 The adult human brain weighs approximately one and a half kilograms, making up less than 2% of a person's body weight. And yet, it defines our humanity and makes us the individuals that we are. The brain is responsible for the generation of language and thought, attention, consciousness, memory, and imagination. In order to fit into the skull and accommodate the massive number of neurons and connections needed, the brain is highly folded. This results in the creation of gyri or ridges and sulci or furrows. If we were to unfold the entire human brain, it would take up approximately one square meter. Perhaps the most impressive feature of the brain is the amount of connections formed between neurons. There are an estimated 86 billion neurons in the brain, each of which forms an average of 7000 connections with other neurons. Resulting in between 100 and 500 trillion synapses within the brain. In an attempt to conceive of the enormity of this system, the number of neurons in the human brain has been equated to the number of stars in the Milky Way. Now that we have begun to appreciate the complexity of the human brain, let's begin to examine its structure. The brain can be divided into functional and anatomical regions. We will now start with an anatomical overview to establish a common terminology and to describe the areas of the brain. The brain has multiple surfaces. Here we have the superior surface and this here is the inferior surface. There are also anterior and posterior aspects to the brain. Here's a section through the brain within the skull and we can appreciate the axes within the CNS. Connections can travel either towards the anterior pole here or towards the inferior end of the spinal cord. Fibers can move either rostrally towards the rostral pole or caudally towards the caudal pole. Now let's look at these component parts of the brain. This is the forebrain, which is composed of the cerebral hemispheres and deep structures. The right and left hemispheres are roughly symmetrical and are connected through corpus callosum, which you can see here. Here we have cut the brain in half, separating the two hemispheres from each other. This is the cut surface of corpus callosum. The largest white matter tract in the brain. Impressively, it consists of an estimated 200 to 250 million projections. This structure here is the thalamus. And together with the hypothalamus here and the subthalamus, which you cannot see, it makes up the diencephalon. The cerebral hemispheres and the diencephalon together comprise the forebrain. We now move into the brainstem, which is caudal to the diencephalon. The brainstem can be divided into three parts. The midbrain, located here just caudal to the thalamus, contains the large fiber bundles with cerebral peduncles that connect the forebrain with all caudal structures. The pons is located caudal to the midbrain. It is connected to the cerebellum through the cerebellar peduncles. The most caudal part of the brainstem is the medulla. It is continuous with the spinal cord as it exits through foramen magnum of the skull. The cerebellum is embryologically part of the pons but its functions are so distinct that it is now considered its own entity separate from the brainstem. The cerebral hemispheres can be divided into lobes along some key surface landmarks. The central sulcus separates the frontal lobe from the parietal lobe. The lateral fissure separates the temporal lobe from both the frontal and parietal lobes. The occipital lobe is separated from the parietal lobe via the parietal occipital sulcus, which you can see on this medial aspect of the brain. You can draw a line onto the lateral surface right here to differentiate between the occipital lobe and the parietal and temporal lobes. In this medial view of the brain, you can identify a continuous strip of cortex which swings around the surface of the brain. This lobe has been dubbed the limbic lobe due to its intimate relationship with the limbic system. It spans the frontal, parietal and temporal lobes. Deep within the brain are spaces filled with cerebrospinal fluid or CSF. This is a 3D reconstruction of the ventricular system. You can see the two C-shaped lateral ventricles in the cerebral hemispheres. They have an anterior horn, deep within the frontal and parietal lobes of the forebrain, a posterior horn, which extends into the occipital lobe, and an inferior horn, which extends into the temporal lobe. These lateral ventricles are connected to the third ventricle in the midline. The third ventricle is connected to the fourth ventricle through the cerebral aqueduct. On this mid-sagittal section of the brain, we can identify different components of the ventricular system. This here is the lateral ventricle. The anterior horn will extend anteriorly here into the frontal lobe, and the posterior horn will extend into the occipital lobe. This here is the third ventricle. The thalamus is on either side of the third ventricle. The third ventricle connects to the fourth ventricle via the cerebral aqueduct, right here. This here is the fourth ventricle at the level of the pons and medulla. The fourth ventricle is going to close off into the central canal. Let's look at the various planes a brain can be sectioned in. This is a very important concept because imaging of the brain uses these planes. I'm going to cut this brain in the coronal plane. So you can now see corpus callosum here, and we've started to look into the ventricular system. So this is the anterior tip of the anterior horn here. This next slice will take us into the anterior pole of the temporal lobe right here. Those pieces might fall off because they're not connected to the plane that I'm cutting in. Here we go. Let's have a look at that. What you can see on this slide is really the lateral ventricle again. We've got the head of the caudate nucleus right there, and the putamen. Here, this is the anterior limb of the internal capsule. Alright, so in this section now you can see the two lateral ventricles, as well as the third ventricle here on either side of the thalamus. Alright, so here in this section now you can again see the two lateral ventricles. Here we're going into the cerebral aqueduct on our way to the fourth ventricle. This here is the inferior horn of the lateral ventricle. Okay, so in this section here again here are the two lateral ventricles. Here again is the inferior horn, the hippocampus, right there in the floor of the inferior horn. All of this here is the thalamus. We're quite posterior now as you can tell in the brain. So all of this is thalamus here. We're actually getting into the posterior parts of the thalamus there. Right here we are in the midbrain and the pons. So it's a bit of an oblique section here through the brainstem. You can see the cerebral aqueduct here and of course this is the pons. Alright, so in this section here you can see that we're now getting into the posterior horn of the lateral ventricle. This is the very, very tail end of corpus callosum there. You can see the cut surface here of the cerebellum. There's the middle cerebellar peduncle moving in there. And here you can see the central canal which is going to open up into the fourth ventricle which we'll like just on the other side of that opening there. In this coronal section we can see both gray and white matter. White matter is the sum of all myelinated axons or tracts as they travel through the CNS. Gray matter is the sum of all nerve cell bodies. It can be seen here along this cortical band on the surface of the brain which is why an increased surface area is so important to accommodate the large number of neurons in the cortex. Gray matter can also be found in these deep nuclei of the forebrain including basal ganglia and limbic structures. We're going to cut this brain in a horizontal orientation. We're now going to cut through the brainstem here in an axial orientation. Here's the midbrain. Here we've cut through the pons in the superior cerebellar peduncle as it projects to the cerebellum. This is the last section here through the pons. You can see some of the deep cerebellar nuclei here. In this cut here you can see the inferior part of the cerebellum and then part of the open medulla. And here you can see the closed medulla as the ventricle has closed over the central canal here. Finally, the brain can be cut along the sagittal plane. Here we have cut the brain along the mid-sagittal plane to separate the two hemispheres. With this overview, we now have a common terminology as we begin our journey through the brain.