Topic 6B - Nervous co-ordination ARN * Flashcards
neurones synaptic transmission muscle contraction
what are charges like when a neurone is in its resting state?
the outside of the membrane is positively charged compared to the inside, there are more positive ions outside
the membrane is polarised
what does it mean if the neurone membrane is polarised?
there’s a difference in charge (potential difference) across it
what is the resting potential?
the voltage across the membrane when it’s at rest
-70mV
how is the resting potential of a neurone created and maintained?
Na+/K+ pumps move 3 Na+ out for 2 K+ in
there is an open K+ channel which allows K+ to diffuse out of the membrane
this creates an electrochemical gradient, outside more positive than inside
how does the sodium potassium ion pump work?
they use active transport to move 3 Na+ out of the neurone for 2 K+ in
using ATP
how does the ‘leaky’ potassium ion channel work?
allow facilitated diffusion of K+ out of the neurone, down their concentration gradient
what are the steps of an action potential?
stimulus depolarisation repolarisation hyperpolarisation resting potential
what does a stimulus do to cause an action potential?
excites neurone cell membrane, causing voltage gated Na+ channels to open
increasing permeability to Na+
Na+ diffuse into neurone down electrochemical gradient
inside of neurone less negative
what is depolarisation?
if potential difference reaches threshold (-55mV)
more voltage gated Na+ channels open
more Na+ diffuse rapidly into neurone
what is repolarisation?
when potential difference reaches +30mV Na+ channels close and K+ channels open
membrane more permeable to K+ so they diffuse out down concentration gradient.
returns membrane back to resting potential
what is hyperpolarisation?
K+ channels are slow to close so there’s a slight overshoot where too many K+ diffuse out of neurone
potential difference becomes more negative than resting potential
how is resting potential restored from hyperpolarisation?
ion channels are reset
Na+/K+ pump returns membrane to its resting potential
what is the refractory period?
Na+ channels closed during repolarisation
K+ channels closed during hyperpolarisation
another action potential can’t occur during this time because ion channels are recovering
what is a wave of depolarisation?
when an action potential happens, some Na+ that enter the neurone diffuse sideways, causing Na+ channels in the next region to open
what direction does the wave of depolarisation move in?
away from the parts of the membrane in the refractory period because they can’t have another action potential
why is the refractory period essential?
time delay between 1 action potential and next so:
action potentials don’t overlap, but pass along as discrete impulses
limit the frequency at which nerve impulses can be transmitted
action potentials are unidirectional
what is the all or nothing nature of action potentials?
once threshold is reached, an action potential will always fire wit the same change in voltage
if threshold isn’t reached, there’s no action potential
how is the size of a stimulus quantified if action potentials are all the same?
a bigger stimulus won’t cause a bigger action potential
it will cause them to fire more frequently
what does it mean if a neurone is myelinated?
it has a myelin sheath
what is a myelin sheath?
an electrical insulator
in the peripheral nervous system it is made of Schwann cells
what is between the schwann cells on a neurone?
tiny patches of bare membrane called nodes of ranvier
Na+ channels are concentrated at the nodes
what happens at the nodes of ranvier?
in a myelinated neurone, depolarisation only happens at the nodes of ranvier
what is saltatory conduction?
the neurone’s cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse jumps from node to node
this is very fast
how do myelinated sheaths affect the speed of conduction of action potentials?
in non-myelinated neurones the impulse travels as a wave along the whole length of the axon membrane this is slower than saltatory conduction
what 3 factors affect the speed of conduction of action potentials?
myelination
axon diameter
temperature
how does axon diameter affect the speed of conduction of action potentials?
action potentials are conducted quicker along axons with bigger diameters
why are action potentials conducted quicker along axons with bigger diameters?
less resistance to flow of ions than in cytoplasm of smaller axon
less resistance = depolarisation reaches other parts of neurone cell membrane quicker
smaller SA: vol, fewer ions leak through K+ channels, membrane potential maintained better
how does temperature affect the speed of conduction of action potentials?
temperature increases = speed of conduction increases
ions diffuse faster
ATP gives energy to Na+/K+ carrier
until 40*C when proteins denature
what is a synapse?
the junction between a neurone and another neurone
or between a neurone and an effector cell
what is the synaptic cleft?
the tiny gap between the cells at a synapse
what is the presynaptic neurone?
the neurone before the synapse
it has a synaptic knob containing synaptic vesicles filled with chemicals called neurotransmitters
what happens when an action potential reaches the end of a neurone?
