The Anatomy of the Anterior Cerebral Artery

Supplying the medial portions of the frontal and parietal lobes, the anterior cerebral artery, also known as the ACA, is one of a pair of arteries that play an essential role in delivering oxygen to the brain. Arising at the termination of the internal carotid artery, its course curves upward and towards the middle of the brain, making up a portion of a ring of arteries located at the base of the brain called the circle of Willis.

Because of its essential function in supplying blood to the brain, disorders or trauma to the anterior cerebral artery can lead to serious consequences. In particular, clotting of this artery can lead to stroke, a dangerous “brain attack” caused by inadequate oxygen supply. In addition, due to the regions supplied, problems here can affect gait, movement of the legs and proximal arms, speech ability, and upper-level reasoning.

Anatomy

Structure

One of the larger arteries tasked with supplying blood to important brain regions, the right and left ACAs are major components of the circle of Willis. These are mainly divided into three sections, some of which have important branches:

• A1: Also known as the horizontal segment, this section runs horizontally from the origin of the ACA for 14 millimeters (mm) to the anterior communicating artery, which plays a role in connecting blood supply between right and left hemispheres. The major branches here are the medial lenticulostriate arteries (a series of smaller arteries) as well as the anterior communicating artery.
• A2: Running vertically from the origin of anterior communicating artery, it courses in front of the lamina terminalis and along the edge of the corpus callosum, terminating at its “genu” or bend. Major branches here include recurrent artery of Heubner (also known as the medial striate artery), the orbitofrontal artery (around the eye socket), and the frontopolar artery (which crosses the surface of the front of each hemisphere of the cerebrum).
• A3: The third segment of the ACA, called the precallosal segment, rounds the genu of the corpus callosum and runs until it bends backward above this brain region. This then branches into the pericallosal and the callosomarginal arteries. Running in parallel orientation, both proceed above the corpus callosum.

Location

Along with the middle cerebral artery, the ACA is a terminal branch of the internal carotid artery, which is the primary source of blood to the brain. It originates from the termination of the internal carotid artery, quickly coursing upward and towards the middle to cross the front of the brain on its way to the corpus callosum (the bundle of nerves in the middle of the brain that divides the right and left hemispheres) above the optic nerve.

Anatomical Variations

Several variations in the structure of the ACA have been observed by doctors. Though relatively rare, they’re clinically significant and include:

• Fenestration of ACA: In 0 to 4% of cases, the A1 section of the ACA displays fenestration, in which segments of the artery are duplicated. This anomaly raises the risk of an aneurysm (bleeding in the brain).
• Trifurcation: This anomaly, in which the second section of the ACA splits into three smaller arteries, is seen in approximately 7.5% of people.
• Azygos ACA: In these cases, the primary supply for the ACA comes from a single trunk in the A2 section. This occurs in roughly 2% of cases.
• Bihemispheric ACA: In cases where the A2 segment never properly forms (called “hypoplasia”), the corresponding segment from the other side’s ACA supplies both sides. This is observed in about 4.5% of cases.
• A1 Segment Absence: Roughly in one in 10 people experience complete absence or hypoplasia of the A1 segment of the ACA on one side. In these cases, the opposite side’s ACA—through the anterior communicating artery—provide supply.
• Asymmetry: The first segment of the ACA can also alter its course and structure as a result of aneurysm, leading to asymmetry.

Function

The ACA plays a central role in providing oxygenated blood to numerous brain regions, most notably the medial portions of the frontal and parietal lobes of the brain. Here’s a quick breakdown of what this artery supplies:

• Orbital Branches: Branches arising from the A2 section of the ACA deliver blood to the gyrus rectus (thought to be related to higher cognitive function) as well as the olfactory complex and the medial orbital gyrus, associated with perception of scent.
• Cortical Branches: Via its frontal branches, the ACA supplies the corpus callosum, which integrates sensory, motor, and cognitive function between the hemispheres as well as the cingulate and medial frontal gyri, which are associated with behavior regulation and emotion.
• Parietal Branches: Branches emerging adjacent to the parietal lobe—one of the four major lobes of the brain—supply the precuneus. This region is involved with episodic memory, visuospatial processing, as well as aspects of consciousness and self-awareness.
• Central Branches: Numerous branches of the ACA, emerging from its A1 and A2 segments, supply the anterior perforated substance, which plays a role in ensuring deeper brain structures access blood. The lamina terminalis, a membrane surrounding the hypothalamus—a small region that regulates the release of hormones in the body—is supplied by these arteries as well. In addition, arteries arising here run to parts of the corpus callosum as well as the putamen and caudate nucleus, which regulate motion and coordination.

Clinical Significance

As with any artery involved with supplying the brain, obstruction or constriction of the ACA due to blood clots or other health conditions such as high blood pressure, diabetes, or atherosclerosis (constriction due to a build-up of plaque) present a clear health risk. Most notable of these is anterior cerebral artery stroke, in which a blockage of the artery prevents enough oxygen from reaching the brain. This, in turn, leads to a “brain attack,” which can be fatal and lead to a range of symptoms, including disrupted cognition, weakness of leg and proximal arm, emotional volatility, memory impairment, incontinence, and speech impairment.

In addition, aneurysm—a bulging of the ACA due to weakened walls—arises as a particularly dangerous. This can lead to rupture of the vessel, and the biggest risk is that blood can then damage surrounding brain areas. These cases are a medical emergency; if treatment isn’t sought out quickly, they can be fatal.