What is neurology? |

The Physiology of the Nervous System

Neurology is the study of the nervous system, an intricate arrangement of electrically conducting nerve cells that permeate the entire animal body. Nervous tissue, which represents a principal means of cell-to-cell communication within animals, is one of four adult tissues found within the organs of most animals. The remaining three adult tissues—epithelia, connective tissue, and muscle—rely heavily on nervous tissue for their proper functioning, particularly muscle tissue.

Nervous tissue is a primary characteristic of most animal species. Evolutionarily, nerve tissue arose from cells that primarily were endocrine in origin. Endocrine cells are hormone producers that secrete hormones for distribution through the organism via the organism’s bodily fluids and circulatory systems. Hormones and closely related chemical messengers called neurotransmitters are molecules that are produced in one part of the organism (such as by an endocrine cell or gland), travel through the organism, and target cells in another part of the organism. Hormones or neurotransmitters usually are composed of proteins (long chains of amino acids, or polypeptides) or of fats such as steroids. These chemical messengers affect gene expression within their target cells. Hormones and neurotransmitters determine whether genes are active or inactive. In active genes, the deoxyribonucleic acid (DNA) of the gene encodes messenger ribonucleic acid (mRNA), which encodes protein. An inactive gene does not encode protein. All events within the target cell are influenced by the presence or absence of gene-encoded proteins.

Hormones are effective cell-to-cell communicative and control molecules within all organisms, including animals. In animals, however, hormones and neurotransmitters have become elaborated as parts of extensive nervous systems. The nervous systems of animals have developed according to the evolution of a specialized nervous cell type called a neuron.

The neuron, a specialized, electrically conducting cell, is the basic unit of the nervous system. A neuron is unlike many other cells because it can assume diverse shapes and can assume (relatively) great lengths, sometimes spanning many centimeters. A neuron consists of a cell body containing a nucleus, where the genetic information resides, and numerous organelles, including the energy-producing mitochondria and protein-synthesizing ribosomes. There may be a few or many cellular extensions of its cytoplasm and membrane that twist the neuron into a very distorted appearance. The two principal types of extensions are axons and dendrites.

A dendrite is an electrically receiving extension of a neuron; it receives electrical information from another neuron. An axon is an electrically transmitting extension of a neuron; it transmits electrical information to another neuron. An axon of one neuron transmits electrical information to the dendrite of another neuron. Yet the two neurons are not in direct contact; the axon does not touch the dendrite. A gap called a synapse separates the axon from the receiving dendrite.

Electrical information crosses the synapse via a special type of hormone called a neurotransmitter, a protein encoded by the genes and synthesized by the ribosomes of the neuron. Electrical information traveling along the axon of a neuron triggers the release of a specified quantity of neurotransmitter proteins at the synapse. The neurotransmitters diffuse across the synapse where, upon making contact with the dendritic membrane of the next neuron, they depolarize the dendrite and allow the electrical information transmission to continue unabated.

Actual electrical conduction in a neuron involves membrane depolarization and the influx of sodium ions and the efflux of potassium ions. Electrical conduction along a neuronal segment involves the depolarization of the membrane with the movement of sodium cations into the neuron. Electrical conduction for a particular neuronal segment ends with repolarization of the neuronal membrane as potassium cations move across the neuronal membrane and out of the neuron. The passing electrical action potential, which is measured in millivolts, involves simultaneous depolarization and repolarization in successive regions of the neuron. The initial depolarization is triggered by neurotransmitters contacting the dendritic membrane. Sodium and potassium ion pumps continue the successive stages of depolarization and repolarization along the dendrites, cell body, and axons of the neuron until neurotransmitters are released from a terminal axon across a synaptic gap to depolarize the membrane of the next neuron.

Within animal nervous systems, neurons are very plastic: they grow in specified patterns, much like crystals, in response to stimuli and contacts with other neurons. A neuron may have one or many axonic and dendritic extensions. A neuron with one dendrite and one axon is termed bipolar. A neuron with many dendrites and many axons is termed multipolar; such a neuron makes many contacts with other neurons for the transmission of electrical information along many different neural pathways.

Animal nervous systems usually consist of centralized, concentrated neurons that form the center of nervous control, called the central nervous system. In vertebrate animals, the central nervous system consists of the billions of neurons composing the brain and spinal cord. Additionally, peripheral neurons extend throughout the animal body, permeating virtually every cell and tissue region and thus forming the peripheral nervous system, composed of billions of dispersed neurons.

