Overview of the Brain

The large brain of humans is perhaps the most important evolutionary advance for the species. At the minimum, it is the characteristic most of us consider the distinguishing characteristic of a human. This module outlines the structural and functional relationships of the human brain.

Superior view of the brain. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).  

Lateral view of the brain. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).  The dominant portion of the human brain is the cerebrum. It is the large upper part of the brain, distinguished by the gyri (folds) and sulci (folds) of the surface. The cerebrum is clearly split into left and right hemispheres; the split is the deep longitudinal fissure. The cerebrum sits atop and around the midbrain, which leads into the brainstem. Situated essentially behind the midbrain and under the cerebrum is the distinctive cerebellum.

The inside of the brain is characterized by regions of gray matter and white matter. The gray matter is mostly cell bodies, dendrites, and synapses and forms a cortex over the cerebrum and cerebellum, and also forms some nuclei deeper in the cerebrum. White matter is myelinated axons forming tracts. (These definitions and components of gray and white matter are similar to the ones for the spinal cord, although their arrangement will be different as you will discover later in this unit.)

The cerebral white matter tracts are classified as

• Projection tracts-from higher to lower, from cerebrum to brainstem and spinal cord
• Commissural– across hemispheres
• Association– within same hemisphere

The gray matter of the cerebral cortex includes:

• Stellate cells– receive sensory input and process information locally
• Pyramidal cells– extend to other parts of the CNS
• Neocortex– 6 layered tissue of recent evolutionary origin

Meninges, Cerebrospinal Fluid and Blood Supply

Like the spinal cord, the brain is covered and partially protected by connective tissue meninges. From outermost (bordering the skull bones) to the innermost (adjacent to the nervous tissue) they are the dura mater, arachnoid mater, and pia mater. The dura mater folds into two layers, a periosteal layer fused to the skull bones, and a meningeal layer. In some areas, these layers are separated by a dural sinus, a space used to collect blood. Some areas may also contain a subarachnoid space or a subdural space.

Meninges.  Cerebrospinal fluid (CSF) is a clear, colorless liquid that bathes the external surfaces of the brain. It is constantly produced, flows through the network of ventricles, and is reabsorbed. CSF functions in cushioning and supporting the brain by buoyance, and in chemical stability of the brain, by transporting nutrients and wastes respectively.

The ependymal cells lining the ventricles produce the CSF. Then the CSF flows throughout the brain in the ventricles. Each cerebral hemisphere contains a lateral ventricle. Each lateral ventricle drains through an interventricular foramen into the third ventricle. The third ventricle sits in the midbrain region. The CSF then flows into the cerebral aqueduct to the fourth ventricle. Before being reabsorbed, the CSF enters one of two lateral apertures or a median aperture, and then fills the subarachnoid space. Reabsorption of CSF occurs there by the arachoid villi and enters the venous blood.

CSF flow through the ventricles.  The choriod plexus is the network of blood vessels and ependymal cells on surface of the ventricles. The ependymal cells of this choroid plexus secrete the CSF. Overall the close proximity of ependymal cells and blood vessels create a blood-brain barrier (BBB). The brain requires large amounts of oxygen and glucose but other items in blood may harm it, hence the barrier. At the capillaries there is a BBB of tight endothelial cells and basement membrane; at the choriod plexus the blood-CSF barrier due to tight junctions between ependymal cells

Next, the main structural areas of the brain will be surveyed with some of their major functions. We will go in general order of most primitive brain region to most evolutionary advanced region, or, in other words, from basic functions to more advanced functions.

Hindbrain and Midbrain

The most inferior part of the brain, the medulla oblongata, appears as a thickening of the spinal cord. Many of the cranial nerves originate here (see below). The medulla oblongata contains nuclei that control many basic functions, including the cardiac center, the vasomotor center, the respiratory centers, and many other involuntary functions such as swallowing, coughing, salivating, sweating, and gastrointestinal secretion.

Posterolateral view of the brainstem.  In humans, the pons is the next most superior feature of the brain; the pons looks like a forward-facing bulge in the brainstem above the medulla oblongata. The pons relays signals between cerebrum and cerebellum, including sleep, hearing, taste, and posture to name a few.

The cerebellum is a smaller, highly folded structure in the back of the brain, behind the pons. Like the cerebrum, it is split into hemispheres, with a flattened area down the center called the vermis. The folds are folia and grooves are sulci. The white matter forms a distinctive arbor vitae (“tree of life”). The cerebellum is concerned with muscular coordination, special perception, and tactile perception, and some planning and scheduling tasks.

The midbrain is a small region of gray matter nuclei involved in different motor and sensory functions and connecting white matter pathways. These structures include:

1. Cerebral peduncles- anchor cerebrum to brainstem
2. Tegmentum- to/from cerebellum for motor control
3. Substantia nigra- inhibitory relay (the area destroyed in Parkinson disease)
4. Central gray matter- pain awareness
5. Tectum- include the inferior and superior colliculi for hearing and vision
6. Red nucleus- subconscious motor commands and muscle tone

The reticular formation is a series of gray matter extensions from the midbrain through the cerebellum. They are involved in

1. Somatic motor control, including the pattern generators
2. Cardiovascular control
3. Pain modulation
4. Sleep and consciousness, including habituation

Forebrain

The forebrain is the large overarching region of the brain. In the center, above the midbrain are the thalamus and hypothalamus.

