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HISTOLOGY 1.13.: NERVOUS TISSUE
Nervous tissue is specialized to generate and conduct impulses.
Origin: neuroectoderm
Tissue components: nerve cells, glial cells and their processes
Blood supply: densely capillarized
The neuron:
Generalized schematic
drawing of a multipolar
neuron
Nissl staining
Silver impregnation
Classification of neurons: on the basis of their processes
Unipolar neurons
Pseudo-unipolar neurons
Bipolar neurons
Classification of neurons: on the basis of their processes
Motoneuron in the
spinal cord (Nissl
staining)
MULTIPOLAR
NEURON-TYPES
Cortical pyramidal cell
(silver impregnation)
Cerebellar Purkinje cell
Classification of neurons: on the basis of their activity
Excitatory cell type: it has spiny
dendrites, its axon makes
asymmetrical synapses
using excitatory neurotransmitters
Inhibitory cell type: it has non-spiny
beaded dendrites, its axon makes
symmetrical synapses using
inhibitory neurotransmitters
Model of a multipolar neuron
within the nervous tissue:
1.
2.
3.
4.
5.
6.
7.
8.
Dendrite
Axon (myelinated)
Nucleus
Nucleolus
Golgi apparatus
rER
Axon hillock and initial segment
Synaptic boutons terminating
on the membrane of the neuron
9. Glial cell endfeet
10.Capillary with erythrocytes
11. Compact neural tissue (neuropil)
Cell body (perikaryon) of the neurons (electron micrograph)
EM
G
M
L
N=nucleus
n=nucleolus
Asterisks label
stacks of rER
LM
(Nissl bodies)
M=mitochondrion
L=lipofuchsin
G=Golgi apparatus
Non-visible on the
picture:
microtubules
neurofilaments
sER
Processes of the neurons: dendrites
Cross-section
Highly branched processes.
Dendrites may contain microtubules,
neurofilaments, sER, free ribosomes
and mitochondria.
Their membranes exhibit postsynaptic
densities (arrows), the sites of synaptic
transmission, thus: dendrites are the
„receiving” processes, accepting impulses
from other neurons.
Transverse section
Processes of neurons: axons
Axon hillock
Initial segment
Long, cylindrical process with few
branches along its course and multiple
terminal branches
(telodendrion).
Axons originate from
axon hillock.
Initial segment: free of
myelin sheath,
receive synapses
from other neurons.
LM
Axon
Terminal bulb,
or synaptic
bouton
Telodendrion
EM
Characteristics of dendrites and axons: a summary
Axon:
1. Extends from cell body or dendrite
2. Begins with initial segment
3. May be absent (amacrine cells)
4. Unique in most cells
5. May be myelinated or no
6. Never contains ribosomes
7. Smooth contours, cylindrical shape
8. The thinnest process at the origin
9. Ramifies by branching at obtuse angles
10. Gives rise to branches of same diameter
11. May extend long distances from soma
12. Neurofilaments predominate in axons
13. Capable of generating action potentials
propagating them and synaptic
transmission
14.Primarily engaged with conduction
and transmission
Dendrite:
extends from cell body
in proximal portion continues cytoplasm
May be absent (dorsal root ganglion)
Usually multiple
Rarely myelinated
Contain rER, or ribosomes
Irregular contours, appendages (spines)
Originates as thick, tapering process
Ramifies by branching at acute angles
Subdivides into smaller branches
Confined to the vicinitiy of cell body
Microtubules predominate in dendrites
Conduct in a decremental fashion
but may be capable of generating
action potentials
Primarily engaged with receiving
synapses
Synapses
A synapse between neurons is a site of morphological specialization
where one neuron is able to influence the excitability of another neuron.
Types of synapses:
1./ Electrical synapse: is a gap junction between the membranes of two
adjacent neurons
2./ Chemical synapse: changes the membrane potential of the postsynaptic
neuron by releasing neurotransmitter molecules.
Electrical synapse (gap junction)
Chemical
synapse
Types of chemical synapses:
A./ On the basis of the postsynaptic site:
Axodendritic
Axosomatic
Axospinous
(Less frequent types
/not shown here/:
Axo-axonic synapse,
Dendro-dendritic synapse,
Dendro-axonic type
Reciprocal synapse)
Types of chemical synapses:
A./ On the basis of the function:
1./ Excitatory (arrow):
Usually asymmetrical
2./ Inhibitory („hands”):
Usually symetrical
Neurotransmitters of the chemical synapses:
Acetyl choline (ACh)
Amino acids:
glutamate (Glu) Amino acid derivatives: serotonin (5-HT)
aspartate (Asp)
dopamine (DA)
glycine (Gly)
g-amino butyric acid (GABA)
histamine (His)
Peptides:
opioids (enkephalins, endorphins, dynorphins, etc.)
neurohypophyseal (vasopressin, oxytocin, neurophysin)
tachikinins (substance P and K, neurokinin, etc.)
gastrins ( gastrin, cholecystokinin-CCK)
Somatostatin (SOM)
Vasoactive intestinal polypeptide (VIP)
Neuropeptide Y (NPY)
etc.
Purins:
adenosin