II-11
REMARKS ON NEUROCONTROL OF THE HAND
AND SIGNIFICANCE OF AFFERENT INPUT
KOMENTARJI O ŽIVČNI KONTROLI ROKE TER O VLOGI AFERENTNEGA DOTOKA
Nejc Sarabon1, Meta M. Dimitrijevic2, 3, Janez Zidar2, Milan R. Dimitrijevic2, 3
1 Institute of Sport, Faculty of Sport, Gortanova 22, 1000 Ljubljana, Slovenia
2 Institute of Clinical Neurophysiology, Neurological Clinic, Clinical Center, Zaloška 7, 1525 Ljubljana, Slovenia
3 Department of Physical Medicine & Rehabilitation, Baylor College of Medicine, Houston, Texas, USA
Arrived 2004-02-16, accepted 2004-05-10; ZDRAV VESTN 2004; 73: Suppl. II: 11–7
Izvleček – Članek podaja nekatere komentarje našega ra-
zumevanja o kontroli gibanja roke. Začenjamo s podajanjem
trenutnega znanja o motoričnem obnašanju in izvajanju
gibanja. Sledi razlaga kompleksne narave kontrolnega siste-
ma za gibanje, ki združuje periferne senzorične informacije,
obstoječe gibalne programe motoričnega spomina subkorti-
kalnih centrov ter informacije različnih primarnih in sekun-
darnih kortikalnih področij, tako motoričnih kot senzoričnih.
Nadalje opisujemo sekvenčno naravo procesov, kot so odloča-
nje, načrtovanje, oblikovanje odgovora in izvedba, ki se zdru-
žujejo v procesu kontrole izvedbe smiselne motorične naloge.
V poglavju, ki sledi, je poudarjen pomen senzoričnega do-
toka, s poudarkom na proprioceptivnem, pri oblikovanju in
vodenju giba. Prispevek je sklenjen z opisom raziskovalnih
pristopov za proučevanje vloge, ki jo imajo senzorične infor-
macije pri spreminjanju živčnih sistemov. Končno je pred-
stavljena metoda električne stimulacije cele roke, ki se zdi
obetavajoča metoda za raziskovanje senzorično-motoričnih
integracijskih mehanizmov roke.
Abstract – This paper outlines some remarks on our under-
standing of motor control of the hand. It begins with current
knowledge about motor behavior of prehension movements.
It further highlights the distributed nature of the control
system that integrates sensory information from the periphery,
existing motor memory from subcortical centers, and infor-
mation from diverse primary and secondary motor as well as
sensory areas. It further explains the sequential nature of the
processes like decision, planning, computing, and execution
involved in neurocontrol of a purposeful hand motor task. In
the successive part it stresses the importance of sensory
input, in particular proprioceptive, for movement setting and
guidance. It concludes by pointing out research concepts used
to study roles of sensory information for modulating states of
neurobiological systems. Finally, a novel method of whole hand
electrical stimulation, which seems promising as a tool for
studying sensory-motor integration mechanisms in human’s
hand, is explained and recent experimental data are provid-
ed.
Introduction
Aristotle described the human hand as ‘an instrument that re-
presents many instruments’. Whereas other animals have limbs
specifically shaped for climbing, running, flying, swimming or
hanging, humans have different specialization and particularly
this is the case in the human hand. Diversely, the human hand
has developed for a range of activities. It is the perfect all-
purpose tool, the Swiss Army knife of limbs, so adaptable that
it solves problems such as hunting, running, flying, swimming,
and so on, by making machines that do all those things. It needs
of course, to be connected to a brain that can control such a
tool, but the brain would also be far less useful without such a
hand connected to it. Brains and hands co-evolved (1).
On the other hand it is remarkable what a role sitting plays in
manual skill. This relation has been illuminated by Phillp V.
Tobias in his essay titled »Man, The Totteting Biped: the evolu-
tion of his posture, poise and skill« (2). In his opinion, bipeda-
lism was not a prerequisite either to effective tool using or to
rudimentary tool making, while there are also other circum-
stances under which a creature’s hands are freed. In fact, most
implemental activities of man and of great apes are carried
out in sitting position in which our bodies find high level of
stability. The later is of basic importance in the development
of manual skill.
