Often during MRI examinations, a Rathke's cleft cyst may be found in patients with no symptoms that suggest its presence. But the cyst can grow and cause symptoms because it is compressing surrounding structures. Headaches may be experienced. When this occurs there may be changes in the field of vision such as bitemporal hemianopia because the cyst compresses the optic chiasma.
The pituitary gland is attached to the hypothalamus by the pituitary stalk (infundibulum; median eminence), and consists of two parts (lobes): the anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis); these two parts produce hormones, but the mechanisms of production and control are quite different. The term 'Neurosecretion' implies that some nerve cells produce hormones that are released in to the circulation and act on a distant (or, in the case of the anterior pituitary, not so distant) target. The diagram below shows two groups of hypothalamic neurosecretory cells: one group has long axons that pass into the posterior lobe where they release hormones directly into the systemic circulation the second group have short axons and release hormones into a capillary bed within the hypothalamus; these hormones pass into the pituitary portal vein and act on hormone producing cells of the anterior lobe.
The anterior lobe of the pituitary (adenohypophysis) receives blood that has already passed through the hypothalamus, where neurons release hormones, called releasing factors, that act on endocrine cells in the anterior pituitary, a short distance away. These cells, in turn, release hormones that enter the general circulation to act on distant endocrine glands, such as the thyroid, adrenal cortex and the gonads.
Pituitary Portal System In most parts of the body, blood passes from the arterial system to the venous system via a single network of capillaries. But in the hypothalamus and pituitary there are two networks of capillaries in series, jointed by a portal vein. Blood flows into the hypothalamus and receives releasing factors from neuroendocrine cells; this blood flows down the pituitary portal vein into the second set of capillaries in the adenohypophysis, where the releasing factors influence the secretion of hormones.
The Hypothalamus is situated below the thalamus and is responsible for the integration of AUTONOMIC and ENDOCRINE functions, which provide for direct control of various organs by this part of the brain. The ventral surface of the hypothalamus is attached to the pituitary gland via the pituitary stalk which joins the hypothalamus at the infundibulum or median eminence. The Optic Chiasma - the crossing of the optic nerves - is a clear feature on the base of the brain and is situated towards the anterior end of the hypothalamus, ahead of the infundibulum. Behind the infundibulum are two mamillary bodies that connect the hypothalamus and the limbic system.
The two halves of the hypothalamus are separated by the third ventricle, which is lined with ependymal cells. However, the blood brain barrier is deficient in this region. Some specialised cells in the walls of the third ventricle, tanycytes, are in contact with the CSF but also send cytoplasmic extensions into some of the hypothalamic nuclei. These are thought to act as sensors for hormones or other chemicals, and have a role to play in homeostasis. At the anterior end of the hypothalamus are the two pillars of the fornix, which form part of the connection with the limbic system and the temporal lobe of the cortex. There is also a band of nerve fibres that join both sides of the hypothalamus -the anterior commissure.
Below the thalamus is a small but important area called the hypothalamus, which is connected to the pituitary gland by nerve fibres (the posterior pituitary) and blood vessels (the anterior pituitary) in a region sometimes called the infundibulum. The whole area is complex, but extremely important physiologically, and in recent years the main functions of the many hypothalamic nuclei has been elucidated. The various nuclei opposite have names as indicated: The Anterior Nucleus is involved in thermoregulation and actively promotes cooling of the body. The Posterior Nucleus is also involved in thermoregulation and actively promotes warming of the body. The Suprachiasmatic Nucleus is important in determing the rhythm of sleep.
The ParaVentricular Nucleus is large and has three parts:
The last of these project, mainly ipsilaterally, on to neurones of :
The paraventricular nucleus, dorsomedial hypothalamus and perifornical area make direct and indirect connections with the dorsal motor nucleus of the vagus, the central grey matter, the parabrachial nucleus, the tractus solitarius, the lateral and ventral medulla and the intermediolateral grey matter of the spinal cord.
These areas of the hypothalamus also receive inputs from a wide variety of sources, including
The parvocellular projection neurones of the ParaVentricular Nucleus run mainly ipsilaterally in three tracts:
The DLF travels near the midline, close to the ventricular system on both sides of the midline, through the periaquductal grey, the brainstem and lateral to Lamina X of the spinal cord.
The Medial Forebrain Bundle joins the limbic system and the hypothalamus.
The mammillotegmental tracts originate in the mammillary nuclei and project to the reticular formation of the pons and medulla.
The hypothalamo-spinal pathway is an oxytocin-ergic pathway to the dorsal horn and intermediolateral columns of the spinal cord.