The Human Brain
The brain is an organ located at the head of the human body that makes all human action possible. It actively monitors and reacts with both the body’s internal organs and the outside environment. It is the human body’s central processing unit (CPU). Together with the spinal cord, it forms the central nervous system, with the spinal cord serving as the supporting structure. It is composed of mostly pinkish-grey colored nervous tissue. The brain is wrinkled like a walnut, with the folds associated with function and specialization. It is a large and complicated mass - the average adult brain weighs 3 pounds - of nervous tissue that is broken down into four main regions: cerebrum, diencephalon, brain stem, and cerebellum.
Nervous tissue is composed of nerve cells (called neurons) and supporting cells (called glia). Neurons conduct electric impulses via their two main types of extensions, dendrites and axons. They receive impulses from other neurons via dendrites and send impulses to other neurons or other types of cells (like muscle cells) via axons. Dendrites are usually located near the nucleus, while axons often extend to a meter in length (even though their cell bodies are micrometers in length). The axons are, for their entire length, enclosed by lipid-rich layers of protection called myelin sheaths. One way to divide nervous tissue is into grey matter and white matter, where grey matter refers to areas of nervous tissue containing mostly neuron cell bodies, their dendrites, and many types of glia that are near them, and white matter refers to areas that contain mostly axons, myelin sheaths, and the types of glia near them that provide support. Both are filled with yellowish or pinkish hues throughout due to the capillaries (blood vessels) that nourish them with nutrients carried by the blood.
The cerebrum is the largest, most developed and superior (top most) and outermost part of the brain. In terms of the types of nervous tissue, it is essentially composed of one outer layer of grey matter supported by an inner layer of white matter, although there are islands of gray matter, called basal nuclei, distributed deep within the white matter throughout the cerebrum. The cerebrum is divided into two halves called the cerebral hemispheres along the groove in the middle of the brain called the medial longitudinal fissure. The right hemisphere controls the body’s left side and the left hemisphere controls the body’s right side. The grey matter of the cerebrum is called the cerebral cortex. It is the outermost part of the brain and accounts for most of the folded parts and therefore the most surface area of the brain. In the folds, the ridges, are dubbed gyri (singular, gyrus) and the valleys sulci (singular, sulcus). Typically, the sulci account for two-thirds of the surface area of the cerebral cortex.
The Functional Lobes
The cerebrum’s white matter is primarily nerve impulse carriers that are important for helping the functional areas of the cerebral cortex to collaborate with each other and send and receive information from the rest of the brain and body. It is not where the actual processing of information is done. Therefore, when discussiong the functions of the cerebrum, the focus is typically on the grey matter, the cerebral cortex. Functionally, each hemisphere of the cerebral cortex is divided into four main sections, called lobes, named for part of the skull they are protected by: frontal lobe, parietal lobe, temporal lobe, and occipital lobe.
- Frontal Lobe - The frontal lobe is the part of the brain that processes the higher mental functions including concentration, planning, judgment, creativity, inhibition, emotional expression, language comprehension, and a working memory. It is the primary area for processing of motor functions like eye movement and orientation, initiation of voluntary muscle movement, and the use of the muscles needed for speech.
- Parietal Lobe - The parietal lobe is the primary somatic sensory area, processing sensations from the muscles and skin (touch) and the tongue (taste) and enabling the association and interpretation of the body’s remaining senses for evaluation of weight, texture, temperature, etc.
- Temporal Lobe - The temporal lobe processes the senses of hearing and smell, serves as the main area for emotion processing, particularly pain and hunger, and enables both short-term memory and written and spoken language comprehension.
- Occipital Lobe - The occipital lobe specializes in processing the sense of sight. While there is a left and right version of each lobe, some functions are processed in one of the cerebral hemispheres and not the other.
- Basal Nuclei - The basal nuclei of the cerebrum, the islands of gray matter embedded deep within the white matter, help regulate voluntary motor activities by modifying instructions sent to the muscles (starting, stopping, etc.)
The majority of brain processing is done in the cerebrum, but the other supporting structures are important for the proper functioning of the human brain. The diencephalon, or the interbrain, rests on the brainstem, and is completely enveloped by the cerebrum. Its major structures are the thalamus, hypothalamus, and the epithalamus.
The thalamus is located at the heart of the diencephalon, just below the corpus callosum, which is part of the white matter of the cerebrum. The thalamus is a center of relay stations that forward nerve impulses to the appropriate places in the cerebral cortex for processing. It also has a role in interpreting those senses as pleasant or unpleasant, even though it is not able to distinguish between sight, hearing, smell, taste, or touch.
