Physiology for Dental Students

357 Pages·1988·9.511 MB·English

by  D. B. Ferguson (Auth.)

Physiology for Dental Students
D. B. Ferguson PhD, BDS, LDSRCS(Eng)
Senior Lecturer in Physiology for Dental Students, Medical School,
University of Manchester
Wright
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First published, 1988
© Butterworth & Co (Publishers) Ltd, 1988
British Library Cataloguing in Publication Data
Ferguson, D. B.
Physiology for dental students.
1. Man. Physiology
I. Title
612
ISBN 0-7236-0725-7
Library of Congress Cataloging-in-Publication Data
Ferguson, D. B.
Physiology for dental students / D.B. Ferguson.
p. cm.
Includes bibliographies and index.
ISBN 0-7236-0725-7 :
1. Human physiology. 2. Dental students. I. Title.
[DNLM: 1. Physiology. QT104F352p]
OP 34.5.F46 1988
612—dcl9
DNLM/DLC
1 for Library of Congress
Photoset by Butterworths Litho Preparation Department
Printed and bound in England by Hartnolls Ltd, Bodmin, Cornwall
Preface
Almost all textbooks of physiology available today significance. These are often taught in courses of
are written, explicitly or implicitly, for medical Oral Physiology or Oral Biology. They include
students. This is hardly surprising since the largest saliva, speech, mastication, swallowing, taste, and
single group of students of physiology is that pain in the area served by the trigeminal nerve. The
studying medicine. It is probably also true that many coverage of these topics is excessive in relation to
of the courses in physiology for non-medical the normal plan of a medical textbook: this
students are based upon those provided for medical distortion is deliberate, and, in my view, appropri-
students and modified mainly by omissions. The ate to a textbook for dental students.
traditional 'systems' approach parallels that of the The most difficult decisions facing a textbook
teaching of clinical medicine and the pattern of writer are those over what to include and what to
specialisation within the medical and surgical omit. I have tried to approach the problem by asking
professions. There are a number of implications myself what I see as the role of a textbook. This
from this. 'Basic' physiology, those principles question is inevitably linked with what one sees as
common to several body systems, tends to be taught the role of a lecture course. Which should provide
in relation to a particular body system and common the irreducible minimum of knowledge necessary for
characteristics are not always identified as such. The the student to pass the examinations? In my
interactions and inter-relationships between body experience the lecture course has usually been
systems are understressed and may never emerge. identified as fulfilling this requirement. If this is so,
The topic of exercise is a good example. Another the textbook is used to enable the student to amplify
implication is that illustrations and examples of and put into context the notes derived from lectures
relevance are almost always drawn from clinical - perhaps to explain more fully what was not clear,
medicine or surgery. perhaps to bring out points not obvious in the
In preparing this textbook I have attempted to lectures. Further, the textbook is always there: it
move away from the traditional pattern and present ought therefore to function as a source of reference
a more generalist and more integrated view of material. 1 do not see the textbook, then, as
physiological mechanisms and physiological containing merely the minimum information neces-
systems. Thus the first section looks at body fluids sary for examination purposes; equally I do not see
and at tissue cells, examining the bases of operation it as a form of instruction to be assessed in isolation.
of these smallest units of body structure. The second In this book, then, I have tried to provide a basis of
section provides an outline of each physiological facts from which physiological ideas can come and I
system, concentrating on the morphological units have further tried to link these facts in a meaningful
and the ways in which they act. The third section sequence.
deals with physiological control and with whole Section I deals with a number of physiological
body physiology. principles and with the physiology of body fluids and
The book is made more specific for dental particular types of cell. In Section II each of the
students in two ways: by the use of examples and physiological systems is described in turn. The
illustrations relevant to clinical dentistry and by the coverage of neuroanatomy varies in different
emphasis given to a small group of topics of oral courses: I have therefore dealt with the nervous
system with a slight bias towards this. The endocrine Ganong, W.F. Textbook of Medical Physiology. Appleton-
system has usually been covered in basics in school Lange, Norwalk, 13th edn (1987)
biology courses: I have therefore tried to summarise Guyton, W.F. Textbook of Medical Physiology. Saunders,
information on the endocrine organs, the endocrines Philadelphia, 7th edn (1986)
Mountcastle, \ .B.Medical Physiology. Mosby, St. Louis,
they produce, and the actions of the individual
14th edn (1980)
hormones. The third section of the book, control
Ruch, T. and Patton, H.D. Physiology and Biophysics.
