Nervous system part 2

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