Arthur Sheridan Lea Sir Michael Foster.

A text book of physiology online

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ment of movement is not the only effect ; a loss or impairment
of sensation is also produced in the part of the body, the move-
ment of which is affected. When, for instance, in the monkey
the area for the arm is removed, not only is the arm, for the time
l»eing, paralyzed, but also the animal does not shew signs of sen-
sation, or shews only feeble signs of sensation when the skin of
the arm is pricked or otherwise stimulated. And so with other
areas. In all cases the loss of movement is accompanied by a
corresponding loss of sensations, of tactile sensations, of sensa-
tions of heat and cold, and even of pain. And the loss or impair-
ment of sensation runs more or less parallel in point of time to
the loss or impairment of movement, and shews the same ten-
dency to be in part lasting. Moreover in the cases mentioned
above where it has been possible to stimulate the cortex of the
human brain, and where this has been done while the subject
was conscious, the production of sensations, often described as
tingling, in the part of the body corresponding to the particular
area, has been at least as striking a result as the production of
moTement in that part. And this is in harmony vnih the fact
that in epileptic attacks which, as we said, illustrate the action
ff cortical areas, the movements which are the objective factoi*s
of an attack are preceded by peculiar sensations, the so-called
'aura,' and these equally with the movement have definite
reUtiona to the area of the cortex, disease of which causes the
attack. In fact there is increasing evidence that the region of
the cortex which we have called the motor area, is connected
not only with the movements carried out by, but also with the



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772 VOLUNTARY MOVEMENTS. [Book in.

sensations derived from the several parts of the body ; we seem
justified in speaking of a topographical distribution of cortical
" sensory " areas, if we may so call them, following very closely
that of the motor areas ; and we know that by the numerous
fibres passing from the cortex to the optic thalamus, if not in
other ways, an anatomical path appears to be afforded for sen-
sory impulses. But to this point we shall return later on.

Meanwhile we may conclude that the loss of movement which
follows the removal of a motor area is not due merely to the
loss of motor elements, it may be as much due to the loss of
sensory elements. Indeed it has been maintained by some that
the loss or impairment of movement in question is not a motor
business at all, but is simply due to a loss of the muscular sense.
We have seen, however, reasons for thinking that the pyramidal
tract is certainly an efferent tract, and injury to it in its begin-
ning in the cortex must lead to failure of efferent impulses.
Moreover, though removal of the cortex does appear to interfere
with muscular sense, it also, and even more clearly, interferes
with cutaneous and other sensations. The conclusion which
we ought to draw from the above facts is perhaps rather this,
that the relations of the cerebral cortex are manifold, and that
the carrying out even of a simple voluntary movement is a very
complicated matter ; even if we assume that the cell in the cor-
tex giving origin to a fibre of the pyramidal tract is in nature a
motor cell, we must also recognize that its work is determined
by ties which bind it to other elements of the cortex and through
them to other parts of the nervous system and indeed of ^e
body. The connections of a sensory nature between a motor
area and the part to whose movements it is related is strikingly
shewn by results which mav make their appearance when stimu-
lation of the cortex is earned on while the animal (dog) is in a
particular stage of the influence of morphia. If a subminimal
stimulus be found, that is a current of such intensity that applied
to a motor area it will produce no movement, but if increased
ever so slightly will give a feeble contraction of the appropriate
muscles, it may be observed th^t a slight stimulus, such as gently
stroking the skin over the muscles in question, will render the
previous subminimal stimulus effective and so call forth a move-
ment. Thus if the area experimented on be that connected with
the lifting of the f orepaw, and the subminimal stimulus be applied
to the area at intervals, after several applications followed by no
movements, a gentle stroke or two over the skin of the paw will
lead to the paw being lifted the next time the stimulus is appUed
to the area. A similar result, but less sure and striking, may fol-
low upon the stimulation of parts of the body other than the part
corresponding to the area stimulated. Then again it has been
observed that in certain other stages of the influence of morphia,
the cortex and the rest of the nervous system are in such a con-



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Chap. II.] THE BRAIK 773

ditioa that the application of even a momentary stimulus to an
area leads not to a simple movement but to a long-continued
tonic contraction of the appropriate muscles. Under these cir-
cumstances, a gentle stimulus, such as stroking the skin, or blow-
ing on the face, applied immediately after the application of the
el^tric stLmulus to the area, suddenly cuts short the contraction,
and bringfs the muscles at once to rest and normal flaccidity.

