Library

Notes: To study gene expression in differentiated adult motoneuron subtypes, we used fluorescent dextrans for both anterograde and retrograde axonal tracing in adult rat and mouse. Application of these dyes to the cut distal and proximal ends of small extramuscular nerve branches revealed both the peripheral ramifications and the cell bodies of subsets of motoneurons. We show that the soleus muscle is innervated by two nerve branches, one of which contains gamma motor and sensory axons but no alpha motor axons. By retrograde tracing of this branch, we selectively labelled gamma motoneurons. In adult rat, the nerves innervating the soleus and extensor digitorum longus muscles contain almost exclusively axons innervating slow (type I) and fast (type 2) muscle fibres, respectively. We selectively labelled slow and fast type motoneurons by retrograde tracing of these nerves. With immunocytochemistry we show that adult motoneurons express several homeodomain genes that are associated with motoneuron differentiation during early embryonic development. Combining selective retrograde labelling with immunocytochemistry we compared the expression patterns in alpha and gamma motoneurons. The homeodomain transcription factors Islet 1 and HB9 were expressed in slow and fast alpha motoneurons and in soleus gamma motoneurons. Motoneurons in each population varied in their intensity of the immunostaining, but no factor or combination of factors was unique to any one population.

Notes: [Auszug] THE adult neuromuscular system continues to offer new and increasingly detailed information about synapse formation. Those interested in development may even trick the system into recapitulating synap-togenesis. All you need to do is to transplant a nerve on to a nonsynaptic region of another ...

Notes: [Auszug] The superficial fibular nerve, which normally innervates flexor muscles in the lower leg, was used as the foreign nerve in our experiments. It was dissected, cut peripherally, and transplanted into the proximal surface of the soleus muscle of young rats. Two weeks later the soleus was deprived of ...

Notes: [Auszug] Fig. 1. A, Generalized thalamic spindle. Rhythmic activity obtained by four microelectrodes (1-4), 1-5 mm apart, placed along a frontal line, and 7-25 mm under the dorsal surface of the thalamus. The vertical lines of the upper four traces indicate the spikes in the corresponding microelectrode ...

Notes: Abstract Neuromuscular junctions on fast and slow skeletal muscle fibers have different properties. Possible reasons for these differences were examined in adult rat soleus (SOL) muscle fibers reinnervated at new ectopic or old denervated sites by fast fibular (FIB) or slow SOL motoneurons. FIB motoneurons formed large ectopic junctions with a high density of nerve terminal varicosities (fast appearance), whereas SOL motoneurons formed small ectopic junctions with a low density of varicosities (slow appearance). Both FIB and SOL motoneurons formed small junctions with a slow appearance when reinnervating old SOL endplates. FIB nerves innervating ectopic sites and SOL nerves reinnervating old sites had the same appearance whether they contacted the SOL fibers alone (single innervation) or together (dual innervation). Continuous stimulation of the FIB nerve at 10 Hz for 3–4 months reduced the size of ectopic FIB and intact extensor digitorum longus (EDL) junctions and caused a modest reduction in density of terminal varicosities in EDL. Junction size and muscle fiber diameter were positively correlated, but the slope describing this relation was steeper for FIB junctions than for SOL junctions. It is concluded that in the present system (1) motoneuron type and not muscle fiber type determines the fast or slow character of the neuromuscular junction, (2) denervated endplates of one type place stable and severe constraints on the termination pattern of reinnervating axons of another type, (3) the appearance of fast EDL junctions undergoes a modest fast to slow transformation when exposed to long-term slow pattern stimulation, and (4) not only the size of the muscle fibers, but also the type and firing pattern of the motoneurons and the spatial constraints at preformed endplates influence the relation between junction size and muscle fiber diameter.

Notes: Summary 1. In rabbits, initially anaesthetized by urethane/chloralose and maintained on urethane alone, the perforant path, contacting the apical dendrites of the dentate granule cells by way of boutons en-passage, was activated by paired stimuli. The effect of the first conditioning stimulus was studied by recording the extracellular field response and the extra- and intracellular responses of single granule cells to a second test stimulus. 2. The monosynaptic test EPSP recorded extra- as well as intracellularly was potentiated by a preceding volley in the perforant path. The rate of rise and the amplitude of the extracellular test EPSP increased by as much as 100% at an optimal conditioning-test interval of about 25 msec. The total duration of the potentiation was 200–300 msec. Sometimes the potentiation was followed by a slight subnormal phase. 3. The EPSP potentiation was not due to a larger presynaptic test volley since (1) the size of the presynaptic fibre potential was not effected, and (2) removal of the entorhinal area did not reduce the effect. 4. The potentiation was not due to the recurrent inhibitory hyperpolarization of the postsynaptic membrane because (1) weak conditioning volleys potentiated the EPSPs without discharging the granule cells or producing any inhibition. (2) Conditioning antidromic volleys produced marked inhibition and IPSPs but had no effect on the test perforant path EPSP. 5. Different mechanisms that may be responsible for the EPSP potentiation are discussed. The potentiation is compared with that observed in other types of synapses. Potentiation of EPSPs may be an important mechanism behind the frequency potentiation of mass responses characteristic of the hippocampal formation.

