medical ,nursing lectures ,nervous system

1. ACTION POTENTIAL
Dr SHAROJ KHAN
MBBS (NGMC,KU)
ANATOMY AND PHYSIOLOGY

2.  An action potential is a short-lasting event in which the
electrical membrane potential of a cell rapidly rises and
falls, following a consistent trajectory. Action potentials
occur in several types of animal cells, called excitable cells,
which include neurons, muscle cells, and endocrine cells.
 In neurons, action potentials play a central role in cell-to-
cell communication by providing for (or assisting in, with
regard to saltatory conduction) the propagation of signals
along the neuron's axon towards boutons at the axon ends
which can then connect with other neurons at synapses, or
to motor cells or glands.

3.  The surface of the cell membrane is usually polarized
(charged), with respect to the inside.
 This polarization arises from an unequal distribution of
positive and negative ions between sides of the
membrane.
 This polarization is particularly important in the conduction
of muscle and nerve impulses

4.  The distribution of ions inside and outside cell
membranes is determined in part by pores or channels
in those membranes.
 Some channels are always open, and others can be
opened and closed.
 Most channels are selective and only allow one type of
ion/molecule through.

5.  Active transport creates a concentration gradient
across the cell membrane of sodium and potassium
ions.
 Na+/K+ pumps work to move Na+ out of the neuron and
K+ into the neuron.
 Uses ATP as energy source
 ATPase breaks down ATP into ADP + P
 Similar effects as those we saw in the ATPase of myosin heads
in muscle fibers

7.  The Na+/K+ pump creates:
 High concentration of sodium outside
 High concentration of potassium inside
 Negative amino acids are found in abundance
inside the cell
 Would make the intracellular fluid more negative, but…
 Chlorine ions (Cl-) are found in abundance outside
the cell
 Counteract the negative of the amino acids

8.  When neurons are excited (stimulated)
 Affect the resting potential in a particular region of a
nerve cell membrane.
 If the membrane’s resting potential decreases (as the
inside of the membrane becomes less negative when
compared to the outside), the membrane is said to be
Depolarizing.

9.  If these were the only factors involved, the neuron
would be neutral
 However, K+ leaks out of the cell through leakage
channels that are always open
 Positives leaving the intracellular space means the
overall charge is negative inside
 (-70mV)

10.  Changes in potential are directly proportional to the
intensity of the stimulation.
 If additional stimulation arrives before the effect of
previous stimulation subsides, summation takes place.
 As a result of summated potentials, a level called
Threshold Potential may be reached.

11.  An action potential can be thought of as the “firing”
of the neuron.
 Action potentials will propagate down the length of a
neuron’s axon
 Action potentials are the electrical signals that move
down a neuron

13.  When threshold is reached:
 1. VGICs that are permeable to Na+ open
 Na+ diffuses into neuron
 Neuron’s membrane potential rises from -70mV to +40mV
(depolarizes)
 2. Na+ channels close as K+ VGICs open
 K+ diffuses out of neuron
 Neuron’s membrane potential repolarizes, going from
+40mV to nearly -85mV

15. 1. Neuron membrane maintains resting
potential.
2. Threshold stimulus is received.
3. Sodium channels in a local region of the
membrane open.
4. Sodium ions diffuse inward,
depolarizing the membrane.

16. 5. Potassium channels in the membrane open.
6. Potassium ions diffuse outward, repolarizing
the membrane.
7. The resulting action potential causes a
local bioelectric current that stimulates
adjacent portions of the membrane.
8. Wave of action potentials travels the length
of the axon as a nerve impulse.

17.  A myelinated axon functions as an insulator and
prevents almost all ion flow through the
membrane it encloses.
 Nodes of Ranvier between adjacent Schwann cells
interrupt the sheath.
 Action potentials occur at these nodes, where the exposed axon
membrane contains sodium and potassium channels.
 Nerve impulses jump from node to node, and are many times faster
than conduction on an unmyelinated axon.

18.  The speed of nerve impulse conduction is proportional
to the diameter of the axon.
 The greater the diameter, the faster the impulse.

19.  Nerve impulse conduction is an all-or-none response.
 If a neuron responds at all, it responds completely.
 A nerve impulse is conducted whenever a stimulus of
threshold intensity or above is applied to an axon, and all
impulses carried on that axon are of the same strength.
 A greater intensity of stimulation does not produce a stronger
impulse, but more impulses per second

23. Chemical
synapses:
the
predominant
means of
communicati
on between
neurons

24. Synapse Structure
 The part of the synapse that belongs to the initiating neuron is
called the presynaptic membrane.
 The part of the synapse that belongs to the receiving neuron is
called the postsynaptic membrane.
 The space between the two is called the synaptic cleft. It is
approx 20-50 nm wide.
 Presynaptic terminals contain numerous synaptic
vesicles
 Synaptic vesicles contain Neurotransmitters,
chemical substances which ultimately cause
postsynaptic changes in the receiving neuron, is
contained within the synaptic vesicles. Common
neurotransmitters include : Acetylcholine, Dopamine,
Norepinephrine (a.k.a., noradrenaline), Serotonin

26. Steps of Synaptic Transmission
Action potential signal arrives at the presynaptic axon terminal
Opening of voltage gated Ca channels
Ca enters into the presynaptic terminal
Ca mediated exocytosis of neurotransmitter
Neurotransmitter binds to the receptors at post synaptic membrane
Opeining of ion channels
Movement of ions through the post synaptic membrane
Change in membrane potential in post synaptic membrane
Synaptic potential generated

28. Fate of the neurotransmitter discharged in
the synaptic cleft
Degradation: Enzymes located in the synaptic
cleft break down the neurotransmitter into a
substance which has no effect on the receptor
channel
Reuptake: The neurotransmitter can reenter the
presynaptic cell through channels in the
membrane.
Diffusion

29. 1. Synaptic delay: All the events involved in
synaptic transmission need some time usually
between 0.5 &1 ms
2. Law of forward conduction: impulse always
travels from presynaptic to postsynaptic
neuron

30. 3a. Spatial summation: If a number of fibers
converging on a single neuron are stimulating
simultaneously with sub-threshold stimuli, the
postsynaptic neuron may fire action potential.
3b. Temporal
summation: If a sub-
threshold stimulus is
repeated several times
in quick succession, the
postsynaptic neuron may
fire.

31. 4a. Facilitation: The effect of stimulating two
nerve fibres may turn out to be greater than
the sum of stimulating either of them
separately.
4b. Occlusion: with stronger stimuli, the
effect of stimulating two nerve fibres may
turn out to be less than the sum of stimulating
either of them separately.

32. 5. Inhibition: Inhibitory pre-synaptic neuron –
release Inhibitory NT
 6. Fatigue: Repeated stimulation of a synapse
leads to gradual diminution and finally
disappearance of the postsynaptic response.