Re: Why do you get goosebumps when you step into a hot bath?

The smooth muscle that is responsible for "goose bumps" is called arrector 
pili or pilimotor muscles.  The are attached to the hair follicle and when 
contacted, raise the hair and pull the skin down in an "attempt" to 
insulate us from cold via trapping air between the hair.  The "bumps" are 
really the indentions due to the muscle pulling the epidermis down and we 
notice the difference in height along the skin.  Since these muscles are 
smooth, they are innervated by the Autonomic Nervous System (ANS).  The 
sympathetic branch of the ANS is the only branch that is connected to these 
muscles.  When activated by the ANS, contraction occurs.  In the dermis of 
the skin, there are at least 2 types of temperature receptors, one for hot 
& one for cold.  Both of these are "phasic", meaning that they react to a 
range of temperatures.  For example, the cold receptors, sometimes called 
Bulbs of Krause, reach threshold between 12 and 35 degrees C whereas the 
hot receptors (Ruffini organs) are activated between 25 & 47 degrees C. 
Below 12 and above 47, pain receptors reach threshold.  Both the cold and 
hot receptors use the same sensory pathway to the brain called the lateral 
spinothalamic tract.  This tract goes through the thalamus and terminates 
in the appropriate location of the post central gyrus of the cerebrum.  
This helps us to locate the area of the body that is being stimulated. 
Temperature changes are mediated by nuclei in the hypothalamus which 
has connections to the thalamus and which then sends messages to the 
medulla.  From there, impulses go down the spinal cord and out spinal 
nerves, which make up the sympathetic division or thoracolumbar outflow.  
These send a nerve impulse to the arrector pili muscles and they contract. 
 It would therefore seem to me that if one gets "goose bumps" from hot 
temps it is due to this pathway being stimulated by a temperature that is 
an overlapping one so that both hot and cold receptors are being activated.