histology Flashcards
(123 cards)
1
Q
define histology
A
is the study of the tissues of the body and how these tissues are arranged to constitute organs. This subject involves all aspects of tissue
biology, with the focus on how cells’ structure and arrangement optimize functions specific to each organ.
2
Q
Plasma membrane
A
Dynamic interface between the internal environment of the cell
and its various external environments.
* Functions as a physical barrier; regulates movement of molecules into and out of the cell; mediates cellular recognition
and interaction.
* Its interaction depends on the specialised function of the cell – selective permeability.
* Membranes of organelles have the same basic structure.
* Made of lipids and proteins - bilayer phospholipid.
* Proteins: receptors, enzymes, cell identity markers.
3
Q
in plasma membrane
A
the integral proteins are channels that extend from one end to the other. the exterinsic proteins are located on the outside of the membrane. a protein and a carbohydrate group together are called glycoprotein. a lipid and carbohydrate group together is reffered to as glycolipid.
4
Q
chromatin
A
a coiled threadlike mass, consists of DNA, protein and
some RNA; when a cell begins to divide it shortens and thickens into
rod-shaped structures – chromosomes (n=46 [23 pairs]).
5
Q
nucleolus
A
a spherical structure within the nucleus; produces
components of the ribosomes, which are important in protein
synthesis.
6
Q
cytoskeleton
A
function is to maintain structural integrity of cells that are dynamic in nature. it is also necessary for movement of cell organelles, cell locomotion and muscle fibre contraction.
7
Q
endoplasmic reticulum
A
ER is a pathway for transportation of substance and storage area for synthesised molecules.
RER is where most protein synthesis occurs; products are enclosed in vesicles for transport
SER mainly involved in lipid synthesis and detoxification
8
Q
golgi aparatus
A
Membranes, cisternae.
Completes post-translational modifications, sorting and packaging
the products synthesised by the cell; cellular secretion → vesicle
(exocytosis).
9
Q
mitochondria
A
Bounded by double membranes: outer smooth, inner – folds/cristae
(surface area for chemical reactions).
performs aerobic respiration, converting carbs and oxygen to carbon dioxide and water, thus generating energy ATP. it has its own DNA and matrilineal inheritance endosymbiotic origin.
10
Q
lysosomes
A
memebrane bounded vesicles of various shapes and sizes produced by the Golgi complex.
Contain more ~ 60 digestive enzymes which act upon macromolecules from within or outside the
cell.
11
Q
what are the 4 basic fundamental tissues
A
epithelial
connective
muscle
nervous
12
Q
epithelial tissue
A
Cells of epithelial tissues: aggregated polyhedral cells. Consists of an uninterrupted layer of tightly packed cells. Holes such as the pores of a gland are an invagination of
epithelium. epithelial tissue is polarised as it has free surface aka the apical surface whihc is exposed to the outside, and 2 the basal surface which is attached to the underlying connective tissue. it could have 1 or several layers of apical and basal layers. In epithelial tissue there is lamina propria which is the layer of connective tissue underlying the epithelium it is bound to epithelium by basement membrane. basement memebrane is formed by secretion of both, epithelial and connective cells. basement memebrane functions as a selective permeable filter between epithelium and connective tissue. Epithelium is avascular – receives nutrients by diffusion from lamina propria which is essentially connective
tissue.
found in a small amount in the extracellular matrix
its main function: lining of surface (skin and gut) and glandular secretion (glands), absorption (intestine), sensation (neuroepithelium), and contractility (myoepithelial cells).
there are two main types of epithelial tissues: 1 surface; : the cells are organised in layers that cover the
external surface or line the cavities of the body. 2 Glandular: the cells specialised to produce secretions
13
Q
connective tissue
A
cells: several types of fixed and wondering cells
abundant amount in extracellular matrix
its main function: support and protection
14
Q
nervous tissue
A
cells: interwining elongated processes
not found in extracellular matrix
main function: transmission of nervous impulses
15
Q
muscle tissue
A
elongated contractile cells
moderate amount in extracellular matrix
main function: movement
16
Q
specialisation of the cell surface
A
1 Microvilli: folds of the plasma membrane- increase surface area, therefore increase absorption
2 Cilia: projections of the plasma membrane. locomotion
17
Q
what are the 3 arrangements of epithelium layers
A
1 simple, pseudostratified, and stratified
18
Q
what are the 3 epithelial cell shapes
A
squamous, cuboidal, and columnar
19
Q
Simple squamous epithelium
A
- flat, irregularly shaped, tightly fitted cells in a mosiac like pattern; with flattened centrally located nucleus.
-function: diffusion and filtration
-location: pulmonary alveoli, kidneys, lining the inner walls of blood and
lymphatic vessels (endothelium), form serous membranes that line the body
cavities and its organs – pericardium, pleura, peritoneum (mesothelium)
20
Q
simple cuboidal epithelium
A
-composed of tightly fitted cube shaped cells, with centrally positioned round nucleus
-function: secretion, filtration, absorption
-location: found in many glands and glandular organs and ducts s (surface of the
ovaries, ducts of the salivary glands and pancreas), lines the kidney tubules.
21
Q
simple columnar epithelium
A
-composed of tall columnar cells of variable height, tightly fitted, nucleus usually located close to the basement memebrane
-existence of goblet cells
-existence of goblet cells
-can be ciliated or not ciliated.
-function: protection, lubrication, secretion, and absorption
-location: lining of intestine, uterine tubes, and bronchioles.
22
Q
pseudostratified epithelium
A
-cells are of different heights with a variable position of the nuclues, giving the appearance of being stratified; goblet cells.
- non-ciliated or ciliated
-functtion: protection, secretion, if ciliated involved in movement of particles
-location, trachea, bronchi, nasal cavity.
