The brainstem contains many neurones that do not belong to well defined groups such as the cranial nerve nuclei or the olivary nuclei, or fibre tracts such as the pyramidal tract, the medial lemniscus or the medial longitudinal bundle. The reticular substance of the brainstem, apart from the the specific nuclei or tracts, appears to consist of many small networks of nerve cells, and axons that pass in many directions, intersecting with each other, rather than travelling together in the form of a tract.
Nevertheless these small neural networks encompass many essential functions. Several are central pattern generators (CPGs) such as those concerned with the regulation of breathing, swallowing or chewing; others provide the autonomic nervous system with a tonic input. Saccadic eye movements - rapid, movements of both eyes simultaneously in the same direction - also appear to depend on a CPG within the optic tectum.
The reticular formation contains groups of neurones with specific histochemical properties - they synthesise and release a specific neurotransmitter. Some of these send their axons into the spinal cord, and examples are the raphe nuclei that contain serotonin and other reticulo-spinal neurones that use Glutamate or GABA at their endings.
Many of the reticular formation neurones receive inputs from different systems - somatosensory, proprioceptive, auditory, visual, etc - i.e. non-specific inputs, not concerned with signalling in a single neural pathway, but integrating the activity of many systems to obtain a view of overall ongoing activity. Some reticular neurones modulate the activity of spinal circuits, in the regulation of muscle tone, autonomic outflow or transmission of nociceptive information in the dorsal horn.
The descending pathways concerned with balance, muscular tone and posture have already been mentioned in the previous chapter. The reticular formation's role in integration and coordination of visual, auditory, and vestibular inputs and its relationship with the cerebellum and the control of eye movements has also been mentioned.
In addition to providing the basic sequences of motor activity in central pattern generators, such as respiratory movements, chewing and swallowing, the brainstem reticular formation is essential for cardiovascular control using medullary 'vasomotor centres'. The Pontine Micturition Centre is another area of the reticular formation that is essential for normal control of micturition.
The descending control of the dorsal horn has also been referred to in Chapter 3. These descending anti-nociceptive pathways regulate transmission of nociceptive transmission in the dorsal horn.