§ 490. The carrying out of a voluntary movement is in fact
a very complex proceeding, and the motor cortex with the pyra-
midal tract is only one part of the whole mechanism. This
complexity is illustrated by the fact that after removing of a
motor area not only purely voluntary but also reflex and other
movements are for a while abolished or impaired; and even
making every allowance for the effects of * shock ' (§ 467) we
cannot account for the latter to the exclusion of the former, by
appealing to such effects. It is further shewn by the fact that
in the case of most voluntaiy movements at least, after removal
of an area recovery is after a while complete, though there is no
regeneration either of the area or the strand of the pyramidal
tract belonging to it; the will finds some other way to the
muscles and to mechanism coordinating the movements of those
muscles. By the following reflection the complexity of the
matter is' also shewn in a different direction. When a gymnast
executes a skilled voluntary movement in which all his four
limbs and other parts as well perhaps of his body are involved,
it is probably the case that changes of the nature of efferent
impulses sweep down his pyramidal tract, and that these im-
pulses, starting in a definite order from his cortex, that is to say
having undergone a certain amount of initial coordination at
their very origin, meet with further coordination in the spinal
grey matter, which serves as a set of nuclei of origin for the
motor nerves concerned in the movement, before they issue as
ordinary motor impulses along the anterior roots. But this is
not all. Should the gymnast's semicircular canals happen to be
injured and his cerebellum thereby be troubled, or mischief fall
on some other part of the brain which like this has no direct
connection with either the pyramidal tract or the motor coiiiex,
the movement fails through lack of coordination, though both
the cortex, the pyramidal tract, and the spinal motor mechanisms
remain as they were before.

I..astly we may note that in the above discussion we have
used the word * will ' in a general sense only. A man may be
brought into a condition, for instance in certain hypnotic phases,
in which he can cany out all the various skilled movements which
he has inherited or which he has learnt ; and yet, according to
some definitions of the word * will,' those movements could not
be said to be initiated by his will. It can hardly be doubted
that in such cases the motor cortex and pyramidal tract play



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774 VOLITIONAL IMPULSES IN THE CORD. [Book ra.

their usual part. But we may pass from such cases as these
through others, until we come to cases where a skilled move-
ment which hAs been learnt and practised by the working of an
intelligent will, may continue to be carried out under circum-
stances which seem to preclude the intervention of any conscious
will at all ; and the transition from one case to another is so
gradual, that it is impossible to suppose that there has been any
shifting of the machinery emploj^ed for carrying out the move-
ment. So that a volitional origin is not an essential feature of
these so-called voluntary movements, and the machinery of the
motor cortex and pyramidal tract is available for other things
than pure volitional impulses.

§ 491. The preceding discussion will enable us to be very
brief concerning a question which has from time to time been
much discussed, and which has acquired perhaps factitious im-
portance, viz. the question as to how volitional impulses leading
to voluntaiy movements travel along the spinal cord. The con-
clusion at which we have arrived, namely, that in the noniial
carrying out of voluntary movements the chief part is played by
efferent impulses passing along the pyramidal tract, carries with
it the answer that volitional impulses travel in the spinal cord
along the pyramidal tract.

