Small Excitatory Junctional Potentials Research Articles

Calcium dependency of ω-AGA IVA effects on large and small excitatory junctional potentials and ultrastructures of nerve terminals producing these potentials

Calcium dependency of ω-AGA IVA effects on large and small excitatory junctional potentials and ultrastructures of nerve terminals producing these potentials

Two excitatory motoneurons differ in quantal content of their junctional potentials in abdominal muscle fibers of the cricket, Gryllus bimaculatus

In abdominal muscles 202 and 203 of the cricket, Gryllus bimaculatus, large and small excitatory junctional potentials (l- and s-EJPs) with similar durations can be recorded from the same muscle fibers. At the normal extracellular calcium ion concentration ([Ca 2+] o) of 5 mM, the amplitudes of l-EJPs in both muscles were larger than the threshold membrane potential for muscle action potentials, which is about −40 mV. Below 0.75 mM [Ca 2+] o, the amplitudes became much smaller and were below the firing level for the action potentials. At 0.5 mM, they fluctuated and decreased to 10.3 and 1.9 mV in muscles 202 and 203, respectively, and at 0.25 mM frequent failures occurred. The amplitudes of s-EJPs at 5 mM [Ca 2+] o were 13.3 and 5.1 mV in muscles 202 and 203, respectively, and the fluctuating amplitudes were far below the threshold for muscle action potentials. Below 0.75 mM, s-EJPs were rarely observed. The relation between log(EJP amplitude) and log([Ca 2+] o) was linear within a certain range of [Ca 2+] o and the slopes of the lines for l-EJPs were about twice as steep as those for s-EJPs in both muscles. In muscle 202, the amplitude distribution of l-EJPs obtained at 0.25 mM and that of s-EJPs at 0.75 mM both showed peaks at once and twice the voltage at the first peak, which were coincident with the voltages at the peaks of amplitude distributions of miniature EJPs recorded simultaneously. The reversal potentials for l- and s-EJPs in muscle 202 were +1.02 and +0.22 mV, respectively. In muscle 202, the decreases in amplitude of both EJPs by l-glutamate were similar and concentration-dependent. The results suggest that the difference in amplitude between l- and s-EJPs is attributable mainly to the difference in quantal contents.

Physiological and Immunocytochemical Determination of the Neurotransmitter at Cricket Neuromuscular Junctions

Abstract An attempt was made to determine electrophysiologically and immunocytochemically the neurotransmitter at neuromuscular junctions (NMJs) in cricket abdominal muscles. One mM bath-applied Lglutamate reduced the amplitudes of large and small excitatory junctional potentials (I- and s-EJPs) reversibly by about 75% of the control on the average. It also produced a slow, transient depolarization of about 10 mV. Joro spider toxin, which is an antagonist of L-glutamate, depressed the amplitudes of I- and s-EJPs almost completely and irreversibly at 3.5 × 10−6 M. By using the antibody to glutamate, glutamate-immunoreactive processes whose configuration resembled that of NMJs revealed by nickel staining were obtained. The present results strongly suggest that the neurotransmitter at cricket NMJs is L-glutamate.

226 Physiological differences between two types of presynaptic terminals respectively generating large and small excitatory junctional potentials

In this paper, the effect of calcium channel blockers on acetylcholine release induced by tityustoxin and ouabain in rat brain cortical slices is described. Cadmium, a non-specific blocker of calcium channels, inhibited the release of ACh induced by tityustoxin. L-type calcium channel blockers (verapamil, nifedipine and diltiazen) had no effect on the release of ACh induced by tityustoxin. The release of ACh was also unaffected by nickel, a T-type calcium channel blocker, and the conotoxins GVIA and MVIIC, blockers of N and Q-type calcium channels. Agatoxin IVA, a specific blocker of the P-type calcium channel, inhibited the release of ACh induced by tityustoxin by 50%. The spontaneous release of ACh as well as ouabain-induced release of ACh was unaffected by any of the calcium channel blockers studied. It is concluded that ACh release induced by tityustoxin is mediated by Ca2+ influx via P-type calcium channels.

Motoneuron reinnervation of phasic uropod muscles in crayfish

Motoneuron reinnervation of the lateral abductor and adductor muscles in the exopodite of the crayfish uropod was obtained by cross-tying the cut proximal ends of one set of uropod nerve roots to the cut distal ends of their contralateral homologues. In normal (nonsurgically treated) animals the abductor was innervated by two excitatory motoneurons and an inhibitor while two excitors innervated the adductor. For each muscle one of the excitors produced large excitatory junction potentials (EJPs) while the other evoked small EJPs. When stimulated repetitively only the smaller EJP in the adductor generated a facilitating response. Within less than 10 weeks postsurgery the muscles were each reinnervated by two excitatory motoneurons. While the abductor motoneurons generated synaptic potentials with similar amplitudes and time courses to those of normal animals, they differed from those in normal animals in that they facilitated when stimulated repetitively. In contrast to the large and small EJPs evoked in the normal animal, the two motoneurons that reinnervated the adductor muscle elicited similar amplitude EJPs, neither of which facilitated in response to repetitive stimulation.

