1. Nerve Impulse Generation & Conduction

2. Two types of physicochemical disturbances
Local, non-propagated potential (Graded potentials)
Propagated potentials, Action potentials or nerve impulse

3. The size of a
graded potential
(here, graded
depolarizations)
is proportionate
to the intensity
of the stimulus.
The duration of a
graded potential
(here, graded
depolarizations)
is proportionate
to the duration of
the triggering event

4. Graded potential spread by passive current flow

6. Electrical Signal Generation
The graded responses produced throughout the dendrites or cell body is summed spatially and temporally, and if the summed response is large enough to pass the threshold by the time it reaches axon hillock, an action potential will be generated at axon hillock.
The axon hillock has the lowest threshold in the neuron because this region has a much higher density of voltage gated Na+ channels than anywhere else in the neurons
Action potential originates at axon hillock

7. Panel 1: Two distinct, non-overlapping, graded depolarizations.
Panel 2: Two overlapping graded depolarizations demonstrate temporal summation.
Panel 3: Distinct actions of stimulating neurons A and B demonstrate spatial summation.
Panel 4: A and B are stimulated enough to cause a suprathreshold graded depolarization, so an action potential results.
Panel 5: Neuron C causes a graded hyperpolarization; A and C effects add, cancel each other out.

8. All-or-None Principle
The all or none feature of action potential implies that stimulus less than certain threshold level of depolarization results in a graded response which would not be transferred. However a stimulus big enough to move the membrane potential beyond the threshold will generate action potential that can propagate to distant regions of the cells
Threshold potential of-55mV corresponds to the potential to which an excitable membrane must be depolarized in order to initiate an action potential

9. Throughout depolarisation, the Na+ continues to rush inside until the action potential reaches its peak and the sodium gates close.
If the depolarisation is not great enough to reach threshold, then an action potential and hence an impulse are not produced.
This is called the All-or-None Principle.

10. One–way propagation of the AP

11. One–way propagation of the AP

12. One–way propagation of the AP

13. AP in neurons is unidirectional
After generating an axon potential at trigger zone, it begins to propagate to neighboring segments of the membrane and depolarize them to threshold triggering action potentials in the next neighboring area and so on
This propagation is unidirectional from axon hillock to axon terminal because in the case of the neuron, the proximal segment just traversed by the action potential just enters a refractory period and thus becomes inexcitable

15. Absolute Refractory Period
The period when a recently activated patch of a membrane is completely refractory (meaning “stubborn” or unresponsive) to further stimulus is known as “absolute refractory period”
Once the voltage gated Na+ channels are triggered to open at threshold, they cannot open again in response to another stimulus no matter how strong until they pass through their “closed and not capable of opening” conformation and they are reset to their “closed and capable of opening” conformation
Absolute refractory period lasts the entire time from threshold to depolarization peak and to hyperpolarization peak

16. Relative Refractory Period
Following the absolute refractory period is a “relative refractory period”, during which a second action potential can be produced only by a triggering event considerably stronger than usual
The voltage gated K+ channels that opened at the peak of action potential are slow to close. During this time, the resultant less than normal Na+ entry in response to another triggering event is opposed by K+ still leaving through its slow to close channels during hyperpolarization
Thus a greater depolarizing triggering event than normal is needed to offset the persistent hyperpolarizing outward movement of K+ and bring the membrane to threshold during the relative refractory period

17. The propagation of the action potential from the dendritic
to the axon-terminal end is typically one-way because the
absolute refractory period follows along in the “wake”
of the moving action potential.

18. Nerve Impulse Conduction
Once initiated action potential are conducted throughout a nerve fiber
Two types of conduction
Contiguous conduction
Saltatory conduction

19. Contiguous conduction

20. Contiguous conduction

21. Saltatory Conduction
In myelinated neurons, charge carriers cannot cross myelinated parts of the axonal membrane
Only at nodes of Ranvier, the axonal membrane is bare and exposed to ECF
Voltage gated Na+ and K+ channels are concentrated at the nodes , whereas the myelin covered regions are devoid of these special passageways
Therefore, action potentials can be generated only at the nodes

23. Saltatory Conduction

24. Saltatory Conduction
Saltatory Conduction: Action potentials jump from one node to the
next as they propagate along a myelinated axon.

25. Myelin acts as an insulating sheath allowing an action potential to spread along the axon until it gets to a node of Ranvier which is a bare portion of axon without mylin. As a result action potential jumps from one node to the next and so on. This is called saltatory conduction

26. Enhancing Speed of Electrical Conduction in Neurons
Speed of electrical conduction will be high if
Internal diameter of axon is increased:
Increase in internal diameter of axon decreases the internal resistance to ion flow and thus increases conduction speed
By myelinating axons:
Myelination increases the resistance of the plasma membrane to charge flow by acting as an insulator. The presence of a myelin sheath greatly increases the velocity at which impulses are conducted along the axon of a neuron. In unmyelinated fibres, the entire axon membrane is exposed and impulse conduction is slower

27. Importance of Myelination
Large myelinated fibers such as those supplying skeletal muscles can conduct action potentials at a velocity of up to 120m/s compared with a conduction velocity of 0.7m/s in small unmyelinated fibers supplying the digestive tract
Speed related to urgency of information being conveyed
Invertebrates have axons with large diameters
In humans, the optic nerve leading from the eye to the brain is only 3mm in diameter but is packed with more than a million myelinated axons. If these axons were unmyelinated, each would have to be about 100 times thicker to conduct impulses at the same velocity, resulting in an optic nerve about 300mm in diameter

28. Properties of Graded vs Action Potential
Graded Potential
Amplitude varies with size of initiating event
Can be summed
Has no threshold
Has no refractory period
Is conducted decrementally i.e. amplitude decreases with distance
Can be a depolarization or a hyperpolarization
It begins with a stimulus (chemical, electrical, mechanical) or a synapse
Mechanism depends on ligand gated channels, or other physical or chemical changes
Action Potential
All or None. Once the membrane is depolarized to threshold, amplitude is independent of size of initiating event
Cannot be summed
Has a threshold which is usually 15mV depolarized relative to resting potential
Has a refractory period
Is conducted without decrement
Is a depolarization initially than a hyperpolarization
Initiated only by a graded potential
Mechanism depends on voltage gated channels