The nervous system is considered to be the most complex
morphological structure of the human body . It consists of over 100
billion neurons that provide integration and coordination of the
body with the external and internal environment. It can be
considered a real computer, which constantly receives information
from external or internal environment, analyzes the information
received and develop appropriate responses.
The nervous system is divided morphologically and functionally
:
- Somatic nervous system receives information from musculo
-articular or cutaneous receptors as well as sensory information (
visual, auditory, gustatory , olfactory or equilibrium) . It is
controlled voluntarily and its main effector organ is the striated
muscle ;- Autonomic nervous system receives information from
osmoreceptors , baroreceptors and chemoreceptors located in the
structure of different tissues or organs . It can not be controlled
voluntarily , and effectors are represented by heart , endocrine or
exocrine glandular cells and smooth muscles of internal organs.
Both the somatic nervous system and the autonomic have two
distinct components: a central component (central nervous system )
and a peripheral component ( peripheral nervous system )
The nervous system is formed of of parenchyma and stroma .CNS
parenchyma consists of all nerve cells. Stroma is composed of glial
cells , blood vessels and fine connective elements .At all levels
of the CNS there are two different structures, morphologically and
functionally :
Gray matter White matter
The gray matter is arranged in columns ( bone marrow ) , islands
(in the brainstem and diencephalon ) or forms the cortex (the area
of the cerebral hemispheres and cerebellum).
Histologically it is formed of:Neuron bodiesDendritesThe initial
unmyelinated portion of the axonGlial cellsCapillary network which
allow a highly oxidative metabolism specific for neurons
Nerve cells have multiple extensions that connect to each other
through various synapses forming a plexiform network called
neuropil.
The white matter is composed of:Myelinated axons arranged
parallel, grouped in bundles or cordsBetween the axons there are
many glial cells , and in particular oligodendrocyte that have a
role in the formation of the myelin sheath surrounding axons .
Capillaries (less numerous in the white matter)
Macroscopically, the spinal cord presents: - Anterior median
fissure; - Posterior median sulcus; - Two collateral grooves
arranged symmetrically on the side of the median plane, places of
emergence of the anterior roots of the spinal nerves; - Two
collateral grooves , arranged a side of posterior median sulcus,
representing the place of penetration of the posterior spinal nerve
roots in the spinal cord.The gray matter is arranged in the center,
surrounded by white matter. In longitudinal section it has the form
of a continuous column for the entire spinal cord length. In
cross-section is typically arranged, symmetrically in relation to
the median plane, in the form of the letter "H" or two crescent
joined together by a strip of gray matter called the gray
commissure. Gray commissure is traversed throughout its length by
the ependymal canal, which continues in the brain with the
ventricular cavities.Each symmetrical half of the gray matter
presents :- An anterior horn, shorter and with a greater transverse
diameter consisting of a head and a base. It contains large motor
neurons, whose axons form the anterior root of the spinal nerve .-
A posterior horn narrower and longer , consisting of a head, cervix
and base. It contains smaller oval sensory or association neurons -
A lateral horn more obvious between C7 - L2 disposed between the
base of the anterior horn and posterior horn base , consisting of
small neurons , spherical , fusiform or oval with autonomic
function . They are sympathetic preganglionic autonomic neurons (
cervico -dorsal region of the spinal cord ) or parasympathetic (
sacral region of the spinal cord ) .- The reticular substance (
Deiters ) consists of small islands of gray matter separated by
fine myelinated nerve fibers, arranged in the angle between the
posterior horn and lateral horn.Histologically, the gray matter
contains neurons, glial cells, amyelinated nerve fibers
representing dendrites and axons, blood vessels and a small amount
of perivascular connective tissue .In anterior horns are many types
of neurons. The most numerous are the alpha neurons . These neurons
are called radicular neurons because their axons pass in white
matter , receive the myelin sheath and enters the anterior spinal
nerve root structure. They are stellate large, multipolar with a
diameter between 60-120 m. Their dendrites make synapses with
pyramidal and extrapyramidal tract axons or with the axons of
sensory neurons in the posterior horns.These motor neurons
innervate striated skeletal muscle fibers .Gamma neurons are
multipolar neurons , small size , with various shapes that
innervate muscle fibers within motor neuromuscular spindles.
