The abdominal ganglion contains about 900 neurons, and it would be an enormous task to catalogue all their interconnections by making recordings with microelectrodes. Another approach, adopted by Lawrence Cohen and colleagues, is to monitor the activity of as many different neurons as possible while the animal performs different behaviours. To do this, Cohen et al. used a chemical (a pyrazolone oxonole) which has been specially developed as a tool in neurophysiological experiments. The dye attaches to cell membranes, and the amount of light that the dye absorbs alters as the voltage across the cell membrane changes. A ganglion is soaked in a solution of this dye and an image of the ganglion is projected by a microscope objective lens onto an array of 144 or 464 tiny photodiodes, each of which measures the amount of light passing through a small part of the ganglion (Fig. 8.1 d). The electrical signal produced by each diode is amplified and processed to produce pulses that indicate the occurrence of spikes. Some neurons are large enough to cover more than one photodiode, and other diodes pick up spikes from more than one neuron. Nevertheless, these experimenters are able to process data in a way that records the occurrence of spikes in about a third of the neurons of the abdominal ganglion at one time, although they cannot assign spikes to identified neurons.
One of the first discoveries made using this technique was that hundreds of neurons in the abdominal ganglion are active during any gill-withdrawal reflex (Zecevic et al., 1989). This observation does not allow us to distinguish between the two arrangements shown in Figs. 8.1b and 8.1c because we need to know whether each neuron is active only during the reflex withdrawals, or during other behaviours as well. Subsequently, three different kinds of gill-withdrawal movements were studied: reflex withdrawal; weak, spontaneously occurring withdrawals; and strong withdrawals that occur during respiratory pumping (Wu, Cohen & Falk, 1994). During each of these movements, between 62 and 72 neurons were usually active and almost all of these neurons were active during all three behaviours. This means that something like 200 neurons in the abdominal ganglion, most of which are interneurons, would be excited during any activity of the gill-withdrawal muscles. The neuronal networks responsible for controlling gill-withdrawal movements seem, therefore, to be organised in a distributed pattern like that in Fig. 8.1c, with few neurons active during one behaviour but not during the other behaviours.
Control of gill movements is a major concern of the neurons in this ganglion and many of the other activities that it controls, such as heart beat, need to be co-ordinated with gill movements. However, the abdominal ganglion is also responsible for controlling a more dramatic defensive response to intense stimulation of the skin, which involves expulsion of a dense cloud of dark purple ink from a special gland. The inking behaviour contrasts with gill-withdrawal in that it is an all-or-none behaviour (Carew & Kandel, 1977; Byrne, 1981). The same sensory neurons that trigger gill withdrawal responses also trigger inking, and they communicate through inter-neurons to three, strongly coupled motor neurons. It would be interesting to investigate whether these three interneurons are dedicated to inking behaviour, or whether they are also active during other behaviours.
The abdominal ganglion of Aplysia is suited to optical monitoring of neuronal activity because it is quite small and transparent. However, a major drawback of this technique is that it cannot be used to stimulate or inhibit an individual neuron. In some other ganglia, the technique is capable of resolving small changes in membrane potential, so that there is a chance that it can be used to trace circuits between neurons by correlating the occurrence of spikes in some neurons with the postsynaptic potentials they cause.
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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.