Ca2+ channels open Ca2+ diffuses synaptic knob vesicles fuse to the presynaptic membrane neurotransmitter released into synaptic cleft they diffuse across bind to specific receptors/ Na+ channels Na+ diffuse in depolarisation
what happens when neurotransmitters bind to receptors on the postsynaptic neurone?
they might trigger an action potential, cause a muscle contraction or cause a hormone to be secreted
how do synapses make sure impulses are unidirectional?
neurotransmitter only released from presynaptic neurone
receptors are only on the postsynaptic membranes
how is a response stopped from repeating at the synapse?
neurotransmitters are removed from the cleft so the response doesn;t keep happening
taken back into presynaptic neurone or broken down by enzymes
what is a common neurotransmitter?
acetylcholine (ACh)
used at cholinergic synapses
what happens when an action potential arrives at the synaptic knob in a cholinergic synapse?
AP arrives at synaptic knob
voltage-gated Ca2+ channels in neurone open
Ca2+ diffuse into synaptic knob
what do the calcium ions do at the cholinergic synapse?
influx of Ca2+ causes synaptic vesicles to fuse to membrane
vesicles release ACh into cleft in exocytosis
what does the acetylcholine do once is has been released into the cholinergic synapse cleft?
ACh diffuses across cleft and binds to specific cholinergic receptors on postsynaptic neurone
Na+ channels open
influx of Na+ causes depolarisation
AP generated if threshold reached
what happens after the postsynaptic neurone has generated an action potential at the cholinergic synapse?
ACh removed from synaptic cleft so response doesn’t keep happening
its broken down by an enzyme called acetylcholinesterase and the products are reabsorbed by presynaptic neurone to make more ACh
what are excitatory neurotransmitters?
they depolarise the postsynaptic membrane, making it fire an action potential if threshold is reached
e.g. acetylcholine at cholinergic synapses in the CNS
what are inhibitory neurotransmitters?
they hyperpolarise the postsynaptic membrane, preventing it from firing an action potential
e.g. ACh at cholinergic synapses in the heart open K+ channels
what is summation?
where the effect of neurotransmitter released from many neurones is added together
what are the 2 types of summation?
spatial summation
temporal summation
what is spatial summation?
many neurones connect to 1 neurone
small amount of neurotransmitter released from each altogether can reach threshold in postsynaptic neurone to trigger/ inhibit an action potential
what is temporal summation?
2 or more nerve impulses arrive in quick succession from the same presynaptic neurone
an action potential is more likely because more neurotransmitter is released into synaptic cleft
what is a neuromuscular junction?
a synapse between a motor neurone and a muscle cell
what neurotransmitter do neuromuscular junctions use?
ACh, which binds to nicotinic cholinergic receptors
how do neuromuscular junctions work differently to a cholinergic synapse between 2 neurones?
postsynaptic membrane has lots of folds that form clefts. these clefts store AChE
postsynaptic membrane has more receptors than other synapses
ACh always excitatory at neuromuscular junction
how can drugs affect synaptic transmission?
agonists antagonists inhibit breakdown of neurotransmitters stimulate release of neurotransmitter inhibit release of neurotransmitter
what are agonists?
drugs that are the same shape as neurotransmitters so they mimic their action at receptors
more receptors are activated
what are antagonists?
drugs that block receptors so they can’t be activated by neurotransmitters
fewer receptors can be activated
what do drugs that inhibit breakdown of neurotransmitters do?
