Download L11Nervous tissue strusture 11

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Lecture 1: Introduction
&Nervous Tissue Organization
Dr. Amjad El-Shanti
MD,MPH, PhD
Assistant professor of Public Health- Epidemiology
2014-2015
Definitions
• Anatomy: Branch of medical science concerned with the
structure and organization of human Body (Cells,
Tissues, Organs, Systems).
• Macro-anatomy: (Shape, Size, Weight, Dimensions,
Color, Content, Surfaces, Location, Nerve supply, blood
supply).
• Micro-anatomy: Microscopic study of Tissues and cells
structures. (Histology, Cytology).
• Histology: The study of minute structure of tissues by
microscope.
• Cytology: The study of molecular structure of cells by
microscope.
• Embryology: The study of development of human
systems and organ since fertilized egg till full term Baby
(Intra-uterine development)
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Physiology
• Physiology: Branch of medical science
concerned with the function of human body
at different levels (Cell, Tissue, Organ,
System) to survive or to keep the state of
homeostasis.
• Homeostasis: refers to attempt of human
body to still balance, equilibrium, and
constant.
Nervous Tissue Functions
• Neurons are the cells in the nervous
system which are responsible for sending
messages.
• They have three major purposes
1. to gather and send information from the
senses such as touch, smell, sight etc.
2. to send appropriate signals to effector cells
such as muscles, glands etc.
3. to process all information gathered and
provide a memory and cognitive ability thus
allowing us to take voluntary action on
information received (Interpretation).
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Organization
• The Human Nervous System consists of the
Central Nervous System & the Peripheral
Nervous System.
• Central Nervous System:
1 - Brain
2 - Spinal cord
• Peripheral Nervous System:
1 - Cranial nerves (12 pair) & their branches
2- Spinal nerves (31 pair) & their branches
Divisions of Peripheral Nervous System
1 - Somatic - supplies & receives fibers (neurons) to & from
the skin, skeletal muscles, joints, & tendons
2 - Visceral - supplies & receives fibers to & from smooth
muscle, cardiac muscle, and glands. The visceral motor
fibers (those supplying smooth muscle, cardiac muscle,
& glands) make up the Autonomic Nervous System. The
ANS has two divisions:
– Parasympathetic division - important for control of 'normal' body
functions, e.g., normal operation of digestive system
– Sympathetic division - also called the 'fight or flight' division;
important in helping us cope with stress
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Histology
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Nervous Tissue consists of only two principal
kinds of cells:
1. Neuroglia
2. Neuron
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Neuroglia serve as a special supporting and
protecting component of nervous system.
Neurons make up the nervous tissue that
forms the structural and functional portion of
the system.
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Neuroglia
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general functions include:
1 - forming myelin sheaths
2 - protecting neurons (via phagocytosis)
3 - regulating the internal environment of neurons.
Types of neuroglia or (Glia cells):
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2.
3.
4.
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2.
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The following four neuroglia are found in the CNS:
Astrocytes have numerous processes that give the cell a star-shaped
appearance. Astrocytes maintain the ion balance around neurons and control
the exchange of materials between blood vessels and neurons.
Oligodendrocytes have fewer processes than astrocytes. They wrap these
cytoplasmic processes around neurons to create an insulating barrier called a
myelin sheath.
Microglia are phagocytic macrophages that provide a protective function by
engulfing microorganisms and cellular debris.
Ependymal cells line the fluid-filled cavities of the brain and spinal cord. Many
are ciliated.
Two kinds of neuroglia are found in PNS:
Schwann cells (neurolemmocytes) wrap around axons to produce an
insulating myelin sheath. Schwann cells provide the same function in the PNS
as oligodendrocytes provide in the CNS.
Satellite cells are located in ganglia where they surround the cell bodies of
neurons.
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Neuron
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Neurons in the human
Nervous systemmvary widely
in their size and shape.
However, all neurons have
some features in common.
A neuron consists of:
1. a cell body, called the soma,
(Perikaryon)
2. with arms that reach out to
connect to the network of
other neurons in the brain.
One arm, the dendrite,
receives signals,
3. while the other arm, the axon,
sends signals.
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All electrical signals are
unidirectional; they flow from
the receiving point on the
dendrite or cell body out
through the axon.
Cell Body
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The cell body of a neuron is called the soma.
It contains the same organelles as other cells in the
body, including the nucleus ,mitochondria ,endoplasmic
reticulum ,ribosomes ,and Golgi apparatus.
There are granules in cytoplasm: (Inclusions and
Characteristic Structures)
Inclusions: Lipofuscin (By-products of lysosomal
activity, increased with age, Clumps of yellowish browm
granules).
Characteristic structures for neuron:
1. Nisssl bodies: (Chromatophilic substances that responsible for
protein synthesis which flow to axon).
2. Neurofibrill: for supporting and transporting.
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The basic functions that: keep the cell alive, such as
replication, respiration, and protein production, are
carried out in the cell body.