In the dog, in which the whole pyramidal tract crosses at the
decussation of the pyramids, we should expect to find that a
break in the pymmidal tract of one side of the cord at any |)oint
along its length caused loss of voluntary movement on the same
side below the level of the break. And experiments as far as
they go support this view. No one it is true has so far succeeded
in dividing or otherwise causing to break in the pyramidal tract
alone, leaving the rest of the cord intact ; and indeed, even if
an injury were limited to the area marked out as the pyramidal
tract, fibres other than pyramidal fibres would be injured at the
same time, since the tract is never a ' pure ' one. But it has
been found that a section of a lateral half of the cord, a lateral
hemisection, or a section limited to the lateral colunm of one
side has for one of its principal effects loss of voluntary move-
ment on the same side in the parts supplied by motor nerves
leaving the cord below the level of the section. We say 'one
of its principal effects ' because, besides the concomitant inter-
ference with sensations concerning which we shall speak pres-
ently, the loss of voluntary movement is not absolutely confined
to the same side ; there is some loss of power on the crossed
side, at least in a large number of cases. vVe must not lay stress
on this crossed paralysis because it is possibly one of the effects
of the mere operation, not a pure 'deficiency' phenomenon, and
indeed appears soon to pass away. But taking into considera-
tion what Avas said above concerning the effects of removing
cortical areas, it is important to note that in the experience of



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Chap. II.] THE BKAIN. 776

many experimenters the loss of voluntary power on the operated
side diminishes after a while, and that the animal if kept alive
and in good health long enough appears to regain almost full
voluntary power over the affected parts. In such cases, as in
other operations on the central nervous system, there is no re-
generation of nervous tissue ; the two surfaces of the section
unite by connective not nervous tissue, and the tracts which as
the result of the section degenerate downwards or upwards are
I)ermanently lost. Hence even if we admit that in the intact
animal a voluntary movement is chiefly carried out by means of
efferent impulses passing along the pyramidal tract right down
to the motor mechanisms of the cord immediately connected
with the motor nerves, we must also admit that the 'will' under
changed circumstances can find other channels for gaining access
to the same mechanisms.

It has been further observed that if in the dog a hemisection
be made at one level, for instance in the lower thoracic region of
the cord, and then, after waiting until the voluntary power over
the hind limb of that side has returned, a second hemisection,
this time on the other side, be made at a higher level, this second
operation is followed by results similar to those of the first; there
is loss of voluntary power on the side operated on, with some
lass of power on the crossed side, and as in the first case this
loss of power not only on the crossed but also on the same side
may eventually disappear. This shews among other things that
the recovery after the first operation was not due to the remain-
ing pyramidal tract doing the work of both. Further, the hemi-
section may be repeated a thij'd time, the third hemisection being
on the same side as the first, and in this case also there may be
at least very considerable return of power over both limbs. That
is to say, under such abnormal circumstances voluntary impulses
may, so to speak, thread their way in a zigzag manner from side
to side along the nmtilated cord until they reach the appropri-
ate spinal motor mechanisms. Such an abnormal state of things
does not however really militate against the view thtit under
normal circumstances volitional impulses normally travel along
the pyramidal tract ; but it does shew, what indeed has already
l)een shewn by the phenomena of strychnia poisoning, § 461,
thit in the central nervous system the passage of nervous im-
jmlses (using those words in the general sense of changes prop-
airdted along nervous material) is not rigidly and unalterably
fixed by the anatomical distribution of tracts of fibres ; in all
such discussions as those in which we are engaged we must bear
in mind that physiological conditions as well as anatomical con-
nections are potent in determining the passage of these impulses.

§ 492. When we reflect on the great prominence of the
pyramidal tract in the spinal cord of man as compared with that
of the dog, we may justly infer not only that the pyramidal tract



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776 VOLUNTARY MOVEMENTS. [Book ra.

is under normal circumstances more exclusively the channel of
volitional impulses in man than in such lower animals, but also,
bearing in mind the discussion in a previous chapter, § 464,
concerning the activities of the spinal cord of man, that the
potential alternatives presented by the spinal cord of the dog
are greatly reduced in that of man. And such clinical histories
of disease or accidental injury in man as we possess support this
conclusion. Lesions confined to one half of the cord, or even
lesions confined to the lateral column of one half, appear to lead
to loss of voluntary power on the same side, and the same side
only, in the parts oelow the level of the lesion ; and the same
symptoms have been observed to accompany disease limited
apparently to the pyramidal tract of one side. Moreover,
though cases of recovery of power have been recorded, we have
not such satisfactory evidence as in animals of the volitional
impulses ultimately making their way along an alternative
route; but here the same doubts may be entertained as were
expressed in discussing the reflex acts of the cord in man.

