Biomedical subjects
Publications and source records attributed to D J Tracey.
The paraterminal ligaments of the distal phalanges have been studied by dissection. They are a normal feature of all distal phalanges in both the hand and foot, and connect the paraterminal spines and paraterminal tubercles of the distal phalanx on both sides. Branches of the proper palmar digital artery and nerve pass under the ligament to reach the matrix of the nail, which they supply.
Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) is a widely used neuroanatomical tracer. When compared with other tracers, WGA-HRP may preferentially label unmyelinated fibers. In agreement with this hypothesis, injections of WGA-HRP in cervical and lumbar dorsal root ganglia resulted in more prominent light microscopical labeling in superficial than deep laminae of the dorsal horn. However, ultrastructural examination of these laminae reveals a paucity of terminal labeling in contrast to the abundance of extracellular tracer in the space surrounding unmyelinated fibers and their terminals, and to the widespread occurrence of transneuronal labeling. These results bear upon the mechanism of preferential labeling in the spinal cord and have implications for the interpretation of the labeling obtained when using WGA-HRP.
The morphology and distribution of spinocerebellar neurones were examined in the guinea pig. Horseradish peroxidase was injected into the cerebellum, and after a survival time of 72 h retrogradely labelled cells were examined in the spinal cord. The distribution of spinocerebellar cells was similar to that previously demonstrated in the rat. Three major groups of neurones were distinguished: the central cervical nucleus (C1-C2), Clarke's column (T2-L3) and spinal border cells (L3-L6). Neurones in the central cervical nucleus were multipolar, had mean equivalent diameters of about 24 microns, and their axons ascended on the contralateral side of the spinal cord. Neurones in Clarke's column were spindle-shaped, approximately 25 X 35 microns, and their axons ascended on the ipsilateral side of the spinal cord. Spinal border cells were multipolar, with mean equivalent diameters of about 35 microns; their axons were predominantly crossed.
1. Excitatory postsynaptic potentials (EPSPs) evoked by impulses in single group I muscle afferents were recorded in dorsal spinocerebellar tract (DSCT) neurons in the spinal cords of anesthetized cats. Fluctuations in the amplitude of these single-fiber EPSPs were determined from measurements of EPSP peak amplitude and contaminating noise (800-4600 trials). 2. In a previous study at this connection, we found that these single-fiber EPSPs fluctuated in amplitude between approximately equal, or quantal, increments. However, these quantal fluctuations could not be described by simple binomial statistics (39). In the present study we have applied further analysis procedures to the same single-fiber EPSPs to formulate a more appropriate probabilistic model of transmission at this connection. 3. In the first stage we have demonstrated that each single-fiber EPSP is composed of the sum of a number (3-30) of uniform quantal events, and that there is extremely little variability in the amplitude of the single quantal event. 4. In a further procedure, we have demonstrated that these quantal fluctuations can be described by a compound binomial model in which each underlying quantal event is associated with a particular, but independent, release probability. The results of this analysis indicate that the probability of transmitter release varies considerably between release sites at this connection. (The use of such a compound binomial model reemphasized previous warnings concerning the interpretation of the results of all statistical models of quantal release. Problems regarding the non-unique nature of N, the total population of quantal events, and other such difficulties are discussed.) 5. A model of transmission at this connection is proposed, in which there are a number of "active" release sites, exhibiting generally high release probabilities, and a number of "reserve" release sites, with zero, or close to zero, release probability. The physiological consequences of such a scheme are discussed.
The dorsal column nuclei and the sensory trigeminal nuclei project not only to the ventrobasal thalamus but also to the cerebellum. In this study the numbers and distribution of neurones projecting to these two regions were examined for the following nuclei: the rostral part of the main cuneate nucleus, the external cuneate nucleus, nucleus x, the principal sensory nucleus of the trigeminal nerve, and the oral, interpolar, and caudal subnuclei of the spinal nucleus of the trigeminal nerve. A thalamic projection from nucleus x and from the external cuneate nucleus was confirmed, and a distinct group of neurones projecting to the ventroposteromedial thalamus was distinguished near the ventromedial aspect of the principal sensory nucleus. Of the 165,000 neurones examined, only one was found to be double labelled. It was concluded that the populations of neurones that project to the ventrobasal thalamus and to the cerebellum are separate, and that somatosensory neurones in the brainstem do not send axon collaterals to both regions.
