The nervous system controls and coordinates the body's activities through electrical and chemical signals transmitted through networks of neurons. Neurons transmit signals through action potentials, which are brief changes in the electrical potential across the cell membrane that allow signals to propagate rapidly along axons. At synapses, neurotransmitters are released by the presynaptic neuron and bind to receptors on the postsynaptic neuron, generating excitatory or inhibitory postsynaptic potentials that influence whether or not an action potential occurs in the postsynaptic cell. This allows neurons to communicate with one another and coordinate the body's complex functions.

1. THE NERVOUS
SYSTEM
DAWN V TOMY M.Pharm
Asst.Professor,
Dept. Of Pharmacology
St.JOSEPH’S COLLEGE OF PHARMACY,
CHERTHALA.
Part -2

2. 2
Cell Membrane Potential
• A cell membrane is usually electrically charged, or
polarized, so that the inside of the membrane is negatively
charged with respect to the outside of the membrane (which is
positively charged).
• This is as a result of unequal distribution of ions on the
inside and the outside of the membrane.

4. 4
Distribution of Ions
• Potassium (K+) ions are the major intracellular positive ions (cations).
• Sodium (Na+) ions are the major extracellular positive ions (cations).
• This distribution is largely created by the Sodium/Potassium Pump
(Na+/K+ pump).
• This pump actively transports 3 sodium ions out of the cell and 2
potassium ions into the cell.

5. 5
Resting Potential
• Resting Membrane Potential
(RMP):
• 70 mV difference from
inside to outside of cell
• It is a polarized
membrane
• Inside of cell is negative
relative to the outside of
the cell
• RMP = -70 mV
• Due to distribution of
ions inside vs. outside
• Na+/K+ pump restores
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AxonCell body
Low Na+
Axon terminalLow K+
High K+
High Na+
(a)
+
+
–
–
+
+
–
–
+
+
–+
–
–
+
–+
–
+
–
+
–
+
–+
–
+
–
+
–
+
–
+
–
+ –
–70 mV
(b)
+
+
––
+
+
––
+
+
–+
–
–
+
–+
–
+
–
+
–
+–
+
–
+
–
+
–
+
–
–70 mV
Low Na+
Low K+ High K+
High Na+
Na+
K+
(c)
Pump
Impermeant
anions

6. 6
Local Potential Changes
• Caused by various stimuli:
• Temperature changes
• Light
• Pressure
• Environmental changes affect the membrane potential by
opening a gated ion channel
• Channels are 1)Leakage channels
2) chemically gated, 3) voltage gated, or 4) mechanically gated.
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Gate-like mechanism Protein
(b) Channel open(a) Channel closed
Cell
membrane
Fatty acid
tail
Phosphate
head

7. 7
Local Potential Changes
• If membrane potential becomes more negative, it is hyperpolarized.
• If membrane potential becomes less negative, it is depolarized.
• Graded (or proportional) to intensity of stimulation reaching
threshold potential.
• Reaching threshold potential results in a nerve impulse, starting an
action potential.

8. 8
Local Potential Changes
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–62 mV
Na+
Na+
Neurotransmitter
(a)
–55 mV
Na+
Na+
Na+ Na+
Na+
Trigger zone (axon hillock)
(b)
Chemically-gated
Na+ channel
Presynaptic
neuron
Voltage-gated
Na+ channel

15. Action Potentials

16. 16
Action Potentials
• At rest, the membrane is
polarized (RMP = -70)
• Sodium channels open
and membrane
depolarizes (toward 0)
• Potassium leaves
cytoplasm and
membrane repolarizes
(+30)
• Threshold stimulus
reached (-55)
• Brief period of
hyperpolarization (-90)
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(a)
Region of depolarization
(b)
Region of repolarization
(c)
–70
–0
–70
–0
–70
–0
K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
K+
K+
K+ K+
K+ K+
Na+ Na+ Na+
Na+ Na+ Na+
Threshold
stimulus
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
K+ K+ K+ K+ K+
K+ K+ K+ K+ K+
Na+ Na+ Na+
Na+ Na+ Na+
K+
K+
K+ K+ K+
K+ K+ K+
Na+ channels open
K+ channels closed
K+ channels open
Na+ channels closed

17. 17
Action Potentials
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(a)
Direction of nerve impulse
+ +
+ +
+
– – – – – – – – –
– – – – –– – – –
– – – – –– – – –
– – – – – – – – –
– – – – – – – – –
– – – – – – – – –
+ + + + + + + +
+ + + + + + + + +
(b)
+ +
+ +
++ + + + + + + +
++ + + + + + + +
(c)
+ +
+ +
++ + + ++ + + +
++ + + ++ + + +
Region of
action potential

18. 18
Action Potentials
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Milliseconds
10
0
+20
+40
2 3 4 5 6 7 8
Membranepotential(millivolts)
Action potential
Hyperpolarization
–40
–20
–60
–80
Resting
potential
Resting potential
reestablished

23. 23
Refractory Period
• Absolute Refractory Period
• Time when threshold stimulus does not start another
action potential
• Relative Refractory Period
• Time when stronger threshold stimulus can start another
action potential

25. 25
All-or-None Response
• If a neuron axon responds at all, it responds completely –
with an action potential (nerve impulse).
• A nerve impulse is conducted whenever a stimulus of
threshold intensity or above is applied to an axon
• All impulses carried on an axon are the same strength

26. 26
Impulse Conduction

28. 28
The Synapse
• Nerve impulses pass
from neuron to neuron at
synapses, moving from a
pre-synaptic neuron to a
post-synaptic neuron.
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Dendrites
Impulse
Impulse
Impulse
Synaptic
cleft
Axon of
presynaptic
neuron
Cell body of
postsynaptic
neuron
Axon hillock of
Postsynaptic neuron
Axon of
presynaptic
neuron

29. 29
Synaptic Transmission
• This is where released neurotransmitters cross the
synaptic cleft and react with specific molecules called
receptors in the postsynaptic neuron membrane.
• Effects of neurotransmitters vary.
• Some neurotransmitters may open ion channels and
others may close ion channels.

31. 31
Synaptic Transmission
• Neurotransmitters are
released when impulses
reach synaptic end bulbs
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Mitochondrion
Synaptic knob
(a)
Synaptic cleft
Neurotransmitter
Axon
Ca+2
Presynaptic neuron
Direction of
nerve impulse
Synaptic
vesicles
Cell body or dendrite
of postsynaptic neuron
Synaptic
vesicle
Vesicle releasing
neurotransmitter
Axon
membrane
Polarized
membrane
Depolarized
membrane
Ca+2
Ca+2

32. 32
Synaptic Potentials
• EPSP
• Excitatory postsynaptic potential
• Graded
• Depolarizes membrane of postsynaptic neuron
• Action potential of postsynaptic neuron becomes more likely
• IPSP
• Inhibitory postsynaptic potential
• Graded
• Hyperpolarizes membrane of postsynaptic neuron
• Action potential of postsynaptic neuron becomes less likely

33. 33
Summation of
EPSPs and IPSPs
• EPSPs and IPSPs are added
together in a process called
summation
• More EPSPs lead to greater
probability of an action
potential
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Nucleus
Neuron
cell body
Presynaptic
knob
Presynaptic
axon

34. Neurotransmitters
34

35. 35
Neuropeptides
• Neurons in the brain or spinal cord synthesize neuropeptides.
• These neuropeptides act as neurotransmitters.
• Examples include:
• Enkephalins
• Beta endorphin
• Substance P

36. 36
Neurotransmitters