When we say that the loss of voluntary power is seen on
the side of the lesion only, we should add that this statement
appears te apply chiefly to the thoracic and lower parts of the
cord. We have seen that in man, in the upper regions of the
cord, the pyramidal tract is only partly crossed ; a variable but
not inconsiderable number of the pyramidal fibres do not cross
at the decussation of pyramids, but running straight down as
the direct pyramidal tract effect their crossing lower down in
the cervical and upper thoracic regions. Hence we should in-
fer that a hemisection of, or a lesion confined te one side of the
cervical cord, would affect the voluntary movements of the
crossed side as well as of the same side, though not to the same
extent. But we have no exact information as to this point
And indeed the purpose of the direct tract is not clear ; there
is no adequate evidence for the view which has been held that
these direct fibres are destined for the upper limbs and upper
part of the body; since they are the last to cross we should
d priori be inclined to suppose that they were distributed to
lower rather than higher parts.

§ 493. We may now briefly summarize what we know con-
cerning voluntary movements. And it will be convenient to
trace the events in order backwards.

Certain muscles are thrown into a contraction which even in
the briefest movements is probably of the nature of a tetanus.
In almost every movement more than one muscle as defined by
the anatomists is engaged, and in many movements a part of
several muscles is employed, and not the whole of each. It is
perhaps partly owing to the latter fact that a muscle which has
become tired in one kind of movement, may shew little or no
fatigue when employed for another movement, though we must



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Chap. II.] THE BRAIN. 777

bear in mind that in a voluntary movement fatigue is much
more of nervous than of muscular origin.

Besides the active muscles, if we may so call them, which
directly cany out the movement, the metabolism of which sup-
plies the energy given out as work done, other muscles, some of
which are antagonistic to the active muscles and some of which
may be spoken of as adjuvant, enter into the whole act. In
flexion for instance of the forearm on the arm it is not the
flexor muscles only but the extensors also which are engaged.
According to the immediateljr preceding position and use of the
arm, and according to the kind and amount of flexion which is
to be carried out, the extensors will be either relaxed, that is to
say inhibited, or thrown into a certain amount of contraction.
And in some of the more complicated voluntary movements the
part played by adjuvant muscles is considerable. Hence in a
voluntary movement the will has to gain access not only to the
active muscles, but also to the antagonistic and adjuvant mus-
cles ; and every voluntary movement, even one of the simplest
kind, is a more or less complex act.

The impulses which lead to the contraction of the active
muscles reach the muscles along the fibres of the anterior roots,
(we may for the sake of simplicity take spinal nerves alone,
neglecting the peculiar cranial nerves,) and such evidence as
we possess goes to shew that the impulses governing the antag-
onistic and adjuvant muscles travel by the anterior roots also;
the question whether the inhibition of the antagonistic muscles
when it takes place, is carried out by inhibitory impulses passing
18 such along the fibres, or simply by central inhibition of pre-
viously existing motor impulses need not be considered now.
These anterior roots are connected as we have seen with the
grey matter of the cord, and in each hypothetical segment of
the cord we may recognize the existence of an area of grey
matter which, though we cannot define its limits, we may, led
by the analogy of the cranial nerves, call the nucleus of the
nerve belonging to the segment ; and we may further recognize
in such a nucleus what we may call its efferent and its afferent
side.

