PS120: Nerves Flashcards
Define neurons
Basic building blocks of the nervous system
What are nerve cells formed of?
- Cell body (soma)
- Cell Processes
What is the soma?
It consists of the nucleus and cytoplasm with its organelles EXCEPT the centrioles.
Why can’t nerve cells divide?
nerve cells cannot divide because of the absence of centrioles.
What are the 2 types of cell processes?
- dendrites
- axons (nerve fibres)
What are dendrites?
with function
- multiple and short processes.
- conduct impulses toward the soma
What are axons?
with function
- single and long process
- conducts impulses away from the soma
What is the first part of the axon and what is it called?
The initial segment. It is the 1mm of the axon and is the most excitable part of the axon because of the high density of voltage-gated sodium channels.
How does the axon end?
It ends in a number of terminal branches, each ending in a number of synaptic knobs.
What do synaptic knobs contain?
They contain granules or vesicles in which the chemical transmitter secreted by the nerve is stored.
What is the lenght of an axon?
It varies, ranging from a few mm to over a metre.
What is an axon’s diametre?
From 0.1 to 20 microns.
What organelles does the axon contain?
- mitochondria
- neurofilaments
- microtubules
- smooth ER
What is the axon covered by?
2 sheaths:
1. Neurilemmal sheath (outer)
2. Myelin sheath (inner)
What is the nature of the neurilemma/outer sheath?
It is a single layer of Schwann cells.
What is the nature of the myelin/ inner sheath?
- Lipoprotein in nature
- many layers Schwann cells’ cell membrane.
Where is neurilemmal sheath found?
It covers ALL axons.
- i.e. all neurons have an outer sheath.
Where is myelin sheath found?
it covers some nerves
- its presence is what classifies nerves into myelinated and unmyelinated nerves.
What does the neurilemmal sheath look like?
continuity
It covers the whole axon, i.e. continuous layer
How does the myelin sheath look?
It covers the axon except: its origin, end, and nodes of Ranvier.
- periodic interruptions 1mm apart, so it is an interrupted layer.
What is the function of the neurilemmal sheath?
regeneration of axon when injured
What is the function of the myelin sheath?
- acts as an insultor to prevent the flow of ions [electric current]
- increases excitability of the nerve fibre
- speeds up the conduction velocity of action potential
What is the resting membrane potential?
It is the electric potential difference between the inner and outer sufrace of the cell membrane during rest.
What is another term for resting membrane potential?
It is also called polarization.
Where is polarization found?
It is present in all cells of the body, with the inner surface of the cell membrane negative relative to the outer surface
What is the value/ magnitude of resting membrane potential?
negative or positive?
It is always negative because the charge of the inner surface is considered the standard.
Examples of the magnitude of RMP?
- in RBCs: -9mV
- in skeletal ms.: -100 mV
- in medium-sized nerve: -70mV
varies from cell to cell
How is RMP recorded?
By a special voltmtre that can measure very small potential differences.
- connected to 2 microelectrodes, one put on the inner surface and the other on the outer surface of the cell membrane.
What causes resting memb potential?
- mainly the selective permeability of the cell membrane
- (to a lesser degree) the Na-K pump
How does relative permeability of the cell membrane affect different ions?
1. Cations: K and Na can leak during rest through the cell membrane’s Na-K leak channels.
- these channels are 50 times more permeable to potassium than sodium
2. Anions: membrane is poorly permeable to Cl- and impermeable to proteins.
Conclusion: cell membrane maintains RMP via cation regulation not anion regulation.
How do the different cations move across the cell membrane for the genesis of a resting memb potential?
Cations:
1. K+: relatively large amount of potassium diffuse to the outside of the cell, leading to the negativity of the inner surface and the positivity on the outer surface of the membrane, creating the membrane potential
2. Na+: relatively smaller amount of sodium diffuse into the cell, decreasing the membrane potential that is created by K+.
What causes the cations’ movements?
- Potassium “leaves the cell” due to: high concentration gradient (30 times), and high permeability of Na-K leak channels
- Sodium diffuses into cell less dramatically due to: low concentration gradient (only 10 times) and low permeability of Na-K leak (1/50 that of K)
How do the different anions move across the cell membrane for the genesis of a resting memb potential?
- Chloride: the membrane is poorly permeable to Cl-. They diffuse from outside to inside in small amounts, having a minor role in creating RMP.
- proteins: no role in creating RMP; the membrane is impermeable to proteins.
What can we conclude about the creation of resting membrane potential?
The net result is that the resting membrane potential is mainly due to the efflux (outflow) of K+.