The synaptic connection between single group I afferents and dorsal spinocerebellar tract (DSCT) neurons in the cat spinal cord has been studied in an attempt to gain insight into the mechanisms of excitatory synaptic transmission in the mammalian CNS. Fluctuations in the amplitude of single group I fiber EPSPs in DSCT neurons were examined using a numerical deconvolution procedure to reduce the effects of contaminating noise. In general, it was found that single fiber EPSPs fluctuate in peak amplitude between discrete levels separated by equal or quantal increments. Many previous studies have proposed simple binomial statistics as a general model of quantal synaptic transmission. In the present study we show that simple binomial statistics do not describe the fluctuations in amplitude of single group I fiber EPSPs in DSCT neurons. It is suggested that nonuniformities in the probability of transmitter release from release site to release site explain the failure of the binomial model to describe the EPSP fluctuation pattern at this synapse. Nonuniform quantal transmission is proposed as a more adequate description of excitatory synaptic transmission in the mammalian CNS.
Proprioceptive information from the hindlimb of the cat is now known to be relayed to the somatosensory thalamus and cortex via axons in the dorsolateral fasciculus and a medullary relay in nucleus z. The aim of this study was to identify nucleus z in the rat, to locate the cells of origin of spinal afferents to nucleus z, and to determine whether they are collaterals of the dorsal spinocerebellar tract. The location and extent of nucleus z were studied by filling the axon terminals of collaterals of the dorsal spinocerebellar tract (dsc) with horseradish peroxidase (HRP), which was injected into the inferior cerebellar peduncle. Nucleus z in the rat was found to be similar in location to nucleus z in other mammals. It was located just below the dorsal surface of the medulla, bounded laterally by the rostral pole of the cuneate nucleus and medially by the nucleus of the solitary tract. The cells of origin of the spinal afferents to nucleus z were studied by using the retrograde transport of HRP. They were located in Clarke's column (dorsal nucleus) and in lamina 10 of the dorsal horn. They were similar in location and morphology to neurons giving rise to the dorsal spinocerebellar tract, but were smaller in average diameter. A double retrograde labeling technique was used to determine whether the spinal afferents to nucleus z are collaterals of neurons giving rise to the dsc. It was estimated that up to 92% of the spinal afferents to nucleus z were collaterals of dsc neurons, while approximately 3% of all dsc neurons gave rise to collaterals terminating in nucleus z.
Single identified group I a and I b muscle afferent fibres were injected with horseradish peroxidase in the lumbar dorsal columns of anaesthetized cats. The morphological details of the axon collaterals and terminal boutons of these muscle afferents within Clarke's column were subsequently reconstructed. The rostro-caudal extent of synaptic terminals from a single afferent fibre within Clarke's column was found to be restricted to less than 1 mm. In the same experiments, dorsal spinocerebellar tract (d.s.c.t.) neurones were retrogradely labelled by injection of horseradish peroxidase into the cerebellum. Synaptic contacts between labelled group Ia and Ib afferent fibres and the soma and proximal dendrites of d.s.c.t. neurones were found. The synaptic contacts from both Ia and Ib fibres varied greatly in size, from 1 X 1 micron up to 'giant' synapses of 20 X 3 micron. Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in d.s.c.t. neurones by impulses in single group I muscle afferent fibres. The fluctuations in peak amplitude of each e.p.s.p. were determined from e.p.s.p. and noise recordings, using a numerical deconvolution procedure. In general, these single-fibre e.p.s.p.s fluctuated between discrete amplitudes separated by an incremental amplitude which was approximately constant. This incremental amplitude did not depend on the average peak amplitude of the particular e.p.s.p. examined. Our anatomical observations of 'giant' boutons arising from Ia and Ib afferent fibres contacting d.s.c.t. neurones raises the possibility of multiple transmitter release sites within an individual synaptic bouton. It is proposed that synaptic transmission between group I muscle afferents and d.s.c.t. neurones occurs with discrete all-or-nothing e.p.s.p.s associated with transmitter release sites.