The hypothalamus, as the name suggests, is located directly under the thalamus. It is considered part of the autonomic nervous system (what we don’t consciously do) and regulates body temperature, water balance, and metabolism. The hypothalamus is also the center of many types of emotions and drives like thirst, appetite, sex, pain, and pleasure. It produces hormones and regulates the pituitary gland, both of which are part of the endocrine system. The pituitary gland hangs from the hypothalamus, connected by mammillary bodies, which are reflex centers involved in the sense of smell. Hormones secreted by the pituitary gland control growth, blood pressure, lactation, reproduction, temperature regulation, pain relief, and metabolism.
Near the thalamus, the main structures of the epithalamus are the pineal gland and the choroid plexuses. The pineal gland is a pine cone-shaped member of the endocrine system that secretes a hormone responsible for affecting sleep patterns. The choroid plexuses produce the cerebrospinal fluid, a colorless liquid between the cerebrum and the skull that adds a layer of protection for the brain, acting as a buffer between it and the skull.
Brainstem and Cerebellum
The brainstem is just below the thalamus and leads into the spinal cord at the bottom. It includes many small grey matter areas and one spread out mass of grey matter extending the entire length of the brainstem called the reticular formation. This grey matter contains neurons that provide the brain with motor control of the visceral organs. This control is obviously part of the autonomic nervous system, as people don’t consciously control such functions. A special type of reticular formation neurons called the reticular activating system (RAS) has a role in consciousness and sleep patterns, and filters the sensory inputs coming from the spinal cord and brain stem daily.
The brainstem also serves as a conduit for the nerve endings (axons) that travel from the main part of the brain to the spinal cord, as all of these pass through the brain stem. The brain stem itself is host to 10 of the 12 cranial nerves that innervate the face and neck – provide the necessary nerves endings to allow sensation and motor activity. The brainstem can be broken into three main parts: midbrain, pons, and medulla oblongata.
Midbrain - The midbrain starts from the mammillary bodies of the hypothalamus and extends downward (inferiorly) to the pons. On the front side (ventral side), it is essentially two nerve highways called the cerebral peduncles (“little feet of the cerebrum”) that simply allow for the brain to communicate with the spinal cord (which in turn communicates with all parts of the body). On the back side (dorsal side), the midbrain has four round protrusions called the corpora quadrigemina that serve as reflex centers for the sense of sight and hearing.
Pons - Located just below the midbrain, the pons (“bridge”) is a semi-round structure located in the middle of the brainstem that essentially serves as nerve ending pathways. It also hosts some nuclei that control breathing.
Medulla Oblongata - Just below the pons and connected to the spinal cord inferiorly, the well-known medulla oblongata contains many neuron nuclei that regulate important visceral activities, control heart rate, breathing, swallowing, vomiting, and many other essential bodily functions. It is connected to the cerebellum posteriorly (on its back side). Because it merges into the spinal cord, as with the rest of the brainstem, it also serves as an axon highway.
The cerebellum (“little brain”) is a cauliflower-shaped combination of white and grey matter. Much like the cerebrum, its outer layer is grey matter and its inner layer is white matter, and it can be broken into hemispheres. Its primary function is to coordinate all motor functions in spite of changes in the configuration of the body and / or its environment. The snapshots taken by the eyes at a few given instants when the body is still don’t require any change or special attention by the cerebellum, but if the body is moving, then the cerebellum ensures that the accuracy and precision of the sensory impulses of the eyes are not affected by the movement of the body by using simultaneous other sensory impulses (hearing, smell, taste, or touch) to verify.
The inner ears, eyes, and other organs have equilibrium apparatuses designed specifically to provide the cerebellum with the information necessary to provide this coordination. The cerebellum can even calibrate the interpretation of sensory inputs if it detects a discrepancy in what it receives from the sense organs. It therefore is vital to all movement, interpretation of any sense, posture, and balance. It is the part of the brain that performs least best under the effect of alcohol or other intoxicants (which is why people lose coordination and balance when they get drunk).
The human brain is a natural marvel, superior to the most advanced supercomputer processors and costing only calories from food. Nervous tissue uses electrical impulses to establish communication between the brain and rest of the body. These impulses make all sensory and motor activity possible, as well as all internal bodily activities that don’t use conscious control. With the help of the spinal cord, the long nerve endings of its neurons, called axons, and the connections between neurons, called synapses, the brain can control every aspect of the human body. Using functional areas in its cerebrum specialized for particular types of sense or human capabilities, the brain breaks complicated tasks into simpler ones and allows further specialization and improvement in the parts of the brain that are used the most by its system of folding upon itself in that functional area. The main processing done in the cerebral cortex is complemented by the function of relay stations (forwarding impulses), pathways, reflex centers (for reactions), support cells (keeping nervous tissue healthy and functional), monitors, interpreters, coordinators, autonomous activity regulators, hormone producers, and other important types of components designed to enable and regulate every activity and react to every kind of internal or environmental change.