Anterior cerebral artery

The anterior cerebral artery (ACA) is one of the major arteries that supplies blood to the brain. It is a branch of the internal carotid artery, which is part of the brain’s arterial network known as the circle of Willis. The ACA is responsible for supplying oxygenated blood to the medial portions of the frontal lobes, superior parts of the parietal lobes, and certain subcortical structures of the brain.

Location

The anterior cerebral artery originates from the internal carotid artery at the base of the brain. It travels medially, passing above the optic chiasm, and then curves around the genu of the corpus callosum. The artery is divided into several segments (A1, A2, etc.), with the A1 segment connecting the internal carotid artery to the anterior communicating artery, while the A2 segment continues anteriorly along the medial surface of the cerebral hemispheres.

Structure and Anatomy

The anterior cerebral artery (ACA) is an essential part of the brain’s blood supply, responsible for providing oxygenated blood to the medial aspects of the cerebral hemispheres. Its anatomy is complex, with multiple segments and branches that serve specific regions of the brain. Below is a detailed description of the artery’s anatomy.

Origin

The anterior cerebral artery originates from the internal carotid artery (ICA). It arises as one of the two major terminal branches of the ICA, with the other being the middle cerebral artery (MCA). The ACA arises at the level of the circle of Willis, near the optic chiasm.

Course

After its origin, the anterior cerebral artery travels medially and slightly anteriorly. It passes above the optic nerve and optic chiasm, then courses into the longitudinal fissure between the two cerebral hemispheres, where it arches over the corpus callosum. It continues along the medial surface of the cerebral hemispheres, providing blood to the superior and medial regions of the frontal and parietal lobes.

Segments of the Anterior Cerebral Artery

The anterior cerebral artery is divided into five main segments, each with its own distinct anatomical course:

A1 Segment (Pre-communicating Segment):

• The A1 segment extends from the origin of the anterior cerebral artery at the internal carotid artery to the anterior communicating artery. This segment runs horizontally, parallel to the optic nerve and the floor of the anterior cranial fossa.
• The A1 segment is responsible for connecting the two anterior cerebral arteries via the anterior communicating artery.

A2 Segment (Post-communicating Segment):

• The A2 segment begins at the anterior communicating artery and runs superiorly and anteriorly, curving around the genu of the corpus callosum.
• This segment is located within the longitudinal fissure and supplies the medial surfaces of the cerebral hemispheres.

A3 Segment (Pericallosal Segment):

• The A3 segment is also known as the pericallosal artery. It continues from the A2 segment, running along the superior surface of the corpus callosum.
• The A3 segment courses around the corpus callosum in close proximity to the cingulate gyrus and supplies the medial portions of the brain’s frontal and parietal lobes.

A4 Segment (Callosomarginal Segment):

• The A4 segment arises from the A3 segment, and it is located more anteriorly along the cingulate gyrus. It is also referred to as the callosomarginal artery when a separate callosomarginal branch exists.
• This segment supplies the superior portions of the medial surface of the frontal lobe and parts of the paracentral lobule.

A5 Segment (Distal Segment):

• The A5 segment is the terminal portion of the anterior cerebral artery. It courses along the upper surfaces of the cerebral hemispheres, reaching the superior margins of the parietal lobe.
• This segment typically gives off smaller branches that supply the most distal portions of the medial cerebral cortex.

Branches of the Anterior Cerebral Artery

The anterior cerebral artery gives rise to several important branches that supply different areas of the brain:

• Anterior Communicating Artery:This artery connects the two anterior cerebral arteries and forms part of the circle of Willis. It allows for collateral blood flow between the right and left sides of the brain.
• Medial Striate Artery (Recurrent Artery of Heubner):The medial striate artery is a small but significant branch of the ACA. It typically arises from the A1 or A2 segment and supplies deep structures, including parts of the basal ganglia and the anterior limb of the internal capsule.
• Orbital Branches:These small branches arise from the A2 segment and supply the medial orbital surface of the frontal lobe, including the olfactory bulb and tract.
• Frontopolar Artery:The frontopolar artery typically arises from the A2 segment and supplies the anterior portions of the frontal lobe, particularly the medial aspects of the superior frontal gyrus.
• Pericallosal Artery:The pericallosal artery runs alongside the corpus callosum and supplies the corpus callosum, cingulate gyrus, and adjacent cortical areas.
• Callosomarginal Artery:In cases where the callosomarginal artery is present as a distinct branch, it arises from the A3 or A4 segment and runs parallel to the pericallosal artery. It supplies the superior medial portions of the frontal and parietal lobes, particularly the paracentral lobule.

Anatomical Relations

• Optic Chiasm and Optic Nerve: The anterior cerebral artery runs above the optic chiasm and optic nerve as it courses medially toward the longitudinal fissure.
• Corpus Callosum: The A3, A4, and A5 segments of the anterior cerebral artery run closely along the superior surface of the corpus callosum, providing blood to this important structure that connects the two hemispheres.
• Cingulate Gyrus: The ACA courses along the cingulate gyrus, which is located superior to the corpus callosum, supplying blood to the medial surface of the cerebral cortex.

Termination

The anterior cerebral artery terminates as smaller branches in the superior and medial surfaces of the frontal and parietal lobes. Its distal branches supply regions involved in sensory and motor control of the lower limbs, as well as areas involved in higher cognitive functions.

Function

The anterior cerebral artery (ACA) plays a crucial role in maintaining the blood supply to essential areas of the brain, particularly those involved in cognitive functions, sensory and motor control, and emotional regulation. Below is a detailed explanation of the functions supported by the ACA, based on the regions it supplies.

