Neurons Flashcards
What are neurons?
Nerve cells present in the nervous system
What are neurons specialised for?
Communication
What do neurons synapse with
- Other neurons
- Muscles
- Glands
What is the axon hillock?
Where the cell body and axon join
Where does input converge on neurons?
Dendrites
Which type of neurons have lots of dendrites?
Purkinje cells
Why does a resting membrane exist across the membrane of all cells?
- Due to the distribution of +ve and -ve charges on each side of the membrane
- In the range of 20 to 90 mV; inside negative relative to outside
- Approximately equal numbers of +ve and -ve charges on each side of the membrane but they are not evenly distributed
Is [Na+] higher in the ICF or ECF?
ECF = 145 mM
15mM in ICF
Is [K+] higher in the ICF or ECF?
ICF = 150 mM
4 mM in ECF
Is [Cl-] higher n the ICF or ECF?
ECF = 110 mM
10 mM in ICF
What does the resting membrane potential arise from?
The RMP arises from the separation of charges on either side of the membrane
What determines the resting membrane potential (RMP)?
- Diffusion of K+ from cell interior through K+ channels
- The Na+K+ ATPase moves unequal amounts of Na+ and K+ (3 Na+ out for 3 K+ in) pumps ions against their conc. gradient
What effect would applying an electric current have to the membrane potential of a cell?
Would alter the membrane potential
Which direction does a hyper-polarising current move the MP?
A hyper polarising current moves the MP further from 0
Which direction does a depolarising current move the MP?
A depolarising current moves the MP nearer to 0
When a current is applied to a cell membrane, the responses are ‘graded’. What does this mean?
The amplitude depends on the size of the stimulus
What happens to a cell when the MP is moved to around -55 mV?
A large depolarisation occurs
Describe how an action potential is generated
- Stimulus applied
- Depolarisation occurs
- Threshold reached ~ -55mV
- This opens voltage-gated Na+ channels. Na+ flows in
- MP moves overshoots 0
- When membrane potential reaches +35 mV, Na+ channels shut and voltage-gated K+ channels open
- K+ leaves the cell, making the MP more negative (repolarisation)
- After a small overshoot, MP returns to original ‘resting’ level
Why is an action potential described as an ‘all or nothing’ event?
Amplitude is independent of stimulus
How do anaesthetics work?
Block Na+ channels -> stops nerve conduction
What is the refractory period?
- After an action potential is initiated, the neuron cannot generate another action potential until the first one has ended
- Period of unexcitability
- It is due to inactivation of voltage-gated sodium channels
What causes the refractory period?
Inactivation of voltage-gated Na+ channels
How is an action potential propagated along an axon?
- An AP in one part of axon depolarises adjacent ‘resting’ parts of the axon
- The AP is regenerated further along the axon
- APs travel along axon as waves of depolarisation
During action potential propagation, why does the action potential not travel back on itself?
- Because of the refractory period
- Refractory period allows the body to maintain uni-directionality of the action potential
What effect does an increase in axon diameter have on the speed in action potential propagation?
Increases the speed of AP propagation
What is the diameter of a giant squid axon?
1mm
What is the maximum conduction velocity of a giant squid axon?
35m/s
What have vertebrates evolved as a means of increasing AP conduction speed whilst decreasing axon diameter?
Myelination
What is myelination and what is its purpose?
Wrapping a layer of myelin round an axon
Increases AP conduction speed
What is the maximum speed of conduction in human axons?
120 m/s
What is myelin?
A fatty layer
How is myelination achieved?
By wrapping the membranes of ‘glial’ cells round the axon
Which substance is used to insulate the axon and, resultantly, increases AP conduction?
Myelin
What are nodes of Ranvier?
- Gaps between myelin sheaths
- Axon membrane is exposed to the ECF at the nodes of Ranvier and ions flow can occur
What is saltatory conduction?
The AP ‘jumps’ from one node of Ranvier to the next. This increases the conduction velocity of APs.
