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- Perikaryon: Cytoplasm surrounding the nucleus.
- Cytoskeleton: Neurofilaments and Neurotubules.
- Nissl Bodies: RER and free ribosomes = gray matter.
- Recieves information from other neurons.
- Carries information towards the cell body.
- Transmit graded potentials, NOT action potentials (usually).
- Axolemma, axoplasm
- Connects to soma at axon hillock.
- First part = initial segment, which generates actions potentials
Major branches of an axon.
Small branches at the end of an axon.
- Ends of the telodendria.
- a.k.a buotons, synaptic end bulbs, synaptic knobs.
- Stores neurotransmitters in synaptic vesicles.
- Release neurotransmitter in response to electrical activity.
Transmembrane Potential (basic definition)
- Electrochemical gradient.
- "Potential" = voltage difference across a membrane.
- Arises from the sum of all chemical and electrical forces acting across the cell membrane.
- Usually reported in millivolts (mV).
- Inside is negative, outside is positive.
Transmembrane Potential (determining factors?)
1. Ion Concentration Differences (ΔC)
- 2. Sodium-Potassium pump (maintains ΔC)
- 3. Membrane permeability diffrerences for ions
- Membrane Channel Types:
- A. Leak Channels
- B. Gated Channels
- 4. Fixed anions (non-diffusible ions; P≠0)
- Mostly negatively-charged proteins and phosphate.
Resting (Membrane) Potential
Voltage difference across the cell membrane for an unstimulated ("resting") cell.
(Review Slides 15-19 and Figure 12-9)
- Local changes in membrane potential due to chemical or physical changes in the membrane.
- DO NOT self-regenerate or spread over long distances.
- Self-regenerating changes in membrane potential due to chemical or physical changes in the membrane.
- Spread over long distances.
Equilibrium Potential (for a particular ion)
- The equilibrium potential is the membrane voltage at which electrical forces and the concentration difference forces acting on an ion are equal.
- No net diffusion of the ion occurs at this membrane potential.
Understanding this concept is REALLY REALLT important!!!!!
- K+=-90 mV
- Na+=+66 mV
- This results in NO NET movement across the membrane.
Equilibrium Potential for K+
- At rest, the membrane is at -70 mV.
- Due to the fact that there is a higher chemical concentration of potassium INSIDE than there is out, K+ will want to flow outside. ↑
- However, since there is a slight negative charge inside of the membrane, the electrical gradient will want to somewhat pull it back in. ⇣
- Forces will move K+ out until it reaches its concentration gradient of -90 mV.
Equilibrium Potential for Na+
- At rest, the membrane is at -70 mV.
- Due to the fact that there is a higher chemical concentration of sodium OUTSIDE than there is in, Na+ will want to flow in. The electrical gradient will also want to pull in Na+ due to the negative charge. ⇊
- Forces will move Na+ into the cell until it reaches equilibrium at +66 mV.
Changes in Transmembrane Potential
- Membrane at rest is "polarized".
- Ion flow can cause changes in potential.
- "Depolarized": Inside of the membrane becomes more positive.
- "Hyperpolarized": Inside of membrane becomes more negative.
Membrane Channel Types
- Ions cross the membrane through:
- 1. Leak Channels: Always open.
- 2. Gated Channels: Open or close.
- a. Voltage-gated channels.
- b. Chemically-gates (ligand-gated) channels.
- c. Mechanically-gated channels.
Leak (Passive) Channels
- Important for establishing resting potential.
- Ions "leak" down their electrochemical gradients (eg. K+ leak channels, Na+ leak channels)
- Size, charge, etc. determine which ions(s) can pass through a channel.
- Determine resting permeabilities for membrane (PK+ at rest, 50-100X greater than PNa+)
- A.K.A Active Channels (does not refer to ATP use)
- Can exist in 3 states:
- 1. Open (activated)
- 2. Closed and cannot be opened (inactivated)
- 3. Closed, but can be opened
Chemically-gated (Ligand-gated) Channels
- Open after binding a specific chemical (ligand)
- Most abundant on cell body, dendrites and motor end plate
- Binding of ACh changes shape of receptor.
- Channel becomes permeable to small ions like Na+ and K+.
- Channels open in response to changes in membrane potential - Threshold.
- Important in action potential conduction, neurotransmitter release from end bulbs.
- Examples: Voltage-gated K+, Na+, and Ca2+ Channels.
- Resting membrane is closed, but can open (-70 mV).
- Opens at -60 mV.
- Closed and inactivated; cannot be opened (-30 mV).
- Open or close in response to physical distortion.
- Examples: touch, pressure receptors.
- A change in membrane potential that decreases with distance.
- Caused by ions entering cell through channels.
- Local depolarization or hyperpolarization.
- Does not spread very far from site of stimulus (unlike action potential).
- Does not involved voltage-gated channels.
- Why don't graded potentials travel very far?
- Cytoplasm resists ion flow.
- The cell membrane is LEAKY TO IONS.
Graded Potentials (present channels).
Leak channels present, Chemically-gated Na+ channels are present, BUT NOT VOLTAGE-GATED CHANNELS.
Depolarization and Hyperpolarization
- Depolarization: Inside is more positive than at rest. (Example=NA+ enters the cell)
- Hyperpolarization: Inside more negative than at rest. (Example=K+ leaves the cell, Cl- enters the cell)
Actions Potentials (Introduction)
A sudden major change in membrane potential.
- An all-or-none phenomenon (EITHER IT HAPPENS, OR IT DOESN'T)
- Occurs when membrane reaches a specific membrane voltage called threshold.
- Does not degrade over long distance (unlike graded potentials).
- Depends upon the presence of voltage-gated Na+ and K+ channels.
- At threshold, voltage-gated Na+ channels open.
Action Potential Recording
Action Potentials and Muscle Cells
Motor End Plate: Muscle cell membrane at neuromuscular junction.
- Contains few voltage-gated Na+ channels.
- Does not generate action potentials (graded potentials ONLY)
- Local current flow spreads to adjacent sarcolemma where action potentials are produced.
- About 100 vesicles, each containing 100,000 ACh molecules, are released into synapse to produce muscle action potential.
Action Potential - Continuous Propagation
Action Potential - Saltatory Conduction
Cholinergic Synaptic Activity