Blood Supply to the Medial and Superior Frontal Lobes

The anterior cerebral artery is responsible for supplying the medial and superior portions of the frontal lobes, which are involved in higher-order cognitive functions. These include:

• Cognitive and Executive Functions:The medial prefrontal cortex, which receives blood from the ACA, is critical for decision-making, problem-solving, and planning. It also supports personality traits and the regulation of emotions. Damage to this area due to disruption in ACA blood supply can lead to issues such as impaired judgment, apathy, and poor decision-making.
• Voluntary Motor Control:The ACA supplies the superior portions of the primary motor cortex located in the medial surface of the frontal lobe, specifically in the region of the paracentral lobule. This area is responsible for controlling voluntary movement, particularly for the contralateral lower limbs. A disruption in ACA blood flow to this region can lead to weakness or paralysis (paresis) of the lower limbs on the opposite side of the body.

Blood Supply to the Paracentral Lobule

The ACA supplies the paracentral lobule, a critical region for both motor and sensory functions:

• Motor Functions:The paracentral lobule, located in the medial portion of the frontal lobe and supplied by the ACA, plays a key role in the motor control of the legs, feet, and pelvis. It is involved in the movement and coordination of these body parts, essential for walking, standing, and maintaining balance.
• Sensory Functions:Adjacent to the primary motor cortex is the primary somatosensory cortex, which processes sensory information from the contralateral lower limbs. Blood flow from the ACA ensures that the sensory inputs related to touch, pressure, pain, and temperature from the legs and feet are appropriately processed.

Blood Supply to the Medial Parietal Lobe

The ACA also provides blood to the medial portion of the parietal lobe, which includes regions responsible for processing sensory information and spatial awareness:

• Sensory Integration:The medial aspect of the parietal lobe, supplied by the ACA, is involved in integrating sensory information from different parts of the body. This region helps with proprioception, the sense of body position, and is particularly important for spatial awareness and coordination of movements involving the lower limbs.
• Motor Planning and Execution:The ACA provides blood to areas involved in motor planning, particularly in relation to the lower limbs. This includes the planning of complex movements, such as walking and running. Damage to these regions due to compromised blood flow from the ACA can result in difficulties with coordination and execution of movements.

Blood Supply to the Corpus Callosum

The anterior cerebral artery plays a vital role in maintaining the function of the corpus callosum, the largest white matter structure in the brain that connects the two cerebral hemispheres:

• Interhemispheric Communication:The corpus callosum is crucial for communication between the left and right hemispheres of the brain. The ACA’s blood supply to this structure allows for the transfer of sensory, motor, and cognitive information between the two sides of the brain, enabling coordination and integration of complex behaviors.
• Cognitive and Motor Integration:The anterior part of the corpus callosum, the genu, is supplied by branches of the ACA. This area is responsible for integrating motor activities and processing higher-level cognitive functions. Disruptions in ACA blood flow to the corpus callosum can lead to disconnection syndromes, where communication between the hemispheres is impaired.

Blood Supply to the Cingulate Gyrus

The ACA provides blood to the cingulate gyrus, a part of the limbic system responsible for emotion formation and processing:

• Emotional Regulation:The cingulate gyrus, especially its anterior portion, is heavily involved in emotional regulation, decision-making based on emotional inputs, and motivation. The ACA’s blood supply to this region helps sustain these emotional and motivational processes. Any disruption can lead to apathy, changes in mood, or emotional instability.
• Pain Processing:The cingulate gyrus also plays a role in the perception and emotional response to pain. The ACA ensures that this region is properly supplied with blood, supporting the brain’s ability to integrate sensory pain signals and respond appropriately.

Blood Supply to the Olfactory Structures

Although less prominent, the anterior cerebral artery supplies portions of the olfactory bulb and tract, which are involved in the sense of smell:

Olfactory Processing:The ACA provides blood to the olfactory structures, ensuring that the brain can process olfactory (smell) inputs. While damage to this part of the ACA’s supply area is rare, it can result in anosmia (loss of smell) if compromised.

Role in Motor and Sensory Pathways

Given the areas it supplies, the anterior cerebral artery is crucial for both motor and sensory pathways, particularly for the lower limbs and the contralateral side of the body:

• Motor Pathway Support:The primary motor cortex, which controls voluntary movement, particularly for the contralateral lower limbs, receives blood from the ACA. The artery’s role in maintaining blood flow to this region ensures proper motor execution, especially for complex activities such as walking or running.
• Sensory Pathway Support:The primary somatosensory cortex processes sensory inputs from the opposite lower limbs. By supplying this region, the ACA ensures proper sensory function, allowing the body to perceive touch, pressure, pain, and temperature sensations from the legs and feet.

Clinical Significance

The anterior cerebral artery (ACA) plays a critical role in the brain’s vascular supply, making it highly significant in clinical settings. Occlusion or disruption of the ACA, often due to ischemic stroke, can lead to a distinct set of neurological deficits collectively known as anterior cerebral artery syndrome. The hallmark symptoms of an ACA stroke include:

• Contralateral Hemiparesis: Weakness or paralysis affecting the contralateral lower limb due to the ACA’s supply to the motor cortex controlling leg movements.
• Contralateral Sensory Loss: Sensory deficits in the lower limb on the opposite side of the body, resulting from impaired blood flow to the sensory cortex.
• Urinary Incontinence: This can occur due to involvement of the paracentral lobule, which is responsible for controlling bladder function.
• Personality and Behavioral Changes: Damage to the frontal lobe areas supplied by the ACA can lead to cognitive and emotional disturbances such as apathy, lack of motivation, or disinhibition.
• Alien Hand Syndrome: In rare cases, damage to the ACA can lead to alien hand syndrome, where a patient loses control over one hand, particularly the left, due to the involvement of the corpus callosum.