However, a picturesque example of human deprived of loco-
motion but able to sit reveals how hand skill potentials can be
expressed. This is the case in paralyzed people due to neuro-
muscular disorders. Namely, manual skills for controlling com-
puters and variety of devices, not to mention skills of art, con-
tinue to be present for reproduction of brain capabilities. If
we provide support for taking advantages of brain-hand ex-
pression in people with paralyses they become independent.
In short, previously disable can through brain-hand capacity
demonstrate new abilities.
Anyhow, the function of the upper extremity in humans is
unique when compared to the function of the forelimb of
other mammals and even to that of primates. The human
upper extremity is fully independent from locomotion in a
body that’s skeletal, muscular, and nervous system control is
able to generate long standing and bipedal locomotion. The
arm and hand are thus devoted to the exploration of extra-
and intrapersonal worlds. Life is full of well defined hand move-
ments which can be divided into topokinetic movements (shak-
ing hands, reaching a latch or a glass of water) and morpho-
kinetic movements (writing or drawing). Control of the arm,
hand, and fingers is part of expressive communication behavior
and is requirement of acceptance into human society. The
hand is also important in expressing emotions through paint-
ing, music, and drama (3).
ZDRAV VESTN 2004; 73: II-11–7
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Besides having different musculoskeletal design than the
lower extremity, the arm and hand are subject to more com-
plex neural control. Before initiating a purposeful motor act
such as grasping an object, the nervous system needs informa-
tion. Vision describes the shape of the object, its location, and
its distance from the body. Proprioceptive input defines the
condition of the inner world and the position of the limbs and
trunk. The brain searches for stored memories of similar situ-
ations. Once the object is touched, cutaneous proprioceptive
sensation updates the recorded memories of weight, surface,
and shape.
The purpose of this paper is to give an overview about neural
control of the hand and significance of the afferent input. We
will review hand motor control related issues like peripheral
receptors as a base for kinesthesia, role of sensory feed back
for on-line movement control, planning and execution of a
hand motor act potentials to use afferent manipulations to
improve motor control.
So we move our hand
However, through daily activities we are not aware that this
implementation of our upper extremity is a part of complex
underlying neurobiological mechanisms of neurocontrol. The
research of motor control has been developed and today this
field covers a wide variety of approaches used to study human
movements, such as hand motor tasks. It all began with obser-
vations of behavior using static techniques such as photographs
(4), over introduction of high-speed cinematography (5) to
neurophysiological measurements. In Lemon’s opinion the
biggest advance in studying motor control of the hand has
come from realization of the fundamental need for start ob-
serving natural movements. Pioneering work (6) based on
complex motor tasks was amenable to in-depth quantitative
study. In spite of this fact motor system physiologists believed
for many years that only simple fractures of a movement could
be analyzed and understood. Almost all the work carried out
was considering single joint system and how it operates. Never-
theless, this is very abstracted model to observe and is far away
from the real situations of everyday life. From the early seven-
ties on, introductory physiological studies presenting natural
movements emerged. Thus, we were introduced the preci-
sion grip (7), pointing tasks (8), precision lift (9), reach and
grip task (10, 11). Since than, all these have been serving as
tools to study normal as well as changed physiology (12–15).
During the recent years, the repertoire of the listed move-
ments with which a human or an animal explores external and
internal environment was enriched by some additives. Wing
and Flanagan (16–19) have been exploring brisk alternative
arm movements during grasping an object while bimanual
‘draw task’ was observed by Wisendanger (20).
Probably the most fundamental hypothesis was proposed by
Jeanerod, speculating about the existence of two major com-
ponents responsible for the guidance in grasping movements:
(i) the transportation of the hand to the vicinity of the object to
be grasped and (ii) the formation of the particular posture of
the fingers (Figure 1). Testing the degree of independence of
each component of motor control is critical for evaluating the
existence of transport and hand-shaping subunits. Based on
studies that were systematically changing entities of an object
a subject was reaching (21, 22) or position of it in space (21,
23, 24), re-examination of the proposed existence of the two
movement components, transport and grasping, was con-
firmed. To sum up, the argumentations above suggest that the
two components of prehension are controlled by diverse path-
ways. Although the two mechanisms for preshaping and trans-
porting the hand, respectively, lie close to each other in the
posterior parietal cortex, they can be dissociated by lesions
(25).
