chapter 2 CELL AND TISSUE.pptx

CHAPTER TWO
THE CELLS AND TISSUES
5/25/2023 By Yibeltal W. 1

The Cell
 is the basic structural and functional unit of the body
 The branch of science concerned with the study of cells is
cytology
 The human body is a social order of 50-100 trillion cells
arranged in to different functional structures
 There are two fundamentally different types of cells:
1- Prokaryotic cells
 Are found only in bacteria.
 These cells are small (1–5 μm long)

 Typically have a cell wall outside the plasmalemma
 Lack a nuclear envelope separating the genetic material
(DNA) from other cellular constituents
 Have no histones (specific basic proteins) bound to their
DNA and usually no membranous organelles
2- Eukaryotic cells
 Are larger and have a distinct nucleus surrounded by a
nuclear envelope
 Histones are associated with the genetic material
 Numerous membrane-limited organelles are found in the
cytoplasm

 A cell can be divided in to three principal parts:-Plasma (cell)
membrane,the nucleus and the cytoplasm
A – Plasma (cell) membrane
 Thin lipid bilayer membrane structure that surrounds the cell
 Separating the cell’s internal component from the extracellular
materials and external environment
Functions of the cell membrane:-
 It functions as a selective barrier
 Regulates the passage of certain materials in to and out of the cell and
facilitates the transport of specific molecules
 It contains receptors for chemical messengers which would activate or
inhibit various cellular functions
Remind: Read about transportation mechanism of plasma membrane.
 Passive transport(diffusion, osmosis, facilitated diffusion), active
transport, bulk transport endocytosis (phagocytosis, pinocytosis &
protein mediated) & exocytosis

B-THE CYTOPLASM
 Aqueous content of a cell inside of the cell membrane but out side the
nucleus
 It includes cytosol, all organelle (except the nucleus) and inclusion
i. Cytosol
 It is a semi fluid portion of the cytoplasm, surrounding the organelles
 Contain water, ion, enzyme…
ii. ORGANELLE = nine
 Are subcellular structures within the cytoplasm that perform specific
functions
MITOCHONDIA
 It is the “ powerhouses’’ of the cell/site of ATP synthesis

 Surrounded by an inner and outer membrane
 The outer mitochondrial membrane is smooth, but the inner
membrane is arranged in a serious of folds called cristae
The cristae
 Increase the internal surface area of mitochondria
 Cells with a high-energy metabolism have abundant
mitochondria with a large number of closely packed cristae
e.g.- cardiac muscle, cells of some kidney tubules
 Cells with a low-energy metabolism have few mitochondria
with short cristae
 Mitochondrial genes usually are inherited only from a mother
 The head of a sperm normally lacks mitochondria

Fig.Mitochondria.

RIBOSOMES
 Are small granules of proteins about 20 x 30 nm in size
 Can be found both free in the cytoplasm and located on the
surface of an organelle called the RER
 Free ribosomes
 Are ribosomes that float in the cytosol
 They have no attachments to other organelle
 The free ribosomes occur singly or in cluster
 Some free ribosomes are clustered as polyribosomes
 Synthesize Proteins for use within the cell cytosol (e.g.,
glycolytic enzymes)
 Other ribosomes attach to a cellular structure called RER

ENDOPLASMIC RETICULUM (ER)
 ER literally means “little network within the cytoplasm”
 ER is a system of interconnected channels called cisternae
enclosed by a unit membrane
 It is a fluid – filled membrane system extensively present
throughout the cytosol
 The two types of ER are:-
A- Rough (granular) ER- membrane factory=b/c produce
components
 In areas called RER, the network is composed of parallel,
flattened sacs covered with granules called ribosomes
 It is continuous with the outer membrane of the nuclear
envelop

 The outer surface of RER contains dark particles
called ribosomes
 RER associated with ribosomes, it synthesizes protein
 Proteins synthesized in the RER can have several
destinations:
 Intracellular storage (e.g., in lysosomes and specific
granules of leukocytes)
 As a component of other membranes (e.g., integral
proteins)

