Excitable tissues: neurons Flashcards
what are neurons?
Neurons (or nerve cells): the principal buildings blocks
and instruments of communication in the brain
what are the two indications for direction of flow of electrical singals?
synaptic potentials
action potentials
how do neurons communicate?
electrical signals (dendrites, cell body, axon)
chemical signals
synapses
What is the resting membrane potential (RMP)
In neurons and their processes, the cytoplasm has a potential that is 50 to 70 mV lower (ie. more negative) than the potential
of the extracellular space.
almost all cells in the body have a ___________
negatice resting membrane potential
Why are neurons and muscle fibres excitable?
Because only neurons and muscle fibres can suddenly respond with a transient
change of this potential (ie. with an action potential) in response to a stimulus
How are the intracellular potentials measured today?
The microelectrode recording technique
The patch-clamp technique
There is more ____________ inside the cell in comparison to the extracellular fluid.
There is more negative charges inside the cell in comparison to the extracellular fluid.
The RMP is due to…?
a) Unequal concentrations of Na+ and K+ inside and outside the cell
b) Unequal permeability of the cell membrane to these ions
[c) Electrogenic action of the Na-K pump – only a small contribution ! ]
what are the two main types of ion channels (which have selective permeability to ions) in neurons?
a) Non-gated (‘leak’) channels
b) Gated channels (voltage-gated, ligand-gated*, or mechanically-gated)
- open at rest
- closed at rest
In cell membrane of neurons, there are
many ______ channels, but very few _______channels.
In cell membrane of neurons, there are
many leak K+ channels, but very few leak Na+ channels.
At rest: PK+ / PNa+ ≈ 40 / 1
How does unequal concentration and unequal cell membrane permeability to Na+ and K+ result in the negative RMP?
The concept of the ‘Equilibrium potential’:
What is the concept of the equilibrium potential?
An intracellular potential at which the net flow of ions is zero, in spite of a concentration gradient and permeability.
How is the equilibrium potential calculated for each ion?
Nernst equation
What is rule #1?
The higher the permeability of the cell membrane to a
particular ion, the greater the ability of this ion to shift the RMP towards its equilibrium potential.
What happens in the neuron’s membrane permeability at rest? Thus?
At rest, in neurons the membrane permeability is much higher to K+ than to Na+; therefore the RMP is closer to the equilibrium potential for K+ ( than the equilibrium potential for Na+ E ( K) ENa).
Thus, in comparison to glia cells, in neurons the RMP is less negative than EK (about - 65 mV). This is due to a small contribution of the leak Na+ channels !
What is the goldman equation?
A way of calculating the value of the RMP taking into account both the concentration gradients and the relative permeability of the resting cell membrane to K+ and Na+ ions.
Why is the potential inside neurons not constant?
Because it changes when ion concentrations change, or when membrane permeability changes!
What is hyperpolarisation? (2)
If the potential becomes more negative
(eg. changes from -70 to -75 mV)
The potential inside the
cell moves closer to EK, and away from ENa
what is depolarisation?
If the potential becomes less negative (eg.
changes from -70 to -60 mV)
The potential inside the cell moves away from EK and closer to ENa
What is the action potential?
A brief fluctuation in membrane potential caused by a transient opening of voltage-gated ion channels, which spreads, like a wave, along axon.
How does action potential occur?
Action potentials occurs after the membrane potential reaches certain voltage called the threshold (~ -55 mV).
Why is the action potential significant? (2)
Information is coded in the frequency of action
potentials. Thus, APs can be regarded as a form of
‘language’ by which neurons communicate !
Action potentials occurs after the membrane potential reaches certain voltage called the threshold (~ -55 mV).
Action potentials are a key element of the process of signal transmission along (often very long !) axons.
ionic mechanisms associated with each stage of AP: When MP reaches the threshold, there is a sudden
activation (opening) __________________
When MP reaches the threshold, there is a sudden activation (opening) of voltage-gated Na+ channels ( PNa+↑ )
AP ionic mechanisms: Opening of _________channels is only short lasting, as these channels ______________
Opening of voltage-gated
Na+ channels is only short lasting, as these channels quickly inactivate
What happens when the voltage threshold is reached?
When the voltage threshold is reached, sodium channels open and Na+ ions move into the cell along both the concentration and
electrical gradient.
influx of Na+ slows down and then stops when? (2)
1) The inside potential becomes positive (moves towards ENa+ ) and
thus attracts Na+
ions less.
2) Na+ channels inactivate.
what is rule #2?
When the current generated by an outside source flows trough the cell membrane from outside to inside → hyperpolarisation (the MP becomes more negative); when it flows from inside to outside → depolarisation (the
MP becomes less negative).
How are APs generated ‘physiologically’ in
CNS neurons ? (3)
● APs are first generated in the axon initial segment (‘axon hillock’) which
has lowest threshold, and thus serves as the ‘trigger zone’ for APs.
