Properties of Cells in Nervous System Flashcards

Question

# *Action Potential* Threshold stimulus intensity = **?** ## Footnote *Properties of Cells in Nervous System*

Answer 1

**Action Potential**: (**a**) Threshold stimulus intensity * Stimulus intensity that is sufficient to produce an action potential (**b**) Unlike local potentials (graded in amplitude and duration), AP is **All-Or-None** * Every time sufficient stimuli are provided, an action potential will be produced. * Stronger stimuli produce action potentials of the ***same*** voltage and duration as the minimally sufficient stimuli. (**c**) APs transmit information over longer distances because they are actively propagated

Answer 2

**Refractory Period**: (**a**) Some channels become inactivated immediately after opening an action potential and require specific amount of time before it can be activated again for another action potential. * **Absolute refractory period**: Another action cannot be stimulated. * **Relative refractory period**: If greater depolarizing current is applied then next action potential can be initiated (**b**) Refractory period promotes forward progression of the action potential and prevents backward flow

Answer 3

**Propagation of Action Potential**: * Once an AP is generated, the change in electrical potential spreads passively along the axon. * Generation of new action potential is repeated along the length of the axon

Answer 4

**Propagation of Action Potential**: (**a**) Faster action potential propagation is dependent upon * Increased diameter of the axon * Speed of transmission is faster in larger-diameter axons (**a**) Myelination * Increases efficiency of conduction of AP by decreasing the inherent leakiness of the membrane

Answer 5

**Myelination**: (**a**) **Myelination**: A sheath of proteins and fats surrounding an axon (**b**) Function: * Provides insulation * Prevents the leakage of current across the axonal membrane * Increases the speed of AP propagation and the distance a current can passively spread (**c**) Thicker myelin leads to faster conduction and greater chances for AP propagation

Answer 6

**Nodes of Ranvier**: (**a**) Small patches which lack myelin * Distributed every 1 to 2 mm along the axon and contain high densities of Na+ and K+ channels (**b**) Specialized for active propagation of an action potential by allowing ion flow across the membrane * AP spreads rapidly along a myelinated region, slows when crossing Nodes of Ranvier * **Saltatory conduction**: quick node-to-node jumping of AP down a myelinated axon

Answer 7

**Glial Cells**: (**a**) Glial cells (or glia): ‘*Glia*’ means glue in Greek, and reflects a presumption that these cells “held the nervous system together” or responsible for “determining the shape of the nervous system.” (**b**) Glial cells essentially form a critical support network for neurons.

Answer 8

**Glial Cells** - Glia are categorized by size and function: (1) **Macroglia** (large) * Astrocytes * Oligodendrocytes * Schwann cells (2) **Microglia** (small) (3) **Glial stem cells**

Answer 9

**Astrocytes**: (**a**) Star-shaped macroglial cells found throughout the CNS, primarily in ***gray matter*** because of close association with neuronal cell bodies, dendrites and synapses. * Can be stimulated by signals from adjacent neurons or by mechanical changes (shape or pressure). * Role in cell signaling

Answer 10

**Function of Astrocytes**: (**a**) Help maintain normal neuronal signaling by acting as scavengers * Taking up extra K+ ions in the extracellular environment, regulating ionic concentration gradient. * Removing chemical transmitters from the synaptic cleft between neurons, and * Cleaning up other debris in the extracellular space (**b**) Connect neurons with capillaries via end-feet, providing nutrition. (**c**) Participate in the formation of scars to fill in cavities following an injury (**d**) Assist in formation of new synapses and circuits

Answer 11

**Oligodendrocytes and Schwann Cells**: (**a**) They form ***myelin sheath*** (*axonal insulation*) and are found primarily in ***white matter***. * Oligodendrocytes myelinate neurons in the **CNS**. * Schwann cells myelinate neurons in the **PNS**. * Subject to immunological insults in certain neurological conditions, causing demyelinating diseases, such as multiple sclerosis (MS) and Guillain-Barre syndrome (GBS)

Answer 12

**Microglial Cells**: (**a**) Are special type of phagocytes that resides in the CNS (**b**) Microglia act as the immune system of the CNS * Activated during development of the nervous system and following injury, infection, or disease * Proteins in dying cells attract microglia to the site; microglia clean up and remove debris, important for healing following stroke, brain injury etc.

