The human brain (and the brains of most higher organisms) is a wonderful, complex organ. The various regions work together to allow the organism to process information, make decisions, and instruct the various body parts to react accordingly. However, none of this functionality is possible without the various "pathways" distributing information between the various regions. A prominent and essential pathway is the paired cerebral peduncles.
The cerebral peduncles are essential parts of the midbrain. Several distinct fiber bundles collect together in a cylindrical shape. These fibers act like a highway between various brain and spinal cord regions, conveying sensory and motor information and refining movement-related impulses.
While the brain's highways are essential for transporting sensory information and motor responses to and from the brain, many highways also synthesize information and "act" accordingly. Below we'll investigate the cerebral peduncles in more detail, examining what they are, why they are essential, how the peduncles function, and what complications they experience.
A peduncle is the stalk or stem of an inflorescence (flower) and connects the "flower head" to the rest of the plant, which makes it an apt name for cerebral peduncles.
Without the cerebral peduncle, an organism would not function correctly. The "cerebral peduncle" refers to the crus cerebri, substantia nigra, and tegmentum, which combine to form paired cylindrical-shaped fiber collections.
Most cerebral peduncle fibers originate in the cerebral cortex and connect to nuclei in the cerebellum (via the pons) or the spinal cord. The fibers are grouped according to their function and which part of the brain they attach to.
These groupings (arrangements) include:
- Frontopontine fibers (beginning at the frontal lobe; these fibers travel to the nuclei of the pons, passing through the inferior colliculus’s anterior limb).
- Corticospinal and corticobulbar fibers travel to various muscle groups (among other regions) throughout the body.
- Temporo-parieto-occipito-pontine fibers (starting in the temporal lobe; these fibers connect the pons with the occipital, parietal, and temporal cortices).
As with all parts of the human body, the cerebral peduncles are critical for the organism's effective functioning.
The cerebral peduncles are located in the midbrain's anterior (towards the front) regionhttps://radiopaedia.org/articles/cerebral-peduncles. The posterior tectum, together with the cerebral peduncles, form the midbrain.
The midbrain forms the topmost spinal cord region and connects the forebrain (via the thalami) to the cerebellum and pons.
The cerebral peduncles "branch off," connecting to either side of the brain and leaving a gap between the two stalks. We refer to this separation as the interpeduncular cistern (a chamber filled with cerebrospinal fluid).
The cerebral peduncles are subdivided into the crus cerebri and the tegmentum (in the posterior). This division is by the substantia nigra.
The cerebral peduncles are mostly white matter tracts. However, the composition of the cerebral peduncles changes along their length (divided into two parts adjacent to the inferior colliculus and the superior colliculus).
Within the inferior colliculus region, we find:
- Crus cerebri (containing four fiber tracts).
- Substantia nigra (made up of the pars reticulata (anterior) and pars compacta (posterior)).
- Tegmentum (which extends into the midline while the crus cerebri terminates).
- Decussation of the superior cerebellar peduncles (where fibers crisscross over the midline).
- Cerebral aqueduct.
- Lemnisci (medial, spinal, trigeminal, and lateral lemniscus).
- Trochlear nuclei.
In addition to the above, within the superior colliculus region of the cerebral peduncle, we find:
- Red nuclei
- Oculomotor nuclei
The cerebral peduncles play a critical role in passing information (nerve impulses) between the various parts of the brain (the forebrain and the cerebellum) and the spinal cord to other parts of the central nervous system.
However, the function of the cerebral peduncles is not limited to “passing on” information. They also play a role in “refining” the instructions passed on from the cerebral cortex.
Once the higher regions of the brain decide on the best course of action, the information moves into the cerebral peduncles. Here, the peduncles “adjust” the instructions to fit the situation.
I.e., nerve impulses emitted by the cortex fail to factor in certain aspects, like where the limbs are, how the body is already moving, etc. The cerebral peduncles receive this information from the other brain regions and adjust the nerve impulses accordingly.
