Spread of Nerve Impulses Flashcards
What is the function of the nervous system?
To detect, relay, integrate and respond appropriately to conditions in the outside world
What role do the glia play in the nervous system?
+ Structural support
+ Shock absorption
What is the nervous system composed of?
Glia and neurons, off which glia are more numerous
What are neurons?
Electrically excitable cells able to convey & integrate and respond by changing the resting membrane potential
What are the components of the structure of a (stylised) neuron?
\+ Axon \+ Dendrites/dendritic branches \+ Cell body or soma nucleus \+ Myelin sheath \+ Internode \+ Node \+ Bouton
What are functional features a (stylised) neuron?
- Information arrives at the cell body via dendrites where it is assimilated and processed
- Processed information is then digitised and transmitted along the axon
- At the end of the axon the information is passed to the target (muscle or neutron) via boutons
What are the variations in neuronal phenotype?
+ Multipolar neuron (most abundant in CNS)
+ Pseudounipolar neuron
+ Bipolar neuron
What are the factors that influence the speed at which the nerve impulse travels along the axon?
- The diameter of the axon
2. If the axon is myelinated
What are is the relationship between diameter size and charge within the axon?
The larger the diameter of axon, the lower the resistance
Passive movement of charge along the axon is easier with less resistance
Larger axons have faster passive charge movement
What are is the relationship between capacitance and charge within the axon?
The more surface area there is on an axon, the higher its capacity to store charge across its membrane
The higher the capacitance the harder it is for charge to cross over the membrane i.e to overcome the repellent face of charge accumulated there
What causes depolarisation/repolarisation for action potential?
Depolarisation: rapid influx of Na+
Repolarisation: efflux of K+
What happens at voltage dependent sodium channels?
- At resting potential Na+ channels are closed - the activation gate is closed
- Depolarisation opens the activation gate and Na+ flows into the cell along it’s electrochemical gradient
- A delayed component of voltage dependent activation is the blocking of the channel by the inactivation gate (after about 0.5ms)
- Repolarisation of the cell resets the two gates to their equilibrium positions
The voltage dependent Na+ channel helps set the refractory period of the action potential. What happens during the refractory period?
The cell cannot be stimulated to its threshold potential - all Na+ channels are closed
During relative refractory period a stronger stimulus than normal could induce an action potential - some Na+ ready but more K+ channels are open than usual (cell still hyperpolarised)
How does an action potential move along an axon?
A combination of passive diffusion currents along the axon & active currents through ion channels
Why are there holes in the axons?
The holes are ion channels which are always open
These are essential to set the resting membrane potential of the cell
What happens regarding action potential if the injected current doesn’t depolarise the membrane to threshold?
No action potentials will be generated
What happens regarding action potential if the injected current depolarises the membrane beyond threshold?
Action potentials will be generated
What is the relationship between action potential firing rate and depolarising current?
A the depolarising current increases, the action potential firing rate increases
Which cells provide myelination in the PNS?
Schwann cells
What are Schwann cells?
They form the myelin sheath around peripheral axons
A single Schwann cell forms the myelin around an axon for a single internode of one neuron
What are oligodendrocytes?
“Cells with quite a few branches”
Form the myelin in the CNS (white matter)
Have different embryological origins and so produce myelin which has differences in chemical composition
What is the relationship between Multiple Sclerosis and oligodendrocyte myelin?
MS attacks oligodendrocyte myelin and so the CNS is differentially affected
What are features of saltatory (jumping) conduction in a myelinated axon?
The local currents can extent further as the normal current leakage is curtailed by the myelin sheath
What are features of saltatory (jumping) conduction in a myelinated axon?
+ Local currents can extend further as normal current leakage is curtailed by myelin sheath
+ Conduction of nerve impulse flows rapidly along inside of axon to the node - it slows and ionic depolarisation (action potential) takes place
+ Fast conduction along inside of axon then resumes afresh
+ Only a few ions are needed to there is an energy saving
+ Action potential only exists at node of Ranvier
What does the action potential (AP) do?
+ AP causes changes in the membrane permeability (ion channels)
+ This sets up a chain reaction causing a wave of depolarisation to travel along the axon
+ The velocity of the travelling wave is dictated by physical characteristics of the nerve fibre, but the amplitude of the wave is constant
Why is the action potential needed to help spread changes in membrane potential along the axon?
+ Axoplasm is a poor conductor
+ Axon membrane is a poor insulator
Therefore, changes in membrane potential do not spread far along the axon without an AP
