Brain stem, which includes midbrain, pons and medulla oblongata, connects cerebrum with spinal cord. It extends from posterior commissure to pyramidal decussating at spino medullary junction. Brain stem gives attachment to III to XII cranial nerves and contains their motor and sensorii nuclei. It gives passage to long ascending sensory tracts and descending motor tracts. It is connected to cerebellum by superior, middle and inferior cerebellar peduncles.
Superior cerebellar peduncle connects midbrain to cerebellum, middle cerebellar peduncle connects pons to cerebellum, and inferior cerebellar peduncle connects medulla to cerebellum.
It is a diamond shaped depression in midline on anterior aspect of midbrain. It is bounded
- Anteriorly by optic chiasma
- Anterolaterally by optic tracts
- Posterolaterally by cerebral peduncles (especially crus cerebri)
- Posteriorly by upper border of pons.
Floor of the interpeduncular fossa presents from before backwards: - Optic chiasma, tuber cinerium, tubal eminences, infundibular stalk, mammillary bodies, and posterior perforated substance, which is pierced by posterior cerebral arteries. The basilar artery divides into right and left cerebral arteries, and posterior communicating artery joins posterior cerebral artery in interpeduncular fossa.
Superior cerebellar artery arises from basilar artery immediately distal to bifurcation. Oculomotor and trochlear nerves intervene between superior cerebellar and posterior cerebral arteries.
- The cerebellum consists of two cerebellar hemispheres joined by vermis. The cerebellum is divided by two transverse fissures into three lobes: Anterior lobe, Posterior lobe and flocculonodular (lateral flocculus and medial nodulus) lobe. Similarly, two longitudinal fissures divide it into the midline vermis and the left and right cerebellar hemisphere.
- The cerebellum has an external cerebellar cortex separated by white matter from the deep cerebellar nuclei. There are four deep cerebellar nuclei (from lateral to medial): dentate, emboliform, globose, and fastigial. The globose and the emboli form nuclei are sometimes lumped together as the interpositus nucleus.
- The cerebellar cortex contains five types of neurons Purkinje cells, granule cells, basket cells, stellate cells, and golgi cells. The axons of purkinje cells are the only output from the cerebellar cortex, generally pass to the deep nuclei. Neurons of deep nuclei form the ultimate (final) efferent output of cerebellum.
Cerebellar cortex is arranged into three layers: -
- External molecular layer contains basket cells and stellate cells.
- Middle Purkinje cell layer contains Purkinje cells
- Inner granular layer contains granule cells and Golgi cells.
Functional divisions of cerebellum
From the functional point of view the cerebellum is divided into three parts: -
- Vestibulocerebellum (Archicerebellum)
- Modulate muscular activity so as to achieve postural equilibrium or balance.
- Coordinate movements of the eyes with movements of head, i.e., vestibulo-ocular reflex.
- Spinocerebellum (palaeocerebellum)
- It consists of the vermis and the adjacent medial portion of cerebellar hemisphere. It receives proprioceptive information from the body as well as a copy of the "Motor plan" from the motor cortex.
- By comparing plan, it smoothens and coordinates movements that are ongoing.
- Cerebro cerebellum (neocerebellum)
- It is the large lateral portion of cerebellar hemisphere. They are the newest from a phylogenetic point of view and therefore also called neocerebellum. They interact with the motor cortex in planning and programming movements.
Three cerebellar peduncles connect the cerebellum with three parts of brain stem, i.e. midbrain, pons and Medulla. Superior cerebellar peduncle connects midbrain to cerebellum, middle cerebellar peduncle connects pons to cerebellum, and inferior cerebellar peduncle connects medulla to cerebellum. All cerebellar afferents and efferents pass through these three peduncles.
A. Superior cerebellar peduncle
1. Anterior spinocerebellar
B. Middle cerebellar peduncle
Pontocerebellar(part of the corticopontocerebellar pathway)
C. Inferior cerebellar peduncle
1. Posterior spinocerebellar
2. Cuneocerebellar (posterior external arcuate fibres)
7. Anterior external arcuate fibres
8. Striae medulla res