The glial cells in the gray matter are the parenchymal glial
cell( macroglia, protoplasmic macroglia, microglia, oligodendrocyte
rare) and epithelial glial cell(ependymal) that guards the
ependymal canal.The posterior horns contain sensory neurons ,
polymorphic , oval , fusiform , of varying sizes , from 6-70
micrometres arranged in several layers ( layer Rolando , the Clarke
etc) incompletely defined. Their dendrites realize synapses in the
posterior horn with axons of the pseudo-unipolar neurons in the
spinal ganglia and their axons pass in white matter , entering the
constitution of the ascending tracts.Other neurons in the posterior
horns are association neurons , which connect sensory neurons and
the motor of the anterior horns at the same level or at different
levels of the neurax.
The lateral horns have autonomic neurons: sensitive in the
posterior half and motor, preganglionic in the anterior half. They
are oval or fusiform neurons with a diameter of 15 to 20 m. Their
dendrites realize synapses with the axons of sensory neurons in the
spinal ganglia. Their axon join the somato-motor fibers of the
anterior roots, forming preganglionic fibers that stop in the
paravertebral sympathetic ganglia or pelvic parasympathetic
ganglia.
Regarding the length of the axon:Golgi I- long axon neurons.
Their axon passes in the white matter of the spinal cord. radicular
neurons- anterior horn and anterior half of lateral horn- forms the
anterior root of the spinal nervecord neurons- posterior horn and
posterior half of lateral horn- ascendant tractsGolgi II- short
axon neurons. Their axon doesnt leave the gray matter. They have
association role.Regarding their function:Sensory neurons
(autonomic and somatic)Motor neurons(autonomic and
somatic)Association neuronsNeuronal extensions (dendrites and
axons) present in the gray matter are devoid of myelin sheath and
participate in the neuropil.The white matter of the spinal cord is
disposed outside the gray matter and is organized into three pairs
of cords :- Anterior cords that communicate with each other through
the white anterior commissure exchanging nerve extensions;- Lateral
cords delimitated by the anterior and posterior roots of the spinal
nerves ;- Posterior cords disposed between the dorsal roots of the
spinal nerves and the posterior median sulcus.
The spinal cords white matter consists of myelinated nerve
fibers grouped into bundles . These fibers are either endogenous or
exogenous .
The endogenous fibers are represented by axons that of neurons
located in the gray matter of the spinal cord at different levels.
These fibers can be: short fibers and long fibers.
Endogenous short fibers are called spino - spinal fibers because
arise in the spinal cord and realize synapses with other neurons in
different levels of the spinal cord. This type of fibers form the
fundamental fascicle .Endogenous long fibers have the perikaryon
located in the dorsal horns of the spinal cord and myelinated axons
pass into the lateral or anterior cord and reach the thalamus ,
cerebellum and brainstem . These fibers form the main ascendant
sensitive fascicles:- Spino - thalamic lateral fascicle located in
the lateral cord, leading thermal and pain sensitivity;- Spino -
thalamic anterior fascicle- anterior cord- tactile sensitivity ;-
Direct spino cerebellar fascicle (Flechsing) and cross spino
cerebellar fascicle (Gowers) located in the lateral cords , leading
unconscious proprioceptive sensitivity
Exogenous fibers are the myelinated axons with the neuronal body
located outside the spinal cord. Regarding their path these fibers
are classified into: fibers ascending and descending fibers .
I. exogenous ascending fibers have the perikaryon located in the
spinal ganglion. Their axons enter the spinal cord through the
posterior root directly into the posterior cord , have an ascendig
path towards the medulla oblongata forming the spino - bulbar
fascicles ( Goll and Burdach ) . These bundles are sensitive and
lead fine tactile sensitivity and conscious proprioceptive
sensitivity.
II . Exogenous descending fibers originate in the upper floors
of the central nervous system ( cerebral cortex , midbrain , pons,
medulla oblongata , cerebellum ) . The descending tracts are large
pyramidal and extrapyramidal pathways. They are effector pathways
controlling voluntary and involuntary motility.
The pyramid tracts called cortico -spinal pathways have the
perikaryon located in the cerebral cortex , the motor neocortex .
