Group 6 - Brain Anatomy

The human brain is the responsible organ for intelligence, creativity, memory and the control of many body functions. It receives the perception signals to see, hear, feel, smell and taste. The perceptions are cognized and then action signals are sent.

The brain and the spinal chord form the central nervous system. Cranial nerves from the brain and spinal nerves from the spinal chord form the peripheral nervous system. The autonomic nervous system is part of the peripheral nervous system and controls most of the human's organ functions. The central nervous system and the peripheral nervous system together are the human nervous system.

The skull protects the brain from injury, being separable into three main areas: anterior fossa, middle fossa, posterior fossa (cp. fig. 1). At the bottom of the skull several holes – called foramina – can be found. Blood vessels and nerves can exit the skull through them. The spinal chord exits the skull through the foramen magnum (cp. fig. 1). [1]

The meninges covers the brain and the spinal chord and forms a protective shell for them. It consists of four parts: the dura mater, the arachnoid mater, the sub-arachnoid space and the pia mater (cp. fig. 2). The dura mater is a thick membrane in direct contact to the skull. The pia mater is a membrane in direct contact to the brain and spinal chord and the arachnoid mater is an elastic tissue in direct contact to the dura mater. The sub-arachnoid space lies between the arachnoid mater and the pia mater and is filled with cerebrospinal fluid. [2]

There is a left and a right hemisphere of the brain. The right hemisphere controls the left part of the body, the left hemisphere controls the right part of the body. Speech, comprehension, arithmetic and writing are functions of the left hemisphere, wheras creativity, spatial ability, artistic and musical skills are functions of the right hemisphere. Both parts communicate with each other via the corpus callosum, a tissue that connects both hemispheres (cp. fig. 8).

Ventricles are cavities within the brain. The cerebrospinal fluid is created in them and flows through canals to the subarachnoid space.

The brain possesses a folded surface, which is called the cortex. The brain has 100 billion nerve cells, of which 70% are located in the cortex. Folds are called gyri, grooves are called sulci (cp. fig. 3). The dense nerve cells on the surface give it a greyish look, therefore the outer brain matter is called grey matter. The neurons are less dense beneath the surface, and the connecting fibres in between give the brain matter a more whitish look, therefore the inner brain matter is called white matter. In addition to nerve cells, the brain matter also consists of glia cells. [1]

Figure 11: artistic representation of the human brain [21]

Figure 1: basal area of the skull [1]

Figure 2: meninges [2]

Figure 3: surface of the brain [1]

Neurons and Glia Cells [3]

Neurons

There are many different types of neurons, each is specialized for a specific task.

Neurons consist of a cell body – called soma – and a nucleus that lies inside. There are two types of processes (outgrowths of tissue from a larger body) at each neuron: dendrites and the axon. Multiple input signals are transported along the dendrites, but only one output signal is sent through the axon. At the end of the axon there is the terminal and at the terminal – as well as sometimes along the axon – there are junctions to the dendrites of other neurons. These junctions are called synapses. At the junction there is a gap between the terminal and the dendrite, signals can pass through this gap by usage of neurotransmitters. Neurotransmitters are chemical substances that transport signals between neurons or between a neuron and a muscle. [4]

Glia Cells

Glia Cells outnumber neurons by 10 to 1, they do neither posses axons nor synapses. There are two types of neuroglia: astrocytes and oligodendrocytes. Astrocytes are star-shaped, oligodendrocytes are much denser and rounder.