Where the movement came from?
Thus, when a voluntary movement of the hand is to be brought
forth, numerous parallel as well as sequential integrative sen-
sory-motor processes are taking part. Like our perceptual skills
reflect the ability of sensory systems to detect, analyze, and
differentiate certain physical stimuli, our dexterity indicates
the capabilities of the motor systems to plan, coordinate, and
execute movements. In fact, these two systems are insepa-
rable. They rather operate in a complementary manner,
though both organized within brain and spinal cord. Just as
there are different modalities of sensation, there are three
diverse categories of movement – reflexive, rhythmic, and
voluntary. While human hand’s primary role is to make
explorative and manipulative motor acts, we will explain its
motor control using an example of a voluntary movement.
So, motor systems operate inversely to sensory ones. Motor
processing namely, begins with an internal representation and
cognition of the movement, within brain mechanisms. Instead
of thoroughly reviewing anatomical and morphological fea-
tures of diverse parts of components of central nervous sys-
tem involved in setting and execution of a purposeful volun-
tary hand movement, we will rather consider their functional
roles. Voluntary movement is organized in the cerebral cor-
tex, although different parts of it communicate to other corti-
cal areas as well as to subcortical areas, cerebellum, and spinal
cord (Figure 2).
The primary motor cortex controls simple features of hand
movements and can be selectively activated by repetitive iso-
lated finger tapping (26). Accordingly, primary motor cortex
plays a major role as an executioner of goal directed move-
ments. More complex motor activity is interactively planned
involving other parts of central nervous system. Major projec-
tions to primary cortex come from premotor cortical areas
that communicate directly to the spinal cord as well. Accord-
ing to their pathways this part of cerebral cortex is divided into
lateral ventral and dorsal premotor areas, supplementary
motor area, and cingulated motor areas. Each of them contri-
butes to different aspects of motor planning. The supplemen-
tary and pre-supplementary motor areas play an important
role in learning sequences of discrete movements. Evidences
for this come from EEG studies (27–29) as well as functional
imaging (26). In the latter study it has been shown that it is
possible to selectively activate medial premotor area when
only mental rehearsing of movements in absence of their
execution. Additionally, researchers demonstrated increased
corticospinal excitability while mental imaging of motor
activity without its execution (30, 31). The lateral premotor
areas contribute to the selection of action and to sensory-
Figure 1. Reaching to grasp representing how transport and
manipulation components are affected. Note the anticipa-
tory opening of the hand that is appropriate for an object to
be grasped. At the same time transport subcomponent remains
unchanged.
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motor transformations. These transformations required for
reaching and grasping involve two different pathways from
the primary visual cortex to the premotor areas. Interestingly,
all of the mentioned cortical areas receive unique and com-
mon subcortical inputs. For instance, motor cortex receives
inputs from the cerebellum via the thalamus, as well as direct
ascendant sensory inputs from periphery (32, 33). Similarly,
there are not only direct projections to motoneuron pools
which derive from primary motor cortex, just the opposite;
there is evidence for such projections from premotor areas.
When we reach out to grasp a small object, the first goal is to
adequately position the fingertips onto its surfaces in order
stable grasp is established. The transport and shaping of the
hand reflect a precise coordination through which spatio-
temporal parameters for reach and grasp are defined (34).
The demand for precise control gradually increases while
approaching contact (35) thus the intertrial variability of fin-
gertip position is attenuated.
After the first contact with an object, necessary finger tip force
should be produced. This action evolves in a series of phases
associated with responses in tactile afferents from the digits
(36, 37). The initial contact marks the beginning of the pre-
load phase. Load phase, transitional phase, and static phase
are afterwards carried out in succession. A specific role of
hand sensory organs is to link the various phases of the lifting
task by informing the central nervous system about the action
flow at the periphery. Finger tip forces are parameterized by
implicit memory information of the friction and weight,
acquired during previous manipulations of an object (38). Thus
tactile input provides information about those two parame-
ters of friction and weight.