In their descending path they go through the cerebral peduncle, the
pons and medulla oblongata , through the bulbar pyramids. Here, the
medial fibers cross forming the decussation of pyramids and the
lateral ones do not cross, thus forming in the spinal cord two
fascicles with symetric arrangement regarding the median plane:
Direct pyramid fascicle- anterior cords of the white matterCross
pyramid fascicle- lateral cord of the white matter The extrapyramid
tracts have the perikaryon located both in the cortex and in
various nuclei located in the midbrain , pons, medulla , cerebellum
. The main extrapyramid tracts are Tecto -spinal fascicle,
Rubro-spinal , nigro -spinal , olivo -spinal , reticulo -spinal etc
.
Descending fibers , both pyramid and extrapyramid make synapses
with motor neurons in the anterior horns of the spinal cord.
Formed of: - medulla - pons - midbrainGray matter: Aggregates of
neurons (known as nuclei or gray matter isles)- the parenchymaWhite
matter: spino-talamic ascendant tracts and cortico-spinal
descendent tracts- myelinated axons
The stroma is formed of glial cells and connective tissue septa
with blood vessels originating in the meninges.
In the brainstem the reticular substance is better developed
than in the spinal cord. It is made up of islands of nerve cells,
different in shape, size and function, separated by many nerve
fibers, more or less organized in fascicles. Regarding the
histologic appearance of the nuclei of the reticular substance two
areas were described: - The median area- rich in neurons: - The
lateral area- poorer in nerve cells. The reticular substances
nuclei receive afferences directly from the spinal cord, the
medulla and pons nuclei, the cerebellum, hippothalamus and even
from the cerebral cortex. They send efferent fibers to the spinal
reticular substance, to autonomic neurons in the brainstem,
hypothalamus, thalamus and cerebral cortex. In the nuclei of the
reticular substance several groups of dopaminergic neurons,
noradrenergic and serotonergic have been observed.
Called " intermediate brain " , diencephalon consists of several
nerve formations : the thalamus , hypothalamus , metathalamus and
epithalamus .
The thalamus is a bulky mass of gray matter located in the
dorsal diencephalon . Its structure is uneven because of the
division of neurons in several groups of nuclei. These nuclei
contain neurons Golgi type I with long axons that project to the
cerebral cortex and Golgi type II neurons that have short axons
with association function.
The hypothalamus , located in the floor of the third ventricle ,
consists of several nuclei : anterior , middle and posterior . It
receives afferences from the retina, the olfactory mucosa , the
reticular substance and cerebral cortex and sends efferent fibers
to the hippocampus, midbrain reticular substance and to the
autonomic nuclei of brainstem and spinal cord. Also , the
hypothalamus is connected structurally with the neurohypophisys
through the anterior nuclei and controls the secretory activity of
the adenohypophysis by middle nuclei . Hypothalamic nuclei also
have autonomic sympathetic and parasympathetic function.
The metathalamusul is made up of four neuronal groups known as
the geniculate bodies :- Medial geniculate corpus - relay station
for auditory pathways ;- Lateral geniculate corpus - relay station
for optical fibers .
The cerebellum is located in the lower floor of the posterior
skull, behind the brainstem , under the occipital lobe of the
cerebral hemispheres .It consists of :- Two cerebellar hemispheres
( neo-cerebelum ) ;- Vermis ( paleo-cerebellum ) an elongated
midian part that joins the cerebellar hemispheres.It is formed of
gray matter and white matter.The gray matter is disposed on the
surface of the cerebellum and forms cerebellar cortex. It has a
thickness of about 1 mm and covers the cerebellar hemisphere and
vermis . Inside the cerebellar masses the gray matter forms islands
called cerebellar nuclei (dentate, emboliform , globose and
fastigii) .The white matter is located on the inside of the
cerebellum consisting of afferent and efferent myelinated nerve
fibers.
The gray matterThe cortex of the cerebellum has a specific
cellular architecture. Thus, from the surface to the depth three
different layers are visible in light microscopy:- Molecular layer;
- Purkinje cell layer; - Granular layer.
The molecular layer is well developed. It consists of few
stellate cells and Golgi type II cell with associative function. It
also contains many dendrites of Purkinje cells from the underlying
layer and axons originating in the granular layer. The fibers are
oriented both perpendicular to the surface of the cerebellar cortex
as well as parallel to it. This layer contains numerous synapses
being an associative layer.
Purkinje cells layer consists of a single row of Purkinje cells
. The dendrites of these cells climb in the molecular layer where
they realize synapses with the " T " axon of the granular cells.
Their axon descends through the granular layer in white matter
where it realizes synapses with the neurons of the dentate nucleus.
