Physiology 2130 Lecture Notes - Lecture 4: Choline, Neuromuscular Junction, Sarcolemma
29 May 2018
School
Department
Course
Professor
Module 4 – Outcomes
Explain the difference between excitable and nonexcitable cells.
• Excitable cells are cells that can generate electrical signals
o Use the resting membrane potential to generate an electrochemical impulse called an action potential
o These excitable cells are the nerve cells and muscle cells
Draw and label the soma, axon, dendrites, axon hillock, myelin sheath, nodes of Ranvier, and terminal boutons of a typical
neuron.
• 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
Describe the "voltage-dependent" sodium and potassium channels.
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
find more resources at oneclass.com
find more resources at oneclass.com
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
Draw a diagram of an action potential and the permeability changes of sodium and potassium, and use it to describe the
ionic mechanisms of the action potential.
• 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
Define depolarization, repolarization, threshold, overshoot, and hyperpolarization.
• The membrane potential rapidly changes from resting (-70 mV) to roughly +35 mV
o If threshold is reached (-55 mV)
o This sudden change to a more positive value is called depolarization
• After the action potential the membrane potential rapidly returns to -70 mV – repolarization
• The membrane then briefly becomes more negative, reaching approx. -90 mV – hyperpolarization or overshoot
• After this the membrane returns to resting levels of -70 mV
Refractory Periods:
• Absolute refractory period: the period of time when, regardless of the strength
of the depolarization, the Na+ gates will not open to fire another AP
o This is caused by the inactivation of Na+ voltage-gated channels
• There is also a second refractory period referred to as the relative refractory
period: this is the period during the action potential where the membrane is
hyperpolarized
o This period is caused by the K+ voltage-gated channels tendency to open
and close slowly
▪ Because K+ can leave the cell even after it is repolarized to -70 mV
▪ During this period, it unlikely to fire another action potential because it would require a stronger
stimulus to reach threshold
How many ions move through the membrane during one action potential?
• Very few ions move through the membrane during the action potential
o Only about one millionth of the ions available participate in the AP
o Therefore, there is no appreciable change in the concentration gradients for the various ions after one AP
good
animation on
slide 11
find more resources at oneclass.com
find more resources at oneclass.com
