Lecture 7 - Nerve Tissue Flashcards
nerve tissue - how cellular, integration and transmission, what is it specialized for, two key attributes
- highly cellular tissue with small amount of connective tissue component
- rapid integration & transmission of information, potentially over long distances & to/from many sites
- tissue specialized for both signal transmission & signal integration
- neurons have two key attributes:
- irritability: respond to stimuli from environment or from other cells
- conductivity: propagate & spread signal electrically to other neurons or non-neuronal cells
nerve tissue - what are functional cells called and what do they do, what about shape, what are support cells called and what do they do overall, what are there differences in
- functional cells are neurons: receive and transmit info
- due to cell shape, many regions of nerve tissue don’t show neuron cell nuclei
- support cells are glial cells
(glia): physical protection and
physiological support - differences in neuron shape and size, and in types of glial cells in various regions of CNS & PNS
neuron - is a neuron a nerve, where is informatino received and how its passed to next cell, what does they cell body contain and what happens there, what are dendrites
Neuron (≠ Nerve):
* information is received by cell &
spread to other parts of the system via neuron processes
* information passed to next cell at synapses
* cell body (soma) contains nucleus, organelles and cytoplasm
* produces materials and
organelles to maintain healthy cell processes
* Nissl bodies/substance = rER
and poly- ribosomes; abundant in
soma
* dendrites are branched &
specialized processes extending
from the soma
neuron - what is an axon hillock and what does it do, what is an axon and what does it do, what is an axon terminal and what does it do
- axon hillock is pyramid-shaped
region of soma; summation of inputs & generation of action potential occurs immediately distal to this point - axon is single (occasionally
branched) long process ending at
synapses - transmits the action potential as
wave of depolarization - axonal terminal (bouton) converts action potential into chemical signal
- participates in synapse with cell
receiving message (another neuron, or a non-neuronal cell)
neuron morphology - is it variable and what are the distinctions, what are most neurons, describe pseudounipolar neurons, functions of bipolar, multipolar and unipolar/pseudounipolar neurons
neuron morphology is variable
* gross anatomical and functional
distinctions
* most neurons are multipolar with extensive branched dendrites & single axon
* pseudounipolar neurons have
branched axon split into
peripheral & central processes
* ‘nerve fibre’ often used since
functionally ‘axon’ or ‘dendrite
-bipolar = cranial species senses,
-multipolar = motor neurons, interneurons
-pseudounipolar/unipolar = spinal sensory neurons
action potential - when is cell polarized, what leads to opening on membrane ino channels, when is cell depolarized, hyperpolarized, how is an action potential formed
- resting membrane potential of neuron has cell relatively –ve charge (-70mV) compared to extracellular: cell is polarized
- stimulus leading to opening of membrane ion channels can reduce relative –ve charge (if Na + channels open) or increase relative –ve charge (if Cl - channels open)
- shift towards +ve membrane potential is excitatory: cell is depolarized
- shift towards more –ve membrane potential is inhibitory:
cell is hyperpolarized - if sum of ion movement at the axon hillock is net +ve charge, a wave of depolarization is propagated down the axon as an action potential or nerve impulse
AP - how is AP propagated, what rapidly repolarizes, what is a refractory period, how is the AP passed onto next cell
- action potential propagated by sequential opening and closing of
voltage-gated ion channels (Na+/K+) - patch of membrane that has just depolarized rapidly repolarizes
- has brief refractory period where it cannot depolarize- ensures action potential propagation is in one direction (from soma towards periphery/axon terminal)
- action potential is passed to next cell in signaling chain at synapse, where electrical potential can be converted into chemical signal
chemical synapses - what is a synapse, direction, what can it contact, where does it convert electrical chemical go, how does it act, what does it lead to
Synapses
* physical structures with swellings (terminal boutons) that make contact with next cell in signalling chain
* transmission is unidirectional
- can contact cell soma,
dendrites or another axon - convert electrical signal
(action potential) from the
presynaptic cell into
