The lateral giant interneuron input and output

Undoubtedly the most striking feature of the initial tail flip is its speed. Within 50 ms, abdominal flexion is completed and the animal has usually moved some distance through the water. The mean delay between the stimulus and the onset of flexor muscle potential is 6 ms (see Fig. 3.1 b). This speed is required because the tail flip is probably a response to a predator that is extremely near to or touching a crayfish. Physiologically, the speed is partly achieved by extensive use of large neurons and of electrical synapses (see section 2.3). The segmentally repeated synapse between successive giant axons is electrical so that spikes are conducted across it with negligible delay and the chain of neurons acts effectively as a single axon. Each segmental lateral giant is also coupled via an electrical synapse with its contralateral partner, and with the motor giant neuron, which provides a pathway that conveys a spike from a lateral giant neuron to a fast flexor muscle in about 2 ms.

In the anterior segments of the abdomen, besides exciting the motor giants, the lateral giants also excite the other fast flexor motor neurons. A lateral giant makes synapses that have a mixed chemical and electrical nature with fast flexor motor neurons (Fraser & Heitler, 1991), but most excitation is conveyed by an indirect route, through an interposed neuron called the segmental giant because of the large size of its dendrite. There is a segmental giant on each side of each abdominal ganglion. Because of the

Figure 3.4 Neuronal circuit for startle behaviour mediated by the lateral giant interneuron. (a) Schematic representation of the excitatory pathway from the mechanoreceptors to the flexor muscles, showing chemical (— and electrical (—|) synapses. Labelled circles represent: the receptors (R); sensory interneurons (SI); lateral giant (LG); segmental giant (SG); motor giant (MoG) and fast flexor motor neurons (FF). The sensory pathways that generate the two components of the compound EPSP (Fig. 3.3c) are labelled a and p. (b) Diagrammatic representation of the arrangement of the above components within the abdominal nervous system. The various components are not drawn to scale and only one segment of the lateral giant is shown; the medial giant and extensor motor neurons are not included. (Modified after Wine & Krasne, 1982).

Figure 3.4 Neuronal circuit for startle behaviour mediated by the lateral giant interneuron. (a) Schematic representation of the excitatory pathway from the mechanoreceptors to the flexor muscles, showing chemical (— and electrical (—|) synapses. Labelled circles represent: the receptors (R); sensory interneurons (SI); lateral giant (LG); segmental giant (SG); motor giant (MoG) and fast flexor motor neurons (FF). The sensory pathways that generate the two components of the compound EPSP (Fig. 3.3c) are labelled a and p. (b) Diagrammatic representation of the arrangement of the above components within the abdominal nervous system. The various components are not drawn to scale and only one segment of the lateral giant is shown; the medial giant and extensor motor neurons are not included. (Modified after Wine & Krasne, 1982).

negligible delay in the transmission of spikes from the lateral giant to fast flexor motor neurons across the two electrical synapses, the existence of the segmental giants was not suspected for many years. An unusual feature of the segmental giants is that they have a blind-ending axon in one of the lateral nerve roots, which might indicate that they were originally motor neurons. The output connections of a lateral giant are summarised in Fig. 3.4, which also shows the sensory input.

The lateral giants receive input exclusively from sensory neurons that are arranged in pairs, each pair being attached to a stout hair. These sensory structures occur on the dorsal surface of the abdomen, including the tail fan. The sensory neuron is a bipolar neuron, meaning that two processes extend out from its cell body, which lies at the base of a hair. One process, the dendrite, attaches to the inside of the hair, and is stretched when the hair is deflected, initiating a chain of events that leads to the production of an electrical signal. The second process is the axon, which carries signals in the form of spikes to the central nervous system. These sensory neurons are sensitive only to touch and to high-frequency water movements (about 80 Hz). They do not respond to low-frequency slosh of the water, which is detected by other hair receptors on the cuticle. Consequently, the hair receptors are well suited to detecting the shock wave in the water produced by the acceleration of a predator towards the crayfish, or even actual contact by the predator. This is a good example of sensory filtering: because these mechanoreceptors respond only to particular types of disturbance, the lateral giants, which they excite, will respond only to imminent danger, and not to water movements caused by waves or movements by the crayfish itself.

The main input pathway to the lateral giants runs from these receptors via sensory interneurons located in the segmental ganglia of the abdomen (Fig. 3.4). Most of these interneurons have relatively large dendrites and axons, from which synaptic potentials and spikes can be recorded readily with microelectrodes. The interneurons are an order of magnitude less numerous than the hair receptors, so there is considerable convergence in the input pathway. At the same time, there is some divergence, because each receptor axon branches to make contact with several interneurons. The receptors make chemical synapses onto the interneurons, which in turn make electrical synapses onto the lateral giant. A small proportion of the receptors make electrical synapses directly onto the lateral giant.

All the synapses on to the lateral giant are thus electrical but, because the postsynaptic neuron is so much larger than the presynaptic one, single spikes cannot generate enough current to depolarise the lateral giant above threshold. Instead, single spikes generate EPSPs which summate in the usual way (see section 2.4); hence, several spikes must arrive within a short time of each other at these synapses in order to trigger a spike in the lateral giant. Following stimulation of the hair receptors, a compound EPSP with two components can be recorded in the lateral giant (see Fig. 3.3c). The first component is due to receptors that synapse directly on the lateral giant, but this input alone is insufficient to drive the giant to threshold. The second component is due to input via the sensory interneurons. When the mechanical stimulus is strong enough, the second component sums with the first to carry the giant's membrane potential beyond threshold.

Thus, the lateral giant has a high threshold for spike initiation. Also, its membrane has a short time constant, which means that individual EPSPs die away quickly, so that many EPSPs must arrive within a short time of each other if they are to sum and trigger a spike. Together with the requirement of the mechanoreceptors for a high-frequency mechanical stimulus, this ensures that the lateral giant responds only to a sudden, abrupt stimulus. These characteristics are reinforced by the occurrence of habituation in tail flips mediated by a lateral giant: when a crayfish is tapped on the abdomen repeatedly, the probability of a tail flip in response diminishes rapidly. Stimulation at the rate of one tap per minute can diminish responsiveness to zero within ten minutes, and then many hours rest are needed for recovery. Physiological analysis has shown that one site at which habituation occurs is in the input pathway. The strengths of the chemical synapses between the receptor axons and sensory interneurons diminish (there is no change in the properties of the electrical synapses). However, habituation of the reflex is not simply due to an automatic reduction in the strengths of these synapses every time the sensory neuron is activated. Experiments by Krasne and Teshiba (1995) indicate that interneurons that originate in the brain exert a powerful influence over habituation, and ensure that habituation only occurs if stimulation of the sensory neurons is sufficiently intense to cause a tail flip.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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