and integration, assumes a knowledge of the various Saunders, Philadelphia, 20th edn (1979)
hormones and therefore the end of the chapter on
and that encyclopaedic reference source, The
the endocrine system is intended as a section to
Handbook of Physiology, published by Williams
which reference back may be made by the reader.
and Wilkins, Baltimore, for the American Phy-
In any scientific subject there must be numerical
siology Society. Sections 1-10, (1977-1986).
information; in human physiology one has to add
There are also a number of textbooks of
the proviso that when a particular value is given it is
physiology as applied to dentistry:
usually a number which is relatively easy to
remember, somewhere towards the middle of a Jenkins, G.N. Physiology and Biochemistry of the mouth.
range, but rarely either a mean or a mode. Blackwell, Oxford, 4th edn (1978)
Biological variation is such that one can only hope to Osborn, J.W., Armstrong, W.G. and Speirs, R.L.
give the student some peg on which to hang an Companion to Dental Studies Volume I, Anatomy,
assessment of normality or an indication of magni- Biochemistry and Physiology. Blackwell, Oxford (1982)
Shaw, J.H., Sweeney, E.A., Cappuccino, C.C. and
tude of a particular variable. When one goes back to
Meiler, S.M. Textbook of Oral Biology. Saunders,
original data one often finds that the investigations
Philadelphia (1978)
themselves were limited in one way or another.
In a new book it seems sensible to use as far as Finally, I must record in print my thanks to all
possible the units and terminology which have been those who have helped me along the very long road
internationally standardised and agreed. British to the appearance of this book in print: Dr H.
schools teach in SI units and use IUPAC terminolo- Richardson and Dr A. Watt, who looked at the
gy in physics and chemistry. I have therefore used earliest written chapters, Dr R. Foster, who taught
both these conventions. The main changes from me to use, successively, two word-processing
traditional physiological textbooks in this respect programmes, Dr J. Waterhouse, who devoted a very
are in the use of newtons for force units, pascals long time to reading, criticising, and offering helpful
(newtons/m2) for pressure units, hydrogen carbon- suggestions towards two consecutive versions of the
ate instead of bicarbonate, and iron II instead of text, and Prof. R. Green, who repeated that
ferrous iron. exercise on the third version. I must thank also Dr J.
At the end of each chapter I suggest further Gilman, of John Wright, who supported me
sources of information. To these should be added a through the early stages of writing, and Mr D.
list of more general textbooks which may, in fact, Kingham, who completed the process. They had to
cover material in many or all of the chapters. These be, and were, very patient. The debt to my wife is
include: too great to be spelt out except in relation to her
professional skill in producing the index. It is
Davson, H. and Segal, M.B. Introduction to Physiology.
perhaps true to say that many others have suffered:
Academic Press, London, Vol.1 1975, vol. 2 1975, vol. 3
1976, vol. 4 1978, vol. 5 1980 to them also my apologies and thanks.
1
Cells and fluids
The cell and its contents muscle cells. The cell pH is usually slightly acid, and
under these conditions most cell proteins behave as
Physiology is the study of how living creatures live; anions and contribute to the total anionic charge of
human physiology the study of the physical and the cell. The other major cell anions are chloride,
chemical processes that make up the life of man. It is phosphate and hydrogen carbonate. Balancing the
based on anatomy, the study of structure or anionic charge are the inorganic cations, principally
morphology; on histology, or micro-morphology; on potassium ions.