Glia cells were originally thought to be only structural support. Nowadays more functions are known, e.g. they fill up empty spaces after injuries and their processes lead new neurons to their final position in the brain. [5]

Brain regions

The outer brain can be separated into three main parts: the cerebrum, the cerebellum and the brain stem. The inner brain is mainly included in the limbic system. For more general and precise explications it is easier to use the three anatomical regions of the human brain, though: the forebrain, the midbrain and the hindbrain. The four main parts of the brain belong entirely or partially to these three regions, but the regions do not only contain neuronal mass, but also the ventricles and some nerves: [6] [7] [8]

The forebrain consists of the diencephalon and the telencephalon. The diencephalon is most of the inner part of the brain, the telencephalon is the outer part of the brain. The biggest part of the telencephalon is the cerebrum.
The midbrain is between forebrain and hindbrain and belongs to the upper part of the brain stem.
The hindbrain consists of the myelencephalon and the metencephalon. The myelencephalon is the same as the medulla oblongata, which is the lower part of the brain stem. The metencephalon consists of the pons – which is the middle part of the brain stem –, the cerebellum and the fourth ventricle (cp. fig. 9).

Cerebrum

The cerebrum is divided into lobes. The frontal lobe is responsible for movement, planning, personality and intelligence. Movement is controlled by the motor cortex at the back of the frontal lobe (cp fig.5 and motor strip, cp. fig. 4). The parietal lobe is located behind the frontal lobe, the somatosensory cortex is located at its front. It is used for physical perception (i.e. touch). The temporal lobe is at the side of the brain. The upper part is the auditory cortex, which is responsible for hearing. The occipital lobe is at the back of the brain, at the back of the occipital lobe the visual cortex is located (cp. fig. 4 and fig. 5).

Non-specialized areas of the brain are called the association corteces. They create a connection between sensory and motor cortices. Probably these cortices are for reasoning and memorizing.

Speech cognition is done in the left hemisphere of the brain, specifically in Wernicke’s area (upper right part of the auditory cortex, cp. fig. 4 and fig. 5) and Broca’s area (in the left of the frontal lobe, cp. fig 4 and fig 5). Wernicke’s area is for speech understanding, whereas Broca’s area is for speech production.

Figure 4: lobes of the cerebrum, cerebellum and brain hemispheres [1]

Figure 5: specialized cortices and the position in the lobes [9]

Limbic System

The limbic system is a group of structures mainly beneath the cerebrum – but some parts of the cerebrum are also considered to be part of the limbic system –, which lies on both sides of the thalamus and surrounds the basal ganglia (cp. fig. 6 and fig. 8). It partly belongs to the diencephalon and the telencephalon. It is the main brain structure for emotions and is related to the function of memory, too. The hypothalamus, the hippocampus and the amygdala are its most important parts (cp. fig. 6).

The hypothalamus is one of the most active brain areas. Its functions are hunger, thirst, pain, pleasure, sexual satisfaction, anger and several others, too. Many of these functions are executed via the release of hormones by the petuitary gland. In addition, the hypothalamus is the main control of the autonomous nerve system.

The hippocampus converts short-term-memories into long-term-memories.

The amygdala enables decision-making and controls aggression. Furthermore, the olfactory bulb – which is responsible for smelling – is mainly connected to the amygdala. [10]

The cingulate gyrus lies above the corpus callosum (the latter is not shown in fig. 6) and the basal ganglia. Its exact functions are not understood, but it contributes to emotion, cognition and motor functions. [15] The fornix, the dentate gyrus and the parahippocampal gyrus are additional elements of the limbic system. The cingulate gyrus and the parahippocampal gyrus are part of the cerebrum, but are considered part of the limbic system, too.[13]

Figure 6: the limbic system and surrounding structures [12]

Video 1: explanation of the limbic system in 60 s [14]

Important structures of the forebrain that are neither part of the cerebrum, nor the limbic system

There are several strucutres of the forebrain that are neither part of the cerebrum, nor the limbic system. The thalamus and the basal ganglia are two important structures that belong to this group.

