Midbrain - Anatomy & Physiology
Also known as: Mesencephalon
Introduction
The midbrain or mesencephalon represents the connection between the brain stem and the higher centres of the brain and is involved in most body systems including sleep/consciousness, vision, hearing and temperature regulation.
The midbrain is located between the diencephalon and the hind brain, or brain stem. More specifically, it can be found ventral to the cerebral cortex and between the cerebral pedicles of the diencephalon and the pons. It is a relatively short portion of the upper brain stem and connects higher brain centres with the lower centres and the spinal cord. In most species the midbrain is found most centrally within the cranial cavity.
Midbrain Development
The midbrain arises from the mesencephalon. One of the key roles of this area of the brain during development is the maintainance of a patent cerebral aqueduct to facilitate the appropriate development of higher brains structures including the cerebral hemispheres.
Midbrain Structure & Function
The midbrain has a stratified structure comprising various layers including the tectum, tegmentum and cerebral peduncle. These structures are found in a dorsoventral sequence. The tectum lies dorsally to the cerebral aqueduct and it has four major rounded surface swellings; colliculi (see below). The tegmentum is the core of the midbrain and a large proportion of it is made up by the reticular formation.
Cranial Nerve Nuclei
The major cranial nerve nuclei within the midbrain are the mesencephalic nuclei of the trigeminal nerves (V), the trochlear nuclei (IV), the principle and parasympathetic oculomotor nuclei (Cranial Nerve III), the 'red nuclei' (so named due to its pronounced vascularity) and the periaqueductal gray nuclei. The periaqueductal gray nucleus is a core of gray nervous tissue located adjacent to the cerebral aqueduct. The oculomotor nerve emerges from the mid brain rostral to the pons.
Cerebral Aqueduct
Within the lumen of the midbrain lies the cerebral aqueduct which acts as a simple passage between the spinal cord and the third and fourth ventricles.
Colliculi
The tectum (roof) has four colliculi, two rostral and two caudal.
Caudal Colliculi
The caudal colliculi are widely spaced and are joined by a substantial commissure. The caudal colliculi act as integration centres for auditory pathways. The caudal colliculi also have a further connection to the thalamus via the 'ipsilateral medial geniculate body'. This body is effectively a swelling of the thalamus.
Rostral Colliculi
The rostral colliculi are placed closer together in comparison to the caudal. The rostral colliculi are also joined to the thalamus, but by the 'lateral geniculate bodies' rather than the medial. The rostral colliculi help to integrate the visual pathways and also are involved in somatic reflexes that are caused by visual cues. The rostral colliculi have also been suggested to be involved in spatial integration.
Substantia Nigra
The substantia nigra is a prominent area of the mid brain and is identifiable on cross sections by its darker pigmentation. This pigmentation is due to the gradual accumulation of pigmentation of neurons and is associated with basal nuclei within the tissue. The substantia nigra nuclei are involved in the control of voluntary movement.
Crura Cerebri
These are visible on the ventral surface of the mid brain and consist of fibre tracts that are in passage between the telencephalon and the brain stem. The oculomotor nerves (see above) also emerge in this region of the mid brain, directly rostral to the pons.
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