Brain Physiology

The Major Structures of the Brain.

The human brain is a mass of pinkish-gray tissue containing a neural network involving approximately 10 billion nerve cells, called neurons. Glial cells serve as the brain's support system, in addition to blood vessels and secretory organs. Weighing in at a mere three pounds, the brain operates as the central control system for movement, sleep, hunger, and thirst. It controls nearly every vital activity necessary for survival. Emotions are controlled by the brain: anger, fear, joy, love, elation, contentment, and happiness find their origin inside the brain. Furthermore, the brain receives and interprets the multitude of signals being sent by other parts of the body and the outside environment. There are three major divisions of the brain: the forebrain, midbrain, and hindbrain.



A. Forebrain:

For anatomical study the forebrain is divided into two subdivisions: the telencephalon and the diencephalon. The primary structures of the telencephalon include the cerebral cortex, basal ganglia, and the limbic system. The diencephalon includes the thalamus and the hypothalamus.



Telencephalon:

Cerebral Cortex: Likened to the bark on a tree, the cerebral cortex surrounds the cerebral hemispheres. The cerebral cortex is the folded, convoluted tissue commonly imagined when an image/thought of the brain is recalled from memory. The folded, crumpled structure contains an enormous amount of small and large grooves (sulci and fissures) and bulges (gyri). This type of structure is beneficial for it greatly increases the overall surface are of the cortex. In fact, because of the convoluted design the area of the cerebral cortex is tripled!



The cerebral cortex is commonly referred to as gray matter. This is based upon the appearance of the cortex which, due to the predominance of cells appears grayish brown. The neurons of the cerebral cortex are connected to other neurons within the brain via millions of axons located beneath the cortex. This area is white in color due to the concentration of myelin; it is often called white matter .



One of the most apparent visible features of the brain is the division between the left and right hemispheres of the cerebral cortex. Through evolutionary advances the functions of each hemisphere have evolved. Mental and emotional differences between men and women are speculated to result from different modes of functioning between the two hemispheres. In most cases the left hemisphere is deemed the dominant half of the brain. This is due to its superior language abilities as well as its analytic, sequential.



In general terms it is well understood that the left hemisphere controls linguistic consciousness, the right half of the body, talking, reading, writing, spelling, speech communication, verbal intelligence and memories, and information processing in the areas of math, typing, grammar, logic, analytic reasoning, and perception of details. The right hemisphere is associated with 'unconscious' awareness (in the sense it is not linguistically based), perception of faces and patterns, comprehension of body language and social cues, creativity and insight, intuitive reasoning, visual-spatial processing, and holistic comprehension. Communication between the two hemispheres takes place through the corpus callosum, which, by the way, is more fully developed in women than men- likely giving rise to women's intuition.



The surface of the cerebral hemispheres is divided into four lobes corresponding to the names of the skull plates that protect them: the frontal lobe, parietal lobe, temporal lobe, and the occipital lobe. In addition to these four lobes, a fifth lobe exists called the insula. This lobe is internal and is not visible from the surface of the brain.



The frontal lobes went through a tremendous evolutionary expansion 50,000 years ago. Subsequently, the capacities for long-term planning, goal development, and the ability to override immediate gratification in favor for future goals greatly expanded. The frontal lobes are sometimes associated with what it means to be human. Absence of the frontal lobes typically results in a person who is deemed emotionally shallow, listless, apathetic, and insensitive to social norms. According to Candace Pert, "If God speaks to man, if man speaks to God, it would be through the frontal lobes, which is the part of the brain that has undergone the most recent evolutionary expansion." Furthermore, the frontal lobes exert a degree of control over the hypothalamus, which controls the autonomic nervous system and endocrine system, as well as organizes survival behavior. Control of movement is associated with the frontal lobes via the primary motor cortex located within this lobe.



The parietal, temporal, and occipital lobes are specialized for perception. Within the parietal lobe is the primary somatosensory cortex which receives information pertaining to the senses of the body: touch, pressure, temperature, and pain. Visual information is received by the primary visual cortex located within the occipital lobe. Hearing is processed in the primary auditory cortex within the temporal lobe. The central sulcus (fissure of Rolando) divides the frontal lobe from the parietal lobe. The lateral fissure (fissure of Sylvius) separates the temporal lobe from the overlying frontal and parietal lobes. The parieto-occipital fissure separates the parietal and occipital lobes.



The corpus callosum is the primary connection between the left and right hemispheres of the cerebral cortex. Connection between the two halves takes place through axons that unite geographically similar regions of the two cerebral cortices.