biochemistry , the study of the chemical processes of
The cell membrane
life; and on biophysics, the study of the physical
parameters of living organisms. In considering The limiting membrane of the cell, often called the
physiological principles it is helpful to look first at plasma membrane, has been much studied because
the basic rules governing the function of all body the properties of this membrane dictate what may
cells, then to apply these to the operation of the enter or leave the cell and hence maintain the
organs of the body, and finally to apply them to the identity of the cell contents. Viewed with the
functioning of the body as a whole. This book electron microscope the typical membrane appears
begins, then, by considering the normal structure of as two electron-dense lines separated by an electron-
cells, their substructures or organelles, and the ways lucent zone. Membranes of this appearance are
in which these contribute to the functioning of the often termed 'unit membranes'. The membrane has
cells. Fig. 1.1 shows two adjacent half cells an average width, after fixation, of around 7.5 nm
containing most of the possible organelles in some of and each of the electron-dense lines is approximate-
their typical arrangements. These are hypothetical ly 2.0nm wide. Chemical analysis of membrane
examples since it is very unlikely that any cell would material and experiments with lipids suggested that
contain all these organelles. a flexible membrane-like structure could be formed
The cytoplasm of a cell surrounds the nucleus and from a bimolecular layer of lipid molecules orien-
the organelles, and is bounded by a membrane. In tated with their hydrophobic fatty acid side chains
histological sections stained with the standard inwards towards each other, and their hydrophilic
haematoxylin and eosin stains it appears as a faintly phosphate or basic groups pointing outwards.
purple-staining, slightly granular background; under Indeed, it was suggested at one time that the cell
the electron microscope it appears clear (or membrane was not a true morphological structure
electron-lucent) with varying degrees of fine elec- but simply a lipid layer on the cytoplasmic surface.
tron-dense granulation and some fine filaments or However, further study of the properties of
rods. Tests with intracellular probes show it to be membranes showed that they were made up of a
either a gel or a sol, and to be more gel-like near the bimolecular layer of phospholipid covered on each
cell membrane. It contains cell water, proteins, side by a protein layer; this model accords with the
small organic molecules, and some inorganic ions. appearance seen with the electron microscope (Fig.
Most cells contain about 85% of water. The protein 1.2).
constituents are in various degrees of aggregation. It is, however, very simple, and explains only the
Some are highly structured to form filaments, as in bounding function of the membrane. Cell mem-
3
4 Cells and fluids
Figure 1.1 Diagram of two half cells as seen under the electron microscope.
These are not actual cells, but have been drawn to demonstrate a number of
organelles. Key: C, centriole; CM, cell membrane; GA, Golgi apparatus; GI,
glycogen granules; L, lysosome; LD, lipid droplet; M, mitochondria; MA,
macula adhaerens; Mf, micro filaments; Mt, microtubules; Mb, microbody;
Mv, microvillus; N, nucleus; NM, nuclear membrane; NP, nuclear pore; Ph,
phagosome; RER, rough endoplasmic reticulum; Rs, ribosomes; SER,
smooth endoplasmic reticulum; SG, secretory granule.
branes have other properties also. They allow the of substances in or out of certain cells can be
passage of lipid-soluble substances and, to a lesser modified precisely and in particular time sequences.
extent, small water-soluble substances. The latter Further, certain substances pass through the mem-
property implies that the phospholipid layer is not brane against concentration and electropotential
continuous, but interrupted by 'pores' of small size. gradients, implying that the membrane is capable of
Studies of the sizes of water-soluble substances able utilising energy to perform work. All these prop-
to cross membranes permit estimation of the erties suggest that the membrane contains proteins
effective pore size - around 0.70-0.85 nm in a red with specific binding functions. In this respect these
blood cell membrane. Comparison of rates of proteins resemble enzymes and so they have
diffusion leads to a calculation that 0.06% of the sometimes been called permeases. Parts of the cell
surface area of these particular membranes is membrane in some instances may enclose external
occupied by the pores. No actual pores have been matter and then incorporate it into the cell; other
detected by electron microscopy even though pores parts of the membrane may apparently be new-
of this size and density ought to be occasionally formed from fusion of the membranes of organelles.
visible. Such pores are probably not actual holes in Any model which suggests complete uniformity of
the membrane but areas in which water-soaked the cell membrane is therefore likely to be
hydrophilic proteins penetrate the lipid layer. The misleading.
