Pilomotor neurons also can be distinguished from most vasoconstrictor neurons by the size of their cell bodies and dendritic arborizations. Thus the pilomotor neurons, on average, are larger than vasoconstrictor neurons in mice and guinea-pigs (Figure 1.6; Gibbins 1991; Gibbins and Matthew 1996). This result is consistent with physiological observations in cats showing that pilomotor neurons generally have faster axon potential conduction velocities than do cutaneous vasoconstrictor neurons (Jänig 1985). The reasons for these differences are not clear, but two points are worth considering in this context:
1. Sympathetic final motor neurons with larger dendritic arborizations tend to have more convergent preganglionic inputs than neurons with smaller arborizations (Purves 1988). Thus, in general, we would expect pilomotor neurons to receive more preganglionic inputs than vasoconstrictor neurons.
2. Sympathetic final motor neurons within pathways that are used only intermittently tend to be larger and have faster action potential conduction velocities than those that are tonically active. Thus within the superior cervical ganglion, for example, cutaneous vasoconstrictor neurons, which are active most of the time, are smaller than pilomotor neurons, which are themselves smaller than salivary secretomotor neurons which probably are activated only rarely.
Most of the information on the organization of pilomotor pathways comes from studies in cats (Langley and Sherrington 1891; Langley 1894; van Rijnberk 1907; Jänig 1985; see also Lichtman et al. 1979 for a study on guinea-pigs). Mostly the pathways that have been studied are those activated during the fear/aggression response. In cats under these conditions, piloerector activity occurs in the facial skin between the eye and ear, a strip about 10 to 12 cm wide extending from the back of the head along the dorsal midline to the base of the tail, and most of the dorsal and lateral part of the tail (Langley and Sherrington 1891; van Rijnberk 1907; Jänig 1985). Although pilomotor muscles themselves are more widely spread than this within cat skin and seem to be innervated, the conditions under which they are normally activated are not really know: presumably they have a role in thermoregulation, but surprisingly, this has not been show explicitly. Certainly, in other species such as rodents (guinea-pigs, rats, mice) and primates (rhesus monkey, human), cold-induced activation of piloerection is widespread over much of the skin.
I have to stop for a moment and consider that strip of skin down the back of the cat that stands the hair up in a fear/aggression display. Just because it is so visible in cats, does this not happen with many animals? I recall a Siamese fighting fish I once kept, who did this upon seeing his own reflection in a glass, every time.
Does this not happen with humans? Is not "get your back up" a common expression? I can often feel a "shudder" and other weird sensations shoot down my back, and wonder if these are related in any way. Pure speculation, I realize..
Dorsal cutaneous nerves innervate that particular strip of skin. I see an implication: could it possibly be, that the sympathetic neurons ... that innervate piloerector muscles in skin... skin which is supplied by dorsal cutaneous nerves... have a closer relationship with the bits of brain that have old mammalian (pre-mammalian even) sympathetic fight/flight reactions?
It makes me wonder about any possible relationship there might be among circulating stress hormones, need to suppress aggression/display submission within our human primate troop (i.e., get along),.. to chronic back pain. However tenuous.
I visualize the cutis/subcutis smooth muscle cells along the spine, a spinal "Mohawk" of smooth muscle lifting with all its might, but alas, barely any hair to lift and struggling all the while against clothing and chair backs, piloerecting like crazy but nothing visible to show for it. Us, barely aware of their efforts to express themselves/our non-conscious responses. Us, completely involved in our conscious, inhibiting, troop-appeasing, socially appropriate descending control of muscular effort to provide deceptive "cover".
Could it be that a "war" between brain modules is constantly raging? Expressed by smooth muscle cells (tiny), against somatic muscles, much bigger and more under conscious control. Dorsal cutaneous nerve rootlets pulled this way and that, and secreting like crazy, all the stress stuff that gives them the neural equivalent of diaper rash (abnormal impulse generating sites) inside their little neural cages - I mean tunnels. Especially in those human primates who are lower ranking in the human primate troop. Having to put up with horrible bossy bosses or just life as a worker in general. Having to fake being cheery even if they don't feel it. Low back pain for no "good" reason, such as a definitive injury, an all too common human fact of human life.
Anyway, just some idle speculation on my part. What else is a blog good for?
The pilomotor pathways have an overall segmental distribution, that loosely follows the sensory dermatomes, especially on the trunk (Langley 12894; van Rijnberk 1907; Jänig 1985). For much of the body, the preganglionic fibres in pilomotor pathways project directly from each spinal segment to the corresponding ganglion; i.e., they do not project up or down the sympathetic chain to a significant degree before connecting with the pilomotor neurons themselves. Similarly the pilomotor neurons project directly from their ganglion into the corresponding spinal nerve. Overall the preganglionic neurons in pilomotor pathways of cats extend from spinal segments T4 to L3 or L4 (Langley 1894). The pilomotor neurons innervating the skin of the head arise from cell bodies in the superior cervical ganglion and receive inputs from preganglionic neurons in upper to mid thoracic level spinal segments (T4 to T6 or rarely T7 in cats, T4 to T6 in dogs, Langley 1894; T1 (rarely) or T2 to T5 in guinea-pigs, Lichtman et al. 1979). In guinea-pigs there is a clear correlation between the level of spinal origin of the preganglionic neurons and the region of skin innervated by pilomotor neurons. Thus, neurons with preganglionic inputs from more rostral spinal levels tend to innervate piloerector muscles on the more rostral and ventral areas of the face whereas neurons with preganglionic inputs from more caudal segments of the spinal cord tend to innervate piloerector muscles on the more caudal and dorsal regions of the head (Lichtman et al 1979).
The central pathways generating piloerector responses clearly must include areas of the hypothalamus associated with thermal regulation, such as the preoptic/anterior hypothalamus, and areas such as the amygdala and the periaquaductal grey involved in generating aggressive or defensive behavioral displays (Schönung et al. 1971; Swanson and Sawchenko 1983; Smith and DeVito 1984; Hilton and Redfern 1986; Holstege 1990; Jordan 1990; Gordon 1993). In particular stimulation of the lateral areas of the periaquaductal grey can cause piloerection as a component of a induced threat display in cats (Bandler, Carrive and Zhang 1991). Although all these areas are known to project directly or indirectly to the spinal cord (see Holstege 1990; Swanson 1991), nothing is known of the specific pathways linking these central areas to preganglionic pilomotor neurons in the spinal cord.
Periaquaductal grey areas are part of the fish brain (basal ganglia) as I recall.. I can hardly wait until people like Gibbins get the pathways more sorted out. I think the implications are quite big, actually.