23
Q
stratified squamous epithelium
A
-apical surface cells are squamous while the cells closer to the underlying connective tissue are cuboidal to columnar.
-function: found in areas subjected to wear and tear (abrasion); top layer cells are rubbed away and replaced via mitosis of the basal cells
-keratinised (dry) contains keratin, a protein that strengthens the tissue; the function: protection, prevents water loss; location epidermis
-nonkeratinised (wet) mucosa; function: protection, secretion; location mouth, esophagus, larynx, vagina, anal canal.
24
Q
transitional epithelium
A
-characterised with cells that change form according to the degree of distension- flat squamous when stretched and cuboidal when relaxed.
-function: protection, distensibility
-location: lining the cavity of urinary bladder, ureters, renal calyces.
25
glandular epithelium
crpts of leiberkuhn
-simle branched tubuar; gastric glands
-simple coiled tubular; sweat gland
-simple acinar; frog's skin
-simple branched acinar; sebaceous glands
-compound tubular
-compound acinar
-compound tubuloacinar
26
endocrine glands
are ductless
and release products, called
hormones, directly into the
bloodstream.
27
exocrine glands
connect to the
surface by ducts, which take the
secretions to the surface or
lumen. The epithelial, or
functional, component of all
glands is parenchyma; the
mainly supportive connective
tissue part is stroma.
28
the glands
These glands are arranged in
cords or clumps of cells, close to
a complex network of
capillaries, for hormone
transport.
29
function of connective tissue
* Provides structural and metabolic support to other tissues; protection, transport, repair,
storage.
* With few exceptions well vascularised( venous blood, lymphatic vessels. heals quickly and efficiently, its waste products are being transported out quickly and effeciently)
* Consists of:
* Cells
* Extracellular matrix (major constituent)
- Fibers
- Ground substance
30
what are the main components of connective tissue
-cells
-ECM
31
connective tissue cells
-all connective tissue develop from embryonic mesenchymal cells
- "blast" (immature) - "cyte" (mature)
-Two types of cell groups:
1. Resident (fixed)
2. Wondering (transient) (migrated from blood as
a result of specific stimuli).
32
connective tissue cells
1 Fixed
-fibroblasts; produce fibres
-adipocytes - store fat
-chondroblasts; cartilage
-osteoblasts; produce bony
2 wondering ( histiocytes):
-mast cells; responsible for an allergy response
-macrophages; responsible for an immune response
-lymphocytes; responsible for immune response
-plasma cells; constitute of the blood cells
-eosinophils; a constitution of the blood cells
33
Mesenchymal Cells
Primitive stem cells
* Differentiate into connective tissue,
bone, blood, lymph, endothelium, and
muscle cells
* Some mesenchymal cells maintain their
plasticity throughout adult life
* Histologically resemble fibroblasts, but
are generally smaller
* Clinical context: sarcomas (tumors of
connective tissue)
34
fibroblasts
* Fibroblasts = basophilic = more organelles and protein
synthesis [blue]
* Fibrocytes = acidophilic – cytoplasm [pink]
* Synthesize collagen, elastic and reticular fibers and other
extracellular material.
* Fibroblasts are
active in the repair of
injury and formation
of scar tissue.
35
fibroblasts nucleus
Fibroblast nucleus = eurochromatic, with
some heterochormaticy shown around the
periphery of the nucleus (towards the nuclear
envelope)
- Eurochromatic: loosely packed chromatin. DNA
accessible for transcription (stains lightly)
-Heterochromatic: tightly packed chromatin.
DNA inaccessible for transcription (dark
appearance)
36
Adipocytes
Two types of adipocytes: brown and white
* White adipocytes:
o Specialised for fat (lipid) storage, energy and insulation;
involved in the synthesis of hormones and growth factors.
o Located throughout loose connective tissue.
o Cytoplasm with flattened nucleus forms a very narrow rim
around a large central lipid droplet.
o Unilocular
Brown adipocytes: less found in adults
o Brown appearance and rich in mitochondria
o In adult life, brown fat persists in back, neck, and thigh
o Function: heat generation and non shivering
thermogenesis
o Multilocular
37
Macrophage – Pacman of the cell
* Develop from monocytes (WBC). they develop from WBC. theyre originally WBC when they reach the connective tissue they differentiate to macrophages
* Irregular shape, short branched projections.
* Destroys bacteria and cellular debris (phagocytosis).
* Fixed and wondering.
38
Mast cells
Develop in bone marrow, differentiate in connective tissue.
* Function: inflammatory and allergic response (granules contain
histamine, heparin), kill bacteria.
39
Extracellular matrix
-fibres; collagen reticular, and elastic
-ground substances; Fluid, semisolid, solid. It consists of water and complex carbohydrates and
glycoproteins - make up a flexible gel.
Function: mechanical and structural support for tissue; biochemical barrier-role in regulating metabolic functions in surrounding cells.
40
types of fibres
1 collagen; most abundant, closely packed and orderly, flexible, high, tensile strength, made of collagen protein
2 reticular; very thin, type 3 collagen, associated with high levels of glycoprotein. provides a supporting framework for the cellular constituents of various tissues and organs- (reticular tissue) stroma.
3 elastic; thin elastic (protein elastin), allows tissue to respond to stretch and distensions; located in skin , lung, and bladder.
41
dense regular connective tissue
* Abundant collagen fibers, few
cells and little ground substance.
* Uniformly aligned densely packed
fibers collagen arranged in
bundles – fibroblasts arranged in
rows between collagen bundles.
* Location: tendons and ligaments.
* Function: reinforces attachment
between organs
42
dense irregular connective tissue
* Mostly collagen fibers, few cells and little
ground substance.
* Tightly packed fibers random in 3
dimensions.
* Location – fasciae, the dermis (reticular or
deep layer) and digestive tract (submucosa).
