- Published on
Pathology - Inflammation and Healing
Acute inflammation
• A swift, non-specific reaction to cellular damage
. • Coordinated by cytokines generated from damaged cells, such as histamine, serotonin, prostaglandins, leukotrienes, and platelet-activating factor. Cytokines stimulate endothelial cells, resulting in the development of an acute inflammatory exudate including fluid, fibrin, and neutrophils. Severe acute inflammation may result in a localized accumulation of pus within a necrotic cavity (abscess). Acute inflammation may resolve, heal with fibrosis, or advance to chronic inflammation.
Chronic inflammation is a sustained type of inflammation characterized by concurrent tissue damage and attempts at healing.
• May originate from acute inflammation or manifest from the onset.
• Defined by the presence of chronic inflammatory cells, specifically macrophages, lymphocytes, and plasma cells.
• More prone to healing with permanent scarring rather than resolution.
Granulomatous inflammation is a distinct form of chronic inflammation marked by the presence of activated macrophages referred to as epithelioid histiocytes. Collections of epithelioid macrophages are referred to as granulomas. Granulomatous inflammation is linked to foreign substances, chronic infections (such as mycobacteria), and conditions of indeterminate etiology (for instance, sarcoidosis).
Restoration
• The procedure of substituting necrotic and impaired tissue with viable tissue
. • May transpire via regeneration or repair. Regeneration (resolution) substitutes injured cells with identical cell types and represents the optimal outcome. This can only happen if the connective tissue framework is intact and if the tissue possesses regenerative capabilities. Repair initiates with the development of granulation tissue, subsequently transformed into a collagen-dense scar. While the structural integrity is preserved, the functionality of the damaged tissue is compromised.
Acute inflammation
• A swift, non-specific reaction to cellular damage
. • Coordinated by cytokines generated from damaged cells, such as histamine, serotonin, prostaglandins, leukotrienes, and platelet-activating factor. Cytokines stimulate endothelial cells, resulting in the development of an acute inflammatory exudate including fluid, fibrin, and neutrophils. Severe acute inflammation may result in a localized accumulation of pus within a necrotic cavity (abscess). Acute inflammation may resolve, heal with fibrosis, or advance to chronic inflammation.
Chronic inflammation is a sustained type of inflammation characterized by concurrent tissue damage and attempts at healing.
• May originate from acute inflammation or manifest from the onset.
• Defined by the presence of chronic inflammatory cells, specifically macrophages, lymphocytes, and plasma cells.
• More prone to healing with permanent scarring rather than resolution.
Granulomatous inflammation is a distinct form of chronic inflammation marked by the presence of activated macrophages referred to as epithelioid histiocytes. Collections of epithelioid macrophages are referred to as granulomas. Granulomatous inflammation is linked to foreign substances, chronic infections (such as mycobacteria), and conditions of indeterminate etiology (for instance, sarcoidosis).
Restoration
• The procedure of substituting necrotic and impaired tissue with viable tissue
. • May transpire via regeneration or repair. Regeneration (resolution) substitutes injured cells with identical cell types and represents the optimal outcome. This can only happen if the connective tissue framework is intact and if the tissue possesses regenerative capabilities. Repair initiates with the development of granulation tissue, subsequently transformed into a collagen-dense scar. While the structural integrity is preserved, the functionality of the damaged tissue is compromised.
- Published on
Terminology of pathology
Nomenclature of diseases
Aetiology denotes the fundamental cause of an illness. Diseases with an unclear aetiology are termed idiopathic, cryptogenic, or essential.
Pathogenesis denotes the mechanism via which the etiological agent induces the signs of a disease.
Incidence denotes the quantity of newly diagnosed instances of an illness within a specified timeframe.
Prevalence denotes the aggregate number of disease cases within a community at a specific point in time.
Prognosis refers to the anticipated progression of an illness.
Morbidity refers to the degree to which a patient's overall health is impacted by a condition. •
Mortality indicates the probability of mortality due to a specific disease
.Acute and chronic denote the temporal progression of a disease occurrence. Acute diseases manifest abruptly.
Chronic illnesses typically exhibit a gradual onset and are more prone to a protracted duration.