Structure and innervation of longitudinal and transverse abdominal muscles of the cricket, Gryllus bimaculatus.

Detailed morphological and physiological studies on the insect abdominal muscles, including their innervation and neuromuscular transmission, are essential for understanding their important role in respiratory movements. There are both longitudinal and transverse muscles in the ventral abdominal segments of the cricket, Gryllus bimaculatus. Muscle 202 was selected as an example of a longitudinal muscle. This muscle is, on average, 1.4 mm long, paired on both sides of the abdomen, and consists of 127 fibers whose mean maximum diameter is 32 microns; the average sarcomere length is 8.1 microns. It is innervated by two ipsilateral motoneurons in the second abdominal ganglion, the axons of which run in the ipsilateral first nerve root of the third abdominal ganglion. Two motor axons run in parallel from the two cell bodies and innervate in close proximity. Accordingly, large and small excitatory junctional potentials (EJPs) are recorded from the same fiber with slightly different thresholds when the first nerve root of the third abdominal ganglion is stimulated. Muscle 203, which is a transverse muscle that extends across the fifth abdominal sternum and is located over the fourth abdominal ganglion and muscle 202 on both sides, is, on average, 2.9 mm long and consists of 86 fibers with a maximum diameter of 33 microns. The average sarcomere length is 7.9 microns. The right or left half of the muscle is innervated mainly by a contralateral motoneuron in the third abdominal ganglion through the ipsilateral first nerve root of the third abdominal ganglion. Nerve branches of the first nerve root also reach muscles 188 and 218. Muscle 203 is additionally innervated by the first nerve roots of abdominal ganglia 1, 2, and 4. These innervations were ascertained both electrophysiologically and histologically. Individual muscle fibers of muscle 203 produced small EJPs in response to stimulation of the first nerve roots of abdominal ganglia 2, 3, and 4 and large EJPs in response to stimulation of the root from the first abdominal ganglion. The large and small EJPs in muscle 203 have properties similar to those in muscle 202.

Examination of the properties of large and small excitatory junctional potentials recorded from the same muscle fibers from the viewpoint of transmitter release

Examination of the properties of large and small excitatory junctional potentials recorded from the same muscle fibers from the viewpoint of transmitter release

Responses to Ca2+ of two types of excitatory junctional potentials in cricket muscles

Cricket (Gryllus bimaculatus) abdominal muscle 202 produced both small and large excitatory junctional potentials (EJPs) depending on the intensity of the stimulation to the nerve. Abdominal muscle 203 generated only small EJPs. The large EJPs, the threshold of which was always slightly higher than the small EJPs, were more sensitive to the Ca2+ concentration at which they were evoked and to concentration changes. There were no marked differences between the two muscles regarding the electrical properties of the muscle fibre membrane and the quantal size of the transmitter. The causes of the differences between the two types of EJPs in these muscles are discussed.

216 Dependency on external Ca2+ of large and small excitatory junctional potentials in insect abdominal muscles

216 Dependency on external Ca2+ of large and small excitatory junctional potentials in insect abdominal muscles

Patterns of electrical activity and neural responses in canine proximal duodenum

The patterns of electrical activity and neural inputs to the proximal duodenum between the pyloric sphincter and the sphincter of Oddi were studied in muscles of the dog. Smooth muscle cells in the most proximal region were electrically quiescent, but slow waves were recorded in all regions distal to the first few millimeters. Electrical activity was recorded from circular muscle cells near the myenteric and submucosal surfaces of the circular layer, and slow wave activity was similar in both regions. The nature of neural inputs was also characterized. With electrical field stimulation, responses in cells near the submucosal surface were predominantly excitatory junction potentials (EJPs); near the myenteric border responses were either inhibitory junction potentials (IJPs) or biphasic responses (i.e., small EJPs followed by IJPs). EJPs were blocked by atropine. IJPs were nonadrenergic and noncholinergic (NANC), and several experiments suggested that nitric oxide (NO), or a NO-releasing compound, serves as the inhibitory neurotransmitter in this region. Exogenous NO caused hyperpolarization of membrane potential. IJPs and the hyperpolarization response to NO were sensitive to apamin. These data describe the myogenic mechanisms and neurogenic apparatus that appear to regulate motility in the most proximal region of the duodenum.

