The Anatomy of the Brainstem

A Small Structure With Vital Functions

The brainstem is continuous wth the spinal cord

Hank Grebe / Getty Images

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The brainstem is the part of the brain that directly connects with the spinal cord. It contains regions that modulate breathing and heart function, as well as pathways for communication between the brain and the spinal cord. The cranial nerves emerge from the brainstem—controlling movement and sensation in and around the face. 

A number of conditions can affect the brainstem, and the symptoms can vary, often including dizziness, double vision, and/or problems with physical movement. 

The brainstem is continuous wth the spinal cord

Hank Grebe / Getty Images

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:

  • Midbrain: Cranial nerves one through 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).

In an average size adult, the brainstem measures approximately 3 inches long.

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.

Examples of these functions include:

  • Movement: The substantia nigra and red nucleus in the midbrain interact with the basal ganglia in the cerebral hemispheres to help control movement.
  • Autonomic functions: The medulla contains nuclei that maintain functions like breathing and regulation of cardiovascular function.
  • Sleep and consciousness: The reticular formation, a group of nerves that extends throughout the brainstem, interacts with the cerebral cortex to mediate states of arousal.

Associated Conditions

Several conditions can affect the brainstem, leading to neurological symptoms. The symptoms correspond to the specific area of the brainstem that’s affected. Sometimes, very small areas of damage can cause profound symptoms. 

Common symptoms of conditions that affect the brainstem include vertigo (a sense that the room is spinning), impaired balance, vomiting, severe head pain, eye movement abnormalities, weakness and/or sensory loss on one side of the body.

Brainstem stroke: A stroke is brain damage that occurs due to interrupted blood flow. This can occur as a result of severe narrowing of the blood vessels or it may occur when a blood clot travels to the blood vessels of the brainstem, obstructing blood flow.

Some of the subtypes of brainstem stroke include lateral medullary syndrome, Weber syndrome (midbrain stroke syndrome), and lateral pontine syndrome.

Multiple sclerosis (MS): The white matter of the brainstem can be affected by demyelination in MS, leading to symptoms that may be reversible, progressively worse over years, or permanent, depending on the type of MS. 

Increased intracranial pressure: When there is swelling in the brain, such as due to a head trauma, a stroke, or an infection, it can place pressure on the brainstem, affecting its function. This can produce life-threatening effects, such as breathing impairment. 

Movement disorders: Several movement disorders, including Parkinson’s disease, are associated with dysfunction and degeneration of certain areas of the brainstem, such as the substantia nigra and the red nucleus.

Brain aneurysm: A brain aneurysm can affect blood vessels in any location of the brain, including the blood vessels near the brainstem. The aneurysm can compress structures in the brainstem, may impair blood supply, or may cause severe effects if it bleeds.

Brain tumor: A primary brain tumor can develop in any region of the brain and cancer from elsewhere in the body can spread to the brain. Some types of primary brainstem tumors include glioma, meningioma, and acoustic neuroma.

Tests

If you have symptoms of brainstem impairment, you should get medical attention promptly. Your doctor will take a careful medical history and you will have a physical examination, which will include a detailed neurological examination.

Your doctor will carefully check your eye movements. Brainstem conditions can cause diplopia (double vision), which may manifest with uneven eye movements. Nystagmus (jumpy eye movements) is also a condition that’s associated with brainstem involvement.

Based on your history and physical, your doctor might order tests to further evaluate your condition. 

Tests you might need include:

  • Brain imaging: A brain magnetic resonance imaging (MRI) test is usually used for visualization of the brainstem.
  • Angiography: If there is concern about a vascular deformity, visualization of the blood vessels near the brainstem might be necessary, with an invasive or a non-invasive test. 
  • Evoked potentials: This is a non-invasive test that measures your response to stimuli such as light or sound. Evoked potential testing like brainstem auditory evoked potentials (BAER) can help in identifying conditions that affect the brain, including the brainstem.
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