A syndrome denotes a collection of symptoms and clinical indications that, when present concurrently, indicate a certain underlying cause(s).
Classification of diseases
Genetic disorders are hereditary conditions resulting from a faulty gene, such as cystic fibrosis.
Infectious disorders arise from the invasion of the body by pathogenic microorganisms, such as malaria.
Inflammatory disorders result from excessive activity of inflammatory cells in an organ, such as rheumatoid arthritis.
Neoplastic illness arises from the unregulated proliferation of cells, such as breast cancer.
• Vascular diseases occur as a result of blood vessel disorders, such as ischemic heart disease.
• Metabolic disorders stem from abnormalities in metabolic processes, such as diabetes mellitus.
Degenerative diseases arise from the destruction and/or loss of specialized cells, such as the loss of neurons from the cerebral cortex in Alzheimer's disease. Iatrogenic disease arises from the consequences of medical treatment, such as osteoporosis resulting from prolonged glucocorticoid therapy
. • Congenital disorders are present at birth, while those that arise postnatally are termed acquired diseases.
Nomenclature of diseases
Aetiology denotes the fundamental cause of an illness. Diseases with an unclear aetiology are termed idiopathic, cryptogenic, or essential.
Pathogenesis denotes the mechanism via which the etiological agent induces the signs of a disease.
Incidence denotes the quantity of newly diagnosed instances of an illness within a specified timeframe.
Prevalence denotes the aggregate number of disease cases within a community at a specific point in time.
Prognosis refers to the anticipated progression of an illness.
Morbidity refers to the degree to which a patient's overall health is impacted by a condition. •
Mortality indicates the probability of mortality due to a specific disease
.Acute and chronic denote the temporal progression of a disease occurrence. Acute diseases manifest abruptly.
Chronic illnesses typically exhibit a gradual onset and are more prone to a protracted duration.
A syndrome denotes a collection of symptoms and clinical indications that, when present concurrently, indicate a certain underlying cause(s).
Classification of diseases
Genetic disorders are hereditary conditions resulting from a faulty gene, such as cystic fibrosis.
Infectious disorders arise from the invasion of the body by pathogenic microorganisms, such as malaria.
Inflammatory disorders result from excessive activity of inflammatory cells in an organ, such as rheumatoid arthritis.
Neoplastic illness arises from the unregulated proliferation of cells, such as breast cancer.
• Vascular diseases occur as a result of blood vessel disorders, such as ischemic heart disease.
• Metabolic disorders stem from abnormalities in metabolic processes, such as diabetes mellitus.
Degenerative diseases arise from the destruction and/or loss of specialized cells, such as the loss of neurons from the cerebral cortex in Alzheimer's disease. Iatrogenic disease arises from the consequences of medical treatment, such as osteoporosis resulting from prolonged glucocorticoid therapy
. • Congenital disorders are present at birth, while those that arise postnatally are termed acquired diseases.
- Published on
Pathology - Cellular modifications
Atrophy
• A decrease in the dimensions of a tissue or organ.
• Can result from a decrease in cell quantity by death or a loss in cell volume through shrinkage.
• Atrophy can manifest as a natural physiological phenomenon, such as thymic atrophy throughout puberty and ovarian atrophy during menopause. Pathological atrophy examples encompass muscle atrophy resulting from denervation and cerebral atrophy caused by cerebrovascular illness.
Hypertrophy
An augmentation in the dimensions of individual cells.
• As a result of an augmentation of cellular proteins and organelles.
• Observed in organs with terminally differentiated cells that are incapable of proliferation, such as cardiac and skeletal muscle. Examples of physiological hypertrophy encompass the myometrium of the uterus during pregnancy and the musculature of a bodybuilder. Pathological hypertrophy examples encompass left ventricular hypertrophy resulting from hypertension or aortic stenosis.
Hyperplasia
• An augmentation in cellular quantity. Examples of physiological hyperplasia encompass the endometrium and breast lobules in reaction to cyclical estrogen stimulation. Pathological hyperplasia examples include benign prostatic hyperplasia and parathyroid hyperplasia.