chemical signal that affects
the postsynaptic cell - act by releasing neurotransmitters: bind
receptors on postsynaptic cell - lead to either opening or closing ion channels or to 2nd messenger cascades in postsynaptic cell
release of neurotransmitter - steps, how does hyperpolarization happen
- action potential in presynaptic cell opens Ca 2+ channels in bouton membrane
- Ca 2+ influx triggers exocytosis of neurotransmitters from synaptic vesicles into synaptic cleft
- neurotransmitters bind receptors on the postsynaptic cell membrane (recipient cell)
- produces either an excitatory or
inhibitory effect at the postsynaptic membrane - Excitatory synapses opens
postsynaptic Na+ channels;
depolarization and action potential in the neuron or effector cell - Inhibitory synapses open anion channels: hyperpolarization
myelinated and unmyelinated axons - what is speed related to, is slow transmission appropriate, what is resistance related to, what is there a limit to, what else reduces resistance,
- speed of action potential is inversely related to resistance of
nerve fibre - slow transmission is appropriate for some signals (regulatory, tonic, etc.) but not for control of motor neurons or for reception of dangerous signals
- resistance related to fibre diameter (smaller diameter = more resistance)
- limit to how large axon diameter can be & how many large axons there is room for
- nerve fibre resistance is also reduced by insulation with myelin
myelin - what is in, what cells in PNS vs CNS, appearance in paraffin sections vs osmium tetroxide in plastic in TEM
- lipid rich covering of multiple wrapping cell membranes of glial cells
- Schwann cells (neurolemmal cells) in PNS and oligodendrocytes in CNS
- in paraffin sections, lipids are extracted so myelinated nerve
fibres are poorly stained - in tissue fixed with OsO4 (plastic
and TEM) lipid is stabilized and
myelin sheath is deeply stained
schwaan cells - where is there a chain, what is the small gap called and for, what is the jumping called
- chain of Schwann cells along length of axon from hillock to
terminal - small gap (node of Ranvier/neurofibril node) between each cell leaves region of axon un-insulated
- node regions of axons are only place ion movement across membrane can occur to trigger
depolarization - action potential ‘jumps’ from node to node via saltatory conduction
4 steps in the formation of a myelin sheath
1 - schwaan cells starts to wrap around a portion of an axon
2 - schwaan cell cytoplasm and PM begin to form consecutive layers around axon
3- the overlapping inner layers of the schwaan cell PM forms the myelin sheath
4- eventually, the schwaan cell cytoplasm and nucleus are pushed to the periphery of the cell as the myelin sheath is formed
unmyelinated axons - are all in the PNS myelinated, where do they sit, how many fibers per schwaan cell, 2 steps for formation
- not all axons in PNS are myelinated, but all are associated
with Schwann cells - unmyelinated fibres sit in groves in Schwann cell
- multiple unmyelinated fibres per Schwann cell
1-schwaan cell starts to envelop multiple axons
2- the unmyelinated axons are enveloped by the schwaan cell but there are no myelin sheath wraps around each axon
myelinated vs unmyelinated - diameter, where they site, how many axons per cell
myelinated axons have larger diameter than unmyelinated
unmyelinated axons sit in grooves within schwaan cells; multiple axons per cell
peripheral nerve organization - how is a peripheral nerve created, what is the endoneurium vs perineurium vs epineurium, what produces the blood nerve barrier, what are large vs small peripheral nerves composed of, what do large nerves run with, what do they establish communication between and what type of fibers do they contain
- axons and Schwann cells are enclosed within layers of connective tissue to create a peripheral nerve
Endoneurium: reticular fibers, capillaries & fibroblasts
* thin layer; surrounds each axon
Perineurium: 1-6 layers of large squamous-like cells (‘fibrocytes’), surrounding axon bundles (fascicles)
Epineurium: dense, irregular CT coat
* covers the exterior of peripheral nerve
* CT extends between fascicle bundles to fill in space
- cell-cell edges sealed by
tight junctions & externally
covering basal lamina to
produce blood-nerve
barrier - large peripheral nerves are composed of multiple fascicles
- smaller nerves may be only single fascicle
- large peripheral nerves frequently run together with large blood vessels (e.g. femoral artery, vein & nerve)
- establish communication between the CNS and sense organs or effectors (e.g., muscle, glands)
- contain both efferent (outflow) and afferent (input) fibers.
