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Physiology 2130 Lecture Notes - Resting Potential, Axon Hillock, Electrochemical Gradient

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Anita Woods

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Module 4: Nerve Cells
Excitable cells (nerve and muscle cells), use resting membrane potential to generate
electrochemical impulse called an action potential
- i.e. nerve cell (neuron)
- action potential is the ―language‖ of nervous system because this is how cells
communicate with one another
- action potentials needed for muscles to contract
Structure of a Nerve Cell
Picture of multipolar neuron
Several type of neurons
- Thin branching processes of cell body that receive incoming signals
- ↑ overall SA of neuron so it can communicate with other neurons
- # dendrites vary depending on where nerve cell is located in the
nervous system
Cell Body (Soma)
- Control centre of nerve cell
- Contains nucleus and all other organelles needed to direct cellular
- Projection of the cell body that carries the outgoing signal to the
target cell in the form of an action potential
- May or may not be myelinated
Myelin sheath
- Layered phospholipid membrane sheath wrapped tightly around the
- Insulated with a fatty acid material called myelin, produced by
special cells: Schwann cells
- Schwann cells: in PNS and oligodendrocytes in CNS
- Effect of this myelin is to insulate the axon so few ions can leak out
through the membrane
Virtually all cells in the body have
membrane potentials
Inside electrically negative
Outside electrically positive
Electrochemical gradients of the major
intercellular and extracellular ions establish
and maintain membrane potential

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- Insulator for the axon forcing the ionic changes that comprise the
action potential to take place in only small exposed regions of the
axon called the Nodes of Ranvier
- This jumping of action potential from node to node results in
significant increase in transmission down the length of the axon
Node of Ranvier
- Small exposed regions of the axon
- Jumping of action potential from node to node increases the speed
of transmission down the length of the axon
- Branching of the axon near its terminal end
- Serve to ↑ # of target cells with which the neuron can interact
Terminal Bouton
(axon terminal)
- Swelling at the end of an axon collateral
- Swelling contains mitochondria & membrane bound vesicles
contain various neurocrin molecules
- Chemical in this terminal facilitate the transmission of the signal
across the synapse to the target cell
Action Potential
Action potential: it is a rapid reversal of the resting membrane
1. Membrane potential rapidly increases from resting (-70mV) to +35mV
- called depolarization
2. Membrane potential rapidly returns to -70mv
- called repolarization
3. membrane briefly becomes more negative, -90mV
- called hyperpolarization
4. Membrane returns to resting membrane potential, -70mV

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Voltage-Gated Channels
Types of channels
1. Voltage gated sodium channel
2. Voltage gated potassium channel
Essential for generation of action potential
Channels are sensitive to changes membrane potential
- They open when inside of cell becomes more +
- i.e. -70mV to -60mV, depolarization
Voltage-Gated Sodium Channel
Na + Steps:
1. Depolarization of the membrane occurs (membrane potential becomes more positive/less
2. Activation gate opens immediately
Q: What causes these rapid changes
in membrane potential?
A: the movement of ions across the
membraneprincipally sodium ions
(Na+) and potassium ions (K+). These
ions are allowed to move across the
membrane through special channels
In neurons, these channels are
found on the axon
Only allows sodium through, when there
is a depolarization of the membrane
(inside +), both gates are on the
intracellular site
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