Propagation of nerve impulses

Propagation of nerve impulses,
PPI

1. PROPAGATION OF NERVE IMPULSE SOMBIR SINGH PhD Research Scholar [email protected]

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

3. TYPICAL NEURON

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

5. Typical neuron

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

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13. GENRATION OF ACTION POTENTIAL

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

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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

17. SALTATORY CONDUCTION 17

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.

20. NEUROTRANSMISSION • Presynaptic neuron • Vesicles • [Calcium channels] • Synaptic cleft • Postsynaptic neuron • Neurotransmitter receptor

21. NEUROTRANSMISSION • Action potential → synaptic knob Ca 2+ → opening of Ca + channels → neurotransmitter vesicles fuse with membrane → release of neurotransmitter into synaptic cleft

22. NEUROTRANSMISSION • Action potential → neurotransmitter vesicles fuse with membrane → release of neurotransmitter into synaptic cleft

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. THANKS End of Presentation