Threshold current for repetitive impulse firing in motoneurones innervating muscle fibres of different fatigue sensitivity in the cat

In anaesthetized cats, alpha motoneurones of m. gastrocnemius medialis were activated by maintained currents that were injected via an intracellular microelectrode. There was a statistically significant correlation between the threshold current for maintained repetitive firing and axonal conduction velocity. These findings confirmed that slow-axoned motoneurones tend to be more excitable than those with faster axons. Among fast-twitch motoneurones of about the same size as judged by their axonal conduction velocity, the average threshold current was about twice as high for cells innervating fatigue-sensitive muscle fibres (FF units) than for those supplying more fatigue-resistant ones (FR units).     

Sizes of soma and stem dendrites in intracellularly labelled α-motoneurones of the cat

The morphology of identified hindlimb motoneurones was studied after intracellular labelling with Procion yellow (59 cells), Procion red (19 cells) or horseradish peroxidase (9 cells). With respect to the measurements performed, all three intracellular labels gave similar results. As judged by their axonal conduction velocity (62–117 m/sec) all included cells were α-motoneurones. The motoneuronal cell bodies had cross-sectional areas of 816–3732 sq. μm, corresponding to diameters of about 32–69 μm. On average each neurone had 12 (5–20) dendritic stems. For all cells together, the number of dendritic stems per neurone was not strongly correlated to soma diameter. In the whole material, the dendritic stem diameters varied between about 0.5 and 19 μm. Stem dendrites of 4–5 μm were common in all kinds of cells, whereas thicker dendritic stems were preferentially distributed to cells with larger somas. The maximum as well as the mean stem-dendrite diameter (d) per cell was clearly correlated to, and roughly proportional to, the diameter of the cell body. The sum of the cross-sectional areas of all the dendritic stems emanating from a cell (‘sum of dendritic holes’= Σ πd²/4) was roughly proportional to the volume of the soma. Quadriceps motoneurones had a markedly greater number of dendritic stems per cell (mean 16.9) than other kinds of hindlimb motoneurones studied (mean 11.5; includes motoneurones of the hamstring muscles, triceps surae and intrinsic foot muscles). The many quadriceps dendrites were, however, also relatively thin, and the average ratio between ‘sum of dendritic holes’ and soma volume was the same for quadriceps motoneurones as for the other cells.     

Relationships between axonal diameter, soma size, and axonal conduction velocity of HRP-filled, pyramidal tract cells of awake cats

Relationships between axonal diameter, soma size, and axonal conduction velocity were examined in intracellularly recorded pyramidal tract (PT) cells of conscious cats using pressure injection of horseradish peroxidase. Positive linear correlations were found between axonal conduction velocities and axonal diameters as well as between axonal conduction velocities and soma sizes. All PT cells had somata located in layer V. Slow PT cells had high densities of dendritic spines in layer III; however, so did some fast PT cells, making this morphologic feature unacceptable for distinguishing between slow and fast conducting PT neurons.     

Rat optic nerve: electrophysiological, pharmacological and anatomical studies during development

Changes in conduction properties and in morphology were studied during rat optic nerve growth from birth (when no myelin is present and the glia have not differentiated) to adulthood (when the optic nerve is essentially 100% myelinated). Myelination begins around the sixth postnatal day and proceeds rapidly so that 85% of the fibers are myelinated at 28 days of age. Mean diameter of optic nerve axons remains about 0.2 micron for the first week and then increases rapidly if the fiber is being myelinated. Those axons not being myelinated remain about 0.2-0.3 micron in diameter. At birth the compound action potential has a single negative peak and a conduction velocity of about 0.2 m/s. The increase in conduction velocity prior to myelination is considerably greater than can be accounted for on the basis of increase in axonal diameter. There is no clear step increase in the velocity of the shortest latency peak correlated with the onset of myelination. During myelination the compound action potential develops multiple short latency components, which evolve into the adult-like 3 component compound action potential by 3-4 weeks of age. Durations of the relative refractory period and supernormal period decrease as age increases, but are not related to myelination in a simple manner. Sodium appears to be the only significant carrier of inward current at all ages. A measureable calcium conductance is not present at any age. Voltage-dependent potassium conductance contributes to the compound action potential at all ages, but the response to 4-aminopyridine in rapidly conducting fibers is apparently smaller than that in slowly conducting fibers. These results show that conduction can occur before myelination or the differentiation of glial cells. Moreover, changes in conduction velocity do not depend entirely on myelination or increases in axonal size. Finally, these results suggest a reorganization of axonal membrane properties during the development of rat optic nerve.     

