Have you ever wondered why you automatically look towards a sudden flash of light, or turn towards a startling sound, or when someone calls your name? Although these actions happen a dozen times a day, they are so automatic and common that you simply ignore them.
These reflexive actions are mediated by four extremely small swellings at the back of your brainstem. They are collectively called the corpora quadrigemina. This article will describe these structures, focusing on their afferent and efferent connections, together with their functions.
Corpora quadrigemina is the Latin terminology for the quadruple bodies, also known as the colliculi. These round eminences are located on the posterior surface of the midbrain, just below the thalamus. There are two superior and two inferior colliculi,with one of each sitting on either side of the midline. The entire region, consisting of the four colliculi, is known as the tectum, or tectal plate. The superior colliculi play an important role in the visual pathway, while the inferior colliculi are important in the auditory pathway.
Each quadruple body is related to a bundle of nerve fibres called the brachium. The superior brachium establishes the connection between the two superior colliculi, the lateral geniculate body and the optic tract. The inferior brachium links the two inferior colliculi with the medial geniculate body, which in turn is connected to the auditory cortex. The geniculate bodies are relay centers which play a role in vision and hearing. They project posteriorly from the pulvinar of the thalamus and contain sets of nuclei carrying the respective names.
Overall, the afferent pathways reaching the superior colliculi start in the retina, inferior colliculi, spinal cord (spinotectal tract) and cortex. More specifically, the superior colliculi consist of alternating layers of gray and white matter which are arranged into seven layers from superficial to deep, as given below:
- Zonal - composed of fibers which originate primarily from Brodmann area 17 (primary visual cortex) and subsequently arrive as the external corticotectal tract.
- Superficial grey - consists of multipolar interneurons, hence it is the synapsing site of the corticotectal fibers.
- Optic - this layer receives fibers originating in the optic tract, more precisely the retina. They are specifically arranged in a particular pattern, resulting in a retinotopic map of the contralateral (opposite) field of vision.
- Intermediate grey - this layer receives fibers from a variety of sources. Firstly, it receives fibers mainly from Brodmann area 18 (visual association cortex). Therefore, the zonal layer allows you to see an image, while the intermediate grey layer allows you to interpret it. In addition, fibers from the spinal cord (spinotectal tract), the inferior colliculi, locus coeruleus and raphe nuclei also arrive in the intermediate grey layer.
- Deep grey - this layer, together with the deep white one forms the parabigeminal nucleus. The deep grey and the deep white are the main sites where efferent information leaves the superior colliculi.
- Deep white
- Periventricular strata
Generally speaking, the efferent pathways exiting the superior colliculi travel to the retina, pulvinar, lateral geniculate bodies (via the superior brachium), pretectum, brainstem and spinal cord. Two of the most important ones are the tectospinal and tectobulbar tracts.
The tectospinal tract travels through the brainstem in close association with the medial longitudinal fasciculus, ending in the anterior white column of the cervical portion of the spinal cord. The tectobulbar tract travels close to the tectospinal one, ending in the pontine nuclei of the pons and motor nuclei of the cranial nerves.
One of the most important functions of the superior colliculi is in visual body reflexes. Such automatic and involuntary movements include: scanning during the act of reading, covering your eyes in very bright areas or turning your head towards sudden and unexpected visual stimuli.
During these reflexes, the stimuli reach the superior colliculi via the visual pathway and are relayed to the anterior gray column of the spinal cord and the motor nuclei of the cranial nerves via the tectospinal and tectobulbar tracts, respectively.
The inferior colliculus consists of a central nucleus of gray matter. The nucleus has two zones (dorsomedial and ventrolateral) that are surrounded by a dorsal cortex, composed of four neuronal layers (I to IV), and an external cortex. The inferior colliculi receive auditory information from the cochlear nuclei via the terminal nerve fibers of the lateral lemniscus.
The majority of efferent fibers leaving the inferior colliculi travel through the inferior brachium and project onto the ipsilateral (same side of the body) medial geniculate body. A portion of those synapse within the geniculate body, while others continue directly through it. In the end, both sets travel to the auditory cortex in the superior temporal gyrus via the auditory radiation.
The inferior colliculi are involved in the reflex component of the auditory pathway. This involvement is mediated by small efferent projections that travel through the superior colliculi, connecting with the tectospinal and tectobulbar tracts. Similar to the visual body reflexes, the auditory reflex is automatic and involuntary, allowing you to turn your head towards unexpected acoustic stimuli, such as someone calling your name or a startling sound.