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 underling 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 answer 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, this 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: 6

1. Structure-function relationships. The integration of structure and function extends across

all level 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 binds 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 to 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.

2. 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 suppling 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.

3. 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 form 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.

4. Homeostasis. Organisms that survive in challenging habitat cope with external variability by

keeping their internal environment relatively stable, an ability know 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 exchange 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

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foreign substances by intercepting substances that enters through the intestines and lungs or

through a break in the skin. In addition, immune system is closely associated with the circulatory

system.

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 fall 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