Lloyd, 1941; Preston & Whitlock, 1960, pyramidal pathway in the primate, already established by

21 Pages·2006·2.01 MB·English

J. Physiol. (1962), 161,pp. 91-111 91
With 12 text-ftgures
Printedin Great Britain
MINIMAL SYNAPTIC ACTIONS OF PYRAMIDAL IMPULSES
ON SOME ALPHA MOTONEURONES OF THE BABOON'S
HAND AND FOREARM
BY S. LANDGREN*, C. G. PHILLIPS AND R. PORTER
From the University Laboratory ofPhysiology, Oxford
(Received 4October 1961)
The method ofdirect electrical stimulation ofpopulations of pyramidal
neurones by weak surface-anodal shocks (Hern, Landgren, Phillips &
Porter, 1962) creates specially favourable conditions for reinvestigating
the questionwhetherpyramidalneurones are connected monosynaptically
with alpha motoneurones in the spinal cord.
The scope of experiment has been deliberately restricted to the 'arm
area'ofthecortexandtothealphamotoneuronesoftheforearmandhand.
The arm area was chosen partly because it is more readily exposed than
the leg area, and partly because the range and precision of hand move-
ments, even in the baboon, would lead one to expect a powerful cortical
command of these final common paths. This expectation is supported
by the fact that the cortical electrical threshold for hand movements is
lower than the threshold for other movements (Liddell & Phillips, 1950,
1951). To detect minimal synaptic transmission by pyramidal impulses
and to measure its quantity and timing, we have made intracellular
records from alpha motoneurones innervating the forearm and hand.
Bythus restricting stimulus andresponse to their minima, and by attend-
ing only to the earliest responses, which are, in fact, the only responses to
be recorded at all under these conditions, it is possible to be reasonably
surethatoneisdealingwithpurepyramidal actions, uncomplicated bythe
so-called extrapyramidal effects that would inevitably be stirred up by
stronger and more prolonged electrical stimulation. Experience of these
delicate methods has justified our beliefthatthe extrahazards of surgical
isolation ofthe pyramidalpathway bytransecting the hind brain, sparing
only the medullary pyramids (Lloyd, 1941; Preston & Whitlock, 1960,
1961), can be avoided in these special circumstances.
Our experiments have confirmed the existence of a monosynaptic
pyramidal pathway in the primate, already established by previous work
* Onleave ofabsencefromthe Department ofPhysiology, Kungl. VeteriIiarhogskolan,
Stockholm 51, Sweden.
92 S. LANDGREN, C. G. PHILLIPS AND R. PORTER
which differed from ours instronger corticalstimulationandmoremassive
spinal response (Cooper & Denny-Brown, 1927; Bernhard, Bohm & Peter-
sen, 1953; Bernard & Bohm, 1954a, b; Preston & Whitlock, 1960, 1961).
The occurrence of inhibitory synaptic action at a slightly longer latency
(Preston & Whitlock, 1960, 1961) has also been confirmed. We have also
found that, when pyramidal impulses are repeated, there is a growth of
the transmitting potency ofexcitatory pyramidal synapses. This new fact
explains the specially effective motor property of cortical stimulating
pulses oflong duration (Wyss & Obrador, 1937; Liddell & Phillips, 1951):
such long pulses set up repetitive pyramidal impulses, which depolarize
the spinal motoneurones steeply.
METHODS
The methods ofpreparation, measurement of stimulus parameters, brain charting and
recordingfromthelateralcorticospinaltractandfromC7-T1motoneuroneshavebeenfully
describedinanotherpaper(Hernetal.1962). Forinvestigatingtheprecisetimingandsequence
ofsynapticactions,thatistosay,forstudyingthesimplestcortico-motoneuronalconnexions,
briefS+ pulses(0-2msecduration),singly or inshort trains, were most useful. Forunder-
standing the conditions in which electrical stimulation ofthe brain can cause movement,
however, theactionsofpulses of5-0msecdurationwereofspecialinterest.
RESULTS
Timing ofarrival ofpyramidal impulses in cervical region
Figure 1 shows a tract wave recorded in the dorsolateral white matter
at C5-6 level following a short S+ shock to the cortex, and a diagram,
on the same scale, showing the times of arrival of impulses in 68 single
corticospinal fibres recorded at this level in six experiments. The sharp
transition from initial positivity to the main negative-going component
of the wave was taken as the sign of first activity at the recording site.