Metaplasia
A transformation in which one cellular type is replaced by another. • Considered to arise from progenitor cells differentiating into a novel cell type rather than through direct morphogenesis from one cell type to another. • Predominantly observed in epithelial cells, frequently as a reaction to chronic injury. • Metaplasia is designated based on the newly formed cell type; for instance, a transition from non-squamous to squamous epithelium is referred to as squamous metaplasia. Common locations of squamous metaplasia encompass the endocervix, forming the transformation zone associated with cervical neoplasia, and the bronchi of smokers. Common locations of glandular metaplasia encompass the lower esophagus in certain individuals with reflux illness, resulting in a discernible Barrett's esophagus.
Metaplasia serves as an indicator of chronic epithelial injury that may, in certain instances, progress to epithelial dysplasia and ultimately cancer.
Atrophy
• A decrease in the dimensions of a tissue or organ.
• Can result from a decrease in cell quantity by death or a loss in cell volume through shrinkage.
• Atrophy can manifest as a natural physiological phenomenon, such as thymic atrophy throughout puberty and ovarian atrophy during menopause. Pathological atrophy examples encompass muscle atrophy resulting from denervation and cerebral atrophy caused by cerebrovascular illness.
Hypertrophy
An augmentation in the dimensions of individual cells.
• As a result of an augmentation of cellular proteins and organelles.
• Observed in organs with terminally differentiated cells that are incapable of proliferation, such as cardiac and skeletal muscle. Examples of physiological hypertrophy encompass the myometrium of the uterus during pregnancy and the musculature of a bodybuilder. Pathological hypertrophy examples encompass left ventricular hypertrophy resulting from hypertension or aortic stenosis.
Hyperplasia
• An augmentation in cellular quantity. Examples of physiological hyperplasia encompass the endometrium and breast lobules in reaction to cyclical estrogen stimulation. Pathological hyperplasia examples include benign prostatic hyperplasia and parathyroid hyperplasia.
Metaplasia
A transformation in which one cellular type is replaced by another. • Considered to arise from progenitor cells differentiating into a novel cell type rather than through direct morphogenesis from one cell type to another. • Predominantly observed in epithelial cells, frequently as a reaction to chronic injury. • Metaplasia is designated based on the newly formed cell type; for instance, a transition from non-squamous to squamous epithelium is referred to as squamous metaplasia. Common locations of squamous metaplasia encompass the endocervix, forming the transformation zone associated with cervical neoplasia, and the bronchi of smokers. Common locations of glandular metaplasia encompass the lower esophagus in certain individuals with reflux illness, resulting in a discernible Barrett's esophagus.
Metaplasia serves as an indicator of chronic epithelial injury that may, in certain instances, progress to epithelial dysplasia and ultimately cancer.
- Published on
Pathology - Cellular death
Necrosis is an inadequately regulated kind of cell death characterized by the loss of membrane integrity, resulting in the spilling of cellular contents and an inflammatory reaction.
• Coagulative necrosis is the predominant type, marked by the absence of cell nuclei while maintaining the overall structural integrity. The necrotic tissue appears macroscopically pale and firm.
Liquefactive necrosis results in the total destruction of cellular architecture, transforming it into a mushy, semi-solid mass. This is commonly observed in the brain subsequent to cerebral infarction.
Caseous necrosis is a form of necrosis characterized by dead tissue that macroscopically resembles cream cheese. It can manifest in several diseases, but is predominantly linked to tuberculosis.
• Gangrene denotes necrotic tissues altered by air exposure, leading to desiccation (dry gangrene) or infection (wet gangrene). In cases of critical limb ischaemia, toes that experience blood deprivation typically exhibit dry gangrene.
Apoptosis is a regulated process of cellular demise when no intracellular components are expelled from the dying cell, hence preventing any inflammatory response. Apoptosis can occur naturally, such as in the elimination of cells during embryogenesis and those with DNA damage, or pathologically. Apoptosis can be initiated via two primary mechanisms: the activation of surface death receptors, such as Fas or TNF-α (extrinsic pathway), or through cellular damage (intrinsic pathway). The final outcome is the activation of protease enzymes known as caspases, which deconstruct the cell's cytoplasm and nucleus.
