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Action Potential & Synaptic Transmission

Dr. Subhash Meena

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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

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TYPICAL NEURON

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Neuron

  • Dendrite - conducts “signal” toward the cell body -- [input zone]
    • often short, numerous & highly branched
    • signal comes from sensory cell or neighbouring neuron

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  • Axon - usually a single fiber -- [conducting zone]
    • conducts signal away from cell body to another neuron or effector cell

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  • Axon Ending
    • a cluster of branches (100’s to 1000’s)
    • each with a bulblike synaptic knob
    • relays signal to next neuron / effector cell

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The Neural Impulse

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The Neural Impulse

  • Ions
    • Charged molecules
  • Resting Potential
    • When more negative ions are inside the neuron than outside
    • Charge is approximately -70mV
    • Neuron is not transmitting information

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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.

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  • When a neuron is not being stimulated, it maintains a resting potential

Ranges from –40 to –90 millivolts (mV)

Average about –70 mV

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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

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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

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The Neural Impulse

  • Polarization
    • When the electrical charge of a cell moves away from zero

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  • Depolarization
    • When the electrical charge of a cell moves toward zero

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The Neural Impulse

  • Action Potential
    • Sudden, massive change in charge in the neuron
    • Occurs when depolarization reaches the threshold of excitation
    • Ions flow across cell membrane

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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

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GENRATION OF ACTION POTENTIAL

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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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PROPAGATION OF ACTION POTENTIAL

  • Two ways to increase velocity of conduction
    • Axon has a large diameter
    • Axon is myelinated
      • Action potential is only produced at the nodes of Ranvier
      • Impulse jumps from node to node
      • Saltatory conduction

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SALTATORY CONDUCTION

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The Neural Impulse

  • Graded Potentials
    • Subthreshold depolarization
    • Many subthreshold depolarizations are added together to produce an action potential (a process known as summation)

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The Neural Impulse

  • All-or-None Law
    • A neuron either fires or it does not
    • When it does fire, it will always produce an impulse of the same strength
    • Intensity of a stimulus is coded by the frequency of action potentials

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The Neural Impulse

  • Absolute refractory period
    • Period immediately after an action potential when another action potential cannot occur
  • Relative refractory period
    • Period following absolute refractory period when a neuron will only respond to a stronger than normal impulse

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NEUROTRANSMISSION

  • Electrical [no synapse]
    • common in heart & digestive tract - maintains steady, rhythmic contraction
    • All cells in effector contain receptor proteins for neurotransmitters

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  • 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

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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.

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NEUROTRANSMISSION

  • Presynaptic neuron

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  • Vesicles

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  • [Calcium channels]

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  • Synaptic cleft

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  • Postsynaptic neuron

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  • Neurotransmitter receptor

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The Synapse

  • Synaptic space (synaptic cleft)
    • Tiny gap between neurons
  • Terminal button
    • Enlarged area at the end of an axon
  • The synapse
    • Composed of the terminal button of one neuron, the synaptic space, and the dendrites or cell body of the receiving neuron

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Transmission Between Neurons

  • Synaptic vesicles
    • Sacs in terminal button that release chemicals into synaptic space
  • Neurotransmitters
    • Chemicals released by synaptic vesicles
  • Receptor sites
    • Location on receptor neuron for specific neurotransmitter

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NEUROTRANSMISSION

  • Action potential

→ synaptic knob

→ opening of Ca+ channels

    • neurotransmitter vesicles fuse with membrane
    • release of neurotransmitter into synaptic cleft

Ca2+

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NEUROTRANSMISSION

  • Action potential
    • neurotransmitter vesicles fuse with membrane
    • release of neurotransmitter into synaptic cleft

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NEUROTRANSMISSION

  • Action potential
    • binding of neurotransmitter to protein receptor molecules on receiving neuron membrane
    • opening of sodium channels
    • triggering of new action potential

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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

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Some Well-Known Neurotransmitters

  • Acetylcholine (ACh)
    • Released at the neuromuscular junction
    • Plays an important role in arousal and attention
    • Loss of ACh producing cells is linked to Alzheimer’s Disease
  • Dopamine
    • Affects neurons associated with voluntary movement
    • Plays a role in learning, memory, and emotions
    • Loss of dopamine-producing cells causes symptoms of Parkinson’s Disease

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Some Well-Known Neurotransmitters

  • Serotonin
    • Found throughout the brain
    • Appears to sets an “emotional tone”
    • Low serotonin levels are implicated in depression
  • Endorphins
    • Reduce pain by inhibiting or “turning down” neurons that transmit pain information

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Neural Plasticity

  • The brain can be changed, both structurally and chemically, by experience
  • Rat studies show that an “enriched” environment leads to larger neurons with more connections
  • Has also been shown in humans
  • Recent research has uncovered evidence of neurogenesis, or the production of new brain cells, in human brains

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Thank You