7.3 - Lecture - Nerve Flashcards
What are the three neuron classifications
- unipolar (pseudounipolar)
- bipolar
- multipolar
What defines each of the three classification types
- they are based on the relationship of the cell body (soma) to the peripheral processes, i.e. axons and dendrites
Nissl Bodies
- in the LM sometimes help to recognize cell bodies of neurons
- representing large amount of basophilic ER
Axodendritic Synapses
- axon synapsing on a dendrite
Presynaptic element
- part of synapse that contains a region specialized for the vesicular release of NT
Postsynaptic element
- part of synapse that contains membrane receptors that convert the chemical signal from the NTs into a membrane potential
Axosomatic synapse
axon synapsing on the soma of a neuron
axoaxonic synapse
- axon synapsing on the exposed portion of an axon - nearer to the soma on the axon
What are the two locations/names for presynaptic elements
1) Bouton en passant - if the presynaptic element occurs along the length of the axon
2) terminal bouton if the presynaptic element occurs at the end of the axon
What is a Dendritic spine in relation to a synapse
- postsynaptic element normally occurs here
Axon hillock
- typically the area of a cell at which an AP (spike) is generated
Describe an AP in Myelinated axons
- spikes propagate via saltatory conduction
- spikes are regenerated at nodes of Ranvier
Describe an AP in unmyelinated axons
- spikes travel more slowly due to the lack of a myelin sheath
Oligodendrocyte
= the support cell of the CNS responsible for making myelin
Schwann Cell
= the support cell of the PNS responsible for making myelin
What are the two appearances of myelin sheaths in EM
alternating:
1) major dense lines
and
2) Intraperiod lines
Major Dense Lines
= two apposed cytoplasmic faces of the unit membrane
Intraperiod lines
= two apposed extracellular faces of the unit membrane
Inner mesaxon
= ending of cell membranes contributing to the myelin
Outer mesaxon
= ending of cells membrane contributing to the myelin that connects to the cell body of the support cell
Glial cells
= non-neuronal cells
- fxns: scaffolding, metabolic support of neurons, contributing to the blood-brain barrier
include:
- astocytes, fibrous astrocytes, protoplasmic astrocytes, microglia
Astocytes (astroglia)
- in the CNS
- contain unique intermediate filament - glial fibrillary acidic protein (GFAP) - useful for recognition by antibody staining
Glial Fibrillary acidic protein (GFAP)
- a unique intermediate filament
- found in astrocytes in the CNS
Ganglia
- collections of cell bodies outside the CNS
Satellite cells
- support the neuronal cell bodies in the ganglia
Nucleus (not organelle)
- a discrete group of cell bodies inside the CNS (in the brainstem mostly)
e. g. brainstem nuclei, paraventricular nucleus, lateral geniculate nucleus
Grey matter
- CNS organization largely containing neuronal cell bodies, neuropil, and glial cells
Neuropil
mesh of axons and dendrites
white matter
- CNS organization largely containing axons + oligodendrocytes
The three covering of the brain
are meninges - three layers of fibrous tissue
1) pia mater - thin/wispy
2) Arachnoid mater
3) Dura mater - extremely tough
Subarachnoid space
- common site of brain hemorrhage
Two classifications of neurons in the periphery
1) Sensory
2) Motor
Motor neurons in the PNS
- motor neurons of the spinal cord - directly control skeletal muscle
- axons of motor neurons from specialized NMJs with the muscle fibers and post-synaptic region of the muscle cell plasmalemma= motor end plate
- leave the spinal cord and pass through ventral roots
Sensory Neurons in PNS
- pass through the dorsal root ganglion
- cell bodies are located in the DRG
Motor end plate
= the post-synaptic region of the muscle cell plasmalemma
ANS
- innervates organs and glands
- has 2 divisions
1) parasympathetic
2) Sympathetic
3) Enteric NS - in the gut
Sympathetic ganglia
located between the spinal cord and organ of innervation
Parasympathetic ganglia
located immediately adjacent to the innervated organ
Perineurium
- CT encircles large nerve fibers
Epineurium
= CT that is adventitial between the nerve fiber and surrounding structures
Endoneurium
= fine CT immediately surrounding axons
Endoneurial tubes (Schwann tubes)
= tubes in which axons travel made by schwann cells
- important structures in axon regeneration