The control of grasping and manipulation relies on distributed
processes in the central nervous system, engaging most areas
to be involved in sensory-motor control (39). During the exe-
cution of the task, primary motor cortex is likely the main
actor, which is probably most obvious when concerning skilled
hand tasks (40). Different subcomponents of the supraspinal
neural control of the hand movement can be studied by com-
paring neural states reached by motor areas. Non-invasive trans-
cranial magnetic stimulation (TMS) enabled researchers to
probe cortico-motoneuronal excitability. Additionally, it has
been reported that the amplitude of motor evoked potentials
evoked by TMS may to some extent be dependent on the
level of cortical activity (41–44).
In their communication Lemon, Johansson and Westling (45)
report about striking changes in the amplitude of EMG re-
sponses to TMS delivered to the hand area of the motor cortex
during different phases of a natural task of reaching, grasping
and lifting. This modulation may well reflect brisk changes in
corticospinal excitability.
Lemon et als’ (45) results suggest that the cortical representa-
tions of extrinsic hand muscles, which act to orientate the hand
and fingertips, were subjected to a strong supraspinal excit-
atory drive throughout the reach. This drive was also observed
for brachioradialis and anterior deltoid which contribute to
transport of the hand. In contrast, the intrinsic hand muscles
appear to receive their strongest cortical drive at the digits
closed around the object and just after the subject first touched
the object at the onset of manipulation.
To infer, tactile inputs are known to be essential for appro-
priate coordination of this task (9, 36, 46) and therefore pos-
sible that this strong effect partly results from a central inter-
action between these inputs and TMS. Indeed, Johansson et al.
(12) have provided evidence that such interactions may occur
while subjects respond to small, unpredictably occurring step
load increases imposed to an object restrained between thumb
and index finger. Accordingly, tactile inputs are known to exert
excitatory effects on a large proportion (around 58 percent)
of monkey motor cortex neurons related to hand movement
(33). To sum up, using natural movements it is possible to
demonstrate changes in corticospinal excitability, while the
task has been carried out. These changes are likely, at least in
part, to be modulated by ascending afferent flow while
cutaneous inputs are shown to be strongly implicated in the
initiation and scaling of the grip force. For example, in early
catch-up grip response (47) cutaneous information was those
to initiate the long-latency ‘catch-up’ grip response (48).
If we conclude, there are sequences of neural processing car-
ried out that finally generate motor information necessary for
the execution of a purposeful volitional movement of the hand
(Figure 2). Each cortical area receives unique inputs. Visual
information from the environment is specifically detected and
conveyed to the primary visual cortex. In order this incoming
information to be interpreted; it goes further to the frontal
cortex. It is a site where planning of the movement takes place
and therefore it is an integrative area that receives wide range
of inputs. From basal ganglia come ‘history information’ on
what the brain already know about movements like the one to
be planned. On the other hand, somatosensory area passes
information on proprioception and body scheme. Using all
these data a plan for intended movement is shaped and con-
veyed through premotor areas to the primary motor cortex.
SARABON N, DIMITRIJEVIC MM, ZIDAR J, DIMITRIJEVIC MR. REMARKS ON NEUROCONTROL OF THE HAND AND SIGNIFICANCE OF AFFERENT INPUT
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These project downstream to the proper segmental interneu-
rons and motoneurons of the spinal neuron pool. As a result of
spatially and temporally coordinated activation of these final
neuronal stations coordinated hand movement is born. How-
ever, reafference copy of motor cortical output back to pari-
etal cortex and constant afferent refreshment from periphery
is crucial for on-line governing of such skilful movement.
Nothing happens without knowing
about peripheral state
During the early nineteenth century, philosophical approach
named positivism was proposed by Comte who was influenced
by British empiricists. They suggested all knowledge be ob-
tained through sensory experience. That is when psychology
was seceded from philosophy, the first being concerned with
studies of sensory signal and perception. Although objects we
reach and grasp are perceived as phenomenal entities, senso-
ry systems are known to detect features not objects (49). The
sensory systems of the hand encode four elementary attributes
of stimuli – modality, location, intensity and timing. They are all
manifested in an integrated sensation. Sensory modality is de-
termined by the form of stimulus energy, so that specific hand
receptors transduce specific types of energy into electrical
signals. The last are conveyed along spinocerebral pathways
crossing different relay nuclei. In 1931 Edgar Adrian and Keith
Lucas demonstrated how a sensory nerve, even though it trans-
mits nerve impulses of fixed strength, can still convey a com-
plex message (50). Today we know that complementary sen-
sory systems have a common plan they follow in order to make
it brain understand and respond what periphery sees.