If the instructions were not tailored to fit the need, our responses would be exaggerated, insufficient, and ungainly.
The cerebral peduncles also assist in learning new motor skills and maintaining balance and posture through interpreting proprioceptive stimuli.
The cerebral peduncles are a product of their various components. Each component is responsible for different functions and information processing; some of these include:
The two horns of the cerebral peduncles (located in the midbrain's anterolateral regions) are separated from the tegmentum by the substantia nigra.
The crus cerebri contains four fiber tracts:
- Frontopontine fibers. These fibers, located most medially, originate in the frontal lobe and terminate at the pontine nuclei.
- Corticospinal fibers. These motor fibers originate in the primary motor cortex. This region is the bridge between the upper and lower motor neurons. The upper fibers connect the brain and spinal cord, while the lower fibers connect the spinal cord to the muscles.
- Corticobulbar tracts. The fibers here are also motor fibers from the primary motor cortex and originate in the precentral gyrus (the origin of several motor pathways). This tract innervates the cranial nerves linked to the head and face.
- Temporopontine fibers. These fibers originate in the cerebral cortex (within the temporal lobe) and run posterolateral. From the sublenticular limb of the internal capsule, these fibers join the crus cerebri and terminate in the pons.
The crus cerebri connects the cerebral hemispheres to the cerebellum through a network of neurons. These neurons are mostly the white matter (axons covered in myelin sheaths) of motor neurons.
The crus cerebri is critical for transporting nerve impulses from the brain, down the spinal cord, and eventually to the relevant muscles.
Together the anterior pars reticulata and the posterior pars compacta from the substantia nigra, a collection of pigmented nuclei. The substantia nigra separates the crus cerebri from the tegmentum.
- Due to the presence of melanin, the pars compacta provides the region with dark pigmentation. The cells in this area produce/synthesize dopamine.
These cells mediate movement and motor coordination (usually inhibiting neurons, particularly those in the caudate nucleus and putamen).
- Cells in the pars reticulata receive impulses from the pars compacta and push the impulses along to the thalamus. This region is important for eye movements, thinking, and learning.
These cells also have an inhibitory action through the gamma-aminobutyric acid present in their cells.
This inhibitory effect prevents/blocks impulses from your brain from reaching the muscles.
The pars compacta and reticulata (together with the globus pallidus) create the striatum.
The substantia nigra plays a fundamental role in assisting with movement, facilitating connections in the brain (as part of the basal ganglia), and controlling various chemical components (e.g., dopamine).
Separated from the crus cerebri by the substantia nigra and located in the posterior region of the midbrain (but still anterior to the tectum), the tegmentum continues down through the pons and up through the midline.
The tegmentum consists of fiber tracts and two differentiated color areas:
- The red nucleus assists in movement and the coordination of sensorimotor information (where sensory information is synergized/integrated with motor responses and affects voluntary and reflexive actions).
Nerve fibers (crossed fibers) from the cerebellar peduncle surround and (some) terminate in the red nucleus. These fibers receive information from the motor cortex.
- Periaqueductal gray matter. Behavioral (like pain and stress) processing and responses occur in this midbrain region. This region receives its color from the lack of axons (i.e., the greater concentration of neuron bodies).
This region deals with pain suppression through endorphin production.
Although some sources view the substantia nigra as part of the tegmentum, others view it as a separate structure in the midbrain.
The tegmentum is essential for alertness, movement, and processing of pain.
As with most brain regions, the cerebral peduncles are not exempt from experiencing injury and diseases. When negatively affected by these complications, the cerebral peduncles severely impact the organism's ability to function correctly.
Some complications include:
- A lack of proprioception.
- Loss of balance.
- Unrefined motor skills.
- Various degrees of paralysis.
Trauma is a significant cause of complications of cerebral peduncles. When the cerebral peduncles are injured, the symptoms present in the corresponding body part (i.e., the part of the body that the damaged nerve cells connect to).