Characterization of hindlimb motoneuron membrane properties in acute and chronic spinal cats

The purpose of this study was to determine if changes in hindlimb motoneuron membrane electrical properties occur 4–6 months after spinal transection in the adult animal. Eight acute and nine chronic animals were spinalized at T₁₂. Intracellular recordings from motoneurons innervating the triceps surae were performed. Membrane electrical properties, including resting potential, action potential peak amplitude, afterhyperpolarization duration, rheobasic current, input resistance and axonal conduction velocity were measured. There were no statistical differences found between group means or frequency distributions in the membrane properties of motoneurons assessed from acute and chronic spinal animals. Thus, alteration of motoneuron membrane properties does not appear to be a major contributing factor to the hyperexcitable hindlimb reflex activity demonstrated by chronic spinal animals.     

Morphology, Electrophysiology and Pathophysiology of Supragranular Neurons in Rat Primary Somatosensory Cortex

Intracellularly biocytin-labelled neurons in layers 11/111 of adult rat primary somatosensory cortex were analysed for their morphological and electrophysiological properties and studied for their response pattern to transient hypoxia under in vitro conditions. The largest dendritic region is formed by the basal dendrites, which constitute an average area of 0.06 mm² and which can receive synaptic inputs over horizontal distances of more than 300 μm. The dendritic territories formed by the oblique dendrites situated on the apical trunk and by the apical tuft are much smaller. The spine density is highest on the apical trunk, suggesting that large numbers of excitatory synapses are present in this region of the cell. All neurons revealed intrinsic membrane properties of typical regular spiking cells and received an excitatory and a strong biphasic inhibitory input. Whereas a significant correlation could be detected between the cell's input resistance and soma area, no correlation existed between the cell's total dendritic length and input resistance or membrane time constant/input resistance. Neurons responded to transient hypoxia either with an anoxic hyperpolarization with an apparent reversal potential of -82.4 mV, or with a gradual anoxic depolarization which reversed at -56 mV. Oxygen deprivation caused a significant reduction in the extent of axonal collaterals, whereas dendritic proportions and spine density were unaffected. The present study indicates that the dendritic tree is well preserved under in vitro conditions, whereas axonal connections are diminished by oxygen deprivation. Our results further suggest that certain structural properties correlate with the cellular physiology, but that the cell's morphology does not determine its responsiveness to hypoxia.     

Local axonal arborization patterns of distinct neuronal types in the caudal nucleus of the tractus solitarius

Neurons in the caudal nucleus of the tractus solitarius (cNTS) are quite heterogeneous in cell size (50 to 450 microm(2) in somal area) and other morphologic characteristics. For a more objective classification of cNTS neurons, their morphologic features were analyzed quantitatively based on reconstructed biocytin-filled cells after whole-cell patch-clamp recordings. According to the patterns of axonal branching behaviors, cNTS cells could be classified into two groups: smaller cells (94.1 microm(2) in mean somal area, range 62-120 microm(2), n = 22) and larger cells (245 microm(2) in mean somal area, range 142-411 microm(2), n = 23). Extensive axonal arborization with numerous possible synaptic boutons was specifically associated with smaller neurons, while larger cells possessed no or few axon collaterals, suggesting their distinct roles as local circuit neurons (or interneurons) and projection neurons, respectively. With regard to somatodendritic characteristics, the following correlations with cell size were found: smaller cells had larger form factors than larger cells (P < 0.05). Larger neurons had more extensive dendritic arborization, expressed by total dendritic length (P < 0.01) and number of dendritic branching points (P < 0.01), than smaller cells. It was suggested that small cNTS neurons contribute specifically to an integration of input information generated in the local circuits, while large neurons convey the integrated information to other autonomic brain regions.     

