The posterior fossa is a cup-shaped compartment located at the base of the skull, forming the deepest and most posterior of the three cranial fossae. This bony basin acts as a protective shell for some of the most sensitive and functionally concentrated components of the central nervous system. Its primary role is to house and shield the structures responsible for controlling fundamental life-sustaining processes and coordinating movement.
Anatomical Placement and Boundaries
This cranial compartment is situated in the lower, back portion of the head. It is defined superiorly by a tough, shelf-like fold of dura mater known as the tentorium cerebelli, which separates the posterior fossa from the overlying cerebrum. The tentorium cerebelli creates a firm roof for this space, allowing communication with the upper brain regions only through a central opening called the tentorial notch.
The boundaries of the fossa are formed by a rigid collection of bone, primarily the occipital and temporal bones. The posterior wall is composed of the curved, squamous part of the occipital bone. Laterally, the compartment is bordered by the petrous and mastoid parts of the temporal bones, which are some of the densest bones in the body.
In the front, the floor slopes upward, formed by the clivus, which is made up of the sphenoid and occipital bones. This upward slope provides structural support to the brainstem nestled above it. At the bottom of this fossa lies the foramen magnum, the largest opening in the skull, which serves as the passage where the brainstem transitions into the spinal cord.
The bony structure of the posterior fossa also contains several smaller openings, or foramina, that allow for the passage of nerves and vessels. For instance, the jugular foramen transmits the internal jugular vein and several cranial nerves, including the vagus and glossopharyngeal nerves. The internal acoustic meatus, found on the posterior surface of the temporal bone, is the pathway for the facial and vestibulocochlear nerves.
Essential Contents and Structures
The space within the posterior fossa is largely occupied by two major neurological components: the cerebellum and the brainstem. These two structures account for the importance of this region to bodily function. The cerebellum, which is Latin for “little brain,” occupies the largest volume of the fossa and is primarily dedicated to coordinating voluntary movements.
The cerebellum refines motor activity, ensuring that movements are smooth, precise, and balanced. It constantly processes sensory input from the body and fine-tunes motor commands sent from the cerebrum. This process is what allows a person to maintain posture, walk steadily, and perform complex tasks that require fine motor control.
Positioned anteriorly, the brainstem is a column of nervous tissue that connects the cerebrum and cerebellum to the spinal cord. It is subdivided into three segments: the midbrain, the pons, and the medulla oblongata. The brainstem is the body’s central control system for basic, involuntary life functions, often referred to as the vegetative functions.
The medulla oblongata, the lowest part of the brainstem, is responsible for controlling heart rate, breathing, and blood pressure. Above it, the pons acts as a bridge, relaying signals between the cerebrum, cerebellum, and the medulla. The midbrain, the topmost segment, is involved in regulating movement and processing auditory and visual information.
In addition to these major components, segments of nearly all cranial nerves from the third through the twelfth either originate from or pass through the posterior fossa. These nerves control functions such as eye movement, hearing, facial sensation, and tongue movement, and they exit the skull through the various foramina in the fossa’s bony walls. The fourth ventricle, a cavity filled with cerebrospinal fluid, is also located here, nestled between the cerebellum and the brainstem.
Clinical Significance of Restricted Space
The rigid, fixed nature of the bony posterior fossa makes it a sensitive area when pathology develops. Unlike the main part of the brain, the posterior fossa is tightly constrained by its bony boundaries and the dural shelf of the tentorium. This anatomical limitation means that any increase in volume quickly leads to compression of the vital contents. A tumor, bleeding, or swelling creates a mass effect, rapidly increasing localized pressure. Because the brainstem controls respiration and consciousness, even a small mass can cause neurological deterioration by squeezing these life-sustaining centers.
One frequent complication of pathology in this region, particularly with tumors, is the development of obstructive hydrocephalus. The fourth ventricle, located in the posterior fossa, serves as a narrow passageway for cerebrospinal fluid (CSF) to exit the brain’s ventricular system. A mass can block the outflow of CSF, causing the fluid to back up and accumulate in the other ventricles, leading to a dangerous rise in intracranial pressure.
This blockage is a common presentation for posterior fossa tumors, especially in children, where the incidence of hydrocephalus can be as high as 70 to 90 percent at the time of diagnosis. The rapid onset of symptoms from either direct brainstem compression or secondary hydrocephalus highlights why conditions affecting the posterior fossa require immediate medical attention.