2 Neural signalling Flashcards
4 factors contribute to membrane resting potential
- intracellular charged proteins
- Na+/K+ pump
- sodium ions
- potassium ions
intracellular proteins
negative groups on surface = -ve
Na+/K+ pump
3 Na out
2 K in
= overall net negative p.d
sodium ions
membrane not very permeable to Na ions
membrane is only slightly permeable to Na+, so its effects on resting potential are small
net inward diffusion of Na+ slightly adds to the positivity of the cell = brings up to -65mV
both conc + electrical gradient pull Na+ in
potassium gradient
K+ out = conc gradient through channels
K+ in = electrical gradient pulls in due to -ve charge in cell
cell resting membrane potential =
close to but not equal to the potassium equilibrium potential as there is also a small leak for sodium ions
an action potential is…
away from cell body -> towards axon terminal
the means by which a neuron sends information down its axon, away from the cell body.
The action potential (spike/impulse) is an explosion of electrical activity that is created by a depolarising current
at RP
-65mV
all voltage gated Na+ and K+ channels closed
non-voltage dependent K+ channels are open
phases of AP 1
Na+ channels open
Na+ enters nerve cell
Membrane potential rises towards zero
phases of AP 2 = DEPOLARISATION
If threshold potential reached, voltage gated Na+ channels open
Na+ ions flow into cell
Action potential spike results
phases of AP 3 = REPOLARISATION
happen after reversal potential
Na+ channels close when Na+ equilibrium potential is reached
Voltage gated K+ channels open and K+ ions flow out of cell
Membrane potential reverses
phases of AP 4 = HYPERPOLARISATION
K+ ions continue to flow out of cell while Na+ channels closed
Hyperpolarisation results
AP = all or nothing principle
Membrane has to be depolarised beyond threshold for an AP to be generated
Further increase above threshold -> higher AP frequency not larger AP amplitude
absolute refractory period
= on graph the spike up to before hyperpolarisation
no further action potentials can be elicited = ensures AP propagation is one way
relative refractory period
= on graph starts at hyperpolarisation (end of repolarisation)
a larger stimulus can result in action potential
non-myelinated neuron AP conduction
wave along entire length of axon
myelinated neuron AP conduction
jumps along between Nodes of Ranvier = faster
Receptor potential caused by
detection of stimulus by receptor which then causes an AP is above threshold
stronger the RP
higher freq of AP generated
2 main sensory receptors in muscle
- muscle spindle
2. golgi tendon organ (GTO)
muscle spindle and GTO are both
mechanoreceptors = pressure changes proprioceptors = position and movement
muscle spindle stimulated when
muscle is passively stretched
GTO responds to
tension = stimulated when its associated muscle contracts or stretches in response to tension
muscle spindle found in
muscle = modified muscle fibre
GTO found in
tendon
muscle spindle description
bundle of modified skeletal muscle fibres (intra-fusal fibres) enclosed in connective tissue capsule
Intra-fusal fibres detect stretch/initiate reflex
GTO description
small bundles of tendon (collagen) fibres enclosed in a layered capsule with dendrites coiling between and around the fibres
stimulated when the associated muscle contracts
sets up reflex causing muscle to relax and removing stimulation
spindle is activated when
muscle is stretched passively = initates a reflex
-> when muscle contracts and shortens it’s switched off
purpose of muscle spindle
prevent muscle from being overstretched
Knee-jerk reflex pathway
= monosynaptic pathway as only 1 synapse
- Muscle stretch stretches spindle - increased discharge of sensory nerves
- Increased firing of the motoneurone and the muscle contracts
- NO SPINAL INTERNEURONE involved in this case.
- Effect - reduce the stretch of the muscle.
- Specific for the muscle stretched
GTO activated when
muscle is actively contracted (also at times when passively stretched)
purpose of GTO
tension detector that protects muscle against excess load
GTO
inverse stretch reflex
Stimulated by excessive tension during muscle contraction or passive stretch
Causes a reflex inhibition of the muscle ……relaxation
2 types of synapse
- electrical
2. chemical
electrical synapse
direct passage of current via ions flowing through gap junctions
occurs in some parts of CNS
chemical synapse
release of vesicles containing chemical transmitter which has an effect on receptors on a target cell
gap junctions in electrical synapses
pores between cells effectively mean that the cytoplasm of the two cells is in continuity
formed by channels called connexons
connexons are made of protein molecules (connexins)
process of chemical transmission across synaptic cleft
- AP reaches axon terminal of presynaptic cell
- triggers Ca 2+ entry into cell
- triggers release of neurotransmitter from vesicles which fuse with presynaptic membrane
- transmitter diffuses across the synaptic cleft
- acts on specific receptors in the postsynaptic membrane (cell body or dendrite)
neurotransmitter =
a substance shown to be released by a neurone and have a physiological action on specific receptors on a target cell
neuromodulator =
a substance that is released and modifies the action of a transmitter, but doesn’t have a direct action itself
neuroactive substance =
a neutral term if a substance is known to have an effect in the CNS but its precise action is not known
different types of neurotransmitter substances
- amines
- amino acids
- peptides
- purines (ATP)
7 major neurotransmitters
acetylcholine (ACh) norepinephrine dopamine gamma-aminobutyric acid (GABA) glutamate serotonin histamine
amine neurotransmitters…
Dopamine (DA) Noradrenaline (Norepinephrine)(NA) Adrenaline (Epinephrine) Histamine Serotonin
amino acid neurotransmitters…
Gamma-aminobutyric acid (GABA)
Glutamate (Glu)
Glycine (Gly)
peptide neurotransmitters…
Dynorphin Enkephalins Neuropeptide Y (NPY) Calcitonin gene-related peptide (CGRP) Somatostatin Galanin Substance P (SP) Thyrotropin-releasing hormone (TRH) Vasoactive intestinal polypeptide
Ionotropic receptor
= ion channels and comprise mostly 4 or 5 similar protein subunits arranged around a central pore that is normally closed to ion movements. When the transmitter (ligand) binds it causes a conformational change that briefly opens the pore and ions pass through to cause a rapid change in the resting potential of the underlying cytoplasm
effect is to hyperpolarise or depolarise the postsynaptic cell
Metabotropic receptor
= single, long protein molecules, mostly crossing the cell membrane 7 times (7 trans-membrane domains)
- no ion pore
- ligand binds there is a conformational change in the molecule that causes the intracellular part to interact with a G-protein that then sets off a chain of intracellular events, that may include opening of ion channels
ionotropic vs metabotropic receptor response speed
ionotropic = faster
EPSP =
excitatory post-synaptic potential
IPSP =
inhibitory post-synaptic potential
binding of what causes EPSP
glutamate, ACh
binding of what causes IPSP
GABA, glycine
EPSP what ion influx
Na+ = depolarise cell towards threshold potential and may initiate an AP
IPSP what ion influx
Cl- = hyperpolarise cell and make may initiation of an AP less likely i.e. inhibition
purpose of EPSP
Decrease resting membrane potential (make more +ve)
i.e. closer to threshold for depolarization
EPSP’s add up to produce threshold potential to generate AP
purpose of IPSP
Hyperpolarize post synaptic membrane
Increase membrane potential
i.e. moving it further from threshold for depolarization
affect of same postsynaptic neurone can receive numerous excitatory and inhibitory inputs to its cell body
net effect of these EPSPs and IPSPs that determine whether it will fire an action potential
