Physiology 2130 Lecture Notes - Lecture 4: Resting Potential, Axon Terminal, Membrane Potential
24 May 2018
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Module 4 – Nerve Cells
Intro:
• Nerve and muscle cells are the excitable cells in our body – that is, cells that can generate electrical signals
o Use the resting membrane potential to generate an electrochemical impulse called an action potential
• These excitable cells are the nerve cells and muscle cells, and their functions are frequently linked
• Electrical signals are one type of communication that the body uses, and this type of message is sent incredibly fast
• The action potential is considered to be the laguage of the ervous system because this is the way nerve cells
communicate with each other
• Action potentials are also necessary for muscle contraction
Structure of a Nerve:
• Dendrites
o This branching process of the cell body
o Function is to receive incoming signals
o Increase the overall SA of the neuron so that it can communicate with many neurons
o Number of dendrites varies depending on where in the NS the cell is located
• Cell body (soma)
o The control center of the nerve cell
o Contains the nucleus and all necessary organelles for directing cellular activity
• Axon
o Projection of the cell body which carries the outgoing signal to the target cell in the form of an AP
• Myelin sheath
o Layered phospholipid membrane sheath wrapped tightly around the axon
o Acts as an insulator for the axon and forces action potentials to only be released at the nodes of Ranvier
• Nodes of Ranvier
o Small uncovered areas of the axon
o Where action potentials are released
• Collaterals
o Branches of the axon near its terminal end
o Serve to increase the number of possible target cells with which the neuron can interact
• Terminal Bouton or axon terminal
o Swelling at the end of an axon collateral
o Contains mitochondria and membrane bound vesicles containing various neurocrine molecules
o These chemicals facilitate the transmission of the signal across the synapse to the target cell
The Action Potential:
• The action potential is a rapid reversal of the resting membrane
• Phases:
o The membrane potential rapidly changes from resting (-70 mV) to roughly +35 mV
▪ This sudden change to a more positive value is called depolarization
o After the action potential the membrane potential rapidly returns to -70 mV – repolarization
o The membrane then briefly becomes more negative, reaching approx. -90 mV – hyperpolarization
o After this the membrane returns to resting levels of -70 mV
• What causes this rapid change in membrane potential?
o The movement of ions across the membrane
o Specifically, sodium ions (Na+) and potassium ions (K+)
o These ions move across the membrane through specific
channels
find more resources at oneclass.com
find more resources at oneclass.com
Voltage Gated Channels:
• Two specific types of channels found in nerve and muscle cells
o Voltage-gated sodium channels
o Voltage gated potassium channels
• The channels are generally found in the axon and are essential for the generation of an
action potential (AP)
• These channels are sensitive to changes in the membrane potential and open when the
inside of the cell becomes more positive – depolarization
o Membrane changes from -70 mV to -60 mV
Voltage Gated Sodium Channels:
• This channel is specific to sodium and will allow no other molecule through
• This channel only opens when there is a depolarization of the membrane
(inside becomes more +ve)
• Involves two gates: activation and inactivation
• Summary of process
o Depolarization of the membrane occurs – membrane potential
becomes more +ve
o Activation gate opens immediately allowing Na+ into the cell
o Na+ flow into the cell, down the concentration gradient
o Inactivation gate closes and Na+ can no longer flow into the cell; the
channel cannot open
▪ This occurs after about a tenth of a millisecond after
opening
o Channel returns to resting configuration – Inactivation gate open
and activation gate closed
o Channel is now ready to receive another depolarization and open again
• Inactivation of Na+ voltage-gated channel leads to the absolute refractory period (#3 in figure)
o During the period when the inactivation gate is closed, the channel will not open, regardless of the strength
of the stimulation
Voltage Gated Potassium Channels:
• Involves one gate – which opens when the membrane depolarizes
• This gate does’t ope iediatel like the Na+ gate) instead it begins to open when the Na+
gate starts to become inactivated
• This difference between the two gates is essential in the generation of an action potential
• Summary of process:
o Depolarization of the membrane occurs – membrane potential becomes more +ve
o After a brief pause, K+ voltage gated channels open
o K+ ions flows out of the cell, down their electrical and chemical gradients
o Gate closes, and channel returns to resting configuration
o Channel is now ready to receive another depolarization and open again
• Unlike the Na+ channels, these channels do not have an inactivation period
The Action Potential Events:
• Strong depolarization at the axon hillock (or initial segment, this is the most electrically sensitive area of the nerve)
triggers the opening of most Na+ voltage gated channels
• Na+ rushes into the neuron, down its electrochemical gradient
• Membrane depolarizes rapidly to ~+35 mV
• Na+ channels become inactive, while K+ channels begin opening
• K+ rushes out of the cell, down its electrochemical gradient
• Membrane begins repolarizing back to normal (+35 mV → -70 mV)
• K+ continues to rush out of the cell and membrane hyperpolarizes (-90 mV)
• K+ channels begin to close and K+ no longer leaves the cell
• Membrane potential slowly returns to resting value of -70 mV
good
animation
on slide 11
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Intro: nerve and muscle cells are the excitable cells in our body that is, cells that can generate electrical signals, use the resting membrane potential to generate an electrochemical impulse called an action potential. These excitable cells are the nerve cells and muscle cells, and their functions are frequently linked: electrical signals are one type of communication that the body uses, and this type of message is sent incredibly fast. The action potential is considered to be the (cid:862)la(cid:374)guage(cid:863) of the (cid:374)ervous system because this is the way nerve cells communicate with each other: action potentials are also necessary for muscle contraction. Structure of a nerve: dendrites, this branching process of the cell body, function is to receive incoming signals. Terminal bouton or axon terminal: swelling at the end of an axon collateral, contains mitochondria and membrane bound vesicles containing various neurocrine molecules, these chemicals facilitate the transmission of the signal across the synapse to the target cell.
