"LES PETITES MAINS" - DEVELOPMENT OF THOSE LITTLE HANDS PART - I : BY DR. VANDANA PATEL (PT)

    

    Upper extremity function is important in successfully moving through the activities of the day. It forms the basis for the fine motor skills important to activities like feeding, dressing and grooming as well as it plays an important role in gross motor skills such as crawling, walking, and the ability to recover balance and protect the body from injury when balance recovery is not possible. Human hand is useful for prehension, communication and non- prehensile functions. Effective use of hand depends on the hand skills, postural mechanisms, visual perception and cognition.

    Development occurs with maturation of different parts of nervous and musculoskeletal systems as well as with experience.

EMBRYOLOGICAL (PRENATAL) DEVELOPMENT OF UPPER-EXTREMITY:

    The upper limb buds appear on day 24 as small bulges on the lateral body wall at about the level of C5 to T1. By the end of the 4^th week, the upper limb buds have grown to form pronounced structures protruding from the body wall. Each limb bud consists of a mesenchymal core of mesoderm covered by an epithelial cap of the ectoderm. Along the distal margin of the limb bud the ectoderm thickens to form an apical ectodermal ridge. This structure maintains outgrowth of the limb bud along the proximal distal axis.

    By 33 days the hand plates are visible at the end of the lengthening upper limb buds. By the end of 6^th week, the segments of the upper limb can be distinguished. Digital rays appear on the hand plates during 6^th week. A process of programmed cell death occurs between the rays to free the fingers. By the end of 8^th week, all the components of upper limbs are distinct.

    The bones, tendons and other connective tissues of the limbs arise from the lateral plate mesoderm, but the limb muscles and endothelial cells arise in the somatic mesoderm and migrate in to limb buds. In general, the muscles that form on the ventral side of the developing long bones become the flexors and pronators of the upper limbs. These muscles are innervated by ventral branches of the ventral primary rami of the spinal nerves. The muscles that form on the dorsal side of the long bones generally become the extensor and supinator muscles of the upper limbs. However, some muscles of the limbs shift their position dramatically during the development.

    The dermatomes of the skin, which represent the tactile system, begin to develop as early as 7 week of gestation. Proprioceptive system develops in utero with the differentiation of the articular skeleton and muscle system, beginning around the 7^th week of gestation. The prenatal period ends with full development of the upper limbs. Incomplete development of the central nervous system and lack of nerve myelination prevent full upper limb control at birth.

ROLE OF REFLEXES:

    The new born infant is dominated by primitive reflexes that provide most of the initial response to stimuli from the external world. Tactile and proprioceptive stimuli elicit the reflexes that most influence early hand function.

    These reflexes are described in table below:

NEURAL CONTROL OF UPPER EXTREMITY:

    In primates, neural pathways controlling movements of the arms are different from those that control movements of fingers and hand. The two systems develop at different times.

    ARM CONTROL, which is mainly coordinated at brainstem level, develops earlier than hand and finger control, which is coordinated at cortical level.

    The capacity to use the hand with skill, in hand– object interactions represents an evolutionary ability characteristic of the behavior of higher primates.

    Three fundamental prerequisites are necessary for this function:

1. Capacity for independent control over the fingers
2.  A sophisticated somatosensory system to guide finger movements
3. Ability to transform sensory information concerning object properties into appropriate hand configurations.

    Each of these prerequisites is served by separate but interconnected areas of the cerebral cortex. This includes the primary motor cortex, primary somatosensory cortex, parietal cortex, and premotor cortex.

    Ability to move the fingers individually is thought to result from direct cortico-spinal connections primarily from neurons in the motor cortex to the alpha motor neuron of hand muscles in the ventral horn in the spinal cord.

    The ventral horn of the spinal cord is divided into two main sections, an inter-neuron zone and the motor neuronal pool or “final common pathway” to the muscle. The motor neurons in the ventral horn are not randomly distributed but are clustered into cell columns, a medial cell column that contains the motor neurons for the trunk, shoulder girdle, and hips, and a lateral cell column that contains motor neurons for the distal extremities. This direct path is fast and thought to be important in moving the hand with speed and skill. These special connections also are thought to be preferentially related to the intrinsic hand muscles. The intrinsic hand muscles provide the ability to handle small objects with precision.

    Primary motor cortex is important structure for the execution of independent finger movements. Damage to motor cortex result in deficit of fine manual coordination.

    The hand is both a motor and sensing organ and there is a tight interplay between these two functions. The primary somatosensory cortex helps to appreciate complexity of information processing within this area, particularly for the hand. Afferent fibres from the dorsal columns project mainly to area 3b for cutaneous input and area 3a for deep, proprioceptive information. Tactile information from fingers is necessary to adjust grip to weight and friction of an object. The transformation of the visual image of an object into an appropriate hand opening and orientation is processed in the posterior parietal lobe.

    As the maturation of pyramidal tract occurs at about 9 to 13 months of age,infants are able to control fractioned finger movements and thus develop more difficult grasping skills such as pincer grasp.

    Sensory inputs from the visual system(eye hand manipulation) go through two parallel pathways involved in goal directed reaching activities: one related to what is being reached for (perception and object recognition) and the other related to where the object is in the space (localisation) and the action system involved in manipulation of object. The perceptual pathway goes from visual cortex to temporal cortex (ventral stream pathway), while the localisation and action pathway goes from visual cortex to the parietal lobe (dorsal stream pathway).

    Higher centres in the cortex take this information and make a plan to generate a movement. This plan is also sent to cerebellum and basal ganglia and they modify it to refine movement. 

This is just the basics stay tuned for part II to learn more about hand deformities and its management...!!👉👉👉👉

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References:

 Henderson A. & Pehoski C. Hand functions in the child: foundations for remediation. 2^nd edition. Elsevier & mosby: 2006; 3-21.

Alexander R., Boehme R. Normal development of functional motor skills; the first year of life. 1^st edition; Therapy skill builders: 1993; 11-154.