Grey Matter
What is grey matter?
Grey matter corresponds to the cell bodies of neurons in the central nervous system.
Grey matter can be found on the surface of the brain (called the cortex), but also as structures deep inside the brain where they are called nuclei.
Grey matter plays an important role in information processing and modulates systems that allow us to control our movements, thoughts and emotions, among others.
The cerebrum
The cerebrum is the largest part of the brain. It’s located above the tentorium cerebri (meningeal fold) and is responsible for a wide range of functions including thought, speech, emotion, memory and higher motor functions. It’s important to understand that the cerebrum is only part of the brain, which it makes up with the cerebellum (located under the tentorium cerebri) and the brainstem.
The cortex
The cortex is a grey-matter structure that lies at the surface of the cerebrum and comprises mainly the layers of neurons and glia. It is folded into gyri and sulci, which are the terms used for the ridges and the grooves of the brain, respectively.
- Gyrus = brain tissue ridge
- Sulcus = groove separating the ridges
The cortex is then organized into five different lobes: the frontal lobe, the two temporal lobes, the parietal lobe and the occipital lobe. However, some resources include a sixth lobe, the insular lobe, found deep to the temporal lobe.
Different parts of the cortex have been mapped to different functions. Some important examples include:
- Pre-central gyrus (primary sensory cortex) : part of the cortex that is responsible for sensation.
- Post-central gyrus (primary motor cortex ) : part of the cortex that is in charge of the voluntary control of muscles in the body.
- Primary visual cortex : located in the posterior occipital lobe, this part of the cortex is responsible for interpreting visual information coming from the eyes.
- Wernicke’s area : found in the superior temporal lobe of the dominant side (usually left for right-handed individuals), this cortical region is one of the areas involved in our ability to understand language.
- Broca’s area : found in the inferior frontal lobe of the dominant side (usually left for right-handed individuals), this cortical region is one of the areas responsible for our ability to express language (ie. Speech).
*Extra concept: Different parts of the body are not equally represented in the motor and sensory cortex. Certain areas, such as the hands or face, are allotted many more neurons than the less sensitive trunk of the body, for instance. This distribution is called the motor and sensory homunculus. (insert image)
Furthermore, certain gyri and sulci serve as important landmarks when navigating brain slices, such as the central sulcus, the lateral sulcus, the occipital-parietal sulcus and the calcarine sulcus.
- The central sulcus : separates the frontal lobe from the parietal lobe, as well as the primary motor cortex (anterior to the sulcus) from the primary sensory cortex (posterior to the sulcus).
- The lateral sulcus (Sylvian Fissure) : separates the frontal lobe from the temporal lobe that lies inferiorly. If this sulcus is pried open, the insular cortex can be found underneath.
- The parieto-occipital sulcus : separates the parietal lobe from the occipital lobe.
- The calcarine sulcus : divides the primary visual cortex (in the occipital lobe) into a superior and an inferior half.
- The marginal sulcus : is a great landmark to identify because it’s the only sulcus that cuts directly into the cingulate gyrus on mid-sagittal view. The sulcus immediately in front of it is the central sulcus, which is not always easy to find on sagittal views.
Try exercises 4.1 and 4.2 out to see if you can identify these important cortical structures!
