BBTA Tutor: Clare Fraser
The intricate manipulation of objects by the human hand represents a testament to the sophisticated physiological control mechanisms within the human nervous system. The seamless interplay of tactile, proprioceptive, and perceptual information, augmented by processes such as stereognosis, two-point discrimination, localisation of touch, and lateral inhibition, forms the foundation of hand dexterity and functional manipulation.
At the heart of managing an object in our hand, lies the processing of ascending sensory information, a multifaceted journey that contributes significantly to the control of object manipulation and function.
Tactile Information:
The journey of ascending sensory information through our neural pathways begins with the activation of sensory receptors (mechanoreceptors such as Merkel cells, Meisner's corpuscles, Ruffini endings etc.) embedded in the skin, which are exquisitely sensitive to changes in pressure, texture, and vibration.
As the hand encounters an object these receptors initiate the relay of sensory data through ascending pathways, and ascending through the spinal cord they synapse in the medulla before reaching the thalamus. Here, the sensory information undergoes further modulation and refinement before being relayed to the primary somatosensory cortex in the parietal lobe.
Therefore the brain receives critical information about the object's surface characteristics and constructs a detailed representation of tactile features (and proprioceptive features), setting the stage for motor planning and execution.
The dorsal column-medial lemniscal pathway emerges as a central player here, ascending and transmitting discriminative touch and proprioceptive information to the somatosensory cortex of the brain.
Proprioception information:
Tactile information isn’t the only detail we need about the object though. The proprioceptive system, deeply embedded in muscles, tendons, and joints, ensures an inherent awareness of limb position and movement. Golgi tendon organs and muscle spindles convey real-time feedback about muscle tension and joint angles to the brain.
This proprioceptive information plays a pivotal role in coordinating the intricate muscle contractions required for precise object manipulation. Proprioceptive information is ascended up through the medial lemniscal pathway (as previously described) as well as the essential spinocerebellar pathway, taking proprioceptive sensory information directly to the cerebellum for processing.
Perceptual Information:
Perceptual information processing is a complex neural task involving the integration of tactile and proprioceptive inputs. The parietal lobe, particularly the primary somatosensory cortex, undertakes the intricate task of combining these sensory modalities to construct a coherent representation of the object in question. This perceptual mapping from the raw sensory data is foundational for subsequent motor planning and execution.
Stereognosis:
Stereognosis, the ability to recognise and comprehend the nature of an object through touch alone, emerges as a critical aspect of skilled manipulation. The posterior parietal cortex is instrumental in stereognostic processing, integrating tactile and proprioceptive inputs to generate a cognitive representation of the object's size, shape, and texture. This cognitive map facilitates adaptive motor responses, tailored to the object's specific properties. Without this ability we are unable to fully understand the nature of the object that our hand is engaged with, what affordance the object has and how we can utilise it to function within our body schematic peri-personal space.
Localisation of Touch:
The localisation of touch, governed by the somatosensory cortex, enables pinpoint accuracy in identifying the location of tactile stimuli on the skin. This intricate mapping facilitates the discrimination between different points of contact, allowing for nuanced adjustments during object manipulation. The precise localisation of touch is indispensable for tasks demanding high precision, such as threading a needle or manipulating delicate instruments.
Two-Point Discrimination:
Two-point discrimination, a measure of spatial acuity, is finely tuned by the somatosensory system. Merkel cells and Meisner's corpuscles, densely packed in areas with heightened sensitivity, enable precise discrimination between two distinct stimuli. This heightened spatial acuity is paramount for tasks requiring subtle adjustments during manipulation, such as writing with a pen or handling small objects.
Lateral Inhibition:
Lateral inhibition, a neurophysiological phenomenon rooted in inhibitory interneurons, refines the perception of tactile stimuli. In the context of object manipulation, lateral inhibition sharpens the contrast between adjacent tactile inputs, enhancing the discrimination of fine details. This process fine-tunes the sensory information relayed to the brain, optimising the perception of object features critical for dexterous manipulation.
On our Advanced Bobath Course at Walkergate Park Rehab Unit in Newcastle we brought these marvellous physiological control mechanisms, governing object manipulation in the human hand, into clinical practice with our neurological patients.
Bringing the theory into our therapy practice is essential for our patients recovery of hand control, which is such a pressing goal for all of our patients. We were able to work on components of sensory and movement integration with objects and task performance, while linking this to the postural control required to stabilise the trunk, and arm, for the manipulative task.
Maximising the sensory motor linkage within the hand through our therapy sessions brought exciting results for the patients on the course, feeling that their hands were stimulated and ‘more part’ of them and more able to explore and interact with the world around them.
Remembering that tactile, proprioceptive, and perceptual information converge to form a comprehensive representation of the object, facilitating the seamless execution of dexterous tasks, is a fundamental starting point when working with our patients hand recovery. Working to rehabilitate stereognosis, localisation of touch, two-point discrimination, and lateral inhibition to refine this representation, takes our patients closer to the remarkable precision with which the human hand navigates and interacts with its surrounding environment.
If you would like to deepen your understanding of the relevant neurophysiology underpinning and linking to your clinical practice, why not join a BBTA Basic or Advanced Bobath Course this year? Look at our courses on www.bbta.org.uk or get in touch on: info@bbta.org.uk. We would love to welcome you to the exciting world of practically applying neurophysiology into your rehabilitation practice!