What does the mnemonic 'have sympathy for the long dilators' refer to in relation to nerves?

The phrase "have sympathy for the long dilators" serves as a mnemonic to help remember the characteristics of sympathetic post-ganglionic neurons. In the context of the autonomic nervous system, sympathetic post-ganglionic fibers are known for having long axons. These neurons typically originate from the thoracolumbar region of the spinal cord and travel long distances to reach their target organs, which allows them to exert effects on various distant structures.

The term "dilators" typically refers to the physiological response of smoothing muscle fibers that dilate blood vessels or the pupil in response to sympathetic stimulation. Hence, the mnemonic effectively ties together the notion of sympathy and the characteristic length and function of sympathetic post-ganglionic fibers, emphasizing their role in mobilizing the body in times of stress or emergency.

The other options do not align with this mnemonic. Parasympathetic post-ganglionics, for instance, are usually short and target organs near their origin. Cranial nerves and spinal reflex arcs pertain to different aspects of the nervous system structure and function, making this mnemonic specifically applicable to sympathetic post-ganglionic fibers.