Describe the major steps in conscious control of skeletal muscle.
- CNS stimulates a motor-neuron (emanates from SC).
- Action potential propagates down the axon and reaches the synapse.
- Transmission of action potential to surface of muscle cell via an intermediate 'transmitter'.
- Action potential on surface of muscle cell triggers contraction.
List the most numerous cellular components of the CNS.
- Mainly neurons and glial cells.
- The most numerous glial cells are:
- Microglial cells
- Ependymal cells
- Microglial cells
- Ependymal cells
How do neurons encode and transmit information?
- As electrical activity
- Electrical charge can be conducted both passively and actively
Describe the difference between passive and active conduction.
- Passive conduction is very lossy, for example, heat travelling down a rod.
- Active conduction involves the generation of an action potential by opening of ion channels.
Nb. Lossy = the dissipation of electrical or electromagnetic energy.
How long does depolarisation last?
1/1000th of a second.
What causes depolarisation?
Rapid influx of Na+
What causes repolarisation?
Efflux of K+
Describe the action of voltage-gated sodium channels.
- At RMP, Na+ channels are closed - the activation gate is closed.
- Depolarisation opens the activation gate and Na+ flows into the cell along its electrochemical gradient.
- A delayed component of voltage dependent activation is the blocking of the channel by the inactivation gate (after ~0.5ms).
- Repolarisation of the cell re-sets both gates to their equilibrium positions.
Describe how an action potential is propagated.
- Voltage-gated Na+ channels open and an action potential is generated - causes depolarisation of the membrane potential.
- Passive current flows to the next voltage-gated Na+ channels - if they are too far apart, the signal cannot propagate passively, so there is an optimum distance between these channels.
- The passive current opens the voltage-gated Na+ channels and another action potential is generated.
What happens during the refractory period of a voltage-gated sodium channel?
- Voltage-gated Na+ channels are blocked by the inactivation gate.
- This restores the membrane potential to threshold.
- Action potentials cannot be generated during this time.
What happens to a cell during the absolute refractory period?
- The cell cannot be stimulated to its threshold potential.
- All Na+ channels are closed.
What happens to a cell during the relative refractory period?
- An action potential could be induced, but only by a stronger stimulus than normal.
- Some Na+ channels are open but more K+ channels are open than usual.
- Cell is still hyperpolarised.
Describe the effect of the refractory period on the direction of an action potential.
- Voltage-gated Na+ channels open and allow positive sodium ions into the axoplasm.
- These ions set up a more positive potential which, when great enough, causes the next Na+ channel to open.
- The refractory nature of Na+ channels keeps the impulse moving in one direction.
Why is the refractory membrane always behind the action potential in progress?
To prevent backward spread and propagation of the action potential, allowing the membrane behind to reset.
What effect does resistance have on action potential propagation?
- Passive movement of charge along the axon is easier with less resistance.
- The larger the diameter of the axon, the lower the resistance.
- Larger axons have faster passive charge movement.
- Optimise diameter of axon to have lower resistance to passive flow.
What effect does capacitance have on action potential propagation?
- The higher the capacitance, the harder it is for charge to cross over the membrane.
- The greater the surface area there is on an axon, the higher its capacity to store charge across its membrane.
- Greater opportunity to cross the membrane = increased capacitance.
- It is more difficult to induce depolarisation across the membrane when there is a large capacitance.
Which cells myelinate axons in the central nervous system?
Which cells myelinate axons in the peripheral nervous system?
What happens to action potentials at a node of ranvier?
Action potentials are regenerated at the nodes of ranvier through the opening of voltage-gated sodium channels and the flow of ions through these channels.
Note that the action potential only exists at the node of ranvier.
What is synaptic transmission?
The transfer of signal from a neuron to a target cell.
How is an electrical synapse bridged?
Via a gap junction.
How is a chemical synapse bridged?
Via an 'intermediate' between the presynaptic and postsynaptic cell - a neurotransmitter.
What happens in the presynaptic cell when the membrane depolarises?
Voltage-gated calcium channels open.
Describe postsynaptic ionotropic receptors.
- Ion channel linked receptors.
- Excitatory or inhibitory effects on excitability of post-synaptic cell membrane depending on selectivity for ions.
Describe postsynaptic metabotropic receptors.
- Enzyme linked receptors.
- May be excitatory or inhibitory on excitability of postsynaptic cell if they modify action of separate ion channels.
- May trigger other processes (i.e. contraction of smooth muscle).
Which type of receptors are affected by acetylcholine at the neuromuscular junction?
To which category do these receptors belong?
Nicotinic acetylcholine receptors.
These are ionotropic receptors.
Briefly describe what happens when an action potential reaches the neuromuscular junction.
- AP in motor-neuron triggers release of acetlycholine (ACh) from nerve endings.
- Synapse with muscle fibre is a very well-defined region (NMJ).
- ACh acts on nicotinic acetlycholine receptors which are ionotropic.
- Influx of sodium on target cell is excitatory.
- Sufficient quanta of ACh = sufficient postsynaptic excitation = generation of AP on muscle fibre.
Describe spatial summation.
Multiple synapses from different neurons stimulating the same postsynaptic cell.
Describe temporal summation.
The same synapse repeatedly stimulating the sam postsynaptic cell.