Nervous Tissue Flashcards
What is nervous tissue?
o a communication system in the body that collects stimuli of various types
o transforms or transduces these into electrical impulses
o these pass into a large, highly organized reception and correlation area
where they are interpreted
o and then appropriate responses or sensations are formed
What is the CNS?
Brain
Spinal Cord
What is the function of CNS?
1) integrate afferent sensory information
- exteroceptive (touch, temperature, pain)
- interoceptive (e.g. distension)
- proprioceptive (e.g., stretch)
2) initiate and coordinate efferent responses
3) brain carries out higher mental functions
What is the PNS?
- Cranial nerves (12 pair)
- Spinal nerves (31 pair)
- Peripheral nerves (many)
• Nerves are collections of neuronal cell processes - Ganglia
• ganglia are collections of neuronal cell bodies
What is the function of PNS?
- interconnect tissues/organs with the CNS
• sensory (afferent) fibers
• motor (efferent) fibers
What are the two components of PNS?
Somatic
Autonomic
What is the somatic system?
- regulates receipt of sensation and formation of appropriate motor responses to all parts of body except viscera, smooth muscle and glands
- control here is conscious/voluntary
What is the autonomic system?
- regulates activity in viscera, smooth muscle and glands
- control here is subconscious/involuntary
What is the fundamental functional cell in nervous tissue?
a neuron
What are the functions of neurons?
o designed to receive stimuli and conduct electrical impulses
o arranged in series as part of extensive communications network
What is the general structure of a neuron?
Cell body
Dendrites
Axons
What is a cell body also called?
perikaryon
What is the cell body?
- trophic center for the neuron
- contains the nucleus and most of the organelles
- variable in size and shape
- may receive synapses directly
What is a synapse?
• specialized cell junctions which allow direct cell to cell communication
What are dendrites?
- carry electrical impulses toward the cell body
- usually numerous short cell processes extending from the cell body
- may be highly branched
- major site where synapses are received by the neuron
What are Axons?
- carries electrical impulses away from the cell body
- usually a single long process extending from the cell body
- usually branching is limited to the terminal arborization
- may receive synapses directly
What are the different classifications of neurons?
Bipolar
Pseudounipolar
Multipolar
What are bipolar neurons?
has one axon and one dendrite
typical morphology of neurons of special senses
What are pseudounipolar neurons?
- has one process that seems to run tangentially past the cell body
- typical morphology of sensory neurons
What are multipolar neurons?
- has one axon and multiple dendrites
- most common neuron morphology (99.9%)
- typical morphology of motor neurons and interneurons
Are neurons metabolically active?
Yes, Neurons are highly metabolically active:
- must maintain a very large surface area of cell membrane
- constantly require energy to develop electrochemical gradients
How do cell body’s appear in the microscope?
- Cell body-may appear round, star-shaped or pyramidal
- shape maintained by an elaborate cytoskeleton
- Golgi apparatus and many mitochondria present
- nucleus is usually found near middle of cell body
- euchromatic with a prominent round central nucleolus
What is nissl substance?
intensely basophilic regions of the cell body containing RER
What do dendrites look like in the microscope?
- contain the same organelle constituents as the cell body (except the nucleus)
- usually covered with thorny spines (gemmules)
- sites of synaptic (:Ontact on the dendrite
What do axons appear like in the microscope?
- may be up to 1 meter long
- maintained by an extensive cytoskeletal network running lengthwise in axon
many-.mitochondria located throughout and in the axon terminal
What is an axon hillock?
pale staining region in cell body where axon originates
What is a nucleus?
group of neuronal cell bodies in the CNS
What is a ganglion?
group of neuronal cell bodies in the PNS
What is a tract?
group of neuronal processes (axons) in CNS
What is a nerve?
group of neuronal processes in the PNS
What is a plexus?
network or tangle of nerves and/or ganglia
What is axoplasmic (axonal) transport?
- axon relies on cell body for maintainence
- proteins and other macromolecules that are depleted by synaptic or metabolic activity are synthesized exclusively in cell body
- neuron must have a mechanism for transporting materials through the axon
What is anterograde flow?
from cell body to axon terminal
What is slow stream (anterograde flow)?
- travels 1-5 millimeters per day
* carries sufficient cytosolic and cytoskeletal proteins to the axon terminals for continuing growth and maintenance
What is fast stream (anterograde flow)?
moves about 100 times faster than slow stream (up to about 400 mm/day)
* carries the constituents needed in the axon terminals to replace the macromolecules expended during synaptic transmission
What speed are mitocondria transported at?
Intermediate speed
What is retrograde flow?