Apoptotic cells undergo shrinkage and fragmentation into apoptotic bodies, each maintaining an unbroken cell membrane. • Apoptotic entities are promptly eliminated by neighboring cells. • Dysregulated apoptosis is believed to play a crucial role in various significant disease mechanisms, especially carcinogenesis.
Necrosis is an inadequately regulated kind of cell death characterized by the loss of membrane integrity, resulting in the spilling of cellular contents and an inflammatory reaction.
• Coagulative necrosis is the predominant type, marked by the absence of cell nuclei while maintaining the overall structural integrity. The necrotic tissue appears macroscopically pale and firm.
Liquefactive necrosis results in the total destruction of cellular architecture, transforming it into a mushy, semi-solid mass. This is commonly observed in the brain subsequent to cerebral infarction.
Caseous necrosis is a form of necrosis characterized by dead tissue that macroscopically resembles cream cheese. It can manifest in several diseases, but is predominantly linked to tuberculosis.
• Gangrene denotes necrotic tissues altered by air exposure, leading to desiccation (dry gangrene) or infection (wet gangrene). In cases of critical limb ischaemia, toes that experience blood deprivation typically exhibit dry gangrene.
Apoptosis is a regulated process of cellular demise when no intracellular components are expelled from the dying cell, hence preventing any inflammatory response. Apoptosis can occur naturally, such as in the elimination of cells during embryogenesis and those with DNA damage, or pathologically. Apoptosis can be initiated via two primary mechanisms: the activation of surface death receptors, such as Fas or TNF-α (extrinsic pathway), or through cellular damage (intrinsic pathway). The final outcome is the activation of protease enzymes known as caspases, which deconstruct the cell's cytoplasm and nucleus.
Apoptotic cells undergo shrinkage and fragmentation into apoptotic bodies, each maintaining an unbroken cell membrane. • Apoptotic entities are promptly eliminated by neighboring cells. • Dysregulated apoptosis is believed to play a crucial role in various significant disease mechanisms, especially carcinogenesis.
- Published on
Medical Physiology - Fundamental Mechanism of Muscle Contraction
The commencement and implementation of muscular contraction transpire in the subsequent sequential steps: 1. An action potential propagates along a motor neuron to its terminals on muscle fibers, where each terminal releases a minute quantity of the neurotransmitter acetylcholine. 2. Acetylcholine interacts with a specific region of the muscular membrane to activate acetylcholine-gated cation channels, permitting primarily sodium ions, along with calcium ions, to enter the muscle fiber, resulting in localized depolarization. The local depolarization subsequently causes the opening of voltage-gated sodium channels, culminating in an action potential. The action potential propagates over the muscle fiber membrane, prompting the sarcoplasmic reticulum to release calcium ions into the myofibrils that were previously stored in the sarcoplasmic reticulum. 4. Calcium ions instigate attractive interactions between the actin and myosin filaments, resulting in their sliding together; this constitutes the contractile process. 5. Calcium ions are continuously transported back into the sarcoplasmic reticulum, where they are stored until a muscular action potential occurs; this expulsion of calcium ions from the myofibrils results in the cessation of muscle
contraction.
The commencement and implementation of muscular contraction transpire in the subsequent sequential steps: 1. An action potential propagates along a motor neuron to its terminals on muscle fibers, where each terminal releases a minute quantity of the neurotransmitter acetylcholine. 2. Acetylcholine interacts with a specific region of the muscular membrane to activate acetylcholine-gated cation channels, permitting primarily sodium ions, along with calcium ions, to enter the muscle fiber, resulting in localized depolarization. The local depolarization subsequently causes the opening of voltage-gated sodium channels, culminating in an action potential. The action potential propagates over the muscle fiber membrane, prompting the sarcoplasmic reticulum to release calcium ions into the myofibrils that were previously stored in the sarcoplasmic reticulum. 4. Calcium ions instigate attractive interactions between the actin and myosin filaments, resulting in their sliding together; this constitutes the contractile process. 5. Calcium ions are continuously transported back into the sarcoplasmic reticulum, where they are stored until a muscular action potential occurs; this expulsion of calcium ions from the myofibrils results in the cessation of muscle
contraction.