Answer 13

**Microglial Cells**: (**a**) Are special type of phagocytes that resides in the CNS (**b**) Microglia act as the immune system of the CNS * Activated during development of the nervous system and following injury, infection, or disease * Proteins in dying cells attract microglia to the site; microglia clean up and remove debris, important for healing following stroke, brain injury etc.

Answer 14

**Glial Stem Cells**: (**a**) Glial stem cells are also found throughout the developing and adult brain * These cells are immature and undifferentiated cells. * Retain the capacity to proliferate and generate additional precursors or differentiated glia and, in some cases, neurons (**b**) Characteristics include the ability to: * Self-renew; differentiate into most types of glial cells and neurons

Answer 15

**Neuroinflammation** - Beneficial and Harmful Effects: (a) **Neuroinflammation**: * The response of the CNS to infections, diseases and injuries * Response is mediated by ***reactive microglia*** and ***astrocytes***. (b) **Beneficial effect**: * When reactive microglia clean up and remove debris; (c) **Harmful effects**: * Microglia and astrocytes become excessively activated. * Death of neurons and oligodendrocytes, inhibition of neural regeneration. * Correlation between abnormal glial activity and neural damage in stroke.

Answer 16

**Neuroinflammation** - Beneficial and Harmful Effects: (a) **Neuroinflammation**: * The response of the CNS to infections, diseases and injuries * Response is mediated by reactive microglia and astrocytes (b) **Beneficial effect**: * When reactive microglia clean up and remove debris; (c) **Harmful effects**: * Microglia and astrocytes become excessively activated * Death of neurons and oligodendrocytes, inhibition of neural regeneration * Correlation between abnormal glial activity and neural damage in stroke

Answer 17

**Clinical Considerations - Myelin**: * Critical to conduction of information in the CNS * Damaged myelination ***slows and impedes the conduction of AP in the CNS*** * Such as multiple sclerosis or the PNS (e.g., Guillain-Barre syndrome)

Answer 18

**Multiple Sclerosis**: (**a**) Damage to myelin sheath in brain and SC * Immune system disorder (autoimmune disorder) * Oligodendrocytes are destructed * Patches of demyelination * Plaques * Women more affected (**b**) **Signs and symptoms**: * Lack of coordination and balance * Impaired and/or double vison * Impaired sensation (paresis/ paralysis) * Speech difficulties * Spasticity (increased reflexes (UMN)) * Gait difficulties (**c**) **Notes**: * Diagnosis is difficult; variable symptoms, MS usually manifests with one sign that may completely resolve. * 20% present with visual symptoms as the first symptom but 80% will have visual symptoms at some point

Answer 19

**Multiple Sclerosis**: (**a**) Onset common between 20 and 40 years; women are three times more frequently affected (**b**) Four types of MS, all named according to the course of disease progression: (1) **Relapsing/remitting** (most common - 85%): * Alternate between relapse(s) /remissions * Relapse - New signs/sxs appear & old signs worsen. * Remission - Full or partial recovery * Without treatment, most cases transition to ***secondary progressive***. (2) **Secondary progressive**: * Continuous decline with fewer or no relapses. (3) **Primary progressive** (10%): * Steady functional decline from time of onset (4) **Progressive relapsing** (5%): * Progressive Relapsing * Steady functional decline with relapse/ partial remissions * Function never fully recovers in remissions

Answer 20

**Summary of Key Elements**: (**I**) Neurons generate ***electrical signals*** to convey information over substantial distances and to transmit it to other cells. (**II**) The distribution of a specific ion depends on * Concentration gradient of the ion and * The electrical force acting on the ion (**III**) An action potential occurs when a transient rise in Na+ ***permeability*** allows a net flow of Na+ to enter the axon (i.e., influx), causing depolarization of the membrane. * Followed by the efflux of K+, a secondary, transient rise in membrane K+ ***permeability*** that repolarizes the neuronal membrane and produces a brief undershoot of the action potential. (**IV**) Astrocytes and microglia provide nutritive and clean up functions throughout the CNS, while when overactive, can cause damage to nervous system function (**V**) Oligodendrocytes and Schwann cells contribute to the myelination of neurons. * Destruction to the former impedes conduction of electrical signals along sensory and motor pathways of the CNS, and to the latter along the pathways of the PNS