The diagram shows that the majority ofimpulses arrive between 1-6 and
2*0 msec from the start of the cortical shock, but that the pyramidal
volley is already spreading out in time as it travels through the cervical
region. In one experiment the wave was recorded at C7-8 as well as at
C5-6; the earliest arrival at C7-8 junction was 0 2 msec later than at
C5-6 junction. More waves are illustrated in Fig. 5. Their time course
was remarkably similar in all six experiments. The actual level at which
pyramidal axons destined for C7-T1 segments enter the grey matter is
not known, nor the length oftheir tapering intramedullary branches and
presynaptic arborizations. These uncertainties, together with the tem-
poral scatter of impulses (Fig. 1), make it impossible to state the exact
time of arrival of pyramidal impulses at their synaptic terminals from
measurements made from the dorsolateral white matter.
MINIMAL PYRAMIDAL ACTIONS 93
The onset of synaptic action on sample motoneurones, however, is
abrupt (Fig. 2), and can be precisely timed in relation to the start of a
short S+ cortical shock. In 49 out of 66 motoneurones of C7-T1 seg-
ments the delay was 2-5-2-7 msec; in 16 out of 66, 3 0 msec, and in 1 it
was 3-5 msec.
Intwelve oftheserecordsaverysmall deflexionwas clearlyvisible inthesuperimposed
traces, betweentheendoftheshock artifact andthestart ofthesynaptic potential. Such
deflexions are distinctly seen inthe records onthe left ofFig. 2, and in Fig. 6; one isjust
visible in the top record ofFig. 11. Measured from their point ofreversal frompositivity
tonegativity, theyprecededthestartofthesynaptic potentials by 0.5-0-8msec. In some
experiments records were taken after withdrawal of the micro-electrode from the cell;
thesamesmalldeflexionswerethenrecordedextracellularly. Thenatureofthesedeflexions
isuncertain; they may be smallpresynaptic spike potentials, or attenuated records ofthe
tract wave in the adjacent lateral column. Their absence from records obtained from the
samemotoneuronesbystimulatingGroupIafibres(Fig.2b),andtheintervalof0-50-08msec,
whichislong forsynaptic delay, areinfavourofthe secondexplanation.
@10:
0
0
z0 5 _
0 1 2 3
msec
Fig. 1.Above: WaverecordedfromlateralcorticospinaltractatC5-6inresponseto
cortical stimulation, S+, 2-8mA, 0-2msec, at 1.0c/s (about 20 superimposed
records). Downward deflexion indicates negativity of micro-electrode. Below:
diagram shows times of arrival of impulses at C5-6 in 68 corticospinal fibres
inresponse to S+ shocksto cortex.
The fact that the majority of pyramidal impulses arrive at C5-6 only
0S6-08 msec before the start of the majority of synaptic potentials at
C7-T1 establishes that these are monosynaptically generated. Further
evidence comes from the form of the synaptic potentials and fromtheir
abilitytofollowhighratesofpyramidalstimulation.
94 S. LANDGREN, C. G. PHILLIPS AND R. PORTER
Excitatory post-synaptic potentials (EPSP) set up bypyramidal
volleys in motoneurones of C7-T1 segments
Strong, brief cortical shocks can cause prolonged depolarization of
spinal motoneurones (Preston & Whitlock, 1961). To avoidthis complica-
tion we have deliberately used weak shocks and have found that these
cause brief synaptic actions ofthe type seen in Fig. 2. Figure 2a shows
the EPSPs set up in a radial motoneurone by pyramidal volleys; Fig. 2b
shows, for comparison, EPSPs of similar small amplitude set up by
Group Ia volleys in the central end of the cut radial nerve, by shocks
belowthreshold forthemotoraxon. Thetime courses aresimilar, showing
thatthere was a well-synchronized synaptic impact in each case.
a b
*.... .......................
Fig. 2. Radial motoneurone, K2S04 electrode, membranepotential 63mV. Note
similarity between Group Ia monosynaptic EPSP, elicited by weak stimulation
ofradialnerve, andEPSPevokedbybriefS+ cortical pulse.
a, EPSPevoked bybriefS+ cortical pulse, 1-25mA, appliedto best point on
lipofRolandicfissure; b, GroupIamonosynapticEPSP;c,cortexstimulatedata
point 3mm from best point; note inhibitory notch following excitatory peak;
d, rectangular 1-5mVstep applied to input. Time, 1000c/s.
Figures 3 and 4 show that short trains ofrepetitive pyramidal volleys,
even at 400 c/s (the highest frequency tested), cause repetitive synaptic
potentials at the same frequency, and that after the last response of the
series the motoneurone repolarizes smoothly. The high rate of driving
andtheabsenceofpersistingsynapticaction,indicativeofsynapticstimula-
tion by stirred-up spinal interneurones, are again in favour ofmonosyn-
aptic action.
