histology of nervous system...muscles) and those of the autonomic nervous system •somatic...
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Nervous Tissue
Dr. Heba Kalbouneh
Associate Professor of Anatomy and Histology
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Nervous Tissue • Controls and integrates
all body activities within limits that maintain life
• Three basic functions
1. sensing changes with sensory receptors
2. interpreting and remembering those changes
3. reacting to those changes with effectors (motor function)
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The PNS is divided into :
1- Somatic nervous system
(SNS)
2- Autonomic nervous system
(ANS)
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Sensory (Afferent) vs. Motor (Efferent)
e.g., skin
e.g., muscle
Gray’s Anatomy 38 1999
sensory (afferent) nerve
motor (efferent) nerve
(pseudo-) unipolar neurons conducting impulses
from sensory organs to the CNS
multipolar neurons conducting impulses
from the CNS to effector organs (muscles & glands)
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Organization
6
Integration Motor Sensory
SNS
(Sensory)
ANS
(Sensory)
Brain
Spinal
cord
SNS
(Motor)
ANS
(Motor)
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Neuron has three parts:
(1) a cell body: perikaryon or soma
(2) dendrites
(3) an axon
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Neurons
cell
body
dendrites
axon with
myelin sheath
synapses
Schwann
cell
Moore’s COA5 2006
• Dendrites: carry nerve impulses toward cell body
• Axon: carries impulses away from cell body
• Synapses: site of communication between neurons using chemical
neurotransmitters
• Myelin & myelin sheath: lipoprotein covering produced by glial cells (e.g.,
Schwann cells in PNS, oligodendrocytes in the CNS) that increases axonal
conduction velocity
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Notice that action potential propagation is unidirectional
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Neurons
1. Cell body
a) Nissl bodies
b) Golgi apparatus
c) Neurofilaments (IFs)
d) Microtubules
e) Lipofuscin pigment clumps
2. Cell processes a) Dendrites b) Axons
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Structure of neurons • Axoplasm: cytoplasm of axon
• Axolemma: cell membrane of axon
• Axon hillock: where axon originates from soma
• Synaptic boutons: swelling of axon terminal
• Synapse: junction axon makes with cell acting upon
• Synaptic vesicles
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Axon
• Nearly constant diameter
• Much Longer
• Branches less profusely
• Distal end forms terminal arborization and
terminal boutons
• Mostly myelinated, could be unmyelinated
• Axoplasm contains mitochondria,
microtubules, neurofilaments and SER but
not RER and ribosomes
• Bidirectional transport along the axon
Dendrite
• Becomes much thinner (tapering)
• short
• Branches profusely
• The cytoplasm of its base is similar to cell
body
• Typically unmyelinated
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Presynaptic neuron
Synaptic vesicles
contain the neurotransmitter
Synaptic cleft
Postsynaptic neuron
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Axonal transport
Anterograde: movement away from soma
Retrograde: movement up toward soma
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Cells of nervous tissue
Neuroglia
Neurons
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Structural classification of neurons
Multipolar neurons
Usually have several
dendrites and one
axon
Motor neurons
Bipolar neurons
Have one main dendrite
and one axon
The retina of the eye
Unipolar neurons
(pseudounipolar
neurons)
Sensory neurons
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Copyright © McGraw-Hill Companies
Figure 9-4
Anaxonic neuron:
CNS
Lack true axon
Don’t produce action potential
Regulatory function
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1. Tissues: neurons vs. glia
2. Position: CNS vs. PNS
3. Function 1: sensory vs. motor
4. Function 2: somatic vs. visceral
Gray’s Anatomy 38 1999
neuron
glial cell
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Continuous versus Saltatory Conduction
1. Continuous conduction
(unmyelinated fibers)
2. Saltatory conduction
(myelinated fibers)
A.P. Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
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Saltatory Conduction
• Nerve impulse conduction
in which the impulse jumps
(Salta) from node to node A.P. Na
Na
Na
Na
Na
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Types of synapses
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Bundles
of
Axons
Clusters of
Neuronal
Cell
Bodies
Cell
body
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1. A ganglion (plural is ganglia) a cluster of
neuronal cell bodies located in the PNS.
2. A nucleus: a cluster of neuronal cell
bodies located in the CNS.
Clusters of Neuronal Cell Bodies
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• A nerve: is a bundle of axons that
is located in the PNS.
Cranial nerves connect the brain to
the periphery
spinal nerves connect the spinal cord
to the periphery
• A tract: is a bundle of axons
located in the CNS.
Tracts interconnect neurons in the
spinal cord and brain.
