Unit 7 Flashcards
What is the main function of neurones?
receive and process information from neurones and sensory receptors
send information to next neurone in chain of command or effector organs
What is the surface of dendrites lined with?
synaptic receptors
-> allow to receive stimulation
What are dendritic spines?
cover many dendrites
increase surface area
Are synapses also possible on surface of cell bodies?
yes
-> also covered in synaptic receptors
Axon
thin fibre of constant diameter
covered in myelin sheath
interruptions in myellin sheath
-> nodes of ranvier
How many axons do neurones have?
1
-> can have many branches
Presynaptic terminal
swelling at end of each axon
-> allows release of neurotransmitters into synapse
What does the surface of neurones consist of and what is its purpose?
cell membrane
-> impermeable
separates inside of cell from external environment
What is extermely important for creating a nerve impulse?
protein channels
-> allow controlled flow of important molecules (e.g. water, oxygen, etc.)
What type of transmission does our nervous system use to communicate information?
electrochemical transmission
How did the nervous system evolve to fix the issue of an electrical impulse losing strength over longer distances?
By regenerating the message at various points along the axon
How fast are impulses conducted?
1-100 m/s
What are Ions?
Two parts with exactly opposite charge
can be seen when electrolytes are dissolved in water
Which forces act on ions?
Electrostatic pressure
Diffusion
Electrostatic pressure
causes ions to move towards opposite electrical charge
What is electrical potential?
difference between areas of positivity and negativity
Diffusion
causes ions to move towards areas of relatively lower concentration
What is the difference in concentration of two ions refered to and what is the difference in electrical potential refered to?
concentration gradient
electrical gradient
What is membrane potential produced by and what is it?
forces of electrostatic pressure and diffusion
difference in electrical charge between inside and outside of cells
What is the membrane at rest and what does that mean?
polarised
electrical potential inside membrane being slightly negative
What is resting potential?
voltage between inside and outside of cells at rest
usually around 70 mV
Selective permeability
some molecules can pass through it freely
others need membrane channels
Concentration of ions at rest
more Na+ on outside and k+ inside the cells
inside more negative
Na+ wants to enter
-> wall impermeable to Na+
K+ at equilibrium
-> electrostatic pressure and diffusion equally at play
How does a neurone deal with Na+ and K+ ions leaking every once in a while?
sodium-potassium pumps
3 Na+ out
2 K+ in
How does the inside of the cell stay negative?
organic anions (A-)
unable to pass through cell membrane
Chloride ions (Cl-)
-> however mostly outside the cell body
-> again at equilibrium
Why does the cell use so much energy at sodium-potassium pumps? (about 40%)
cells are always ready for action
What happens when you apply a negative charge to the cell membrane?
hyperpolarisation
What happens when you apply a positive charge to the cell membrane?
depolarisation
What happens when the threshold of excitation is reached?
massive depolarisation
causes voltage-gated Na+ channels to open
-> rapid influx of Na+
potential shoots up further towards positivity
-> action potential
All-or-none law
any depolarisation reaching threshold of excitation produces action potential
What can vary between action potentials of neurones and what stays consistent?
strength and speed vary between neurones
stay the same for each neurone
-> neurones either fire, or don’t
What happens at the end of the action potential?
membrane potential reversed (inside more positive than outside)
-> Na+ channels snap shut
When do K+ channels open?
at slightly greater levels of depolarisation
-> a bit later than Na+ channels
What else helps the neurone repolarise?
Cl- influx
What is the process of returning to negativity known as?
repolarisation
What produces hyperpolarisation?
K+ channels remain open for a while longer
-> more K+ leave cell to lower concentration
-> repolarisation continues past resting potential
How is the resting potential finally restored?
closing of K+ channels
sodium-potassium pump ensures right concentration of Na+ and K+ on each side
refractory period
time during which a neurone is unable or less likely to produce another action potential
initial part: absolute refractory period
-> impossible to produce another AP
second part: relative refractory period
-> neurones require larger than normal stimulation
How does action potential move down the neurone?
moves from cell body to end of axon
Na+ flow inside axon to neighboring areas of negativity
-> causes neighboring area to become more positive and reach threshold of excitation
-> AP regenerated
=> proagation of AP
only in direction of axon terminal because of refractory period
Can AP occur anywhere?
no, only at nodes of ranvier due to absence of NA+ channels in myelinated parts of axon
Na+ diffuses towards next node of ranvier where AP is generated again.
How is this form of conduction called?
saltatory conduction
Why is saltatory conduction efficient?
speeds up propagation of AP
-> don’t have to occur at every point along axon
maintains strength of impulse
-> because of periodical regeneration
conserves energy
-> less sodium-potassium pumps needed
Multiple sclerosis
autoimmune disease
immune system attacks myelin sheath
most axon potentials die out between nodes of ranvier due to absence of Na+ channels in unmyelinated parts
Symptoms: visual impairments, poor muscle coordination, fatigue, etc.
postsynaptic potential
change in membrane potential of postsynaptic neurone after receiving stimulation from presynaptic neurone
Why is postsynaptic potential known as a type of graded potential?
stimulation of postsynaptic neurone doesnt always cause mebrane to reach threshold of excitation
Action potentials along axon vs postsynaptic potentials
along axon: always depolarisations
postsynaptic: depolarisation (excitatory) or hyperpolarisations (inhibitory)
Excitatory postsynaptic potential
EPSP
result of an influx of Na+ into postsynaptic membrane
quickly decays if threshold of excitation isnt reached
What is temporal summation?
if time between multiple ESPS is short enough they add up
-> helps reach the threshold of excitation
What is spatial summation?
Several presynaptic neurones stimultion postsynaptic neurone
Inhibitory post-synaptic potential
IPSP
pushes postsynaptic potential further away from threshold of excitability
decreases likelihood of postsynaptic neurone firing
What did Loewi find out and how?
most presynaptic neurones communicate messages via neurotransmitters
Stimulated vagus nerve in one frog
-> decreases heart rate
collected fluid and transferred it to second frog’s heart
-> heart rate also decreased
Where are smaller and bigger neurotransmitters synthesised?
Smaller: axon terminal (e.g. monoamines)
larger: cell body and carried down (e.g. neuropeptides)
The activation of which type of voltage-gated channels causes and influx leading to the release of neurotransmitters in the presynaptic terminal?
Ca2+ channels
influx of Ca2+
What causes postsynaptic ion channels to open?
neurotransmitter diffusing to receptors
What are the methods used to remove neurotransmitters from the synapse to prevent overstimulation?
Breaking them down
-> e.g. acetylcholine broken down by acetylcholinesterase into acetate and choline
-> choline re-synthesised in terminal
Reuptake: for other neurotransmitters like some monoamines, acheived by using transporter proteins
Diffusing away from synapse
Negative feedback
used to limit release of neurotransmitters of presynaptic neurone
2 ways:
1) many have autoreceptors that detect release of neurotransmitters and inhibit further release
2) some postsynaptic neurones release chemicals (e.g. nitric oxide) to inhibit further release