- Published on
Medical Physiology - Physiological Anatomy of Skeletal Muscle
ZApproximately 40% of body mass comprises skeletal muscle, with an additional 10% consisting of smooth and cardiac muscle. Numerous concepts of contraction are applicable to all three muscle types. Skeletal Muscle Fiber In the majority of muscles, the fibers span the full length of the muscle. Each fiber is innervated by a single nerve terminal. Myofibrils consist of actin and myosin filaments. Each muscle fiber comprises hundreds to thousands of myofibrils, with each myofibril consisting of approximately 1500 myosin filaments and 3000 actin filaments arranged laterally. These filaments are substantial polymerized protein molecules that facilitate muscle contraction. The thick filaments consist of myosin, whereas the thin filaments comprise actin; observe the subsequent characteristics:
• light and dark bands.
The myosin and actin filaments partially interdigitate, resulting in the myofibrils exhibiting alternating bright and dark bands. The light bands consist solely of actin filaments and are referred to as I bands. The dark regions known as A bands comprise myosin filaments along with the terminal portions of actin filaments. The A band length corresponds to the length of the myosin filament
. • Cross-bridges. The diminutive extensions from the myosin filament sides are termed cross-bridges. They extend from the surfaces of the myosin filament over its whole length, with the exception of the core. Myosin cross-bridges engage with actin filaments, resulting in contraction.
• Z the disc. The termini of the actin filaments are affixed to Z discs.The Z disc traverses the myofibril, connecting and aligning the myofibrils throughout the muscle fiber.
The complete muscle fiber exhibits alternating light and dark bands, resulting in a striated appearance in skeletal and cardiac muscle.
• Sarcomere. The segment of a myofibril situated between two consecutive Z discs is referred to as a sarcomere. At rest, actin filaments exhibit ideal interdigitation with myosin filaments in skeletal muscle, while in cardiac muscle, the interdigitation is somewhat less than optimal.
ZApproximately 40% of body mass comprises skeletal muscle, with an additional 10% consisting of smooth and cardiac muscle. Numerous concepts of contraction are applicable to all three muscle types. Skeletal Muscle Fiber In the majority of muscles, the fibers span the full length of the muscle. Each fiber is innervated by a single nerve terminal. Myofibrils consist of actin and myosin filaments. Each muscle fiber comprises hundreds to thousands of myofibrils, with each myofibril consisting of approximately 1500 myosin filaments and 3000 actin filaments arranged laterally. These filaments are substantial polymerized protein molecules that facilitate muscle contraction. The thick filaments consist of myosin, whereas the thin filaments comprise actin; observe the subsequent characteristics:
• light and dark bands.
The myosin and actin filaments partially interdigitate, resulting in the myofibrils exhibiting alternating bright and dark bands. The light bands consist solely of actin filaments and are referred to as I bands. The dark regions known as A bands comprise myosin filaments along with the terminal portions of actin filaments. The A band length corresponds to the length of the myosin filament
. • Cross-bridges. The diminutive extensions from the myosin filament sides are termed cross-bridges. They extend from the surfaces of the myosin filament over its whole length, with the exception of the core. Myosin cross-bridges engage with actin filaments, resulting in contraction.
• Z the disc. The termini of the actin filaments are affixed to Z discs.The Z disc traverses the myofibril, connecting and aligning the myofibrils throughout the muscle fiber.
The complete muscle fiber exhibits alternating light and dark bands, resulting in a striated appearance in skeletal and cardiac muscle.
• Sarcomere. The segment of a myofibril situated between two consecutive Z discs is referred to as a sarcomere. At rest, actin filaments exhibit ideal interdigitation with myosin filaments in skeletal muscle, while in cardiac muscle, the interdigitation is somewhat less than optimal.
- Published on
Medical Physiology - Distinctive Features of Signal Transmission in Nerve Trunks
Large nerve fibers are myelinated, whereas small nerve fibers are unmyelinated. The axon constitutes the central core of the fiber, and its membrane facilitates the conduction of the action potential. A thick myelin sheath, formed by Schwann cells, encases the bigger axons. The sheath comprises several layers of cellular membrane that include the lipid sphingomyelin, an effective insulator. At the interface between two consecutive Schwann cells, a brief noninsulated segment measuring only 2 to 3 mm in length persists, allowing ions to move freely between the extracellular fluid and the axon. This region is the node of Ranvier. Saltatory conduction transpires in myelinated fibers. Although ions cannot traverse the thick sheaths of myelinated neurons considerably, they can pass with relative ease through the nodes of Ranvier.
