6B Nervous Coordination and Muscles Flashcards
What is a wave of depolarisation?
- when Na+ ions diffuse into the neuron and some of the ions move sideways
- this sideways movement opens adjacent sodium ion channels
What do waves of depolarisation ensure?
- ensures a unidirectional impulse
- this is because the region behind the wave is still vulnerable to the refractory period
Describe the all or nothing principle
- action potentials will fire with the same ferocity regardless of the size of the stimulus
- a larger stimulus will not give a larger action potential
Does a larger stimulus affect the frequency of action potentials in a neuron?
- yes
- larger stimuli will result in more frequent action potentials
- larger stimuli reduce the duration of the refractory period
- allows another action potential to be generated more quickly
Describe how myelin is involved with neurons and how it is made
- mammalian nerves are covered in a myelin sheath
- myelin is a fatty substance that insulates the axon
- myelin is made from specialised cells called Schwann cells
- there are small gaps between Schwann cells called nodes of Ranvier
Describe the relationship between nodes of Ranvier and saltatory conduction
- Na+ ions can only diffuse through the axon at these nodes
- as the ions diffuse across the axon, it depolarises the adjacent node
- this jumping between nodes is called saltatory conduction
Explain how the diameter of a neuron affects the impulses that travel through it
- impulses are faster through axons with larger diameters
- a larger diameter means more internal volume relative to its surface area
- this means there’s less resistance to the flow of ions
Explain how temperature affects speed of impulses
- ions diffuse quicker at higher temps
- only up until around 40 degrees then protein channels begin to denature
Describe how impulses begin to be transmitted across synapses
- incoming action potential causes depolarisation in the presynaptic knob
- voltage-gated calcium channels open and calcium ions flood into the presynaptic knob
What happens in synaptic transmission after calcium ions flood into the presynaptic knob?
- the influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane
- this releases nuerotransmitter into the cleft via exocytosis
- calcium ions cause the release of neurotransmitter
What happens in synaptic transmission after neurotransmitter is released into the synaptic cleft?
- neurotransmitter (acetylcholine) binds to the receptor sites on the postsynaptic membrane after diffusing across the cleft
What happens in synaptic transmission when acetylcholine binds to the receptors on the postsynaptic membrane?
- ligand-gated sodium ion channels open due to a conformational change
- this causes sodium ions to flood into the postsynaptic knob and the neuron becomes depolarised
What happens in synaptic transmission when the postsynaptic neuron becomes depolarised?
- the depolarisation must be above a threshold value
- if the threshold is reached, a new action potential will be generated and sent along the axon
What is a synapse?
- the junction between 2 neurons
- could also be found at the junction between a motor neuron and an effector
What occurs at synapses?
- electrical info is converted into chemical info
- when the ap reaches the end of a neuron, it stimulates the release of neurotransmitters
- neurotransmitters then bind to receptors on the postsynaptic neuron and trigger an ap in the new neuron
What doe synapses allow for and how?
- the unidirectional transmission of impulses
- receptors for neurotransmitter are only found on the postsynaptic membrane
What is acetylcholine?
- an example of a neurotransmitter
What are synapses that use acetylcholine referred to as?
- cholinergic synapses
Describe the whole process of synaptic transmission
- an ap arrives at the end of the presynaptic neuron
- this change in voltage opens calcium ion channels
- calcium ions enter the presynaptic knob and signal vesicles containing acetylcholine to migrate and fuse with presynaptic membrane
- acetylcholine is released into the synaptic cleft through exocytosis, it diffuses across the cleft and binds to receptors on the postsynaptic membrane
- these receptors open ligand-gated sodium ion channels that are also on the postsynaptic membrane
- sodium ions enter the postsynaptic knob leading to its depolarisation and an ap is triggered in the new neuron
- acetylcholine is reabsorbed back into the presynaptic knob through channel proteins and broken down by acetylcholinesterase
Describe the effect of excitatory neurotransmitters
- lead to depolarisation of postsynaptic knob
- if this depolarisation reaches threshold then an ap is created
Give an example of an excitatory neurotransmitter and state its specific effect
- acetylcholine
- leads to depolarisation at CHOLINERGIC SYNAPSES
Describe the effect of inhibitory neurotransmitters
- lead to hyperpolarisation of the postsynaptic knob
- prevent creation of an ap
Give an example of an inhibitory neurotransmitter and state its specific effect
- GABA (gamma aminobutyric acid)
- opens chloride ion channels which leads to hyperpolarisation
Explain how chloride ions lead to inhibition of impulses
- the postsynaptic membrane has chloride ion channels which are made to open
- leads to an influx of chloride ions making the inside of the postsynaptic membrane even more negative than when it’s at resting potential
- this means it is less likely that a new ap will be created
How do weak stimuli affect action potentials?
- they are unlikely to be triggered
- they will only release small quantities of neurotransmitter into the synaptic cleft
- resulting influx of sodium ions is unlikely to exceed threshold
Describe spatial summation
- neurotransmitters from 2 or more neurons are combined
- cumulative effect of multiple neurons may offer a depolarising effect that is sufficient to depolarise the postsynaptic neuron
Describe temporal summation
- occurs when 1 neuron fires several impulses in quick succession
- leads to release of more neurotransmitter than usual
- the increased volume of neurotransmitter may be sufficient to depolarise the postsynaptic neuron
What is a neuromuscular junction?
- a synapse between a motor neuron and an effector
- specialised type of cholinergic synapse
Where does the neurotransmitter bind in a neuromuscular junction and what neurotransmitter is it?
- acetylcholine
- binds to receptors on the postsynaptic knob called nicotinic cholinergic receptors
Contrast neuromusclar junctions and traditional cholinergic synapses
- neuromuscular junctions have more receptors on their postsynaptic membrane
- acetylcholine has an excitatory effect in neuromuscular junctions, this isn’t always true in cholinergic synapses
- neuromuscular junctions contain junctional folds
Explain the structure and function of junctional folds
- store acetylcholine
- increase the likelihood of a depolarisation leading to a muscular response
Describe and explain the effect of agonists on synapses
Give examples of these drugs
- similar shapes to the genuine neurotransmitter so they mimic the action of the neurotransmitter when they bind to receptors
- leads to more frequent depolarisations
eg. nicotine, morphine, heroin
Describe and explain the effect of antagonists on synapses
Give examples of these drugs
- block receptors by binding to them so the receptor can’t be activated which leads to more infrequent depolarisations
eg. curare, naltrexone
Describe and explain the effect of nerve gases on synapses
Give examples of these drugs
- usually work by inhibiting enzymes such as acetylcholinesterase so acetylcholine remains in synaptic cleft for much longer leading to repeated depolarisations
eg. Sarin, VX
Describe and explain the effect of stimulants on synapses
Give examples of these drugs
- encourage release of neurotransmitter which leads to more frequent depolarisations
eg, caffeine, amphetamines
Describe and explain the effect of sedatives on synapses
Give examples of these drugs
- inhibit release of neurotransmitter leading to more infrequent depolarisations
eg. alcohol
