Located in the midbrain, the pedunculi cerebri are composed of the cerebral peduncles (crura cerebri) and the midbrain cap (tegmentum mesencephali). Lesions in these areas can be associated with a variety of conditions, depending on which structures are affected. For example, Parkinson’s disease results from atrophy of the substantia nigra in the tegmentum and typically leads to tremor, rigor, bradykinesis, and postural instability.
What are the pedunculi cerebri?
The pedunculi cerebri are also known as the (cerebral) peduncles and are located in the midbrain (mesencephalon). In the two halves of the brain, they form a symmetrical structure and can be anatomically divided into cerebral peduncles (crura cerebri) and midbrain peduncles (tegmentum mesencephali), with the tegmentum having a significantly larger proportion. More rarely, the literature equates the pedunculi cerebri with the crura cerebri without including the tegmentum. The midbrain consists not only of crura cerebri and tegmentum mesencephali, but also has a third part: the midbrain roof or tectum mesencephali. The aquaeductus mesencephali separates the tectum from the tegmentum and is filled with fluid.
Anatomy and structure
The pedunculi cerebri are located in the midbrain and are composed of the crura cerebri and the tegmentum mesencephali. The crura cerebri are divided into right and left sections; the fossa between these two sections is the fossa interpeduncularis. Nerve fibers from the internal capsula pass through the crura cerebri and connect the cortex with deeper brain areas. The capsula interna unites different nerves of the brain and thus represents a significant data pathway in the human brain. The tegmentum mesencephali includes the substantia nigra, which is part of the extrapyramidal motor system and is located next to the crura cerebri. In addition, the tegmentum mesencephali contains parts of the formatio reticularis and other nuclear areas consisting of gray matter: In them, the density of cell bodies is particularly high. These nuclear areas form the origin of cranial nerves, which travel from them through the brain. Nerve tracts can carry both afferent fibers, which carry information from the peripheral nervous system to the brain, and efferent fibers, which carry signals from the central nervous system to the periphery. The nuclei of the tegmentum include:
- Nucleus ruber
- Nucleus nervi oculomotorii
- Nucleus accessorius nervi oculomotorii
- Nucleus nervi trochlearis
- Nucleus mesencephalicus nervi trigemini
Function and tasks
Although the pedunculi cerebri anatomically form a contiguous complex, their functions differ according to area. Numerous nerve tracts traverse the capsula interna in the crura cerebri, and three parts of the internal capsule can be distinguished. The crus anterius is formed by the nerve tracts that run between the caudate nucleus and the putamen/pallidum. These include the thalamic peduncle and the tractus frontopontinus, which transmits information from the frontal lobe to the pons. The crus posterius of the internal capsule includes the pyramidal tract, which is responsible for movement control, and fibers of the auditory and visual pathways. Between the crus anterius and the crus posterius lies the genu (“knee”) of the internal capsule. In the tegmentum mesencephali are the substantia nigra and the nucleus ruber, which belong to the extrapyramidal motor system. On the other hand, the fibers of the third cranial nerve originate from the nucleus nervi oculomotorii and the nucleus accessorius nervi oculomotorii. This controls movements of the eyes and is also known as the oculomotor nerve. Its three branches are the superior ramus, the inferior ramus, and the ramus to the ciliary ganglion. The latter is a collection of nerve cell bodies in the peripheral nervous system located in the orbit. Another cranial nerve is the trochlear nerve. Its origin is also in the tegmentum mesencephali of the pedunculi cerebri: the nucleus nervi trochlearis is responsible for the nerve pathway. As the fourth cranial nerve, the trochlear nerve is also responsible for eye movements. In contrast, the mesencephalic nucleus nervi trigemini is a sensitive nucleus and belongs to the trigeminal nerve (cranial nerve V).The nucleus mesencephalicus nervi trigemini processes proprioceptive perceptions from the external eye muscles, the masticatory muscles, the temporomandibular joint, and the periodontium.
Diseases
Because of the various structures located in the pedunculi cerebri, numerous disorders and diseases associated with the cerebral peduncles are possible. The substantia nigra is located in the tegmentum mesencephali; in Parkinson’s disease, the substantia dwindles and causes dopamine deficiency in the brain. Dopamine is an important neurotransmitter and is essential for the transmission of information from one nerve cell to another in the extrapyramidal motor system. Therefore, typical symptoms of PD include muscle tremor (tremor), muscle rigidity (rigor), slowing of movements (bradykinesis), and postural instability (postural instability). To compensate for the lack of dopamine, doctors can use the drug L-dopa. This is a dopamine precursor that can cross the blood-brain stage. Other treatment options include catechol-O-methyltransferase (COMT) inhibitors, which can slow the breakdown of dopamine or L-dopa, or targeted brain stimulation using brain pacemakers. Other neurological problems that can arise in connection with the pedunculi cerebri are lesions of the capsula interna. Because many important nerve pathways run through it, different symptoms can develop depending on the location. Impairments of the pyramidal tract may lead to hemiparesis of the contralateral half of the body. One possible cause is a stroke: If an artery that supplies the brain with oxygen, energy and nutrients closes, there is initially a loss of function in this area. Persistent undersupply leads to the death of the affected nerve cells.