Composite excitatory postsynaptic potentials were elicited in lumbar motoneurones by Ia afferents from muscles of the triceps surae group. These excitatory postsynaptic potentials were examined in the same cell before, during and after interruption of descending spinal pathways. After transection or cold block of the spinal cord at T12-T13, the amplitude of composite excitatory postsynaptic potentials showed no significant change for a period of up to seven hours after transection. However, there was a reduction in amplitude of the monosynaptic reflex in the extensor motoneurones which may be due to an observed hyperpolarization and reduction in membrane time constant in these neurones. The reduction in amplitude of the monosynaptic reflex observed in spinal shock can be attributed to the effects of these changes, rather than to a decrease in the size of the monosynaptic excitatory postsynaptic potential.
Intracellular recordings were made from Renshaw cells in the cat. Recurrent excitatory postsynaptic potentials (EPSPs) were elicited by antidromic stimulation of the ventral roots. These EPSPs had a simple monophasic time course without a spike-like peak reported by previous authors. No hyperpolarization could be detected following averaged EPSPs. However, after bursts of impulses an afterhyperpolarization could be detected which probably contributes to a pause in firing after the initial response. Depolarizing current pulses elicited a burst of impulses similar in pattern to the initial response to stimulation of the ventral roots.
Extensor muscles of the fore- and hindlimb were stretched in the lightly anaesthetized monkey (Macaca fascicularis) and cat. The resulting electromyogram contained a short latency peak consistent with a monosynaptic, segmental pathway; this peak was identified as the M1 peak of Tatton et al. [21]. A longer latency peak, identified as M2 and said to involve a pathway including supraspinal centres, was also present. After spinal section at a high cervical level, the electromyogram was reduced in amplitude, but both short (M1) and longer latency (M2) peaks were present. It is concluded that neither the cerebral cortex nor the cerebellum are necessary parts of the neural circuitry generating longer latency components of the electromyographic response to muscle stretch.
1. Records were made from axons in the dorsal columns and cells in the cuneate nucleus which responded to stimulation of the wrist joint nerve. 2. A sample of twenty-five axons activated by the wrist joint nerve was recorded in the dorsal columns at the level of the third cervical segment. All twenty-five were post-synaptic fibres as judged by response latency, burst length, and maximum frequency of following. Nineteen of the twenty-five had convergent inputs from the wrist joint nerve and the cutaneous superficial radial nerve. 3. While no primary wrist joint afferent fibres were recorded in the dorsal columns, their presence was demonstrated by recording single units in the wrist joint nerve which were antidromically activated by microstimulation in the cuneate fasciculus. 4. The majority of cells recorded in the cuneate nucleus were activated not only by stimulation of joint afferents, but also by skin and muscle afferent fibres. 5. About half of the cells in the cuneate nucleus responded to wrist movement in animals with partially denervated forelimbs, where the intact wrist joint nerve was the only afferent channel providing information about natural, imposed wrist movements. The majority of the cells had phasic responses, which were weak and irregular in comparison with the responses of primary wrist joint afferents to the same movements. 6. Only two of thirty-four cells tested could be shown to project directly to the ventrobasal thalamus, using collision of antidromic and peripherally activated impulses as the criterion.
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In seven monkeys (M. fascicularis), recordings were made from neurones in subnuclei VPLo and VPLc of the thalamus. The peripheral inputs to these cells from the forelimb were established by stimulating muscle, skin and joint nerves, and cutaneous receptive fields were examined by natural stimulation of the skin. The projection of the same cells to motor and sensory cortex was examined by stimulation of the cortex to demonstrate antidromic responses and by retrograde axonal transport of horseradish peroxidase injected into these regions of cortex. Of the 139 neurones which were driven by stimulation of peripheral nerves, 73 were located in VPLo. This suggests that there is a significant projection of forelimb afferents to VPLo in the monkey. Of the 73 neurones located in VPLo, 60 responsed to stimualtion of muscle or joint nerves. The projection to VPLo is therefore predominantly from deep receptors. Twenty-one of these 73 neurones had convergent inputs. The latencies of activation of units in VPLo were short (4--8 ms) and consistent with a lemniscal pathway via the dorsal column nuclei. Injection of horseradish peroxidase into the arm area of the motor cortex labelled cells in VPLo and not in VPLc. The neurones from which recordings were made in VPLo were located within the population of cells labelled with horseradish peroxidase after injections into the primary motor area. This suggests that this subnucleus is the thalamic relay for the sensory input from peripheral receptors to cells of the motor cortex.
