GENERAL FEATURES OF AUTONOMIC PATHWAYS TO CUTANEOUS EFFECTORS
From the preceding account, it is obvious that the skin is a major target of the autonomic nervous system in most vertebrate groups. In mammals with hairy skin, up to 25% of neurons in the paravertebral sympathetic ganglia lie in pilomotor pathways in addition to various populations of vasomotor neurons. Although few details are known, there is likely to be at least as large a pool of neurons supplying the dense innervation to the sophisticated pennamotor system of birds. Mammals with extensively innervated and widely distributed sweat glands, such as humans, have up to another 15-25% of their paravertebral sympathetic neurons in sudomotor pathways. Similarly, 25-50% of neurons in the sympathetic ganglia of anuran amphibians lie in cutaneous secretomotor pathways. Overall the skin represents a substantial target of the autonomic outflows, rivaled in size and number of neurons only by the sympathetic vasoconstrictor pathways that supply virtually all of the vasculature in most vertebrate species.
Despite significant and marked differences in the details of the cutaneous autonomic pathways, some common features are apparent. Most obvious of these is that the final motor neurons in the cutaneous pathways tend to run out to the skin in a segmental fashion, travelling with the sensory fibres in cutaneous branches of the spinal nerves to the dermatomes. The autonomic dermatomes usually are not as well defined as the sensory ones, but they have the potential to allow a unique insight into the organization of peripheral autonomic function. This characteristic has been utilized well in studies of the control of colour change in teleost fish and in abnormalities of sweating function in humans.
The final motor neurons in cutaneous pathways show surprisingly constant differences in the pathway-specific expression of their morphology and neurochemistry (Figure 1.11). In all species examined to date, neurons in vasoconstrictor pathways have the smallest cell bodies. Moreover the smallest of the vasoconstrictor neurons are those projecting to cutaneous vascular beds. As in the case elsewhere in the nervous system, it is generally accepted that the size of a sympathetic motor neuron and the complexity of its dendritic arborisation is related to the number of synaptic inputs it receives. It is likely therefore, that cutaneous vasoconstrictor neurons receive less synaptic input than do pilomotor neurons in mammals or cutaneous secretomotor neurons in frogs. The size of neurons cutaneous vasoconstrictor neurons generally have slower conduction speeds than autonomic motor neurons in other cutaneous pathways. Why this should be so is not clear. However, one potentially important factor is that the vasoconstrictor pathways tend to be tonically active, whereas the pilomotor and secretomotor pathways tend to be activated only in specific circumstances. Synaptic transmission in vasoconstrictor pathways tends to be very reliable, often requiring only one suprathreshold preganglionic input. In contrast, it might be predicted that the larger pilomotor and secretomotor neurons are only activated after summation of several preganglionic inputs. This prediction remains to be tested.
The neurochemical differences between different functional classes of neurons in cutaneous autonomic pathways provides unambiguous evidence for the presence of highly specific pools of neurons projecting to well defined effectors. Within the cutaneous vasculature it is clear that there are separate populations of neurons projecting to the proximal vessels, small distal vessels and AVAs and veins. It is also clear that pilomotor neurons form a well-defined population distinct from any of the vasomotor neurons. Furthermore, it is likely that cutaneous vasodilator neurons, when present, are distinct from sudomotor neurons. These results are consistent with more recent studies on the central pathways responsible for autonomic activity which indicate that there is a series of distinct areas in the periaquaductal grey, hypothalamus, and medulla that activate specific autonomic pathways in response to well defined changes in the external or internal environment. Furthermore, it is clear from physiological and anatomical studies that there are separate pools of preganglionic neurons projecting to different functional population of cutaneous motor neurons. Although we still do not know how the central areas are connected to the final autonomic motor and premotor neurons responsible for generating the appropriate effector activity, it is absolutely clear that there is no such thing as a generalized autonomic outflow, and that the widespread activation of different cutaneous effectors must require the coordinated recruitment of multiple independent autonomic motor pathways.