- from axon terminal to cell body
- occurs about half as fast as fast anterograde flow (i.e.,-200mm/day)
- returns unused or recycled materials
What is the mechanism of axonal transport?
Kinesin
Dynein
What is kinesin?
Microtubule associated protein ATPase
- moves along microtubules in a minus to plus direction
What is the function of kinesin?
functions in anterograde transport
What is Dynein?
Microtubule associated protein ATPase
• moves along microtubules in a plus to minus direction
What is the function of dynein?
’ functions in retrograde transport
What are general types of synapses?
Axodendritic synapse -axon synapses with dendrite
Axosomatic synapse -axon synapses with cell body
Axoaxonic synapse -axon synapses with another axon
What is the 2 natures of the synapse?
Chemical synapse
Electrical synapse
What is a chemical synapse?
- most common type
• unidirectional communication utilizing neurotransmitters
What are electrical synapse?
-much less common than chemical synapse
• potentially bidirectional communication utilizing gap junctions
‘ limited to smooth and cardiac muscle
What are are the components of a chemical synapse?
Synaptic bouton
Presynaptic terminal
Postsynaptic terminal
Synaptic cleft
What is a synaptic bouton?
- swollen terminal end of the axon
- closely applied to the target cell
- contains many mitochondria and neurotransmitter vesicles
What is presynaptic terminal?
region on the synaptic bouton from which NT is released
What is the postsynaptic terminal?
- region on the target cell membrane
- contains NT receptors and voltage gated channels
What is the synaptic cleft?
20-~0hm space between the pre-and postsynaptic terminals
What is the baseline electrical gradient across the cell membrane maintained by?
Ion pumps
What is the purpose of voltage gated channels in the synaptic activity?
voltage-gated channels, when stimulated by ion pumps causing a threshold, allow dramatic
shifts in sodium and potassium concentrations across cell membrane
What does the influx of sodium lead to?
a rapid membrane depolarization
What is an action potential?
self-propagating transitory wave of depolarization that sweeps
rapidly across the neuron cell membrane (voltage-gated channels) followed by a
return to the electrically polarized resting state (ATP dependent ion pumps and
voltage gated K+ channels)
What happens when an action potential is propagateddown an axon to presynaptic terminal?
voltage-gated calcium channels open briefly at the presynaptic terminal
What does the opening of the voltage-gated calcium channels lead to?
influx of calcium releases stored synaptic vesicles to fuse with the membrane
What does the fusion of synaptic vesicles to the membrane lead to?
neurotransmitters are released into the synaptic cleft by exocytosis
What does the release of neurotransmitters into the synaptic cleft cause?
neurotransmitters diffuse across synaptic cleft and bind to receptors on the
postsynaptic membrane which elicits an electrical change in this membrane
What is an excitatory synapse?
- NT binding triggers a depolarization on the postsynaptic membrane
- mediated by indirectly opening postsynaptic voltage gated cation channels
- promotes generation of action potentials in the postsynaptic neuron
what is an inhibitory synapse?
- NT binding hyperpolarizes the postsynaptic membrane
- mediated by indirectly opening postsynaptic voltage gated anion channels
- suppresses the generation of action potentials in the postsynaptic neuron
What dictates wheather a postsynaptic neuron will generate impulses on its own?
summation of the excitatory and inhibitory impulses
What are the components of the peripheral nervous system?
Nerves
Nerve endings
Ganglia
What are nerves?
- bundles of nerve fibers, their investing cells and CT wrappings
- may contain both afferent/efferent and myelinated/unmyelinated neurons
what are nerve endings?
afferent and efferent endings
What are ganglia?
- nodular masses of neuron cell bodies and support cells
- encapsulated by dense irregular CT
what are sensory ganglia?
contain cell bodies of afferent neurons (pseudounipolar)
ie: dorsal root ganglion
What are autonomic ganglia?
- contain cell bodies of postsynaptic efferent neurons (multipolar)
- ie: sympathetic chain ganglion
What are the connective tissue layers around a peripheral Nerve?
Epineurium
Perineurium
Endoneurium
What is the function of connective tissue layers around a peripheral Nerve?
provide structural support and contain the vasculature
What is epineurium?
- fibrous sheath surrounding moderate to large nerves as a whole
- dense irregular CT organized generally longitudinally on nerve
What is perineurium?
- extensions of the epineurium surrounding fasicles
- dense irregular CT organized generally circularly around each fasicle
What is endoneurium?
- extensions of the perineurium surrounding individual axons
- loose CT organized generally longitudinally on each axon
what will the speed of conduction along nerves limited by?
electrical capacitance and resistance of the axon
What will increasing the diameter of axons do towards speed of conduction?