Soma size and oxidative enzyme activity in normal and chronically stimulated motoneurones of the Cat''s spinal cord

In normal adult cats we measured the density of staining for the activity of succinate dehydrogenase (SDH staining) in ventral horn cells of different sizes. The measurements were restricted to that part of the lumbar ventral horn (L6-L7) which is known to contain motoneurones of the peroneal nerve. A statistically significant tendency was found for the SDH staining to be denser in smaller than in larger neurones within the size range of a motoneurones (soma diameter greater than 40 microns). These results are consistent with recently published evidence for ventral horn cells of rats and qualitatively similar relationships between size and SDH staining have also been observed among skeletal muscle fibres (confirmed for mixed muscle of cat in present study). In hindlimb muscles, size as well as SDH staining are known to be markedly activity-dependent. We tested whether this is the case for peroneal motoneurones as well by analyzing the effects of chronic nerve stimulation on the properties of neurones within the appropriate region of the ventral horn. Prior to the final acute experiment, these cats had been subjected to a left-side dorsal rhizotomy and hemispinalization. By aid of a portable mini-stimulator, the left-side common peroneal nerve was activated by repetitive pulses during 50% of total time per day (intra-activity rate: 10, 20 or 40 Hz). After 8 weeks of such treatment, cell sizes as well as the densities of SDH staining showed hardly any differences between peroneal ventral horn cells of the experimental and control sides of the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional organization of the spinal reflex pathways from forelimb afferents to hindlimb motoneurones in the cat. II. Conditions of the interneuronal connections

The interneuronal conditions of the descending pathways from forelimb afferents to hindlimb motoneurones were investigated by testing spatial interactions in these pathways and between these pathways and segmental lumbar reflex pathways. In high spinal unanaesthetized cats hindlimb motoneurones were intracellularly recorded and spatial interactions were tested between effects evoked by stimulation of pairs of ipsi- and contralateral forelimb nerves or pairs of a forelimb and an ipsilateral hindlimb nerve. The excitatory and late inhibitory pathways from forelimb afferents projecting to most of the hindlimb motoneurone pools, showed an interactive pattern which was distinctly different to the fast inhibitory pathway projecting specifically from ipsilateral forelimb afferents to flexor digitorum and hallucis longus (FDHL) motoneurones. Stimulation of homonymous or heteronymous pairs of two forelimb nerves of both sides evoked generally a distinct spatial facilitation of the excitatory and late inhibitory effects, while the specific early IPSPs to FDHL motoneurones were not facilitated. Paired stimulation of two forelimb nerves of one side only produced spatial facilitation of EPSPs or late IPSPs if low strength stimuli were used, using higher strength which induced larger effects, generally caused occlusion instead. In case of large IPSPs this may be due to the vicinity to the equilibrium potential. Except for an inhibition of cutaneous reflex pathways, the spatial interaction of the excitatory and late inhibitory pathways onto segmental lumbar reflex pathways was weak and variable. The fast inhibitory pathway to FDHL motoneurones showed a partial spatial facilitatory interaction with lumbar reflex pathways from cutaneous and group II muscle afferents. The second IPSP wave evoked by this pathway was inhibited by antidromic stimulation of the ventral root L7S1 and of the alpha-efferents of the antagonistic peroneal nerve. From the results conclusions are drawn on the interneuronal organization of the descending pathways from forelimb afferents to hindlimb motoneurones.     

