Grasping is an automated movement pattern that is planned in the motor cortex of the brain. From there, the movement plan for reaching is transmitted to the voluntary muscles via the brain‘s pyramidal pathways. Impaired reaching movement may indicate neurodegenerative diseases.
What is reaching?
Grasping is an automated movement pattern that is planned in the motor cortex of the brain. To grasp, a person actively closes his or her hand around something near his or her body. In this process, the brain thus plans, realizes, and controls a purposeful and usually voluntary hand movement involving the fingers. All grasping movements are so-called precision movements and thus belong to fine motor skills. Grasping can be done consciously as well as unconsciously. An unconscious type is present, for example, in reflex movements. An unconscious grasping reflex can also be observed in newborns. Already in the womb, the fetus is given the necessary prerequisites for carrying out grasping movements. However, it is not until children over one year of age that they grasp things in their environment with precision and good timing. Various anatomical structures are involved in grasping movements. In addition to the muscles of the arm and hand, the spinal cord and various areas of the brain are primarily involved in grasping. In addition to the motor cortex for voluntary movements, the perceptual system in the brain also plays a role in grasping movements. For example, the planning of grasping is related to visual and spatial perception.
Function and task
Humans purposefully grasp things from a few dozen to a few hundred times every day. Already in early childhood, the automation of the grasping movement begins. Grasping is incorporated into comprehensive patterns of action that are stored in the brain and are henceforth retrievable in an automated manner. For example, a person reaches for a glass, brings it to the mouth and tilts it upward to drink. Since this sequence of movements takes place several times a day, it is automated by the brain. As a result, the person no longer has to concentrate on the individual movements or explicitly and consciously commission the individual movements in thought. Considering the frequency with which people reach for something, this automation is an important protection against overload. The compilation of movement patterns from simple and single movements takes place in the motor cortex, which makes up a part of the cerebral cortex. This posterior zone of the frontal lobe forms the superimposed control system for the spinal pyramidal tracts. The central eye field also feeds into this area of the brain, as it plays an important role in motion planning. Thus, movement sequences are planned and automated in the motor cortex. In the pyramidal tracts, the movement plans of the motor cortex are finally switched over via a complex switching system and from here reach the voluntary muscles. The extensor and flexor muscles in particular are involved in grasping movements. At an age of only two months, infants can stretch their arms out for something. At this stage, however, they are not yet able to grasp, because the extension of the arm is not yet linked to the opening and closing of the hand. After the first attempts to grasp, patterns of innervation become consolidated in the spinal cord. These patterns develop into variable and self-organizing systems of motor control, which from then on are practiced further and become more and more secure. Even before an infant is six months old, he reaches for objects with an open hand, but the grasping movement still takes place more or less chaotically. From then on, higher levels of the central nervous system are continuously involved in the grasping movement. From then on, specialized programs of innervation develop for variable external conditions, which in the further course become more and more stable and can be executed more and more automatically.
Diseases and disorders
Precision movements such as grasping are impaired in various neurodegenerative diseases. One example is Parkinson’s disease. Purposeful and voluntary grasping movements become less and less possible to perform as Parkinson’s progresses. Including the planning and final control of the grasping process, the motocortex of a healthy patient needs about 800 milliseconds for grasping. Even in the early stages of Parkinson’s, the values for affected individuals are significantly higher than these figures.However, cerebral infarction can also make grasping impossible. Cerebral infarction with vascular occlusion in the middle cerebral artery causes most lesions of the motor cortex, which is responsible for fine motor planning and realization. Lesions of the motor cortex can thus make grasping difficult, prevent it, or disrupt automated movement patterns. Paralysis or ataxia are therefore common symptoms of cerebral infarction. In some circumstances, grasping can be retrained after a cerebral infarction. In stroke cases, for example, the tissue around the damaged area can take over the tasks of the defective areas through specific training. Diseases such as multiple sclerosis can also deautomatize or paralyze reaching movements. Not only inflammation in the brain, but also inflammation in the pyramidal tracts can damage the motor system in multiple sclerosis. Inaccurate and powerless grasping movements can be an early sign of current inflammation in the relevant areas. For example, if people drop things out of their hands more often than usual or if objects are regularly missed when grasping, this is sometimes interpreted as a subtle clue to a possible MS diagnosis.