this pdf file enrolls the basic concept of nerve impulse

1. NERVE IMPULSE

2. CONTENTS
• Structure of a nerve cell
• Resting Potential
• Action Potential
• Formation of an action potential
• Propagation of Action
• Saltatory conduction
• Neurotransmission: Jumping the Synaptic Cleft

4. qDendrite - conducts “signal” toward the cell body -- [input zone]
– often short, numerous & highly branched
– signal comes from sensory cell or neighboring neuron
q Axon - usually a single fiber -- [conducting zone]
– conducts signal away from cell body to another neuron or effector cell
q Axon Ending
– a cluster of branches (100’s to 1000’s)
– each with a bulblike synaptic knob
– relays signal to next neuron / effector cell

5. 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

6. qDuring resting potential there is an ion displacement between the
inside and the outside of the neuron (i.e. on either side of the neuron
cell membrane) as follows:
– There are more Na+ ions on the outside than on the inside
– There are more K+ ions on the inside than on the outside
– There are many large organic anions (-ve charged ions) locked inside
since they are too big to pass through the neuron’s cell membrane

7. Na+/K+ PUMP
• Powered by ATP this pump actively “ pumps” Na+ ions out of the cell
and K+ ions into the cell
• Other two leaky channels: K+ channels >>> Na+ channels.
• more K+ ions leak out of the cell as opposed to Na+ leaking into the
cell.
• As a result, a negative charge on the inside of the membrane.
• The neuron, of course, is not actually "resting" because it must
produce a constant supply of ATP to fuel active transport.

10. Action potential
• The signal that conveys information over distances in the nervous
system—the action potential.
• the inside of the membrane becomes positively charged with respect
to the outside. ( Wave of depolarization and repolarization)
• also often called a spike, a nerve impulse, or a discharge.
• In a cell,
• all similar in size and duration,
• and they do not diminish as they are conducted down the axon.
• so rapidly—100 times faster than the blink of an eye—a special type
of voltmeter, called an oscilloscope

11. UPS AND DOWN
• Finally, there is a gradual restoration of the resting potential.
• From beginning to end, the action potential lasts about 2 milliseconds (msec).

12. • the thumbtack enters the skin,
• the membrane of the nerve fibers in the skin is stretched,
• Mechano Na-permeable channels open (membrane Stretching)
• Reaches to threshold potential. (–55 mV)
• The critical level of depolarization that must be crossed in order to
trigger an action potential is called threshold.
• Action potentials are caused by depolarization of the membrane
beyond threshold.

13. qVoltage-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
q• Voltage-gated K+ channels
– Single activation gate that is closed in the resting state
– K+ channel opens slowly
– Efflux of K+ repolarizes the membrane

14. ALL
or
none
law

15. • 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

17. Recovery/Refractory Potential
• Immediately following an action potential, a neuron is unable to
conduct a nerve impulse until it has recovered because its Na gates
won’t open
• is impossible to initiate another for about 1 msec. This period of time
is called the absolute refractory period.
• relative refractory period relatively difficult to initiate another action
potential for several milliseconds after the end of the absolute refractory
period.
• The membrane potential is returning to its resting value of -70mV

18. PROPAGATION
• Thus the nerve
impulse is the
movement of the
action potential along
the neuron cell
membrane

19. Two ways to increase velocity of conduction
qAxon has a large diameter
• Less resistance to current flow
• Found primarily in invertebrates
qAxon is myelinated
• Action potential is only produced at the nodes of Ranvier
• Impulse jumps from node to node
• Saltatory conduction

21. 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