Neurophysiology revision Flashcards
(41 cards)
What are the functions of neurons?
They carry out cell processes, interconnect cells, transmit information and use electrical signals
How do neurons work
Information arrives at the cell body via dendrites (and cell bodies) where it is assimilated and processed
Processed information is then digitised into action potentials which are transmitted along the axon
At the end of the axon th information is passed to the target (muscle or neuron) at boutons
How do action potentials occur?
Action potentials are an all or nothing response, they can be more intense or spread out. They are graded by frequency more or less rapid (bullets from gun or vibration)
They occur mainly in axons and start at the cell body (axon hillock)
They are transmitted towards the end of the axon (synapse)
What is the charge inside a neuron and how does this affect transmission of action potentials?
Inside a neuron has a negative potential of about -70mV which is called the resting membrane - maintained by ion pumps.
The resting potential means that we need enough positive sodium ions to come into the cell to reach threshold so we can trigger an action potential.
Why is the refractory period important?
It ensures that only 1 impulse is generated at a time forcing it to travel in one direction
What are the steps to trigger and action potential?
1). Na + channels open and Na + begins to enter the cell (rapid depolarisation)
2). K + channels open and K + begins to leave the cell
3). Na + channels come refractory and no more Na + enters the cell
4). K + continues to leave the cell causing a repolarisation and the membrane to return to resting level (hyperpolerisation occurs with overshoot)
5). K+ channels close and Na + channels reset
6). Extra K + outside diffuses away
Describe how the voltage dependent sodium channels work?
At resting potential Na + channels are closed - the activation gate is closed
Depolarisation opens the activation gate and Na + flows into the cell along the electrochemical gradient
A delayed component of voltage dependent activation is blocking of th channel by th inactivation gate (after about 0.5ms - in built timer)
Depolarisation of the cell re-sets the two gats to their equilibrium positions
How does the voltage dependent sodium channel affect the refractory period?
The voltage dependent sodium channel helps to set the refractory period of the action potential
During the absolute refractory period the cell cannot b stimulated to its threshold potential - all Na channels closed
During relative refractory period a strong stimulus than normal could induce an action potential - some Na + ready but more K_ channels are open than usual - cell still hyper polarised
How can we increase the speed of conduction?
Increasing the diameter
increases the speed of conduction as there is decreased resistance
To increase speed for a diameter the axon is insulated in a fatty sheath called myelin
What helps to build myelin?
Schwann cells in the PNS
Oligodendrocytes in the CNS
How do local currents travel in myelinated axons
By salutatory (jumping) conduction
local currents can extend further as the normal current leakage is restricted with the addition of the myelin sheath (decreased capacitance)
This causes the nerve impulse to flow rapidly along the inside of the axon to the node where ionic depolarisation takes place. This requires fewer ions and is energy saving. and then fast conduction along side of the axon resumes
Action potentials only exit at the node of Ranvier
How do unmyelinated nerves work?
Unmyelinated fibres, larger diameter = faster conduction
Why do we have nerves myelinated ?
To keep the nervous system compact so small diameter myelinated fibres can conduct faster than large unmyelinated ones
If conduction in a myelinated axon slowed down what would be the reason for this?
Demyelination
What is the most basic way to describe a neuron?
They are elongated secretory cells - polarised (axon - apex; dendrites - base)
What do secreted chemical signals into a narrow gap cause?
They influence another cell, whether another neuron or muscle to make in impulse
This occurs in the synapse
How do neurons communicate?
The axon terminal of the presynaptic cell forms a swelling called a bouton. The bouton is next to the synaptic cleft where the postsynaptic cell will be on the other side of this cleft.
The bouton contains vesicles which transports molecules of neurotransmitters
How are vesicles sat at the bouton and released?
Vesicles and the presynaptic membrane recognise each other.
v-Snare and t-Snare complex folds strongly to draw the vesicle membrane and pesynaptiic membrane close
The vesicle is then docked on the membrane (exocytosis cannot happen here as it is ‘clamped’ by complexin). This created a pool of docked vesicles at the synapse and helps synchronising release.
Action potentials triggers calcium influx at end bulb. Calcium induces synaptotagmin to displace complexin - which triggers exocytosis
How can toxins affect SNARE mechanisms?
Toxins target regulators of synaptic SNARE mechanisms
E.g botulinum toxins - stop vesicle release - global paralysis caused
Tetanus toxins - stop vesicle release - glycine/GABA (tetany)
How are neurotransmitters recycled?
They are released by exocytosis and diffuse to the post-synaptic membrane where they bind to receptors - then inactivated by diffusion, re-uptake or enzyme inactivation
How can we class our receptors?
How they are activated;
- Ionotropic - directly gate ion flow
or
- metabotropic - indirectly gate ion flow or activate other pathways
Positive/negative effects;
- excitatory
or
- inhibitory (depending on the ions they let into/out of the cell)
How does ionotropic receptors work?
1). Neurotransmitter binds to the outside of the channel
2). this causes a conformational change and the channel opens
3). ions flow across the membrane
How does metabotropic receptors work?
1). Neurotransmitter binds to the outside of a g-protein linked receptor
2). this causes the G-protein to be activated
3). The G-protein subunits or intracellular messengers modulated ion channels
4). The subunits opens the channel
5). ions flow across the membrane
How does neurotransmitters effect cells postsynaptically?
In their target cells (neurons and muscles) the neurotransmitter causes a change in the potential f the post synaptic membrane
This is NOT an action potential and has a special name
Post synaptic potential of which there are 2 types;
- Post synaptic excitatory potential (EPSP)
- Post synaptic inhibitory potential (IPSP)
