M02.02.01 The midbrain

The midbrain, also known as the mesencephalon, is a region of the brainstem that plays a crucial role in various sensory and motor functions. Here’s an overview of the external anatomy of the midbrain, including its tectum and peduncles, as well as its blood supply:

1. External Anatomy:
   • Location: The midbrain is located between the diencephalon (thalamus and hypothalamus) above and the pons below.
   • Size and Shape: It is a small, cylindrical structure with a length of about 2 to 2.5 centimeters.
   • Structures: The midbrain consists of several prominent structures, including the tectum, cerebral peduncles, and the substantia nigra.
2. Tectum:
   • The tectum is the dorsal (upper) region of the midbrain and is involved in sensory processing, particularly visual and auditory functions.
   • Superior Colliculi: These paired structures are located at the posterior aspect of the tectum and are involved in visual processing and orienting movements of the eyes and head in response to visual stimuli.
   • Inferior Colliculi: These paired structures are situated below the superior colliculi and play a crucial role in auditory processing and sound localization.
3. Peduncles:
   • Cerebral Peduncles: The cerebral peduncles are located on the ventral (lower) aspect of the midbrain and contain bundles of nerve fibers known as corticospinal tracts. These tracts carry motor information from the cerebral cortex to the spinal cord and control voluntary movements.
   • Crus Cerebri: The crus cerebri is a part of the cerebral peduncles and contains descending motor fibers originating from the cerebral cortex.

Blood Supply:

• Posterior Cerebral Arteries: The midbrain receives blood supply primarily from branches of the posterior cerebral arteries, which are branches of the basilar artery.
• Superior Cerebellar Arteries: The superior cerebellar arteries, also arising from the basilar artery, contribute to the blood supply of the midbrain.
• Perforating Arteries: Numerous small perforating arteries, such as the paramedian branches and quadrigeminal arteries, originate from the posterior cerebral and basilar arteries and provide blood supply to specific regions of the midbrain.

Understanding the external anatomy of the midbrain, including its tectum and peduncles, is essential for identifying and localizing various neurological structures and functions. The blood supply to the midbrain is crucial for its proper function, and any disruption can lead to ischemia, stroke, or other related conditions. Diagnosis and treatment of midbrain-related disorders often involve neuroimaging techniques like MRI or CT scans, as well as careful evaluation of neurological symptoms and deficits.