C2.2 Neural signalling (year 6) Flashcards
Describe the structure of a neuron
Nucleus surrounded by cell body with dendrites and axon attached to it.
Dendrites are multiple short fibres whereas axons are long single fibres that end with synaptic terminals (release neurotransmitters to continue the impulse chemically)
Describe the sequence of an action potential that allows nerve impulses to travel
An action potential is a sequence of events that allow an impulse to travel through a neuron:
- When a neuron is ready to send an impulse, it is POLARIZED and has a RESTING POTENTIAL across the membrane
- the resting potential is created by active transport of Na+ and K+ ions, with the Na+ ions actively transported out of the axons cytoplasm to the intercellular fluid, and K+ ions from the outside of the cell into the cytoplasm. (Sodum potassium pump)
- the Na-K pump transports 3 Na+ ions out for ever 2 K+ pumped in. ATP is required.
- There are also -ve charged organic ions permanently located in the cytoplasm of the axon = the cell becomes +ve charged in relation to the inside.
- this potential difference in charge across cell membrane creates the membrane potential.
Nerve impulse propagated through neurons > Na-K pump to restore resting potential
This loop repeats to allow the neuron to propagate another action potential right away
Compare the speed of nerve impulses in myelinated/non-myelinated fibres and larger/smaller nerve fibres
Myelinated>non-myelinated
- when myelin sheaths are present, action potentials skip from one node of Ranvier to the next as the impulse progresses along the axon towards the synaptic terminals
- action potential does not have to undergo the time-consuming and energy-expensive ion movements in the membrane underneath the myelin material.
- Advantages: impulse travels faster as in/out movement of ions only occurs at the nodes of Ranvier, and less ATP is expended because Na-K pump only is working at the nodes.
Larger>smaller axons
- axons with a greater diameter have a faster transmission velocity than those with a smaller diameter
Describe what happens when an action potential reaches the terminal buttons/synapse
- The action potential arrives at the terminal buttom = depolarization of the presynaptic membrane and uptake of Ca2+ ions into the terminal buttons
- Ca2+ ions act as a signalling chemical, activating a pathway that moves vesicles containing the neurotransmitter through the cell.
- the vesicle fuses with the presynaptic membrane
- Neurotransmitter is released from the fused vesicles into the synaptic cleft
- Neurotransmitter binds with a protein receptor on the postsynaptic neuron membrane
- This results in an ion channel opening, with Na+ ions diffusing in through the channel
- This initiates the action potential to begin moving down the postsynaptic neuron, which is now depolarised
- Neurotransmitters are released back into the synaptic cleft and degraded by enzymes
- Ion channel in postsynaptic membrane closes to Na+ ions
- Neurotransmitter fragments diffuse back across the synaptic cleft to be reassembled in the terminal buttons of the presynapatic neuron.
Example of a neurotransmitter used in the synaptic cleft
Acetylcholine is found at the synapses btwn a neuron and muscle cell.
Acetylcholinesterase is an enzyme that breaks down acetylcholine into fragments, so that the action potential from the presynaptic membrane to the postsynaptic membrane occurs only once
(in-depth) describe the process of neuron depolarization/repolarization
Depolarization;
1. Na+ ions are actively transported (against conc. gradient) to the outside of the membrane, creating a conc. gradient.
2. When the voltage changes acorss the membrane (due to an action potential), the voltage-gated Na+ channel opens and Na+ ions diffuse in down the conc. gradient into the axon cytoplasm.
3. This reusults in the inside of the axon becoming temporarily positive in relation to the outside
4. the neuron membrane is DEPOLARIZED.
5. the depolarized area of the axon then initiates the next region of the axon to open the voltage-gated channels for Na+, and so the action potential continues down the axon.
Repolarization:
1. A neuron cannot send another action potential until the RESTING POTENTIAL is reached once again.
2. During depolarization, for a short period, both Na+ and K+ ions are inside the cell so the inside of the membrane is positive relative to the outside.
3. immediately after depolarization, voltage-gated K+ channels open to K+ ions which diffue out of the axon.
4. the inside of the axon becomes -ve charged relative to the outside.
5. the Na-K pump begins to actively transport Na and K ions across the membrane to reach resting potential
Describe the properties (?) of an action potential
- self-propagating (once started at the dendrite end of a neuron, will self-propaate to the axon end of the cell to the synaptic terminals)
- must reach a minimum threshold potential to be self-propagated. (e.g. in retinal cels, a minimum intensity of light must be reached)
- there is no such thing as a strong or weak impulse due to the all-or-nothing action of depolarization
What is an oscilloscope?
Electronic test instrument that graphically displays changing voltages.
- used in the study of axons to demonstrate the voltage changes that occur as part of an action potential
Describe how myelinated axon achieve faster impulses
SALTATORY CONDUCTION:
- the phenomenon where an action potential jumps from one node of Ranvier to the next.
- specialized cells called SCHWANN CELLS produce myelin by wrapping themselves around an axon multiple times, and are spaced evenly along any one axon.
- the gaps btwn schwann cells are nodes of Ranvier.
- Saltatory conduction enables a faster transmission velocity of action potentials as ion movements ONLY OCCUR at the nodes of Ranvier, such that depolarization and repolarization only occur at the nodes = allows action potential to jump along the length of the axon.
Advantages: requires less ATP to fuel the Na-K pump, compared to non-myelinated axons where the whole axon must be depolarized and then repolarized
Describe the effects of exogenous chemicals on synaptic transmission + 2 examples
NEONICOTINOID insecticides:
- binds to the postsynaptic receptors that normally accept acetylcholine.
- Instead of the opening of Na+ channels and propagation of action potentials that acetycholine causes, Neonicotinoid molecules do not propagate the action potential.
- the neonicotinoid molecules are also not released by the receptor and not broken down in the synaptic cleft, permanently blocking the receptor.
- this results in the paralysis and death of the affected insect
Cocaine:
- affects the neurotransmitter dopamine, preventing the removal of dopamine from the synapse.
- stimulate dopamine-releasing neurone to release dopamine that is usually held in reserve
- Normally, dopamine is removed from the synaptic cleft by a dopamine transporter.
- when cocaine is present, it attaches to the dopamine transporter and blocks the dopamine removal process = build-up of dopamine in the synaptic cleft, flooding the brain with an elevated response
- as cocaine use continues, a tolerance may develop so that higher doses and more frequent use are sought
Diff btwn exogenous and endogenous
Exogenous - chemicals produced outside the body
Endogenous- chemicals produced within the body
Describe excitatory neurotransmitters
- generate an action potential by increasing the permeability of the postsynaptic membrane to +ve ions.
- this causes Na+ ions that are in the synaptic cleft to diffuse into the postsynaptic neuron = postsynaptic neuron is depolarized locally by the influx of +ve ions
- the inside of the neuron develops a net +ve charge compared to the outside, generating an action potential
Describe inhibitory neurotransmitters + 1 example
- cause hyperpolarization of the neuron, inhibiting action potentials
- this is as the inside of the neuron becomes more -ve than normal, making it difficult for an action potential to be generated
- inhibitory neurotransmitter binds to specific receptor > -ve charged Cl- ions move across the postynaptic membrane into the neuron/+ve charged K+ ions move out of the neuron = hyperpolarization
GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter
Describe how we perceive pain
Stimulus (e.g. high temp) are sensed by protein receptors on the membrane of sensory receptors > ion channels open and ions flow across the cell membrane > if stimulus is sufficient, the threshold potential is reached > nerve impulse propagated > sensory neurons carry the action potential to CNS > brain allows for perception of the action potentials = pain
