Nervous Tissue Flashcards
Cell body
Contains a large, round nucleus with a nucleolus
Perikaryon
Cytoplasm of neuron
Cytoskeleton of neuron
Neurofilaments and neurotubules
Neurofibrils
Bundles of neurofilaments that extend into dendrites and axons
Nissl bodies
Clusters of RER and free ribosomes
Grey matter
Areas containing neuron bodies
Dendrites
Extend and branch out from the cell body
Axon
Propagates an action potential
Axoplasm
Cytoplasm of the axon
Axolemma
Plasma membrane of the axon
Initial segment
Base of the axon
Axon hillock
Thickened region
Collaterals
Side branches that enable a single neuron to communicate with several other cells
Telodendria
Fine extensions
Synapse
Where a neuron communicates with another cell
Axonal (axoplasmic) transport
Movement of materials between the cell body and axon terminals
Anterograde transport
Cell body to axon terminal carried by kinesin
Retrograde transport
Axon terminal to cell body carried by dynein
Passive chemical gradients
K+ high inside
Na+ high outside
Both move through leak channels along chemical gradient
Active Na+/K+ pumps
Maintain the concentration of gradients of sodium across the plasma membrane
Passive electrical gradients
K+ leaves more quickly than Na+ enters
More positive outside p.m and negative inside = electrical gradient
Current
Movement of charges to eliminate a potential differences
Resistance
A measure of how much the membrane restricts ion movement
Electrochemical gradient
The sum of the chemical and electrical forces acting on an ion across the plasma membrane - potential energy
Equilibrium potential
Membrane potential at which there is no net movement of a particular ions across the plasma membrane
Resting membrane potential for most neurons
-70mV
Leak channels
Passive ion channels that are always open
Gated channels
Active channels that open and close in response to specific stimuli
3 classes of gated channels
Open or close in response to:
- Chemically (ligand)-gated: bind to specific ligands
- Voltage-gated: changes in membrane potential
- Mechanically gated: physical distortion
3 states of voltage-gated channels
- Closed but capable of opening
- Open (activated)
- Closed and incapable of opening (inactive)
Graded potentials
Changes in the membrane potential that cannot spread far from the site of stimulation
Chemically gated sodium ion channels
- Depolarisation: Na+ enters cell, membrane potential becomes more positive
- Local current: Na+ outside move towards open channels, parallel movement of inner and outer surfaces
- Repolarisation: restoration of normal membrane potential after depolarisation
Gated potassium ion channels
- Hyperpolarisation: K+ flows out of cell, inside becomes more negative than outside, increase in negativity of resting membrane potential
- Local current
Action potentials
Nerve impulses
Threshold
Membrane potential at which an action potential begins
Typical axon threshold
-60mV to -55mV
All-or-none principle
A stimulus either triggers a typical action potential or none at all
Generation of action potential
- Graded depolarisation to threshold that opens voltage-gated sodium channels (-60mV)
- Activation of sodium ion channels and rapid depolarisation (+10mV)
- Inactivation of sodium ion channels and activation of potassium ion channels starts repolarisation (+30mV)
- Time lag in potassium ion channels closure leads to temporary hyperpolarisation (-90mV)
Refractory period
Period when the plasma membrane doesn’t respond normally to additional depolarising stimuli from the time an action potential begins until the resting membrane potential has been established
Absolute refractory period
Voltage-gated sodium channels either are already open and or are inactivated
Relative refractory period
Requires larger than normal stimulus
Continuous propagation
Unmyelinated axon
Action potential spreads by depolarising adjacent region of axon membrane
Saltatory propagation
Myelinated axon
Action potential jumps = much faster
Classes of axon
Type A fibres: largest myelinated
Type B fibres: smaller myelinated
Type C fibres: unmyelinated
Electrical synapses
There is direct physical contact between the cells
Pre and postsynaptic membranes joined by gap junctions
Chemical synapses
One neuron sends chemical signals to another cell
Synaptic cleft
Separates the two cells
Neuromuscular junction
Synapse between a neuron and skeletal muscle cell
Neuroglandular junction
Neuron that controls or regulates the activity of a secretory cell
Cholinergic synapses
Synapses that release ACh
Synaptic delay
Occurs because calcium ion influx and the release of neurotransmitter takes a while
Choline
Released during the breakdown of ACh in the synaptic cleft
Reabsorbed and recycled by the axon terminal
Synaptic fatigue
Occurs when stores of ACh are exhausted
Excitatory neurotransmitters
Cause depolarisation and promote the generation of action potentials
Inhibitory neurotransmitters
Cause hyperpolarisation and suppress the generation of action potentials
What determines the effect of a neurotransmitter on the postsynaptic membrane?
The properties of the receptor, not the neurotransmitter
Adrenergic synpases
Release norepinephrine (NE)
Norepinephrine (NE)
Has an excitatory depolarising effect on the postsynaptic membrane
Neuromodulates
Influence postsynaptic cell’s response to neurotransmitters
Information processing
Excitatory and inhibitory stimuli are integrated through interactions between postsynaptic potentials
Excitatory postsynaptic potential (EPSP)
A depolarisation caused by a neurotransmitter
Summation
Individual EPSPs combine
Temporal summation
Occurring at a single synapse when a second EPSP arrives before the effects of the first have disappeared
Spatial summation
Resulting from the cumulative effects of multiple synapses at various location
Inhibitory postsynaptic potential (IPSP)
Hyperpolarisation of the postsynaptic membrane
Most important determinants of neural activity
EPSP-IPSP interactions
Presynaptic inhibition
GABA released at an axoaxonic synapse inhibits the opening of voltage-gated calcium ion channels in the axon terminal, reducing the amount of neurotransmitter released when an action potential arrives at the axon terminal
Presynaptic facilitation
Activity at an axoaxonic synapse increases the amount of neurotransmitter released when an action potential arrives at the axon terminal, prolongs the effects of neurotransmitters on the postsynaptic membrane