brainstem- midbrain

what is an alternative name for the midbrain?

mesencephalon

briefly summarise the external features of the midbrain

smallest and most superior portion of the brainstem acting as a conduit between the forebrain and the pons and cerebellum

passes through tentorium cerebelli as it ascends

can be divided into the tectum (posterior to cerebral/mesencephalic aqueduct) and paired cerebral peduncles (separated interiorly by the substantia nigra)

describe the tectum

consists of four round structures called colliculi that lie inferior to the pineal gland, separated by the cruciform sulcus in 2 inferior and 2 superior colliculi

quadrigeminal brachia extend laterally from each colliculus connecting the superior colliculi to the retinas (superior quadrigeminal brachium), and carrying fibres from the inferior colliculus to the medial genticulate nucleus in the thalamus (inferior quadrigeminal brachium)

describe the cerebral peduncles

paired round structures that originate at the cerebral cortex and converge at the pons

separated anteriorly in the midline by the interpeduncular fossa

oculomotor nerve (CNIII) exits between the peduncles

describe the internal anatomy of the midbrain at the level of the inferior colliculi

the crus cerebri (anterolateral surface of midbrain) contains four fibre tracts:

frontopontine fibres (most medial);

corticospinal fibres (from primary motor cortex);

corticobulbar fibres (from primary motor cortex);

temporopontine fibres (posterolateral)

the substantia nigra consists of the anterior pars reticulata and the posterior pars compacta and is located posteriorly where it separates the cerebral peduncles

tegmentum is posterior to the substantia nigra and is continuous at the midline

cerebral/mesencephalic aqueduct is located on the midline and is surrounded by periaqueductal grey matter containing the mesencephalic nucleus of the trigeminal nerve and the nucleus of the trochlear nerve

medial, spinal, lateral and trigeminal lemnisci are visible between periaqueductal grey matter and substantia nigra

describe the interior anatomy of the midbrain at the level of the superior colliculi

much the same as above except the lateral lemniscus is no longer visible

paired red nuclei visible

oculomotor nucleus visible, oculomotor nerve projects anteriorly

where does the midbrain derive its arterial blood supply from?

the basilar artery and its branches

(posterior cerebral artery, interpeduncular branches of basilar artery, superior cerebellar artery, peduncular branches of the posterior cerebral artery, posterior choroidal artery)