Neuropathy in the spontaneously hypertensive rat: An electrophysiological and histological study

Introduction: Hypertension is identified as a risk factor for development of polyneuropathy. In this study we examined nerve conduction and morphological alteration of peripheral nerves in spontaneously hypertensive rats (SHR). Methods: Motor nerve conduction velocity (MNCV) in the sciatic–tibial nerve and sensory nerve conduction velocity (SNCV) in the sural nerve were measured. Pathological investigations included spinal cord, dorsal root ganglion, and hindlimb nerves in SHR and Wistar–Kyoto rats (WKY) aged 4–64 weeks. Results: Blood pressure was significantly higher in SHR than WKY animals at 4 weeks and elevated further with aging. MNCV and SNCV were significantly slower in SHR compared with WKY after age 24 weeks. Prominent morphological changes in SHR nerves included axonal atrophy and myelin splitting. SHR also had endoneurial microangiopathy with reduplication of basement membrane. Conclusions: SHR showed slowed nerve conduction velocity and pathological abnormalities of hindlimb nerves. Sustained severe hypertension may cause axonal atrophy and endoneurial microangiopathy. Muscle Nerve 54 : 756–762, 2016     

Visibility of synaptically induced conductance changes: theory and simulations of anatomically characterized cortical pyramidal cells

A recent report has provided evidence that there are no significant increases in the neuronal input conductance during the response of cortical cells in cat visual cortex to non-preferred visual stimuli (Douglas et al., 1988). A criticism of experiments of this kind is that changes in the membrane conductance occurring in the dendritic tree may not be visible from electrodes that impale the soma. Our paper describes theoretical and numerical results concerning the visibility of synaptically induced conductance changes from intracellular electrodes, in both ideal and anatomically well-characterized cortical neurons. Based on earlier work by Rall (1967), we here derive theoretical expressions for the change in input conductance at any location in a passive dendritic tree resulting from activation of a single synapse and obtain bounds for the effects of multiple synapses. We find that the conductance change measured at the cell body is always less than the sum of the synaptic conductance changes and that this observed conductance change does not depend on the synaptic reversal potential. For the case of an infinite dendritic cylinder, the change in input resistance due to a single synaptic input decays exponentially with distance of the synapse from the recording site. Numerical simulations of synaptic inputs that change approximately as fast as the membrane time-constant produce an increase in input conductance that is only slightly less visible than that of a constant input. We also compute the changes in somatic input conductance of 2 morphologically identified pyramidal cells from cat visual cortex during activity of a single inhibitory basket cell with known synaptic input locations. We find that the increase in conductance due to the activity of the inhibitory basket cells is clearly visible from the cell body of the pyramidal cells and that a 70% reduction in the amplitude of excitation is associated with at least a 30% increase in somatic input conductance, which would be visible in intracellular recordings. Taken together with the negative experimental evidence of Douglas et al. (1988), our results cast doubt on a large class of models of direction selectivity that rely on synaptically mediated inhibitory conductance increases to veto or block excitatory conductances increases.     

The cause of slowed forearm median conduction velocity in carpal tunnel syndrome.

