6B Nervous Coordination Flashcards
What is the charge like in a neuron at resting state?
Outside of membrane is positively charged to inside the membrane (membrane is polarised)
What is the resting potential across the neurons membrane?
-70mV
How is the resting potential maintained in a neuron?
- Sodium potassium pump
- Potassium ion channels
What does the sodium potassium pump do?
- Pumps 3 Na+ out of the neuron
- Pumps 2 K+ into the neuron
Is the neuron membrane permeable to Sodium and Potassium?
No Sodium
Yes Potassium
How do Potassium ions diffuse through the membrane?
Diffuse out of neuron through Potassium ion channels
What is created when sodium moves out of the cell?
Sodium ion electrochemical gradient
What are the stages when a cell membrane is stimulated?
1) Stimulus
2) Depolarisation
3) Repolarisation
4) Hyperpolarization
5) Resting potential
What happens when there is a stimulus?
- Excites cell membrane causing sodium channels to open
- Sodium ions diffuse into cell making is less negative
What happens during depolarisation?
- Occurs after stimulus
- If threshold of -55mv is reached more sodium channels open
- Rapid diffusion into membrane
What happens during repolarization?
- Occurs at potential difference of about +30 mv
- Sodium ion channels close and potassium ion channels open
- K+ ions diffuse out of cell so PD goes back to resting potential
What happens during hyperpolarization?
- After repolarization K+ channels slow to close so there is a ‘overshoot’
- PD becomes more negative (less than -70 mv)
What happens after hyperpolarization?
- Ion channels are reset
- Sodium potassium pump returns membrane to resting potential
Where does the action potential move to?
Move along the neuron like a wave or depolarization
When an action potential happens how do some of the sodium ions diffuse?
They diffuse sideways
Why do sodium ions diffuse sideways during an action potnential?
It causes sodium ion channels in next region to open (wave of depolarisation)
What occurs after an action potential?
The refractory period
What is the refractory period?
Where sodium and potassium ion channels are recovering and can’t be opened
What is the purpose of the refractory period?
- Means action potentials don’t overlap
- action potentials are unidirectional (one directional)
- Limits the frequency at which nerve pulses can be transmitted
What is an action potentials all or nothing nature?
If the threshold inst met an action potential wont fire
Does the size of the stimulus affect the size of the action potential?
No, but it will cause it to become more frequent
What are the 3 factors that affect the speed of an action potentials?
- Myelination
- Axon diameter
- Temperature
How does the axon diameter affect action potentials speed of conduction?
- A larger diameter means action potential are conducted quicker
Why does having a larger diameter increase action potentials speed of conduction?
Less resistance to flow of ions - therefore depolarization reaches other parts of cell membrane quicker
How does temperature affect action potentials speed of conduction?
Speed of conduction increases with the temperature
Why does temperature increase action potentials speed of conduction?
Because ions diffuse faster (up to 40 degrees where they start to denature)
How does myelination affect the speed of conduction of action potentials?
It increases it
What structure do myelinated neurons have?
- Myelinated sheath (electrical insulator)
- Sheath made up of Schwann cells
- Between Schwann cells are nodes of Ranvier
- Sodium ions are concentrated at the nodes
Why is conduction of action potential faster in myelinated neurons?
- Depolarization occurs at nodes of ranvier
- Neuron’s cytoplasm conducts electrical charge to depolarise the next node, makes impulses jump from node to node (which is faster)
What are nodes of Ranvenir?
Gaps in between the myelin sheath
Why does the myelin sheath cause impulses to move faster?
The myelin sheath is an electrical insulator
What is Saltatory conduction?
Where the neuron’s cytoplasm conducts electrical charge to depolarise the next node, makes impulses jump from node to node
Why do impulses travel more quickly along mylenated neurons than non-myelenated neurons?
Non-mylenated –> impulses have to travel as a wave the whole way length of the axon membrane
Slower process than saltatory conduction
Can there be multiple types of neurotransmitters?
Yes
They can be excitatory, inhibatory or both
How do excitatory neurotransmitters act and what does this cause?
They depolarise the postsynaptic membrane
Causes it to fire an action potential if the threshold is reached
What is a synapse?
A junction between a neurone and the next cell or between a neurone & an effector cell
e.g. a muscle or glad cell
What is the synaptic cleft?
The tiny gap between the cells at a synapse
What is the presynaptic neurone like?