It iwll increase the speed of conduction but becomes inefficient because giant axons require a high metabolic upkeep
what is an alternative that occurs instead of increasing diameter of axons to increase the speed of conduction?
- to insulate large regions of the axonal membrane
- allows the electrical impulse to ‘hop’ down the axonal membrane
What is a Schwann cell?
support cell found only in PNS
where are schwann cell located?
- located inside the endoneurial tube
- wraps itself around the axon to “insulate” it
what is myelin?
multiple concentric layers of Schwann cell membrane around an axon
What is neurilemma (sheath of schwann)?
- outermost layer of the Schwann cell
- contains most of the cytoplasm and organelles
True or false, a Schwann cell will myelinate the full length of an axon and requires minimal amounts of Schwann cells?
False, each Schwann cell myelinates a limited length of the axon and myelinating an entire axon requires many Schwann cells in series
What are nodes of ranvier?
spaces between adjacent Schwann cells on a myelinated axon
What does the myelin insulate?
the internodal regions of axolemma from the tissue fluid
Ar nodes of ranvier, are axolemma exposed to tissue fluid?
Yes, at nodes of Ranvier, axolemma is freely exposed to tissue fluid
Where are voltage-gated ion channels found in mylenated peripheral nerve fibers?
- voltage-gated ion channels are located at nodes of Ranvier
- provides for a rapid type of impulse conduction involving flow of local currents at nodes of Ranvier in rapid succession
- each current flows (through axoplasm and tissue fluid) from one depolarized node to the next polarized node, depolarizing that node, and so on
is the current leakage between nodes prevalent?
no it is low due to myelin insulation
What is saltatory conduction?
discontinuous form of impulse conduction relating to nerve impulses effectively jump from one node to the next
Are all peripheral nerve fibers myelinated?
no, not all peripheral nerve fibers are myelinated
are unmyelinated nerve fibers surrounded by Schwann cells?
yes, unmyelinated nerve fibers are still surrounded by Schwann cells
Why is no myelin produced in unmyelinated nerve fibers?
- the Schwann cell does not wrap around these cells so no myelin is produced
- the axons are freely exposed to tissue fluid along their entire length
what are remak bundles?
unmyelinated nerve fibers
how do impulses differ in unmyelinated nerve fibers in comparison to myelinated nerve fibers?
impulses travel in a continuous manner which is less rapid than saltatory conduction
which type of impulse conduction requires more energy?
Unmyelinated nerve fibers because energy is needed to pump ions out of the whole length of the cell (as opposed to only pumping out ions at the nodes of Ranvier in myelinated axons)
Can mature neurons be replaced?
mature neurons are generally considered nonmitotic and cannot be replaced
Do damaged neurons have a good capacity to regenerate?
No, damaged neurons have limited capacity to regenerate
- any disruption of axonal transport due to peripheral nerve injury deprives distal portion of axon of essential components synthesized in cell body
What initiates cell body (perikaryon) changes immediately following distal axonal injury?
- chromatolysis
- increased volume
- nucleus shifts to a peripheral position
what inital axon changes occur proximal to injury site?
minimal retrograde degeneration (1-2 internodal segments)
what initial axon changes occur distal to injury site?
- complete anterograde degeneration of the distal axon and associated myelin
- endoneurial macrophages clean up the debris
- the Schwann cells survive and will eventually proliferate
- endoneurium remains as a tubular structure
How is the functional recovery of the PNS susequent to nerve damage?
- PNS regeneration is fairly good (especially when endoneurium is intact)
- Schwann cells seem to promote axon regeneration
How is the functional recovery of the CNS susequent to nerve damage?
CNS regeneration is minimal at best
- astrocytes seem to inhibit regeneration (think about glial scars)
What neuronal changes occur 2-3 weeks following axonal injury?
- the cell body begins to resume its normal morphology
- begins functioning as a trophic center to sustain regenerative growth
- many neurites begin sprouting from the proximal stump of the damaged neuron
What neuronal changes occur 2-3 weeks following axonal injury if endoneurial sheaths remain intact?
- the chance for successful regeneration is reasonably good
- the Schwann cells proliferate distally to form a column to the target
- a neurite successfully crossing the damaged site will be guided back
to the target by this column of cells
What neuronal changes occur 2-3 weeks following axonal injury if endoneurial sheaths are severed?
- the chance for successful regeneration is much more remote
- Schwann cells will proliferate to try and bridge the gap
- neurites will be free to grow laterally instead of just distally
- the likelihood of neuroma formation is great
What is nerve grafting?
- provides endoneurial tubes through which regenerating neurites may grow, sometimes effective
can normal function be restored if sensory neurites grow down a motor column?
