SENSORY ORGANS AND RECEPTION 337 



receptors are also known to occur in fish muscles and in invertebrates. 

 Compared with tactile receptors, tension receptors are usually very slowly 

 adapting. An instance of a slowly-adapting pressure-receptor and proprio- 

 ceptor, responsive to fin movements and pressure deformation in selachians, 

 has been noted in the previous section. 



Stretch receptors in selachian muscle, histologically unidentified, give 

 rise to a maintained discharge in afferent nerve fibres when a load is applied 

 to the muscle. The frequency of discharge increases with the load, the 

 relationship between frequency and logarithm of tension being linear. 

 On stretching the muscle there is initially a high-frequency discharge, which 

 declines over a period of some 20 sec, owing to adaptation, and is succeeded 

 by a steady rhythmic discharge so long as the tension is maintained. These 

 stretch receptors continue to function rhythmically under constant tension 

 for over an hour. When the tension is suddenly decreased, there follows 

 a silent period before the discharge resumes at a new frequency level 

 corresponding to the reduced tension. In a fin at rest there is a resting 

 discharge from the muscle receptors; bending the fin one way, and then 

 the other, increases and decreases the discharge from a given receptor. 

 This differential response signals the degree of muscular contraction, and 

 the sign and magnitude of fin movements (47). 



Various kinds of proprioceptors have been described in different 

 Malacostraca. These are: A. organs in the extensor muscles of thorax and 

 abdomen (Malacostraca, except Brachyura): (a) muscle receptor organs; 

 (b) N-cells in the ordinary thoracic muscles. B. pereiopod organs (Deca- 

 poda): {a) rows of nerve cells ending on connective strands in the joints; 

 (b) muscle receptors spanning the thoracico-coxal articulation; (c) in- 

 nervated strands associated with levator and depressor muscles of the 

 pereiopods; (d) "myochordotonal organs" of Barth. 



The joints of the legs (pereiopods) contain at least seven organs consisting 

 of rows of nerve cells (category B (a) above). One of these organs — that 

 occurring at the propodactylopodite joint — has been subjected to physio- 

 logical study. It consists of many nerve cells ending in an elastic strand 

 terminating at the joint. The connective tissue strand is in a stretched 

 condition at all positions, and the amount of stretch increases during 

 flexion. In the afferent nerves coming from the receptor organ there is a 

 resting discharge which depends on the length of the organ. Sudden change 

 in length, and vibrational stimuli, evoke bursts of impulses in the sensory 

 axons (Fig. 8.23). The organ signals rate and extent of movement at the 

 joint; it furthermore appears that some fibres may signal static position 

 and direction of movement (24, 158). 



Muscle receptor organs occur in the dorsal body wall of thoracic and 

 abdominal segments of malacostracans (Fig. 8.24). There are two organs 

 on each side, lying near the dorsal extensor muscle. Typically, each organ 

 consists of a long thread-like muscle plus sensory, motor and accessory 

 nerve fibres. The cell body of the sensory neurone lies near the muscle, 

 and its dendrites terminate in connective tissue intercalated in the muscle. 