Why does hyperpolarization make the inside surface of the cell more negative?
Because there will be a greater efflux of K ions, making the inside much more negative and the outside much more positive than in regular polarization.
What proves that RMP is mainly caused by K+ outflow?
- RMP is affected markedly by variations in K+ conc. in ECF. ex. decrease in ECF K causes hyperpolarization, a more negative RMP, causing a decrease in excitability
- RMP is close to equilibrium potential for K+
What is equilibrium potential?
It is the membrane potential created when the cell is only permeable to K+ and mobility reaches a state of equilibrium (no net movement of potassium) where K efflux by concentration gradient causes an influx by electrical forces.
- in other words, it is when the chemical and electrical gradients of an ion are equal but in opposite directions.
How is the equilibrium potential calculated?
Using the Nemest equation:
Ek= 61.5 log (conc. outside/conc. inside)
What is the equilibrium potential of potassium?
[K] on outside: 4
[K] on inside: 145
Ek=61.5log(4/145)= -90m.V
-90mV is the equilibrium potential of potassium, which is close to the normal RMP.
What is the sodium-potassium pump?
- it is an active transport pump.
- present in all body cells
- moves 3 Na+ ions outside the cell while bring 2 K+ ions inside.
What is the mechanism of the sodium-potassium pump?
- Carrier protein features: 3 receptor sites for sodium on inner surface & 2 receptor sites for potassium on outer surface. Carrier contains ATP-ase activity, breaking down ATP.
- Binding process: When 3 Na+ and 2 K+ ions bind to the protein, ATP-ase is activated, splitting ATP into ADP and energy
- Conformational change: the energy released causes a change in the protein’s shape, enabling the movement of 3 Na+ outside and 2 K+ inside.
What are the functions of the sodium potassium pump?
- electrogenic pump for RMP: contributes to the RMP (by -4mV to -10 mV) to create a charge difference across the membrane.
- maintenance of ion concentration: regulates sodium and potassium concentration differences
- cell volume control: prevents excessive accumulation of Na+ inside the cell, thus avoiding cell swelling and bursting
Why is the sodium-potassium pump so important?
Vital for various cellular functions, including maintaining ion gradients, which are essential for nerve impulse transmission and muscle contraction.
How does the potassium leak channel filter other ions out?
Through the selectivity filter that allows K+ ions as well as some Na+ ions to leak out/in the cell.
What is action potential?
action potential refers to the changes in the electric potential difference between the inner and outer surfaces of the membrane during activation.
Explain what the resting is.
- The resting membrane potential is typically -70mV
- at rest, the inside of the cell is negatively charge compared to the outside due to the distrubtion of ions (mainly Na+ outside and K+ inside).
- Voltage-gated sodium channels and potassium channels are closed.
Explain what depolarization is?
- a stimulus causes Na+ gated channels to open, increasing the permeability 1000, which is 5000 times during the resting state.
- Na+ rushes into the cell, making the inside more positive.
- the potential difference changes from -70mV to +35mV. This is called depolarization.
Explain what repolarization is:
- after reaching 35mV, Na+ channels close, and K+ channels open.
- K+ moves out of the cell, restoring the inside to a negative state
- the potential returns to its resting level (-70mV)
Explain what hyperpolarization is:
- membrane potential becomes slightly more negative than the resting state due to excess K+ leaving the cell before the channels close.
What is the magnitude of the action potential?
the total change in membrane potential is approximately 105mV, from -70mV at rest to +35mV at depolarization and back to -70mV after repolarization.
What is the voltage-gated sodium channel?
It is a channel that plays a crucial role in initating and propagating the action potential (nerve signal).
How does voltage-gate Na channel operate?
gates
operate with 2 gates:
1. activation gate: controls when the channel opens
2. inactivation gate: controls when the channel closes after activation
How does the sodium channel act during the resting state?
- activation gate is closed, preventing sodium from entering the cell.
- inactivation gate is open, but no effect since the activation gate is closed
What are the events during the resting state?
- BOTH Na and K voltage-gated channels are closed
- the membrane is polarization, with negative charge inside the cell.
What are the events during depolarization?
- stimulus cause Na channels to open at ~-55mV (threshold)
- Na+ influx makes the inside of the cell more positive, rapidly depolarizing the cell membrane to +35mV
What are the events during repolarization?
- Na+ channels inactivate, stopping Na+ inflow.
- K+ channels open allowing K+ to leave the cell, restoring a negative membran potential.
What are the events during hyperpolarization?
K+ channels close slowly, causing an overshoot where the membrane potential becomes more negative than the resting potential.