Information transmitted to the brain from mechanoreceptors
of the hand enables us to feel the shape and texture of objects
as well as permits us to execute finely coordinated motor tasks
such as threading the sewing needle. According to their loca-
tion mechanoreceptors can be divided into cutaneous and
musculotendinous. The later involves muscle spindle and
Golgi tendon organ. Physiologically, muscle spindle is a recep-
tor for muscle length and is unique among sensory organs
with its sensitivity being descendently controlled from supra-
spinal centers. When its function was studied using different
methodological approaches, it was recognized that muscle
spindle with associated neural connections respond both, to
phasic changes in muscle length (tendon-tap reflex) as to to-
nic input of vibrations (tonic vibration reflex). Anyhow, in both
cases afferent input influences agonistic motoneuron pool by
means of facilitation. Zoological studies however emphasize
muscle spindle to be paramount designed for detecting vib-
rations. On the other hand, Golgi tendon organ is a receptor
for muscle tension. While it responds to active production of
muscle force (voluntary contraction) much more than to
passive stretch it is sometimes also considered as a sensor for
level of muscle contraction. Under relaxed resting conditions,
Golgi tendon organ activation inhibits agonistic motoneuron
pool. Touch is mediated by mechanoreceptors in the skin,
whereby these differ in morphology and skin location (Figure
3). These receptors are purposefully distributed over the area
of a ‘skin glove’ and hence different sensitivity for different
stimulus modalities at various sites of the hand surface (51).
Experiments using anaesthesia (52) documented crucial role
of cutaneous sensory input for perception of hand body
schema and neurocontrol of the hand movements, oppositely
such sensory restriction at knee joint caused actually no
deficit. On the contrary, joint and muscle mechanoreceptors
of the hand seem to play a secondary role.
To reconsider, peripheral sensors are designed to transducer
physical energy to nerve impulses, spikes. According to the
all-or-none rule, amplitude of an action potential in a single
afferent fibre is constant as it once depolarizes. Thus, ‘informa-
tion code’ is defined by population of sensory fibres activated
and frequency of nerve impulses on one hand (stimulus
strength encoded), and by the duration of the input (tonic vs.
phasic events). Once generated, this afferent code is conveyed
centrally to spinal cord and upstream to supraspinal centers,
crossing relay nuclei. Anyhow, interpretation and utilization of
this incoming information is dependent on central states at
different levels.
The control of upper extremity movement and posture is be-
lieved to involve particular neural signals, some of which have
access to consciousness. Such signals, encompassed within the
terms kinaesthesia and proprioception were originally pro-
posed by Bastian and Sherington and have become virtually
synonymous that covered a loose meaning of almost anything
concerned with the control of movement. These signals arise
from activity in mechanoreceptors, from centrally generated
motor commands, and from interactions between these affe-
rent and efferent signals (53). Kinesthesia and propriocep-
tion today denote a group of sensations. First is the traditional
sensation of position and movement of the limbs and trunk.
Second, there are sensations related to muscle force, includ-
ing effort, tension, heaviness, and stiffness. Third, sensations
exist for timing of muscle contractions. Fourth, there is a sen-
sation of body posture and size as a part of a ‘schema’ enclos-
ing more than one joint. The term ‘input’ refers to the dis-
charge of afferents from muscle, joint, and cutaneous recep-
Receptor
Inner- Adap-
vation tation
Modality
Ruffini corpuscle SA II slow moderate, static touch
Pacinian corpuscle PC / FA II rapid light, dynamic touch (vibration)
Hair follicles Hair rapid light, dynamic touch
Meissner corpuscle RA / FA I rapid light, dynamic touch (flutter)
Merkel cells SA I slow light, static touch
Figure 3. The location and morphology of receptors
in hairy and glabrous skin. Note the exact location of
different receptors and free endings relative to the
distance from the skin surface. Deep receptors have
bigger receptive fields as compared to the superficials.
