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Lecture 4
Nerve Cells, Action Potentials, Synapses
Undergraduate 3

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Regions of a Nerve Cell/Cluster
  • Soma: cell body, contains nucleus and most organelles
  • Dendrites: reception of incoming information
  • Axon: transmits action potentials
  • Axon Hillock: axon originates here, and action potentials are initiated here.
  • Synapse: site of communication between two neurons or between neuron and effector organ.
Ion Channels in Neurons
  • Leak Channels: found throughout a neuron; always open; important in resting membrane potential.
  • Ligand-Gated Channels: open/close in response to ligand binding; found in dendrites and soma; involved in synaptic potentials.
  • Voltage-Gated Channels: open/close in response to changes in the membrane potential.
    • Sodium/Potassium Channels: found throughout, but more in the axon (especially the hillock), and involved in action potentials.
    • Calcium Channels: mainly in the axon terminal; involved in the release of neurotransmitters.
Equilibrium Potentials
  • RMP of a cell is -70 mV.
  • More negative charges inside, more positive charges outside.
  • Membrane Potential is always given in terms of voltage inside the cell relative to outside.
  • 2 Factors Critical to Resting Membrane Potential
    • Ion Concentration Gradients
    • Membrane Permeability to these Ions (Ion Channels)
  • Equilibrium Potential is when electrical force is equal but opposite to the chemical force, resulting in a net force of zero.
  • Equilibrium Potential of K is -94 mV.
  • Equilibrium Potential of Na is +60 mV.
Na+/K+ ATPase Pump
  • 20% of RMP is due directly to the pump's action.
    • 3 Na out for every 2 K brought in.
    • Net +1 charge out.
  • 80% of RMP is due indirectly to the pump's action, since it produces and maintains concentration gradients.
  • [Na] is high outside, low inside.
  • [K] is low outside, high inside.
K+ Equilibrium Potential
  • -94 mV
  • Cell is more 25 times more permeable to K+ than Na+
  • Since [K+] is low outside and high inside, chemical driving force is out of cell.
  • As it diffuses out, inside of cell becomes more negative, and the electrical driving force "pulls" [K+] back in.
NaEquilibrium Potential
  • +60 mV
  • Chemical driving force is in, so it diffuses in.
  • This creates creates a more positive inside, and the electrical driving force "pushes" it out.
The Resting Membrane Potential of Neurons
  1. Chemical Driving Forces: K+ out, Na+ in.
  2. More K leaves the cell than Na enters, so the inside becomes negative.
  3. Electrical Forces Develop, drawing Na and K into the cell: K outflow slows, Na inflow speeds.
  4. Eventually the membrane potential stabilizes at -70 mV.
  5. There's some Na and K leak at rest, but the sodium pump maintains the gradient at -70 mV.
    1. Membrane Potential is not equal to Equilbrium Potential for Either Ion.
    2. There's a strong net ECF into the cell for Na, but the membrane is not permeable to Na.
    3. There's a weak net ECF out of the cell for K, and the membrane is highly permeable to K.
Mechanically Gated Channel
  • makes the cell change its membrane potential, but not generate an action potential.
  • Response to pressure/light/pain
  • Exists in sensory receptors.
  • As they are stretched they can further open and close.
Graded Potentials
  • small change in membrane potential; communicate short distance
  • initiated by a stimulus
  • magnitude varies (graded)
  • some are depolarizing, some are hyperpolarizing.
  • these determine whether an action potential will occur or not (threshold=level of depolarization necessary to elicit an action potential; all-or-none).
  • temporal: same stimulus repeated close together in time
  • spatial: different stimuli that overlap in time


Action Potentials
  • rapid, large depolarization used for communication.
  • In neurons, action potentials travel along axons from the soma to the axon terminal (or, if an afferent neuron, from receptor to terminal).
  • Excitable membranes have the ability to generate action potentials.
Phases of an Action Potential
  1. Depolarization (-70 to +30 mV): Permeability of Na goes way up (Na channels open)
  2. Repolarization (+30 to -70 mV): Permeability of Na decreases, Permeability of K increases
  3. After-Hyperpolarization (-70 to -85 mV): Permeability of Na is low, Permeability of K peaks then declines.
Na and K Gating During an Action Potential
  • When threshold is reached, a rapid opening of Na channels happens, followed by a slow closing of Na channels and a slow opening of K channels.
  • Voltage-Gated Na Channel
    • 2 Gates: Activation and Inactivation
    • Activation: voltage dependent, positive feedback.Open during threshold and depolarization.
    • Inactivation: voltage and time dependent, open during resting membrane potential and depolarization.
  • Voltage-Gated K Channel: one gate, voltage and time dependent, and operates on negative feedback.
Refractory Period
  • the period of time following an action potential marked by decreased excitability.
  • Can be absolute or relative.
  • Absolute:
    • Spans all of depolarization and most of the repolarization.
    • Second action potential cannot be generated.
    • Na gates are inactivated.
  • Relative:
    • Spans the last part of repolarization and hyperpolarization.
    • Second action potential can be generated, but you have to have a stronger stimulus.
    • Some Na gates are closed, some are inactivated.
Frequency Coding
the coding of stimulus intensity by the frequency of action potentials in a neuron, in which a stronger depolarizing stimulus above threshold causes the action potential frequency to increase.
Relationship between Axon Diameter and Length of Refractory Period

the larger the diameter, the shorter the refractory period (allowing for more rapid propagation of the action potential).


the smaller the diameter, the longer the refractory period (causing a slower propagation of the action potential).

Myelin Forming Cells
  • Oligodendrocytes (CNS): one oligodendrocyte forms several myelin sheaths, myelinating sections of several axons.
  • Schwann Cells (PNS): one Schwann cell=one myelin sheath (myelinates one section of the axon).
  • Saltatory Conduction: very fast; type of action potential conduction that occurs in myelinated axons, where action potentials "jump" from node to node.
  • Presynaptic Neuron=axodendritic; Postsynaptic Neuron=Axosomatic
    • excite or inhibit the postsynaptic neuron, nonselectively.
  • Syanptic Cleft=axoaxonic
    • excite or inhibit one synapse, selectively. 
    • They are modulatory synapses that can either presynaptically facilitate or inhibit.
  • Electrical Synapses
    • two neurons linked together by gap junctions
    • functions in the nervous system only.
    • rapid, bidirectional commmunication.
    • excitation and inhibition at the same synapse.
  • Chemical Synapses
    • most prevalent
    • neurons and effector organs
    • neurotransmitters
    • excitation and inhibition
Steps of Communication Across a Synapse
  1. Action Potential
  2. Voltage-Gated Calcium Ion Channels Open
    1. Synaptic Delay=0.5-5 msec from when the AP arrives and the Vm changes.
  3. The Calcium triggers exocytosis
  4. The neurotransmitter diffuses and binds to the receptor.
  5. There is a response in the cell.
    1. Can be EPSP (excitatory postsynaptic potential) that brings the membrane potential closer to threshold and then depolarization.
    2. Can be IPSP (inhibitory).
  6. The response is terminated by removing the neurotransmitter from the synaptic cleft.
  7. Then it can be degraded, reuptaken, or diffuse away.
Postsynaptic Membrane Potential Stabilization
  • Stabilization of membrane potential against changes via "inhibitory" Chloride Ion Channels.
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