What are the events to return to resting state?
both channels close, and the sodium-potassium pump restores ion gradients (sodium out, potassium in)
Review: when is the resting membrane potential restored?
after hyperpolarization and when the voltage-gated channels become inactive and the Na-K pump become active again to bring the potassium back into the cell and restore ion gradients.
When are sodium channels active?
- active early
- inactive during depolarization
- reset during repolarization.
When are potassium channels active?
- active late
- stay open during repolarization to help terminate the action potential.
What does synchronized activity of these voltage-gated channels do?
It allows neurons to generate and propagate electrical signals efficiently.
Label all periods of action potential.
- stimulus artifact
- latent period
- local potentials
- rapid depolarization
- overshoot
- rapid repolarization
- after depolarization
- after hyperpolarization
Describing an action potential
- an action potential consists of the spike and after-potentials.
- when electrical changes are recorded using two microelectrodes, one on the outer surface and the other on the inner surface of a cell, stages of action potential are observed.
What is the stimulus artifact period?
- indicates the time of application of the stimulus.
- caused by current leakage from the electrical stimulus to the recording electrodes.
What is the latent period?
represents the time between the application of the stimulus and the recording of the action potential.
- during this period, nerve impulse travels from the stimulus point to the recording electrodes.
What does the latent period depend on?
- velocity of nerve conduction
- distance between the stimulus and electrodes
What is the spike?
- it is a rapid phase, lasting about 2ms, with high magnitude changes (105mV change).
What does the spike consist of?
- ascending limb: depolarization
- descending limb: repolarization
What is the ascending limb of the spike?
depolarization, occurring as the membrane potential changes from -70mV to +35mV
What causes the ascending limb?
due to influx of sodium from voltage-gated channels
- initially the depolarization is slow: in the first 15mV increase from -70 to -55mV=== SLOW
- but then, at the firing level of -55mV, most voltage-gated Na+ channels open, accelerating depolarization and causing the spike.
What is the descending limb?
repolarization, with the membrane potential returning to near resting levels.
What causes the repolarization phase?
- inactivation of sodium channels, stopping Na influx.
- activation of potassium channels, allowing K efflux.
What are after-potentials?
after-potentials are periods of slower change (long duration) with low-magnitude changes.
What are the 2 phases that are considered after-potentials?
- after depolarization (negative after potential)
- after hyperpolarization (positive after potential)
What is after depolarization?
It is the last 30% of repolarization.
- when the repolarization is 70% complete, the rate of repolarization slows as the resting potential approached.
What happens during after depolarization?
a slowed outflow of K+ ions due to:
1. partial closure of voltage-gated K+ channels
2. accumulation of K+ ions just outside the membrane.
How long does after-depolarization last?
about 4 miliseconds
What is after hyperpolarization?
after reaching the RMP, the membrane becomes slightly hyperpolarizated and then gradually returns to RMP.
What are the causes of after hyperpolarization?
- primarily due to the many voltage-gated potassium channels that remain open for several milliseconds after repolarization, allowing excess K+ to diffuse out.
- hyperactivity of the sodium-potassium pump: this pump is strongly stimulated when the excess sodium accumulates inside the cell. Following depolarization, the pump becomes more active to expel the excess sodium from the cell.
How long does after hyperpolarization last?
about 40 milliseconds, much longer than after depolarization
Summarize action potential in 3 steps.
- depolarization: Na ions enter the cell to make the inside less negative, leading to a spike.
- repolarization: K ions leave the cell to restore the negative resting potential.
- after-potential: residual ionic movement slows the return to full resting state.
How is biphasic action potential recorded?
it is recorded using two microelectrodes placed on the outer surface of a nerve.
What are the 5 stages of biphasic action potential?
- at rest
- stimulation
- depolarization reaching between the electrodes
- depolarization reaching the second electrode
- depolarization wave leave the second electrode
What do the stages of biphasic action potential create?
they result in a biphasic waveform, where the recorded potential shows an initial upward deflection followed by a downward deflection before returning to baseline.
What happens at rest?
biphasic
No potential difference is recorded between the 2 electrodes.
What occurs during stimulation?
biphasic
when the nerve is stimulated, depolarization reaches the first electrode, causing an upward deflection in the recorded potential.
What occurs during depolarization that reaches between the electrodes?
potential difference returns to zero as the wave moves between the two electrode.
What occurs during depolarization the reaches the second electrode?
A downward deflection is recorded.
What occurs when depolarization wave leaves the second electrode?
The potential difference returns to 0.