Functionally, neurons are of three principal types: sensory, motor, and internuncial neurons. Sensory neurons detect stimuli and transmit the electrical information from the stimulus toward the central nervous system. The sensory neurons are arranged one after another, transmitting the electrical action potential from axon to neurotransmitter to dendrite, and so on. The central nervous system processes this information, usually utilizing an intricate array of connected internuncial neurons and specialized neuronal regions devoted to specific bodily functions. Once the central nervous system has processed a response to the stimulus, the response is affected by motor neurons, which transmit electrical information back to the body. Motor neurons will transmit electrical information between each other in the same fashion as sensory neurons. The motor neurons, however, often will terminate at some effector tissue, usually a muscle. Neurotransmitters released from the last motor neuron will depolarize the muscle membranes and trigger the biochemical and physical contraction of muscle. The muscle responds to the stimulus.

The primary purpose of nervous systems in animals is to respond to stimuli, both internal and external. Sensory neurons detect stimuli and direct this information to the central nervous system, where internuncial neurons process the information to appropriate decision centers, which direct a response along a chain of motor neurons to a muscle or muscles that physically respond to the initial stimulus. This chain of nervous communication is called a reflex arc. Virtually every activity in the body requires reflex arcs involving central and peripheral nervous system sensory, internuncial, and motor neurons.

The neurons of vertebrate animal nervous systems are very plastic and make trillions of neuron-to-neuron interconnections for the accurate processing of information, the reception of stimuli information, and the direction of response information along reflex arcs. Within the central and peripheral nervous systems of the human body, millions of information transfer processes occur by reflex arcs every second along trillions of neuronal interconnections. The number of such electrical information transfers that must occur accurately every moment within the body is staggering, yet the human nervous system accomplishes these amazingly intricate tasks with ease. No supercomputer yet devised even comes close to the complexity and efficiency of the vertebrate animal nervous system.

Science and Profession

Neurologists attempt to understand the structure of the nervous system, including the functioning of the neuron, neuronal plasticity, supporting nerve cells (neuroglia and Schwann cells), neurotransmitters, neuronal patterning in learning, how vision and hearing occur, nerve disorders, and the embryological development of the nervous system.

Neurons are among the most flexibly specialized cells in animal tissues. Animal nerve tissue is derived from embryonic ectodermal tissue. The ectoderm is a tissue layer of cells formed very early in animal development. Very early in development following conception, all animals undergo a blastula stage, in which the embryo is a hollow sphere of roughly five hundred cells. A region of the blastula called the blastopore folds to form a channel of cells through the blastula, thus initiating the gastrula stage.

The gastrula has three embryonic tissues: ectoderm, mesoderm, and endoderm. The ectoderm and endoderm continue to divide and differentiate into epithelial cells. Mesodermal cells multiply and differentiate into muscle and connective tissue cells. Dorsal ectodermal cells (cells that will become the back side of the organism) fold inward to form a nerve cord. The neurons of this nerve cord multiply and differentiate into central and peripheral nervous tissue.

In humans, the dorsal nerve cord becomes the billions of centralized neurons composing the spinal cord. The neurons in the anterior region of the spinal cord will fold, multiply, and differentiate into the various brain regions. The complete, fully functional human brain has an estimated one hundred billion neurons that grow plastically and form trillions of interconnections for the accurate processing of electrical information.

The embryonic brain consists of three principal enfolded regions: the prosencephalon (forebrain), the mesencephalon (midbrain), and the rhombencephalon (hindbrain). Each of these three embryonic regions folds and differentiates further. The prosencephalon neurons multiply and differentiate to become the cerebrum, thalamus, and hypothalamus. The mesencephalon region becomes the corpora quadrigemina and cerebral peduncles, areas that connect other brain regions and coordinate sensory and motor impulses for basic reflexes. The rhombencephalon becomes the cerebellum, pons, and medulla oblongata; these are brain regions that control basic bodily processes such as coordination, prediction of movements, and maintenance of heart rate and respiration.

Furthermore, special regions of brain tissue develop into external sensory apparatuses: eyes for vision, ears for hearing and balance, and nasal and tongue chemoreceptors for smell and taste. Millions of sensory neurons flow from these special sense organs to highly complicated brain regions that analyze, interpret, learn from, and react to these sensory stimuli. Neurophysiologists attempt to decipher the mechanisms by which the brain processes information. For example, the hundreds of thousands of retinal neurons in the eye collect light images reflected from objects, convert these diverse stimuli into thousands of bits of electrical information, and combine this information along an optic nerve. The optic nerve then transmits the electrical information of vision to the posterior occipital region of the cerebrum within the brain, where millions of visual processing neurons position the inverted, reversed visual image and interpret it.