Presynaptic spike broadening reduces junctional potential amplitude

Presynaptic modulation of action potential duration may regulate synaptic transmission in both vertebrates and invertebrates. Such synaptic plasticity is brought about by modifications to membrane currents at presynaptic release sites, which, in turn, lead to changes in the concentration of cytosolic calcium available for mediating transmitter release. The 'primitive' neuromuscular junction of the jellyfish Polyorchis penicillatus is a useful model of presynaptic modulation. In this study, we show that the durations of action potentials in the motor neurons of this jellyfish are negatively correlated with the amplitude of excitatory junctional potentials. We present data from in vitro voltage-clamp experiments showing that short duration voltage spikes, which elicit large excitatory junctional potentials in vivo, produce larger and briefer calcium currents than do long duration action potentials, which elicit small excitatory junctional potentials.

Actions of neuropeptide Y on innervated and denervated rat tail arteries.

1. Neuropeptide Y caused a dose-dependent contraction and depolarization of the smooth muscle of the rat tail artery. 2. 30 nM-neuropeptide Y increased the contraction caused by either nerve-released noradrenaline or smooth muscle action potentials. 3. 30 nM-neuropeptide Y did not change the amplitude or rate of rise of the smooth muscle action potential. It did not change the amplitude of small excitatory junction potentials, suggesting that it did not affect neurotransmitter release. 4. 30 nM-neuropeptide Y increased the contraction caused by exogenous noradrenaline, 5-hydroxytryptamine and K in concentrations that gave submaximal contractions. It did not affect the response to higher concentrations that gave maximal or near-maximal contractions.

Motor Control of Tail Spine Rotation of the Horseshoe Crab, Limulus Polyphemus

ABSTRACT Limulus polyphemus (L.), the horseshoe crab, rotates its tail spine in order to right tself and to keep itself balanced. Eight muscles, discrete bundles of muscle fibres, move the tail spine. Fibres of the muscles contract in sequence and thereby pull consecutively on the several tendons of each muscle in order to rotate the tail spine in either a clockwise or counterclockwise direction. Motoneurones in nerves to different muscles, fibres within a muscle and units in a nerve to a single muscle fire in different sequences during clockwise and counterclockwise rotation of the tail spine. The firing pattern consists of a major burst of small to large neurons which fire in clusters, and a minor burst of small neurones which appear to fire randomly. Motoneurones in both bursts are excitatory. The major burst develops tension, the minor burst acts during extension of the muscle and presumably impedes relaxation in order to produce stable deflexion and smooth rotation of the tail spine. Muscle fibres respond to motor output with small excitatory junctional potentials of < 5 mV. E.j.p.s sum and show facilitation, and in some cases develop spike-like potentials of 10–20 mV. Both spiking and the greatest increase in tension occur during the clustered firings in major bursts. Muscle fibres have sarcomere lengths of 6·5 ± 0·8 –m and diameters of 10–60 –m. Nerve fibres range from less than 3 to 32 μm in diameter in large nerve branches which contain between 50 and 100 fibres. These findings indicate that two different motor programs evoke contraction of muscle fibres in opposite sequences. Sequential contraction of fibres within a muscle means that muscle fibres which are activated together, rather than whole muscles, are the functional contractile entities.

Neuromuscular transmission in a spider and the effect of calcium

1. 1. Investigations on the neuromuscular transmission of a spider, Eurypelma hentzi Girard, were performed for the first time. The composition of an artificial saline solution is given. The membrane of the fibers of the musculus flexor metatarsi bilobatus (MFMB) behaves as a K +-electrode. Each fiber receives three motoraxons. 2. 2. Efferent stimulation of motoraxons produces small excitatory junction potentials (EJPs) of three types, which are very similar in height and rise time, nevertheless the contraction evoked is quite different. The EJPs show summation and facilitation; no inhibition was found. 3. 3. An increase in the external Ca 2+ concentration from 2 to 12 mM/l increases the amplitude of all junction potentials significantly; above that there is a decline in the height of the EJPs. The membrane potential is not affected. 4. 4. In high Ca 2+ concentrations the EJPs usually reach the critical depolarization (30 mV) for spike discharge. Action potentials vary in amplitude; sometimes there is a positive overshoot. They make the membrane partly inactive for a closely following second action potential, but not for a junction potential. 5. 5. The contraction is always augmented in higher Ca 2+ concentrations. Both action potentials and junction potentials can elicit twitches of the muscle with single shocks, the ones produced by action potentials are always much more powerful.