* Function - strength, resists stretch/tearing
in multiple directions.
43
elastic dense connective tissue
* Predominantly elastic fibers; fibroblast in the spaces btw
the fibers.
* Yellowish in colour; stretching capability; lungs, elastic
arteries, some ligaments.
44
loose areolar tissue
* Most common type; all three types of fibres, abundant ground
substance, several diff. cells; delicate consistency, flexible, and
well vascularised.
* While it allows for movement between adjacent structures, it
inhibits distorsion.
* Subcutaneous layer, surrounds lymph and blood vessels,
nerves and body organs, lamina propria of mucous
membranes, dermis of skin (superficial layer).
45
Loose - Reticular connective
tissue
Intertwining reticular fibers.
* Forms the stroma, binds smooth muscle cells; reticular
fibers filter blood and lymph.
46
cartilage
o Plays a critical role in providing structural support for soft tissues and a sliding
area for joints
o Avascular
o Firm consistency (plastic-like)
o Allows for the bone to grow in length
o Highly hydrated (70-75%) water, collagen (15-20%) and proteoglycans (2-
10%) – hence glossy appearance
o Cells: Chondroblasts > Chondrocytes (in matrix cavities – lacunae; synthesise
and secrete e. m.) and an extensive extracellular matrix.
o Types (matrix composition):
– Hyaline,
– Fibrocartilage,
– Elastic
47
Hyaline cartilage
* Bluish white, glossy.
* Location: at ends of long bones, anterior ends of ribs, nose, parts
of larynx, trachea and bronchi and fetal skeleton.
* Function: cushioning, smooth low friction surface for joints, flexibility
and support in respiratory system, frame for ossification (epiphyseal
plate).
* Growth and repair very limited in adults.
48
fibrocartilage
* Combination of dense regular
connective tissue and cartilage
* Chondrocytes and fibroblasts
embedded in bundles of collagen
fibers and proteoglycans.
* Collagen fibres lie parallel to lines
of stress
* Location: pubic symphysis,
intervertebral discs, menisci of knee.
* Function: support and fusion,
resists deformation under stress.
* Limited ability to repair in
adults.
49
elastic cartilage
* Normal components of hyaline
cartilage with addition of an
abundant network of elastic fibres
* Support with flexibility.
* Location; external ear, external
auditory canal, epiglottis.
50
cartilage growth
Cartilage growth occurs in two ways:
1) appositional (width): chondroblast cells secrete
matrix against the external face of existing cartilage
2) interstitial (length): chondrocytes divide and secrete
new matrix expanding cartilage from within
51
function of bone
1. mechanical support
2 protection
3movement
4 storage
5acid base balance
6 blood formation: hemopoiesis
52
what is bone tissue composed of?
1 Cells: Osteogenic
* Osteoblasts
* Osteocytes
* Osteoclasts
2 Extracellular Matrix (mineralised): * Major organic component: 90%
collagen fibers; type I; other
organic components - type V collagen, proteoglycans and
glycoproteins
* Difference to other connective
tissue: mineral deposition in matrix
leading to calcification
(hydroxyapatite Ca10 (PO4)6(OH)2)
53
function of Cells: Osteogenic
* Osteoblasts
* Osteocytes
* Osteoclasts
Osteogenic : - Unspecialised stem cell developed from mesenchyme (undergo cell division); prominent role in
growth and remodelling.
- Give rise to osteoblasts
- Located in the inner portion of the periosteum, endosteum and in the canals that contain blood
vessels.
Osteoblasts: - Responsible for synthesis of the organic component of matrix and influence deposition of
inorganic components.
- Present only on the surface: cells cuboidal/columnar or flat.
- When completely surrounded by matrix osteoblasts become osteocytes
Osteocytes: - Osteocytes maintain bone matrix.
- Can respond to mechanical forces and lay down new matrix or remove matrix.
- Located within lacunae
- Communicate with neighbouring osteocytes via canaliculi
osteoclast: - Arise by the fusion of monocytes or macrophages.
- Multinucleated, branched, motile, phagocytic; bone resorption. - Use lysosomes to break down the matrix - resorption. Osteoclasts are hormonally influenced.
54
bone formation
formed by either intramembranous or endochondral ossification.
Intramembranous: occurs in areas of ordinary mesenchyme where osteoblasts/bone forming cells differentiate directly within richly vascularized mesenchymal tissue (flat bones of skull, mandible, and clavicles)
Endochondral ossification: occurs in preexisting hyaline cartilage models. Here mesenchymal cells differentiate into osteoblasts and the fetal skeleton acts like a cartilage template is modified to facilitate mineralization, vascular invasion and replacement by bone
55
intramembranous ossification
Occurs in areas of ordinary mesenchyme where osteoblasts/bone forming cells
differentiate directly within richly vascularized mesenchymal tissue
* Location: flat bones of skull, mandible, and clavicles.
* Intramembranous bone formation begins during gestation when mesenchymal
cells aggregate at sites of richly vascularized connective tissue and differentiate
into osteoblasts.
* The osteoblasts secrete osteoid, which is an organic matrix of proteoglycans and
type I collagen fibres.
* Osteoblasts also secrete alkaline phosphatase, which induces mineralization of
osteoid via precipitation of inorganic calcium phosphate salts.
56
Endochondral Ossification
Bone forming from cartilage of the fetal skeleton. This is the hyaline cartilage
model
* The perichondrium already surrounding the hyaline cartilage, has
mesenchymal cells in it.
* The mesenchymal cells within the perichondrium differentiate into
osteoblasts.
* This forms a bony collar – i.e. it ‘chokes’ the hyaline cartilage.
* Because the hyaline cartilage is receiving no blood supply at this point, the
cells swell up, secrete calcified cartilage and die
* Blood vessels make their way to the new formed bone, and come in to the
dying zone the osteoblasts and osteoclasts.