No, if sensory neurites grow down a motor column (or vice versa) normal function cannot be restored
what will happen if a peripheral nerve is partially severed?
Collateral sprouting will likely occur
ie:
- 75 of 100 fibers in peripheral nerve are cut/damaged
- remaining 25 fibers sprout nerve collaterals (at nodes of Ranvier)
What happens when collateral sprouting occurs?
Collaterals assume the position of the damaged fibers and reinnervating the lost motor or sensory filed
What are the advantages and disadvantages of collateral sprouting?
The remaining nerve fibers have expanded spheres of influence and thus more sensory receptors/ muscle fibers are innervated by a single neuron (returning function)
Disadvantage: less fine motor control or discrimination of sensory information
What is phantom pain?
sense pain as coming from the limb that has been amputated
who experiences phantom pain?
experienced by people who have had a limb amputated
What is the cause of phantom pain?
sensory nerves which used to receive stimuli from amputated limb are now being stimulated by irritation to amputated stump
True or false, cns has a large amount of connective tissue?
False, CNS has very little connective tissue
- CT present only around blood vessels
- minimal CT leaves CNS very soft
what are neuroglia or glial cells?
Support cells for neurons
what are the differences between neurons and glial cells?
approximately 10 glial cells/neuron
glial cells are mitotic; neurons are not mitotic
-glial cells do not form synapses
- glial cells do not have action potentials
- special techniques are required to see whole glial cells
- H&E only allows visualization of glial cell nuclei
what are the different types of neuroglial cells?
oligodendrocytes
Atrocytes
Microglia
Ependymal cells
where are oligodendrocytes found?
found in both gray and white matter
what is the appearance of oligodendrocytes?
- cell body contains most/all the organelles
- cell processes radiate away from the cell body
what is the function of oligodendrocytes?
- produce segmental myelin sheaths around CNS neurons
- one oligodendrocyte can myelinate segments of several CNS neurons
- each cytoplasmic process may wrap around sections of different neuronal axons or multiple sections of a single neuronal axon
what is multiple sclerosis/
- produce segmental myelin sheaths around CNS neurons
- one oligodendrocyte can myelinate segments of several CNS neurons
- each cytoplasmic process may wrap around sections of different neuronal axons or multiple sections of a single neuronal axon
what are astrocytes?
primary supporting cell in CNS, largest glial cell type
what are fibrous astrocytes?
most common in white matter
- relatively few, straight processes
What are protoplasmic astrocytes?
most common in gray matter
- numerous short branching processes
what are endfeet?
- expanded tips of the astrocyte cell processes
- cover large areas on the outer surface of CNS blood vessels (blood brain barrier)
- cover nodes of Ranvier and synaptic clefts
- form a layer on the outer surface of the CNS (glia limitans)
what is the function of astrocytes?
- during development, form structural framework to guide migration of developing neurons
- provide physical support to CNS cells and structures
- expedite the metabolic exchange between neurons and blood vessels
- assist in the removal of extracellular neurotransmitter
What is gliosis (glial scar)?
- occurs in response to neuronal death & subsequent macrophage cleanup
- astrocytes rapidly proliferate and fill the defect with an astrocytic scar
what is microglia?
seem to be a special kind of resting macrophage
What is the appearance of microglia?
smallest glial cell type
- thought to be derived from monocytes
what is the function of microglia?
normally found in relatively low numbers but they are mitotic
become actively motile and phagocytic if the CNS is damaged
what are ependymal cells?
line brain ventricles and central canal in spinal cord
what is the morphology of edendymal cells?
- cuboidal to columnar cells with cilia and microvilli
- no basement membrane
- desmosomes present but no occluding junctions
what is the function of ependymal cells?
participate in cerebrospinal fluid production and circulation
What is cilia?
hair-like projections on the apical surface of some lining cells
- apical cytoplasmic extension covered by cell membrane (3-10 m long, 250 nm wide)
- microtubule axoneme at the core anchored at basal body
what is the blood-brain barrier and it’s function?
- this blood-brain barrier protects CNS neurons from toxic drugs, bacterial toxins and many other potentially harmful substances occasionally seen in the -bloodstream
can antibiotics and other large or electrically charged moltecules penetrate the blood brain barrier?
No, antibiotics, such as penicillin, and other large or electrically charged molecules (not lipid soluble) do not penetrate most parts of the CNS to any extent from the bloodstream
is CSF restricted like the blood brain barrier?
- access via CSF is not similarly restricted i.e., CSF and CNS extracellular fluid seem to be in equilibrium
- therefore, the barrier seems to be capillary dependent
Anatomical feutresfactors that contibute to the blood brain barrier?