Describe the characteristic nuclear morphology of a neuron
- large spherical euchromatic nucleus
- contains very prominent nucleolus (tells you the nucleus is active)
What are Nissl Bodies and the Nissl Stain
- correspond to stacks of ER that fill the neurons because they are very actively producing proteins
- shown by the Nissl Stain - accumulating in cytoplasmic globules of dye = nissl bodies - charcateristic of neurons
Golgi Stain
- stains only 1-5% of the neurons –> allows visualization of a neuron and its processes
- myelin disturbs Golgi stain
Describe the typical characteristics/morphology of a local process or dendrite
- multiple dendrites emerge from soma at different sites and branch extensively at less than 90 degree angles
- dendrites tape with increasing distance from cell body
- appears studded with Golgi stain
Describe the typical characteristic/morphology of a distal process or axon
- one axon
- emerges from cell body at a specialized region = axon hillock
- maintains fix diametere throughout its long course
- branches emerge at 90 degree angles
- appears smooth in Golgi stain
Describe the fine structures of dendrites
- microtubules
- mitochondria
- large dendrites - contain ER, ribosomes, neurofilaments
- dendritic spines
What are dendritic spines
= specialized evaginations of dendritic plasmalemma that receive signals
- each spine can move (increase or decrease in size) - they are highly dynamic - each receives a synapse
Describe the fine structures of an axon
- neurofilaments
- microtubules
- stray mitochondria
- no rER or free ribosomes
Describe the morphology of axons in Nissl stains
- axons don’t stain with nissl (because do not contain ER)
Describe the morphology of axons in H and E stains
- eosinophilic core = axon
- white space = myelin
- can see perpindicular line = indicating node of Ranvier of myelinated axons (in longitudinal sections)
Describe the morphology of axons in osmium and what osmium stains for
- Osmium is a heavy metal stain for lipids
- therefore it will show myelin staining around myelinated axons
Compare myelination of axons in the CNS and PNS
- in PNS 1 schwann cell contributes 1 internode of myelin to a single axon
- in CNS 1 oligodendrocyte contributes internodes to multiple axons
Describe the morphology of unmyelinated axons in H and E stains and their relation to the myelin-producing cells
- they appear as froth in H and E stains
- many unmyelinated axons are enveloped by a single supporting cell - which offers partial support (not to the level of myelination though)
- i.e. Schwann cells that are not involved in myelinating an axon encapsulate a group of unmyelinated axons
Describe the different axonal terminations
- axo-dendritic (on a dendrite)
- axo-spinous (on the dendrite spine)
the above ones are the most common - axo-somatic (on the soma)
- axo-axonic (on the inital segment of an axon)
- in rare cases dendrites can have vesicles and be presynaptic to other vesicles
What are the two types of synapses and are they differentiable in EM?
- The two types are excitatory and inhibitory and they are differentiable in EM
Describe the characteristics of Excitatory Synapses
use the acronym WEAR
- Wide synaptic cleft
- Excitatory
- Asymmetric synapse
- Round Vesicle
Describe the characteristics of Inhibitory Synapses
use acronym PINS
- Pleomorphic vesicles
- Inhibitory
- Narrow Synaptic Cleft
- Symmetrical Synapse
Describe the multipolar classification of neurons
- Multiple processes into and out of the cell body- Multiple dendrites + multiple axons
ex. Chandelier cells with “candles” axons - effective rapid inhibitory neuron - shuts down cells very effectively
ex. Purkinje Neurons
Describe the Bipolar classification of neurons
- morphologically simlified neurons
- 1 input process
- 1 output process
- only found in retina and olfactory bulb
Describe the Unipolar classification of neurons
- only found in sensory ganglia of the PNS –> DRG
- AP initiation and summation happens at sensory end
- AP signal doesnt have to go though the cell body (unlike multipolar or bipolar neurons)
- almost always myelinated
- have satellite cells (subclass of Schwann cell)
2 Differences of CNS and PNS
- supporting cells/elements
- response to injury
Describe the relative size of a supporting cell
- essentially equivalent to nucleolus of neuron –> (might only apply to unipolar neurons???)
