Appunti di Fisiologia (Human physiology, ENG)

Human physiology biomedical engineering

Course of Human Physiology, Prof. M. Fabri, a.a. 2016-2017. Notes revised by students: A. Bracchetti, L. Buscarini, L. Migliorelli, L. A. Pettinari, A. Tigrini.

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Index

• Introduction ............................................................................................................................... 6
• Membrane and transport mechanisms ............................................................................ 8
• General organization of the body .............................................................................................. 8
• Homeostasis ................................................................................................................................ 9
• The cell membrane .................................................................................................................... 11
• Transport mechanisms ............................................................................................................. 12
• Neurons ....................................................................................................................................... 15
• Organization of nervous system .............................................................................................. 15
• Neuron anatomy ........................................................................................................................ 17
• Support structures .................................................................................................................... 18
• Synaptic transmission .......................................................................................................... 21
• Membrane potential ................................................................................................................... 21
• Action potential ......................................................................................................................... 22
• Synapse ...................................................................................................................................... 24
• Synthesis and storage of neurotransmitters .......................................................................... 25
• Receptors ................................................................................................................................... 28
• Muscles ....................................................................................................................................... 31
• Muscle tissues ........................................................................................................................... 31
• Skeletal muscle .......................................................................................................................... 31
• Neuromuscular junction ........................................................................................................... 37
• Muscle contraction .................................................................................................................... 39
• Smooth muscle .......................................................................................................................... 41
• General brain anatomy and function ........................................................................... 44
• Central nervous system ............................................................................................................ 44
• Brain ........................................................................................................................................... 47
• Spinal cord ................................................................................................................................. 52
• Brain function ............................................................................................................................ 54
• Motor control ........................................................................................................................ 61
• Reflexes ...................................................................................................................................... 61
• Descending motor pathways .................................................................................................... 65
• Motor cortex ............................................................................................................................... 72
• Cerebellum ................................................................................................................................. 76

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• Basal ganglia ............................................................................................................................. 83
• Motion processing ..................................................................................................................... 86
• Somatosensory system ....................................................................................................... 89
• Properties of sensory systems ................................................................................................ 89
• Somatic senses ......................................................................................................................... 91
• Somatosensory pathways ........................................................................................................ 94
• Touch and thermoception ........................................................................................................ 96
• Nociception ................................................................................................................................ 97
• Eye and vision ......................................................................................................................... 102
• The eye ..................................................................................................................................... 102
• Eye movements and sensorimotor integration .................................................................... 103
• Phototransduction ................................................................................................................... 110
• Vision and perception ............................................................................................................. 113
• Visual pathways ....................................................................................................................... 116
• Auditory and vestibular systems ................................................................................. 120
• The ear ...................................................................................................................................... 120
• Hair cells ................................................................................................................................... 123
• Auditory pathways .................................................................................................................. 125
• Vestibular system .................................................................................................................... 126
• Autonomic nervous system .............................................................................................. 131
• Autonomic division ................................................................................................................. 131
• ANS neurotransmitters and receptors .................................................................................. 133
• Cardiovascular system ..................................................................................................... 136
• The heart .................................................................................................................................. 136
• Cardiac muscle tissue ............................................................................................................. 138
• Cardiac cycles ......................................................................................................................... 143
• Circulation ................................................................................................................................ 146
• Heart innervation ..................................................................................................................... 152
• Respiratory system ............................................................................................................. 155
• Structure and function ............................................................................................................ 155
• Mechanics of respiration ........................................................................................................ 158
• Gas exchange and transport .................................................................................................. 162
• Control of ventilation .............................................................................................................. 169

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Introduction

Physiology, which literally means “knowledge of nature” (from the Greek physis, “nature”, and logos, “knowledge”) is the branch of biology that studies the normal functioning of a living organism and its component parts, including all its chemical and its physical processes. Physiology has a wide field of study, dividing in animal and plant physiology (respectively part of the disciplines of zoology and botany), cellular physiology (with cytology they constitute the branch of cellular biology), microbial, bacterial and viral physiology (part of the field of study of microbiology). Human physiology is focused on the study of human body functioning and processes.

Many complex systems – including those of the human body – possess emergent properties, which are properties that result from complex, non-linear interactions of different components and cannot be predicted to exist based only on the knowledge of these latter. Among the most complex emergent properties in humans, there are emotion, intelligence, and other aspects of brain function: nevertheless, none of them can be predicted from the complete knowing of the individual properties of nerve cells. Thus, a special focus of physiology is to find the missing link among several levels of organization to explain and outline the oddest and most indescribable property featured by complex organisms. This is why physiology is said to be an integrative science, with a considerable overlap between different fields of study: such as molecular biology to ecology. Moreover, at the level of organism, physiology is closely tied to anatomy: the structure of a tissue or organ must provide an efficient physical base for its function. For this reason, it is nearly impossible to study human physiology without understanding the underlying anatomy.

