The Structural Basis of an Innate Behavioural Pattern

Abstract

The abdominal nervous system of the crayfish contains six serially homologous ganglia, each containing approximately 650 neurones. No two ganglia are identical, and the ganglia interact extensively. Studies confined to intraganglionic interactions thus yield limited and sometimes misleading information. Each ganglion contains intrinsic (local) interneurones, motor neurones and projecting interneurones in roughly equal numbers, except in the specialized terminal ganglion where the ratio of these cells is approximately 3:2:1. Although the number of nerve cell bodies in a ganglion is small enough to be tractable, integration occurs in the neuropile, which contains terminals from interneurones and afferents that outnumber the neurones originating in the ganglion by at least ten to one. The abdominal nervous system responds almost exclusively to a variety of mechanosensory stimuli. It has very limited light sensitivity. Other modalities, notably chemosensitivity, are undescribed and may be lacking. The effectors of the abdomen consist of fast axial muscles (used for tailflip-powered escape), slow axial muscles (for setting abdominal posture), appendage muscles (for swimmeret beating), and slow muscles of the intestine and rectum (that control gut emptying). The fast and slow muscles of the tailfan are specialized homologues of the axial and appendage muscles. The abdominal nervous system represents only 3–4% of the 100 000 neurones within the crayfish central nervous system (CNS). Most sensory information gathered in the abdomen is transmitted to the rostral CNS for processing, and many abdominal motor programmes are activated by descending commands. Nevertheless, a surprising degree of autonomy is present, and at least some motor programmes of every motor system can be activated in isolated abdomens. Tailflip escape behaviour illustrates the integrative properties of the crayfish nervous system. Ninety pairs of efferents and eighteen pairs of interneurones have been identified within the abdominal portion of the escape circuit. A cell-by-cell analysis has so far provided neurophysiological explanations, in varying states of completeness, for ethological concepts such as innate releasing mechanisms, spatial patterning of movement, serial order in behaviour, and alterations in responsiveness to a constant stimulus.