Considering the central sulcus

Introducing functional organization in somatic sensory and motor cortex

Introduction

The central sulcus is one of the most important landmarks in the human brain because it precisely divides the somatic sensory cortex of the parietal lobe from the somatic motor cortex of the frontal lobe. Furthermore, an appreciation of the structure of the central sulcus—actually, the structure of the gyri that define the central sulcus—will help you understand how the opposite side of the body is represented in the somatic sensory and motor areas that reside in these gyral formations. So this exercise will be worth some time, careful study and repetition in the days and weeks ahead.

So how should one recognize the central sulcus? Surprisingly, the most reliable way to find the central sulcus is not by inspecting the lateral surface of the brain, where this is one of the longest and deepest sulci of the human cerebral cortex (Figure 1). Rather, the best way to find the central sulcus is to start on the medial surface of the hemisphere; so get your hands on a brain sectioned through the sagittal plane.

Figure 1. Lateral and midsagittal views of the human brain, emphasizing the division of the cerebral cortex into four lobes (identified with color). Note the location of the central sulcus (A) and the cingulate sulcus (B). (Figure A from an Neuroscience, 6th Ed)

If (when) a "midsagittal brain" is not available, you can complete this study using your digital atlas of the human brain, Sylvius4Online.To do so, view the medial surface of the brain by navigating to Surface Anatomy, Photographic Atlas, and then click on either Unlabeled or Sulci and Fissures.

Lab Protocol 1: Finding the central sulcus

(refer to Figure 1)

1. Locate the cingulate gyrus, which is the gyral structure that sits just dorsal to the corpus callosum (the massive bundle of white matter that interconnects the two hemispheres).

2. Next, identify the cingulate sulcus, which is the sulcus (space) formed by the dorsal bank of the cingulate gyrus, and follow the course of this sulcus posteriorly. Just past the middle of the hemisphere, there is a sharp, dorsal curve in this sulcus called the “marginal branch (or dorsal ramus) of the cingulate sulcus” where the cingulate sulcus turns toward the dorsal surface of the hemisphere.

The gyral structure bounded by the marginal branch of the cingulate sulcus on the dorsal midline of the hemisphere is important functionally. This structure, named the paracentral lobule, contains the somatic sensory and motor representations of the contralateral foot (more on this in a later session). Find the paracentral lobule on the brain in your hand and confirm this localization with an instructor.

3. Keep your eye (or a probe) on this marginal branch of the cingulate sulcus and identify the very first sulcus that terminates in the gyral formation just anterior to the marginal branch; this first sulcus anterior to the marginal branch of the cingulate sulcus is the central sulcus (see Figure 1) 4. Next, follow the course of the sulcus that you just identified as the central sulcus and you should see that it passes along the lateral surface in a gentle anterior progression as you trace it from the dorsal midline toward its inferior margin in the lateral (Sylvian) fissure.

5. Look for a lazy “S”-shaped bend in the central sulcus near the middle of the cerebral hemisphere.

This “S” shape is a cerebral “hot-spot” for clinicians and neuro-imagers: this is where the somatic sensory and motor representations of the contralateral arm and hand are localized.

6. Next, notice that the central sulcus “straightens out” between the “S”-shaped bend and the lateral fissure.

The contralateral face is localized to the inferior segment of the central sulcus below that lazy “S” shape (not where you might expect it if the body where mapped continuously). As you take all of this in, remember: in each of these regions of the central sulcus, somatic sensation is represented on the posterior or parietal side of the central sulcus in a gyrus called the postcentral gyrus, and motor control is localized to the anterior or frontal side of the sulcus in a gyrus called the precentral gyrus. Find the pre- and post-central gyri on the brain in your hand and confirm this localization with an instructor.

Now that you’ve found the central sulcus on the gross specimen (and in Sylvius4Online) let’s progress to two additional exercises based on this first achievement.

Lab Protocol 2: Locate the central sulcus in axial sections

1. Open the Unlabeled image set in the Sectional Anatomy group and view the most dorsal horizontal (axial) section in the set.

At this level and in this plane, the central sulcus is usually the deepest sulcus near the middle of the hemisphere.

2. Look for a conspicuous Ω shape (i.e., “omega-shape”) in the depths of the central sulcus formed by an interdigitation of the walls of the pre- and post-central gyri.

This gyral feature is what forms the “S” shape that can be appreciated when the course of the central sulcus is viewed from the lateral surface of the hemisphere. More importantly, the somatic motor andsensory representation of the contralateral hand invariably includes this distinctive Ω shape deep in the central sulcus.

To check your identification of the central sulcus and the localization of this important Ω-shape (which is sometimes called the “hand knob” by neurologists and neuroradiologists), repeat the exercise above using the Motor Systems image set in the Sectional Anatomy group. The Ω-shape is mainly attributed to a posterior outgrowth of the precentral gyrus, which—as you should now easily see—harbors the primary motor cortex. Evidently, this morphological feature reflects the “over-representation” of the hand in the human brain and the need to instantiate this important functional representation with as much cortical structure and circuitry as can be packaged into a cramped space. This outgrowth of the precentral gyrus is especially prominent in the right hemisphere of the Sylvius4 brain.

3. Now, repeat this procedure using the MR Atlas in Sylvius4Online (click on the thumbnail image labelled “Axial 09” for starters).