The Anatomy of the Brainstem

The brainstem is a stalk-like structure in the brain that connects a part of the brain called the cerebellum to the spinal cord.

Although small, the brainstem performs many critical functions, including controlling the heart and breathing rate, providing motor and sensory functions to the face and neck, regulating the sleep cycle, and conveying nerve signals back and forth from the brain to the rest of the body.

The brainstem can be affected by diseases like stroke, aneurysm, multiple sclerosis, and tumors. When this happens, the functions of the brainstem can be compromised, leading to muscle weakness, abnormal heart rhythms, vision changes, and loss of balance.

The article explains the anatomy and function of the brainstem and its various parts. It also describes conditions that can affect the brainstem and how they are diagnosed.

Anatomy and Parts of the Brainstem

The brainstem is the lowest part of the brain located towards the back of the neck. It is a stem-shaped structure roughly 3 inches in length extending from the middle of the brain to the spinal cord.

The brainstem is protected by a three-layered membrane called the meninges, which stabilizes the brainstem, absorbs shock, and supports blood vessels and nerves that service the brain. Cerebrospinal fluid (CSF) flows between the meninges and brainstem, providing nourishment and protection.

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

Parts of the Brainstem 

The brainstem is composed of three main parts, each of which has specific functions. Each section serves as a nerve gateway, relaying motor and/or sensory information to and from different parts of the body.

The sections of the brainstem include the following:

• Midbrain: This is the smallest part of the brainstem, measuring roughly 3/4 of an inch. Located at the very top of the brainstem, the midbrain is associated with vision, hearing, sleep, wakefulness, arousal, and temperature regulation.
• Pons: This section measures roughly 1 inch and is situated just below the midbrain. It is tasked with facial sensations and expressions, eye movements, chewing, and the secretion of saliva and tears. It also processes sensory information for balance.
• Medulla oblongata: Also known simply as the medulla, this is the part of the brainstem that connects to the spinal cord. The medulla oblongata regulates breathing, heart rate, blood pressure, and reflexes such as vomiting, coughing, and sneezing.
  

What Does the Brainstem Do?

The brainstem has several functions regulated by different nerve pathways. The functions can be broadly described as:

Locomotion and Coordination

All information relayed to and from the cerebellum must pass through the brainstem. The cerebellum, also known as the "little brain," is responsible for coordinating movement, balance, and proprioception (the awareness of your body in space).

Motor and Sensory Function of the Head and Neck

The brainstem is where 10 of the 12 pairs of cranial nerves originate. These nerves relay signals to and from the head and neck, enabling smell, vision, taste, swallowing, facial sensations, and the movement of the head, face, shoulder, eye, and tongue

The cranial nerves in the brainstem are distributed as follows:

• Midbrain: Cranial nerves 3 through 4
• Pons: Cranial nerves 5 through 8
• Medulla: Cranial nerves 9 through 12

Consciousness

The ascending reticular activating system (ARAS) is a nerve pathway originating in the brainstem that regulates consciousness. This includes functions like the sleep-wake cycle, alertness, arousal (wakefulness) awareness, and conscious emotional and behavioral responses informed by your subconscious stimuli.

Autonomic Functions

These are involuntary body functions regulated by the larger autonomic nervous system. The brainstem integrates nerve pathways, primarily in the medulla, that regulate key autonomic functions like breathing, heart rate, blood circulation, blood pressure, and reflexes.

What Conditions Can Affect the Brainstem?

Several conditions can directly or indirectly affect the brainstem. Because the brainstem is responsible for many key functions, the list of possible symptoms is extensive, ranging from uncontrollable eye movements and vertigo (spinning sensations) to one-sided facial paralysis and loss of muscle control.

These are some of the conditions known to affect the brainstem:

• Brainstem stroke: A stroke is brain damage caused by interrupted blood flow, typically when a blood vessel is blocked by a clot (ischemic stroke). Brainstem strokes cause hallmark symptoms of all strokes but can also cause double vision, slurred speech, and decreased consciousness. Severe cases can lead to "locked-in syndrome" in which survivors can move only their eyes.
• Multiple sclerosis (MS): This is an autoimmune disease in which the immune system attacks the protective coating of nerve cells, called myelin, causing nerves to malfunction in different parts of the body. When the brainstem is affected, MS can cause blurred vision, double vision, trouble swallowing, slurred speech, dizziness, loss of coordination, and decreased sensations.
• Increased intracranial pressure (ICP): When there is swelling in the brain, such as due to head trauma, encephalitis (brain inflammation), or infection, it can place extreme pressure on the brainstem. Symptoms of ICP include head pain, blurred vision, nausea, vomiting, sleepiness, change in personality, and problems walking or speaking.
• Movement disorders: Several movement disorders, including Parkinson’s disease and Huntington's disease, are associated with dysfunction of the brainstem. If cranial nerves are affected, you might experience a loss of muscle control (ataxia) or involuntary twitches or spasms (myoclonus) of the neck, eyelids, tongue, or lower face.
• Brain aneurysm: This is the bulging of the wall of an artery in the brain. An aneurysm in the brainstem can compress nerves, causing a diverse range of neurological symptoms, or spontaneously burst, leading to hemorrhagic stroke, coma, and even death.
• Brain tumors: These are abnormal growths that can either be benign (non-cancerous) or malignant (cancerous). Types that can affect the brainstem include glioblastomas, meningiomas, and acoustic neuromas. The symptoms vary by the size and location of the tumor.
  

Diagnosing Brainstem Problems

If you have symptoms of a brainstem problem, seek prompt medical attention. Your healthcare provider will take a careful medical history and perform a physical exam, including an in-office evaluation known as a neurological exam.

Eye movements are a key focus of the exam as brainstem diseases can often cause diplopia (double vision), nystagmus (jumpy eye movements), or myokymia (eyelid twitching).

Based on your history and physical, your medical provider may order tests to further evaluate your condition, including:

• Brain imaging: This typically involves non-invasive magnetic resonance imaging (MRI) which creates highly detailed images of soft tissues and blood vessels in the brain using powerful magnetic and radio waves.
• Cerebrovascular angiography: This is a type of X-ray that uses an injected contrast dye to visualize blood vessels in the brain. Among other things, the test can detect blockages leading to ischemic stroke or ruptures leading to hemorrhagic stroke.
• 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 identify conditions that affect the brainstem, including multiple sclerosis.

Summary

The brainstem is a roughly 3-inch structure in the brain that connects the cerebellum to the spinal cord. It is composed of three sections called the midbrain, pons, and medulla oblongata that regulate numerous functions in the body, including movement, coordination, balance, taste, smell, facial expression, reflexes, heart rate, breathing rate, blood pressure, and the sleep-wake cycle.

Conditions adversely affecting the brainstem include stroke, aneurysm, multiple sclerosis, brain tumors, Parkinson's disease, Huntington's disease, and increased intracranial pressure.