The thalamus lies above the brain stem and on both sides of the third ventricle (cp. fig. 6 and fig. 9). It regulates and translates neural impulses to the cerebrum, there they are experienced as touch, pain and temperature. In addition, it transmits information into the limbic system. [16]

The basal ganglia lies above the hypothalamus, hippocampus and amygdala and lies beneath the cingulate gyrus (cp. fig 6). It mainly consists of the caudate nucleus and the globus pallidus. The globus pallidus is surrounded by the putamen. [11] Movement decisions, repetitive behaviors, reward experiences and focusing attention is its function. [10]

Cerebellum

The cerebellum is a structure underneath the occipital and temporal lobes (cp. fig. 7). It takes motor commands from the motor cortex of the cerebrum, modifies them and thus makes them more precise. The cerebrum has the following functions: maintenance of balance and posture, coordination of voluntary movements and motor learning. In addition, it has cognitive functions, too, which are not yet understood. [17]

Brain Stem

The brain stem transports information to the body via its connection to the spinal chord. The thalamus connects the brain stem to the limbic system, the pons is connected to the cerebellum (cp. fig. 7). It consists of the midbrain, the pons and the medulla oblongata. In addition to their function of transporting the information from the cerebrum, the cerebellum and the limbic system, the parts of the brain stem have some specific functions, too. The midbrain connects visual, auditory and motor system information, the pons controls breathing and sleep, whereas the medulla oblongata controls breathing and sleep, too, as well as the heart rate and several functions that belong to the autonomic nervous system. [18]

Figure 7: location and regions of the brain stem and location of the cerebellum [18]

Cerebrospinal Fluid System

The ventricles produce cerebrospinal fluid and are hollow cavities in the brain. The fluid flows from the ventricles through canals into the subarachnoid space (cp. fig. 9). Being in the subarachnoid space, the fluid protects the brain within the skull and is continously replaced by the ventricles. If fluid production and absortion is disbalanced, the ventricles and the the cisterna can enlarge, which is a specific disease pattern.

The lateral ventricles are beneath the corpus callosum (cp. fig. 8), the third ventricle is above the brain stem and between the thalamus. The lateral ventricles are connected to the third ventricle by the foramen of Monro. The fourth ventricle is within the pons and in the upper part of the medulla. It is connected to the lateral ventricles by the aqueduct of Sylvius. Cerebrospinal fluid can flow from the fourth ventricle into the cisterna magna, which is a bigger storage area for the fluid. (cp. fig. 9). [1]

Figure 8: coronal cross-section of the brain [1]

Figure 9: sagittal view of the brain, showing the main areas of the cerebrospinal fluid system [1]

Blood Supply and Blood-Brain-Barrier

The arteries that go into the skull are divided into one main blood supply for the cerebrum and one for the cerebellum and brain stem. At the basal area of the skull, these two main supplies are connected in the Circle of Willis. If one supply is blocked, blood can flow from one blood supply to the other through the Circle of Willis to prevent brain damage. The blood flows from the brain to the meninges, as the veins are integrated into the dura mater. [1]

The cells that form the hull of the blood vessels are called endothelial cells. Outside the brain, endothelial cells possess small openings (medical term: fenestrae) to enable the diffusion of hydrophilic solutes. In the brain, the endothelial cells do not possess such openings, as they are connected by tight junctions. In addition, nearly all endothelial cells are enclosed by the ends of the processes of astrocytes. Together – both endothelial cells and astrocytes – they form the blood-brain-barrier (cp. fig. 10). [19]

The blood-brain-barrier excludes pathogens (e.g. viri, bacteria) and cells of the immune system as well as immune system antibodies and large proteins. In a regular immune response, tissue swells and heats up. As the space is limited in the skull, this is very dangerous for the brain. Therefore, a regular immune response is tried to be prevented. In addition, macrophages (white blood cells that digest unwanted cells, e.g. cancer cells) cause damage to neurons, too, and should not be allowed to enter the brain. Glia cells are supposed to do clean up and repair tasks in the brain. Because of this, practically no large molecules and 98% of the small molecules of the blood do not pass the blood-brain-barrier. Oxygen and carbon-dioxide are able to pass the blood-brain-barrier, though, as well as glucose – the main energy supplier for neurons. [20]