Consequently, the nerve impulse traverses from node to node along the fiber, which is the basis for the term "saltatory." Saltatory conduction is advantageous for two reasons: • Enhanced speed. This method enhances the pace of nerve transmission in myelinated fibers by facilitating the depolarization process to leap across extensive intervals along the nerve fiber axis, achieving increases of 5 to 50 times. • Energy conservation. Secondly, saltatory conduction conserves energy for the axon as only the nodes undergo depolarization, resulting in a significantly reduced ion loss—potentially a hundredfold less—than would be required otherwise, thereby necessitating minimal energy to restore the sodium and potassium concentration gradients across the membrane following a sequence of nerve impulses. The conduction velocity is highest in large, myelinated nerve fibers.
The conduction velocity of action potentials in nerve fibers ranges from a minimum of 0.25 m/sec in diminutive unmyelinated fibers to a maximum of 100 m/sec in substantial myelinated fibers. The velocity increases roughly with fiber diameter in myelinated nerve fibers and about with the square root of fiber diameter in unmyelinated fibers.
Large nerve fibers are myelinated, whereas small nerve fibers are unmyelinated. The axon constitutes the central core of the fiber, and its membrane facilitates the conduction of the action potential. A thick myelin sheath, formed by Schwann cells, encases the bigger axons. The sheath comprises several layers of cellular membrane that include the lipid sphingomyelin, an effective insulator. At the interface between two consecutive Schwann cells, a brief noninsulated segment measuring only 2 to 3 mm in length persists, allowing ions to move freely between the extracellular fluid and the axon. This region is the node of Ranvier. Saltatory conduction transpires in myelinated fibers. Although ions cannot traverse the thick sheaths of myelinated neurons considerably, they can pass with relative ease through the nodes of Ranvier.
Consequently, the nerve impulse traverses from node to node along the fiber, which is the basis for the term "saltatory." Saltatory conduction is advantageous for two reasons: • Enhanced speed. This method enhances the pace of nerve transmission in myelinated fibers by facilitating the depolarization process to leap across extensive intervals along the nerve fiber axis, achieving increases of 5 to 50 times. • Energy conservation. Secondly, saltatory conduction conserves energy for the axon as only the nodes undergo depolarization, resulting in a significantly reduced ion loss—potentially a hundredfold less—than would be required otherwise, thereby necessitating minimal energy to restore the sodium and potassium concentration gradients across the membrane following a sequence of nerve impulses. The conduction velocity is highest in large, myelinated nerve fibers.
The conduction velocity of action potentials in nerve fibers ranges from a minimum of 0.25 m/sec in diminutive unmyelinated fibers to a maximum of 100 m/sec in substantial myelinated fibers. The velocity increases roughly with fiber diameter in myelinated nerve fibers and about with the square root of fiber diameter in unmyelinated fibers.
- Published on
Medical Physiology - Re-establishing sodium and potassium ion gradients following action potential completion: significance of energy metabolism
By spreading each impulse along the nerve fiber, the concentration variations between the inner and out-side of the membrane are virtually eliminated. Before the ion concentration changes to the point where action potential conduction stops, nerve fibers can send between 100,000 and 50 million impulses. Still, with time it becomes essential to restore the differences in sodium and potassium membrane concentration. The Na+-K+ pump acts to do this.
By spreading each impulse along the nerve fiber, the concentration variations between the inner and out-side of the membrane are virtually eliminated. Before the ion concentration changes to the point where action potential conduction stops, nerve fibers can send between 100,000 and 50 million impulses. Still, with time it becomes essential to restore the differences in sodium and potassium membrane concentration. The Na+-K+ pump acts to do this.
