Nerve Conduction Flashcards
Introduction to nervous system.
- The nervous system (NS) is complex and comprises neurons(10^11) and glial cells (10^12). Glia are non conducting supporting cells and essential ‘partners’
- Neurons have a great variation in structure & biochemistry and are also referred to as secretory cells
- Neurons are specially adapted to generate and propagate impulses.
NB basic morphology of the neuron = Cell body, hillock, axon, and dendrites
Functional zones of the neurons
Somato-dendritic zone - receive primary input
Somatic zone - integration and chemical encoding
Axon hillock zone - electrical encoding
Axonal zone - Signal propagation
Presynaptic zone = signal output
All cells are polarized, meaning?
- If you put a Recording device across cell membrane you’ll record potential difference or charge across the membrane = polarized
Nerve and muscle the polar nature changes transiently or over a short period of time
- Polarized cells are a characteristic of life.
- Polarity of excitable tissue changes when stimulated. This forms the basis of communication in NS.
The plasma membrane plays an instrumental role in this process.
The special role of the cell membrane.
The cell membrane is a continuous lipid bilayer with proteins.
-The lipid bilayer effectively separates two conducting media.
[ECF* & ICF# are electrolytic solutions]
- The bilayer is a non-conductor & acts as a dielectric.
- This geometric arrangement (ECF + Bilayer + ICF) approximates that of a capacitor
-A capacitor stores an electrical charge between the two plates. It also discharges.
A charge, therefore, exists across the membrane of each living cell.
……
Cell membrane perfect insulator.
Resting membrane potential
The charge or potential difference across the membrane is due to the separation of the 2 fluid compartments and the unequal distribution of ions across the cell membrane.
This is called the Resting membrane potential (RMP)
This potential difference varies amongst cell types but is often quoted as ~ -70mV
To maintain the resting membrane potential [or this potential difference]
To maintain this PD:
1) T° needs to be within the physiological range 2) Ionic imbalance maintained 3) Cell membrane needs to ~25 A° thick (d), constant in cells
NB capacitance, c = ε A /d
where A = area, d = distance, ε = dielectric constant
Mammalian neuron
More Potassium in intracellular fluid than extracellular
More Sodium in Extracellular fluid than intracellular
#Na you’re out, K you’re in
More chloride out than in
More calcium out than in
-Let’s not forget the large concentration of negative anions with the cell (106.5mM)
Ionic pumps
- To maintain RMP, constant gradients need to be maintained with respect to Na+, K+ & Cl- ions
- Gradients maintained by ionic pumps
- Na+ & K+ transported by a common transporter → Na/K pump (Na+/K+ ATPase)
- Consists – 2 subunits = α & β
-The α unit → specialized & spans membrane many times:
EC surface binds K+
IC surface binds Na+ + site for phosphorylation
The pump is sensitive to IC [Na+]. Na+ binds with the receptor intracellularly and induces a conformational change → cycle commences (ATP)
In the Na and K pump. The ins and outs
- For each stroke 3 Na+ exported and 2 K+ imported (3:2 coupling ratio)
- Each cycle of the pump there is a net loss of positivity from the cell hence referred to as an electrogenic pump.
- β unit is a glycoprotein that offers stability to the pump
- Alpha unit also has receptors on extracellular surfaces that bind with ligands/ substances e.g. ouabain + glycosides
The sodium-potassium pump is usually coupled with which exchange mechanisms.
This pump is often coupled with other exchange mechanisms :
1) Cl‾ levels maintained
Chloride- bicarbonate
exchanger → 1 Na+ + 1 HCO‾imported →1 Clˉ ion pumped out
2) Sodium – Calcium exchanger with a 3:1 ratio.
This is one of the important mechanisms that is targeted therapeutically in cases of heart failure.
Nernst to Goldman equations
Na+, K+, Cl- ions are “major” role players in establishing PD. 2 considerations:
1) Chemical gradient 2) Equilibrium (electrochemical + electrostatic)
The Potential Difference at equilibrium described by Nernst:
EMF = -61 log [conc. outside] / (divided by) [conc. inside]
Mammals: K+ = - 102 mV
Na+ = + 56 mV
Cl- = - 76 mV NB Nernst equation → considers single ion!
Membrane → ∆ permeability rates for different ions!
-
Goldman - Hodgkin - Katz (G-H-K)
The Goldman-Hodgkin-Katz (G-H-K ) derived with these considerations
vrest= rt / f In pk out +pna out+ pcl in divided all these ps opposite.
P = permeability, [ ] = concentration, R gas constant,
T = Temp in K, F = Faradays constant,
Note RT/F =constant = 61.5mV
Important points of GHK
- RMP depends on the concentration gradients and on the relative permeabilities to Na+, K+ and Cl-.
- The Nernst potential for an ion does not take membrane permeability into account.
- The GHK equation describes a steady-state condition, not electrochemical equilibrium.
- There is a net flux of individual ions, but no net charge movement.
- The cell must supply energy to maintain its ionic gradients.
An important consideration on the membrane and different types of channels.
Membrane contains:
Passive (leak) channels that are always open
Gated channels that open and close in response to stimuli – essential for nerve function
There are 3 types of gated channels:
- Chemically regulated channel
- Voltage-regulated channel
- Mechanically regulated channel
Voltage gating in ion channels
- Voltage-gated ion channels display a voltage sensor (transmembrane helix) containing a number of positively charged amino acids along one side of the helix.
- ∆ electrical potential across the membrane induces a conformational change in amino acids causing the voltage sensor helix to change its position, thereby affecting the pore dimension of the channel.
depolarization causes the opening of the ion channel