OBJECTIVES: Attempting to answer a debate concerning the etiopathogenesis of the decreased forearm median motor conduction velocity (FMMCV), we tried to use proximal stimulation at the wrist, elbow, mid-arm and axillary regions to determine segmental median motor conduction velocity (MMCV). We also correlated the FMMCV with median motor distal latency (MMDL) and compound muscle action potential (CMAP) amplitudes of the abductor pollicis brevis (APB) muscle in order to assess whether the conduction block of large myelinating fibers or retrograde axonal atrophy was the major cause of the decreased FMMCV. BACKGROUND: The cause of the decreased FMMCV resulting from either the conduction block of the large myelinating fibers at the wrist or distal compression with retrograde axonal atrophy remains an unresolved issue at the moment. Animal models have supported the hypothesis that the retrograde axonal atrophy might also occur in humans. Other authors believe the standard FMMCV is calculated by subtracting the distal latency which may not represent an exact assessment of FMMCV but rather the velocity of small fibers that persist through the carpal tunnel. SUBJECTS AND METHODS: Patients with the clinical symptoms and signs of carpal tunnel syndrome (CTS) confirmed using standard electrodiagnosis were included. The patients were arbitrarily divided into two groups based on the FMMCV, one with reduced FMMCV (n = 20, FMMCV < 50 m/s) and the other with normal FMMCV (n = 35, FMMCV> or =50 m/s). Age-matched volunteers served as controls. We explored motor conduction proximally at wrist, elbow, mid-arm and axillary stimulation, and recorded at the APB muscles. Based on the latency differences, we calculated the FMMCV, distal arm MMCV (DAMMCV) and proximal arm MMCV (PAMMCV), and compared the conduction velocity (CV) differences of DAMMCV-FMMCV, PAMMCV-FMMCV and PAMMCV-DAMMCV in the two patient groups and the control. Furthermore, we correlated FMMCV with MMDL and CMAP amplitudes of APB muscle because MMDL and CMAP amplitudes might reflect the integrity of the large myelinating fibers. RESULTS: CMAP amplitudes of APB muscle at wrist stimulation and MMDL were not correlated with FMMCV in either of the two patient groups; however, the CMAP amplitude was markedly decreased and MMDL was significantly prolonged when compared with normal controls. The significant increase of CV gradient of DAMMCV-FMMCV and PAMMCV-FMMCV without an equal increase of CV gradient of PAMMCV-DAMMCV only occurred in the reduced FMMCV patient group, suggesting that the conduction block is not the primary cause. The CV gradient of DAMMCV-FMMCV and PAMMCV-DAMMCV did not show any significant difference between patients with the normal FMMCV and the control group. CONCLUSION: The retrograde axonal atrophy, not selective damage of the large fibers at the wrist, was the direct cause of the decreased FMMCV.     

Motor unit function during evolution of proximal axonal swellings.

beta,beta'-Iminodipropionitrile (IDPN) impairs axonal transport of neurofilaments; their accumulation leads to the formation of proximal swellings in motor axons. Similar proximal swellings are a feature of some cases of motor neuron disease such as amyotrophic lateral sclerosis (ALS). Motor units in IDPN-treated animals were assessed to determine their relative susceptibilities to impaired function and whether the functional changes resulting from proximal axonal swellings share certain electromyographic features with ALS. Intrinsic properties of medial gastrocnemius motoneurones (MN) and contractile responses of their motor units were examined during the evolution of proximal axonal swellings in cats administered IDPN (50 mg/kg once weekly) for 7, 14 or 35 days. While conduction velocities were significantly decreased in all motor unit types by 35 days, the conduction slowing was greater in fast fatigable (types FF and FI) motor units than in fatigue resistant (types FR and S) motor units. Normal correlations between axonal conduction velocity and MN input resistance (Rin) and the inverse relationship between Rin and rheobase were lost with progression of the neuropathy. Twitch and maximum tetanic tension developed by fast-fatigable motor units declined early in the neuropathy, whereas fatigue-resistant units did not show similar changes until later stages of the intoxication. In some motor units, irregular and abnormal tetanic tensions were elicited by repetitive MN discharge. At 14 and 35 days, a novel, intermediate motor unit response classified as slow and fatigable (SF) was observed. Conduction block, characterized by repetitive MN firing without a corresponding contractile response, was observed in some type FF and S units by 35 days. Morphometric analysis of muscle fiber types showed significant atrophy, particularly in the type I fibers at 14-35 days; the atrophy reversed following cessation of IDPN administration. The influence of proximal axonal swellings on motor unit function in IDPN neuropathy is discussed in terms of reported electrophysiological alterations in motoneurone disease.     

Ultrastructure of Golgi-impregnated and gold-toned neurons in the central nucleus of the inferior colliculus in the cat.