B-Smooth ER
 It does not have ribosomes hence looks “smooth.”
 The cisternae are more tubular in shape, and they
branch more extensively
 Their cisternae are continuous with those of the RER
 It is the site of fatty acid, phospholipids, and steroid
synthesis; detoxify lipid soluble drugs
 It is relatively abundant in liver and kidney cells
 Skeletal and cardiac muscle contain extensive
networks of SER that store calcium and release it to
trigger muscle contraction

Schematic representation of a small portion of the RER to show the shape of its
cisternae and the presence of numerous ribosomes which are part of polyribosomes.

The ER is an anastomosing network of intercommunicating channels and sacs formed
by a continuous membrane. Note that the SER (foreground) is devoid of ribosomes,
the small dark dots that are present in the RER (background). The cisternae of the
SER are tubular, whereas in the RER they are flat sacs.

GOLGI COMPLEX (GOLGI APPARATUS)
 It is extensive in cells with high secretory activity
 It consists of flattened sacs called cisterns, stacked up on
each other with expanded bulges at their edges
 One side of the stack faces the ER and the opposite side
faces towards the plasma membrane
Functions of the Golgi complex:-
 It processes, sorts, and packages proteins and lipids
 It packages proteins produced by RER to make secretory
vesicles (or secretory granules)

 It packages the digestive enzymes of the cell to form
lysosomes
LYSOSOMES
 Are sites of intracellular digestion and turnover of cellular
components
 Are membrane- enclosed sacs containing as many as 40
kinds of powerful digestive (hydrolytic enzymes)
 Digests and removes unwanted cellular debris and foreign
materials such as bacteria
 Are particularly abundant in cells exhibiting phagocytic
activity (e.g.- macrophages, neutrophilic leukocytes)
Functions of lysosome
 Digest bacteria and other substances that enter the cell.

 hydrolyzes proteins, nucleic acids, complex
carbohydrates, phospholipids, and other substances
 Example- in liver, lysosomes breakdown stored
glycogen to release glucose in to the bloodstream.
PEROXISOMES: has oxidative & catalase enzymes
 Contains several specific enzymes that promote
oxidative reaction
 They resemble lysosomes but contain different
enzymes and are not produced by the Golgi complex
 They occur in nearly all cells but are especially
abundant in liver and kidney cells
 They neutralize free radicals and detoxify alcohol and
other drugs, breaking down long chain fatty acids

THE CYTOSKELETON
 It is a complex protein network that act as the “bone
and muscle” of the cell
 It is a collection of protein filaments and cylinder that
 determine the shape of a cell,
 lend it structural support,
 organize its contents,
 move substances through the cell, and
 contribute to movement of the cells a whole
 It is composed of microfilaments (mov’t), intermediate
filaments (strength), and microtubules (shape)

Centrosome
• Spherical non membranous organelle near the
nucleus
• Contain peripheral centrosome matrix and inner
pair of centrioles (for forming cilia and flagella)

C- THE NUCLEUS
 It is the largest organelle
 It contains the DNA or genetic material of the cell and
thus directs the cell’s activity-protein synthesis
 The nucleus also contains one or more nucleoli
 Nucleoli are center for production of ribosomes which
are the sites of protein synthesis.
 Appears as a round or elongated structure, usually in
the center of the cell
 Contain most of the hereditary units of the cell, called
genes, which controls cellular structure and direct
many cellular activities

 The nuclear genes are arranged in single file along
structures termed chromosomes
 Human somatic (body) cells have 46 chromosomes;
23 inherited from each parent.
 Most cells in the body have a single nucleus, but there
are exceptions:
 Mature RBCs have none; they are a nuclear.
 A few are multinucleate - e.g. skeletal muscle cells,
and osteoclasts
 Some are binucleated - e.g. some cardiac muscle
cells
 Its main components are: the nuclear envelop,
nucleolus, chromatin and nuclear matrix.