Notes: Summary 1. In rabbits, anaesthetized with urethane/chloralose, stimulation with tungsten microelectrodes was employed to initiate a volley in the perforant path fibres which made en-passage contacts with the apical dendrites of dentate granule cells. The ensuing activation pattern was studied by recording the extracellular field potentials in the dentate area and the extra and intracellular responses of single granule cells. 2. The afferent perforant path volley appeared as a small triphasic potential followed after 0.8 msec by a monosynaptic wave, which was maximally negative in the middle third of the molecular layer, corresponding to the region of the perforant path synapses on the granule cell dendrites. The wave became abruptly positive in the inner third of the molecular layer. After removal of the CA1, reversal occurred also in the outer third of the molecular layer, indicating an active synaptic sink restricted to the middle third of the dendritic region. 3. A perforant path volley, propagating at a speed of about 3.3 m/sec, discharged granule cells lying in a horseshoeshaped segment of the dentate area. The dentate area is thus divided into a series of segments or lamellae by the perforant path input. Laterally, the perforant path fibres run through a bottle neck, deep to the angular bundle, before fanning out to enter the various segments of the dentate area of the dorsal hippocampus. 4. A stimulus applied to the lateral region of the angular bundle activated the perforant path directly as well as indirectly. The indirect activation was presumably mediated by commissural fibres to the entorhinal area from which the perforant path originates. 5. The negativity recorded extracellularly in the synaptic layer had the same onset as, but was phase advanced with respect to the intracellularly recorded EPSP. The monosynaptic extracellular negative wave was therefore interpreted as reflecting the synaptic current generating the intracellular EPSP and is termed the extracellular EPSP. 6. When the perforant path volley was sufficiently large, a compound spike became superimposed on the extracellular EPSP. The spike was maximally negative in the granule cell body layer and positive in the outer dendritic region. Occasionally, multiple single cell discharges could be recorded in the granular layer. These unitary discharges always coincided in time with the compound spike and their number parallelled the size of the compound spike. The latter, therefore, reflects the number of nearly synchronously discharged cells and is termed the granule cell population spike. 7. Typically, the granule cells discharged only once in response to a single perforant path volley. Subsequent discharges were blocked by inhibitory postsynaptic potentials, IPSPs. The postsynaptic excitatory response time varied between 5 msec and 1.4 msec depending on the size of the perforant path volley. A value of about 2 msec was most commonly observed in response to moderate or strong stimuli. 8. The granule cells were discharged by a perforant path volley of increasing size only after a considerable growth of the extracellular EPSP had taken place. Apparently, the discharge requires summation of a large number of relatively small individual EPSPs. This may be a mode of synaptic activation characteristic of pathways with numerous boutons en-passage making contact with spines of profusely branching dendrites.

Notes: Summary Following selective activation of four afferent paths that terminate exclusively on dendrites, only a small proportion of pyramidal cells in the hippocampal fields CA1 and CA3 discharged impulses. Following a single afferent volley, an EPSP was never observed even in cells synaptically excited. On tetanic stimulation (about 10/sec), a large EPSP developed, but this was not a prerequisite for an action potential. Studies of the extracellular field potentials corresponding to the EPSP and the population spike potential, indicated that the EPSP was generated across the dendritic membrane and that the spike was initiated in the neighbouring part of the dendritic tree, propagating from there along the thicker dendrites towards the soma. This conduction had an average velocity of 0.4m/sec, and, presumably, a relatively low safety factor. In certain cases, the intrasomatic electrode recorded small all-or-nothing spikes which presumably were generated in the dendritic tree. These small spikes (D-spikes) invaded the soma only if assisted by some additional depolarization, for example by frequency potentiation of excitatory synapses. The results indicate two functional types of pyramidal dendrites, the conducting and the synaptic type.