CNS capillary endothelial cells are unique in that they have zonula occludens
most capillary endothelial cells in the body have fascia occludens
- CNS capillary endothelial cells are unfenestrated
- many capillary endothelial cells in the body are fenestrated
- CNS capillary endothelial cells have few pinocytotic vesicles
- astrocyte endfeet are known to encircle CNS capillaries
what are the solute characteristics?
- size of molecule seems to be important
- lipid solubility enhances transport of substances
What is gray matter?
primarily contains cell bodies of neurons and neuroglia
What is white matter?
- composed primarily of myelinated neuronal processes
- no neuronal cell bodies present, some glial cell bodies present
- sometimes nuclei of gray matter are seen (in brain)
what are the differences between gray and white matter?
- both are vascular; gray matter more vascular than white matter
- distinguishable grossly in both brain and spinal cord
What are neuropil?
histological description of gray matter
- includes dendrites, proximal unmyelinated axons and glial processes
- CNS area in which most synapses occur
In the spinal cord, where does gray matter occupy and contain?
- occupies central region (“H” shaped)
- bilateral anterior and posterior horns (or ventral/dorsal horns)
- combined anterior and posterior commisures
- central canal (possibly obliterated)
In the spinal cord, where does white matter occupy and contain?
- completely surrounds the central gray matter
* ascending and descending tracts .
What divides the cerebrum into 2 hemispheres?
Deep longitudinal fissure
What causes deep folds of the cerebrum to increase surface area?
Gyri and Sulci
What are gyri?
Rounded elevations
What are sulci?
Grooves between the gyri
What are the 6 distinct superficial layers of gray matter in the cerebrum?
I molecular layer II outer granular layer Ill outer pyramidal layer IV inner granular layer V inner. pyramidal layer VI .fusiform layer
What is the function of sensory cortex of the cerebrum?
registe’r and interpret afferent (sensory) impulses
What is the function of the motor cortex of the cerebrum?
efferent neurons controlling voluntary movements and
skeletal muscle tone
Where is the gray matter in the cerebrum?
Located peripherally
Where is the white matter in the cerebrum?
central in the brain
• consists of neuronal tracts running to/from cortical areas
What is the function of the cerebellum?
- cerebellum is responsible for subconscious fine tuning of voluntary
movements and skeletal muscle tone - coordinates contraction of different groups of skeletal muscles, enabling
such movements to be carried out smoothly and with precision
How many layers are the gray matter of the cerebellum organized into?
gray matter organized into 3 layers
What are the 3 layers of gray matter in the cerebellum?
Molecular layer
Purkinje layer
Granular layer
What is the molecular layer of gray matter in the cerebellum?
outermost layer of small neurons
What is the purkinje layer of gray matter in the cerebellum?
distinctive neurons with flask shaped cell bodies and
extensively branching dendrites that extend up into molecular layer
What is the granular layer of gray matter in the cerebellum?
innermost layer of small neurons
What is the white matter of the cerebellum?
medullary core in each of the large cerebellar folds
What are meninges?
connective tissue coverings around brain and spinal cord
What is the pia matter?
o thin layer of loose CT continuous with CNS perivascular CT
o tightly adherent to all CNS gyri and sulci
o the surface facing the arachnoid is lined by a thin squamous epithelium
o this layer is adjacent to the glia limitans
- innermost layer
What is the arachnoid layer?
- middle layer
- membranous roof and irregular trabeculae just below roof
- bridges over the CNS gyri and sulci
What is the subarachnoid space?
- between pia and the membranous arachnoid
- vasculature is located here
- lined by a thin squamous epithelium
- filled with CSF
What is the cerebrospinal fluid?
- cushioning fluid of the CNS
o fills brain ventricles, subarachnoid space and remnants of central canal (
o modified plasma dialysate with low protein content and few lymphocytes
Where is cerebrospinal fluid made?
Formed by choroid plexuses
- usually wide leaky capillaries- covered by ependymal cells
- ependyma modifies the plasma dialysate
- ependyma helps CSF circulate through ventricles and subarachnoid space
What is dura mater?
- outermost layer; dense irregular CT
o relatively thick and very inextensible
o fuses with the periosteum of the cranial bones
o forms folds which compartmentalize the cranium and support parts of the brain
o contains endothelially lined venous sinuses
What is a subdural hematoma?
potential space between dura and arachnoid
What is an extradural hematoma?
potential space between dura and cranial bones
What is a subarachnoid hemmorage?
bleeding into the subarachnoid space
What is meningitis?
inflammation of the CSF in the subarachnoid space
