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.
How does the neurilemmal sheath look?
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?
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.