In the granular layer, the Purkinje cell axons emit collaterals to
neighboring making axo - axonal synapses. The body Purkinje cell
synapses with ' terminal panier " cells present in the granular
layer and with the hanger" fibers originating in the pons
nuclei.
The granular layer is made up of a large number of granular
cells ( about 2 million/mm3 ) . Dendrites of these cells form a
special synapse called "cerebellar glomerulus". Cerebellar
glomerulus is a cluster of synapses made between granular cell
dendrites and axons of "muscle" cells or Golgi cell axons, wrapped
by glial capsule.Axons ascend to the molecular layer where they
bifurcate in " T " and create synapses with Purkinje cell dendrites
.The white matterContains myelinated afferent and efferent nerve
fibers. These fibers are either endogenous or exogenous .Exogenous
fibers are afferent fibers that have the perikaryon in the spinal
cord , the medulla , the pons, or even brain cortex. They form
spino - cerebellar tracts, bulbo - cerebellar , reticulo cerebellar
or cortico- ponto cerebellar tracts .Some of these afferent fibers
, called hanger fibers and muscle fibers, performs special synapses
in the cerebellar cortex.
Hanger fibers originate in the medullas ovlivar nucleus, they
pass through the cerebellar white substance as myelinated fibers
and penetrate the cerebellum cortex where they lose their myelin
sheath. After traversing the granular layer give numerous branches
ending "clinging like vines" around Purkinje cell dendrites or
body. They have direct excitatory action on Purkinje cells and
Golgi cells.
Muscle fibers are thick , wavy , highlighted more easily . They
come from many fascicles ( spino - , Ponto- , vestibulo - and
reticulo - cerebellar ) . In the cerebellar cortex they lose their
myelin sheath and give numerous branches forming the cerebellar
glomerulus. On their length they have short and thin branches like
muscle bushes. They have excitatory effect on granular
cells.Endogenous fibers originate in the cerebellar cortex or
cerebellar nuclei. Purkinje cell axons, cross the granular layer,
they gain the myelin sheath and head towards the cerebellar nuclei
where they realize synapses. The axons of these neurons form tracts
that go to the vestibular nuclei in the medulla, to the reticular
substance of the brainstem or to the red nucleus or towards the
thalamus.
Glial cells of the cerebellum are: - Fibrous and protoplasmic
astrocytes are more numerous in the cerebellar cortex; - The
microglia present in both the gray matter and white matter; - The
oligodendrocyte is present especially in the white matter; -
Fananas glial cell in the molecular layer; - Bergman glial cell
also known as radial epithelial cells present in the Purkinje cells
layer.
Cerebral hemispheres are the most developed segment of the CNS ,
they occupy over 80 % of the skull. They are formed of: - The gray
matter formes the cerebral cortex on the surface of hemispheres
with a thickness of 3-5 mm and the striatum at the base of the
cerebral hemispheres;- White substance disposed within the cerebral
hemispheres , composed of myelinated nerve fibers that come and go
from the cerebral cortex or linking different areas of the cerebral
hemispheres.
The gray matterThe cortex represents the largest concentration
of neurons and synapses. It is considered that it could contain
between 14 and 26 billion neurons. The number of synapses is huge ,
some authors showing that there are about 1012 synapses/cm3 of
cerebral cortex . The total area of the cerebral cortex was
estimated at 1924 cm2.
In terms of structural , functional and phylogenetic in the
cerebral cortex there are two areas :paleo-cortex or allo-cortex ,
the oldest representing 1/12 of the surface of the cortex . It has
a cellullar arangement forming two layers of neurons and is located
in the hippocampus ;neo-cortex or isocortex , the largest area (
dealing 11/12 of the surface of the cortex ) and the structurally
complex . It is composed of 6 layers of neurons .
Histologically, the cerebral cortex is formed of:- Parenchyma ,
formed by all neurons and neuronal extensions;- Stroma composed of
glial cells , blood vessels and fine perivascular connective tissue
Cytoarchitectonics of the cerebral cortexThe distribution of
neurons in the cerebral cortex is extremely varied, which made many
authors identify 20 to over 200 different cortical areas
morphologically different. With all this great variability in the
isocortex six layers of overlapped nerve cells can be identified.
