Brainstem

The brainstem is an anatomical structure of the brain. It links the forebrain, the cerebellum, and the spinal cord. The brainstem plays a vital role in bodily functions such as breathing and heart rate. It is mostly involved in regulating involuntary processes. The brainstem includes three main parts: the midbrain, pons, and medulla. These parts help perform vital functions and send signals between the brain and the spinal cord. The brainstem also acts as a pathway that carries signals traveling to and from the brain.

Background

The brainstem is part of the central nervous system (CNS). The CNS is the system that controls all other parts of the body. The two main parts of the central nervous system are the spinal cord and the brain. The brain is housed and protected inside the bony skull. The spinal cord is protected inside a canal that runs through the vertebrae of the spine. The spinal cord is a long structure of nervous tissue that sends signals between the brain and the body. The spinal cord is about 43 to 45 centimeters (17 to 18 inches) long and runs from the brainstem down to the lower back (lumbar region). When the spinal cord enters the skull, it continues as the medulla oblongata, which is part of the brainstem.

Two types of cells make up the parts of the central nervous system: neurons and glial cells. Neurons are cells that send signals throughout the CNS and other parts of the body. Glial cells make up most of the CNS, and they protect neurons, provide nutrients to neurons, and create and move cerebrospinal fluid. Neurons and glial cells are highly specialized, and many different types of each exist in different parts of the body. All neurons have the same basic structure. A neuron has a main part called the cell body. It has dendrites, which are small branches that project out from the cell body. It also has an axon, which stems from one side of the neuron. Axons can be very long. They send signals from neurons to other cells.

The cells that make up the CNS are coated in a material called myelin. This material is white and is made mostly of lipids and proteins. The coating helps insulate nerve fibers and helps signals travel faster. The brainstem, other parts of the brain, and the spinal cord are also bathed in cerebrospinal fluid. This clear, colorless fluid helps protect the CNS by cushioning blows and other injuries. The fluid is produced inside the brain and is absorbed by the blood so that it can be replaced to ensure any waste products are taken away from the brain.

Overview

The brainstem begins when the structure of nervous tissue that is the spinal cord enters the bony skull. It is located below the thalamus and in front of the cerebellum. The section of the brainstem that is located nearest the spinal cord is called the medulla. The medulla, or medulla oblongata, is the lowest part of the brainstem. The medulla helps maintain some of the vital functions of the human body, such as breathing and heartbeat. It also helps control many reflex functions such as vomiting, coughing, and sneezing. This part of the brainstem includes the medullary pyramids, which are structures that include many corticospinal axons.

The pons, the middle section of the brainstem, is positioned above the medulla. This section of the brainstem is involved in motor control, including control of some facial muscles, and sensory perception, such as understanding where one is in space. The pons also have parts that help regulate consciousness and sleep.

The midbrain, which is also called the mesencephalon, is the highest part of the brainstem. The top of the midbrain is called the tectum (or tectal area), which helps process visual and auditory information. This part of the midbrain connects to the cerebellum. The bottom of the midbrain is called the tegmentum, which is involved in movement and alertness. The midbrain is important for body movement, eye movement, vision, and hearing. It also plays a role in voluntary body movement. The brainstem also contains a network called the reticular formation, which helps control consciousness, alertness, and sleep–wake cycles.

The cerebellum is a brain structure that lies behind the brainstem. The brainstem is connected to the cerebellum via three peduncles. The inferior peduncle connects the cerebellum to the medulla. The middle peduncle, which is the largest, connects the cerebellum to the pons. The superior peduncle connects the cerebellum to the midbrain.

The brainstem is also the part of the brain where many cranial nerves (CNs) connect to the brain. Humans have twelve different pairs of CNs. Most cranial nerves begin in the brainstem, except for the first and second pairs, which arise from the forebrain. CNs are nerves that send and receive motor and sensory signals between the brain and other parts of the body.

Ten CNs connect to various parts of the brainstem:

• CN III (oculomotor nerve) connects to the midbrain. It helps control eye movement and pupil constriction.
• CN IV (trochlear nerve) connects to the midbrain. It also helps control eye movement.
• CN V (trigeminal nerve) connects to the pons. It sends sensory information from the face and head.
• CN VI (abducens nerve) connects to the pons. It helps control eye movement.
• CN VII (facial nerve) connects at the border between the pons and the medulla. It helps control muscles in the face.
• CN VIII (vestibulocochlear nerve) connects to the pons. This nerve sends auditory information and information about where a person is in space.
• CN IX (glossopharyngeal nerve) connects to the medulla. It sends sensory and motor information to parts of the tongue and throat.
• CN X (vagus nerve) connects to the medulla. It sends sensory and motor function information to some glands and organs.
• CN XI (spinal accessory nerve) connects to the medulla. It helps control head movement.
• CN XII (hypoglossal nerve) connects to the medulla. It helps control the muscles of the tongue.

The brainstem is one of the evolutionarily oldest parts of the brain. Many animals have brains that include parts similar to the human brainstem. Reptiles’ brains include parts similar to the human brainstem, but they have less developed higher brain regions, such as a smaller and simpler cerebrum. Damage to the brainstem can be life-threatening because it controls essential functions such as breathing and heart rate.

Bibliography

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“Cerebellum: What it is, Function & Anatomy.” Cleveland Clinic, 7 July 2022, my.clevelandclinic.org/health/body/23418-cerebellum. Accessed 31 Mar. 2026.

Kinser, Patricia Anne. “Brain Structures and Their Functions.” Serendip Studio, 2 May 2018, serendipstudio.org/bb/kinser/Structure1.html. Accessed 31 Mar. 2026.

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Schröder, Hannsjörg, et al. The Human Brainstem: Anatomy and Pathology. Springer International Publishing, 2023.