Action Potentials, Axon Conduction, and Neuromuscular Junction

  >   Rahul's Noteblog   >   Notes on Neurology   >   Action Potentials, Axon Conduction, and Neuromuscular Junction

Graded Potentials

• Caused when a stimulus causes a ligand-gated or mechanically-gated channel to open or close.

• More Polarized (hyperpolarized): Inside is more negative compared to outside.

• Less Polarized (depolarized): Inside is less negative compared to outside.

• "Graded" means that electrical signals vary in amplitude (size) depending upon strength of stimulus.

• Signal is localized; it travels only a short distance before dying out.

• Useful for communication when distance is small.

• Graded potentials most often occur in dendrites of cell body and less often in axon because ligand-gated and megnanically gated channels are present in these areas more often.

• There is no refractory period.

Action Potentials

• Occur where there are voltage-gated channels.

• Take place in 2 phases: depolarization and repolarization.

• Depolarization: membrane potential decreases to zero.

• Repolarization: membrane potential is restored to -70mV.

• During action potentials, first Na channels open and Na+ rushes into the cell causing depolarization.

• Then K channels open causing K+ to rush out of the cell causing repolarization.

• APs occur with all-or-none principle. Depolarization magnitude of -55mV must occur for depolarization to start.

• Na channels have two gates: activation and inactivation gate. During resting state, the inactivation gate is open and activation gate is closed; during activated state, all gates are open.

• As Na channels open, more and more channels open caused by positive feedback.

• Na that enters the cell is considerably less than present outside the cell; the entering Na is easily bailed out by Na-K pumps.

• Na channels: active>inactive>resting. To go from inactive to active, channel must pass resting phase.

• After Na has entered, Na channels enter the inactivated state, and K pumps also open, much more slowly however.

• There are two types of refractory periods: absolute and relative.

• Absolute: AP cannot be initiated.

• Relative: AP can be initiated but only with larger-than-normal stimulus.

• The above type of conduction is called continuous conduction.

Continuous vs Saltatory Conduction

• In saltatory conduction, impulse conduction is faster.

• Axons are myelinated. There is uneven distribution of voltage-gated channels.

• There is no myelin over nodes of Ranvier.

• Nerve impulses at the first node generate ionic currents in cytosol and extracellular fluid that depolarize the membrane to threshold, opening Na channels at the other node.

• Myelinated conduction is more faster and efficient.

Axon Diameter

• Large diameter axons conduct impulses faster.

• A-fibers: large diameter myelinated; activated by touch, pressure, position of joints, and some thermal sensations.

• B-fibers: mid diameter myelinated; ANS motor and ANS brain > spinal cord.

• C-fibers: small diameter unmyelinated.

Stimulus Intensity

• Stimulus intensity varied by frequency of nerve impulses and number of sensory neurons recruited.

Synapses

• Synapses change depending on learning.

• Communication flows presynaptic neuron > postsynaptic neuron.

• Electrical synapses: action potentials (through ions) travel from one cell to another through gap junctions. This type of communication is faster and under better control.

• Chemical synapses: When the action potential reaches the synaptic end bulb, Ca2+ ions rush inside causing the neurotransmitter to be released. The neurotransmitter binds receptors in postsynaptic neurons and the AP resumes.

• Information flows in one direction because only synaptic end bulbs of presynaptic release neurotransmitter, and only postsynaptic neuron's membrane has the correct receptor.

• The postsynaptic neuron can either depolarize (due to inflow cations/Na, K, Ca2+) or hyperpolarize (anions / Cl- or outflow of K+).

Neuromuscular Junction

• When APs flow across the sarcolemma of muscle fibers, the muscle fibers contract.

• Somatic motor neuron > NMJ > skeletal muscle fiber.

• A synaptic cleft separates the two cells.

• The neurotransmitter here is ACh, suspended within the synaptic end bulbs of the motor neuron.

• The muscle part of the NMJ is called motor-end plate, with ACh receptors (integral ligand-gated ion channel proteins).

Additional Reading:

Basic Neurology

1. Peripheral Nervous System
2. Central Nervous System
3. The Ventricular System
4. The Spinal Cord
5. The Brain Stem
6. The Cerebellum
7. Visual Pathways
8. Diencephalon
9. Basal Ganglia
10. Cerebral Cortex
11. Sleep Disorders
12. Autonomic Nervous System
13. Cranial Nerves and Parasympathetic Ganglia
14. Cells of the Nervous System
15. Cerebrospinal fluid
16. Additional short notes on Cerebrum
17. Functions and Diseases of Cerebrum
18. Subcortical Grey Matter
19. Notes on The Spinal Cord
20. Regulation of Heart Rate by Autonomic Nervous System
21. Action Potentials, Axon Conduction, and Neuromuscular Junction
22. Types of Seizures
23. What is a Cough Reflex?
24. Notes on Congenital Prosopagnosia
25. Findings in Parkinson's Disease
26. Types of Heat Strokes
27. Types of Strokes
28. What is Benign Intracranial Hypertension?
29. What is Cauda Equina Syndrome?
30. Cranial Nerve Locations in Brain Stem

Neurology Videos

1. Video of Neurology Examination in a Clinical Setting

Medical Images

Useful Medical Images & Diagrams (link opens in a new window)

Related Topics

1. Nervous System Disorders
2. Histology of Nervous Tissue
3. Cranial Nerve Reflexes
4. Motor System Examination

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