Midbrain - Anatomy & Physiology

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/conciousness, 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 diecncephalon 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 to 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 maintainence 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).

Oculomotor Nerves

The oculomotor nerves (Cranial Nerve III) have their superficial nuclei within the midbrain.

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. The caudal colliculi are widely spaced and are joined by a substancial commissure. The caudal colliculi act as integration centres for auditory pathways. The caudal colliculi also have a further connection to the thalamus via the 'ipselateral medial geniculate body'. This body is effectively a swelling of the thalamus.

Cranial nerves III and IV have their nuculi within the midbrain.

Structure and Function

	Structures Present	Function
Midbrain (Mesencephalon)	Colliculi Occulomotor Nuculi	Colliculi - Processing visual and auditory information. Occulomotor Nuculi - Eyeball mm., more cranial nerves.

Oculomotor Nuclei

• See blood suppply to the brain and the relevent species differences.

• The cerebral (mesencephalic) aqueduct is a ventricular tube running through the midbrain which connects the third and fourth ventricles. It contains cerebral spinal fluid which offer it immunological protection.

See immunology.

Species Differences

• Other than size the brain of most species is fairly consistant.