Basal Ganglia: The basal ganglia are a collection of subcortical nuclei situated beneath the anterior portions of the lateral ventricles; they are involved with the control of movement. Parkinson's disease has an effect upon the basal ganglia resulting in poor balance, rigidity of the limbs, tremors, weakness, and difficulty with initiating movements. Some anatomists consider the amygdala (primary component of the limbic system) a part of the basal ganglia given its location.



The Limbic System: The limbic system is a collection of brain structures involved with emotion, motivation, multifaceted behavior, and memory storage and recall. The hippocampus (sea horse) and the amygdala (almond), along with portions of the hypothalamus, thalamus, caudate nuclei, and septum function together to form the limbic system. [see question #4 for further information].



Diencephalon:

The diencephalon is the second major division of the forebrain. The principle structures include the thalamus and hypothalamus.



Thalamus: The thalamus is the relay station for incoming sensory signals and outgoing motor signals passing to and from the cerebral cortex. With the exception of the olfactory sense, all sensory input to the brain connected to nerve cell clusters (nuclei) of the thalamus. The thalamus consists of two large connected lobes. The massa intermedia serves as a bridge connecting the two lobes of the thalamus. It is comprised of gray matter and is deemed a non-critical part of the brain; absence of which is outwardly unnoticeable.



Hypothalamus: The hypothalamus is comprised of distinct areas and nuclei which control vital survival behaviors and activities; such as: eating, drinking, temperature regulation, sleep, emotional behavior, and sexual activity. It is located just beneath the thalamus and lies at the base of the brain. The autonomic nervous system and endocrine system are controlled by the hypothalamus. The anterior pituitary gland is directly connected to the hypothalamus via a special system of blood vessels. Neurosecretory cells released by the hypothalamus act upon the anterior pituitary gland which then secretes its hormones. Most hormones secreted by the anterior pituitary gland control other endocrine glands. Because of this the anterior pituitary gland is sometimes referred to as the Master Gland. Hormones of the posterior pituitary gland are also governed by the hypothalamus.





B. Midbrain, The Mesencephalon:

Two primary parts comprise the midbrain : the tectum and the tegmentum.



Tectum: The primary structure of the tectum include the superior colliculi and the inferior colliculi. The superior colliculi form part of the visual system. The inferior colliculi are part of the auditory system. The structures appear as four small bumps located on the brain stem. Function in mammals relates to visual reflexes and reaction to moving stimuli.



Tegmentum: The tegmentum is situated below the tectum. The reticular formation, periaqueductal gray matter, and the red nucleus and substantia nigra are part of the tegmentum. The reticular formation is comprised of more than 90 nuclei and an interconnected neural network located at the core of the brain stem. It receives sensory information and is involved with attention, sleep and arousal, muscle tonus, movement, and various vital reflexes.



The periaqueductal gray matter consists of neural circuits that control sequences of movements constituting species-typical behavior. The red nucleus and substantia nigra are parts of the motor system. The red nucleus serves as one of two major fiber systems bringing motor information from the brain to the spinal cord. The substantia nigra affects the caudate nucleus via dopamine-secreting neurons.





C. The Hindbrain:

Metencephalon:

Cerebellum (little brain): The cerebellum's primary function involves control of bodily movements. It serves as a reflex center for the coordination and precise maintenance of equilibrium. Voluntary and involuntary bodily movements are controlled by the cerebellum. Visual, auditory, vestibular, and somatosensory information is received by the cerebellum, as is information on the movements of individual muscles. Processing of this information results in the cerebellum's ability to guide bodily movements in a smooth and coordinated fashion.



Pons: The pons appear as a large bulge in the brain stem between the mesencephalon and the medulla oblongata. The pons contain a portion of the reticular formation as well as nuclei believed important in the role of sleep and arousal.



Myelencephalon:

The myelencephalon is comprised of one structure: the medulla oblongata (oblong marrow). It is the origin of the reticular formation and consists of nuclei which control vital bodily functions. The medulla oblongata is the control center for cardiac, vasoconstrictor, and respiratory functions. Reflex activities, including vomiting, are controlled by this structure of the hindbrain. Appearing as a pyramid-shaped enlargement of the spinal cord, damage to this area typically results in immediate death.





Part Two: The Neuron:



A. Basic Neuron Description:

A neuron, also known as a nerve cell, is the information processing and transmission device of the nervous system. They come in a variety of shapes, sizes, and types. In the human body, certain neurons reach up to three feet long. While differences exist between particular neurons given their specialization, most neurons are comprised of four primary structures: the soma; dendrites; axon; and terminal buttons.



Soma: The soma is the cell body of the neuron. It houses the nucleus and the majority of cell components which sustain the life processes of the cell. The shape of the cell body varies greatly between the different types of neurons.