Only skin receptors that are most important for the
control of hand movements are provided. For more
detailed data on the above classification see (54, 55).
d09.p65 5.6.2004, 3:1614
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tors. ‘Kinaesthetic signals’ include the contribution from mo-
tor command signals as well as the mentioned inputs. Motor
commands encompass the general term corollary discharge
as well as the specific form called an efference copy.
There has long been controversy about how to reference kines-
thetic signals. It is now clear that limb muscle receptors influence
sensations of limb movement just as cutaneous inputs influence
sensations of objects on the skin. Appropriate referencing of
motor command signals must also occur. As we can see, the full
array of specialized muscle, joint, and cutaneous afferents are
potential kinesthetic inputs. On the other hand, an afferent class
may have more than one kinesthetic role. It is not purposeful to
extensively review types of receptors and modalities of infor-
mation they transfer, while there are clusters of such literature
available elsewhere (54, 55).
If we depict a simple scheme for kinesthetic signals, we can
say that afferent and efferent kinesthetic signals have no sin-
gle role, they contribute to kinesthesia and motor learning and
they also reflexly change movements. Their contribution to
motor control proceeds with or without conscious awareness.
In the short and long term, kinesthetic signals mould an adap-
tive model of the musculoskeletal system (53). Inclusion of
sensations of muscle timing and the body schema in the defini-
tion of kinesthesia recognizes their essential role in the con-
trol of movement.
Hand motor control can be affected
using afferent manipulations
As we could see sensory mechanisms through which central
system is informed about relative positions of body’s parts, the
rates of their movements, the forces they exert, and the rela-
tive timings of their various contractions, are essential for or-
ganization of neurocontrol. Therefore, having controlled arti-
ficial access to this input could enable us to control setting
mechanisms of the control. Projections of individual types of
proprioceptors to central nervous system are inseparable
when natural movement is carried out. In order to establish
these projections one must be able to excite receptor belong-
ing to that type selectively. This can be done using appropri-
ate mechanical stimulation (56) of the muscle or joint or by
graded electrical stimulation of their sensory nerves (57, 58).
However, with either technique it is difficult to excite recep-
tors of one type and not other.
To date, different neurophysiological and neuroimaging tech-
niques were used to test effects of electrically evoked by affe-
rent input on activity of various neural structures and their excit-
ability. Excitability of the corticomotoneuronal system is modu-
lated by peripheral afferent stimulation tested by TMS and mea-
sured by elicited motor evoked potentials (MEPs). In healthy and
adult subjects, both facilitation (59, 60) and inhibition (61, 62) of
MEPs have been shown to take place 100 ms following a single
electrical stimulus above the threshold for sensory perception
given at periphery. On the other hand, decreased motor cortex
excitability has been observed at intervals longer than 200 ms
(63, 64). Reports are in agreement that in the single condition-
ing ES paradigm, applied below threshold for perception, there
is no modification of corticospinal excitability tested by TMS.
There is actually no report on the effect of prolonged electrical
stimulation with a train of ES above or below sensory perception
on the corticospinal excitability.
Our previous studies (65–67) showed that increased muscle
tone and impaired volitional movements of the spastic hand in
the spinal cord, head injury and stroke subjects can be amelio-
rated if the whole hand is stimulated by sustained electrical
stimulation (train of 50 Hz, for 30 minutes) below the level for
sensory perception. The hand is stimulated by a glove made of
conductive wires that act as a common anode to a pair of car-
bon rubber cathodes placed over the dorsal and palmar sur-
faces of the forearm (Figure 4). This finding that sustained
stimulation of the spastic and paretic hand below perception
of tingling sensation within whole hand can induce functional
modifications, rise two questions: (i) which structures of the
hand are stimulated? Do these externally induced volleys, when
the whole hand is target of stimulation, modify motor cortex
excitability?
