Brainstem

What is the brainstem?

The brainstem, also written brain stem, is the lower part of the brain that connects to the spinal cord. It helps regulate breathing, heart rate, blood pressure, basic reflexes, and arousal, while carrying sensory and motor information between the spinal cord and higher brain regions. Its three main structures are the midbrain, pons, and medulla oblongata.

Anatomy

• Midbrain

  The midbrain, also known as the mesencephalon, is the uppermost part of the brainstem. Its tectum includes the superior colliculi, which help orient responses to visual events, and the inferior colliculi, which are important in auditory processing. The tegmentum contributes to motor functions, arousal, and basic reflexes. The substantia nigra and red nucleus also contribute to motor control.

• Pons

  The pons is located between the midbrain and the medulla oblongata. It serves as a relay center for information passing between the cerebellum and the cerebrum, and it is involved in the regulation of sleep, arousal, and facial movements. The pons also contains the nuclei of several cranial nerves that control functions such as facial sensation, taste, and eye movements.

• Medulla Oblongata

  The medulla oblongata is the lowest part of the brainstem and connects to the spinal cord. It controls vital autonomic functions, such as heart rate, breathing, and blood pressure, and houses the nuclei of several cranial nerves that regulate functions like swallowing, gagging, and coughing. The reticular formation extends through the medulla and other parts of the brainstem. This network contributes to arousal, sleep, and reflex coordination.

Brainstem functions

• Autonomic Functions

  The brainstem is responsible for regulating many essential autonomic functions, such as heart rate, breathing, and blood pressure. Together, these functions help maintain a stable internal environment, or homeostasis.

• Reflexes

  Basic reflexes, such as swallowing, coughing, sneezing, and vomiting, are coordinated by the brainstem. These reflexes are important for protecting the body from harm and maintaining normal physiological processes.

• Sensory and Motor Relay

  The brainstem serves as a relay center for sensory and motor information passing between the spinal cord and higher brain regions. These pathways connect incoming information with responses elsewhere in the nervous system.

• Consciousness and Sleep

  The reticular formation within the brainstem plays a crucial role in regulating arousal, consciousness, and sleep. Damage to the reticular formation can result in coma or other disorders of consciousness.

Associated Disorders

• Brainstem Stroke

  A brainstem stroke occurs when blood flow to the brainstem is disrupted, leading to damage or death of brain tissue. Brainstem strokes can result in severe neurological impairments, including paralysis, difficulties with speech and swallowing, and disruptions in autonomic functions like breathing and blood pressure regulation.

• Locked-In Syndrome

  Locked-in syndrome is a rare neurological disorder characterized by complete paralysis of voluntary muscles, with vertical eye movements and blinking often preserved in the classic form, while consciousness remains intact. This condition is typically caused by damage to the pons, which disrupts motor pathways in the brainstem.

• Brainstem Tumors

  Brainstem tumors are abnormal growths that develop in the brainstem. These tumors can cause various symptoms, depending on their location and size, including difficulties with coordination, weakness, and problems with speech, swallowing, and vision.

Sources and evidence

• OpenStax: Anatomy and Physiology 2e: The Central Nervous System. An introductory anatomy source; vital functions rely on interconnected circuits.
• Genetic and Rare Diseases Information Center: Locked-in syndrome. Presentations vary; preserved eye movement is not identical in all cases.