<P> Autonomic functions of the brain include the regulation, or rhythmic control of the heart rate and rate of breathing, and maintaining homeostasis . </P> <P> Blood pressure and heart rate are influenced by the vasomotor centre of the medulla, which causes arteries and veins to be somewhat constricted at rest . It does this by influencing the sympathetic and parasympathetic nervous systems via the vagus nerve . Information about blood pressure is generated by baroreceptors in aortic bodies in the aortic arch, and passed to the brain along the afferent fibres of the vagus nerve . Information about the pressure changes in the carotid sinus comes from carotid bodies located near the carotid artery and this is passed via a nerve joining with the glossopharyngeal nerve . This information travels up to the solitary nucleus in the medulla . Signals from here influence the vasomotor centre to adjust vein and artery constriction accordingly . </P> <P> The brain controls the rate of breathing, mainly by respiratory centres in the medulla and pons . The respiratory centres control respiration, by generating motor signals that are passed down the spinal cord, along the phrenic nerve to the diaphragm and other muscles of respiration . This is a mixed nerve that carries sensory information back to the centres . There are four respiratory centres, three with a more clearly defined function, and an apneustic centre with a less clear function . In the medulla a dorsal respiratory group causes the desire to breathe in and receives sensory information directly from the body . Also in the medulla, the ventral respiratory group influences breathing out during exertion . In the pons the pneumotaxic centre influences the duration of each breath, and the apneustic centre seems to have an influence on inhalation . The respiratory centres directly senses blood carbon dioxide and pH . Information about blood oxygen, carbon dioxide and pH levels are also sensed on the walls of arteries in the peripheral chemoreceptors of the aortic and carotid bodies . This information is passed via the vagus and glossopharyngeal nerves to the respiratory centres . High carbon dioxide, an acidic pH, or low oxygen stimulate the respiratory centres . The desire to breathe in is also affected by pulmonary stretch receptors in the lungs which, when activated, prevent the lungs from overinflating by transmitting information to the respiratory centres via the vagus nerve . </P> <P> The hypothalamus in the diencephalon, is involved in regulating many functions of the body . Functions include neuroendocrine regulation, regulation of the circadian rhythm, control of the autonomic nervous system, and the regulation of fluid, and food intake . The circadian rhythm is controlled by two main cell groups in the hypothalamus . The anterior hypothalamus includes the suprachiasmatic nucleus and the ventrolateral preoptic nucleus which through gene expression cycles, generates a roughly 24 hour circadian clock . In the circadian day an ultradian rhythm takes control of the sleeping pattern . Sleep is an essential requirement for the body and brain and allows the closing down and resting of the body's systems . There are also findings that suggest that the daily build - up of toxins in the brain are removed during sleep . Whilst awake the brain consumes a fifth of the body's total energy needs . Sleep necessarily reduces this use and gives time for the restoration of energy - giving ATP . The effects of sleep deprivation show the absolute need for sleep . </P>

Identify the smallest diameter structure within the ventricular system of the cns