Every voluntary movement, even the simplest, is as we have
repeatedly insisted a coordinated movement, and in its coordi-
nation afferent impulses play an important part. The study of
reflex actions, § 462, has led us to suppose that each spinal seg-
ment presents a nervous mechanism in which a certain amount
of coordination is already present, in which efferent impulses
are adjusted to afferent impulses. But the results obtained by
stimulating separate anterior nerve roots shew that, in the case
of most muscles at all events, the especially active muscles of
the limbs for instance, each muscle is supplied by fibres coming
from more than one nerve root, that is to say the spinal nucleus,



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778 VOLUNTARY MOVEMENTS. [Book iii.

or at least the spinal motor mechanism for any one muscle,
extends over two or three segments. Hence d fortiori in a vol-
untary movement, involving as this does in most cases more
than one muscle, the spinal mechanism engaged in the act
spreads over at least two or three segments, thus allowing
of increased coordination. In that coordination the impulses
serving as the foundation of muscular sense play an important
part, but other afferent impulses, such as those from the adjoin-
ing skin, also have their share in the matter; and it is worthy
of notice that not only is the skin overljdng a muscle serveo,
broadly speaking, by nerve roots of the same segment as the
muscle itself, afferent in one case, efferent in the other, but
in the parts of the body where coordination is especially com-
plex, in the fingers for instance, not only is each muscle sup-
plied from more than one segment, but also each piece of skm
is supplied in the same way by the posterior roots of more than
one nerve.

In the case of the frog it is clear that in reflex movements
a large amount of coordination is carried out by these various
spinal mechanisms ; and as we have urged, we may safely infer
that in the voluntary movements of the frog, the will makes
use of this already existing coordination, whatever be the exact
path by Avhich in this animal the will gains access to the spinal
mechanisms. In the dog we may conclude that in voluntary
movements the spinal mechanisms, with coordinating functions,
are also set in action, in this case by impulses passing straight
from the coitex to the mechanisms by the pyramidal tract,
though, apparently in the absence of the pyramidal tract,
the will can work upon the mechanisms by changes travelling
through other parts of the cerebrospinal axis. And in the
monkey and man, subject to the doubts already expi*essed as
to the potentialities of the human spinal cord, we may prob-
ably also infer that in each voluntary movemeht some, perhaps
we may say much, of the coordination is carried out by the
spinal mechanism set into action through impulses along the
pyramidal tract. We m.ay probably further infer that a care-
ful adjustment obtains between the beginnings of the p)Tra-
midal tract in the cortex and its endings in the cord, so that
the topography of ' areas ' or * foci ' in the cortex above is an
image or projection of the spinal mechanisms below.

The complex chamcter, on which we insisted just now, of
almost every voluntary movement necessitates that in eveiT
such movement a large area of spinal mechanism is involved.
But this is not all. The movements of any part, of the legs
for instance, are not determined, nor is the coordination of the
movements effected, simply by Avhat is going on in the legs and
the part of the spinal cord belonging to them. The discussion
in a previous section has shewn that much of the coordination



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Chap. II.] THE BRAIN. 779

of the body is carried out by the middle portions of the brain,
and on these the motor area must have its hold as well as on
the spinal mechanisms.

The details of the nature of that hold are at present un-
known to us; but it must be remembered that not all the
fibres passing down from the motor region, not all those even
proceeding from the densest and most clearly defined motor
areas, are pyramidal fibres. With the pyramidal fibres are
mingled fibres having other destinations, and some of these
probably pass to the thalamus and so join the great tegmental
region. Aloreover the motor region must have close ties with
other regions of the cortex whence fibres pass to the pons to
make connections with the cerebellum. On the other hand,
the cerebellum is especially connected with what we may fairly
consider the afferent side of the spinal cord and bulb. These
facts must merely be taken as indicating the possibilities by
which the motor region is kept in touch with the great coordi-
nating mechanism ; it would be venturesome at present to say
much more.

In an ordinary voluntary movement an intelligent conscious-
ness is an essential element. But many skilled movements
initiated and repeated by help of an intelligent conscious voli-
tion may, when the nervous machinery for carrying them out
has acquired a certain facility, (and in all the higher processes
of the brain we must recognize that, in nervous material at
ail events, action determines structure, meaning by structure
molecular arrangement and disposition) be carried out under
appropriate circumstances with so little intervention of distinct
consciousness that the movements are then often spoken of as



Online LibraryArthur Sheridan Lea Sir Michael FosterA text book of physiology → online text (page 87 of 148)