---The only structure within the myelencephalon is the
The medulla is about 3 cm long and is continuously
All the descending and ascending fibre tracts that
the brain must pass through the medulla. Many of
these fibre tracts cross to the contralateral side in
elevated, triangular structures in the medulla called the
pyramids, so that the left side of the brain receives
sensory information from the right side of the body
and vice versa. Similarly, because of the decussation
of fibres, the right side of the body controls motor
activity in the left side of the body and vice versa.
---There are many important nuclei within the medulla.
Several nuclei are involved in motor control, giving
rise to axons within cranial nerves VIII, IX, X, XI and XII. The vagus nuclei (there is one on each lateral side
of the medulla), for example, give rise to the highly important vagus (X) nerves. Other nuclei relay sensory
information to the thalamus and then to the cerebral cortex.
---The medulla contains groupings of neurons required for the regulation of breathing and of cardiovascular
response, hence, they are known as the vital centres. The X centre controls the autonomic innervation of
blood vessels, the cardioinhibitory centre controls the parasympathetic innervation (via the vagus nerve)
of the heat (there does not appear to be s separate cardioaccelerator centre); and the respiratory centre of
the medulla acts together with centres in the pons to control breathing.
---The reticular formation is a complex network of nuclei
and nerve fibres within the medulla, pons, midbrain,
hindbrain, thalamus, and hypothalamus that functions as the reticular activating system or RAS. Because of
its many interconnections, the RAS is activated in a non-specific fashion by any modality X sensory
information. Nerve fibres from the RAS, in turn, project diffusely to the cerebral cortex; this results in
non-specific arousal of the cerebral cortex to incoming sensory information.