Topic 6B: Nervous Coordination Flashcards
What is a neurone like at rest?
- Resting potential (around -70mv)
- Polarised -> outside is positive - more +ve ions
How is the resting potential maintained?
- Sodium-potassium pump
- 3Na+ moved out - membrane not permeable to them - can’t diffuse back in - build up outside - sodium electrochemical gradient
- 2K+ moved in - membrane more permeable to them so diffuse back out through K+ channels
- Needs energy from ATP
How does depolarisation of a neurone occur?
- Stimulus excites the neurone cell membrane opening voltage gated Na+ channels - Na+ move in (facilitated diffusion) - membrane becomes less negative - generator potential
How does an action potential form from a depolarised neurone?
- If threshold reached - action potential formed
- Membrane becomes more permeable to Na+ - more channels open - Na+ rush in by facilitated diffusion - further depolarisation
How does repolarisation occur?
- Voltage gated Na+ channels close and K+ open
- Na+ cannot enter but more K+ leave by facilitated diffusion - so axon becomes more negative
How does hyperpolarisation happen?
- Voltage gated K+ channels slow to close - axon briefly too negative - more negative than the resting potential
How is the resting potential reset?
- Ion channels reset
- Sodium-potassium pump returns the membrane to the resting potential
What is the refractory period?
- After an action potential - neurone cannot immediately be excited - ion channels are recovering - can’t be made to open
- Na+ channels closed, K+ closed
Describe a wave of depolarisation
- With an action potential - some Na+ move sideways (diffuse) - causes Na+ channels there to open and Na+ to enter
- Wave moves away from the part of membrane in the refractory period - these cannot produce an action potential
How are impulses made discrete?
- Refractory period - ion channels recovering - acts as a time delay between action potentials
- Action potentials do not overlap
- Limit frequency impulses can be transmitted
- Action potentials unidirectional
How are action potentials all or nothing?
- If the threshold reached - action potential always happens
- Same change in voltage - always same size
How is a bigger stimulus expressed?
- More frequent action potentials
What is myelin?
- Electrical insulator
- Schwann cells
What are patches of bare membrane on neurones called?
- Nodes of Ranvier
How does myelination help impulses?
- Depolarisation happens only at the nodes of Ranvier - impulse jumps - faster - only areas Na+ can move through
What is the movement of impulses in myelinated neurones called?
- Saltatory conduction
What is an advantage of myelination?
- Less ATP needed - resting potentials only re-established at the nodes - less work for sodium-potassium pump
How do impulses move in unmyelinated neurones?
- Wave of depolarisation must pass through every section of the membrane
How does axon diameter affect the speed of impulses?
- Wider = faster impulse transmission - less resistance to flow of ions in the cytoplasm
- Smaller SA:V = fewer ions leak - action potentials propagate easier
How does temperature affect the speed of impulses?
- Ions diffuse faster - have more kinetic energy
- Enzymes in respiration work faster - more ATP for active transport in sodium-potassium pump
How is synaptic transmission unidirectional?
- Receptors only on post synaptic membrane
- Vesicles of neurotransmitter only in presynaptic neurone
What does an action potential do in the presynaptic membrane?
- Action potential arrives at presynaptic knob
- Voltage gated Ca2+ channels open
- Ca2+ move in
Make vesicles of neurotransmitter (ACh) move to the membrane, fuse and release into the synaptic cleft
What does neurotransmitter do once released?
- ACh diffuses across the cleft, binds to complimentary receptors on postsynaptic membrane
- Na+ channels open, N’a+ move in
- Membrane depolarised
- Action potential if threshold reached
What happens to ACh after the impulse has been transmitted?
- Removed from cleft so the response stops
- Acetylcholinesterase breaks it down - products reabsorbed into presynaptic neurone to reform ACh
What do excitatory neurotransmitters do?
- Depolarise the postsynaptic membrane
- Create action potential if threshold reached
- e.g. Neuromuscular junction and Na+ channels open
What do inhibitory neurotransmitters do?
- Hyperpolarise the postsynaptic membrane
- Prevent an action potential by making it more negative
- e.g. ACh in heart - K+ channels open and move out
What is temporal summation?
- Multiple impulses from 1 presynaptic neurone in succession
- Inc conc neurotransmitter in cleft
- Inc likelihood of an action potential
What is spatial summation?
- Multiple impulses from multiple presynaptic neurones all applied to one postsynaptic neurone
- Add together for an action potential
What is the sarcolemma?
- Membrane of muscle cell
What is the sarcoplasm?