* Start to build bone all bone except the epiphyseal plate
57
bone growth in length
* The growth plate is the cartilaginous portion of long bones where the longitudinal growth of the bone takes place.
* Its structure comprises chondrocytes suspended in a collagen matrix that go through several stages of maturation until they finally die, and are replaced by osteoblasts, osteoclasts, and lamellar bone.
* Chondrocytes progress from a resting state to enter the phases of proliferation and hypertrophy. Under the influence of oestrogen, the proliferation of chondrocytes decreases as the resting chondrocytes are consumed
* During the terminal phase of differentiation, cartilage is replaced by blood vessels and organized bone tissue, and once chondrocytes have died, the longitudinal growth of the bone ceases and the growth plate
closes
58
bone tissue classification
* Primary (immature, woven): temporary, with few exceptions replaced by mature in adults; irregular array of collagen fibres, low mineral content, high proportion of osteocytes.
* Secondary (mature, lamellar): collagen fibres arranged in lamellae. Two subtypes: compact and spongy.
59
bone structure (compact)
Haversian system = osteon, a cylindrical unit with
concentric lamellae of bone matrix surrounding a
central canal.
Long axis of osteon usually parallel to long axis of
bone.
Haversian canals contain 1 or 2 capillaries, lymph
vessels and nerves.
Areas between haversian systems called interstitial
lamellae; outer- circumferential lamellae.
Cementing substance surrounds haversian system
(mineralised matrix, few collagen fibres)
60
bone structure spongy
No osteons; lamellae arranged in an irregular
lattice of thin columns called trabeculae; spaces
in btw contain red or yellow bone marrow
(numerous blood vessels)
Within each trabecula – osteocytes (within
lacunae) connected with canaliculi.
Blood supply directly from the surrounding
blood vessels.
61
bone remodeling
Stress to periosteum of bony
prominences→ osteoblasts
activation.
Absence of stress loss
of bone tissue.
Wolff’s law: bone in a healthy person
or animal will adapt to the loads it is
placed under.
62
function of muscle tissue
* Responsible for movement of the body
and its parts
* Responsible for changes in the size and
shape of internal organs
* Responsible for contraction
63
muscle tissue is composed of
1- Cells (fibers) specialized for contraction
2- Moderate to small amount of ECM
Special names for the organelles in muscle
tissue:
* Sarcoplasm for cytoplasm
* Sarcoplasmic reticulum for smooth ER
* Sarcolemma for cell membrane and its
external lamina
* External lamina:
o Basal lamina when it forms a
peripheral cellular investment, as in
muscle cells, adipocytes and
peripheral nerve supporting cells
(Schwann cells)
64
muscle tissue classification
Morphologic/histologic classification:
1- Striated muscle
* Skeletal muscle
* Cardiac muscle
2- Smooth muscle
Functional/physiologic classification:
1- Voluntary muscle
* Skeletal muscle
2- Involuntary muscle
* Cardiac muscle
* Smooth muscle
65
skeletal muscle cells/fibres :
Skeletal muscle cells/fibers:
* Long
* Cylindrical
* Multinucleated
o Peripherally located nuclei
* Dimensions:
o Diameter from 10 to 100 μm
o Lengths up to 30 cm
66
Development of Skeletal Muscle
Derived from mesenchymal cells
* Differentiation of mesenchymal cells into
myoblasts
* Fusion of myoblasts together to form
myotubes
* Synthesis of myofilaments and
appearance of striations
* Displacement of the nuclei against the
sarcolemma due to the formation of
functional myofilaments
67
development of skeletal muscle
Some myoblasts remain undifferentiated to
form satellite cells
o Acting as stem cells and form new
muscle fibers following injury
68
Organization of a Skeletal Muscle
* Epimysium:
o A thick layer of a dense irregular
connective tissue surrounding the
entire muscle
o Septa of this tissue extend inward,
carrying the larger nerves, blood
vessels, and lymphatics of the
muscle
* Perimysium:
o A thin layer of dense connective
tissue layer that immediately
surrounds each bundle/fascicle of
muscle fibers
* Endomysium:
o A very thin layer of delicate
connective tissue surrounding each
muscle fiber
69
myotendinous junction in skeletal muscle
Continuation of the dense collagen fibers
of the tendon with those in the three
connective tissue layers around muscle
fibers
* Forms a strong unit that allows muscle
contraction to move other structures
70
Organization Within Skeletal Muscle Fibers
* Striations of alternating light and dark
bands in longitudinal section:
o Dark bands: A bands (anisotropic or
birefringent in polarized light
microscopy)
o Light bands: I bands (isotropic, do
not alter polarized light)
* Sarcoplasm filled with myofibrils (long
cylindrical filament bundles)
* Myofibrils: end-to-end repetitive
arrangement of sarcomeres
71
ultrastructure of myofibrils
* Myofibrils: end-to-end repetitive
arrangement of sarcomeres
* Sarcomere:
o Repetitive functional subunit
extending from Z disc to Z disc
o About 2.5-μm long in resting muscle
* Z disc:
o A dark transverse line bisecting each
I band
* Characteristic pattern of transverse
striations in entire muscle:
o Formed by lateral registration of
sarcomeres in adjacent myofibrils
72
Molecular Arrangement of Myofibrils/Sarcomeres
* Regular arrangement of thick myofilaments
(myosin) and thin myofilaments (F-actin)
* Thick myofilaments (myosin):
o Occupy the A band at the middle region
of the sarcomere
o Composed of two heavy chains (tail) and
two pairs of light chains (head)
* Thin myofilaments (F-actin):
o Run between the thick filaments
o Composed of G-actin monomer
containing a binding site for myosin
o Have two tightly associated regulatory
proteins:
§ Tropomyosin
§ Troponin: a complex of three
subunits (TnT, TnC, and TnI)
q TnT: attaches to tropomyosin
q TnC: binds Ca2+
q TnI: regulates the actinmyosin interaction
