, the fibers always end on interneurons, never directly to the anterior horn cell. A complete damage to the motor cortex of a page thus never leads in almost all non-primate to Plegien but to contralateral disturbances of posture and righting reactions.

In humans, in particular the control of the hand and the speech musculature has been refined in its evolutionary development. He also has a uniquely high potential in the animal kingdom, life- long learning new movements.

History

In principle, since 1870 it was known that the stimulation of certain cortical areas leads to defined motor responses: Gustav Theodor Fritsch and Eduard Hitzig had then performed revealing experiments on dogs, the results of David Ferrier could be confirmed and substantiated by experiments on monkeys. However, John Hughlings Jackson led solely from the close observation of partial seizures true theories about the organization of motor systems from (especially the above-described and named after him Jackson seizures) substantially. The discovery of the importance and somatotopic organization of the primary motor cortex in humans goes back to the Canadian neurosurgeon Wilder Penfield. By weak electrical stimulation of the cerebral cortex of awake patients with open skull (the brain itself is not sensitive to pain ) he could clarify the situation of some functions. In 1949, he discovered so that it is possible by stimulation of the precentral gyrus trigger spasms in specific muscle groups.

Importance

The extraordinary importance of the motor areas - also with regard to philosophical considerations - is that they represent a sort of interface between consciousness and matter. Only through this connection is the man capable of purposeful and directed to influence his environment to get around and make contact with other individuals. Impressively, the importance of the motor cortex revealed when at a complete loss of its function - is lost, any arbitrary control over the body - usually caused by lesion of the descending ( efferent ) pathways. Patients with locked-in syndrome known as fully conscious and perceive their environment, but can no longer respond and are thus almost completely enclosed in itself (locked in ). Only on vertical eye movements is still an understanding possible.

View

Currently, there are promising attempts paralyzed people with intact motor cortex return ( eg following a cross-sectional injury) by so-called brain-computer interfaces, a (very limited) framework for action. The neuromotor prostheses are placed directly on the brain surface in the area of the primary motor cortex. They consist of an array of small electrodes, which tap the potentials that arise (movement -related potential ) during a movement. Although the physiological conduction is interrupted, so possibly can in the future to a limited extent tetraplegic paralyzed policy options are given, as here, such as the control of a cursor on a computer screen or control of a robot arm.