membrane is able to permit certain ions and A number of modifications to the original model
molecules to pass selectively at rates greatly in have been suggested. None is entirely satisfactory
excess of those predicted from diffusion data. This but each presents solutions to some of the difficul-
property is linked with protein, or, more precisely, ties with the original model. It is clear that
carrier or enzyme activity. This selective movement membranes differ from one type of cell to another
The celt membrane 5
Rigid model
Outer surface
protein ^^ Protein passing through
O>N lo 000Ά f° f ?? ? ? ???? oo o
Phospholipid
Fluid model
Figure 1.2 Models of the cell membrane ,(a) The rigid model with a pore filled
with protein; on the right, its appearance under the electron microscope, (b) A
section through a fluid membrane model at a single instant of time, a protein
molecule spanning the membrane again providing a pore for water and small
solutes to cross the membrane.
and also from point to point on the surface of one Junctions between cells
cell. For example, the protein to lipid ratio is
different in red blood cell membranes from that in Cells are usually separated from each other by a
the membranes of nerve cells; indeed, the composi- space about 20 nm wide filled with proteoglycans.
tion of the lipid material itself is different. However, a number of membrane specialisations
Membranes vary in thickness and in biological provide either a means of attaching one cell to
activity. The current model is of a fluid structure in another or of barring the space between the cells
which the lipid and protein molecules are relatively (Fig. 1.3). The term zonule is used to describe a
mobile, so that the appearance of the membrane junction which extends round a cell, and the term
differs from moment to moment. The structure may macula to describe a junction involving only a small
become more stable or more rigid when particular area of cell membrane. Either of these may be
protein units are performing particular functions. 'adhaerens', so that the cells are simply attached to
Chemical substances or drugs may cause changes in each other, or 'occludens', when the attachment
the movement of membrane components. Some effectively obliterates the intercellular space.
proteins may extend across the membrane, some There are three types of junction frequently seen
may travel from one side to the other, whilst still between mammalian cells: desmosomes, tight junc-
others may be associated with only the inner or the tions, and gap junctions. The desmosome (Fig. 1.3a)
outer side of the membrane. is a macula adhaerens. It is a small discoid
6 Cells and fluids
reform, apparently in response to specific stimuli.
They are particularly sensitive to increases in
intracellular calcium ion concentration, which cause
them to close and disappear. This response may be
protective: it could prevent products of damage or
malfunction passing from one cell to its neighbours.
Cell organelles and inclusions
(a) (b) The nucleus
Within the cytoplasm are varying numbers of
organelles, each enclosed by membrane material.
The cell nucleus is bounded by a nuclear membrane
consisting of two unit membranes 7-8 nm thick
separated by a 30-50 nm space. The paired mem-
brane fuses in places to form 'pores' approximately
65 nm in diameter and octagonal in shape. Up to
one-third of the nuclear membrane may be occupied
by these pores. The nucleus contains the genes
which are here associated with strands of chromatin
composed of desoxyribonucleic acid (DNA) with
(c) (d)
some ribonucleic acid (RNA) and nucleoproteins.
Figure 1.3 Types of cell junction, (a) The macula Within the nucleus there is a large clear area, the
adhaerens - a desmosome, (b) a zonula occludens - a tight nucleolus, containing many granules which are
junction, (c) a close or gap junction, (d) a close or gap
precursors of those on the outer surface of the
junction at higher magnification. The protein plugs joining
nuclear membrane. These granules are the ribo-
the two cells can be seen on the inner face of the
somes: they are about 15 nm in diameter and are
membrane of the cell on the right.
made up of nucleic acid and nucleoproteins. Similar
granules are found on the rough endoplasmic
attachment between neighbouring cells, found reticulum and are described later. Functionally the
particularly in epithelial tissues. Dense clusters of nuclear membrane serves to preserve the identities
filaments are situated in a pair of electron-dense of the nucleoplasm and the cytoplasm. In addition it
spots on adjacent membranes of two cells and the is the route by which RNA leaves the nucleus after
space between the cells is filled with fibrous its synthesis against the template of DNA, the
material. A tight junction (Fig. 1.3b) is a zonula nuclear genetic material. There is continuity be-
occludens, usually extending round a cell. In a tight tween the outer layer of the nuclear membrane and
junction the membranes of the two cells are fused the endoplasmic reticulum.
together so that their outer layers become one; the
membrane thus appears to have three electron-
The endoplasmic reticulum
dense lines separated by two electron-lucent lines.