The remarkable growth in synaptic action with repetitive stimulation
at 400 c/s (Fig. 3) requires an explanation. The upper record of Fig. 3
shows that the amplitude of the successive EPSP upstrokes increases;
thus the general upward trend is not to be explained merely by the fact
that at 200 c/s each wave begins before repolarization from the previous
wave has finished. In Fig. 4 the dotted curves are obtainedby adding the
MINIMAL PYRAMIDAL ACTIONS 95
appropriate number of single-volley curves at 5 msec intervals. The
responses actually elicited by repetitive volleys at 200 c/s were always
largerthanthecurvesexpectedbymereaddition,thedisparitybeinglarger
insome motoneuronesthaninothers. Therefore, the quantity of synaptic
action set offby the successive cortical shocks increasedprogressively.
This might have been due to recruitment of pyramidal neurones at
the cortex, forexample, as aresultoffacilitation ofneighbouringneurones
3
-
2-
0
2-
/% E ~~~~~~~~~~~1
LI~~I
2
Fig. 3 Fig. 4
Fig. 3. Medianmotoneurone, toshowrepetitiveEPSPsfollowingcorticalstixnula-
tion with 02msec pulses, S+, 045mA, at 200c/s (above) and 400c/s (below).
Membranepotential -60to -66mV, calibration 3mV, KG1electrode.
Fig. 4. Each set oftracings, from superimposed sweeprecords fromsingle moto-
neurones, shows, below (full line) monosynaptic potential evoked by single
S+ 0-2msec cortical pulse; above (full line), series of monosynaptic potentials
evoked by same pulses repeated at 200c/s. Interrupted line shows response
expected from mere addition of monosynaptic curves. Time: 5msec separates
shockartifacts. Above,medianmotoneurone,membranepotential-70mV,shocks
1-3mA; centre, median motoneurone, membrane potential -72 mV, shocks
1-9mA;noteinhibitoryactionfollowing crestofexcitatoryactioninlastresponse
of repetitive series. Below, ulnar motoneurone, membrane potential -63mV,
shocks 1-3mA.
96 S. LANDGREN, C. G. PHILLIPS AND R. PORTER
by the recurrent axon collaterals of the discharging population (Phillips,
1961). In that case the sizeofthepyramidal volleys shouldincreasewith
repetitive stimulation. Figure 5, which shows repetitive waves in the
lateralcorticospinaltract,showsnoappreciablechangeinsizeofthevolleys
with repetition, although the volleys were submaximal and could readily
be enlarged by increasing stimulus strength (Fig. 5a, b). Thus the in-
creasing EPSPs are due to an increasing transmitting potency of the
to
a _ ; ;
W0
b i > >
C
Fig. 5. Superimposed records ofwaves in lateral corticospinal tract, caused by
single and repetitive surface-anodal shocks (duration 0-2msec), to pre-central
gyrus near central fissure and about equidistant between superior and inferior
pre-centralfissures;negativedeflexiondownwards;voltagescale,05mV.a,stimulus
2*0mA, single and repetitive, 250c/s; b, same experiment, stimulus 3-25mA;
time marker, msec for fast and slow sweeps for a and b; c, another experiment,
C7-8level; singles 2-9mA, tetani 2-8mA; 250c/s; timemarker, msec.
pyramidal synapses with repetition. In this property they resemble the
Group Ib synapses applied to the cells of origin of the ventral spino-
cerebellar tract (Eccles, Hubbard & Oscarsson, 1961).
Small, smooth delayedwaves ofdepolarization have beeninfrequently seentowardsthe
end ofa train offour or five volleys at 200c/s, when we were stimulating cortical points
afew millimetres awayfrom those from which the monosynaptic potentials were elicited.
These are presumably due to interneuronal activity and have not been systematically
investigated. They could, however, contribute, in an unknown degree, to the increasing
discrepancy betweenthe dotted curves andthefullcurves ofFig. 4. Butsuchbackground
depolarization would not account for the increasing amplitude of the successive mono-
synapticupstrokes. For, given a constant quantity ofsynaptic actionpervolley, thesuc-
cessive upstrokes should become 8maller as the membrane potential is driven towards the
equilibriumpotentialforexcitatorysynapticaction.Thisprovesthatthequantityofmono-
synaptic actiondoes, infact, increase withsuccessive pyramidalvolleys.