Nuclear envelope
 It is a double membrane (outer and inner membrane)
that separates the nucleus from the cytoplasm
 The outer membrane (has ribosomes) is continuous
with the RER in the cytoplasm
 At various points, the inner and outer membranes of
nuclear envelop are fused together by structures
called pore complexes
Nuclear pore
 a central opening in each nuclear pore complex
 It is through which the exchange of substances
between the nucleus and cytoplasm takes place

CHROMATIN
 Is the thread like material that makes up the chromosomes
 Is composed of DNA and protein
NUCLEOLUS
 They are spherical structures that are rich in rRNA and
protein, site of ribosomal subunits assembled
 Is not enclosed by a membrane
CELL INCLUSION
 Are temporary structures in the cytoplasm that contain
secretions and storage products of the cell
 Are not enclosed in a unit membrane
 They are not essential to cell survival

Are of two kinds:
 stored cellular products such as
glycogen granules, pigments, and fat
droplets
 foreign bodies such as
dust particles, viruses, and intracellular
bacteria.
Remind : read about cell life cycle.

PART II – TISSUE
 Tissues are made of cells
 The science that study about the tissues of the body
and how these tissues are arranged to constitute
organs is histology
The different types of tissues
 The human body is composed of only four basic types
of tissue: epithelial, connective, muscular, and
nervous
1- Epithelial tissues (ET)
 The principal functions of epithelial tissues are:
 The covering and lining of surfaces (e.g., skin,
intestines)
 absorption (e.g., intestines)

 Sensation (e.g., gustative and olfactory
neuroepithelium)
 Contractility (e.g., myoepithelial cells)
Specializations of the Cell Surface
 The free or apical surface of many types of epithelial
cells contain specialized structures that increase the
cell surface area or move substances or particles
stuck to the epithelium
Microvilli
 Are finger like extensions measuring about 1 µm high
and 0.08 µm wide
 found in absorptive cells, such as
 the lining epithelium of the small intestine and the cells
of the proximal renal tubule

Stereocilia
 Are long and branched microvilli
 It increase the cell surface area, facilitating the
movement of molecules into and out of the cell
 Found in epididymis, vas deference & inner ear
FLAGELLA & CILIA
Flagella
 The flagellum is a single whip like structure that
propels the sperm cell through its environment
 In flagella, the projections are few and long in
proportion to the size of the cell
 Found only in sperm cells.

Cilia
 Tiny hair like structures that project from the surface of
a cell
 Found on the apical surface of epithelial cells in the
respiratory and female reproductive tracts
 In the respiratory system
 The cilia transport strands of mucus to the pharynx,
where the mucus can either be swallowed or
expectorated
 In the female reproductive tract
 Ciliary movements in the epithelial lining of the uterine
tube draw the ovum (egg) in to the tube and move it
towards the uterus.

Types of Epithelia
 two main groups: covering epithelia and glandular
epithelia.
1- Covering Epithelia
 In covering epithelia the cells are organized in layers
that cover the external surface or line the cavities of
the body
 They can be classified according to the number of cell
layers and the morphological features of the cells in
the surface layer
 Simple epithelium contains only one layer of cells
 Stratified epithelium contains more than one layer
 According to the cell shape : squamous, cuboidal, and
columnar

Simple epithelia
 consists of a single layer of cells.
 Found at interfaces involved in selective diffusion,
absorption and/or secretion
 They provide little protection against mechanical
abrasion and thus are not found on surfaces subject to
such stresses
 Based on cell shape, simple epithelia can be
 squamous, cuboidal, or columnar.