How the brain neurons process such information is not well understood and is the subject of intense study. While neurophysiologists have a fairly good understanding of nervous system structure, nervous system function represents a tremendous challenge to investigating scientists.

Structurally, neurons are supported by nerve cells called neuroglia in the central nervous system and Schwann cells in the peripheral nervous system. Neuroglia include four cell types: astrocytes, ependyma, microglia, and oligodendrocytes. Astrocytes stabilize neurons, ependyma allow cerebrospinal fluid exchange between brain ventricles and neurons, microglia clean up dead and foreign tissue, and oligodendrocytes insulate neurons by wrapping around them and secreting an electrically insulating protein called myelin. In the peripheral nervous system, Schwann cells behave much like oligodendrocytes; they wrap around axons and electrically insulate the axons with myelin for the efficient conduction of electrical information.

Specific neurological research is focused on neuronal plasticity in learning, the effects of various neurotransmitters upon neural activity, and diseases of the central nervous system. Various models of neuroplasticity have been proposed to explain how learning occurs in higher vertebrates, including humans and other mammals. Most of these neuronal processing models involve the spatial patterning of neural bundles, which orient information in space and time. The plastic growth of these neurons in specified directions and locking patterns contributes to memory, learning, and intelligence in higher mammals such as primates (which include humans and chimpanzees) and cetaceans (dolphins and whales).

Neurotransmission can be affected by a variety of physical states and chemical influences. The extensive use and misuse of pharmaceuticals and drugs can have serious effects upon the nervous system. Furthermore, developmental errors of the nervous system and aging can contribute to various diseases and disorders.

Perspective and Prospects

The nervous system of humans and higher vertebrate animals presents a tremendous variety of exciting research possibilities. The brain, the seat of human consciousness, represents a mystery to scientists even with the intense scientific scrutiny devoted to this organ. The brain is studied to understand how humans learn and how they might accelerate this exceptional ability. The intricate connections between billions of very plastic cerebral cortical neurons enable millions of electrical information impulses to direct millions of simultaneous activities every second. Brain structure, neural pathways, and techniques of learning and cognition are studied indirectly in human subjects and more directly in other intelligent mammals such as chimpanzees, gorillas, dolphins, and whales. These studies include analyses of the senses as well as poorly understood extrasensory perceptions that may be linked to exceptional nervous system activity.

Researchers in the field of artificial intelligence attempt to generate cellular automatons, machines that can think and self-replicate. Artificial intelligence research began with the work of the physicist and computer pioneer John von Neumann, who attempted to mimic the human nervous system within computer systems—systems that have been called von Neumann machines. The best supercomputers yet devised by humans may process data far more rapidly than the human brain, but they are no match for the human brain’s capacity to process millions of data items simultaneously.

While the basic physical and chemical mechanisms of neuronal function have been deciphered by neurological scientists, research into neurotransmission across synaptic gaps continues. One principal neurotransmitter at muscular junctions is acetylcholine, which triggers muscle contractions following a motor neural impulse. When acetylcholine is not needed, it is destroyed by a molecule called acetylcholinesterase. Two types of molecular poisons can affect neuromuscular activity: acetylcholine inhibitors, which compete with acetylcholine; and antiacetylcholinesterases, which inhibit acetylcholinesterase and, therefore, accelerate acetylcholine activity. Acetylcholine competitors (such as atropine, nicotine, caffeine, morphine, cocaine, and valium) block acetylcholine at neuromuscular junctions, thereby stopping muscular contractions and producing flaccid paralysis; death can result if the heart or respiratory muscles are affected. Antiacetylcholinesterases, such as the pesticides sevin and malathion, leave acetylcholine free to contract muscles endlessly, thereby causing convulsions.

Neurology also is devoted to understanding the biochemical and genetic basis for various neurological disorders, including Alzheimer’s disease, parkinsonism, seizures, abnormal brain wave patterns, paralysis, and coma. The field has also increasingly overlapped with psychiatry, which traditionally covered mental illnesses, as research reveals the lack of distinction between the biological and psychological origins of various disorders. Neurological research also is concerned with the nature of pain, the sense organs, and viral diseases of the nervous system, such as meningitis, encephalitis, herpes simplex virus 2, and shingles. As neurology uncovers how the nervous system works in greater detail, it develops the potential for new drugs or other treatments not only for disorders, but to possibly control or enhance many aspects of individuals' lives. This inevitably leads to ethical debates that span a host of related issues. The complexity of the human nervous system continues to inspire an enormous variety and quantity of research.

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