The Neurophysiological Systems Involved in Shaping Our Grasp, Even Before Reaching Begins.
BBTA Tutors: Clare Fraser & Debbie Strang
The art of reaching and grasping objects is an extraordinary display of the human brain's neurophysiological systems working together to achieve an essential functional task. The moment we set our sights on an object, from the pre-shaping of our hand for the appropriate grasp, to the initiation of reach, a symphony of descending systems come together seamlessly.
This blog, from our ‘Unravelling’ series, based on the Advanced Bobath Course in Newcastle this year, will delve into the fascinating neuroscience behind this process, with a focus on object recognition and the goal of grasping hold of that object.
At the core of initiating a reach are the interplay between visual object recognition and the intended goal. Our visual system, particularly the dorsal and ventral streams, helps us identify the object and its key features (see blog 5). Simultaneously, the prefrontal cortex processes the goal of the task, such as picking up the water bottle to take a drink. This integration of object recognition and the task required to achieve the goal, lays the foundation for the subsequent neural events.
Key neuroscience authors, such as Jeannerod, Grafton, Alessandro Farnè and Christian T. Santini, have supported the idea that the pre-shaping of the hand for grasp begins even before the initiation of the reach pattern.
Initially the motor plan is formulated in the pre-frontal cortex, and the posterior parietal cortex, which is often described as a ‘sensorimotor interface’. The posterior parietal cortex has two dominant functional roles: to merge information from multiple senses such as vision, auditory, and touch, and to plan and execute actions based on transformations of this sensory input.
Then, descending systems, including the corticospinal and corticofugal tracts, play a vital role in the ‘grasp shaping’ process. Signals are sent, via the corticospinal tract, to the hand's intrinsic muscles, to pre-shape it for the specific grasp needed to interact with the object, this is based on the initial motor plan devised for the proposed task.
Interestingly, the initiation of the reach occurs distally at the hand before the arm itself moves. This phenomenon is known as the "hand preshaping" hypothesis. The motor commands for hand pre-shaping are generated before the arm starts to move, ensuring that the hand adopts the appropriate configuration to grasp the object efficiently. And indeed, knowledge of the object, and the proposed task, will create the stimulus needed to shape the hand in readiness for the object and stimulate the excitation of the reach pattern.
As the reach is initiated, the corticospinal tract (primarily) conveys motor signals from the motor cortex to the muscles of the arm and hand. This orchestration of motor commands, (together with other descending tracts), ensures the hand aligns perfectly with the object's size, shape, and positional orientation, enabling a precise and stable grasp.
In conclusion, the neurophysiological systems involved in the initiation of reaching, and the pre-shaping of the hand for grasp, are a testament to the intricacy of our brain's skill at generating motor control. Through object recognition and goal planning, coupled with the interplay of descending systems and tracts, we achieve remarkable dexterity and precision.
Understanding these neural mechanisms not only sheds light on the wonders of human movement skill, but also opens avenues for advancing neurorehabilitation with our neurological patients. The importance of pre-shaping the hand for grasp efficiency, based on the objects properties, and its position in relation to the person, has profound impact on the way we help our patients to practice a task in our therapy sessions.
As we continue to unveil the physiology of the brain, our appreciation for the remarkable coordination of precision in reaching and grasping grows even more profound.
Do you want to understand more about how you can help your patients develop their ability to reach and grasp, within a functional task? Do you feel confident facilitating your patient’s task practice in treatment sessions? Why not join us on a Bobath Course to update and up-skill yourself; contact us on www.bbta.org.uk.