Neurophysiology Revision Flashcards
What is the resting potential of a neuron?
-64mV
What are the resting intracellular values for sodium and potassium in a neuron
Sodium: 15mM
Potassium: 140mM
What are the resting extracellular values for sodium and potassium?
Sodium: 150mM
Potassium: 5mM
Describe the 3 principles of transmission of an action potential
1) Self propagates
2) Travels in one direction
3) No volume control, just on or off
Describe the evolution of an action potential
- The neuron reaches the threshold level of excitation (depolarisation) and voltage gated sodium channels open and Na+ enters
- K+ channels open and K+ begins to leave the cell
- Na+ channels become refractory, no more Na+ enters the cell
- K+ continues to leave the cell, causing the membrane potential to return to resting level
- K+ channels close and Na+ channels reset
- Extra K+ outside the cell diffuses away
Describe the mechanisms of voltage dependent sodium channels
- At resting potential, Na+ channels are closed, their 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 about 0.5ms)
- Repolarisation of the cell re sets the two gates to their equilibrium positions
Why can the cell not be stimulated to its threshold potential in the refractory period?
All of the Na channels are closed (although a stronger than normal stimulus could induce an action potential - some Na+ ready but more K+ channels are open than usual)
How can you increase the speed of transmission in a neuron?
- Increase the diameter
* Myelination
What cells are responsible for myelination?
- In the PNS, the Schwann cells
* In the CNS, oligodendrocytes
Describe saltatory conduction
- With a myelin sheath, local currents can be extended further as current leakage is reduced by the myelin sheath
- Conduction of the impulse flows rapidly along the inside of the axon to the node where it slows and the ionic depolarisation takes place (only a few ions needed so is energy saving)
- Then, fast conduction inside of the axon resumes (action potential only exists at the nodes of ranvier)
What is normal conduction velocity?
50-60 meters/second
What is the conduction speed of the fastest neurones and what are they?
- 120m/s
* Alpha motor neurons
What is the terminal swelling of a presynaptic cell called?
Bouton
What is the size of the synaptic gap?
Approx 20nm (twice the size of a membrane)
How do vesicles and presynaptic membranes recognise each other?
• V-SNARE on the vesicle complex pairs with T-SNARE at the presynaptic membrane
Describe synaptic docking and fusion
- SNARE complex folds and strongly draws the membrane close
- Vesicle becomes docked on the membrane
- Action potential triggers calcium influx at the end bulb, inducing synaptotagmin to displace complexin and exocytosis proceeds
What is the effect of the Botulinum toxin?
The botulinum toxin targets the SNARE membrane, inactivating acetylcholine related processes causing paralysis
How are neurotransmitters inactivated?
- Diffusion
- Re-uptake
- Enzymal inactivation
What are the different types of receptors that recognise neurotransmitters?
- Ionotropic receptors (directly gate ion flow)
* Metabotropic (indirectly gate ion flow or activate other pathways) e.g. activate a G protein
What type of receptor is the acetylcholine receptor?
Ionotropic
What is an EPSP?
- Excitatory post synaptic potential
- Net flow of positive ions into the cell depolarises the membrane bringing it closer to the threshold
- Influx of Na+
What is the typical amplitude of an EPSP?
1-5mV
What is an IPSP?
- Inhibitory post synaptic potential
- Causes a hyper polarisation, making the neuron more resistant to an action potential
- Influx of Cl- ions
What is spatial summation?
- Multiple EPSPs
- Can reach the threshold so an action potential can be generated
- EPSPs arrive at the same time but at different locations on the axon
What is Temporal summation?
- Multiple EPSPs
- Can reach the threshold so an action potential can be generated
- EPSPs arrive in succession at the same point
What ions are SNARES dependent on?
Calcium ions
How are action potentials generated in a sensory neuron?
- Most receptors will code duration and magnitude of the external signals as a generator potential
- A stronger external signal will result in a higher frequency of Action Potentials in the axon
- Sensory receptors can adapt to a stimulus with time (not all adapt in the same way)
How can we measure nerve conduction?
Clinically we don’t measure individual action potentials, we measure the sum of all action potentials in a nerve or the resulting stimulus inside a target such as a skeletal muscle
• Electrodes at two points
In a nerve conduction study, what would a reduced amplitude mean?
Less axons
In a nerve conduction study, what would a slowed conduction velocity mean?
Reduced myelin
What is a SNAP?
Sensory nerve action potential
What is a CMAP?
Compound muscle action potential
What happens in carpal tunnel syndrome?
- The median nerve is compressed at the wrist resulting in numbness/pain.
- Entrapped by the transverse carpal ligament
What are the types of inherited channelopathies?
- Neurological
* Cardiac
What are the autoimmune channelopathies?
- Myasthenia gravis
- Lambert-Eaton myasthenia syndrome (associated with small cell lung cancer)
- Limbic encephalitis (memory loss; with an underlying neoplasm)
What is Myasthenia Gravis?
- Involved acetylcholine receptors
- Prevents the muscles from contracting e.g. weakness in eye muscles, weakness in arms, legs etc.
- Sometimes in cases of severe weakness, may cause respiratory failure
Describe the involvement of muscle spindles in muscle contraction
Mediate stretch reflexes to enable muscle fibres to oppose forces that elongate them.