What are the non-neuronal cells of the CNS
- generally known as glia (glue)
- support neuronal function and provide structure
1) Macroglia
1A) Astrocytes
1B) Oligodendrocytes
2) Microglia
Describe astrocytes
- star shaped marcoglial cells of CNS
- provide structure to brain and fill space between neuronal processes (CNS doesnt have CT)
- Contains intermediate filaments - GFAP in cytoplasm to provide structural support
- maintain stable ionic medium around nucleus by taking up excess NT and regulating ionic composition
- provide metabolic support to neurons
- forms glial scar - by proliferating and forming scar in regions of CNS damage
What are the types and localizations of the two types of astrocytes
1) Protoplasmic Astrocytes - in grey matter
2) Fibrous Astrocytes - in white matter
Describe Oligodendrocytes
- macroglial cells of CNS
- each cell gives rise to several processes that each forms myelin internode on a different axon
Describe Microglia
= phagocytic cells that patrol the brain + spinal cord
- are part of the Mononuclear phagocytic system (MPS)
- derived from bone marrow - from monocytes (like macrophages, alpha-synoviocytes, osteoclasts)
- incredibly dynamic cells - highly motile within nervous tissue
- responsible for immune surveillance
What are the nonneuronal cells of the PNS
- nerves in periphery are ensheathed, organized, supported by CT + the microenvironment bathing the neuronal elements in PNS is strictly controlled by: Epineurium Perineurium Endoneurium External Lamina Schwann Cells Satellite Cells
Function of epineurium in PNS - in relation to support cells it provides
- dense CT sheath around perineurium
- separating perineurium from rest of tissues
Function of perineurium in PNS - in relation to support cells it provides
- collagen fibers with squamous cells
- surround and support nerves
Function of endoneurium in PNS - in relation to support cells it provides
- type III collage fibers (reticular fibers) - between myelin sheaths
Function of external lamina in PNS - in relation to support cells it provides
- basal lamina surrounding schwann cells
Function of Schwann cells in PNS - in relation to support cells it provides
- surround myelinated and unmyelinated axons in peripheral nerves
Function of Satellite cells in PNS - in relation to support cells it provides
- a subclass of Schwann Cell
- encapsulate neuronal cell bodies in peripheral ganglia (DRG)
- protect cell - joined together by gap JXNs - to separate it from ECF
Which support cell is responsible for the glial function of physical support in CNS and PNS
CNS - Astrocyte
PNS - CT
Which support cell is responsible for the glial function of Control of Microenvironment in CNS and PNS
CNS - Astrocyte
PNS - Schwann Cell/satellite cell
Which support cell is responsible for the glial function of Myelin synthesis in CNS and PNS
CNS - Oligodendrocyte (multiple internode per cell)
PNS - Schwann Cell (1 internode per cell)
Which support cell is responsible for the glial function of Defense/Immune in CNS and PNS
CNS - Microglia
PNS - Immune System
What happens during peripheral nerve injury with respect to retrograde RXN?
Retrograde RXN
- produce cell that completely cahnges morphology
- Nissl substance dissolves
- undergoes chromatolysis
What happens during peripheral nerve injury with respect to Local Changes?
- Axon terminals are completely dependent on the soma organelles
- so the axon and its associated wrapped myelin cytoplasm are phagocytosed (but the schwann cell remains intact)
What happens during peripheral nerve injury with respect to Anterograde RXN?
- Distal Axon and myelin are phagocytosed
- Schwann cells proliferate and form tubes = Schwann tubes = Band of Bungner
- cell body begins heightened production and proximal axon sprouts and growth into the Schwann tubes
- Schwann cells wil re-myelinated and guide the axon to its target
What happens if their is failure of regeneration
- traumatic neuroma occurs - results is sprouting axon cannot reach Schwann cell tubes and therefore cannot reach its target
Describe Central Nerve (CNS) Regeneration
- Regeneration in CNS is extremely rare
- ECM (External lamina) does not exist in CNS and cannot guide axon sprouts
- Schwann cells secrete chemical factors that attract and encourage growth of axon sprouts in PNS, but oligodendrocytes do not
- Central myelin = a potent inhibitor of axonal growth in CNS
- After CNS injury - astrocytes fill in the spaces and form a physical barrier (gliotic scar) to regeneration
Why does regeneration work in the PNS but not CNS following nerve damge
- largely due to the CT organization in the PNS
- external lamina
- ECM
- schwann cells - which secrete chemical factors to aid targeting and encourage growth as well as form Schwann tubes/Bands of Bungner
==> all of these factors aid/guide the axonal growth and are not present in CNS