Physiology is defined as the normal functioning of the body, but it is necessary to distinguish the difference between function and mechanism. The function of a physiological system or event describes the interaction and the functioning of different parts of an organism in response to the requests of the organism itself and the solicitations of the environment, so that the adaptive significance is the prevalent aspect taken in account. In other words, the function answers to the question why things happen in such ways, but not how: red blood cells carry oxygen to the tissues because they need to. On the other hand, the mechanism studies exactly how a certain process occurs in the human body, without caring the significance of it in the whole frame of functioning of the organism. In fact, this approach studies that red blood cells contain haemoglobin, a molecule that allows the oxygen to reversely bind with, without explaining the implications of the presence of oxygen inside tissues (and the damages of its absence). Generally, these two aspects are complementary: mechanisms are completely useful to study if they are not contextualized with a unanimous function, and functions would not have been so detailed without an in-depth analysis of the manifold biochemical and microbiological mechanisms that occurs inside an organism. The main themes of physiology are:

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• Structure-function relationships. The integration of structure and function extends across all levels of organization, from the molecular level to the intact body. This theme subdivides into two major ideas: molecular interactions and compartmentation. The first one is the ability of individual molecules to bind to or react with other molecules. A molecule’s function depends on its structure and shape, and even a small change to those may have significant effects on the function featured by the molecule. Many physiologically significant molecular interactions involve the class of biological molecules called proteins. Functional groups of proteins include enzymes that favour chemical reactions, signal molecules, receptor proteins (that bind selectively with the signal molecules to accomplish a certain function) and specialized protein (that serve as biological pumps, filters, motors or transporters). The second regards the division of space into separate compartments, which allow a cell, a tissue, or an organ to specialize and isolate functions. At macroscopic level, the tissues and organs of the body form discrete functional compartments, such as body cavities or the insides of hollow organs. At the microscopic level, cell membranes separate cells from the fluid surrounding them and also create tiny compartments within the cell called organelles.
• Biological energy use. Growth, reproduction, movement, homeostasis – these and all the other processes that take place in an organism require the continuous input of energy. This theme answers to the questions related to the modalities of supplying energy to the organism, its storage and usage. In the organism, energy is stored in chemical bonds, so it is always in the form of chemical energy and is used for various tasks: building/breaking molecules, transport of molecules across cell membranes, creating movement within muscles.
• Stream of information. Information flow in living systems ranges from the transfer of information stored in DNA from generation to generation (genetics) to the flow of information within the body of a single organism. At the organismal level, information flow includes translation of genetic code into proteins that are responsible for cell structure and function as well as the many forms of cell-to-cell communication that coordinate the functioning of a complex organism. In the human body, information flow between cells takes the form of either chemical signal or electrical signals. Information may go from one cell to its neighbours (local communication) or from one part of the body to another (long-distance communication). Some molecules are able to pass through the barrier of cell membrane, other ones need a specific signal molecule and a receptor that makes available a specific channel for them, and other ones cannot pass the membrane at all.
• Homeostasis. Organisms that survive in challenging habitat cope with external variability by keeping their internal environment relatively stable, an ability known as homeostasis, which is a key theme physiology revolves around, describing the physiological control systems along with their mechanisms of regulation inside the organism.

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Membrane and transport mechanisms

General organization of the body

Human physiology is focused on the study of the processes, functions and mechanisms regarding apparatus, organs, tissues and cells of human individuals. Human body can be presented like a complex system that exchanges air, nutrients, water and waste with the external environment.

In this diagram, it is shown the general organization of the human body, divided in systems. Four systems exchange materials between the internal and external environments. The respiratory (pulmonary) system exchanges gases; the digestive (gastrointestinal) system takes up nutrients and water and eliminates wastes; the urinary (renal) system removes excess water and waste material; and the reproductive system produces eggs or sperm. The remaining four systems extend throughout the body. The circulatory (cardiovascular) system distributes materials by pumping blood through vessels (note the relative supporting anatomical structures of these systems between brackets). The nervous and endocrine systems coordinate body functions and regulations. The one system not illustrated in the diagram below is the diffuse immune system, which includes but is not limited to, the anatomical structures known as the lymphatic system. The specialized cells of the immune system are scattered throughout the body. They protect the internal environment from foreign substances by intercepting substances that enter through the intestines and lungs or through a break in the skin. In addition, immune system is closely associated with the circulatory system.

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Homeostasis

Homeostasis (from Greek, homeo, “like, similar”, and stasis, in this instance translated as “condition”) specifies the condition of dynamic steady state of the organism, that is, the maintenance of several variables in a limited range of values. To maintain homeostasis, the human body monitors certain key variables, such as body temperature, blood pressure and heart rate, and tends to regulate them by physiological control mechanisms that kick in if the variable ever strays too far from its set-point, or optimum value. If the body fails to maintain homeostasis by keeping limited this group of physiological variables, then normal function is disrupted and a disease state, or pathological condition (from pathos, “suffering”) may result. Disease falls into two general groups according to their origin: those in which the problem arises from internal failure of some normal physiological process, and those that originate from some outside source. Internal causes of disease include the abnormal growth of cells, which may cause cancer or benign tumours; the production of antibodies by the body against its own tissues (autoimmune diseases); and the premature death of cell (necrosis) or the failure of cell processes. Inherited disorders are also considered to have internal causes. External causes of disease include toxic chemicals, physical trauma, and foreign invaders such as viruses and bacteria. In both internally and externally caused diseases, when homeostasis is disturbed, the body attempts to compensate. If the compensation is successful, homeostasis is restored. If compensation fails, illness or disease may result. The study