Zusammenfassung der Ressource
Basic
neurophysiology
- types of neurones
- unipolar sensory
neurons-one long axon
which connects receptors
to the spinal cord or brain
- multipolar inter- and
motorneurons-have many
dendrites and one axon
- bipolar neurons-
one dendrites and
one axon
- resting potential
- neurons process info in the form of
electrical signals (Ap's). Proteins
act as ion channels & ion pumps in
plasma men & move ions across
men
- inside the cell, [K+] high, balanced by a high
conc of -vely charged proteins & other anions
(A-).In ECF (outside the cell) [Na+]
high,balanced by Cl-
- cell mems of neurone have high
selective permeability for K+- diffuses out
of cell, impermeable anions left behind
creating a electrical potential.
- Inside of cell now has net negative charge,
electrical force starts to pull K+ back in.
- When certain potential difference
reached, electrical force pulling K+ in
exactly counterbalances conc
grad=equilibrium
- the electrical potential diff that exactly balances
an ionic conc grad is called EQUILIBRIUM
POTENTIAL (measured using microelectrode
technique)
- can calc equilibrium constant using nerst equation: v=(RT/zF)ln (CR/Ci).
For K+, [K+]out=5mM & [K+]in=150mM. EK+=61.5 x log10 (5/150)=-91mV
- neurons are also permeable to Na+- large electrochemical gradient
for Na+ drives Na+ into the cell. Makes resting mem potential more
positive (-70mV)
- Goldman equation
- At -70mV, K+ leaks out neuron, Na+ leaks in. The Na-K pump
actively transports ions to compensate for Na+ & K+ leaks using
ATP (3 Na+ out, 2 K+ in)
- nonexcitable
cells-mem potential
depends only on
K+.
- electrical & chemical forces
combine to become electrochemical
gradient causing movement of ion
- ACTION POTENTIAL
- in inter- and motor neurone, info flow is
directional-incoming signals are intergrated & if signal
large enough=AP generated & conducted along axon
- Depolarisation phase
- some voltage gated Na+ channels open=Depolarisation of mem to
threshold (-55mV)=open more voltage gated Na+
channels-rapid depolarisation.
- peak depolarisation,Na+
channels
inactivated-because of +ve
charge inside cell & conc
grad-K+ voltage gated
channels open-K+ moves
out=REPOLARISATION
- resting mem reached but
some K+ channels still open,
more K+ move
out=HYPERPOLARISATION
(below -70mV)
- Na/K+ pump restores
K+ to 150mV and Na+
to
15mV=RESTORATION
PHASE. All voltage
gated channels
closed
- K+ leak channels
always open but effect
overcome by Na+
- Na channels
- At resting mem potentials=
closed/ as neuron
depolarisaed=open/ at fixed
time after channel
opens=inactivates
- absolute refractory period
- after the neurone has
generated an AP it cannot
generate another one (From
the moment Na+ channels
open at threshold until
inactivation)
- relative refractory period
- immediately after absolute refractory
period, neuron can develop AP but only if
it is depolarised to a value MORE
POSITIVE than threshold
- Hodgkin-Huxley model of
excitability (from
experiments with giant
squid axons)
- 1.AP caused by inward
Na+ & outward K+
- currents flow through
voltage-gated conductance
channels which undergo a
conformational change upon
changes in mem
potential-alters their
conductance
- AP is ALL or NONE
- only INA & IK are
needed to produce an
AP.All other gated
channel currents can
indirectly modify INA &
IK, altering mem
potential & therefore
conduct
- Conduction of APs in Axons
- myelinated-APs can jump between Nodes
of Ranvier-faster propogation (saltatory
conduction)=quick response.
Unmyelinated-moves slower in current
loops
- Speed much faster
when diameter of axon
bigger
- if temp higher=AP moves
quicker/if cold can block AP
- impulses travel along neurons at
speed from 1 to 120 m/s
- fibres
- A fibres-large myelinated
axons that conduct AP at the
speed 15-120m/s
- B fibres-medium diameter myelinated axon
that conducts Ap at speed of 3-15m/s
- C fibres: unmyelinated axons that
conduct AP at speed <3m/s
- chemicals can
affect electrical
signals in neurons
- Tetrodotoxin-puffer fish/Saxitoxin-from
marine organisms that cause red
tide/Procaine-local anasthetic=all block
VOLTAGE GATED NA+ CHANNELS
- local anasthetics
block pain by
blocking
transmission to the
brain
- Tetraethylammonium chloride TEA-BLOCKS
VOLTAGE GATED K+ CHANNELS
- Quabain-from ripe seeds of African
plants-BLOCK SODIUM-POTASSIUM PUMP