Simple squamous epithelium
 Is composed of flattened, irregularly shaped cells
 found lining surfaces involved in passive transport
(diffusion) of either gases (as in the lungs) or fluids (as
in the walls of blood capillaries)
 It also forms the delicate lining of the pleural,
pericardial and peritoneal cavities where it allows
passage of tissue fluid into and out of these cavities
 Function- Facilitates the movement of the viscera
(mesothelium), active transport by pinocytosis
(mesothelium and endothelium), secretion of
biologically active molecules (mesothelium)

Simple cuboidal epithelium
 The epithelial cells appear square
 The nucleus is usually round and located in the centre
of the cell
 usually lines small ducts and tubules that may have
excretory, secretory or absorptive functions
examples are
 Collecting tubules of the kidney and
 Small excretory ducts of the salivary glands and
pancreas

Simple cuboidal epithelium

Simple columnar epithelium
 Is similar to simple cuboidal epithelium except that
 the cells are taller and appear columnar in sections at
right angles to the basement membrane
 The height of the cells may vary from low to tall
columnar depending on the site and/or degree of
functional activity
 The nuclei are elongated and may be located towards
the base, the centre or occasionally the apex of the
cytoplasm: this is known as polarity of the nucleus
 is most often found on absorptive surfaces such as in
the small intestine

Simple columnar ciliated epithelium
 Some simple columnar epithelia have surface cilia on
the majority of the cells
 Cilia are much larger than microvilli and are readily
visible with the light microscope
 The waving motion of the cilia propels fluid or minute
particles over the epithelial surface
 Is found mainly.
 in the female reproductive tract (eg. fallopian tube)

FIG. Simple ciliatedcolumnar epithelium.

Stratified epithelium
 Are defined as epithelia consisting of two or more
layers of cells
 Is classified according to the cell shape of its
superficial layer:
 Squamous, cuboidal, columnar, and transitional.
Pseudostratified epithelium forms a separate group.
Stratified squamous epithelium
 Is found primarily in places subject to attrition (skin,
mouth, esophagus, and vagina)
 Its cells form many layers, and the cells closer to the
underlying connective tissue are usually cuboidal or
columnar

 As they move progressively closer to the surface the
cells become irregular in shape and flatten, becoming
very thin and squamous.
Stratified squamous epithelium can be:
1- keratinized stratified squamous epithelium
 Covers dry surfaces such as the skin (epidermis).
2- Nonkeratinized stratified squamous epithelium
 which covers wet surfaces

Stratified squamous epithelium

Transitional epithelium or urothelium
 Is a form of stratified epithelium found only in the
urinary tract in mammals
 Highly specialised to accommodate a great degree of
stretch and to withstand the toxicity of urine
 This epithelial type is so named because
 it has some features intermediate (transitional) b/n
stratified cuboidal and stratified squamous epithelia

Transitional epithelium

Stratified cuboidal and stratified columnar
epithelia
 Are rare
 Stratified columnar epithelium
 Can be found in the conjunctiva lining the eyelids,
where it is both protective and mucus secreting
 Stratified cuboidal epithelium
 Is restricted to large excretory ducts of sweat and
salivary glands, where it apparently provides a lining
more robust than that of a simple epithelium

Pseudostratified epithelium
 So called because the nuclei appear to lie in various
layers, all cells are attached to the basal lamina,
although some do not reach the surface
 The best-known example of this tissue is
 Ciliated pseudostratified columnar epithelium in the
respiratory passages
 Main functions- Protection, secretion; cilia-mediated
transport of particles trapped in mucus

Pseudostratified epithelium

2- Glandular Epithelia
 Are formed by cells specialized to produce secretion
 The molecules to be secreted are generally stored in
the cells in small membrane-bound vesicles called
secretory granules
 Glandular epithelial cells may
 Synthesize, store, and secrete proteins (e.g.,
pancreas), lipids (e.g., adrenal, sebaceous glands), or
complexes of carbohydrates and proteins (e.g.,
salivary glands)

Glands
 Is an organ that secretes substances for use
elsewhere in the body or releases for elimination from
the body.
 Are composed predominantly of epithelial tissue.
 They originate as invaginations of a surface
epithelium.
 Glands can be exocrine and endocrine glands.
Exocrine glands
 Retain their connection with the surface epithelium
from which they originated by way of a duct
 Have a secretory portion and ducts.

Endocrine glands
 They are ductless
 Their secretions are picked up and transported to their
site of action by the bloodstream.
 Endocrine cells typically aggregate as cords, or as
follicles in the case of the thyroid gland.

