2. CONTENTS
1.Structure of a nerve cell
2. Resting Potential
3. Action Potential
(a) Formation of an action
potential
4. Propagation of Action
Potentials as an Impulse
(b) Saltatory conduction
5. Neurotransmission: Jumping
the Synaptic Cleft
4. Neuron
• Dendrite - conducts “signal” toward the cell body -- [input zone]
– often short, numerous & highly branched
– signal comes from sensory cell or neighboring neuron
• Axon - usually a single fiber -- [conducting zone]
– conducts signal away from cell body to another neuron or effector cell
• Axon Ending
– a cluster of branches (100’s to 1000’s)
– each with a bulblike synaptic knob
– relays signal to next neuron / effector cell
6. RESTING POTENTIAL
Resting potential may be defined as the
difference in voltage between the inside and
outside of the cell as measured across the
cell membrane.
• When a neuron is not being stimulated, it
maintains a resting potential
Ranges from –40 to –90 millivolts (mV)
Average about –70 mV
7. RESTING POTENTIAL
• Two major forces act on ions in establishing
the resting membrane potential
1. Electrical potential produced by unequal
distribution of charges
2. Concentration gradient produced by
unequal concentrations of molecules
from one side of the membrane to the
other
8. RESTING POTENTIAL
• Sodium–potassium pump creates significant
concentration gradient
• Concentration of K+
is much higher inside the
cell
• Membrane not permeable to negative ions
• Leads to buildup of positive charges outside
and negative charges inside cell
• Attractive force to bring K+
back inside cell
• Equilibrium potential – balance between
diffusional force and electrical force
8
9. ACTION POTENTIAL
• Action potential may be defined as the entire series of
changes which contribute towards the changes in
membrane potential.
Action
potentials:-
– Result when depolarization reaches the threshold
potential (–55 mV)
– Depolarizations bring a neuron closer to the
threshold
– Hyperpolarizations move the neuron further from
the threshold
– Caused by voltage-gated ion channels
• Voltage-gated Na+
channels
• Voltage-gated K+
channels
10. ACTION POTENTIAL
• Voltage-gated Na+
channels
– Activation gate and inactivation gate
– At rest, activation gate closed, inactivation gate
open
– Transient influx of Na+
causes the membrane to
depolarize
• Voltage-gated K+
channels
– Single activation gate that is closed in the resting
state
– K+
channel opens slowly
– Efflux of K+
repolarizes the membrane
11. ACTION POTENTIAL
• The action potential has three phases
– Rising, falling, and undershoot
• Action potentials are always separate, all-or-
none events with the same amplitude
• Do not add up or interfere with each other
• Intensity of a stimulus is coded by the
frequency, not amplitude, of action potentials
11
14. PROPAGATION OF ACTION
POTENTIAL
• Propagation of action potentials
– Each action potential, in its rising phase,
reflects a reversal in membrane polarity
– Positive charges due to influx of Na+
can
depolarize the adjacent region to threshold
– And so the next region produces its own
action potential
– Meanwhile, the previous region repolarizes
back to the resting membrane potential
• Signal does not go back toward cell
body
16. PROPAGATION OF ACTION
POTENTIAL
• Two ways to increase velocity of conduction
–Axon has a large diameter
• Less resistance to current flow
• Found primarily in invertebrates
–Axon is myelinated
• Action potential is only produced at the
nodes of Ranvier
• Impulse jumps from node to node
• Saltatory conduction
16
18. NEUROTRANSMISSION
• Electrical [no synapse]
– common in heart & digestive tract - maintains steady,
rhythmic contraction
– All cells in effector contain receptor proteins for
neurotransmitters
• Chemical - skeletal muscles & CNS
– presence of gap (SYNAPTIC CLEFT) which prevents action
potential from moving directly to receiving neuron
– ACTION POTENTIAL (electrical) converted to CHEMICAL
SIGNAL at synapse (molecules of neurotransmitter) then
generate ACTION POTENTIAL (electrical) in receiving
neuron
19. Overview of Transmission of Nerve Impulse
• Action potential
→ synaptic knob
→ opening of Ca+
channels
→neurotransmitter vesicles fuse with
membrane
→release of neurotransmitter into synaptic cleft
→binding of neurotransmitter to protein receptor
molecules on receiving neuron membrane
→opening of ion channels
→triggering of new action potential.
23. NEUROTRANSMISSION
• Action potential
→binding of
neurotransmitter to
protein receptor
molecules on receiving
neuron membrane
→opening of sodium
channels
→triggering of new
action potential
24. NEUROTRANSMITTERS
• Amino acid derived Neurotransmitters
– Derived from amino acid tyrosine
norepinephrine, epinephrine
• Amine Neurotransmitters
– acetylcholine, histamine, serotonin
• Amino Acids
– aspartic acid, GABA, glutamic acid, glycine
• Polypeptides
– Include many which also function as hormones
– endorphins
25. References
• Cell and Molecular Biology by Gerald Karp
• Cell and Molecular Biology,8th
ed.E.D.P.
Robertis and E.M.F. De Robertis
• Net source:- www.freeman.karp.in
• For any query contact-9896543665
