Nerves 4 Flashcards
what is the RMP?
-70mV
what is the usual treshold needed to be reached to fire an AP?
-55mV
what is the potential at sensory neurone called?
generator potential
what is the potential at the skeletal muscle which determines whether the AP will be fired or not?
endplate potential
describe steps of the AP when stimulus is activated (4)
- stimulus causes sudden depolarisation and an overshoot to around +30 in a short space of time
- repolarisation then occurs (AP falls back down to below RMP)
- refractory period occurs
- cells is at RMP again
at rest, what is the Na and K permeability like?
Na permeability is low
K permeability is high (due to leaky K channels)
Describe ionic basis of AP when treshold is reached. (4)
when threshold is reached..
- Na channels open immediately and Na floods in depolarising the cell until its equilibrium potential is reached
- this causes a massive increase in permeability (decrease in resistance)
- K permeability slowly rises as more K channels open (voltage gated/ dependent ones NOT the leaky ones)
- K channels opening and Na channels closing causes repolarisation and later hyperpolarisation and then we are back to where we started at RMP
what is the refractory period?
occurs when there is no response to further stimulation (unresponsive tissue)
what sort of effect occurs if the Na/K pump is poisoned?
delayed effect (it takes time for gradients to run down)
what do local anaesthetics block? (procaine/ lidocaine)
block voltage dependent Na channels and no APs can be transported (some toxins do the same e.g. tetrodoxin)
what happens at depolarisation phase?
- Na channels open and Na floods the cell very quickly
what happens at repolarising phase?
- Na permeability drops
- VOLTAGE gated K channels open (more gradual, slow and stays open for longer), making cell more negative (drop back down)
what are main properties of APs?
- all or none treshold
- can’t encode stimulus in their amplitude, only in FREQUENCY
- they self propagate
- once AP is made, they are all the same size
- mediated by voltage gated channels
- have a refractory period
what is meant by APs being self-propagated?
- non-decremental (good for long distances)
- depolarisation can only sweep in ONE direction (forward) along the axon
- signal goes on until it reaches the muscle
what can conduction velocity be improved by?
myelination
what effect do large axons have on depolarisation?
-evokes a bigger AP as depolarisation evoked at one channel can spread further
why do larger axons give a bigger AP?
depolarisation can happen in wider space since Na channels are more spread out and can reach their “neighbour” channels to reach treshold. The bigger the passive spread, the signal travels for longer
what takes longer, the signal travelling or opening/closing of channels?
opening/closing of channels
what is myelin sheath wrapped around?
the Schwann cells which wrap around axons
in which part of the axon, do the GPs summate (integrate) which trigger an AP?
at the axon hillock/ initial axon segment (by reaching the treshold)
what is saltatory conduction?
propagation of APs along myelinated axons from one node to the next, increasing conduction velocity of APs
what is the site of APs on a neurone?
at nodes of Ranvier (AP decays as it moves along the axon)
what glia bodies help with myelination?
oligodendrocytes
why is myelination good for AP signal?
- makes axon less leaky
- increases action resistance (allows AP to spread like a local current)
- depolarisation spreads quicker and increases velocity of conduction
what disease is assocaited with demyelination?
multiple sclerosis (APs are not effectively passed as it decays quicker and leaks out, no depolarisation occurs at next node and treshold is not reached (conduction fails).
what is a compounda action potential?
mammals have big and small myelinated and big and small unmyelinated axons. Therefore an extracellular recording of axons (nerve trunk) evokes a compound AP
what is the biggest peak AP on a graph?
An AP that travels the fastest (smaller ones arrive slightly later)
which type of axons give the BIGEGST peak?
big myelinated axons
which type of axons give the SMALLEST peak?
small unmyelinated axons
What is the order of fibres from: (Most sensitive to anoxia and least sensitive to local anaesthetics) to (Least sensitive to anoxia and most sensitive to local anaesthetics)
- proprioception, motor neurones
- touch, pressure receptors
- touch,cold, fast pain,
- preganglionic, autonomic fibres (unconscious actions)
- heat, slow pain
which fibres are more sensitive to anaesthetics?
small ones are affected first by anaesthetics
which fibres are most sensitive to absence of oxygen?
myelinated fibres
Summary of graded potentials
- graded
- encoded by AMPLITUDE
- can summate
- no treshold
- no refractory period
- decremental
- de or hyper polarising
- ligand gated channels
- “decides” if cell will fire AP
- occurs in cell body and dendrites
- initiated by entry of ions through the channels
- usually Na,Cl and CA ions involved
- input signal
Summary of Action potentials
- all or none
- encoded by FREQUENCY
- cannot summate
- are evoked at treshold
- has refractory period
- self-propagating
- depolarising
- voltage gated channels
- sends electrical signal over long distances
- occurs in initial/trigger segment of the axon
- initiated by above treshold GP at the trigger zone
- usually Na and K ions involved
- conduction signal
what gives limits on APs?( so they happen one at a time)
refractory periods
does depolarising lead towards the treshold?
yes
does hyperpolarising lead away from treshold?
yes
