7.Excitability and excitation: general characteristics. Propagation of action potentials along the nerve fibers. Nerve fiber types. Flashcards
What are the main biological properties of neurons
Excitability and conductivity
What is the significance of excitability and conductivity
transmission of information
afferent fibers transmit from peripheral organs to CNS
Efferent fibers transmit from CNS to periheral organs
How does propagation of action potentials occur
spread of local currents from active regions to inactive regions
ELECTROTONIC CONDUCTION
What is conduction velocity
The speed at which action potentials travel along nerve fiber
What are the cable properties that are major concepts in conduction velocity
Time constant and length constant
the higher the permeability, the lower the resistance, and vice versa.
Longitudinal resistance (Rl ): is the resistance that counteracts electric
current in the cytosol and ECF.
What are 2 factors that affect time constant
membrane resistance + membrane capacitance
Time constant is highest when both of these are high
How can you increase the conduction velocity of a nerve
Increasing nerve diamter- the larger the fiber, the lower the internal resistance
Myelination- myelin increases membrane resistance but decreases membrane capacitance. High resistance means it forces the action potential to take path of least resistance
At nodes of Ranvier, the resistance is low-saltatory conduction
What is rheobase
The lowest intensity of stimulus that can cause A.P after prolonged stimulation
What is chronaxie
Time requires for a stimulus that is 2x rheobase to induce a A.P
Rehobase is the minimum intensity of stimulus to induce AP
What is the Erlanger and Gasser classification of nerve fibres
A- alpha, beta, delta, gamma
B
C
Which is the fastest nerve fiber
A
largest diameter
skeletal muscle, receptors of muscle spindle etc
If the action potential is stimulated in the middle of the nerve fiber, how will the impulse travel
Action potential travels in both directions, centrally and peripherally
What is the threshold potential
-55mv
What determines the excitability of given excitable tissue
Difference between resting membrane potential (-70mV) and threshold potential (-55mV)
What is the law for all or nothing stating
Upon threshold or above threshold all APs have the same amplitude
What is the biological significance of refractory periods
- Limit the max frequency of effective stimuli
- determine direction of nerve propogation
Can an AP generated in 1 nerve fibre be transmitted to neighbouring nerve fibers within a single nerve
NO IT CANT- law for isolated conduction
the resistance in the extracellular space is much lower than the transmembrane resistance of adjacent unexcited nerve fibers
Differences between myelinated and unmyelinated AP conduction
unmyelinated - Na+ channels are evenly distributed along axonal membrane- slow conduction
Myelinated- myelin sheath increases transmembrane resistance. The nodes have the highest conc of Na+ channels with lowest resistance
What is time constant
The time constant (τ) gives us an idea about the time it takes for initially
applied voltage to drop to 37% of its original value.
It is usualy 2-15 ms
How can physiological continuity of nerve be interrupted
Blocking voltage gated Na+ channels eg lidocaine
Cooling
Demyelination leading to decrease of length constant
Compression of nerve leading to resistance for local currents
Mechanism of neuromuscular transmission
- Depolarization of presynaptic terminal by AP
- Voltage gated Ca2+ channels open and Ca2+ flows into presynaptic terminal
- Ach extruded by exocytosis into synaptic cleft
- Ach binds to receptors on postsynaptic membrane (motor end plate)
- Na+/K+ channels open leading to depolarization and excitory postsynaptic potential
- This EPSP causes generation of AP
in the adjacent muscle tissue, were
voltage-gated Na+ channels are
again available. - Ach is then degraded to choline
and acetate by acetylcholinesterase
(AchE).
