neurones Flashcards
what is voltage
the difference in electric potential energy per unit charge between two points
what is current
a flow of electric charge through a medium (through a surface)
what is a resistance
the opposition to the passage of an electric current; the inverse quantity is electrical conductance (g), measuring how easily electricity flows along a certain path
what is conductance
1/R – ability to conduct electric current
what is ohms law
V = I x R
I = V x g
(if you multiply the force on an ionic species (voltage) by the ability of it to be conducted forward (conductance = g), the result is the current that actually can flow per second
what is membrane potential
A measurement of the overall electrical potential energy across the membrane
what does membrane potential depend on
Vm depends on the relative electrical current and conductance of different ions
what happens when ion channels open- neurones
[Na+] is higher outside the cell so when the channels open, Na+ flows inside the cell and the membrane tends to become positive (inside)
[K+] is higher inside the cell so when the channels open, K+ flows to outside and the membrane becomes negative (inside)
what is equilibrium potential
whenever ion channels for that ion open, the membrane potential moves toward that ion’s equilibrium potential
what is Ena
+60mV
what is Ek
-90mV
what is Eca
+123mV
what is ECl
-40mV (-65mV neurones)
what is action potential
rapid changes in the membrane potential that spread rapidly along the nerve fibre membrane
at rest what is the membrane potential and what is the membrane more permeable to
-70 to -90 mV
membrane more permeable to K+
at action potential what is the membrane potential and what is the membrane more permeable to
+20 to +40mV
membrane temporarily more permabe to Na+, Ca2+ or both
what are the stages of action potential
stimulus reaches threshold
depolarisation
repolarisation
refactory period-hyperpolarisation
resting
what is the resting stage
: K+ that is going out of cell clamps the membrane potential negative
what is the initial depolarisation
Depolarisation: inside the cell the voltage becomes less negative (or more positive)
Could be a nearby cell depolarising
Could be synaptic transmission where a neurotransmitter opens a ligand-gated channel
what happens if the stimulus is below the threshold
Background K+ permeability pulls Vm back resting stage
This is a “failed initiation”
what is the positive feedback of depolarisation
The initial depolarisation causes a few of the Na+ channels to open
The additional current of Na+ going into the cell
more depolarisation
This acts as a positive feedback loop
When the voltage goes above the threshold voltage the cell is committed to an AP
what is repolarisation
Na+ channel inactivation decreased Na+ current going in
Delayed rectifier K+ channels open increased K+ going out
These cause the membrane to be less positive and more negative inside
what is hyperpolarisation
the voltage inside temporarily becomes slightly more negative than at rest
increased K+ permeability and decreased Na+ permeability
the membrane potential becomes even more negative
causes refractory period
what happens to the synapse when an action potential is caused
When Vm becomes positive inside during action potential, calcium channels open
Increase influx of Ca2+
what is a neurotransmitter
Endogenous chemical
Released extracellularly by a neuron
Used to signal to other neurons, myocytes, endocrine cells,…
Under physiological conditions
what is the life cycle of a neurotransmitter in the presynaptic neurone
Synthesis
In the soma (cell body) or locally in the axon terminal
Sometimes instead of synthesis it is recycled (after use)
Storage
Transported into a vesicle for storage (synaptic vesicle)
Membrane depolarisation and activation of voltage-gated Ca2+ channels
Vesicle Transport and Fusion with the membrane
Release
what is the life of a neurotransmitter at the post synaptic neurone
Diffusion across the synaptic cleft
Action on post-synaptic cell
Activation of specific receptors
Excitatory or inhibitory inputs
Open ion channel – if Na+, will cause depolarisation in the post-synaptic cell
Any of the different types of receptors
Signal termination
Removal of transmitter from synaptic cleft
Degradation by enzymes
Recycled into the presynaptic cells - reuptake
when does synthesis of neurotransmitters stop
Synthesis stops depending the enzymes present in the neuron / cell
what is noradrenaline secreted by
sympathetic nerve terminals
what is adrenaline secreted by
secreted by chromaffin cells in the adrenal medulla
what are types of adrenoreceptors
a and b
what are features of α-adrenoceptors
α1, α2 (post-/pre-synaptic)
Blood vessels of organs and tissues except skeletal muscle vessels
Agonists: NA, Adr, isoprenaline
Antagonist: phentolamine
what are features of β-adrenoceptors
β1 (myocardium), β2 (bronchi, uterus, muscle and coronary vessels), β3 (adipose tissue, bladder, gallbladder)
Agonists: Isoprenaline, Adr, NA
Antagonist: propranolol
Sensitive to up/down regulation
what is the 2nd messenger system of a1 receptors
Gaq
what is the 2nd messenger system of a2 receptors
Gai
what is the 2nd messenger system of B1/2/3
Gas
which adrenoreceptor causes smooth muscle contraction/ vasoconstriction
A1- in blood vessels
which adrenoreceptor causes GI smooth muscle relaxation
A1
which adrenoreceptor causes GI sphincteric muscle / bladder sphincter contraction
a1
which adrenoreceptor causes glycogenolysis in the liver
a1/b1
which adrenoreceptor causes decreased neurotransmitter release in the presynaptic sympathetic neurones
a2
which adrenoreceptor causes increased heart rate and force of contraction
B1
which adrenoreceptor causes dilation/relaxation in the bronchi and blood vessels
b2
which adrenoreceptor causes tremor and glycogenesis in skeletal muscle
b2
which adrenoreceptor causes lipolysis, thermogenesis in adipose tissue and skeletal muscle
b3
which adrenoreceptor causes relaxation - preventing urination in the bladder
b3
what is the pathway of ACH
Synthesis in the cytosol
Storage
Vesicle Transport and Fusion with the membrane
Release
Diffusion across the synaptic cleft
Action on post-synaptic cell – receptors
Signal termination
Metabolism by cholinesterase:
what are metabotropic receptors (GPCR)
Metabotropic (GPCR)
Located postsynaptically in the smooth muscle, cardiac muscle, glands
Agonists: ACh, muscarine
Antagonist: atropine
what are ionotropic receptors
Ionotropic (pentameric cations channel)
Autonomic ganglia, motor endplate, CNS
Agonists: ACh, nicotine
Antagonist: curare (tubocurarine)
what is the 2nd messenger system for nicotinic receptors
ion channel
what is the 2nd messenger system for muscarinic M1 (neural)
Gaq
what is the 2nd messenger system for Muscarinic m2 (cardiac)
Gai
what is the 2nd messenger system for muscarinic M3 (glandular/ smooth muscle)
Gaq
what do muscarinic receptors do in the airway smooth muscle
bronchoconstriction
what do muscarinic receptors do in the gut smooth muscle
constraction
what do muscarinic receptors do in salivary glands
increased salivation
what do muscarinic receptors do in the bladder smooth muscle
contraction
what are features of preganglionic neurones
Cell bodies in the grey matter of the brain stem OR in the lateral horn of spinal cord grey matter
Long, synapse with postganglionic neurons at or near organ
Release ACh to activate nicotinic receptors on postganglionic neurons
what are features of post-ganglionic neurones
Cell bodies are in indistinct ganglia near or on their final target organ (further away from the spinal cord)
Short, synapse on the target organ
Release ACh to activate muscarinic receptors on the target organ
what are the effects of the sympathetic nervous system on sweat glands mediated by
muscarinic receptors because in this special case the postganglionic neurones are also cholinergic
what are drugs effect on the autonomic nervous system
Drugs enhance or mimic the autonomic nervous system or block the effects of theneurotransmitters at their receptor sites