Bundles of Axons
Bundels of Axons
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Copyright © McGraw-Hill Companies
Figure 9-26
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Peripheral Nerve
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Myelinated Axons
LM EM
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Myelinated Nerve Fiber
Myelin
Sheath
The increased lipid content of the
myelin sheath provides electrical
insulation for the underlying axon.
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Peripheral nerve
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Bundles of axons
(Fasiculi)
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Epineurium
Perineurium
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Axon
Myelin
Nerve fiber
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Peripheral nerves
Consist of Cranial and Spinal nerves connecting brain and spinal cord to peripheral tissues.
Peripheral nerves consist of parallel bundles of nerve fibers, Myelinated or Unmyelinated, surrounded by connective tissue sheaths.
Endoneurium: a layer of loose connective tissue around the nerve fiber
Perineurium: A fibrous connective tissue that surround bundles of axons
Epineurium: is the outermost layer of dense irregular connective tissue surrounding a peripheral nerve
Spaces between bundles usually contains fat.
Myelinated fiber = axon + myelin
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somatic sensory nerve (GSA)
somatic motor nerve (GSE)
spinal
nerve
skin
(dermatome)
muscle
(myotome)
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Structure of Spinal Nerves: Somatic Pathways
dorsal root dorsal root ganglion
ventral root
spinal nerve
dorsal ramus
ventral ramus
gray ramus communicans
white ramus communicans
sympathetic ganglion
dorsal horn
ventral horn
somatic sensory nerve
somatic motor nerve
CNS inter-
neuron
Mixed Spinal
Nerve
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Structure of Spinal Nerves: Somatic Pathways
dorsal root dorsal root ganglion
ventral root
spinal nerve
dorsal ramus
gray ramus communicans
white ramus communicans
sympathetic ganglion
dorsal horn
ventral horn
somatic sensory nerve
somatic motor nerve
CNS inter-
neuron
Mixed Spinal
Nerve
ventral ramus
Somatic sensations
• touch, pain, temperature,
pressure
• proprioception: joints, muscles
Somatic motor activity: innervate
skeletal muscles
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Structure of Spinal Nerves: Dorsal & Ventral Rami
spinal nerve
dorsal ramus
somatic sensory nerve
somatic motor nerve
Territory of Dorsal Rami
(everything else, but head,
innervated by ventral rami)
ventral ramus
Stern Essentials of Gross Anatomy
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Impact of Lesions
Disruption of motor
(efferent) neurons
(paralysis)
somatic sensory nerve
somatic motor nerve
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Impact of Lesions
Disruption of sensory (afferent) neurons (paresthesia)
somatic sensory nerve
somatic motor nerve
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Impact of Lesions
somatic sensory nerve
somatic motor nerve
Disruption of motor
(efferent) neurons
(paralysis)
Disruption of sensory (afferent) neurons (paresthesia)
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Impact of Lesions
somatic sensory nerve (GSA)
somatic motor nerve
(GSE)
Disruption of sensory (afferent) neurons (back paresthesia)
Disruption of motor
(efferent) neurons
(paralysis of deep back muscles)
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SNS
PNS
ANS
Sensory
Motor
Sensory
Motor Parasympathetic
Sympathetic
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• ANS is the subdivision of the peripheral nervous
system that regulates body activities that are generally
not under conscious control
• Visceral motor innervates non-skeletal (non-
somatic) muscles
45
To repeat… Composed of a special group
of neurons serving: Cardiac muscle (the heart)
Smooth muscle (walls of
viscera and blood vessels)
Glands
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Divisions of the autonomic nervous system
• Parasympathetic division
• Sympathetic division
Serve most of the same organs but cause
opposing or antagonistic effects
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Parasysmpathetic: routine maintenance
“rest &digest”
Sympathetic: mobilization & increased metabolism
“fight, flight or fright” or “fight, flight or freeze”
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Basic anatomical difference between the motor pathways of the voluntary somatic nervous system (to skeletal muscles) and those of the autonomic nervous system
• Somatic division:
– Cell bodies of motor neurons reside in CNS (brain or spinal cord)
– Their axons (sheathed in spinal nerves) extend all the way to their skeletal muscles
• Autonomic system: chains of two motor neurons – 1st = preganglionic neuron (cell body in brain or cord) – 2nd = postgangionic neuron (cell body in ganglion outside CNS) – Slower because lightly or unmyelinated
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CNS ganglion
preganglionic
neuron
postganglionic
neuron
glands
smooth
muscle
cardiac
muscle
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• Axon of 1st (preganglionic) neuron leaves CNS
to synapse with the 2nd (ganglionic) neuron
• Axon of 2nd (postganglionic) neuron extends to
the organ it serves
49 Diagram contrasts somatic and autonomic
Autonomic
Somatic
This autonomic
ganglion is motor This dorsal root
ganglion is sensory
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Sympathetic
CNS ganglion
short preganglionic
neuron
Parasympathetic CNS ganglion
long preganglionic
neuron
Overview of the Autonomic Nervous System Differences between Sympathetic & Parasympathetic
Relative Lengths of Neurons
long postganglionic
neuron
target
target
short postganglionic
neuron
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Parasympathetic
Sympathetic
Ach
Ach
Ach
NE
Physiological
effect
Overview of the Autonomic Nervous System Differences between Sympathetic & Parasympathetic
Types of neurotransmitters
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Ganglia
• Ganglia Are Masses Of Neuronal Somas,
Usually Defined As Being Outside The Central
Nervous System. They Seem To Act As
Coordinating Way Stations.