From the surface to the depth of the cerebral hemispheres, there
are :- Molecular or plexiform layer , just under the meninges , is
poor in nerve cells, but is rich in dendrites and axons of neurons
belonging to the underlying layers . Neurons are fusiform ,
pyriform cells Cajal and Golgi II cells with short axon . This
layer has association function.- External granular layer is richer
in neurons than the above layer . It consists of small granular
cells, particularly neurons Golgi type II, Martinotti cells with
ascending axon and small and medium pyramidal neurons. It has
association function .- External pyramidal layer consists mainly of
small and medium-sized pyramidal cells and rare granular neurons.
Dendrites of pyramidal cells move towards the surface of the
cerebral cortex and they branch especially in the molecular layer.
The axon goes towards the white matter of the cerebral hemispheres.
It has motor function.- Internal granular layer has a high density
of granular neurons and few medium size pyramidal cells small . It
has receptor function.- Internal pyramidal layer contains mainly
pyramidal cells small , medium and large ones ( Betz cells ).
Because of these large cells , some authors have called the
internal pyramidal layer the ganglionar layer. It has the motor
function.- Polymorphic layer consists of a mixture of the various
cell sizes and shapes . It is rich in nerve fibers and has
association funtion.
The uneven distribution of nerve cells in the isocortex led to
its division: - Homeotypic isocortex (containing all 6 cell
layers), subdivided into: - isocortex of frontal type- pyramidal
cells; - isocortex of parietal type- granular cells; - Heterotypic
isocortex (characterized by the almost complete absence of some
layers): - Agranular isocortex type present in the motor area,
characterized by the absence of the granular layer and the presence
of a large number of pyramidal cells; - Granular isocortex type
characterized by the absence of pyramidal cells and granular layers
pronounced development.
The allocortexul or arhipalium is characterized by a reduced
development and absence of stratification. Its structure has only
layers 1 and 6.Myelo-architectonics of the cerebral cortexIs formed
by the arrangement of neuronal extensionsNeuronal extensions
(dendrites and axons) in the cerebral cortex are the amyelinic
nerve fibers, afferent, efferent or association. These fibers are
classified into:- Tangential fibers- horizontal, parallel to the
surface, grouped in layers- Superficial stria or tangential plexus
of Exner in the molecular layer- Lamina difibrosa located in the
external granular layer (less developed); - Bechterew stria located
in the superficial part of the external pyramidal layer; - External
Baillarger's stria located in the internal granular layer; -
Internal Baillarger's stria located in the internal pyramidal
layer; - Infrastriated layer located in the polymorphic layer.-
Radial fibers are formed mainly by the dendrites and axons of
neurons that form afferent and efferent cortical fascicles
The white matterRepresents the most voluminous component of the
cerebral hemispheres. Disposed under the cortex, the white matter
is formed only of nerve fibers wrapped in myelin sheath and glial
cells. In its structure different nerve fibers categories can be
observed: - Afferent fibers that form the sensory-sensory pathways
specific and nonspecific; - Efferent or motor fibers that emerge
from the cortex; - Association fibers: Unilateral association
fibers Bilateral association fibers passing from one hemisphere to
the other, found in the structure of the corpus callosum and white
commissure (anterior and posterior).Glial cells of the cerebral
hemispheresParenchymal glial cell- astrocyte, microglia and
oligodendrocyteEpithelial (Ependymal) glial cells- covers the
cavities of the CNS, forming in the lateral ventricle the choroid
plexus.
Choroid PlexusAre found in roof of 3rd & 4th ventricles and
walls of lateral ventricles. Composed of fibrovascular core of
loose connective tissue and blood vessels covered by specialized
ependymal cells. These cells contain microvilli and cilia on their
apical surface.Main function is to produce cerebrospinal fluid. CSF
passes from ventricles to subarachnoid space and then enters the
venous circulation via the arachnoid villi.
1. Dura mater (pachymeninges) dense irregular connective tissue.
There is an epidural space above and a subdural space below.2.
Arachnoid avascular connective tissue covered on both sides by a
simple squamous epithelium. Sends down fibrous trabeculae into the
subarachnoid space to connect the arachnoid with the pia mater.
Subarachnoid space is filled with CSF and communicates with the
ventricles of the brain; it also contains blood vessels
(subarachnoid hemorrhage).Arachnoid villi (Pacchioni granules)
perforate through the dura mater, allowing CSF to flow into the
venous sinuses. In old persons they can calcify generating
nodules.3. Pia mater loose connective tissue, very vascular.