73
Molecular Arrangement of Myofibrils/Sarcomeres
* A bands:
o Contain both the thick filaments and
the overlapping portions of thin
filaments
* I bands:
o Consist of the portions of the thin
filaments that do not overlap the
thick filaments in the A bands
* H zone:
o A lighter zone in the center of A
band
o Corresponding to a region with only
the rodlike portions of the myosin
molecule and no thin filaments
* M line:
o Bisecting the H zone
o Containing:
§ Myomesin: a myosin-binding
protein that holds the thick
filaments in place
§ Creatine kinase:
q catalyzes transfer of
phosphate groups from
phosphocreatine (a
storage form of highenergy phosphate
groups) to ADP
q Helping to supply ATP for
muscle contraction
74
Sarcoplasmic Reticulum & Transverse Tubule System in Skeletal Muscle Fibers
* Sarcoplasmic reticulum (smooth ER):
o Contains pumps and other proteins
for Ca2+ sequestration
o Surrounds the myofibrils
o Calcium release from cisternae of the
sarcoplasmic reticulum through
voltage-gated Ca2+ channels is
triggered by membrane depolarization
produced by a motor nerve
* Transverse or T-tubules:
o Tubular infoldings of the sarcolemma
o Long fingerlike invaginations of the cell
membrane penetrate deeply into the
sarcoplasm
o Encircle each myofibril near the
aligned A- and I-band boundaries of
sarcomeres
o To trigger Ca2+ release from
sarcoplasmic reticulum throughout the
muscle fiber simultaneously and
produce uniform contraction of all
myofibrils
* Triad:
o Complex of a T-tubule with two
terminal cisternae in longitudinal TEM
sections
* Allows depolarization of the sarcolemma
in a T-tubule to affect the sarcoplasmic
reticulum and trigger release of
Ca2+ ions into cytoplasm around the
thick and thin filaments, which initiates
contraction of sarcomeres
75
Mechanism of Contraction in Skeletal Muscle Fibers
* Neither the thick nor the thin filaments
change their length during muscle
contraction
* Contraction occurs as the overlapping thin
and thick filaments of each sarcomere slide
past one another
76
Mechanism of Contraction in Skeletal Muscle Fibers
* Contraction is induced when an action potential
arrives at a synapse, the neuromuscular junction
(NMJ)
* Action potential is transmitted along the Ttubules to terminal cisternae of the
sarcoplasmic reticulum to trigger Ca2+ release
* In a resting muscle, the myosin heads cannot
bind actin because the binding sites are blocked
by the troponin-tropomyosin complex on the Factin filaments
* Calcium ions released upon neural stimulation
bind troponin, changing its shape and moving
tropomyosin on the F-actin to expose the
myosin-binding active sites and allow cross
bridges to form
* Binding actin produces a conformational change
or pivot in the myosins, which pulls the thin
filaments farther into the A band, toward the Z
disc
* Energy for the myosin head pivot that
pulls actin:
o Provided by hydrolysis of ATP bound
to the myosin heads, after which
myosin binds another ATP and
detaches from actin
o In the continued presence of
Ca2+ and ATP, these attach-pivotdetach events occur in a repeating
cycle (each lasting about 50
milliseconds) which rapidly shorten
the sarcomere and contract the
muscle
o A single muscle contraction results
from hundreds of these cycles
* When the neural impulse stops and
levels of free Ca2+ ions diminish:
o Tropomyosin again covers the
myosin-binding sites on actin and
the filaments passively slide back
and sarcomeres return to their
relaxed length
* In the absence of ATP:
o Actin-myosin cross bridges become
stable, which accounts for the rigidity
of skeletal muscles (rigor mortis) that
occurs as mitochondrial activity stops
after death
77
Muscle Spindles & Tendon Organs
* Sensory receptors
* Acting as proprioceptors providing the
central nervous system (CNS) with data
from the musculoskeletal system
Muscle spindles:
* Location: among the muscle fascicles
* Structure:
o Encapsulated by modified perimysium,
with concentric layers of flattened cells
o Containing interstitial fluid and a few thin
muscle fibers filled with nuclei and
called intrafusal fibers
o Penetrated by several sensory nerve
axons wrapping around individual
intrafusal fibers
* Function:
o Stretch detectors
o Changes in length (distension) of the
surrounding (extrafusal) muscle fibers
caused by body movements are
detected by the muscle spindles and the
sensory nerves relay this information to
the spinal cord
o Help maintain posture and to regulate
the activity of opposing muscle groups
involved in motor activities such as
walking
Golgi tendon organs:
* Much smaller encapsulated structures that
enclose sensory axons penetrating among
the collagen bundles at the myotendinous
junction
* Detect changes in tension within
tendons produced by muscle contraction
* Act to inhibit motor nerve activity if
tension becomes excessive
* Because both of these proprioceptors
detect increases in tension, they help
regulate the amount of effort required to
perform movements that call for variable
amounts of muscular force
78
cardiac muscle
* Rather than fusing into multinucleated
cells/fibers as in developing skeletal muscle
fibers, cardiac muscle cells form complex
junctions between interdigitating
processes
* Cells within one fiber often branch and join
with cells in adjacent fibers
* Consequently, the heart consists of tightly
knit bundles of cells, interwoven in spiraling
layers that provide for a characteristic wave
of contraction that resembles wringing out of
the heart ventricles
* Usually has only one or two centrally
located nucleus
79
organisation of cardiac muscle
* Endomysium:
o Surrounding the muscle cells
o a delicate sheath of connective tissue
with a rich capillary network
* Perimysium:
o Separates bundles and layers of
muscle fibers
o In specific areas, forms larger
masses of fibrous connective tissue
comprising the “cardiac skeleton.”
