Nervous tissue Flashcards
nervous system
allows rapid and specific communication between widely spaced areas of the body
specialised nerve cells
neurons gather and process information and generate appropriate response signals
2 parts of nervous system
central nervous system
peripheral nervous system
CNS
brain and spinal cord
PNS
nerves running between CNS and other tissues
nerve relay stations - ganglia
neurons functions
network of highly specific connections between different groups of cells
gather information from sensory receptors
process information and provide memory
generate appropriate signals to effector cells
what are neurons characterised by?
cell body
axon
dendrites
synapses
neuron cell body
contains nucleus and most organelles
axon
long cell process stretching from cell
transmitting signals from neurons to cells
dendrites
numerous short cell processes
increase SA available for connecting with axons of other neurons
synapses
specialised cell junctions between axon and other cells to allow direct communication
determination of functional attributes of the nervous system
determined by network of connections between neurons, not on specific structural features of individual neurons
neuron activity
highly metabolically active
maintain big SA of membrane
energy to develop electrochemical gradients
histological appearance of neurons
large rounded nucleus large central nucleolus RER Nissl substance - purple stained granules Golgi mitochondria lysosomes residual bodies containing lipofuscin
Nissl substances
purple stained granules in neurons cell body (perikaryon) and dendrites, not axon
types of neurons
motor neurons
sensory neurons
interneurons
motor neurons
large cell body
metabolic support for large axon
many dendritic processes - multipolar neurons
sensory neurons
unipolar cells
one major process
divides into 2 branches, one running to CNS and one to sensory area of body
interneurons
small, simple cells
short processes providing local connections within CNS
bipolar - 2 main processes of equivalent size, one dendritic and one axonal
intermediate filament of neurons
neurofilament protein
acts as internal scaffold to maintain shape of axon and cell body
neurofilaments in axons
membrane proteins are anchored in place in organised pattern by attachment to cellular neurofilaments
microtubules in neurons
transport substances and organelles up and down axon
metabolic maintenance of long cell process of axon
requires transport system for organelles, enzymes and metabolites from cell body
slow axonal transport
enzymes and elements of cytoskeleton transported down axon at 1-5mm/day
anterograde fast axonal transport
membrane bound organelles transported at speeds of 400 mm/day
mediated by microtubular transport mechanisms using kinesin as motor
retrograde axonal transport
effete organelles and recycled membrane from synaptic ending back to neuronal cell body at 300 mm/day
mediated by mechanisms using dynein as motor
how is neuronal signalling controlled?
electrical (ionic) gradient across membrane
neuronal firing
depolarisation of membrane
resting axon has negative membrane potential (-70mV)
depolarisation is propagated along axon at 100m/s
membrane ion pumps
maintain baseline electrical gradient between inside and outside of cell
widely distributed
ion channel proteins
modify electrochemical gradient across membrane
form gates/pores which switch permeability to ions in response to specific signals
gated channels
gates switch permeability to ions in response to specific signals
ligand gated channels
close or open in response to binding to chemical transmitter substances
synapses
voltage gated channels
explosive and rapid depolarisation occurring as cells fire
widely distributed in cell membrane
depolarisation and small current
locally depolarised and small current - no gated channels open
flows down axon by passive local spread for small distance
dissipates due to leakage from membrane
depolarisation and large current
Na+ and K+ gated channels open and lead to explosive change in membrane potential (action potential)
determination of passive local spread
resistance and capitance of axon
larger diameter, greater speed
synapse
special type of cell junction allowing direct communication between cells
transmitter substance secreted in highly localised fashion
synaptic bouton
terminal end of axon swollen
applied to surface of target cell
synaptic cleft
20nm gap between synaptic boutons
cell membranes on synaptic cleft
contain special membrane proteins and receptors involved in neurotransmission
ultrastructure of cell membrane on synaptic cleft
slightly thickened
synaptic bouton contains mitochondria, microtubules, neurofilaments and neurosecretory vesicles
synaptophysin
glycoprotein in membrane of neurosecretory granules
chromogranins
proteins involved in packaging of transmitter into dense core vesicles
what does the wave of depolarisation trigger?
release of a transmitter substance from neurosecretory granules by exocytosis
transmitter substance diffuses across the synaptic cleft and interacts with receptors in the postsynaptic membrane of target neuron
membrane of vesicle recovered as coated pit and recycles to endosome compartment in the nerve terminal
types of neurosecretory vesicles
arrive in axon terminal by transport from cell body
formed locally by budding off from endosome compartment in synapse
synaptobrevin
anchoring protein in membrane of synaptic vesicle
ties vesicle to docking protein in presynaptic membrane (syntaxin) via linking proteins
syntaxin
docking protein in presynaptic membrane
linking proteins
SNARE, alphaSNAP, betaSNAP, SNAP25 and NSF
membrane fusion proteins
in synaptic vesicle membrane and in presynaptic membrane
allow exocytosis
synaptotagmin
calcium sensitive trigger protein
prevents linking complex from allowing calcium fusion
action potential - voltage sensitive Ca2+ channels open
releases trigger protein, allows exocytosis
3 possible effects of a released transmitter binding to receptors on postsynaptic cells
depolarisation, hyperpolarisation and altered cell sensitivity
depolarisation
target cell depolarises if transmitter substance binds to ligand-gated receptor and causes it to open, allowing ions to diffuse into the neuron
if many receptors are activated at once, the alteration in membrane potential causes activation of voltage gated ion channels - action potential
acetylcholine and glutamate
hyperpolarisation
target cell hyperpolarises if transmitter substance binds to ligand gated receptor admitting small negative ions into cell
inhibits depolarisation
gamma aminobutyric acid
glycine
altered sensitivity of the cell
transmitter substance binds to a non channel linked receptor
generate secondary messengers (e.g. cAMP) within target neuron to modify overall sensitivity of cell to depolarisation
monoamines
small neuropeptides
neuromodulation
generating secondary messengers within target neuron to modify overall sensitivity of cell
types of synapses
axodendritic
axosomatic
axoaxonic
serial axoaxonic
neuromuscular junction - specialised synapse between motor nerve junction and skeletal muscle
factors affecting speed of conduction
electrical capitance and resistance of axon
increased if leakage of current is minimised by insulation
width and capitance
wide axons have lower capitance than narrow ones
increasing diameter increases speed of conduction
inefficient as they have high metabolic upkeep
myelin
produced by oligodendrocytes in CNS and Schwann cells in PNS
lipid rich insulating layer
Schwann cell
myelinates only one axon
oligodendrocyte
myelinates several adjacent axons
staining of myelin
affinity for lipid or protein components of sheath
difference between grey and white matter
grey matter
neuron rich areas low in myelin
white matter
tracts of axons with abundant myelin
myelin sheath
spiral layers of cell membrane around axon
each layer linked to neighbour by cell adhesion proteins
formation of myelin sheath
begins with invagination of axon into support cells, bringing its outer cell membranes into close opposition and seals them together to form sheet of internal membrane (mesaxon)
mesaxon
fused outer leaflets of myelin forming cell’s cell membrane
intraperiod line
line of fusion mediated by proteins in outer surfaces of cell membrane
formation of spiral
support cell wraps layers of mesaxon around axon
tight spiral composed of double thickness membrane fused together forms - cytoplasm of support cell is excluded from space between layers
major dense line
inner surfaces of cell membranes fuse together
form dense line
lipids in myelin
galactocerebroside
nodes of Ranvier
small bare areas of axon between myelin sheaths
where does the cytoplasm of the myelin-forming support cell stay in the myelin sheath?
adjacent to axon (inner collar