In our recent studies (68, 69) we demonstrated how temporal
summation, 50 Hz train for 30 and 120 minutes respectively,
of afferent input by stimulus below the level of sensory per-
ception can induce modification of motor cortex excitability
(Figure 4). Furthermore, pure spatial summation, whole hand
instead of nerve trunk stimulation, of a single conditioning
peripheral electrical stimulus however did not affect state of
corticospinal tract. It seems so far most possible slowly adapt-
ing Ruffini endings are activated by the low intensity mesh
glove stimulation, though to induce an effect they recruit need
to be fed as a train. Anyhow, until now we can not explain
unambiguously what population of peripheral sensory system
is recruited by subsensory whole hand stimulation. Namely,
Figure 4. 120-minute subthreshold mesh
glove stimulation was evaluated by pre-post
measurements using TMS as a diagnostic
tool. Standard whole hand mesh glove sti-
mulation was employed by means of com-
mon distal anode and cathodes placed on
the palmar and dorsal area of the forearm.
Botzmann equation was employed to ob-
serve changes in corticospinal excitability.
Before and after the whole hand condition-
ing, muscle responses (first dorsal interos-
seus) to TMS of different intensities were
recorded. On the graph above we can see
average data for 10 healthy subjects before
(solid line for the calculated function and
red circles for data at single relative inten-
sity) and after (dashed line and blue trian-
gles). All the key parameters of the mathe-
matical method showed statistically signi-
ficant modulations of corticospinal excit-
ability after 2-hour whole hand subsensory
stimulation; thus left-up shift of the curve
(68, 69).
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two crucial physiological characteristics are interdependently
responsible for what is going to be depolarized first. On one
hand, diameter of an afferent fiber that innervates certain sen-
sor – large fibers are depolarized at lower current intensity.
On the other hand, spatial relation between stimulation site
and underlying neural structure is important. Given electrical
current being reduced by square of distance, deep neural struc-
tures are going to be much less prone to respond to electrical
stimulation applied over skin than those more superficially.
Studies in progress will probably clarify, using methods of
mathematical modeling, what the precise physical stimulation
characteristics of the mesh glove stimulation are and which
sites of stimulation are the most likely to be activated. It seems
promising for our future research to use the mesh glove as a
unique tool to study sensory-motor integration mechanisms of
the hand. We namely believe that more functional controlled
whole hand afferent input is diversely ‘understood’ by the brain
from that of the stimulation of the nerve trunk.
Moreover, this demonstration how hand receptors’ afferents can
be stimulated using electrical current of the intensity below the
level for sensation, opens a new avenue in the research of motor
control. Possibility to modify corticomotoneuronal excitability
in such a way seems promising also for other research work such
as learning. We are up to ask ourselves some new questions. Can
we optimize the execution of a certain motor skill? Can we con-
tribute to faster and more efficient acquiring of new motor skills?
At present this is an alternative research direction in motor con-
trol which might be leading us to new training procedures use-
ful also for people practicing sports.
Moreover, demonstration of selective external control of pro-
prioceptive and exteroceptive afferent input by the whole
hand mesh-glove electrical stimulation provide experimental
background for neurophysiological studies of preparatory brain
motor control functions. Activation of different regions of the
brain activity, after movement was planned by frontal lobe,
there is setting phase of brain motor control, consist of distri-
bution of activity within different parts of the brain in order to
set components of complex ‘code’ for execution of the move-
ment. Descending volleys via multi-parallel direct and indirect
long brain-spinal cord descending pathways will reach pre-
motor spinal cord network and common final pathway and
generate desired skillful or automatic movement. This new
possibility to modify proprioceptive skin input at the level
below sensory perception by stimulation of the part of the
body instead of the nerve trunk it opened new avenue in ex-
ternal modification of the preparatory brain motor functions.
Therefore, above described mesh-glove studies in addition to
beneficial application to the people with impaired movements
due to neurological conditions can become a method for modi-
fication of preparatory brain motor control functions in
people working toward or practicing improvements of motor
performances in the variety of sports.
A poet Sir Philip Sidney once wrote a poem about a group of
pre-lapsarian animals who, despite living in a perfect harmony,
nevertheless bagged Jove for a king to rule over them (70).
Each animal contributed its best part to the new ruler, and so
the lion gave its heart, the elephant its memory, the parrot its
tongue, the cow her eyes, the fox its craftiness, the eagle its
vision, and finally, the ape gave the ‘instrument of instruments,
the hand’. Since than human’s hand driven by the brain en-
ables us to develop ourselves by means of exploration of exist-
ing environment as well as creating a new one.
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