The fusion of the membranes means that the The endoplasmic reticulum is present in all cells
intercellular space is obliterated. Tight junctions are except mature red blood cells and platelets. It is
seen in cell layers which line body cavities, where particularly well developed in cells involved in
they prevent movement of materials into or out of protein synthesis and secretion. There are two types
the cavities by intercellular pathways. The gap - rough endoplasmic reticulum with ribosomes on
junction, or nexus (Fig. 1.3c), is a macula occludens, the surface, and smooth endoplasmic reticulum
that is, a spot junction closing the intercellular gap. which lacks these granules. The reticulum is a
The membranes of neighbouring cells approach system of interconnecting tubes 40-70 nm in dia-
closely, to within some 2nm, and are linked by meter whose walls appear as single electron-dense
special membrane proteins which form channels lines about 1.4 nm in thickness (Fig. 1.4). In some
between the cells along which transfer of ions and areas of the rough endoplasmic reticulum the system
small molecules and electrical charge can take place. of tubules shows close apposition and the tubules
They are found in some excitable tissues such as are flattened to form cisternae. On the outer surface
smooth muscle and cardiac muscle, where they of the rough endoplasmic reticulum are the granules
provide pathways for electrical stimuli to pass of RNA and protein which constitute the ribosomes.
rapidly across sheets of cells so that cell activities can At these sites amino acids are linked to the transfer
take place in a highly integrated manner. Gap RNA to be assembled into new protein molecules
junctions are labile structures: they close and under the combined action of the messenger RNA
Cell organelles and inclusions 1
4. Secondary lysosomes formed by the coalescence
of primary lysosomes with auto- or heterophago-
somes.
The secondary lysosome digests its contents and the
products may pass back into the cytoplasm. When
digestion is completed the vesicle is termed a
post-lysosome; this may remain within the cell or it
may release its contents to the environment through
the cell membrane (Fig. 2.7). The enzymes found in
lysosomes are a group of about 15, which, if free in
the cytoplasm, would be capable of digesting the cell
itself. They include acid phosphatase, proteolytic
Figure 1.4 The rough endoplasmic reticulum seen in three
dimensions. A section through the reticulum appears as in enzymes known as cathepsins, a-glucosidase, and
Fig. 1.1 as a series of parallel tubes. The granules scattered enzymes capable of splitting RNA and DNA. The
over the surface are ribosomes. typical enzymes of lysosomes are hydrolases. The
relatively high protein content of the lysosomes
usually renders them electron-dense. Their bound-
and the ribosomal nucleoproteins. Some of the new ing membranes are thicker than those of most cell
protein molecules then pass into the lumen of the organelles, being around 9nm in thickness. The
tubules. The smooth endoplasmic reticulum trans- lysosomes are thought to develop as vesicles on the
fers some enzymes from the rough endoplasmic Golgi apparatus, although the enzymes are original-
reticulum to the Golgi apparatus. It appears to be ly synthesised on the rough endoplasmic reticulum.
important in cholesterol metabolism and possibly in The heterophagosomes are results of pinocytosis
general lipid metabolism. Cells involved in the (incorporation of the external medium by an
synthesis of steroids have a much more extensive invagination of the cell membrane) or phagocytosis
smooth endoplasmic reticulum. (incorporation of solid particles by invagination of
the cell membrane). The properties of pinocytosis
and phagocytosis are shown by the phagocytic cells
The Golgi apparatus
of the blood, by their equivalent cells in tissues, and
The Golgi apparatus is a filamentous or plate-like by cells with a 'brush border' - a membrane with
smooth membrane reticulum (Fig. 1.1). Although many small projections termed microvilli. The
present in all cells, it is most highly developed in autophagosome is produced when smooth mem-
cells involved in secretion and storage. The secre- brane surrounds an organelle or cytoplasmic area
tory cells of the pancreas may have as much as 10% and then fuses with a primary lysosome. Secondary
of their volume occupied by the Golgi apparatus. In lysosomes perform their digestive function, small
such cells the apparatus may show cisternae and molecules escape across their walls and they become
vacuolar and vesicular regions. The formation of post-lysosomes. Some of these are retained indefi-
enzyme-containing vesicles, lysosomes and enzyme nitely, as in the cells of the skin, but others fuse with
precursor (zymogen) granules takes place in the the cell membrane and release their contents to the
Golgi system. There are some enzymes located extracellular environment. In the liver, for example,
there, and the linking of sugars and proteins to form waste material is transferred to the bile.