80
Intercalated discs
A unique characteristic of cardiac
muscle
* Transverse lines that cross the fibers at
irregular intervals
* Where the myocardial cells join
* Represent the interfaces between
adjacent cells
* Consist of many junctional complexes
o Transverse regions: composed of
many desmosomes and fascia
adherens junctions
o Longitudinally oriented regions:
run parallel to the myofibrils and are
filled with gap junctions;
§ Serve as electrical synapses
81
cardiac muscle
* Dyads:
o Junctions between terminal
cisterns of sarcoplasmic reticulum
and T-tubules typically involve only
one structure of each type
82
Smooth Muscle/Visceral Muscle
* Specialized for slow, steady contraction
under the influence of autonomic nerves
and various hormones
* Major component of blood vessels and of
the digestive, respiratory, urinary, and
reproductive tracts and their associated
organs
* Fibers:
o Fusiform, elongated, tapering, and
unstriated cells
o Each of which is enclosed by an
external lamina and a network of type
I and type III collagen fibers
comprising the endomysium
o Contain single elongated nucleus
located centrally
o Linked by numerous gap junctions
o Close packing is achieved with the
narrow ends of each cell adjacent to
the broad parts of neighboring cells
§ Cross sections of smooth
muscle show a range of cell
diameters, with only the largest
profiles containing a nucleus
o Have rudimentary sarcoplasmic
reticulum
o Lack T-tubules
o Have caveolae (small plasmalemma
invaginations) containing the major
ion channels:
§ Control Ca2+ release from
sarcoplasmic cisternae at
myofibrils that initiates
contraction
* Different organization of myofibrillar arrays
of actin and myosin
o Bundles of thin and thick
myofilaments crisscross the
sarcoplasm obliquely
o Troponin is replaced with calmodulin
o Tropomyosin is replaced with Ca2+-
sensitive myosin light-chain
kinase (MLCK)
* Insertion of actin myofilaments into
anchoring cytoplasmic and plasmalemmaassociated dense bodies (contain αactinin and are functionally like the Z
discs)
* Intermediate filaments, composed of
desmin, which attach to the dense bodies
83
Nervous tissue:
Distributed throughout the body as an
integrated communications network
Composed of:
1- Cells:
I. Nerve cells/Neurons
II. Supporting cells/Neuroglial
cells
2- Extracellular matrix (ECM)
o Very small amount
84
nerve cell/neurons
Respond to environmental changes
(stimuli) by altering the ionic gradient
that exists across their plasma
membranes
Neurons are excitable or irritable cells:
* Cells that can rapidly change the
electrical potential (ionic gradient
across the cell membrane) in
response to stimuli (eg, neurons,
muscle cells, some gland cells)
Action potential (nerve impulse):
* Reversal of the ionic gradient as
membrane depolarization
(depolarization wave) spreading in
the neuron’s plasma membrane
85
The functional unit in both the CNS and
PNS
Composed of 3 parts:
1- Cell body (or perikaryon or soma)
2- Neurites (nerve fibers)
I. Dendrites
II. Axon
86
Cell body (Perikaryon or Soma)
* Containing the nucleus and most of
the cell’s organelles
* Large, with a large, euchromatic
nucleus and well-developed
nucleolus indicating intense
synthetic activity
* Serves as the synthetic or trophic
center for the entire neuron
* Contains chromatophilic substance
called Nissl substance or Nissl
bodies:
o Large masses of free
polysomes and RER
o Basophilic
o Abundant in large nerve cells
such as motor neurons
* Golgi apparatus: located only in the
cell body
* Mitochondria: can be found
throughout the cell and are usually
abundant in the axon terminals
* Microtubules, actin filaments and
intermediate filaments in
perikaryon and its processes (axon
and dendrites):
o Neurofilaments (or
neurofibrils): intermediate
filaments
* Contain inclusions of pigmented
material, such as lipofuscin
87
dendrites
* Numerous elongated processes
* Extend from the perikaryon
* Specialized to receive stimuli from
other neurons at unique sites
called synapses
* Usually covered with many synapses
* Become much thinner as they
branch
* Contain dendritic spines in CNS:
o Dynamic membrane
protrusions along the small
dendritic branches
88
axon
* Axolemma: plasma membrane of
the axon
* Axoplasm: cytoplasm of the axon
* A single long process ending at
synapses
* Specialized to generate and conduct
nerve impulses to other cells (eg,
nerve, muscle, and gland cells)
Originated from axon hillock of the
perikaryon
* Initial segment:
o Just beyond the axon hillock
containing concentrated ion
channels that generate the
action potential
* Undergo terminal arborization:
o Each small axonal branch ends
with a dilation called
a terminal bouton
89
classifications of the neurons
Function:
* Sensory (afferent)
* Motor (efferent)
* Interneuron
Location:
* Cortical
* Spinal
Neurotransmitter:
* Glutamatergic
* Cholinergic
Morphology:
* Pyramidal
* Granule
* Mitral
OR
* Multipolar
* Bipolar
* Unipolar or pseudounipolar
* Anaxonic
90
Morphological Classification of the Neurons
1- Multipolar neurons:
* One axon and two or more dendrites
* The most common including all motor
neurons and most CNS interneurons
2- Bipolar neurons:
* One dendrite and one axon
* Comprise the sensory neurons of the
retina, the olfactory epithelium, and
the inner ear
3- Unipolar or pseudounipolar neurons:
* A single process that bifurcates close to
the perikaryon, with the longer branch
extending to a peripheral ending and
the other toward the CNS
* Include all other sensory neurons
4- Anaxonic neurons:
* Many dendrites but no true axon
* Found only in certain CNS interneurons
