Neuroscience: Neuron Refractory Period

Questions and Answers

What is the primary function of microglia in the nervous system?

To transmit nerve impulses

To support the structural integrity of neurons

To remove debris from injury and infection (correct)

To form myelin around axons

Which type of glia is responsible for myelin formation in the central nervous system?

Schwann cells

Microglia

Oligodendrocytes (correct)

Astrocytes

What is a consequence of myelin loss in conditions like multiple sclerosis?

Increased synaptic transmission

Delayed or blocked conduction (correct)

Enhanced nerve conduction speed

Formation of new neurons

Which of the following statements is true regarding Schwann cells?

They create segments of myelin sheath around single axons. Signup and view all the answers

Charcot-Marie-Tooth disease is primarily caused by mutations in which of the following?

Myelin protein genes Signup and view all the answers

What occurs during the absolute refractory period?

No stimulus can excite the nerve, regardless of its strength. Signup and view all the answers

Which statement correctly describes the role of electrotonic depolarization?

It initiates a local response by drawing positive charges into the area of negativity. Signup and view all the answers

During which phase of the spike potential is the neuron most refractory to stimulation?

During the rising and much of the falling phases of the spike potential. Signup and view all the answers

What phenomenon occurs in myelinated axons regarding action potential propagation?

Depolarization appears to ‘jump’ from one node of Ranvier to the next. Signup and view all the answers

What happens to the nerve cell membrane during an action potential?

The polarity of the membrane is completely reversed for a brief period. Signup and view all the answers

Study Notes

Action Potential and Refractory Periods

During local response, the neuron's excitation threshold decreases; during spike potential phases, it becomes refractory to stimulation.
Refractory period consists of two phases: absolute (no stimulus can evoke an action potential) and relative (stronger stimuli can result in excitation).
Absolute refractory period spans from the firing level to about one-third completion of repolarization.

Nerve Membrane Polarization

At rest, nerve cell membranes are polarized, having positive charges outside and negative charges inside.
During action potential, membrane polarization is reversed temporarily.
Positive charges flow into the area of negativity, reducing polarity and initiating local responses.

Conduction and Myelination

In myelinated axons, depolarization appears to jump between nodes of Ranvier (saltatory conduction).
Oligodendrocytes (CNS) and Schwann cells (PNS) are responsible for myelin formation around axons.
Oligodendrocytes sheathe multiple axons, while Schwann cells wrap around a single axon.

Microglia and Immune Response

Microglia act as immune system scavenger cells, removing debris from injury and disease.
They originate from macrophages and are distinct from other neural cell types.
The presence of microglia is notable in diseases like multiple sclerosis (MS) and neurodegenerative disorders.

MS is characterized by patchy myelin destruction in the CNS, leading to delayed conduction.
Autoimmune reactions to myelin proteins (P0 and PMP22) can result in Guillain–Barré syndrome.
Charcot-Marie-Tooth disease is linked to mutations in myelin protein genes, causing peripheral neuropathies.

Neurotrophins and Neuronal Growth

Nerve Growth Factor (NGF) is crucial for the growth and maintenance of cholinergic neurons, suppressing apoptosis.
Other neurotrophins include Brain-Derived Neurotrophic Factor (BDNF), Neurotrophin 3 (NT-3), and NT-4/5, each supporting various neuron types.
BDNF can rapidly depolarize neurons, while NGF is important for skin nociceptive neurons.

Receptor Mechanisms

Neurotrophins bind to high-affinity tyrosine kinase (Trk) receptors, initiating signaling in neurons.
Low-affinity p75 receptor binds all neurotrophins; its activation without neurotrophins can lead to apoptosis.

Multiple Sclerosis Diagnosis and Treatment

MS diagnosis is challenging, often requiring evidence of multiple episodes and MRI to visualize scarring/plaque.
Common treatments include corticosteroids for inflammation, β-interferons to suppress immune response, and other immunomodulatory agents like glatiramer acetate and natalizumab.
Emerging therapies include B cell-depleting therapies and oral medications like fingolimod.

Nerve Fiber Classification

Nerve fibers are categorized into types A, B, and C based on axonal diameter and conduction velocity.
Sensory afferent fibers have a numerical classification (Ia, Ib, II, III, IV), reflecting diverse functionalities and hypoxia sensitivity.

Transport Mechanisms

Orthograde transport along axons requires dynein and kinesin, functioning at fast (400 mm/day) and slow (0.5–10 mm/day) rates.
Retrograde transport moves at about 200 mm/day from nerve endings to cell bodies.

Glial Cells Functions

There are two main glial types: microglia (scavengers) and macroglia (oligodendrocytes, Schwann cells, astrocytes).
Astrocytes maintain ionic balance and neurotransmitter concentrations, supporting neuronal health.

Description

Explore the fascinating dynamics of neuron refractory periods through this quiz. Learn about the absolute and relative refractory periods, including their significance during the spike potential. Test your understanding of how these phases impact neuronal responsiveness to stimulation.