5. Membrane potential
• A potential difference exists across all cell membranes
– This is referred to as “Resting Membrane Potential” (RMP)
– Inside is negative with respect to the outside
6. What are excitable tissues?
• They are capable of generating electrochemical
impulses and transmitting them along the
membrane
8. Resting Membrane Potential
• This depends on following factors
– Ionic distribution across the membrane
– Membrane permeability
– Other factors
• Na+/K+ pump
9. Factors contributing to RMP
• One of the main factors is K+ efflux (Nernst Potential:
-94mV)
• Contribution of Na influx is little (Nernst Potential:
+61mV)
• Na/K pump causes more negativity inside the
membrane
• Negatively charged protein remaining inside due to
impermeability contributes to the negativity
• Net result: -70 mV inside
11. Na/K pump
2 K+
ATP
3 Na+
ADP
• Active transport system for Na-K exchange using
energy
• It is an electrogenic pump since 3 Na influx coupled
with 2 K efflux
• Net effect of causing negative charge inside the
membrane
12. Action Potential (A.P.)
• When an impulse is generated
– Inside becomes positive
– Causes depolarisation
– Nerve impulses are transmitted as AP
14. Physiological basis of AP
• When the threshold level is reached
–
–
–
–
–
–
–
–
Voltage-gated Na channels open up
Since Na conc outside is more than the inside
Na influx will occur
Positive ion coming inside increases the positivity of the
membrane potential and causes depolarisation
When it reaches +35, Na channels closes
Then Voltage-gated K channels open up
K efflux occurs
Positive ion leaving the inside causes more negativity inside
the membrane
– Repolarisation occurs
15. Physiological basis of AP
• Since Na has come in and K has gone out
• Membrane has become negative
• But ionic distribution has become unequal
• Na/K pump restores Na and K conc slowly
– By pumping 3 Na ions outward and 2 K ions inward
16. m gate
-70
Na+
Threshold level
Na+
+35
Na+
outside
outside
outside
inside
inside
inside
h gate
• At rest: the activation gate is closed
• At threshold level: activation gate opens
– Na influx will occur
– Na permeability increases to 500 fold
• when reaching +35, inactivation gate closes
– Na influx stops
• Inactivation gate will not reopen until resting
membrane potential is reached
17. -70
At +35
n gate
outside
outside
inside
inside
K+
K+
– At rest: K channel is closed
– At +35
• K channel open up slowly
• This slow activation causes K efflux
– After reaching the resting still slow K channels may
remain open: causing further hyperpolarisation
18. Propagation of AP
• When one area is depolarised
• A potential difference exists between that site
and the adjacent membrane
• A local current flow is initiated
• Local circuit is completed by extra cellular fluid
19. Propagation of AP
• This local current flow will cause opening of
voltage-gated Na channel in the adjacent
membrane
• Na influx will occur
• Membrane is deloparised
20. Propagation of AP
• Then the previous area become repolarised
• This process continue to work
• Resulting in propagation of AP
25. AP propagation along myelinated
nerves
• Na channels are conc around nodes
• Therefore depolarisation mainly occurs at
nodes
26. AP propagation along myelinated
nerves
• Local current will flow one node to another
• Thus propagation of A.P. is faster. Conduction
through myelinated fibres also faster.
