What is a Gyrus?
A gyrus (plural: gyri) is a ridge on the surface of the brain. Each ridge is surrounded by fissures known as sulci (singular: sulcus). Gyri are unique structures that have an important evolutionary function; they increase the surface area of the brain up to an impressive 2000 centimeters squared. This large surface area gives us better cognition without having to increase the size of the brain, which is of course trapped within the skull and hence limited by size.
There are many specific gyri which are important to the brain’s functions. For example, the superior temporal gyrus contains the Wernicke’s area, which is vital in the computation of language, while the precentral gyrus acts as the primary motor center of the brain. As gyri are important brain structures, they have a great deal of clinical significance, with various abnormalities leading to disorders such as epilepsy.
A gyrus is a ridge-like elevation found on the surface of the cerebral cortex. Gyri are surrounded by depressions known as sulci, and together they form the iconic folded surface of the brain. Gyri are made up of the gray matter of the cerebral cortex, which mainly consists of nerve cell bodies and dendrites. The size and layout of gyri vary from person to person, although there are certain important gyri that are found in everyone. However, even the exact size and location of these gyri can vary slightly between individuals.
There are certain gyri in the brain that are of particular importance. These are outlined below:
- Precentral gyrus - Found on the lateral surface of the the frontal lobe and acts as the primary motor area of the brain.
- Inferior frontal gyrus - Found on the lateral surface of the frontal lobe, involved in speech production.
- Anterior paracentral lobule - Found on the medial surface of the frontal lobe and is a continuation of the precentral gyrus.
- Gyrus rectus/orbital gyri - Found on the inferior surface of the frontal lobe.
- Postcentral gyrus - Found on the lateral surface of the parietal lobe, and acts as the primary somesthetic area of the brain.
- Superior parietal lobule - Found on the lateral surface of the parietal lobe, involved in somatosensory activity.
- Inferior parietal lobule - Found on the lateral surface of the parietal lobe, divided into the following:
- Supramarginal gyrus (Involved in the integration of sensory information)
- Angular gyrus (Involved in receiving visual information)
- Posterior paracentral lobule precuneus - Found on the medial surface of the parietal lobe; simply a continuation of the postcentral gyrus.
- Superior temporal gyrus - Found on the lateral surface of the temporal lobe, contains the Wernicke’s area which allows us to compute language.
- Middle and inferior temporal gyri - Found on the lateral surface of the temporal lobe.
- Transverse temporal gyri (of Heschl) - Found on the superior surface of the temporal lobe; forms the primary auditory cortex.
- Fusiform gyrus - Found on the inferior surface of the temporal lobe; attaches to the parahippocampal gyrus.
- Superior and inferior occipital gyri - Found on the lateral surface of the occipital lobe.
- Cuneate gyrus (cuneus) - Found on the medial surface of the occipital lobe, separated by the calcarine fissure which borders the primary visual cortex.
- Lingual gyrus - Found on the medial surface of the occipital lobe.
- Short and long gyri - Found on the lateral surface of the insular lobe (insula), involved in the sensation of taste.
- Parahippocampal gyrus - Found on the medial surface of the limbic lobe.
- Hippocampal formation - Found on the medial surface of the limbic lobe; comprises the hippocampus, dentate gyrus, and subiculum.
- Subcallosal, parolfactory, and preterminal gyri - Found on the medial surface of the limbic lobe; referred to as the subcallosal area.
Gyral abnormalities can lead to the cerebral cortex being smoother than it should be, resulting in pathologies. These pathologies can lead to diseases which can be categorised within a spectrum called the lissencephaly-pachygyria spectrum. Issues with gyri can also be a factor in the development of epilepsy.
Lissencephaly, meaning smooth brain, is often characterised by agyria (absence of gyri) or pachygyria (the presence of overly broad gyri). It results from abnormal migration of neurons during gestation, which inhibits the normal development of gyri and sulci in the brain.
There are two main types of lissencephaly. Type 1 lissencephaly is more common, and causes hypotonia (lack of muscle tone) and lack of movement. Type 2 lissencephaly can lead to muscular dystrophy-like symptoms, causing diseases such as Fukuyama syndrome and muscle-eye-brain (MEB) disease.