glycoproteins takes place in this area. Secretory granules may be regarded as special
forms of lysosome.
Vesicles
A vesicle is a membrane-enclosed sac. Many vesicles Microbodies
have been found to have special functions or to be
Microbodies are membrane-bounded spherical
identifiable with special stains. The larger vesicles
structures containing dense granular or even crystal-
are termed vacuoles.
line material. They are present in large numbers in
Lysosomes are one group of vesicles with special
liver and kidney cells. Their diameters range from
characteristics and functions. Four types are de-
0.3 to 0.6 μιτι and their unit membranes are about
scribed:
6.5 nm wide. Microbodies contain enzymes: these
1. Primary lysosomes containing enzymes include catalase (most consistently, although its
2. Phagosomes containing materials or particles function here is unknown), urate oxidase and
taken up at the cell surface by invagination of the D-amino-oxidase. The presence of these two en-
membrane (also called heterophagosomes) zymes has led to the suggestion that the microbodies
3. Autophagosomes containing cell organelles due are concerned in amino acid and uric acid metabol-
for destruction, and ism. Because of their enzyme content microbodies
8 Cells and fluids
have also been called peroxisomes or uricosomes. apparatus along pathways laid out by the microtu-
They are probably formed from the endoplasmic bules to the cell surface (see Chapter 2). Microtu-
reticulum. bules also form the mitotic spindle during cell
Cells of calcifying tissues contain structures division. Characteristic patterns of microtubules are
similar to microbodies which are able to concentrate found in ciliated cells to form structures capable of
calcium ions and form an amorphous calcium producing movement of the cilia. It is probable that
phosphate. This later transforms to apatite, the the microtubules are assembled by the endoplasmic
characteristic crystal of calcium phosphate in nearly reticulum.
all mammalian calcified tissues. These structures are Microtubules form the walls of the centrioles - a
referred to as membrane vesicles whilst they are still pair of cylindrical bodies about 400 nm long and
inside the cells, but when they leave the cell they are 150 nm in diameter. The centrioles lie near the
known as matrix vesicles. nucleus and are an essential part of the mitotic
apparatus during cell division.
Specialised microfilaments are found in the
Microtubules and microfilaments
cytoplasm of many cells. The most obvious exam-
Thin cylindrical membranes known as microtubules ples are the ordered microfilaments and filaments of
are found in various forms in different cells. They the actin and myosin of skeletal and cardiac muscle,
are usually 20-40 nm in diameter with walls 5-10 nm but actin filaments are also found in smooth muscle,
thick made up apparently of very fine fibrils. One of in platelets, and in the cilia and processes of many
their functions in cells appears to be that of other cells.
maintaining rigidity - in blood platelets, for
example, they maintain the discoid shape. Microtu-
Mitochondria
bules also stabilise the long narrow extensions, or
processes, which extend into the tubules of the The production of high energy phosphate, in the
dentine from the odontoblast cells. Another func- form of adenosine triphosphate (ATP), takes place
tion appears to be that of defining pathways for in another membranous organelle: the mitochon-
movements of cell constituents, as in secretory cells drion (Fig. 1.5). Each mitochondrion is ellipsoidal in
where synthesised material passes from the Golgi section and consists of an outer double boundary
Outer membrane
Inner membrane
Respiratory stalks
Crist ae
Ribosome
DNA
Figure 1.5 A mitochondrion with a portion removed to show the internal
structure. This can be compared with the sections through mitochondria in Fig.
1.1.