It has a swelling called a synaptic knob - this contains synaptic vesicles filled with chemicals called neurotransmitters
What happens when an action potential reaches the end of the presynaptic neuron?
It causes neurotransmitters to be released into the synaptic cleft
They diffuse into across the postsynaptic membrane & bind to specific receptors
What happens when neurotransmitters bind to receptors?
They might trigger an action potential (in a neurone), cause muscle contraction (in a muscle cell), or cause a hormone to be secreted (from a glad cell)
Why can impulses only travel in one direction?
The receptors are only on the postsynaptic membranes, synapses make sure impulses are unidurectional
Why are neurotransmitters removed from the synaptic cleft?
So the response doesn’t keep happening
e.g. they’re taken back into the presynaptic neurone or they’re broken down by enzymes
Give some examples of different neurotransmitters
Acetlycholine & noradrenaline
What are synapses that use acetylcholine called?
They are called cholinergic synapses
Where does acetylcholine transmit nerve impulses?
Across a cholinergic synapse
How is a nerve impulse transmitted across a cholinergic synapse?
1 - Action potential (AP) arrives at synaptic knob of the presynaptic neurone
2 - AP stimulates voltage-gates calcium ion channels in presynaptic neurone to open
3 - Ca2+ ions diffuse into the synaptic knob
4 - Influx of Ca2+ ions into the synaptic knob causes synaptic vesicles to move to the presynaptic membrane. They then fuse with presynaptic membrane
5 - Vesicles release the NT acetylcholine (ACh) into synaptic cleft - this is called exocytosis
6 - ACh diffuses across the synapticcleft & binds to specific cholinergic receptors on the postsynaptic membrane
7 - Causes Na2+ ion channels in postsynaptic neurone to open
8 - Influx of sodium ions into postsynaptic membrane causes depolarisation. An AP on postsynaptic membrane is generated if threashold is reached
9 - ACh removed from synaptic cleft so response doesn’t keep happening - broken down by enzyme called acetylcholinesterase (AChE) & products are re-absorbed by presynaptic neurone & used to make more ACh
Give an example of an excitatory NT
Acetylcholine, it’s found at cholinergic synapses in the CNS
It binds to cholinergic receptors to cause an action potential in the postsynaptic membrane & and at neuromuscular junctions
What do inhibitory neurotransmitters do?
They hyperpolarise the postsynaptic membrane (makes potential difference more -ive), preventing it from firing an action potential
Give an example of an inhibitory NT
Acetylcholine is inhibitory at cholinergic synapses in the heart
When it binds to receptors here, it can cause potassium ion channels to open on the postsynaptic membrane, hyperpolarising it
What are the two types of summation?
- Spatial summation
- Temporal summation
What is summation?
Is where the effect of NT released from many neurones (or one neurone that’s stimulated a lot in a short period of time) is added together
What is spatial summation?
Where many neurones summate at one neurone
What happens in spatial summation?
1 - Sometimes many neurones connect to one neurone
2 - The small amount of NT released from each of these neurones can be enough altogether to reach the threshold in the postsynaptic neurone & trigger an action potential
3 - If some neurones release inhibitory NT then the total effect of all the NTs might be no AP
What is temporal summation?
Is where two or more nerve impulses arrive in quick succession from the same presynaptic neurone
Makes an AP more likely because more NT is released into the synaptic cleft
What are skeletal muscles like?
They are the type of muscle you use to move - also called voluntary muscles
They are attached to bones by tendons
How do muscles work in antagonistic pairs?
Pairs of skeletal muscles contract & relax to move bones at a joint
The bones of the skeleton are INCOMPRESSIBLE (rigid) so act as a lever - give the muscles something to pull against
What are antagonistic pairs?
Muscles that work together to move a bone
What does the agonist muscle do?
It is the contracting muscle
What does the antagonist muscle do?
It is the relaxing muscle
What is skeletal muscle made up of?
Long muscle fibres
How are muscles stimulated to contract?
By neurones - they act as effectors to the stimulus
What is the sarcolemma?
The cell membrane of muscle fibre cells
What are the folds in the sarcolemma called?
Transverse (T) tubules
What are T tubules?
Where bits of the sarcolemma fold inwards across the muscle fibre into the sarcoplasm to create folds
What do T tubules do?
