Periaqueductal Grey

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(PAG) Also known as the CENTRAL GREY or midbrain central grey, this structure is wrapped around the CEREBRAL AQUEDUCT as it travels through the MIDBRAIN and BRAINSTEM. It is composed of small neurons, with some larger cell groups embedded with in it (such as the LATERODORSAL TEGMENTAL NUCLEUS, which contains the Ch6 cell group [see CH1–CH8]).The largest inputs come from structures higher along the NEURAXIS. Principal inputs are from the HYPOTHALAMUS, PREOPTIC AREA, ZONA INCERTA, AMYGDALA, SUPERIOR COLLICULUS and INFERIOR COLLICULUS, with BRAINSTEM sites (for example RAPHE NUCLEI, PARABRACHIAL NUCLEI and LOCUS COERULEUS) making a smaller contribution. There are also some inputs from both the CEREBRAL CORTEX (mainly FRONTAL CORTEX) and the SPINAL CORD.

There are also a great many intrinsic connections between different parts of the PAG. The outputs are for the most part reciprocal with the inputs: the largest outputs go to the hypothalamus and related structures such as the preoptic areas and zona incerta. Numbers of sites in the PONTINE RETICULAR FORMATION and MEDULLARY RETICULAR FORMATION also rcceive input from the PAG, as does the PEDUNCULOPONTINE TEGMENTAL NUCLEUS. The PAG has been associated with a variety of integrative and AUTONOMIC functions, many to do with the control of AGGRESSION, DEFENCE and FEAR. VOCALIZATION and RAGE reactions can both be stimulated from here. Hormonal release is also to some extent regulated by the PAG, and it has been associated strongly with ANALGESIA and the perception of PAIN.

See also: fight-or-flight

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