How permeable are different things
Different molecules have different permeabilities
Hydrophobic molecules have the highest permeability, such as O2, CO2, N2, steroids, and hormones
Small uncharged polar molecules such as H2O, urea, and glycerol are the next highest
Large uncharge polar molecules such as glucose and sucrose are next
Last are ions, such as H+, Na+, HCO3-, K+, Ca2+, Cl-, Mg2+
what is the nervous system composed of
- Nervous system mainly composed of neurons and glial cells
- Neurons send or receive electrical impulses, subdivided into sensory neurons, motor neurons, and interneurons
- glial cells: most abundant variety of types
describe the anatomy of a neuron
- cell body similar to other cells
- dendrites receive signals
- axons transmit signals, terminate in synaptic boutons
- junction is the synapse
how is membrane potential created
Cytosol and extracellular space have different concentrations of ions
this leads to membrane potential
what is the concentration of different ions inside and outside the cell
Inside the cell:
- [Sodium] = 15 mM
- [Potassium] = 125 mM
- [Chloride] = 13 mM
Outside the cell:
- [Sodium] = 145 mM
- [Potassium] = 5 mM
- [Chloride] = 150 mM
How does membrane potential work
The cytosol has a much higher concentration of potassium ions (K+) and impermeable anions (M-) relative to the extracellular fluid
AS K+ ions diffuse out of the cell the impermeable anions are left behind, creating a membrane potential. The magnitude of the membrane potential increases until an equilibrium is reached
how is the plasma membrane leaky
The plasma membrane is normally leaky to K+, but not the -ve counterion
electrochemical equilibrium
Describe the Nerst equation
Ex = RT/zF * ln([X]Outside/[X]Inside)
Ex = equilibrium potential for ion X in volts
R = gas constant (1.978 cal/mol-degree)
F = Faraday constant (23 kcal/V-mol)
T = temperature (in K)
z = charge
what does the Nerst equation do
Describes the relationship between an ion gradient and the equilibrium potential when membrane is only permeable to one ion.
what membrane potentials does the Nerst equation give
The Nerset equation gives membrane potentials of
+60 mV for sodium
-85 mV for Potassium
-65 mV for Chloride
what is the relative ion concentrations
Potassium ions are high in cell, sodium and chloride ions are high in extracellular fluid
So their respective electrochemical gradients are different
At rest significantly more potassium leak channels are open compared to sodium leak channels. Creating the membranes preferential permeability to potassium.
What does the Goldman equation do
The Goldman Equation Describes the combined effects of ions on Membrane Potential
- Even in the resting state, the cell is a little permeable to sodium, chloride, and potassium ions
- The Nerst equation doesn’t account for leakage of sodium and chloride into the cell; it deals with just one ion at a time
- It is helpful to consider the steady state ion movements across the membrane
What is the Goldman equation
- The Goldman equation, unlike the Nerst equation includes terms for permeability of the ions involved
- In this case, Pk, PNa and PCl are the relative permeabilities for each ion
- Except under special circumstances, the contribution of other ions to membrane potential is negligible
Vm = RT/F (Pk)[K+]out + (PNa)[Na+]out + (PCl)[Cl-]in)/((Pk)[K+]in + (PNa)[Na+]in + (PCl)[Cl-]out)
how are the Goldman and nerst equations related
- When the relative permeability of one of the ions is very high, the Goldman equation reduced to the Nerst equation for that ion
- For instance, if we ignore the effect of Cl-, as we can when PNa»_space;> PK it becomes the Nerst equation where X = Na+
how are permeabilities assigned
- To estimate resting membrane potential in a squid axon, we use known steady state concentrations and relative permeabilities of the three ions
- K+ can be assigned a permeability value of 1.0, and the others are determined relative to that.
- The relative permeability of Na+ is 4% (0.04) and that of Cl- is 40% (0.4)
what are the stages in an action potential
- Resting state: all gated Na+ and K+ channels are closed. The resting potential is approximately -60 mV
- Depolarizing phase: Na+ channels open. An action potential begins when the neuron is depolarized by about 20 mV to its threshold potential. The potential rapidly becomes positive, reaching a value of +40 mV
- Repolarizing phase: Na+ channels inactivated and K+ channels open. Once the cell reaches its peak positive potential, it repolarizes, returning to a negative membrane potential
- Hyperpolarizing phase (undershoot): K+ channels remain open and Na+ channels close. Often the membrane potential becomes more negative than the resting potential
what does the membrane experience in an action potential
The membrane experiences a Depolarizing stimulus, followed by Absolute refractory period, finally a relative refractory period
what are voltage gated ion channels
- When membranes depolarize, they become more positively charged on the cytoplasmic side
- this causes helix S4 to move into the up position, opening the channel
- N-terminal 20 amino acids form a structure that can block the channel
How do ion channels work
Closed: At the resting potential, the channel is closed
Open: in response to a nerve impulse, the gate opens and Na+ enters the cell
Inactivated for a brief period following activation, the channel does no open in response to a new signal
what is an action potential
- membrane potential changes in response to changes in ion permeability
- action potential is a “brief but large electrical depolarization and repolarisation of the neuronal plasma membrane
- Na+ in and K+ out
how is an action potential transmitted
- At the start, the membrane is completely polarized
- When an action potential is initiated, a region of the membrane depolarizes. As a result, the adjacent regions become depolarized
- When the adjacent region is depolarize to its threshold, an action potential starts there
- Repolarization occurs due to the outward flow of K+ ions. The depolarization spreads forward, triggering an action potential
- Depolarization spreads forward, repeating the process
what does myelination do
Axons are surrounded by either Schwann cells or Oligodendrocytes.
Axons are surrounded by either Schwann cells or Oligodendrocytes.
what are the two types of myelination and what do they do
Schwann cells are found in the peripheral Nervous system and wrap around a single axon
While Oligodendrites are found in the Central Nervous system and extend to wrap around multiple axons.
how does myelination work
- In myelinated neurons, an action potential is usually triggered at the axon hillock, just before the start of the myelin sheath. The depolarization then spreads along the axon
- Because of myelination, the depolarization spreads passively to the next node
- The next node reaches its threshold and a new action potential is generated
- This cycle is repeated, triggering an action potential at the next node
- the process continues