CONNECTIVE TISSUE
Functions of the CT
 Binding of organs. E.g.
 Tendons bind muscle to bone,
 ligaments bind one bone to another,
 fat holds the kidneys and eyes in place, and
 fibrous tissue binds the skin to underlying muscle.
 Support
 Bones support the body, and cartilage supports the
ears, nose, trachea, and bronchi
 Physical protection
 The cranium, ribs, and sternum protect delicate
organs such as the brain, lungs, and heart

 Immune protection - connective tissue cells attack
foreign invaders
 Movement
 Storage
 Heat production - Brown fat generates heat in infants
and children.
 Transport - Blood transports gases, nutrients, wastes,
hormones, and blood cells.
Characteristics of CT
 Originate from the mesenchyme, an embryonic tissue
formed by elongated cells, the mesenchymal cells
 Contain blood and lymphatic vessels

 Is formed by three classes of components:
 Cells, fibers, and ground substance
 Extracellular matrix consist of protein fibers (collagen,
reticular, and elastic) and ground substance
Cells of the Connective Tissue (CT)
 Fibroblasts, Chondroblasts and osteoblasts
 Originate locally from undifferentiated mesenchymal cells
and spend all their life in this tissue
 Important for production of fibers and ground substance
 Mast cells, macrophages, and plasma cells
 Originate from hematopoietic stem cells in the bone marrow
 Circulate in the blood, and move to connective tissue, where
they remain and execute their functions

 Leukocytes
 Transient cells of CT, also originate in bone marrow
 Usually migrate to CT where they reside for a few days and
die
Classification of connective tissue
 Connective tissue can be classified as:
1. Connective tissue proper (loose & dense CT)
2. Cartilage
3. Bone
4. Blood
1. Connective Tissue Proper
 There are two classes of connective tissue proper:
 loose and dense.

a. Dense connective tissue (regular, irregular & elastic)
 Is adapted to offer resistance and protection
 Is less flexible and more resistant to stress than is loose CT
 Dense connective tissue can be:
i-Dense irregular connective tissue- tension resistant
 Example- reticular layer of dermis, fibrous capsule...
ii- Dense regular connective tissue - stress resistant
 Example- Tendons, ligament, aponurosis
iii-Elastic Tissue
 Is composed of bundles of elastic fibers
 The abundance of elastic fibers in this tissue is the cause of its great
elasticity.
 Present in the yellow ligaments of the vertebral column (lig.nuchea &
flavum),in the suspensory ligament of the penis, in the wall of large
arteries

b. Loose connective tissue (areolar, reticular &
adipose) i. areolar LCT
Supports many structures that are normally under
pressure and low friction.
Has a delicate consistency; it is flexible, well
vascularized, and not very resistant to stress.
Is also found in the:
 Papillary layer of the dermis (areolar)
 In the hypodermis (adipose)
 In the serosal linings of peritoneal and pleural cavities,
and
 In glands and the mucous membranes supporting the
epithelial cells (areolar)

ii. Reticular Tissue
 Is a specialized CT consisting of reticular fibers.
 It provides the architectural framework that creates a special
microenvironment for hematopoietic organs and lymphoid
organs (bone marrow, lymph nodules and spleen).
 The reticular cells are dispersed along this framework and
partially cover the reticular fibers and ground substance
with cytoplasmic processes.
 The resulting cell-lined trabecular system creates a sponge
like structure within which cells and fluids are freely
mobile.

iii. Adipose tissue
 Is tissue in which adipocytes are the dominant cell type.
 Adipocytes usually range from 70 to 120 µm in diameter,
but they may be five times as large in obese people.
 Functions are:
 Body’s primary energy reservoir, provides thermal
insulation, and it contributes to body contours such as the
female breasts and hips.
There are two main types of adipose tissue:
a- White adipose tissue
 It comprises up to 20% of total body weight in normal, well
nourished male adults and up to 25% in females.