* Do not produce action potentials, but
regulate electrical changes of adjacent
CNS neurons
91
Synapses
* Sites where nerve impulses are
transmitted from one neuron to
another, or from neurons and other
effector cells
* Transmission is unidirectional
* Types of synapses:
o Chemical
o Electrical
92
Chemical Synapses
1- Presynaptic axon terminal
(terminal bouton):
* Contains mitochondria and
numerous synaptic vesicles from
which neurotransmitter is released
by exocytosis
2- Synaptic cleft:
* A 20- to 30-nm-wide intercellular
space
* Separates these presynaptic and
postsynaptic membranes
3- Postsynaptic cell membrane:
* Contains receptors for the
neurotransmitter, and ion channels
or other mechanisms to initiate a
new impulse
93
Morphological Types of Chemical Synapses
1- Axosomatic synapse
2- Axodendritic synapse
3- Axoaxonic synapse
94
Glial Cells
Support neuronal survival and activities
10 times more abundant than neurons in
the mammalian brain
Participating in the formation of neuropil
of CNS:
* Network of fine cellular processes
emerging from neurons and glial
cells
* The fibrous intercellular network of
CNS tissue superficially
resembles collagen by light
microscopy
95
Types of Glial Cells
Glial cells present in CNS:
1. Astrocytes
2. Oligodendrocytes
3. Microglia
4. Ependymal cells
Glial cells present in PNS:
1. Schwann cells
2. Satellite cells
96
Oligodendrocytes
* Predominant glial cells in white
matter
* Extend many processes, each of
which becomes sheetlike and wraps
repeatedly around a portion of a
nearby CNS axon
* Forming myelin sheath
* An axon’s full length is covered by
the action of many oligodendrocytes
* Oligodendrocytes in routine light
microscope staining:
o Small cells with rounded,
condensed nuclei and
unstained cytoplasm
97
astrocytes
Have a large number of long radiating,
branching processes
Two types:
1- Fibrous astrocytes:
* With long delicate processes
* Abundant in white matter
2- Protoplasmic astrocytes
* With many shorter processes
* Predominate in the gray matter
function:
1- Extending processes that associate
with or cover synapses, affecting the
formation, function, and plasticity of
these structures
2- Regulating the extracellular ionic
concentrations around neurons
3- Extending fibrous processes with
expanded perivascular feet that cover
capillary endothelial cells and modulate
blood flow and help move nutrients,
wastes, and other metabolites between
neurons and capillaries
4- Forming a barrier layer of expanded
protoplasmic processes, called the glial
limiting membrane, which lines the
meninges at the external CNS surface
5- Filling tissue defects after CNS injury
by proliferation to form an astrocytic scar
98
Ependymal Cells
Columnar or cuboidal cells that line:
1- Fluid-filled ventricles of the brain
2- Central canal of the spinal cord
Characteristics of the apical ends of the
ependymal cells in some regions:
1- Cilia: facilitate the movement of
cerebrospinal fluid (CSF)
2- Long microvilli: likely involved in
absorption
Characteristics of the basal ends of the
ependymal cells in some regions:
1- No basal lamina
2- Elongated and extend branching
processes into the adjacent neuropil
99
Microglia
Consist of small cell bodies from
which radiate many long, branched
processes
* Evenly distributed throughout
specific regions of gray and white
matter
* Constitute the major mechanism of
immune defense in the CNS,
removing any microbial invaders and
secreting several immunoregulatory
cytokines
100
Schwann Cells / Neurolemmocytes
* Counterparts to oligodendrocytes of
the CNS
* Function:
o Form myelin sheathes in the
PNS
o Unlike an oligodendrocyte, a
Schwann cell forms myelin
around a portion of only one
axon
101
Satellite Cells of Ganglia
* Form a thin, intimate glial layer
around each large neuronal cell body
in the ganglia of the PNS
* Function:
o Trophic or supportive effect
on these neurons
o Insulating, nourishing, and
regulating neurons
microenvironments
102
nervous tissue and nervous system
Nervous tissue:
1- Cells:
I. Nerve cells/Neurons
II. Supporting cells/Neuroglial cells
a. Astrocytes
b. Oligodendrocytes
c. Microglia
d. Ependymal cells
e. Schwann cells
f. Satellite cells
2- Extracellular matrix (ECM)
o Very small amount
Nervous system:
o Nervous tissue
o Associated tissues (meninges, blood
vessels, connective tissue, CSF, …)
1- CNS
2- PNS
103
central nervous system
CNS:
o Brain
* Cerebrum
* Cerebellum
* Brain stem
o Spinal cord
Protectors of CNS:
o Skull and vertebral column
o Connective tissue (Meninges)
o Cerebrospinal fluid (CSF)
104
glial cells
Glial cells present in CNS:
1. Astrocytes
2. Oligodendrocytes
3. Microglia
4. Ependymal cells
Glial cells present in PNS:
1. Schwann cells
2. Satellite cells
105
CNS
Structural features (according to differential
distribution of lipid-rich myelin):
o White matter:
* Myelinated axon (tracts)
* Oligodendrocytes (myelin-producing
cells)
* Astrocytes
* Microglia
o Grey matter
* Neuronal cell bodies
* Dendrites
* Astrocytes
* Microglia
* Where most synapses occur
106
white matter vs grey matter
Location of white matter:
o Inner part of the brain (medulla)
o Outer part of spinal cord
Location of grey matter / neuronal cell bodies:
o Outer part of the brain (cerebral cortex)
o Inner part of the spinal cord
o Islands of grey matter in inner part of the brain
(nuclei)
o Complexes of neuronal cell bodies associated
with PNS (ganglia)
o Retina
o Olfactory epithelium
107
Cerebral Cortex
Made up of six layers of neurons with different
sizes and shapes:
o Pyramidal neurons
* Integration of sensory information
* Initiation of voluntary motor responses
108