Axon
initial segment
EPSPs
● Depolarisation to threshold is evoked by excitatory postsynaptic
potentials (EPSPs), which spread mainly passively from dendrites.
● Once generated, APs are transmitted actively along the axon, away
from the cell body.
What are the two types of axons?
a) Unmyelinated axons: small diameter (~ 1 µm);
transmission of APs slow, continuous
b) Myelinated axons: larger diameter (5-10 µm);
transmission of APs fast, ‘saltatory’ (in large steps)
What are two stages of action potential transmission
(in both types of axons) ?
(1) Passive spread
(2) Generation of action potentials
What is the ‘passive’ spread of current?
- (Subthreshold) depolarisation at one region of the membrane
- Passive current flow
(inside and outside the axon) - Depolarization of adjacent parts of membrane (cf. Rule #2)
How far does this deprolarisation spread?
Current can spread passively only over
a short distance
what is the structure of neurons with myelinated axons
Myelin sheath formed:
- by oligodendrocytes in
the CNS
-by schwann cells in the PNS
oligodendrocites and
Schwann cells are two types
of_________?
gilia cells
how does myelination increase speed of AP conduction?
Myelination increases speed of AP conduction by
increasing the efficiency of
passive spread, and the fact that APs do not need to be regenerated at every part of cell membrane.
Where are APs generated?
APs are generated only
at nodes of Ranvier
(current flows passively
between nodes).
how are Aps generated in sensory neurons?
The concept of ‘the receptor potential’
what is the receptor potential?
● When a stimulus acts on receptors in sensory neurons (eg. a mechanical
stimulus acting on muscle spindles), it does not immediately evoke APs.
● First it evokes a graded depolarisation, known as ‘the receptor potential’.
● The receptor potential spreads passively to more distally located ‘trigger zone’
where APs are generated.
● APs then spread along the axon (myelinated or unmyelinated) towards the CNS.
● Information about the strength of the stimulus is coded in the amplitude of the
receptor potential and the frequency of APs (analog–to-digital converter !)
How is a ‘message’ transmitted from one
neurone to another neurone (or to muscle fiber)? (2)
Synaptic transmission
between neurons
Synaptic transmission between
a neuron (motoneuron) and a muscle fiber
(Neuromuscular junction = ‘End plate’)
What are the two main types of chemical synapses
in the CNS?
Excitatory synapses
Inhibitory synapses
Excitatory synapses?
depolarisation of the postsynaptic membrane called
the Excitatory Postsynaptic Potential ( EPSP )
Inhibitory synapses
hyperpolarisation of the postsynaptic membrane
called the Inhibitory Postsynaptic Potential (IPSP )
what are the excitatory synapses
Neurotransmitters?
mainly glutamic acid
(glutamate) or ACh
what are the Inhibitory synapses
Neurotransmitters?
mainly GABA (gamma-aminobutyric acid) or glycine
Ionic mechanism of EPSPs?
transient opening of channels permeable
to Na
+, K
+ and sometimes Ca2+
Ionic mechanism of IPSPs?
usually transient opening of
K
+
channels
What are the classification of neurotransmitters?
- Small molecule neurotransmitters (‘Classical’ neurotransmitters)
- Neuropeptides (‘Neuromodulators’)
what are small molecule neurotransmitters features? (4)
Usually fast action (within milliseconds), and direct on postsynaptic receptors
- Amino acids: glutamate, GABA, glycine
- Acetylcholine (ACh)
- Amines: serotonin (5-HT), noradrenaline,
dopamine
What are Neuropeptides features? (3)
Large molecule chemicals that have an indirect ( ‘metabotropic’) action on postsynaptic receptors, or modulatory action on the effects of other neurotransmitters
Slow (seconds to minutes) and usually more diffuse action
Several dozens of neuropeptides have been identified which may be involved in communication between neurons
what are the three factors that determine the synaptic factor?
The type of neurotransmitter / neuromodulator
The type of neurotransmitter receptor/ channel complex expressed in the postsynaptic membrane
The amount of neurotransmitter receptor present in
the postsynaptic membrane – ’Synaptic plasticity’: LTP or LTD (*)
Neurotransmitter inactivation (and recovery)? (4)
Diffusion away from the synapse
Enzymatic degradation in the synaptic cleft
(eg. Acetylcholine esterase degrades ACh )
Re-uptake (for most of the amino acids and amines) and recycling !
Involvement of specific neurotransmitter transporters
in the presynaptic membrane;
Integration of synaptic inputs by neurons
Each neuron receives thousands (!) of synapses; some excitatory, some inhibitory
Each individual synapse, when activated, produces only very small (only ≈ 0.1 mV) postsynaptic potentials at axon initial segment (Note: the decay of the potential when they are passively conducted from dendrites)
In order to depolarise the initial segment to the threshold, EPSPs need to be enhanced !