peripheral nerve neurons - where are neuron cell bodies found, afferent (cell body location and where they enter), efferent (cell bodies and where it leaves)
neuron cell bodies are only found in gray matter of CNS or in ganglia
Afferent:
* cell bodies of pseudounipolar
sensory neurons are in dorsal
root ganglia on ‘dorsal root’
(posterior root) of spinal nerve
* axons enter spinal cord in dorsal root and synapse in grey matter of spinal cord
Efferent:
* cell bodies of multipolar motor neurons are in ventral horn of spinal cord grey matter
* axons leave spinal cord in
‘ventral root’ (anterior root)
ganglia - what are they are where are they found, what is the neuron cell body surrounded by, what do the ganglia have, sensory afferent ganglia impulses and associations, autonomic efferent ganglia impulses and association
aggregates of neuronal cell bodies outside CNS
* neuron cell body surrounded by glial cells called satellite cells
* ganglia have delicate dense CT with or without capsule
Sensory (afferent) ganglia:
* receive impulses that go to
CNS
* associated with cranial
nerves and dorsal roots of
peripheral (spinal) nerves
* involves pseudounipolar
neurons
* glanglion has distinct CT
capsule
Autonomic (efferent) ganglia:
* associated nerves affect smooth
muscle, some glands, heart
* two-neuron systems:
pre-ganglionic neuron fibres from
CNS synapse with post-ganglionic
neurons in ganglia
* involves multipolar neurons
* generally less well developed CT
capsule; can also be microscopic
structures (in walls of organs)
satellite cells - what is each cell body enclosed by, what do they form in sensory ganglia vs autonomic ganglia
- in sensory ganglia satellite cells
form layer over surface of pseudounipolar neuron cell bodies
-Each cell body is fully enclosed by it own sheath of satellite cells - in autonomic ganglia satellite cells form layer over surface of multipolar neuron cell bodies
what are four CNS glial cells and their functions
oligo - nerve fiber myelination in CNS
astrocytes - regulation of BBB, support for neurons
microglial - resident macrophages of CNS
ependymal cells - lines cavities in CNS, regulate CSF levels
CNS glial cells - oligodendrocytes (what are they similar to, diameter, unmyelinated axons), ependymal cells (what are the like and what do they line, what do they give rise to)
Oligodendrocytes (oligodendroglia)
* similar to Schwann cells but each one myelinates a few (rather than single) axons
* axons in CNS tend to be smaller
diameter
* unmyelinated axons don’t associate with oligodendrocytes: naked
Ependymal cells
* epithelial-like cells lining spinal
central canal & brain ventricles
* ciliated cells involved in regulating cerebral spinal fluid by absorbing excess
-also give rise to choroid plexus which produces CSF
microglia - derived from what, how many compared to neurons, where is it distributed, how they function, what happens when activated
- monocyte-derived antigen-
presenting cells of the CNS - less numerous than astrocytes but nearly as common as neurons
- evenly distributed in both gray
and white matter - microglia not interconnected; are motile cells, constantly
involved immune surveillance of CNS - when activated, loose processes and become phagocytic
astrocytes - how % of glial cells do they make up, what is their functions
- ‘star’ shaped cells; 20-40% of
CNS glial cells - astrocytes cover CNS capillaries- help induce/maintain restricted
permeability in brain capillaries:
blood-brain barrier - astrocytes also modulate neuron health and functions
(e.g. release neurotransmitters
ANS - divisions, type of cell bodies within and where they are, 2nd neuron cell bodies where
parasympathetic
-cell bodies of preganglionic neurons (1 st neuron) in brain
stem or spinal cord
* 2 nd neuron cell bodies in small
ganglia nearby (cranial nerves) or within wall of target organ
-sympathetic
* cell bodies of preganglionic
neurons (1 st neuron) in spinal cord
* 2 nd neuron cell bodies in ganglia along the vertebral column (sympathetic chain) or in large visceral ganglia