Partially follows blood vessels that enter the brain, resulting in
a perivascular space around these vessels lined by pia mater. On
its inner surface it is doubled by a glial membrane formed of glial
cells extensions.
Blood-Brain Barrier functional barrier composed of endothelial
cells with tight junctions, their basement membranes, and astrocyte
vascular processes. It has selective permeability.
Is formed of nerves and nervous ganglia
Nerve Fibers -- an axon or collection of axons, plus any
surrounding sheaths of ectodermal origin. The presence or absence
of a sheath, and the nature of the sheath (cytoplasm vs. myelin)
are used to further characterize nerve fibers.1. Unmyelinated
Fibersa. Small axons in the PNS are embedded in clefts of the
cytoplasm of Schwann cells. Each Schwann cell can sheathe (but not
myelinate) a dozen axons. b. In the CNS, the majority of axons are
unmyelinated. The axons are NOT sheathed by any type of cell. 2.
Myelinated FibersAxons that become myelinated are generally larger
than 1 M in diameter. Myelin is deposited just beyond the axon
hillock and continues to near the region of termination of the
axon. a. Myelin is laid down by Schwann cells. The membranes of the
Schwann cell wrap around the axon several times and fuse to form a
myelin sheath. The sheath is interrupted by gaps called nodes of
Ranvier which represent the spaces between adjacent Schwann cells
along the length of the axon. A Schwann cell can only myelinate a
single axon; however, each axon is myelinated by several Schwann
cells.b. In the CNS, myelin is laid down by oligodendrocytes. Each
cell can myelinate several axons.
Nerves = groups or bundles of nerve fibers covered by connective
tissue.1.Epineurium the outer layer of dense connective tissue. It
surrounds several nerve bundles. Contains blood vessels and
lymphatics.2.Perineurium connective tissue and flattened epithelial
cells that surrounds each bundle (fascicle) of nerve fibers. Cells
are joined by tight junctions to provide a barrier to passage of
most macromolecules (blood-nerve barrier).3.Endoneurium thin layer
of reticular fibers that surround individual nerve fibers.
On the trail of the posterior spinal nerve root in the spinal
canal , there is one small , oval structure called the spinal
ganglion. In longitudinal section the following histological
structures are observed:- Fibrous connective capsule on the outside
;- A cortical area ;- Axial zone .The capsule is a connective,
fibrous structure tissue, composed of collagen fibers arranged in
lamellae and rare reticulin fibers and fibroblasts . Fine
connective tissue septa and blood vessels penetrate inside the
ganglion forming the organs stroma. Cortical area is disposed just
under the capsula and it is formed of a cluster of neurons and
glial cells. There are two types of neurons :- Pseudounipolar
somatosensory neurons ;- Pseudounipolari autonomic-sensoty neurons
;The perikaryons are spherical shaped with sizes ranging from 15 to
70 and even 120 microns . They have a single amyelinic extension
that forms a glomerulus surrounding the neuronal body and then
divides in two branches: The axon penetrates the spinal cord
through the posterior rootThe dendrite- towards the periphery,
forming the sensory nervous endingBoth the neuronal body and its
extension are covered by a continuous glial capsule formed of
satellite peri-neuronal cells (isolation role)
They are formed of groups of neurons located on the trail of
vegetative nerves. Thus , one side and the other of the spine there
are two paravertebral sympathetic chain ganglia; around some blood
vessels, or in the vicinity of some organs there are autonomic
nerve plexus ( celiac plexus , heart , etc. ) and in the walls of
internal organs there are intramural ganglia.
Autonomic ganglia, whether sympathetic or parasympathetic ,
presents a similar structural organization.
At the periphery the ganglion has a fibrous capsule which sends
inner connective septa containing blood capillaries .The neuronal
bodies are arranged in small islands between connective bundles and
bundles of myelinated axonal fibers ( preganglionic ) and amyelinic
(postganglionic ) . Perineuronal satellite cells form a capsule
enclosing each perikaryon isolating it from the connective
tissue.
Autonomic neurons are multipolar neurons with a variable
diameter of from 15 to 60 microns. They have many dendrites of
variable length and a single axon is distributed towards effector
organ ( smooth muscle cells , glandular cells ).In autonomic
ganglia we can find cells with endocrine function, secreting
catecholamines (adrenaline and noradrenaline ).