A branch of the dorsal interosseous nerve is described, which innervates the dorsal aspect of the wrist joint capsule of the cat. Recordings of the whole wrist joint nerve showed no tonic activity until the wrist was flexed to within 45 degrees of full flexion (about one third of the normal range). There was no tonic activity at full extension or at intermediate postions. 110 single afferents from the wrist joint capsule were isolated from dorsal root filaments. Their responses to movement of the wrist and other mechanical stimuli were examined, and their conduction velocities measured. 31 were slowly adapting, 41 were phasic, 9 were Pacinian corpuscle-like, 7 were weakly activated, and 22 were not activated by the mechanical stimuli which were used. Slowly adapting receptors responded only when the joint was in a flexed position as would be expected from the whole nerve data. Conduction velocities for the 4 classes of responding mechanoreceptors were not significantly different, and fell in the group II range. Fibre diameter histograms of myelinated axons in the wrist joint nerve showed peaks in the group II and group III range. Intravenous injection of succinylcholine showed no increase in whole nerve activity attributable to muscle spindles, and had a negligible effect on identified receptors in the wrist joint capsule.
The properties of some receptors with afferent fibres in the cat's posterior knee joint nerve have been examined, especially those discharging tonically with the joint in intermediate positions between full flexion and extension. Some of these receptors behave like muscle spindles, and respond to manoeuvres which stretch popliteus muscle. Both in single unit and whole nerve recordings their discharge pauses during a popliteus twitch, and can be strikingly augmented by tetanic stimulation of a number of popliteus fusimotor fibres isolated from ventral root filaments. The action of succinylcholine on these receptors closely resembles its effect on popliteus spindle units with fibres sited normally in the popliteus nerve. Other units with properties suggesting origin from popliteus tendon organs were also observed; their fibres and those of the spindle units conducted at Group I velocity. It is concluded that some afferent fibres from popliteus spindles and possibly tendon organs commonly pursue an aberrant course in the posterior articular nerve of the knee joint.
1. The identity of the receptors and afferent nerve fibres that mediate the sense of touch varies somewhat with body location. Those that have been most intensively characterized are associated with the distal glabrous skin of the limbs and, in primates, mediate the sense of touch in the fingertips and palms. In this glabrous skin region, there appear to be three or four principal classes of tactile sensory nerves that fall into two broad groups. One group, the so-called slowly adapting (SA) receptors and afferent fibres, is responsive to static mechanical displacement of skin tissues and is made up of two classes, the type I (SAI) fibres that innervate Merkel receptors and the type II (SAII) fibres that innervate Ruffini endings. The second broad group displays a pure dynamic sensitivity to tactile stimuli and also falls into two principal classes, the rapidly adapting (RA) tactile fibres that are associated with Meissner corpuscle receptors and the Pacinian corpuscle (PC)-associated class of tactile afferent fibres. 2. In other regions of the skin, such as the hairy skin of the arms, legs and trunk, there are similar functional classes of tactile sensory nerves, although the receptor endings differ somewhat from those of the glabrous skin. 3. Receptors in close association with the long bones of the limbs include groups of Pacinian corpuscles distributed along the interosseous membranes. These are highly sensitive to dynamic forms of mechanical stimuli, in particular vibrotactile disturbances. However, despite their close association with bone, these receptors probably cannot be legitimately considered 'osseoreceptors'. 4. Both the periosteum and the bone marrow are richly supplied by nerve fibres. However, much evidence indicates that these are largely or entirely in the fine-diameter category of nerve fibres, whose roles may be confined to either nociception or to the efferent autonomic regulation of bone-associated blood vessels. 5. In conclusion, it remains uncertain whether any aspects of our innocuous touch or kinaesthetic senses, in either the limbs or in orofacial regions, can be ascribed to 'osseoreceptors' located in the periosteum or within the bone marrow itself.