nerve transmission Flashcards
what are action potentials
nerve impulses
what do nerve cells require
a membrane potential
whats a membrane potential
electrical charge difference across cell membrane – like a battery
what are ion channels
allow ions to move by diffusion. These generate the membrane potential. They can be passive transport
If no action potential then resting cell has resting membrane potential of what
minus 70mv
explain Intra/extra cellular ion concentrations and The resting membrane potential
When membrane resting, so nerve not responding to a stimulus.
theres a particular distribution of high levels of sodium ions in extracellular environment and a high level of potassium ions in intracellular environment
When resting, more potassium ions leave cell than sodium ions entering cell, due to distribution of ions and charge across cell membrane
Sodium potassium transporter pump actively pumps these ions across membrane by ATP breakdown so actively
what channels do we find at resting membrane potential
leakage channels
explain leakage channels
Leakage channels = channels that assist membrane permeability to certain ions, but always work in maintaining resting membrane potential
- cytosol high in K+ & interstitial fluid high in Na+ (sodium –potassium pumps)
Leakage lets K+ through easily and Na+ poorly
- inside is negative relative to outside
Actual RMP depends on the relative leakage channel numbers
explain action potential
Series of active events, that take the action along a nerve
during the events: channels actively open & close
Some initial event is required to reach a voltage threshold (~ - 55 mv)
this is a Stimulus
- any event bringing membrane to threshold
what is the voltage threshold in mv
minus 55mv
After the arrival of a stimulus, and once threshold reached, the action potential goes into what
depolarizing phase
and then action potential moves to a repolarizing phase
explain depolarizing phase
membrane potential rises and becomes positive. So polarized membrane lost
explain Repolarizing phase
potential restored to resting value
may overshoot = hyperpolarizing phase
then recovery to rest. Resting value is around minus 70mvs
what happens If Stimulus is strong enough to reach threshold
- Na+ channels open
- Na+ ions enter the cell from extracellular fluid
- depolarization of membrane occurs (positive membrane potential) - K+ channels then open
- K+ leave the cell
- repolarization (negative membrane potential) - Sodium/potassium pump
restores original ion concentrations
With hyperpolarization the stimulus allows potassium channels to what
open, hence internal environment becomes more negative as those ions leave
an action potential can lead to the opening of the sodium channels leading to a degree of depolarization but isnt always what
sufficient enough to reach the threshold
explain what happens in the full generation of an action potential, (on a graph)
Theres opening of sodium channels but in this case the stimulus is strong enough to allow more sodium channels to open. The net result is a movement of membrane potential from negative to positive, and action potential is generated
After peak achieved, cell repolarizes as sodium channels close and potassium channels open, going into a hyperpolarized phase where the membrane potential goes slightly lower than resting. This is then normalized with activity of sodium potassium pumps
If threshold is reached then the action potentials are always what
the same size
Stimulus must be sufficient enough for what to start
threshold to start
After one AP there is a short period before next can be triggered
what is this called
refractory period – put in place to make sure that direction in which it travels is the same and impulse doesn’t come back on itself
why does a strong stimulus creates a greater response
response because the frequency of action potentials is higher. Not to do with the size of the peak
what substances can affect nervous transmission
caffein
alcohol
Local Anaesthetics –for treating pain etc
explain hoe caffein affects nervous transmission
a stimulant found in coffee, tea, cocoa, cola etc.
acts by lowering the threshold level of the nerve
increases the possibility for a stimulus to result in a nerve potential
explain how alcohol affects nervous transmission
acts as a nervous system depressant
increases the nerve threshold level, so may be minus 30mv compared to minus 55mv, so stimulus has to be stronger to reach the threshold levels
decreases the possibility of a stimulus to result in a nerve action potential. As takes more effort for nerve threshold to be achieved
explain how Local Anaesthetics –for treating pain etc, affect nervous transmission
Generation of nerve action potentials can be prevented by local anaesthetics
such as novocaine and xylocaine.
These drugs prevent the opening of sodium gated channels in the nerve cell membrane
- pain messages are not carried from the site of injury back to the CNS. So feel no pain
how is a nerve impulse conducted
Each section triggers the next by local currents
refractory period keeps it going the right direction
explain the conduction of a nerve impulse in an Unmyelinated fibre
continuous conduction
explain the conduction of a nerve impulse in a myelin fibre
saltatory conduction – action potential jumps from unmyelinated regions to myelinated regions. So its faster
can only be triggered at Nodes of Ranvier
what fibres conduct nerve impulses fastest
Myelinated fibers are faster & move larger neurons faster
explain synaptic transmission
Sequence of events at synapse
if action potential strong enough to reach synapse then a new series of events occurs
These are Triggered by voltage change of the action potential
if 2 neurons close to one another and communicating with one another:
- Sending neuron –
the first is called, presynaptic - Receiving neuron
the second neurone is called postsynaptic - Neurotransmitter
chemical that carries signal across synaptic cleft
Between the pre and post synaptic neurons theres a synaptic cleft
explain the presynaptic events
- Action potential arrives at presynaptic end bulb
- Opens voltage gated Ca2+ channels
- Ca2+ flows into cell from synaptic cleft
- increased Ca2+ concentration in presynaptic cell,
- causing exocytosis of synaptic vesicles - Neurotransmitter (NT) released into cleft
diffuse across to postsynaptic cell membrane
explain the Postsynaptic Events
- Once NT binding at postsynaptic receptors
- Binding causes a Chemical trigger of ion channels, to open
- May depolarize or hyperpolarize postsynaptic cell membrane, depending on which channels open, due to the nature of the neurotransmitter
- If threshold reached at axon hillock
- then postsynaptic cell action potential results, and the evets occur again but in post synaptic cell
how is Termination of Synaptic Transmission done
by removing the neurotransmitter
how is the neurotransmitter removed to allow termination of Synaptic Transmission
Diffusion
Destroyed by enzymes in cleft
Transported back into presynaptic cell
Destroyed by neuroglia
name some neurotransmitters
Acetylcholine (Ach)
- common in PNS most common in peripheral nervous system
- may be stimulatory (by causing depolarization)or inhibitory (by causing hyperpolarization)
Amino Acids
- glutamate, aspartate, gamma aminobutyric acid (GABA), glycine
Modified amino acids
- norepinephrine (NE), dopamine (DA), serotonin (5-HT)
Neuropeptides
- endorphins
Nitric oxide (NO)
name some chemicals affecting synaptic transmission
Botulinum toxin – a neurotoxin protein
Curare – plant extract
Nerve agents
explain Botulinum toxins affect on synaptic transmission
its a neurotoxin protein
inhibits release of acetylcholine
inhibits muscle contraction (leading to paralysis)
One of Most acutely lethal toxins
explain Curare affect on synaptic transmission
competes for acetylcholine receptor sites on muscle cells
muscle relaxant, so muscle function lost ( used in surgical procedures previously) in high doses it can lead to death
explain nerve agents affect on synaptic transmission
inactivates acetylcholinesterase. By blocking activity of enzymes that break down acetylcholinesterase
leads to tetanus (paralysis)
Can lead to death even mins after exposure