It is distributed throughout the body particularly in the
deep layers of the skin.
Functions:
 Is an important energy store.
 Acts as a thermal insulator under the skin and functions as
a cushion against mechanical shock in such sites as
around the kidneys.
b- Brown adipose tissue.
Is found in newborn mammals and some hibernating
animals, where it plays an important part in body
temperature regulation.
Only small amounts of brown adipose tissue are found in
human adults.

2. Cartilage
 Is a specialized form of CT
 Is avascular
 Cartilage has no lymphatic vessels or nerves
 Cartilage derives from the mesenchyme
Functions of cartilage
 Allows the tissue to bear mechanical stresses without
permanent distortion.
 Give support to soft tissues. E.g In the respiratory system.
 It is a shock-absorbing and sliding area for joints and
facilitates bone movements.
 It is also essential for the development and growth of long
bones both before and after birth.

Types of cartilage
a. Elastic Cartilage
 Characterized by its great pliability, contains significant
amounts of the protein elastin in the matrix
 Is found in the
 Auricle of the ear
 The walls of the external auditory canals, the auditory tubes.
 The epiglottis, and the cuneiform cartilage in the larynx.

b. Hyaline Cartilage
 The most common of the three forms.
 In the embryo, it serves as a temporary skeleton until it is
gradually replaced by bone
 In adult mammals, hyaline cartilage is located in the:
 Articular surfaces of the movable joints.
 Walls of larger respiratory passages (nose, larynx,
trachea, bronchi).
 In the ventral ends of ribs.
 In the epiphyseal plate.

c. Fibrocartilage
 Is a tissue intermediate b/n DCT and hyaline cartilage.
 Is characterized by a matrix containing a dense network of
coarse type I collagen fibers.
 Present in regions of the body subjected to pulling forces
 It is found in the:
 Interverteberal disks
 Attachments of certain ligaments to the cartilaginous
surface of bones, and in the symphysis pubis.
The perichondrium
 Is a sheath of dense connective tissue that surrounds
cartilage.
 It harbors the vascular supply for the avascular cartilage and
also contains nerves and lymphatic vessels.

Fig. Distribution of cartilage in adults.

Muscle tissue
 Is composed of differentiated cells containing contractile
proteins
 Most muscle cells are of mesodermal origin
 It consists of highly specialized cells, which provides motion,
maintenance of posture and heat production
 Classification of muscles is made by structure and function
 Are grouped in to:
 skeletal, cardiac and smooth muscle tissue.
Some muscle cell organelles have names that differ from
other cells.
 The cytoplasm of muscle cells is called sarcoplasm.
 The smooth endoplasmic reticulum is called sarcoplasmic
reticulum.

 The sarcolemma is the cell membrane, or plasmalemma.
Skeletal muscle tissue
 are attached to bones
 the tissue can undergo limited regeneration.
 it is voluntary
 cylindrical, multinucleated & striated.
Cardiac muscle tissue
 It forms the wall of the heart
 Has cross-striations
 Is composed of elongated, branched individual cells that lie
parallel to each other.
 Its contraction is involuntary, vigorous, and rhythmic.
 Has no mitosis and regeneration.

 intercalated disks is dark-staining transverse lines that cross
the chains of cardiac cells at irregular intervals.
 possesses only one or two centrally located nuclei.
Smooth muscle tissue
 Located in the wall of hallow internal structure like blood
vessels, stomach, intestine, and urinary bladder.
 It is involuntary and non-striated.
 Consists of collections of fusiform cells.
 They may range in size from 20 µm in small blood vessels to
500 µm in the pregnant uterus.
 Could regenerate

Nerve Tissue
The human nervous system is formed by a network of
more than 100 million nerve cells (neurons), assisted by
many more glial cells
Nerve tissues develop from embryonic ectoderm
Structurally, nerve tissue consists of two cell types:
1- nerve cells, or neurons
 Usually show numerous long processes
Nerve cells, or neurons, are responsible for:
The reception, transmission, and processing of stimuli
Triggering of certain cell activities
The release of neurotransmitters and other
informational molecules

2- Glial cells
 Have short processes
 Support and protect neurons
 Participate in neural nutrition, and the defense processes of
the CNS
Most neurons consist of two parts:
 The cell body and process (axon& dendrites)
Cell Body (perikaryon)
 Is the part of the neuron that contains the nucleus and
surrounding cytoplasm.
 Receives a great number of nerve endings that convey
excitatory or inhibitory stimuli generated in other nerve
cells.
 contains a highly developed RER, polyribosomes etc.

