Nervous Tissue consists of of cells
Transcript of Nervous Tissue consists of of cells
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Nervous Tissue consists of 2 types of cells
• 1 - Neurons – main cells (basic functional
units), specialized to
• perception of sensory stimuli,
• processing received information and
• transmission it further to other neurons in form
of nerve impulses
• 2 - Neuroglia-(glial cells) (supporting cells)
• they support,
• nourish and
• protect neurons
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Neuron Structure
1. Cell body = perikaryon = contains nucleus and is the metabolic center of the cell
2. Processes – that extend from the cell body (dendrites and axon)
3. Nerve endings (synapses, special receptors)
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Neuron
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Cell body has:
Nucleus with large nucleolus
Neurofibrils
“Nissl bodies” (chromophilic substance)
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Neurofibrils are present in the perikaryon, dendrites and axon and are unique to neurons. = “Skeleton” of the neurons
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Nissl bodies
- large clumps of basophilic
material around the nucleus,
an aggregation of many parallel
cisternae of the rough
endoplasmic reticulum with
the rosettes of free polisomal
ribosomes
Function – protein synthesis
(neurotransmitters)
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Neuron processes - Extensions outside the
cell body
Slide 8
Dendrites –conduct impulses toward the cell body
Axons – conduct impulses awayfrom the cell body (usually only 1!)
All processes end with the nerve endings
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• Axons are covered with a fatty material called myelin.
• Axons in the PNS are heavily myelinated.
• This is done by the Schwann Cells
• These Schwann cells layer around the axions and
squeeze their cytoplasm out creating many layers of
plasma membrane tissues (proteins/lipids) surrounding
the axion. This is the Myelin sheath.
• Areas of neuron not covered are called Nodes of
Ranvier.
• Myelin insulates the nerve fibers and greatly increases
the speed of neurotransmission by nerve fibers.
12-9
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• Each axon terminal (synaptic knob) is seperated from the cell
body or dendrites of the next neuron by a tiny gap…synaptic
cleft.
• Neurotransmitters are released into the synaptic cleft and
diffuse across to bind to membrane receptors on the next
neuron..initiating an electrical surrent or synaptic potential.
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12-11
Axonal Transport • many proteins made in soma must be transported to axon and
axon terminal
– to repair axolemma, serve as gated ion channel proteins, as enzymes or
neurotransmitters
• axonal transport – two-way passage of proteins, organelles, and
other material along an axon
– anterograde transport – movement down the axon away from soma
– retrograde transport – movement up the axon toward the soma
• microtubules guide materials along axon
– motor proteins (kinesin and dynein) carry materials “on their backs” while
they “crawl” along microtubules
• kinesin – motor proteins in anterograde transport towards outside
• dynein – motor proteins in retrograde transport towards center
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(1) Structural Classification of
Neurons - According to amount of processes
1. Unipolar neurons – are found during early embryogenesis. They have one axon
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(1) Structural Classification of
Neurons
2. Bipolar neurons – one axon and one dendrite
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(1) Structural Classification of
Neurons
3. Pseudounipolar neurons – have a short single process leaving the cell body
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(1) Structural Classification of
Neurons
4. Multipolar neurons – many extensions from the cell body
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(2) Functional Classification of
Neurons• 1. Sensory (afferent) neurons
Carry impulses from the sensory receptors to the cell body
• 2. Motor (efferent) neurons
Carry impulses from cell body which lie in the central nervous system to effector cells
• 3. Interneurons (=association neurons) -
99,9% in the central nervous system
Connect sensory and motor neurons
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Supporting Cells
(Neuroglia or Glia) =
Macroglia + Microglia
Glial cells of the CNS=
Astrocytes
Oligodendrocytes…myelination
Microglial
Ependymal cells
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Supporting cells (glial cells) of
the PNS
• Schwan cells
• Satelite cells
• These supporting “glial” brace and protect
the fragil neuron cells
• Act as phagocytes
• Control the chemical environment around
the nerve cells.