- Cytoplasm of muscle cells
What is the sarcoplasmic reticulum?
- Endoplasmic reticulum of muscle cells
What is a neuromuscular junction?
- Synapse between motor neurone and muscle cell
What neurotransmitter is used and what does it bind to?
- ACh
- Nicotinic choligernic receptors
What are the differences between a cholinergic synapse and a neuromuscular junction?
- Postsynaptic membrane has folds - clefts that store acetylcholinesterase
- Postsynaptic membrane has more receptors
- ACh always excitatory - normally triggers a response
What are 5 ways a drug can impact synapses?
- Same shape as neurotransmitter - mimic action - more receptors activated - agonist
- Block receptors so they cannot be activated - fewer receptors activated - antagonist
- Inhibit enzyme that breaks down neurotransmitter in cleft - more left in cleft
- Stimulate release of neurotransmitter from presynaptic neurone - more receptors activated
- Inhibit release of neurotransmitter from presynaptic neurone - fewer receptors activated
What are antagonistic muscles?
- Work in pairs to move a bone
- One contracts, the other relaxes
What is the general structure of a muscle?
- Large bundles of long cells - muscle fibres
What are t-tubules and what do they do?
- Bits of sarcolemma fold inwards into the sarcoplasm
- Helps spread electrical impulses throughout the sarcoplasm to all parts of the muscle fibre
What does the sarcoplasmic reticulum do?
- Stores and releases Ca2+
What organelles do muscle cells have that make it specialised?
- Lots of mitochondria - ATP for contraction
- Multinucleated - DNA to code for proteins and enzymes
What is myosin?
- thick filament
What is actin?
- Thin filament
What is the A band?
- Dark bands
- Myosin and overlapping actin
What is the I band?
- Light bands
- Actin only
What is the H zone?
- Slightly dark
- Only myosin
What is the Z line?
- Ends of sarcomeres
- Centre of actin and I band
What is the M line?
- Middle of myosin and H zone
What is a sarcomere?
- 1 contracting unit of the myofibril
What happens to the sarcomere length when it contracts?
- Gets shorter
What happens to the A band when it contracts?
- Same length
What happens to the I band when it contracts?
- Gets shorter
What happens to the H zone when it contracts?
- Gets shorter
What are myosin heads like?
- Hinged globular heads - move back and forth
- Has binding site for actin and ATP
What are binding sites on actin like?
- Has binding sites for myosin
- Blocked by tropomyosin
How does the muscle begin to contract?
Action potential to cross bridge formation
- Nerve impulse arrives at neuromuscular junction - ACh released into cleft - binds to complimentary receptors on sarcolemma - Na+ channels open and Na+ move in
- Action potential produced in sarcolemma and travels along t-tubules
- Sarcoplasmic reticulum more permeable to Ca2+ - diffuse out into sarcoplasm - bind to tropomyosin
- Binding sites on actin revealed, myosin binds and cross bridges are formed
How do muscle filaments move past each other?
- Myosin heads change angle - powerstroke
- Actin pulled across myosin - ADP released
How do myosin heads detach?
- ATP binds to head, detaches, Ca2+ activates ATP-ase to hydrolyse ATP providing energy so myosin heads return to their original position - recovery stroke
How does contraction continue?
After myosin detached
- Myosin attaches to new site, continues
What happens when muscle contraction is done?
- Ca2+ pumped back into the sarcoplasmic reticulum
What is the role of ATP in muscle contraction?
- Allow myosin to attach
- Cause head to move back to original position
- ADP+Pi released from myosin head to cause it to ,move - powerstroke
What are slow twitch muscles and what are they for?
- Contract slowly
- Endurance
What are features of slow twitch muscles?
- Fatigue slowly
- Lots of mitochondria - aerobic respiration
- Lots of blood vessels - O2 supply
- Dark colour - myoglobin
What are fast twitch muscles and what are they for?
- Contract quickly
- Fast movement
What are features of fast twitch muscles?
- Fatigue quickly
- Fewer mitochondria and blood vessels
- Anaerobic respiration - use glycogen stores
- Light colour - less myoglobin
How does aerobic respiration release energy?
- Oxidative phosphorylation from electron transport chain
- Needs O2
How does anaerobic respiration release energy?
And features of this
- Glycolysis
- Produces lactate - muscle fatigue
- Short periods only
How does the ATP phosphocreatine system release energy?
- Phosphorylate ADP using phosphate from PCr - stored in cells
- Stored incells
- Generates ATP quickly
- Anaerobic and alactic