MINIMAL PYRAMIDAL ACTIONS 97
Inhibitory synaptic actions ofpyramidal volleys
Minimal inhibitory post-synaptic actions are evident in the records
ofFigs. 2 and4. InFig. 2athepost-synapticpotentialelicitedbystimula-
tionofthebestpointonthecortexisapureEPSP,resemblingtheGroupIa
EPSP shown in the same figure. Record c, however, elicited by shocks
applied to a cortical point 3 mm away from the best point, is impure.
The rising phase of the EPSP is similar, but descent from the summit is
notched by inhibitory synaptic action.
Inhibitory action is favoured by repetition of the pyramidal volleys,
and K2SO4-filled micro-electrodes favour its detection. In Fig. 3 the
synapticpotentialsshownoevidenceofit,andtheupperandlowertracings
Fig. 6. Ulnar motoneurone, showing pyramidal inhibitory action. Above, anti-
dromic impulse; initial membrane potential -57mV, spike peak +11 mV.
Below,high-gainrecordofresponsetofivecorticalshocks,S+,0-2msec,3-2mA,at
200c/s.Membranepotentialinitially -57mV. Inlastresponseexcitatorysynaptic
action begins at 2-6msec, inhibitory action at 40msec after last shock. Time
marker, msec; voltage scale, 30mV.
ofFig. 4 are pure EPSPs. The fourth response ofthe middle record, how-
ever, shows inhibitory erosion ofits repolarizing phase. Such sharpening
ofthelaterpeaksinarepetitiveseriesofsynapticactionsiscommon, andis
assumedto indicate an inhibitory component. Figure 6 shows an example
in which inhibitory synaptic action is revealed against a background of
low membrane potential. The timing of events is best seen after the last
shock of the series. The EPSP begins at 2-6 msec after the beginning of
the stimulus, and at 4 0 msec the peak ofthe EPSPis abruptly cut short
by the steep onset of the IPSP. Figure 7 shows that in the depolarized
stateresultingfromgrossinjurytoamotoneurone, theinhibitorysynaptic
component is displayed to advantage, as if, by deliberate passage of
depolarizing current, the membrane potential had been driven away
fromtheequilibriumlevelforinhibition (Coombs,Eccles& Fatt, 1955).The
progressive increase in the size ofthe successive IPSPs is obvious, and is
7 Physiol. 161
98 S. LANDGREN, C. G. PHILLIPS AND R. PORTER
illustrated also in Figs. 6 and 8. In every experiment in which IPSPs
have been evoked, their latency has been longer than that of the mono-
synaptic EPSPs. The values of 2-6 msec for the EPSP and 4-0msec for
the IPSP seen in Fig. 6 are typical. Thus, either the inhibitory actions
are due to a delayed component of the pyramidal volley, or there is an
interneurone in the inhibitory pathway (Eccles, 1957). If the former
explanation were correct, the histogram ofFig. 1 and the waves ofFigs. 1
and 5 would be expected to show evidence oftwo volley components.
+15
Eirk~~~~~~~~~~~~
O , , -o~- 6
i _ -~~~39
-61
b cd
Fig. 7. Median motoneurone, to show exaggeration ofcortically-evoked IPSPs
indepolarizedstate. a,antidromicimpulse;b,c,d,responsestocorticalstimulation,
S+, 0-75mA, 0-2 rmsec, at 170c/s; simultaneous low-gain d.c. records (above)
and higher-gain a.c. records (below). b, membrane potential -61mV; last two
shocks give excitatoryfollowed byinhibitory synaptic action. c, micro-electrode
dislodged during Isec interval between this sweep and the previous sweep;
membrane potential neax zero (see upper trace). d, injured cell re-entered spon-
taneouslybetween this sweep andthelast; membranepotential-39mV, IPSPs
exaggerated. K2SO4electrode.
Our cortical maps give abundant evidence of different topographical
localization of the optimal points for evoking excitatory and inhibitory
synaptic sections on a test motoneurone. An instance has been given in
Fig. 2. Figure 8 shows two more, one with repetitive stimulation, the
other with a longpulsewhich presumably caused asynchronous repetitive.
firing in a population ofpyramidal neurones (see below). Since we have
MINIMAL PYRAMIDAL ACTIONS 99
not, in these first experiments, subdivided the peripheral nerves in order
toidentifyantidromicallythemotoneurones ofspecificmuscles,wecannot,
as yet, interpret our findings in terms of reciprocal innervation (cf.
Bernhard & Bohm, 1954b). The areas from which different motoneurones
can be affected by minimal stimulation overlap freely in every brain.