• Known as Saltatory Conduction
42. Synapse
• A gap between two neurons
• More commonly chemical
• Rarely they could be electrical (with gap
junctions)
43. ♦ Although an axon conducts bothways,
conduction through synapse is oneway
♦ presynaptic neuron to postsynaptic neuron
♦ A neurone receives more than 10000
synapses. Postsynaptic activity is an
integrated function
44. Presynaptic terminal (terminal knob,
boutons, end-feet or synaptic knobs)
♦ Terminal has synaptic vesicles and mitochondria
♦ Mitochondria (ATP) are present inside the presynaptic
terminal
Vesicles containing neurotransmitter (Ach)
45. Presynaptic terminal (terminal knob,
boutons, end-feet or synaptic knobs)
♦ Presynaptic membrane contain voltage-gated Ca
channels
♦ The quantity of neurotransmitter released is
proportional to the number of Ca entering the terminal
♦ Ca ions binds to the protein molecules on the inner
surface of the synaptic membrane called release sites
♦ Neurotransmitter binds to these sites and exocytosis
occur
47. • Postsynaptic membrane contain receptors for
the neurotransmitter released
• eg: Acetylcholine receptor
Na+
Ach
•This receptor is Ach-gated
Na+ channel
•When Ach binds to this,
Na+ channel opens up
•Na+ influx occurs
48. • Na+ influx causes depolarisation of the
membrane
– End Plate Potential (EPP)
• This is a graded potential
• Once this reaches the threshold level
• AP is generated at the postsynaptic membrane
49. Ach release
• An average human end plate contains 15-40 million
Ach receptors
• Each nerve impulse release 60 Ach vesicles
• Each vesicle contains about 10,000 molecules of Ach
• Ach is released in quanta (small packets)
• Even at rest small quanta are released
• Which creates a minute depolarising spike called
Miniature End Plate Potential (MEPP)
• When an impulse arrives at the NMJ quanta released
are increased in several times causing EPP
50. Ach vesicle docking
• With the help of Ca entering the presynaptic terminal
• Docking of Ach vesicles occur
• Docking:
– Vesicles move toward & interact with membrane of
presynaptic terminal
• There are many proteins necessary for this purpose
• These are called SNARE proteins
• eg. Syntaxin, synaptobrevin etc
51. Axoplasmic transport
• The process by which mitochondria, synaptic vesicles
and other cytoplasmic constituents travel to and from
the cell body
• axon contains 100 times the volume of the cell body
• proteins for neurotransmitters and membrane repair
are constantly transported anterogradely
– to maintain normal axonal function
• If the axon is deprived of proteins because it is
severed or crushed
– the segment that is distal to the injury cannot support itself
and will degenerate
52. NMJ blocking
• Useful in general anaesthesia to facilitate
inserting tubes
• Muscle paralysis is useful in performing surgery
53. Earliest known NMJ blocker - Curare
•
Curare has long been used in South America
as an extremely potent arrow poison
•
Darts were tipped with curare and then
accurately fired through blowguns made of
bamboo
•
Death for birds would take one to two minutes,
small mammals up to ten minutes, and large
mammals up to 20 minutes
•
NMJ blocker used in patients is tubocurarine
•
Atracurium is now used
54. Neuromuscular blocking agents
• Non-depolarising type (competitive)
–
–
–
–
–
–
–
–
Act by competing with Ach for the Ach receptors
Binds to Ach receptors and blocks
Prevent Ach from attaching to its receptors
No depolarisation
Prolonged action (30 min)
Ach can compete & the effect overcomes by an excess Ach
Anticholinesterases can reverse the action
eg.
•
•
•
•
Curare
Tubocurarine
Gallamine
Atracurium
55. Neuromuscular blocking agents
•
Depolarising type (non-competitive)
– Act like Ach, but resistant to AchE action
– Bind to motor end plate and once depolarises
– Persistent depolarisation leads to a block
• Due to inactivation of Na channels
– Two phases
• Phase I – depolarisation phase – fasciculations
• Phase II – paralysis phase
–
–
–
–
Ach cannot compete
Quick action (30 sec), short duration (10 min)
Anticholinesterases cannot reverse the action
eg.