Dendrites
 Multiple elongated processes
 Are usually short and divide like the branches of a tree
 Specialized in receiving stimuli from the environment,
sensory epithelial cells, or other neurons.
 They receive many synapses and are the principal signal
reception and processing sites on neurons.

Axons
 Is a single process specialized in generating or conducting
nerve impulses to other cells (nerve, muscle, and gland
cells).
 An axon is a cylindrical process that varies in length and
diameter according to the type of neuron.
 All axons originate from a short pyramid-shaped region, the
axon hillock, which usually arises from the perikaryon.
 The plasma membrane of the axon is called the axolemma;
its contents are known as axoplasm.

Parts of Motor neuron.

Based on the number of their processes, most neurons
can be placed in one of the following categories:
1- Multipolar neurons
 Have more than two cell processes, one process being the
axon and the others dendrites.
 Most neurons of the body are multipolar.
2- Bipolar neurons
 With one dendrite and one axon.
 Are found in the cochlear and vestibular ganglia as well as
in the retina and the olfactory mucosa.

3- Pseudounipolar neurons
 Have a single process that is close to the perikaryon and
divides into two branches.
 In Psudounipolar neurons:
 Are found in the spinal ganglia (the sensory ganglia located
in the dorsal roots of the spinal nerves).
 They are also found in most cranial ganglia.

Neurons can also be classified according to their
functional roles:
Motor (efferent) neurons
 Control effector organs such as muscle fibers and exocrine
and endocrine glands.
Sensory (afferent) neurons
 Involved in the reception of sensory stimuli from the
environment and from within the body.
Interneurons
 Establish relationships among other neurons, forming
complex functional networks or circuits (as in the retina).

Synaptic Communication
 Synapses are sites of functional contact:
b/n neurons
b/n neurons and other effector cells (e.g., muscle and gland cells).
 Most synapses transmit information by releasing
neurotransmitters during the signaling process.
Types of synapse
 Axosomatic - if an axon forms a synapse with a cell body.
 Axodendritic - If axon forms a synapse with a dendrite.
 Axoaxonic - If axon forms a synapse with an axon.

Synapse has the following components:
1-Presynaptic axon terminal (terminal bouton)
 From which neurotransmitter is released
2- Postsynaptic cell membrane
 Have receptors for the transmitter and ion channels or other
mechanisms to initiate a new impulse.
3- The synaptic cleft
 20–30 nm wide intercellular space separating the
presynaptic and postsynaptic membranes

Fig. Types of synapses.

Glial Cells
 Are 5-10 times more abundant in the mammalian brain than
neurons.
 They surround both cell bodies and their axonal and
dendritic processes that occupy the interneuronal spaces.
1- Oligodendrocytes
 Produce the myelin sheath that provides the electrical
insulation of neurons in the CNS.
2- Schwann Cells (Neurolemmocytes)
 Produce the myelin sheath that provides the electrical
insulation of neurons in the PNS.
3- Astrocytes
 Are star-shaped cells with multiple radiating processes.

 It bind neurons to capillaries and to the pia mater.
4- Ependymal Cells
 Ependymal cells are low columnar epithelial cells lining the ventricles
of the brain and central canal of the spinal cord.
 In some locations, ependymal cells are ciliated, which facilitates the
movement of CSF.
5- Microglia
 Are small elongated cells with short irregular processes.
 Is phagocytic cells that represent the mononuclear phagocytic system
in nerve tissue.
 Are derived from precursor cells in the bone marrow.
 They are involved with inflammation and repair in the adult CNS.
6-Satellite cell – in PNS ; for repairing

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