drugs that inhibit the enzyme that breaks down neurotransmitters
means there are more neurotransmitters in the synaptic cleft to bind to receptors and they’re there for longer
what affect do drugs that stimulate the release of neurotransmitter from presynaptic neurone have?
more receptors are activated
what do drugs that inhibit release of neurotransmitter do?
fewer receptors are activated
what is skeletal muscle?
the type of muscle used to move
attached to bones by tendons
what do ligaments do?
they attach bones to other bones to hold them together
how do we move?
pairs of skeletal muscles contract and relax to move bones at a joint
bones are incompressible so act as levers, giving the muscle something to pull against
what are antagonistic pairs?
muscles that work together to move a bone
agonist - contracting muscle
antagonist - relaxing muscle
what is skeletal muscle made up of?
large bundles of long cells called muscle fibres
what is the cell membrane of a muscle fibre cell called?
sarcolemma
what is the cytoplasm of a muscle fibre cell called?
sarcoplasm
what are the holes in the muscle fibre made from the sarcolemma folding inwards called?
transverse tubules
T tubules
what do T tubules do?
they help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre
what is the sarcoplasmic reticulum?
a network of internal membranes that run through the sarcoplasm
what does the sarcoplasmic reticulum do?
it stores and releases calcium ions that are needed for muscle contraction
why do muscle fibres contain lots of mitochondria?
to provide the ATP that’s needed for muscle contraction
what are some properties of muscle fibres?
they have lots of mitochondria
they are multinucleate
they contain myofibrils
what does multinucleate mean?
they contain many nuclei
what are myofibrils?
long, cylindrical organelles
made up of proteins and are highly specialised for contraction
what do myofibrils contain?
thick and thin myofilaments
what are thick myofilaments made of?
myosin
what are thin myofilaments made of?
actin
what does a myofibril look like under a microscope?
it looks stripy
it is dark where the myofibrils over lap - A-band
it is light in places where there is only 1 myofibril
I-bands = only thin actin filaments
H-zone = only myosin
what are myofibrils made up of?
many short units called sarcomeres
what does a sarcomere contain?
the ends are marked with a Z-line with actin attached
in the middle there is an M-line which is the middle of the myosin filaments
what happens when sarcomeres contract?
myosin filaments pull actin inwards
what do myosin filaments have?
globular heads that are hinged so they can move back and forth
what does each myosin head have?
a binding site for actin
a binding site for ATP
what does actin have that allows myosin to move along it?
actin-myosin binding sites
tropomyosin is another protein wrapped around actin which has to move out of these binding sites
what does tropomyosin do in resting muscle?
the actin-myosin binding site is blocked by tropomyosin
myofilaments can’t slide past each other because the myosin heads can’t bind to the actin-myosin binding site on the actin filaments
what happens when an action potential from a motor neurone stimulates a muscle cell?
it depolarises the sarcolemma, depolarisation spreads down the T-tubules o the sarcoplasmic reticulum
what happens when the sarcoplasmic reticulum is depolarised?
it releases stored calcium ions into sarcoplasm
what do calcium ions do to tropomyosin?
Ca2+ bind to protein attached to tropomyosin, causing it to change shape, pulling tropomyosin out of actin-myosin binding site
meaning an actin-myosin cross bridge can be formed
what does calcium do in the sarcomere?
bind to tropomyosin
activate ATP hydrolase
what do myosin heads do in a muscle contraction?
when myosin binds to tropomyosin it changes shape and releases ADP, causing myosin head to swivel, pulling actin inwards - power stroke
ATP binds to myosin breaking cross bridge
ATP hydrolysed by ATPase region of myosin head, causing it to recock ready for next power stroke
what happens when a muscle stops being stimulated?