• Two types:
1. Sensory. 2. Autonomic
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Sensory ganglion
Ganglion cells in dorsal root ganglia do not receive synapses
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Autonomic ganglion
Autonomic ganglia do contain synapses
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Autonomic ganglia
with multipolar
neurons are less
organized than
Sensory ganglia
(dorsal root
ganglia) with
pseudounipolar
neurons.
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Neuroglial cells (Nerve glue)
• Non-neuronal cells of CNS & PNS.
• Can divide during adult life, in response to trauma or disease to fill the spaces previously occupied by neurons.
• Held nervous tissue together (support).
• Neuroglial cells of CNS: – Astrocytes = star cells
– Oligodendrocytes = few tree
– Microglia = small
– Ependyma = above garment
• Neuroglial cells of PNS: – Schwann cells
– Satellite cells
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Oligodendrocytes Small glial cells with few processes Myelin-forming cells of CNS
Neuroglial cells of CNS
Astrocytes
The most abundant glial cells of the
CNS
Are characterized by numerous
cytoplasmic processes
Astrocytes are an important part of the
blood-brain barrier (BBB), regulating entry
of molecules and ions from blood into CNS
tissue
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Blood brain barrier BBB
Consists of:
1- Tight junctions that seal together the endothelial cells of brain blood capillaries
2- Thick basement membrane
3- Astrocytes processes
A few water soluble substances (glucose) cross the BBB by active transport
Proteins and most antibiotic drugs do not pass into brain tissue
Lipid soluble substances (oxygen, carbon dioxide, alcohol, most anesthetic agents
cross freely
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Microglia
Are monocyte-derived, antigen-presenting
cells of the CNS
Ependymal cells
Are epithelial-like cells that form a single
layer lining the fluid-filled ventricles and
central canal of the CNS.
Neuroglial cells of CNS
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Ventricles are CSF-filled cavities within the brain
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Astrocytes
Neuroglial cells of CNS
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Oligodendrocytes
Neuroglial cells of CNS
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Microglia
Neuroglial cells of CNS
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Ependyma
Neuroglial cells of CNS
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Neuroglial cells of PNS
Schwann cells
Flattened cells
Myelin-forming cells of PNS
Satellite cells Flattened cells arranged around cell bodies of neurons within ganglia. Support neurons in PNS ganglia.
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Satellite cells
Neuroglial cells of PNS
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Schwann cells
Neuroglial cells of PNS
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Myelin formation
• Myelin is not part of the neuron but formed by the Neuroglial cells.
• Begins during 2nd trimester of pregnancy and continues well into the 2nd decade
• Myelin increase the speed of impulse conduction.
Nerve fibers are either: Myelinated: Impulse conduction is saltatory (jumping from node to node) with a maximum speed of 120m/s. Unmyelinated: Impulse conduction is continuous with a maximum speed 15m/s.
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Axon
Myelin sheath
(Schwann cell)
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Myelination in the PNS:
Formed by Schwann cells Each Schwann cell myelinates only one internodal segment of one axon
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Myelination in the PNS:
• Formed by Oligodendrocytes.
• Each cell can myelinate internodal segments of about 60 axons (or internodal segments)
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Copyright © McGraw-Hill Companies
Figure 9-21
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Copyright © McGraw-Hill Companies
Figure 9-21
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Copyright © McGraw-Hill Companies
Figure 9-21
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Copyright © McGraw-Hill Companies
Figure 9-21
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Figure 9-22
Axon
Myelin sheath
Schwann cell nucleus
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Copyright © McGraw-Hill Companies
Figure 9-25
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Figure 9-30