The combined Golgi/electron microscope technique was used to analyse the cytoarchitecture and the fine structure of the central nucleus of the inferior colliculus in the cat. The analysis of Golgi-impregnated sections discriminates three major neuronal types, according to somato-dendritic morphology, and to presence or absence of dendritic spines. Two major types (spiny and aspiny) might be further subdivided into large, medium-large, medium-small and small, whilst the third ("mixed") type is subdivided into large and medium-large types. The large, medium-large and medium-small cells of the major types appear to be efferent (relay) neurons, whilst the small spiny, and especially the small aspiny cells are interneurons. In agreement with previous data, the subdivision of the relay neurons in disc-shaped and stellate cells is confirmed but the disc-shaped neurons are further subdivided in typical and atypical. The dendritic fields of the latter neurons correspond greatly but not exclusively to the fibrodendritic laminae of the central nucleus. In addition to the axonal ramification of the local circuit neurons, the axons of most (if not all) types of relay cells emit a moderate to scant, rarely--a substantial number of collaterals. The collaterals of the large spiny neurons (atypical disc-shaped cells) occasionally innervate also the cell of origin. Parallel to the light microscopic discriminations of the different neuronal types, the electron microscopic observations confirm that the ultrastructural characteristics might be very distinct. Especially evident are the differences between the large neuronal types, concerning the amount and arrangement of the granular endoplasmic reticulum, and the mode of the perikaryal, dendritic, and axonal innervation by various synaptic bouton types. Along with the unequivocal discrimination of the neurons in the central nucleus of the inferior colliculus according to the dendritic orientation, we suggest also a more detailed classification of the neuronal types according to the perikaryal size, fine dendritic morphology, and ultrastructural characteristics. Further hodological experiments, combined with the presently explored technique, will help to clarify the complicated synaptic events in the central nucleus of the inferior colliculus.     

Morphology and physiology of the polyaxonal amacrine cells in the rabbit retina.

We examined the morphology and physiological response properties of the axon-bearing, long-range amacrine cells in the rabbit retina. These so-called polyaxonal amacrine cells all displayed two distinct systems of processes: (1) a dendritic field composed of highly branched and relatively thick processes and (2) a more extended, often sparsely branched axonal arbor derived from multiple thin axons emitted from the soma or dendritic branches. However, we distinguished six morphological types of polyaxonal cells based on differences in the fine details of their soma/dendritic/axonal architecture, level of stratification within the inner plexiform layer (IPL), and tracer coupling patterns. These morphological types also showed clear differences in their light-evoked response activity. Three of the polyaxonal amacrine cell types showed on-off responses, whereas the remaining cells showed on-center responses; we did not encounter polyaxonal cells with off-center physiology. Polyaxonal cells respected the on/off sublamination scheme in that on-off cells maintained dendritic/axonal processes in both sublamina a and b of the IPL, whereas processes of on-center cells were restricted to sublamina b. All polyaxonal amacrine cell types displayed large somatic action potentials, but we found no evidence for low-amplitude dendritic spikes that have been reported for other classes of amacrine cell. The center-receptive fields of the polyaxonal cells were comparable to the diameter of their respective dendritic arbors and, thus, were significantly smaller than their extensive axonal fields. This correspondence between receptive and dendritic field size was seen even for cells showing extensive homotypic and/or heterotypic tracer coupling to neighboring neurons. These data suggest that all polyaxonal amacrine cells are polarized functionally into receptive dendritic and transmitting axonal zones.     