- Published on
Medical Physiology - Propagation of the Action Potential
Usually activates nearby areas of the mem-brane, an action potential generated at any one site causes propagation of the action potential. The depolarization process thus follows the whole length of the fiber. A nerve or muscle impulse is the depolarization process transmitted along a nerve or muscle fiber. • Propagation direction of interest. An excitable membrane lacks a single direction of propagation; instead, the action potential moves both away from the stimulus in both directions. • All-or-nothing concept. Once an action potential has been generated at any place on the membrane of a normal fiber, the depolarization process travels over the whole membrane if conditions are correct, or it might not travel at all if conditions are not right.
Usually activates nearby areas of the mem-brane, an action potential generated at any one site causes propagation of the action potential. The depolarization process thus follows the whole length of the fiber. A nerve or muscle impulse is the depolarization process transmitted along a nerve or muscle fiber. • Propagation direction of interest. An excitable membrane lacks a single direction of propagation; instead, the action potential moves both away from the stimulus in both directions. • All-or-nothing concept. Once an action potential has been generated at any place on the membrane of a normal fiber, the depolarization process travels over the whole membrane if conditions are correct, or it might not travel at all if conditions are not right.
- Published on
Medical Physiology - Nerve Action Potential
Nerve signals are conveyed through action potentials, which are fast alterations in membrane potential. Each action potential commences with an abrupt transition from the typical resting negative potential to a positive membrane potential, followed by a nearly instantaneous return to the resting negative potential. The sequential phases of the action potential are as follows:
• Resting phase. This represents the resting membrane potential prior to the initiation of the action potential.
• Depolarization phase. At this moment, the membrane abruptly becomes permeable to sodium ions, permitting a substantial influx of positively charged sodium ions into the axon, resulting in a quick increase in potential in the positive direction.
• Repolarization phase. Within a few ten-thousandths of a second after the membrane attains high permeability to sodium ions, the sodium channels commence closure while the potassium channels open to a greater extent than usual. The swift efflux of potassium ions to the exterior reinstates the typical negative resting membrane potential.
Voltage-gated sodium and potassium channels undergo activation and inactivation throughout the progression of an action potential. The essential element for both depolarization and repolarization of the neuronal membrane during the action potential is the voltage-gated sodium channel. The voltage-gated potassium channel significantly contributes to the acceleration of membrane repolarization. These two voltage-gated channels exist alongside the Na⁺-K⁺ pump and the Na⁺-K⁺ leak channels that determine the membrane's resting permeability.
The events that precipitate the action potential can be summarized as follows.
• In the resting state, prior to the initiation of the action potential, the conductance for potassium ions is approximately 100 times greater than that for sodium ions. This results from significantly increased leaking of potassium ions compared to sodium ions through the leak channels.
• At the initiation of the action potential, the sodium channels rapidly activate, permitting an increase in sodium permeability of up to 5000-fold (also referred to as sodium conductance). The inactivation process subsequently occludes the sodium channels within only fractions of a millisecond. The initiation of the action potential triggers the voltage gating of potassium channels, leading to their gradual opening. Upon conclusion of the action potential, the restoration of the membrane potential to a negative state prompts the potassium channels to revert to their previous configuration, albeit after a delay.
A positive-feedback loop initiates the opening of sodium channels.
Should any event induce the membrane potential to ascend from 90 millivolts towards the zero level, the increasing voltage itself prompts the opening of many voltage-gated sodium channels. This facilitates the fast influx of sodium ions, resulting in an additional increase in membrane potential, so activating more voltage-gated sodium channels. This mechanism constitutes a positive-feedback loop that persists until all voltage-gated sodium channels are engaged. An action potential is initiated only upon reaching the threshold potential. This occurs when the quantity of sodium ions infiltrating the nerve fiber The quantity of potassium ions exiting the fiber exceeds the influx of potassium ions. An abrupt elevation in the membrane potential of a major nerve fiber from –90 millivolts to around –65 millivolts typically triggers a rapid onset of the action potential. A membrane potential of –65 millivolts is identified as the stimulation threshold. An action potential cannot be generated while the membrane remains depolarized from the preceding action potential. Immediately following the initiation of the action potential, the sodium channels undergo inactivation, rendering any excitatory signal supplied to these channels ineffective in opening the inactivation gates. The sole situation that can reactivate them is when the membrane potential reverts to or nearly to the initial resting membrane potential level.
