The Anatomy of the Brainstem

Anatomy

The brainstem is a stem shaped structure, extending down from the posterior (back) part of the brain to the spinal cord. It is protected by the meninges, which are composed of three layers of sheet-like connective tissue that envelop the brain and spinal cord.

Outside the meninges, the brainstem is shielded by the lower part of the skull. Cerebrospinal fluid (CSF) flows between the meninges and the brainstem, providing nourishment and protection.

Structure

From top to bottom, the brainstem includes the midbrain, the pons, and the medulla. Each of these sections contains nerve pathways, many of which travel throughout the whole brainstem. Cranial nerve roots are located in the brainstem, and each pair of the 12 cranial nerves emerge from the brainstem.

The cranial nerve levels are:

Cerebrum: Cranial nerves one and two

Midbrain: Cranial nerves three and four

Pons: Cranial nerves five through eight

Medulla: Cranial nerves nine through 12

The deeper portion of the brainstem is composed of grey matter, and the remaining nerve pathways of the brainstem are primarily are composed of white matter, which is more heavily myelinated (protected by a type of fat that insulates nerves).

The brainstem receives blood supply from several arteries, including the vertebral arteries, basilar artery, and pontine arteries.

Location

Located towards the back of the neck, the brainstem is the lower part of the brain, and it is continuous with the spinal cord. Behind the brainstem, the cerebellum (the part of the brain largely responsible for coordination) is also protected by the lower portion of the skull.

Anatomical Variations

The most common variations of the brainstem generally involve asymmetry of the blood supply or of the cranial nerves. These variations are usually minor, and they typically don’t cause clinical effects.

Aneurysms, which are defects in a blood vessel, can be congenital, and can develop in the blood vessels near the brainstem. Brain aneurysms near the brainstem may cause serious effects due to compression or bleeding.

Function

The brainstem contains nerves and tracts (nerve pathways) that provide motor and sensory functions throughout the body. Nerve tracts are composed of a sequence of nerves that rapidly send messages along a specific route.

Major nerve pathways in the brainstem include:

Spinothalamic: This tract runs at the outer portion of the brainstem, relaying messages of sensation that originate in sensory nerves to the spinal cord, through the brainstem, and to the thalamus in the cerebral cortex.

Corticospinal: This tract runs medially, near the center of the brainstem, sending messages from the motor portion of the cerebral cortex through the brainstem, to the spinal cord, and eventually to the muscles to control movement.

Spinocerebellar: This tract runs in the lateral portion of the brainstem, relaying messages between the cerebellum and the spinal cord to regulate the body’s position.

Some of the structures located in the brainstem work by coordinating with neurotransmitters (chemical messengers) and structures in other parts of the brain and throughout the body to control complex functions.

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