• More about supporting cells later12-18
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• about a trillion (1012) neurons in the nervous system
• neuroglia outnumber the neurons by as much as 50 to 1
• neuroglia or glial cells
– support and protect the neurons
– bind neurons together and form framework for nervous tissue
– in fetus, guide migrating neurons to their destination
– if mature neuron is not in synaptic contact with another neuron is covered by glial cells
• prevents neurons from touching each other
• gives precision to conduction pathways
Neuroglial Cells
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Macroglia in the CNS
1. Ependymal cells
Line cavities of the brain and spinal cordSynthesize cerebrospinal fluid
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2. Astrocytes
most abundant glial cell in CNS
Star-shaped cells
Support neurons
Form barrier between capillaries and neurons (BBB)
Control the chemical environment of the brain (CNS)
2 types: Protoplasmic
and Fibrous
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3. Oligodendrocytes
Produce myelin sheath
around nerve fibers in
the central nervous
system
Nourish neurons
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Microglia
- arise from monocytesof the blood,
Spider-like
Phagocytes
Checked up brain tissue
Dispose of debris
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Supporting Cells of the PNS
Schwann cells - form myelin sheath in the peripheral nervous system
envelope nerve fibers in PNS
assist in the regeneration of damaged fibers
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Supporting Cells of the PNS
Satellite cells – surround cell bodies of neurons in sensory ganglia
provide electrical insulation around the soma
regulate the chemical environment of the neurons
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Nerve fibers
1. Unmyelinated 2. Myelinated
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Myelin• in PNS, Schwann cell spirals repeatedly around a single nerve
fiber
– lays down as many as a hundred layers of its own membrane
– no cytoplasm between the membranes
– neurilemma – thick outermost coil of myelin sheath
• contains nucleus and most of its cytoplasm
• external to neurilemma is basal lamina and a thin layer of fibrous
connective tissue – endoneurium
• in CNS – oligodendrocytes reaches out to myelinate several
nerve fibers in its immediate vicinity
– anchored to multiple nerve fibers
– cannot migrate around any one of them like Schwann cells
– must push newer layers of myelin under the older ones
• so myelination spirals inward toward nerve fiber
– nerve fibers in CNS have no nemma or endoneuriumeuril
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Myelin
• many Schwann cells or oligodendrocytes are needed to cover
one nerve fiber
• myelin sheath is segmented
– nodes of Ranvier – gap between segments
– internodes – myelin covered segments from one gap to the next
– initial segment – short section of nerve fiber between the axon hillock
and the first glial cell
– trigger zone – the axon hillock and the initial segment
• play an important role in initiating a nerve signal
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Unmyelinated nerve fiber:Axones and dendrites are invaginated in Schwann cell
cytoplasm
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Myelinated nerve fibers
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Myelinated nerve fiber structure
Nodes of Ranvier –spaces between 2 Schwann cells –free from myelin
Nodes of Ranvier provide saltatoryconduction of nerve impulse
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SynapseThe specialized region of contact between 2 neurons
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Classification of synapses:by nature:
chemical synapse
electrical synapse
by localisation
axodendritic synapse
axosomatic synapse
axoaxonic synapse
By action:
excitatory synapse
inhibitory synapse
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Sensory Nerve endings(afferent neurons receptors)
Classifications:
By location:1. Exteroceptors,
2. Interoceptors,3. Proprioceptors
By type of stimuli:1. Chemoreceptors,
2. Mechanoreceptors,3. Photoreceptors,4. Thermoreceptors
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Sensory nerve endings
(afferent neuron receptors)
Classification:
By type of the structure:1. A. Free nerve endings
B. Hair follicle nerve ending
C. Merkel nerve endings (Merkel’s disk)
2. Encapculated:
Tactile corpuscle of Meissner
Corpuscle of Pacini
Ruffini endings
3. Muscle spindle
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1. A. Free nerve endings – pain, thermal receptors
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1. B. Hair follicle nerve endings – respond to very light touch
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1. C. Merkel nerve endings – light touch receptors
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1. C. Merkel nerve endings – light touch receptors
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2. Encapsulated = Tactile corpuscle of
Meissner
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2. Encapsulated = Tactile corpuscle of
Meissner
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2. Encapsulated = Tactile corpuscle of
Meissner
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2. Encapsulated. Corpuscle of Pacini
(lamellar body) is specialized to detect
gross pressure changes and vibration
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2. Encapsulated. Corpuscle of Pacini
lamellar body are specialize to detect
vibration
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2. Encapsulated. Ruffini ending
Dense branches of nerve-endings encapsulated in
connective tissue. Is sensitive to skin stretch
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2. Encapsulated. Ruffini ending
Dense branches of nerve-endings encapsulated in
connective tissue. Is sensitive to skin stretch
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