510 Comparative Animal Physiology 



distances for resolution being 2.3 mm. on finger tip, 11.3 mm. on palm of 

 hand, 31.6 mm. on back of hand, and 67 mm. on upper arm, thigh, and 

 middle of back.^ 



A tactile sensory unit appears to be one sensory neurone, together with 

 all those sensory endings whose processes converge on that axon. The most 

 accurate mapping of such an area has been done by recording impulses in 

 single fibers of the long ciliary nerve of the cat; one fiber serves 50 to 200 sq. 

 mm. (1 quadrant) of the cornea, together with adjacent sclera and conjunc- 

 tiva.»2 In the central portion of such a field the threshold is lowest, impulse 

 frequency highest, and adaptation slowest, hence the endings differ over 

 the same field. Cutaneous sensory endings can be stimulated electrically as 

 well as mechanically.^- ^ Areas of touch and prick (pain) are independent, 

 and as the point stimulus moves over a fingertip the stimulation "feels" as 

 though it went by jumps. Any stimuli within 2-15 mm. of each other on the 

 back of the hand are referred to the same locus, and stimulation of two 

 points in that area can summate. There is some overlap of the unit areas as 

 they are delineated during recovery from anesthesia, and two-point discrim- 

 ination probably involves several sensory units. Single sensory fibers serve 

 relatively large overlapping areas; the sensitivity of all the endings in such 

 an area is not the same, and much of the discrimination of tactile sense 

 depends on sorting out impulses of different frequencies and in different 

 afferent fibers within the central nervous system. Overlap of areas served 

 by sensory nerves has been observed in mammals, frogs, and earthworms. 

 Adaptation of Mechanoreceptors. The intensity of any sensory message 

 is given by the frequency of discharge in single units and the number of 

 active units. The frequency increases with increasing tension or pressure 

 on a mechanoreceptor. The characteristic pattern of nerve impulses is an 

 initial burst of high frequency impulses which decline rapidly (adaptation). 

 The maximum frequency is limited by the refractory period of the nerve 

 fiber, but sensory discharge is usually much slower than this, and adaptation 

 is not fatigue in the sense of metabolic failure. Adaptation is the decreasing 

 excitability of sensory endings during maintained stimulation, and the 

 mechanism of adaptation is not well known. Mechanoreceptors differ in 

 rate of adaptation, from those which give only one impulse to those which 

 continue to discharge more or less indefinitely at low frequency during 

 stimulation. In general, tactile receptors adapt rapidly and proprioceptors 

 adapt slowly.^ 



The rapidly adapting endings are best known among tactile hairs and touch 

 receptors of the skin. Each sensory hair along the margin of the telson and 

 uropods of a crayfish gives only one impulse when flexed;^" hair receptors 

 in the skin of a rabbit give 1 or 2 impulses (Fig. 181).^^ Vibration receptors 

 such as the cochlea and lateral line organ must also be considered as rapidly 

 adapting, since on synchronization with a sound one impulse appears per 

 wave front. Fine sensory hairs on the leg of a cockroach adapt rapidly, 

 while stout spinous hairs adapt more slowly. '^^ 



The tactile receptors of vertebrate skin are next in order of decreasing 

 speed of adaptation. The study of these has been aided by the fact that in 

 the dorsal cutaneous nerves of the frog there are occasional antidromic fibers, 

 efferent branches of sensory neurones, so that stimulation of the sensory 



