Lecture 4 Flashcards
The Neurone 1
why do more complex organisms need a NS
- cells on the inside of the body are not in direct contact with the outside world
- cells live in diff environments
- cells have become specialised
what do the cells need to do for the organisms to function ?
coordinate
endocrine system do what ?
specialised to secrete chemicals ( hormones ) into bloodstream
slow
long term
not specific
nervous system
specialised to transmit electrical impulses between 2+ cells
fast
precise co-ordination + communication
specific
neural impulses
basis for constant and rapid communication between cells
( constant and rapid control and adjustment of ongoing cell activities)
what do neurones do
- generate and transmit electrical impulses over great distances
- rapidly
where do electrical impulses reach
specific target cells
- modifies the activity of this cell
- allows selective control of specific target structures
how does the NS have a structured communication system
combination and integration of signals from diff sources
- structured communication
- input used to adjust output
special requirements of neurone - to do with energy
no possibility to store energy
so glucose ( sugar ) and oxygen must be constantly supplied
without glucose being provided for the neurone, what happens
it stops working within seconds
dies within minutes
where do neurones come from
neural stem cells ( they do not divide )
neurogenesis
generation of new neurones - 5 months after conception : after this, dead neurones usually replaced
how many neurones die during maturation
20-80%
Glia cells
provide protected environment for neurones to survive
develop like neurones - from neural stem cells
- as many glia as neurones in brain ( but 10th of a size of neurone )
Astrocytes
star shaped
physical, nutritional support for neurone ( blood brain barrier )
transport nutrients from blood vessels to neurones
waste products away from neurones
hold neurones in place
Microglia
small
mobile for defensive function
produce chemicals that aid repair of damaged neurones
digest dead neurones ( phagocytosis )
Oliogendrogia
large
flat branches
wrap around axons
fatty substance ( myelin sheath ) insulating axon
draw parts of neurone - look at diagram
dendrites
main input area
axon hillock
axon terminals
axon collaterals ( axon split into many branches )
resting potential
ion conc differ between inside and outside of cell
if membrane was non-permeable, electrical potential would remain static
protein channels in cell membrane allow ions to enter / leave cell
if membrane channels were just passive ‘holes’, membrane would depolarise ( electrical potential dissapears )
-70mV
sodium potassium pump + membrane potential - resting potential
active channels work against equilibrium
neurone need energy to maintain their resting potential
3 Na+ out
2 K+ in
conc gradient + electrical gradient try to push in our push out K+ ( resting potential )
concentration gradient : push out
electrical gradient : push in
conc gradient + electrical gradient try to push in our push out Na+ and Cl- ( resting potential )
Na+
conc grad = pull in
electrical grad = pull in
Cl-
conc grad = pull in
electrical grad = push out
ion specific channels can do what? and when ?
open - by chance or response to stimulus
depolarisation happens when
if + ions enter ( or - ions leave ), membrane depolarises
- inside less negative than usual
membrane hyperpolarisation when
if negative ions enter ( positive leave )
inside more negative than usual
electronic transmission happens where
soma and dendrites
electronic transmission
electrical and concentration gradients sweep ions along the membrane
passive and graded ( of diff magnitude - strength )
–> some ions get lost on their way eg. drift out of membrane so signal decays
action potential - explain what is
electrical and concentration gradients push/pull ions across membrane
active and not graded ( always same magnitude / strength of signal)
self-replicating (signal replicates itself to same strength )
constant magnitude
so no decay
action potential process
sodium channels open or close in response to electrical changes at membrane
membrane depolarises ( more negative )
sodium channels open if threshold reached ( -50mV)
sodium ions enter cell
membrane depolarises further
all nearby sodium channels open
membrane fully depolarises total sodium permeability
enough positive ions enter that it becomes more positive inside than outside ( complete depolarisation)
sodium channels close, no more sodium ions enter
potassium channels open
potassium ions rush OUT
threshold potential is also called
Hodgkin-Huxley cycle
Hodgkin-huxley cycle
IF membrane potential at axon hillock remains below -50mV = resting potential returns + signal decays
IF membrane potential at axon hillock depolarises above -50mV
- all sodium channels open
- action potential generated
repolarisation
enough positive ions enter that it becomes more positive inside than outside ( complete depolarisation)
sodium channels close, no more sodium ions enter
potassium channels open
potassium ions rush OUT
membrane repolarises
hyper polarisation
potassium channels close when resting potential is restored
fewer potassium ions inside than out cell = membrane hyper polarises
conduction of action potential
starts from axon hillock and goes down axon
each burst of depolarisation triggers sodium channels in adjacent sections of the membrane of the axon to open
why does AP not travel backwards ?
during hyperpolarisation - (after AP has just passed through) the membrane is difficult to depolarise
BUT
membrane in front of AP is still at resting potential and so more easy to depolarise
properties of AP
doesn’t decay during transmission
always strong enough to depolarise next area of membrane
all-or-nothing phenomenon : either generated or not ( and at same intensities )
cannot be produced continuously - minimum Time of 2-5 ms between each AP
fast
all-or-nothing phenomenon
either generated or not ( and at same intensities ) but can be diff frequencies
can an AP be produced continuously ?
no
2-5 ms between each AP
In mammals, how do the signals travel so fast ?
because sensory and motor neurones are myelinated
what does myelin prevent ?
inflow and outflow of ions because it electrically insulates
electrical charges are transported inside the axon - without need to produce AP
nodes of ranvier
every 1-2mm, myelin sheath has gaps called nodes of ranvier
what happens at the nodes ?
new AP is generated - action potential jumps from node to node ( saltatory conduction )
qualitative - types of info coded how
place in brain where signal received
quantitative - how strong a stimulus is how detected ?
firing rate
- how many AP’s within given time
a strong input will cause neurone to send out signals more quickly
weak stimulus
low frequency of AP
strong stimulus
higher frequency of AP’s