Functional organization of long ascending propriospinal pathways linking lumbo-sacral and cervical segments in the cat

In high spinal cats propriospinal pathways ascending from lumbo-sacral levels of the spinal cord can mediate strong excitatory and inhibitory changes in reflexes to different groups of motoneurones supplying muscles of the forelimb. Discharges evoked by electrical stimulation of hindlimb nerves could be evoked in 41% of experiments in the motoneurones of pectoralis major and minor. The latency of the discharge (8–18 msec) could be shortened by increasing the repetition frequency of the stimulus, the greatest reduction occurring in the range 1–4 Hz. Contralateral hindlimb nerves were less effective and the discharge generally occurred at a latency 1–2 msec longer than for ipsilateral nerves.Monosynaptic reflexes to pectoralis major and deep radial motoneurones supplying the physiological flexor muscles were strongly facilitated by hindlimb nerve stimulation, ipsilateral nerves being more effective than contralateral. Monosynaptic reflexes to latissimus dorsi showed a reciprocal pattern of conditioning, being depressed by ipsilateral and facilitated by contralateral hindlimb extensor nerves, the flexor nerves giving the reverse pattern. Monosynaptic reflexes to median and ulnar nerves supplying physiological extensor muscles were not significantly affected by hindlimb nerve conditioning.Polysynaptic reflexes to pectoralis major and deep radial motoneurones received initial strong facilitation followed by prolonged depression, ipsilateral hindlimb nerves being more effective than contralateral. In latissimus dorsi a reciprocal pattern similar to that for monosynaptic reflex testing was found. Polysynaptic reflexes to median and ulnar motoneurones received only prolonged depression.The hindlimb afferent nerves responsible for the discharge in forelimb motoneurones and for the facilitation and depression of forelimb reflexes include groups II and II muscle afferents and group II skin afferents, especially from quadriceps and sartorius muscles, and sural and superficial peroneal nerves, respectively.The ascending long propriospinal pathways are influenced bilaterally from hindlimb nerves and are located in the lower thoracic segments in the ventrolateral funiculus. The pathways mediate effects on ipsilateral and contralateral forelimb reflex systems, the ipsilateral projections being dominant. Part of the long ascending projection terminates monosynaptically on the motoneurones of pectoralis major. It is likely that group II afferents from ipsilateral quadriceps muscle activate the ascending tract monosynaptically and those from contralateral quadriceps disynaptically.The hypothesis is suggested that long propriospinal paths primarily represent intrinsic links between hindlimb and forelimb ‘motor centres’. The pattern of long ascending influences to groups of forelimb motoneurones corresponds closely to the sequences of hindlimb and forelimb stepping observed in normal cats. A functional role in stepping is therefore proposed for long ascending propriospinal pathways.     

Immunocytochemical analysis of basket cells in rat piriform cortex

Basket cells, defined by axons that preferentially contact cell bodies, were studied in rat piriform (olfactory) cortex with antisera to gamma-aminobutyric acid (GABA)ergic markers (GABA, glutamate decarboxylase) and to peptides and calcium binding proteins that are expressed by basket cells. Detailed visualization of dendritic and axonal arbors was obtained by silver-gold enhancement of staining for vasoactive intestinal peptide (VIP), cholecystokinin (CCK), parvalbumin, and calbindin. Neuronal features were placed into five categories: soma-dendritic and axonal morphologies, laminar distributions of dendritic and axonal processes, and molecular phenotype. Although comparatively few forms were distinguished within each category, a highly varied co-expression of features from different categories produced a "combinatorial explosion" in the characteristics of individual neurons. Findings of particular functional interest include: dendritic distributions suggesting that somatic inhibition is mediated by feedforward as well as feedback pathways, axonal variations suggesting a differential shaping of the temporal aspects of somatic inhibition from different basket cells, evidence that different principal cell populations receive input from different combinations of basket cells, and a close association between axonal morphology and molecular phenotype. A finding of practical importance is that light microscopic measurements of boutons were diagnostic for the molecular phenotype and certain morphological attributes of basket cells. It is argued that the diversity in basket cell form in the piriform cortex, as in other areas of the cerebral cortex, reflects requirements for large numbers of specifically tailored inhibitory processes for optimal operation that cannot be met by a small number of rigidly defined neuronal populations.     