• suxamethonium
• Ach in large doses
• nicotine
58. PHASE I
Membrane depolarizes
resulting in an initial
discharge which
produces transient
fasciculations followed
by flaccid paralysis
+
- + - + -+ -
Na+
+
- +-
+
+
-
Depolarized
+
- - +
+ + -
- - - - - - -+
+ + + -
Na+
Depolarizing Neuromuscular blocking drugs
59. PHASE II
Membrane repolarizes
but the receptor is
desensitized to effect
of acetylcholine
-
-
+
+
+
-
+
-
+
-
-
+
+
-
+
-
Repolarized
+
- - + + +
+
- +
+ -
+
-
Depolarizing Neuromuscular blocking drugs
60. Neuromuscular blocking agents
• AchE inhibitors
– Inhibit AchE so that Ach accumulates and causes
depolarising block
• Reversible
– Competitive inhibitors of AChE
– Block can be overcome by curare
• physostigmine, neostigmine, edrophonium
• Irreversible
– Binds to AChE irreversibly
• , insecticides, nerve gases
61. NMJ disorders
• Myasthenia gravis
– Antibodies to Ach receptors
– Post synaptic disorder
• Lambert Eaton Syndrome (myasthenic syndrome)
– Presynaptic disorder (antibodies against Ca channels)
• Botulism
– Presynaptic disorder
– Binds to the presynatic region and prevent release of Ach
63. Botulinum toxin
• Most potent neurotoxin known
• Produced by bacterium Clostridium botulinum
• Causes severe diarrhoeal disease called botulism
• Action:
– enters into the presynaptic terminal
– cleaves proteins (syntaxin, synaptobrevin) necessary for Ach vesicle
release with Ca2+
• Chemical extract is useful for reducing muscle spasms, muscle
spasticity and even removing wrinkles (in plastic surgery)
65. Organophosphates
• Phosphates used as insecticides
• Action
– AchE inhibitors
– Therefore there is an excess Ach
accumulation
– Depolarising type of postsynaptic
block
• Used as a suicidal poison
• Causes muscle paralysis and death
• Nerve gas (sarin)
66. Snake venom
• Common Krait (bungarus
caeruleus)
– Produces neurotoxin known as
bungarotoxin
– Very potent
• Causes muscle paralysis and death
if not treated
• Cobra
– venom contain neurotoxin
67. Myasthenia gravis
•
Serious neuromuscular disease
•
Antibodies form against acetylcholine nicotinic
postsynaptic receptors at the NMJ
•
Characteristic pattern of progressively reduced
muscle strength with repeated use of the muscle
and recovery of muscle strength following a period
of rest
•
Present with ptosis, fatiguability, speech difficulty,
respiratory difficulty
•
Treated with cholinesterase inhibitors
72. • AP spreads through t tubule into the muscle
tissue
• Close to the sarcoplasmic reticulum, AP
activates a receptor (votage-gated Ca++
channel)
• Ca flows to the myoplasm in the vicinity of actin
& myosin
73. • Ca++ binds to troponin
• Troponin shifts tropomyosin
• Myosin binding sites in actin filament
uncovered
• Myosin head binds with actin
• Cross bridges form
• Filaments slide with ATP being broken down
• Muscle shortens
• New ATP occupies myosin head
• Myosin head detaches
• Filaments slide back
• Cycling continues as long as Ca is available
Actin
Troponin
Tropomyosin
Myosin
ATP
75. Ca++ binds to troponin
Tropomyosin exposes actin
Ca++
Myosin
ATP
Actin
Troponin
myosin head binds to actin
& cross bridge forms
Filaments slide
ATP is broken down
New ATP comes, Ca is removed, ready to detach
79. • Relaxation
– This occurs when Ca++ is removed from myoplasm
by Ca++ pump located in the sarcoplasmic
reticulum
– When Ca++ conc is decreased
– Troponin returns to original state
– Trpomyosin covers myosin binding sites
– Cross-bridge cycling stops
80. slow & fast fibres
• Slow twitch fibre (type I fibre)
• Fast twitch fibre (type II fibre)
81. Slow twitch fibre (type I fibre)
– Slow cross-bridge cycling
– slow rate of shortening (eg. soleus muscle in calf)
– high resistance to fatigue
– high myoglobin content
– high capillary density
– many mitochondria
– low glycolytic enzyme content
– They are red muscle fibres
82. Fast twitch fibre (type II fibre)
– rapid cross-bridge cycling,
– rapid rate of shortening (eg. extra-ocular
muscles)
– low resistance to fatigue
– low myoglobin content
– low capillary density
– few mitochondria
– high glycolytic enzyme content
– fast twitch fibers use anaerobic metabolism
to create fuel, they are much better at
generating short bursts of strength or speed
than slow muscles