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