Ifinhibition ofa median motoneurone is found from an area from which
a radial motoneurone is excited, this need not imply a reciprocal morpho-
logical pattern, since, for example, the dorsiflexors of the wrist are used
as fixators in voluntary flexion ofthe fingers (Beevor, 1904): thus not all
radial motoneurones need be antagonists of all ulnar and median moto-
neurones in all cortically-initiated actions. For the present, the point to
be taken is that a test motoneurone may be optimally excited from one
e
.0~ ~~~~~~~~~~~~~~~~~ I
-
.LLLLJ r
Fig. 8. Two experiments illustrating different proportions ofexcitatory and in-
hibitorysynaptic actionfromstimulating differentcorticalloci.
Above, median motoneurone, K2S04 electrode, membrane potential -66mV;
S+ pulses, 6msec, 035mA. Map (left) showspre-centralexcitatoryfield (dotted
outline). Upper record, superimposed sweeps showing response to stimulating
point e. Lowerrecord, response to stimulating point i, near edge of pre-central
field. Mapalsoshowspartofapost-central inhibitoryfield.
Below, radial motoneurone, K2S04 electrode, membrane potential -63mV,
S+ pulses, 0-2 msec, 0-75 mA, at 200 c/s. Map (left) shows central triangle
for optimal EPSPs with small IPSPs (right, lower record); x, best IPSPs, with
small initial EPSPs (right, upperrecord); 0,noresponse.
Fulloutline, edge offield for minimal excitatoryactiondottedoutline,edgeof
fieldforminimalexcitatoryfollowed byminimalinhibitoryaction. Time marker,
msec; calibration, 3mV.
7-2
100 S. LANDGREN, C. G. PHILLIPS AND B. PORTER
cortical focus and optimally inhibited from another focusgradingintoit.
Thus different populations of pyramidal neurones exert, on any moto-
neurone, a varying mixture ofexcitatory and inhibitory synaptic action.
The inhibitory action may be virtually pure, as in the long-pulse experi-
ment ofFig. 8. Commonly the excitatory action has seemed to be pure,
but this may have been favoured by the use of KCI-filled electrodes in
many experiments. Systematic employment of depolarizing and polar-
izing currents, as well as ofsulphate-filled electrodes, would be necessary
to unmask small inhibitory components in mixed responses.
Interpretation ofexcitatory synaptic actions generated by 5msec
S+ corticalpulses in terms ofthe monosynaptic pyramidalpathway
It has long been known that, for eliciting movements by electrical
stimulation of the cortex, the optimal shocks are of longer duration
thanthe 0x2 msecpulses usedinthis paperforinvestigation ofthe cortico-
motoneuronal pathway (Wyss & Obrador, 1937; Liddell & Phillips, 1950,
1951). It is therefore interesting to analyse the effects of such pulses at
minimal effective strengths.
Figure 9 shows the full range of responses of a single pyramidal fibre
to surface-anodal stimulation ofthe cortex. The fibre was tapped at C5-6
level. The right-hand column shows its responses to stimulation at its best
corticalfocus (point o onmap). Thethresholdfor abriefpulseis 0 35 mA.
It is shown at 0 4 mA following repetitive shocks at 200 c/s with unvary-
ing latency. It is thus more likely that the shocks are directly stimulating
some part ofthe pyramidal cell membrane than that they are exciting it
indirectly via cortical interneurones (Hern et al. 1962).
The action of long pulses (7 msec) on this neurone is shown in the
remaining records ofFig. 9. The threshold was somewhere below 0*15 mA,
for at this strength two impulses were usually discharged, reaching C5-6
level about 5-5 and 8-5 msec after the start of the pulse. Increasing the
current to 0-22mA produced four impulses, advanced them in time and
reduced the variation oflatency. Reasons have already been given (Hern
et al. 1962) for supposing that such records show the rhythmic response
of the pace-maker membrane of the pyramidal neurone (cf. Phillips,
1961) to a steady depolarizing current, anodal at the cortical surface
but acting as a virtual cathode in the region of origin of the pyramidal
axon.
It should benoted that the optimal point forffick movement ofthumb and index is at
m,Fig. 9. Atthethresholdformovement, 1.8mA,thestimulusatmexcitesthispyramidal
neuronetorepetitivefiring, withapreference forthreeparticularlatencies. Theneuroneis
likelytobesomewherenearthelowest-thresholdpointo, whichis 9-5mmdistantfromm.
There is of course no evidence that this pyramidal neurone makes any connexions with
thumb-indexmotoneuronesinthecord. Butthedifficultiesinherentintheuseofelectrical