Ca2+ leave binding sites and are moved by active transport back into sarcoplasmic reticulum
tropomyosin moves back into actin-myosin binding sites
actin filaments slide back to relaxed position
how is ATP generated for muscle contractions?
aerobic respiration
anaerobic respiration
ATP-Phosphocreatine system
how does aerobic respiration provide ATP for muscle contractions?
most ATP is generated via oxidative phosphorylation in the cell’s mitochondria
only works when oxygen is present
how does anaerobic respiration provide ATP for muscle contractions?
ATP made rapidly by glycolysis
lactate can quickly build up in muscles and cause muscle fatigue
how does PCr system provide ATP for muscle contractions?
ATP made by phosphorylating ADP adding phosphate group from PCr
PCr stored inside cells and system generates ATP very quickly
PCr runs out after a few seconds
system is anaerobic and alactic
what does alactic mean?
doesn’t form any lactate
what 2 types of muscle fibre are skeletal muscles made up of
slow twitch
fast twitch
what are the properties of slow twitch muscle fibres?
contract slowly
less powerful contractions
used for posture
good for endurance
work for a long time without getting tired
slow release energy from aerobic respiration, lots of mitochondria and blood vessels
red - lots of myoglobin
what are the properties of fast twitch muscle fibres?
more powerful, rapid contractions
intense activities
thicker and more myosin filaments
high glycogen conc. to hydrolyse into glucose for anaerobic respiration
high conc. of enzymes for anaerobic respiration
phosphocreatine stores - rapid ATP synthase in anaerobic conditions
what is the nervous system?
communication via nerve impulses transmission by neurones rapid transmission impulse sent to specific body parts response is localised, rapid, short lived effect is temporary and reversible
what is the hormonal system?
communication via hormones
transmission through blood
slower transmission
hormones travel around whole body, only target cells respond
response is widespread, slow, long lasting
effect may be permanent and irreversible
what is a nerve impulse?
a self propagating wave of electrical activity that travels along an axon membrane
how is a generator potential produced?
stretch mediated sodium ion channel opened
sodium ions diffuse into axon
depolarisation (generator potential) occurs in this region of the axon
this depolarisation propagates across the neurone via a series of action potentials
what happens in a nerve impulse?
generator potential causes voltage gated Na+ channel proteins to open
Na+ diffuse into axon - depolarisation
if threshold reached, more Na+ channels open
action potential reached at +40mV
voltage gated Na+ channels close, K+ channels open
K+ diffuse out of axon - repolarisation
overshot of K+ - hyperpolarisation
voltage gated K+ channels close and Na+?K+ pump re-establishes resting potential - repolarisation
what does the synaptic knob contain?
vesicles containing neurotransmitter (ACh)
voltage gated Ca2+ channel proteins
smooth ER - store Ca2+
mitochondria
what is the purpose of synapses?
make sure action potentials unidirectional
single stimulus can initiate multiple responses - relay neurones have multiple axons
increase sensitivity to stimuli by spatial summation
allow weak stimuli to be filtered out - temporal summation
what is epilepsy?
an inability to regulate action potentials in brain neurones to prevent over excitation
how do inhibitory neurones work?
AP arrives at synaptic knob
Ca2+ channel proteins open and Ca2+ diffuses in
vesicles fuse to membrane and release inhibitory neurotransmitter
they diffuse across cleft and bind to K+ and Cl- channels in postsynaptic membrane
loss of K+ and gain of Cl- via diffusion results in more negative resting potential
making it more difficult to reach threshold, and preventing APs
what purpose does ATP have in muscular contractions?
providing energy for:
breaking actin-myosin cross-bridges
re-cocking myosin heads for the power stroke
what purpose do calcium ions have in muscular contractions?
activating ATPase
altering tropomyosin tertiary structure to expose myosin binding sites of actin filaments
what happens in muscle relaxation?
nervous stimulation ceases
Ca2+ is actively transported back into the sarcoplasmic reticulum
energy from ATP used to do this
tropomyosin blocks myosin binding sites on actin filaments
myosin can’t bind so contraction ceases
antagonistic muscle can pull actin and myosin filaments apart