They help to spread electrical impulses throughout the sarcplasm so they reach all parts of the muscle fibre
What is the sarcoplasm?
They cytoplasm of muscle fibre cells
What is the sarcoplasmic reticulum and what does it do?
A network of internal membranes that run through the sarcoplasm
It stores & releases calcium ions that are needed for muscle contraction
Why do muscle fibres have lots of mitochondria?
They provide the ATP that’s needed for muscle contraction
What does muscle cells being multinucleate mean?
They contain many nuclei
What are muscle fibres made up of?
Myofibrils - long, cylindrical organelles, made up of proteins specialised for contraction
What are the two types of filaments contained in myofibrils?
- Thick mysoin filaments
- Thin actin filaments
What are thick myofilaments made up of?
The protein myosin
What are thin actin filaments made up of?
The protien actin
What protien is contained in the dark bands (muscles)?
The thick myosin filaments & some overlapping actin filaments - these are called the A band
What protien is contained in the light bands (muscles)?
Thin actin filaments only - these are called I bands
What makes up a myofibril?
Many short units called sarcomeres
What is the Z-line?
The end of each sarcomere (short unit fo the myofibril)
What is the M-line?
The middle of each sarcomere is the M-line
The M-line is in the middle of the myosin filament
What is the H-zone?
It contains ONLY myosin filaments
Around the M-line is the H-zone
What is the A-band?
It contains BOTH thick myosin filaments and thin actin filaments
What is the I-band?
Contains only thin actin filaments
What theory is used to explain muscle contraction?
The sliding filament theory
How does the sliding filament theory work?
- Myosin & actin filaments slide over one another to make sarcomeres contract - myofilaments themselves don’t contract
- Simultaneuos contraction of lots of sarcomeres means myofibrils & muscle fibres contract
- Sarcomeres retun to their original length & muscle relaxes
What happens to each band when the sarcomere contracts (shortens)?
- A-bands stay the same length
- I-band gets shorter
- H-zone get shorter
What are myosin filaments like?
They have globular heads & binding sites
What are the globular heads on myosin filaments like?
They’re hinged, so they can move back and forth
What do each myosin head have?
A binding site for actin & a binding site for ATP
Actin filaments have binding sites for myosin heads, called actin-myosin binding sites
Where is tropomyosin found?
It is found between actin filaments
What does tropomyosin do?
It is a protein that helps myofilaments move past each other
It blocks the bindign sites in resting muscles for the myosin head
Why can’t myofilaments slide in a resting (unstimulated) muscle?
In a resting muscle, the actin-myosin binding site is blocked by tropomyosin
The myosin heads can’t bind to the actin-myosin binding site on the actin filaments
What is muscle contraction triggered by?
An influx of calcuim ions
What is the process of a triggering a muscle to contract? (long version)
1 - When an AP from motor neurone stimulates a muscle cell, it depolarises sarcolemma - depolarisation spreads down T-tubules to the sarcoplasmic reticulum
2 - Causes sarcoplasic reticulum to release stored calcium ions (Ca2+) into the sarcoplasm
3 - Ca2+ ions bind to protein attached to tropomyosin, causes protein to change shape - pulls attached tropomyosin out of actin-myosin binding site on actin filament
4 - Exposes binding site - allows myosin head to bind
5 - Bond formed when mysoing head binds to actin filament –> called actin-myosin cross bridge
6 - Ca2+ ions activate enzyme ATP hydrolase - hydrolyses ATP to provide energy for muscle contraction
7 - Energy released from ATP causes myosin head to bend, pulls acting filament along
8 - Another ATP molecule provides energy to break actin-myosin cross bridge - so myosin head detaches from actin filament once moved
9 - Myosin head reattaches to different binding site further along actin filament - new actin-myosin cross bridge formed & cycle repeats
Cycle will cont. as long as Ca2+ ions are present
How do myosin heads work together to pull the actin filament at the same time?
Many cross bridges form & break very rapidly, pulling the actin filament along - shortens the sarcomere, causes muscle to contract
Describe the sliding filament theory of muscle contraction
- Ca2+ ions cause tropomyosin to move away froma ctin binding sites, allowing myosin head to attach & form a cross-bridge
- Myosin head changes angle, pulling actin filament along & releasing ADP
- ATP attaches to myosin head, allowing it to detach
- ATP hydrolysed, allowing myosin head to return to original position
