Nervous And Chemical Integration

FIGURE 13.5. Cross-section to show major areas of brain. [After R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

responding to stimuli collectively constitute neural integration. Neural integration includes, therefore, the biophysics of impulse transmission along axons and across synapses, the reflex pathways (in insects, intrasegmental) from sense organ to effector organ, and coordination of these segmental events within the central nervous system.

Impulse transmission along axonal membranes and across synapses appears to be essentially the same as in other animals and will not be discussed here in detail. However, the absence of a myelin sheath and nodes of Ranvier precludes the phenomenon of saltatory conduction seen in vertebrates. Following the arrival of a stimulus of sufficient magnitude, an action potential is generated and the impulse travels along the axon as a wave of depolarization. The speed of impulse transmission is a function of axonal diameter so that in giant axons values of 3-7 m per sec have been recorded while in average-sized axons the speed is 1.5-2.3 m per sec. In addition to "spiking" neurons (i.e., those in which an action potential can be generated), there are in the insect central nervous system intragan-glionic "non-spiking" interneurons unable to produce action potentials. Rather, the amount of neurotransmitter released at their synapses (see below) is proportional to the size of their endogenous membrane permeability changes; in other words, they release neurotransmitter (and affect the postsynaptic neuron) in a graded manner. These non-spiking interneurons may have wide importance in the initiation of rhythmic behaviors such as walking, swimming, and chewing (see below).

Transmission across a synapse, depending as it does on diffusion of molecules through fluid, is relatively slow and may take up about 25% of the total time for conduction of an impulse through a reflex arc. Rarely, when a synaptic gap is narrow (i.e., pre- and postsynaptic membranes are closely apposed), the ionic movements across the presynaptic membrane are sufficient to directly induce depolarization of the postsynaptic membrane

(Huber, 1974). Mostly, however, when an impulse reaches a synapse, it causes release of a chemical (a neurotransmitter) from membrane-bound vesicles. The chemical diffuses across the synapse and, in excitatory neurons, brings about depolarization of the postsynaptic membrane. Acetylcholine is the predominant neurotransmitter liberated at excitatory synapes, including those of interneurons and afferent neurons from mechanosensilla and taste sensilla (Homberg, 1994). 5-Hydroxytryptamine (serotonin), histamine, octopamine, and dopamine function as central nervous system excitatory neurotransmitters in specific situations on occasion. These, and other amines, have an excitatory effect when applied in low concentrations to the heart, gut, reproductive tract, etc., and it may be that they also serve as neurotransmitters in the visceral nervous system.

Sometimes a single nerve impulse arriving at the presynaptic membrane does not stimulate the release of a sufficient amount of neurotransmitter. Thus, the magnitude of depolarization of the postsynaptic membrane is not large enough to initiate an impulse in the postsynaptic axon. If additional impulses reach the presynaptic membrane before the first depolarization has decayed, sufficient additional neurotransmitter may be released so that the minimum level for continued passage of the impulse (the "threshold" level) is exceeded. This additive effect of the presynaptic impulses is known as temporal summation. A second form of summation is spatial, which occurs at convergent synapses. Here, several sensory axons synapse with one internuncial neuron. A postsynaptic impulse is initiated only when impulses from a sufficient number of sensory axons arrive at the synapse simultaneously. Divergent synapses are also found where the presynaptic axon synapses with several postsynaptic neurons. In this arrangement the arrival of a single impulse at a synapse may be sufficient to initiate impulse transmission in, say, one of the postsynaptic neurons. The arrival of additional impulses in quick succession will lead to the initiation of impulses in other postsynaptic neurons whose threshold levels are higher. Thus, synapses play an important role in selection of an appropriate response for a given stimulus.

Eventually, an impulse reaches the effector organ, most commonly muscle. Between the tip of the motor axon and the muscle cell membrane is a fluid-filled space, comparable to a synapse, called a neuromuscular junction. Again, to achieve depolarization of the muscle cell membrane and, ultimately, muscle contraction, a chemical released from the tip of the axon diffuses across the neuromuscular junction. In insect skeletal muscle, this chemical is L-glutamate; in visceral muscles, glutamate, serotonin, and the pentapeptide proctolin have all been suggested as candidate neurotransmitters.

In addition to stimulatory (excitatory) neurons, inhibitory neurons whose neurotrans-mitter causes hyperpolarization of the postsynaptic or effector cell membrane are also important in neural integration. When inhibition occurs at a synapse within the central nervous system, it is known as central inhibition. Central inhibition is the prevention of the normal stimulatory output from the central nervous system and may arise spontaneously within the system or result from sensory input. For example, copulatory movements of the abdomen in the male mantis, which are regulated by a segmental reflex pathway located within the terminal abdominal ganglion, are normally inhibited by spontaneous impulses arising within the brain and passing down the ventral nerve cord.In the fly Protophormia the stimulation of stretch receptors during feeding results in decreased sensitivity to taste caused by central inhibition of the positive stimuli received by the brain from the tarsal chemoreceptors. When inhibition of an effector organ occurs it is known as peripheral inhibition. At both synapses and neuromuscular junctions, the hyperpolarizing chemical is y-aminobutyric acid (Homberg, 1994).

Mention must also be made of neuromodulators, a group of chemicals that can modify the effects of neurotransmitters (Orchard, 1984; Homberg, 1994). Typically,

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