Action Potentials

Sensory (Afferent) Neurons

From body to CNS (ex. photoreceptors, mechanoreceptors)

Interneurons

Vast majority of neurons in the CNS

Efferent (Motor) Neurons

From CNS to effectors (ex. corticospinal, PSNS)

Pesudounipolar Neuron

Found Chiefly in ganglia and often sensory

Bipolar Neurons

Found in retina

Anaxonic Neurons

CNS Neurons

Multipolar Neuron

Highly branched but lack long extensions (many dendrites and one axon that branches)

PNS Cells

Schwann Cells, Satellite cells

Schwann Cells

Wrap around axon and form insulating myelin sheath in PNS

Satellite Cells

Supportive capsule in a ganglion (plural ganglia), feed neurons, provide structural support.

CNS Cells

Oligodendrocytes, astrocytes, microglia, ependymal cells

Astrocytes

Take up and release chemical, feed neurons, water K+ balance, and part of bbb, provide structural support

Microglia

Provide immune defense

Ependymal Cells

Source of stem cells

Oligodendrocytes

Myelin sheath in CNS

Slow Transport

1-10 mm a day of materials like cytoskeletal proteins, microtubules, neurofilaments, and polypeptides

Fast Axoplasmic Transport

Up to 1000 mm/day, Amino acids and other materials caried in vesicels alone microtubule tracks

Kinesin

Move materials towards the terminal anterograde (toward axon terminal)

Dynein

Moves material retrograde (back to soma)

Hyperpolarized

More (-) than resting potential, which is caused by inhibitory stimulus

Depolarized

Less negative or more positive than resting potential (caused by excitatory stimulus)

Repolarized

Membrane potential returning to normal

Graded Potentials

Passive event (no energy involved), conducted and not propagated, declining in amplitude with distance and time. May be depolarizing (+) or hyperpolarizing (-). They vary in amplitude in proportion to the stimulus that causes them.

Excitatory Post-Synaptic Potential (EPSP)

Depolarizing stimulus is applied to a neuron

Inhibitory Post-Synaptic Potential (IPSP)

Hyperpolarizing stimulus applied to a neuron

IPSP & EPSP

Equal and simultaneous stimuli lead to little change in resting membrane potential.

Cause of Graded Potentials

Sensory cells generate potentials in response to an appropriate stimulus (light in retina, sound), also caused by NT binding ligand-gated ion channels.

Axon Hillock

If membrane depolarization reaches threshold, it will produce AP

Action Potential (AP)

Excitable cells (nerve, cardiac) serve as long-distance signals that require energy due to propagation. An all-or-nothing electrical signal that travels the length of an axon without loss of amplitude. Initiated by a sufficiently strong (+) graded potential so that the neuron membrane is brought to threshold. Always depol, consisting of rapid depol followed by repol of the membrane.

Graded Potential to AP

Only when it reaches the threshold level of depolarization is an AP generated

Summation of APs

Some inputs excite the neuron, bringing it close to threshold and an AP, while others will inhibit the neuron, thus decreasing its likelihood of firing an AP. This decides whether a neuron will fire an AP.

Depolarization of AP (rising phase)

Rapid (+) increase in membrane potential in mV

Overshoot

Membrane potential becomes (+) charged

Hyperpolarization or Undershoot

Become more negative than resting membrane potential

Positive Feedback Loop

Opening of voltage-gated Na+ channel, causing more depolarization, then more voltage gated-Na+ channels open, causing a wave of depolarization to move across neruons towards axon terminal.

Positive Feedback Termination

Na+ channels have an inactivation gate, which closes as the AP reaches the overshoot range. The Na+ channels will not open again until they have been reset by the membrane returning to the resting membrane potential. K+ voltage-gated channels open, and the influx of K+ causes the membrane to repolarize back to resting potential.

Axon Hillock

The trigger zone, an area of axon that is adjacent to the soma, is especially susceptible to depolarizing stimuli.

Ready for an AP State

Activation gate is closed, but inactivation gate is open

Na+ Channel in AP

Depolarization of membrane about threshold results in activation gated opening

Na+ Channel During Depolarization

Both activation and inactivation gates open

Repolarization Na+ Channel

Inactivation gate closes and the voltage-gated Na+ enter the inactivation state (activation gate open)

Voltage-Gated K+ Channels (Open)

Same depolarization that opened Na+ also open these channels, but they open slowly

Resting Membrane Potential Na+ Channel

Activation gate closed but inactivation gate open

Voltage Gated K+ Channels (Closed)

Repolarization to resting membrane potential. Causes themselves to close, an example of negative feedback.

Absolute Refractory Period

Na+ Channel inactivation gates staying closed - no amount of stimulation will trigger another AP.

Relative Refractory Period

Na+ Channel inactivation gates reopen, the membrane is still hyper-polarized (K+ channels slowly closing). Due to inactivation gates being open but the membrane is hyperpolarized, it requires a greater depolarization to reach threshold.

Conduction Velocity Increases

Large diameter (more ions available; lower internal resistance), and myelin insulation (prevents ion leakage and reduces the membrane capacitance)

If K Channels Decrease

AP becomes more slow to repolarize

If Concentration of Na in Cytoplasm Increases

Amplitude decreases and less driving force required

If K+ Channels Did Not Have Slow Kinetics

Equilibrium meaning may not reach threshold (depolarization is shorter and weaker)

Saltatory Conduction

Propagation of AP with myelin

Myelin

Crucial to support AP (electrical signal) propagation along the axon. Insulation prevents the leakage of electrical charge and reduces capacitance. Propagate faster.