Nuclei of the thalamus.  The thalamus is a set of nuclei mainly involved in the relay of sensory signals. Those will be covered in more depth in sensory units. Other thalamic nuclei are involved in memory and emotions.

The hypothalamus is a set of nuclei situated underneath the thalamus. The main function of the hypothalamus is control of the endocrine system and as such will be covered in more detail there. Other nuclei of the hypothalamus are involved in many autonomic functions such as thermoregulation, food and water intake, biological cycles, and emotions.

The cerebrum is the major anatomic feature of the human brain. The cerebrum is made of lobes. The frontal lobe is from the frontal bone to central sulcus and is involved in voluntary motor functions, planning and foresight, memory, mood, emotion, social judgment, and aggression.

The parietal lobe is the upper part of brain in each hemisphere from the central sulcus to parietal-occipital sulcus; this lobe is primarily involved in sensory reception and integration.

The temporal lobe of each hemisphere sits under the parietal lobe and the lateral sulcus; this lobe has roles in hearing, smell, learning, memory, visual recognition and emotional behavior.

The lobe furthest to the rear of the head is the occipital lobe, and it contains the visual center.

The insula is a mass of cortex underneath the outer lobes, found beneath the frontal and temporal lobes.

Locations of the basal nuclei.  The basal nuclei are clusters of cell bodies found at the bottom (base) of the cerebrum, surrounding the thalamus. They include the caudate nucleus, and the putamen, and globus pallidus, (last two collectively known as the lentiform nucleus and all 3 are sometimes referred to as the corpus striatum) and are involved in motor control.

The limbic system is a loop of cortical structures in temporal lobe surrounding corpus callosum and thalamus. These structures include the hippocampus, amygdala, fornix, and cingulate gyrus. More on the hippocampus and amygdala later in the presentation of higher level functions.

Brain Regions

Higher brain functions are ones generally assigned to regions in the forebrain. Most of these locations were discovered by studying people who had lesions in these regions, and as a result, were defective in one of these functions.

For most functions, there is a localization between the left and right hemispheres, called cerebral lateralization. The left hemisphere generally is stronger in motor, mathematical, and language skills, while the right hemisphere generally emphasizes spatial and tactile skills. The two hemispheres are connected by the corpus callosum.

Cognition is awareness perception, thinking, knowledge, and memory. Its most basic definition is the integration of sensory and motor systems. The cerebral lobes contain most of the regions associated with cognition. An important memory forming center is the hippocampus.

Emotion is deeper feeling, resulting from memory and learned behavior. Many emotions are rooted in the hypothalamus and amygdala.

Sensation is the perception of one of the senses. Here, we will simply point out the importance of the post-central gyrus for the interpretation of general senses. General senses are ones from widely distributed receptors, such as touch, pressure, temperature, pain. These pathways end at the post-central gyrus, or sensory cortex. The cortex exhibits somatotophy: point by point correspondence of body locations to brain locations. The point by point correspondence gives more area of the cortex to regions that are well innervated with sensory receptors, such as fingers, and face; meanwhile regions that do not have large sensory innervation have correspondingly smaller areas on the sensory cortex.

The special senses are interpreted in their own specialized cortical regions, and are discussed in another section of this course.

Motor control refers to the initiation and proper coordination of the movement of a muscle. For a skeletal muscle, the intent to contract a skeletal muscle begins in motor association area (frontal lobe). The signal is then sent to the precental gyrus or primary motor area, which is the origin of the upper motor neuron. The precentral gyrus also exhibits somatotophy. Body areas, such as lips and fingers, which have fine motor control, have a large area dedicated on the primary motor cortex while areas that do not have fine control have a correspondingly smaller area.

Language includes abilities such as reading, writing, speaking, and comprehending words. At least two major areas are involved in the recognition and formation of language. Wernicke’s area, within the parietal and temporal lobes, is involved in the recognition of language. Broca’s area is involved in the formation of words.

Cranial Nerves

The Cranial Nerves provide input to and output from the brain. The numbers, names, and a short functional description are below.

1. Olfactory: sensory for smell
2. Optic: sensory, process visual information
3. Oculomotor: motor, movement of eyes and smooth muscles controlling pupil and lens
4. Trochlear: motor, eye movements
5. Trigeminal: sensory of upper, and mid face and upper jaw; motor for muscles of chewing
6. Abducens: motor, eye movements
7. Facial: motor for facial expression, tears and salivary glands; sensory for taste
8. Vestibulocochlear: sensory, hearing and equilibrium
9. Glossopharyngeal: motor for mouth (swallowing) and for regulation of blood pressure; sensory for tongue and pharynx and outer ear
10. Vagus: motor for swallowing, speech, cardivascular and digestive regulation; hunger and fullness; sensory from visceral organs and taste. Main parasympathetic nerve
11. Accessory: swallowing, and head, neck, shoulder movement
12. Hypoglossal: tongue movements