Dendrites: The dendrites branch out from the soma resembling branches of a tree (dendron is Greek for Tree). With the exception of sensory neurons, the dendrites are the mechanism through which a neuron receives communication, incoming information, from other neurons. Sensory neurons transmit information where the incoming signal is generated by specialized receptors in the skin. Messages between two neurons are transmitted across the synapse, a junction between the receiving dendrites of one neuron and the information sending terminal buttons of another. Communication between neurons is a one-way affair. Signals are sent out by one neuron through the terminal buttons and received by the cell membranes of the receiving neuron.



Axon: The axon is a long, slender tube that carries information away from the soma to the terminal buttons. Axons are usually covered by a myelinated sheath. The axon carries a basic message called termed an action potential. The action potential is a brief electrical/chemical event which starts at the end of the axon near the soma and travels downward to the terminal buttons. The action potential is consistent; I remains the same size and duration even through axonal branches. Each branch of an axon receives a full charge.



As with the dendrites, axons come in different shapes. Furthermore, the three principal types of neurons are classified by the manner in which their axons and dendrites leave the soma. The most common type of neuron is the multipolar neuron which has one axon and many branches of dendrites. Bipolar neurons are depicted by having one axon and one dendritic tree, each located at opposite ends of the cell body. Bipolar neurons are typically sensory. They have a dendrite which receives information from a receptor which gets sent onto the central nervous system informing it of external events. Unipolar neurons, as found in the somatosensory system, consist of one stalk containing terminal buttons at one end and a dendritic tree at the other.



Terminal Buttons: Most axons divide and split many times. At the ends of the branches there are small knobs which are called terminal buttons. The terminal buttons secrete neurotransmitters which affects the receiving cell. Neurotransmitters can be either excitatory or inhibitory. The nature of the neurotransmitter determines whether the receiving cell will send a message down its axon and communicate with the connected to its terminal buttons. A single neuron can receive information from hundreds of other neurons thus creating an intricate neural network. Additionally, the terminal buttons of a neuron can form synapses at the dendrites and/or cell body membranes of adjacent neurons.



Internal Structure: The boundary of the nerve cell is defined by the cell membrane. Within the membrane are protein molecules which serve special functions for the cell. Some of the proteins detect substances outside the cell, such as the presence of hormones, and pass the information onto the interior of the cell. Other proteins serve as the cell's gatekeeper, allowing some substances to pass into the cell while barring others. Some proteins functions as transporters carrying certain molecules into and out of the cell.



At the center of the neuron is the nucleus which is round or oval and covered by a nuclear membrane. Inside are the nucleolus and chromosomes. The nucleolus manufactures small structures that are involved with protein synthesis, called ribosomes. Genetic information is contained on long strands of deoxyribonucleic acid (DNA) which make up the chromosomes. When portions of the chromosomes (genes) are active they cause the production of messenger ribonucleic acid (mRNA). Messenger RNA exits the nuclear membrane and attaches itself to ribosomes where the production of a specific protein takes place. Proteins provide structure and serve as enzymes, directing the chemical processes of a cell by controlling chemical reactions



Cytoplasm makes up the bulk of the cell. It is a jellylike, semiliquid substance that fills the space within the membrane. Cytoplasm streams and flows, it is not static. Contained within it are small, specialized structures essential for the cell to perform its duties. The small structures include the:



Mitochondria: This structures takes food and breaks it down into energy which the cell can use to carry out its job. Because it has its own DNA mitochondria are believed to have been their own organism which later merged inside of larger cells; a process and phenomenon known as symbiogenesis.



Endoplasmic reticulum is a structure that serves as a storage reservoir and channel for transporting chemicals through the cytoplasm. Lipid molecules are also produced here.



The Golgi apparatus assembles some complex molecules made up of simpler, individual molecules. It makes new synaptic vesicles out of the membranes of old vesicles which have served their purpose. In this sense the apparatus functions as the cell's recycling center. The Golgi apparatus also operates as a packaging facility. It prepares and wraps proteins destined for export.



Lysosomes are produced by the Golgi apparatus. They are small sacs which contain enzymes used to break down substances no longer needed by the cell. Lysosomes can cause cell death or suicide.

The microtubule is the scaffolding of the cell. It is the skeleton of the cell and is involved with the transportation of substances from one place in a cell to another.



Unlike most other types of cells found inside the body, neurons cannot be replaced when they die. All the neurons a person will have are present at birth; once a neuron is destroyed it can never be replaced. In addition, neurons possess a very high rate of metabolism requiring a constant supply of nutrients and oxygen. The needs of the neuron must be met by support cells in order for the neuron to survive.