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Intracellular systems for transporting
Axoplasmic Flow
Axonal Transport
Slower
Faster
Convey axoplasm in one
direction from cell body to
axon terminal
Convey materials in both
directions (away cell body ,
and toward cell body)
1-Supplies new axoplasm for 1- Transport organelles and
developing and regenerating materials that form
axons
axolemma, Synaptic end
bulbs, synaptic vesicles.
2- Returning materials to cell
2- Renew axoplasm in
bodies
growing and mature axon.
Dendrite
• There are many types of dendrites ,but, in general, a
dendrite looks like a tree whose trunk ends in the soma.
• Its branches, called dendritic spines, are stretched out to
receive signals from the axons of other neurons.
• Dendrites contain many receptors which can bind to
signaling molecules called neurotransmitters .
• These receptors are sometimes also found on the soma.
• When these receptors bind enough neurotransmitter
molecules, the neuron undergoes a change, becoming
more or less likely to send a signal to the next neuron.
• The type of change depends on the type of
neurotransmitter that is bound. Some neurotransmitters
excite the cell while others depress it. Still others have
cause various biochemical reactions within the
postsynaptic neuron .
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Axon
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The axon is a long, thin structure which sends out signals from the
cell.
The end of the axon is called the terminal bouton . Axon
terminal)Each signal travels along the neuron's axon to the terminal
bouton, where it is then transmitted to the next neuron.
The axon is covered in myelin, a thick phospholipid substance that
insulates the nerve to help transmit the electrical signal along the
length of the axon.
Periodic gaps in the myelin sheath ,called nodes of Ranvier ,allow
the signal to refresh itself as it propagates along the axon.
The small area, like a neck, that lies between the soma and the
axon is called the axon hillock.
Electrical charge builds up at the axon hillock until it is sufficient to
generate an action potential.
When there is enough charge to make an action potential capable of
propelling itself all the way down the axon, the action potential
initiates at the axon hillock and travels down the axon to the terminal
bouton .
Axoplasm: cytoplasm of axon
Axolemma: sheath of axon
Difference between Dendrites and Axon
Axons
1.Take information away from the
cell body
2.Smooth Surface
3.Generally only 1 axon per cell
4.No ribosomes (No Nissl
bodies)= (No protein synthesis)
5. Can have myelin
6.Branch further from the cell
body
7. Thin, Long
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Dendrites
1.Bring information to the
cell body
2.Rough Surface (dendritic
spines)
3.Usually many dendrites
per cell
4.Have ribosomes (Nissl
bodies)= (Prptein syntheisi)
5.No myelin insulation
6.Branch near the cell body
7. Thick, short
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Neuron structure
Nerve fibers and Myelination
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Nerve Fiber: Axon with sheaths.
Myelin sheath :
1.
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3.
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Schwan cell (Neurolemmocyte):
1.
2.
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6.
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increases speed of nerve impulse conduction
Increases insulate and maintain the axon.
White color (white matter in brain ans spinal cord).
Along axon
Encircles axon
It has nucleus, cytoplasm pushed outside layer
Neurolemma= membrane of neuolemmocyte (20-30layers every
cell).
Outer neurolemma responsible for regeneration injured axons.
Schwann cells are located at regular intervals along the process
(axons) & so a section of a myelinated axon would look like this.
Between areas of myelin are non-myelinated areas called the
nodes of Ranvier.
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Myelination
Myelin Sheath and Schwan cells
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Classifications of neurons
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Anatomical (Structural)
Classification: by number of
processes
1. Multipolar neurons are so-named
because they have many (multi-)
processes that extend from the cell
body: lots of dendrites plus a single
axon. Functionally, these neurons are
either motor (conducting impulses that
will cause activity such as the
contraction of muscles) or association
(conducting impulses and permitting
'communication' between neurons
within the central nervous system).
2. Unipolar neurons have but one
process from the cell body. However,
that single, very short, process splits
into longer processes (a dendrite plus
an axon). Unipolar neurons are
sensory neurons - conducting
impulses into the central nervous
system (posterior root ganglia of spinal
cord).
3. Bipolar neurons have two processes
- one axon & one dendrite. These
neurons are also sensory. For
example, biopolar neurons can be
found in the retina of the eye, internal
ear. and olfactory areas.
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Physiological Clssification:Neurons
can also be classified by the
direction that they send
information.
1. Sensory (or afferent)
neurons: send information
from sensory receptors (e.g., in
skin, eyes, nose, tongue, ears)
TOWARD the central nervous
system.
2. Motor (or efferent) neurons:
send information AWAY from
the central nervous system to
muscles or glands.
3. Interneurons: send
information between sensory
neurons and motor neurons.
Most interneurons are located
in the central nervous system
Neuron Types
1. Bipolar
2. Unipolar
3. Multipolar
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