Subsequently, during another brief fraction of a second, the inactivation gates of the channels open, allowing for the initiation of a fresh action potential.
• Absolute refractory phase. An action potential cannot be generated during the absolute refractory period, regardless of the stimulus strength. The refractory period for big myelinated nerve fibers is around 1/2500 of a second, allowing for a maximum transmission of about 2500 impulses per second
. • Relative refractory phase. This interval succeeds the absolute refractory period. During this period, stimuli of greater intensity than usual can activate the nerve fiber, leading to the initiation of an action potential.
Nerve signals are conveyed through action potentials, which are fast alterations in membrane potential. Each action potential commences with an abrupt transition from the typical resting negative potential to a positive membrane potential, followed by a nearly instantaneous return to the resting negative potential. The sequential phases of the action potential are as follows:
• Resting phase. This represents the resting membrane potential prior to the initiation of the action potential.
• Depolarization phase. At this moment, the membrane abruptly becomes permeable to sodium ions, permitting a substantial influx of positively charged sodium ions into the axon, resulting in a quick increase in potential in the positive direction.
• Repolarization phase. Within a few ten-thousandths of a second after the membrane attains high permeability to sodium ions, the sodium channels commence closure while the potassium channels open to a greater extent than usual. The swift efflux of potassium ions to the exterior reinstates the typical negative resting membrane potential.
Voltage-gated sodium and potassium channels undergo activation and inactivation throughout the progression of an action potential. The essential element for both depolarization and repolarization of the neuronal membrane during the action potential is the voltage-gated sodium channel. The voltage-gated potassium channel significantly contributes to the acceleration of membrane repolarization. These two voltage-gated channels exist alongside the Na⁺-K⁺ pump and the Na⁺-K⁺ leak channels that determine the membrane's resting permeability.
The events that precipitate the action potential can be summarized as follows.
• In the resting state, prior to the initiation of the action potential, the conductance for potassium ions is approximately 100 times greater than that for sodium ions. This results from significantly increased leaking of potassium ions compared to sodium ions through the leak channels.
• At the initiation of the action potential, the sodium channels rapidly activate, permitting an increase in sodium permeability of up to 5000-fold (also referred to as sodium conductance). The inactivation process subsequently occludes the sodium channels within only fractions of a millisecond. The initiation of the action potential triggers the voltage gating of potassium channels, leading to their gradual opening. Upon conclusion of the action potential, the restoration of the membrane potential to a negative state prompts the potassium channels to revert to their previous configuration, albeit after a delay.
A positive-feedback loop initiates the opening of sodium channels.
Should any event induce the membrane potential to ascend from 90 millivolts towards the zero level, the increasing voltage itself prompts the opening of many voltage-gated sodium channels. This facilitates the fast influx of sodium ions, resulting in an additional increase in membrane potential, so activating more voltage-gated sodium channels. This mechanism constitutes a positive-feedback loop that persists until all voltage-gated sodium channels are engaged. An action potential is initiated only upon reaching the threshold potential. This occurs when the quantity of sodium ions infiltrating the nerve fiber The quantity of potassium ions exiting the fiber exceeds the influx of potassium ions. An abrupt elevation in the membrane potential of a major nerve fiber from –90 millivolts to around –65 millivolts typically triggers a rapid onset of the action potential. A membrane potential of –65 millivolts is identified as the stimulation threshold. An action potential cannot be generated while the membrane remains depolarized from the preceding action potential. Immediately following the initiation of the action potential, the sodium channels undergo inactivation, rendering any excitatory signal supplied to these channels ineffective in opening the inactivation gates. The sole situation that can reactivate them is when the membrane potential reverts to or nearly to the initial resting membrane potential level.
Subsequently, during another brief fraction of a second, the inactivation gates of the channels open, allowing for the initiation of a fresh action potential.
• Absolute refractory phase. An action potential cannot be generated during the absolute refractory period, regardless of the stimulus strength. The refractory period for big myelinated nerve fibers is around 1/2500 of a second, allowing for a maximum transmission of about 2500 impulses per second
. • Relative refractory phase. This interval succeeds the absolute refractory period. During this period, stimuli of greater intensity than usual can activate the nerve fiber, leading to the initiation of an action potential.
