11) Propagation Of ap Flashcards
Local currents and their direction
Currents on the surface of the membrane between the excited and non excited sections
On external surface_from the unexcited towards the excited
Internal surface- from the excited to unexcited
Local currents loop across the membrane and electrical charge is transported
Membrane polarisation
As the membrane is excited the external surface is negative and internal surface is positive charged
Conduction of excitation
Where excitation occurs the membrane potentials becomes less negative and reaches the threshold value for an action potential to be generated
At the initial excited section repolarisation occurs
These 2 process cause the action potential to propagate across the membrane
Cable equation
Describes the variations of the membrane potential over time and alone the nerve fibre
🔱 = -rn/ri d2🔱/dx2 -rmcm d🔱/dt
Decay of the membrane potential over time
At any fixed point along the nerve fibre the membrane potential decays exponentially over time
Decay rate is determined by the resistance and capitance of the membrane
🔱=🔱o e^-t/that
Distribution of membrane potential along fibre
The membrane potential decreases exponentially away from the point of excitation
The decay rate is determined by the membrane and cytoplasm resistance
🔱=🔱o e^-x/lander
Model validity
The model describes the conduction of the graded local potential in a real nerve fibre
Weakly depolarising blow the threshold
Hyper polarisation potential
Time constant
T = rmcm
T is the time constant of nerve fibre
At time t = T the excitation of the membrane decays by a factor of e= 2.72
Length constant
Lander = root of rm/ri
Lander is the length constant of the nerve fibre
Length constancy grows by
Membrane thickness
Diameter of the nerve fibre
Membrane sensitivity
Nondecremental conduction
The action potential impulses propagate without decay along the membrane
Amplitude is sustained by ion currents across membrane
Increasing conduction speed
Increasing the fibre diameter too much is not feasible
Vertebrates speed up the condition of nerve fibres using myelinated nerve fibres
Myelin
Composed of lipids proteins and cholesterol
Myelin is a good insulator
Myelin sheath structure
Layer surrounding the axon is much thicker than the membrane
It has regular gaps in the myelin called nodes of ranvier
Spaced at 1-3 mm
Myelin sheath function
Depolarisation only occurs at the nodes of ranvier
The excitation jumped from one node to the next and this is known as saltory conduction of the impulse
Advantages of saltatory conduction
High conduction speed of the nerve impulses up to 100m/s
10 times faster than unmyeleineated
More efficient conduction so less energy is used up in traveling across the entire Membrane
Blocking of myelinated fibres
If one mode is blocked by a anaesthetic then excitation jumps over to the next node
If 2 successive nodes are blocked then the impulse can not pass
-depolarisation of the 3rd node can’t reach the threshold value
Demyelination and dismyelination
De is the loss of myelin sheath
Due to disorders of the immune system
-multiple sclerosis
Dis is defective structure of the sheath and function
-genetic mutations
Schizophrenia
Propagation of impulses in expanding fibres
Before expansion the conduction speed decreases
After the expansion the confusion speed increases to a higher value
If fibre diameter increases the impulse may stop
Propagation is impulses in contracting fibres
Before contraction the conduction speed increases
After the conduction the conduction speed decreases to a new lower value
No blocking of the impulse can occur
Nerve trunks
A bundle of nerve fibres bound together by a sheath of connective tissue
Each fibre is an independent communication channel
Impulse propagating in one fibre may influence the membrane potential of adjacent fibres mood
Interaction between the fibres in a nerve trunk
Impulses propagating in one fibre nag give rise to impulses in an adjacent fibre
Interaction becomes stronger if the diameter of the nerve fibre increases