cerebellar cortex
o Coordinates muscular activity
throughout the body
o Organized with three layers
109
spinal cord
White matter: peripheral
Grey matter: deeper H-shaped mass
o Anterior horns:
* Cell bodies of very large motor
neurons whose axons make up the
ventral roots of spinal nerves
* Astrocytes
o Posterior horns:
* Neurons which receive sensory fibers
from neurons in the spinal (dorsal
root) ganglia
* Oligodendrocytes
Grey commissure
o Central canal
* Lined by ependymal cells
* Contains CSF
White commissure
110
Meninges
Three membranes of connective tissue
1- Dura mater
2- Arachnoid mater
3- Pia mater
111
dura mater
o Dense irregular connective tissue
o In brain, organized as:
* Outer periosteal layer continuous
with the periosteum of the skull
* Inner meningeal layer (dura mater
proper)
ü Dural venous sinuses
o In spinal cord, organized as:
* One meningeal layer
ü Epidural space
112
Arachnoid (Mater)
Avascular connective tissue
Two components:
(1) A sheet of connective tissue
* In contact with the dura mater
(2) A system of loosely arranged trabeculae
* Composed of collagen and fibroblasts
* Continuous with the underlying pia
mater layer
Subarachnoid space
Arachnoid villi
o Penetration of dura mater by arachnoid
o Protrusion into blood-filled dural venous
sinuses
o Sites for absorption of CSF into the blood of
the venous sinuses
113
Pia mater
o Consists of flattened, mesenchymally
derived cells closely applied to the entire
surface of the CNS tissue
o Separated from the neural elements by the
glial limiting membrane, or glia limitans:
* Very thin superficial layer of astrocytic
processes
114
Blood brain barrier BBB
A functional barrier that allows much tighter
control than that in most tissues over the
passage of substances moving from blood into
the CNS tissue
Structural component of the BBB:
1- Capillary endothelium
* Occluding junctions
* Little or no transcytosis activity
* Surrounded by the basement
membrane
2- Limiting layer of perivascular astrocytic feet
* Envelops the basement membrane of
capillaries
BBB absent in:
1- Hypothalamus where plasma components
are monitored
2- Posterior pituitary
3- Choroid plexus
115
Choroid Plexus
Highly vascular tissue, elaborately folded and
projecting into the large ventricles of the brain
o Thin layer of well-vascularized pia mater
covered by cuboidal ependymal cells
Location of choroid plexus:
o Roof of the third ventricle
o Roof of fourth ventricle
o Parts of the two lateral ventricular wall
Function of choroid plexus:
o Production of CSF
Function of CSF:
o Providing the ions required for CNS
neuronal activity
o Serving to help absorb mechanical shocks
in arachnoid
116
Peripheral Nervous System (PNS)
Components:
1- Nerves
o Cranial nerves
o Spinal nerves
2- Ganglia
o Sensory Ganglia
o Autonomic Ganglia
* Sympathetic ganglia
* Parasympathetic ganglia
117
Peripheral Nerves
Bundles of nerve fibers (axons) individually
surrounded by Schwann cells and connective
tissue
Nerve fibers:
o Containing axons enclosed within sheaths of
glial cells (Schwann cells/neurolemmocytes )
specialized to facilitate axonal function
* Myelinated fibers
* Unmyelinated fibers
118
Myelinated Fibers
Axons engulfed along their length by a series of
Schwann cells/neurolemmocytes
Process of myelination:
o Fusion of the plasma membrane of each
covering Schwann cell with itself at an area
termed the mesaxon
o Continuation of wide, flattened process of the
cell to extend itself, moving circumferentially
around the axon many times
o Union of the multiple layers of Schwann cell
membrane as a thick myelin sheath
Function of myelin sheath:
o Insulate axons
o Maintain a constant ionic microenvironment
most suitable for action potentials
Myelin clefts / Schmidt-Lanterman clefts:
o Periodic separation of cytoplasmic surfaces
of the Schwann cell membrane along the
myelin sheath
o Allow transient movement of cytoplasm for
membrane maintenance
Nodes of Ranvier / Nodal gaps
o Axon is only partially covered by
interdigitating Schwann cell processes
Internodal segment
119
Unmyelinated Fibers
o Only smallest diameter axons of peripheral
nerves are still enveloped within simple
folds of Schwann cells
o Portions of many axons with small
diameters enclosed by Schwann cells
* No nodes of Ranvier
* No saltatory conduction
o No multiple wrapping to form a myelin
sheath
120
nerve organisation
Endoneurium
o Immediately around the external lamina of
the Schwann cells
o Consisting of reticular fibers, scattered
fibroblasts, and capillaries
Perineurium
o Groups of axons with Schwann cells and
endoneurium are bundled together
as fascicles by a sleeve of perineurium
o Containing flat fibrocytes with their edges
sealed together by tight junctions
* Blood-nerve barrier
Epineurium
o Externally, peripheral nerves have a dense,
irregular fibrous coat
121
ganglia
Ovoid structures containing neuronal cell
bodies and their surrounding glial satellite cells
supported by delicate connective tissue and
surrounded by a denser capsule
Serve as relay stations to transmit nerve
impulses
* At least one nerve enters and another
exits from each ganglion
122
sensory ganglia
Receive afferent impulses that go to the CNS
associated with both cranial nerves (cranial
ganglia) and the dorsal roots of the spinal
nerves (spinal ganglia)
The large neuronal cell bodies (pseudounipolar
neurons) of ganglia are associated with thin,
sheetlike extensions of small glial satellite cells
Sensory ganglia are supported by a distinct
connective tissue capsule
123
autonomic ganglia
Small bulbous dilations in autonomic nerves,
usually with multipolar neurons
