AVS Neurophysiology Part 3

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UI AVS 271

Action Potential

Electrical impulse that initiates synaptic transmission
-Generated by movement of ions across membrane through selective and ion specific channels
-Onset of Action Potential temporarily changes membrane potential

Types of Ion Channels

1. Mechanically Gated
2. Chemically Activated (ligand-gated)
3. Voltage Activated

Neuron Resting Membrane Potential

-60 mV

5 Phases of Neuron Membrane Potential

1. Rising Phase (partial depolarization)
2. Peak Depolarization
3. Repolarization
4. Hyperpolarization
5. Resting State
-Each phase is explained by state of ion channels and movement of ions

Excitatory Post-Synaptic Potentials (EPSPs)

Small depolarizing potentials that result when neurotransmitters from the pre-synaptic neuron bind to receptors on the post-synaptic neuron and cause depolarization of the post-synaptic membrane

Inhibitory Port-Synaptic Potentials (IPSPs)

Small hyperpolarizing potentials that result when neurotransmitters from the pre-synaptic neuron bind to receptors on the post-synaptic neuron and cause hyperpolarization of the post-synaptic membrane
Ex: Opening of K+ channels allowing K+ to leave cell or Cl- channels opening and allowing Cl- to enter cell

The NET Effect

The sum of all EPSPs and IPSPs determines the net effect that the post-synaptic neuron will respond to

Steps of Voltage-Gated Na+ channels in Neurons

Responsible for Depolarization component of Action Potential. State depends on voltage and time.
1. Voltage-gated channel is closed when neuron is at rest
2. Channel switches to activated/open state when cell is stimulated/depolarized to threshold of about -45 mV
3. Rapid influx of Na+ following concentration gradient resulting in the "Rising Phase"
4. Second gate (the Inactivating Gate) closes after a certain amount of time and remains closed during the refractory period
5. Both gates return to resting state

Voltage-Gated K+ channels in Neurons

Opening and Closing of gate both voltage dependent
1. Single gate, opens when reaches threshold (occurs more slowly than Na+ channels)
2. K+ moves out of cell following concentration gradient results in the "Repolarization Phase"
3. No Inactivation gate, gate remains open until membrane is repolarized (in fact, hyperpolarized)

2 Types of Refractory Periods

1. Absolute Refractory Period
2. Relative Refractory Period

The Refractory Period usually plays a role in maintaining a single direction of action potential propagation along nerve cell dendrites and axon

Absolute Refractory Period

Period after initiation of an action potential during which the axon is unable to generate another action potential

Relative Refractory Period

Period of time after the Absolute Refractory Period during which the axon can be induced to initiate a second action potential but only if the membrane is depolarized to a greater degree than the normal threshold

Importance of Threshold in initiating Action Potential

-When the stimulation threshold is reached, the full action potential is generated
-If stimulation is below threshold level, no action potential is generated

Propagation of Action Potential

-Action Potential propagates along nerve without decreasing even over long distances or branching of nerve
-Rate of conduction depends on myelination and nerve fiber diameter (Larger fibers = faster conduction)
-Transmission ranges from 0.5 m/sec to 120 m/sec (268 mph)

Saltatory Conduction

The "Leaping" propagation of action potential from one Node of Ranvier to another along the neuron
-Only occurs in myelinated nerves
-Conserves energy because it is unnecessary to depolarize the entire axonal membrane
-Increases velocity of propagation without diminishing electropotential (signal strength)

2 Groups of Neurotransmitters

1. Low molecular weight transmitters (acetylcholine, dopamine, serotonin, glutamate, GABA, NE)
2. Large neuropeptide transmitters (oxytocin, vasoactive intestinal protein (VIP)

Low Molecular Weight Transmitters

-Synthesized within presynaptic terminals by enzymes that are produced in the soma and transported to the terminal region
-Stored in vesicles within presynaptic terminal buttons, released upon terminal depolarization

Neurotransmitter Release

1. Neurotransmitters stored in vesicles in terminal buttons of Presynaptic Neurons released into synaptic cleft by fusion of vesicles to synaptic face of terminal button (induced by influx of Ca++ after depolarization of terminal buttons)
2. Transmission is terminated when transmitters are removed from cleft by either re-uptake mechanisms into presynaptic button (and repackaged into vesicles) or enzymatic degradation (products of which are also transported into the presynaptic button)

Neural Signal Factors

1. Origin of initiating AP
2. Pre-synaptic neurotransmitter type(s)
3. Post-synaptic receptor type(s)
4. Net effect of all EPSPs and IPSPs
5. Type of neuronal circuit
6. Neuronal tract involved

Autonomic Nervous System (ANS)

Responsible for the involuntary control mechanisms of systemic regulation
2 Components:
-Parasympathetic
-Sympathetic

Sympathetic ANS

-Raises heartbeat rate and amplitude
-Dilates pupil of eye
-Constricts blood vessels
-Dilates bronchi and bronchioles
-Inhibits gastrointestinal motility

Parasympathetic ANS

-Lowers heartbeat rate and amplitude
-Contracts pupil of eye
-Dilates blood vessels
-Constricts bronchi and bronchioles
-Excites gastrointestinal motility

Reflex Arcs

The neural circuitry responsible for a specific reflex
-5 Arc Components:
1. Receptor organ
2. Afferent neuron
3. Interneuron in cord or brain
4. Efferent neuron
5. Effector neuron

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