Electrophysiological and morphological measurements in cat gastrocnemius and soleus α-motoneurones

Intracellular recording and staining with HRP was used to study the electrical properties and anatomical size of medial gastrocnemius (MG) and soleus (SOL) alpha-motoneurones in curarized cats. The MG motoneurones were divided into two groups on the basis of their input resistance (RN), namely low-resistance MG-LR cells (RN less than 1.0 M omega) and high-resistance MG-HR cells (RN greater than 1.0 M omega). Analysis of the voltage transients following applied current pulses indicated that the SOL neurones had longer membrane time constants (tau o) than the MG-LR cells, while the MG-HR group exhibited intermediate values. Using Rall's equivalent cylinder model, a difference in specific membrane resistivity (Rm) between the MG-LR (low Rm) and SOL (high Rm) cells was obtained. This difference was observed also in neurones of similar anatomical size, and was consistent with the observed difference in tau o. In two neurones Rm was in addition calculated directly from anatomy and input resistance according to the general solution for a continuous neurone model with arbitrary geometry given by Rall. The latter method was found to yield significantly lower values for Rm, although the observed difference between the neurone types remained similar. Also the values for electrotonic length (L) were found to differ considerably between the calculations based on voltage transient analysis and those obtained from combined physiological and anatomical measurements. The observed variations in results are discussed in relation to possible sources of error in the experimental techniques and/or in the theoretical assumptions, particularly that of Rm being uniform over the entire soma-dendritic membrane. It is suggested that Rm might be larger in the dendritic regions than in the soma. A crude approximation of the dendrite to soma conductance ratio (Q) indicated that most cells (80%) had Q greater than 5.     

Functional organization of the spinal reflex pathways from forelimb afferents to hindlimb motoneurons in the cat. II. Conditions of the interneuronal connections

The interneuronal conditionsof the descending pathways from forelimb afferents to hindlimb motoneurones were investigated by testing spatial interactions in these pathways and between these pathways and segmental lumbar reflex pathways. In high spinal unanaesthetized cats hindlimb motoneuroneswere intracellularly recorded and spatial interactions were tested between effects evoked by stimulation of pairs of ipsi- and contralateral forelimb nerves or pairs of a forelimb and an ipsilateral hindlimb nerve. The excitatory and late inhibitory pathways from forelimb afferents projecting to most of the hindlimb motoneurone pools, showed an interactive pattern which was distinctly different to the fast inhibitory pathway projecting specifically for ipsilateral forelimb afferents to flexor digitorum and hallucis longus (FDHL) motoneurones. Stimulation of homonymous or heteronymous pairs of two forelimb nerves of both sides evoked generally a distinct spatial facilitation of the excitatory and late inhibitory effects, while the specific early IPSPs to FDHL motoneurones were not facilitated. Paired stimulation of two forelimb nerves of one side only produced spatial facilitation of EPSPs or late IPSPs if low strength stimuli were used, using higher strength which induced larger effects, generally caused occlusion instead. In case of large IPSPs this may be due to the vicinity to the equilibrium potential. Except for an inhibition of cutaneous reflex pathways, the spatial interaction of the excitatory and late inhibitory pathways onto segmental lumbar reflex pathways was weak and variable. The fast inhibitory pathway to FDHL motoneurone showed a partial spatial facilitatory interaction with lumbar reflex pathways from cutaneous and group II muscle afferents. The second IPSP wave evoked by this pathway was inhibited by antidromic stimulation of the ventral root L7S1 and of the α-efferents of the antagonistic peroneal nerve. From the results conclusions are drawn on the interneuronal organization of the descending pathways from forelimb afferents to hindlimb motoneurones.     

F-wave studies on the deep peroneal nerve: Part 1. Control subjects

An electrophysiological method is described for estimating the conduction velocity in the proximal segments (anterior horn cells to the knee) of the motor fibres of the deep peroneal nerve. The method utilizes large numbers of F-waves which are late muscle responses due to antidromically activated motoneurones. The results in 58 healthy subjects are presented. A detailed literature review concerning the nature of F-wave is presented.