Aside from trauma, diseases are the other significant reason for complications in the cerebral peduncles.
Some of these diseases include:
An infraction is when blood vessels leading to a particular organ (or part thereof) become blocked (often through swelling). This blockage leads to the "starvation" of cells in the affected area, which begin to die.
During a study in 2019, researchers discovered that the leading causes for these blockages in blood flow to the cerebral peduncles were due to pressure on the vertebrobasilar artery and its branches through stenosis or occlusion ( a full or partial blockage of a blood vessel).
Stenosis is narrowing spaces in the spine, applying pressure to the spinal cord and causing pain and discomfort, which could eventually lead to paralysis/loss of function.
An occlusion in the intracranial regions of the brain could cause ischemia (lack of oxygen). This ischemia leads to Wallenberg Syndrome, Locked-In Syndrome, and Top-of-the-basilar syndrome, depending on where the blockage occurs.
These diseases range in consequences/symptoms, from ataxia and disturbances in brain function to quadriplegia.
Although not limited to the cerebral peduncles, “cobblestone lissencephaly” is a developmental disorder affecting the midbrain (and, by extension, the cerebral peduncles).
Certain neurons fail to migrate during fetal development (especially during weeks 12 and 24). This failure results in the absence of grooves and folds developing on the brain’s surface.
Individuals affected by these disorders often suffer from:
- Cerebral ocular and muscle defects.
- Muscle-eye-brain and Fukuyama muscular dystrophy.
- Walker-Warburg syndrome.
The term “lesion” is a collective description of any damaged tissue in the brain. Although all tumors are lesions, not all lesions are tumors.
Unfortunately, tumors may develop in the midbrain and the cerebral peduncle. An example is gliomas. This type of tumor develops when glial cells grow out of control. While some patients may not show symptoms, these tumors are frequently cancerous (malignant).
These tumors are often fatal as they are challenging to treat/reach for surgery and may spread to other areas in the brain.
There are three types of gliomas:
- Astrocytomas. The tumors begin in astrocytes and vary in severity. Glioblastomas (a type of astrocytoma) are “aggressive,” and occur most frequently in adults, while astrocytomas are prevalent in children.
- Ependymomas. Ependymomas (originating in ependymocytes) develop in the ventricles of the brain or the spinal cord and spread via the cerebrospinal fluid. These tumors are more common in children.
- Oligodendrogliomas. As the name implies, these tumors originate in oligodendrocytes. Although they usually start slowly, they often become aggressive. These tumors are relatively rare and occur more frequently in adults.
Some symptoms of glioma include:
- Aphasia. Difficulty communicating, especially speaking.
- Behavior and personality changes.
- Difficulty thinking, understanding, and remembering (cognitive issues).
- Dizziness and headaches.
- Movement issues, like walking and balance.
- Nausea and vomiting.
- Vision loss and other changes.
The type of disease/complication affecting the cerebral peduncles and surrounding brain regions dictates the necessary treatment.
In several cases, the treatment includes:
- Radiation therapy (cancer)
For example, benign astrocytomas (a type of tumor) are usually treated with radiation therapy, as surgery is dangerous in this sensitive brain region. However, a study published in 2002 looked at 7 child patients who had their tumors surgically removed.
While most patients were fine in the long term, some patients experienced a loss of motor, oculomotor, or memory function in the short term. After the surgery, only 1 patient experienced a resurgence of benign astrocytomas.
Occasionally there is no treatment, and management of the disease/symptoms remains. Although genetics play a significant factor, prevention is better than trying to fix the issue.
The cerebral peduncles form part of the midbrain, an essential part of the mammalian brain. The cerebral peduncles facilitate sensory and motor impulses from the brain to the muscles and vice versa. The cerebral peduncle is a collection of white and gray matter that play a fundamental role in the brain's movement, pain responses, and chemical composition. They are paramount in refining motor signals from the cerebral cortex.