Cellular Pathology - Cell Injury

• Cell injury refers to morphological (appearance), functional and biochemical changes that occurs in a living cell in response to adverse external stimulus or physiological stress.
• The various forms of injury are hypoxia (most common cause). 
• Other causes include physical agents (eg radiation, burns), chemicals, infectious agents, genetic diseases, immunological reactions (eg atopy) and nutritional deficiencies (anemia, vitamin deficiency)
• A cell can respond in several ways to an injurious stimulus. This depends on the nature of injury, duration of exposure and the nature of the cell itself. 
• The various responses include adaptation (cell undergoes modifications to cope with the adverse stimulus), reversible injury (cell can become normal if the injury is mild or the stimulus is removed eg stopping alcohol intake may restore liver cell back to normal), irreversible injury and cell death (injury makes the cell reach a point of no return)

Reversible Injury

• It should be remembered that the nerve cell (neuron) is the cell type most susceptible to injury.
• The mitochondria is the first cellular organelle whose function gets affected; consequently ATP (source of energy) synthesis, necessary for all cellular activities is affected. 
• Due to lack of ATP, membrane permeability is altered, and cell contents leak out
• Cell appears swollen (hydropic swelling) because lack of ATP disrupts the activity of ATP dependent Na+ K+ ATPase pump with resultant influx of Na+ and water into the cell
• Hydropic swelling of cell is the first noticeable microscopic change in a reversibly injured cell 
• Blebs form on the cell surface due to out-pouching of cell membrane to hold more water in the cell
• Protein synthesis is affected due to swelling of endoplasmic reticulum and detachment of ribosomes

Irreversible injury

The cell transitions from reversible injury to an irreversibly injured state due to failure of adaptive mechanisms. Some of the features of irreversible injury include the following

• Rupture of the cell membrane
• Swelling and rupture of mitochondria
• Mitochondrial calcification
• Myelin figures due to curling up of damaged membranes
• Condensation of chromatin (pyknosis)
• Lysis of chromatin (karyolysis)
• Fragmentation of nuclear material (karyorrhexis)

The morphological changes of the cell in irreversible injury is referred to as necrosis. Apoptosis is another morphological pattern of irreversibly injured cell

Calcium and Acute Cell Injury

• Levels of calcium in the cell increase due to influx from the outside (extracellular fluid), damaged mitochondria and the endoplasmic reticulum. 
• The ionized calcium activated several intracellular enzymes such as ATPase, phospholipase, proteases and endonucleases. 
• The action of these enzymes amplifies the effects of hypoxia and increases the cell injury.
• If the injurious stimulus is prolonged, the injury will become irreversible resulting in cell death

Muscle Spindle

Muscle spindles are sensory receptors embedded within the skeletal muscle. They respond rapidly to change in length of the muscle and convey this information to the CNS

Structure of muscle spindle
Each muscle spindle has three important components

• Specialized intrafusal fibres (static and dynamic nuclear bag fibres and static nuclear chain fibre types) with contractile ends and non-contractile centre

• Group Ia and II sensory nerve fibers.

Group Ia sensory fibers (annulospiral endings) that innervate both nuclear bag and nuclear chain fibers. They are dynamic, rapidly adapting sensory fibers and as such are sensitive to the rate of change in length of the muscle. 
Group Ia afferent fibers end directly on α-motor neurons supplying the extrafusal(the main skeletal muscle fibers) fibers of the same muscle. 
Group II sensory fibers innervate nuclear chain fibers. They are slowly adapting sensory nerve fibers that convey information about muscle length rather than the rate of change of length

• Efferent nerves (gamma motor neurons) supplying the contractile ends of the intrafusal fibres. Dynamic γ-motor neurons innervate dynamic nuclear bag fibers; static γ-motor neurons innervate combinations of chain and static bag fibres.

Role of muscle spindle

• Being in parallel with the extrafusal muscle (the main skeletal muscle fibers that are supplied by alpha motor neurons) fibers, when the main muscle is passively stretched, the spindles are also stretched, referred to as “loading the spindle.” This causes reflex contraction of the extrafusal fibers in the skeletal muscle due to firing of the afferent spindle fibers.
• On the contrary, muscle spindle afferent fibers characteristically stop firing when the muscle is made to contract by electrical stimulation of the α-motor neurons to the extrafusal fibers because the muscle shortens while the spindle is unloaded. 

'Thus, the muscle spindle and its reflex connections form a feedback mechanism that functions to maintain muscle length.
  

Fate of Pyruvate

Pyruvate is the end product of aerobic glycolysis. Pyruvate can be metabolized in any of the following ways.

Fate of Pyruvate

• Under aerobic conditions, pyruvate enters the TCA (tricarboxylic citric acid) cycle through pyruvate dehydrogenase where 2 carbon unit Acetyl CoA is formed
• Under anaerobic conditions, the TCA cycle shuts down and the pyruvate is converted to lactate by lactate dehydrogenase

The lactate thus formed is utilized to regenerate NAD+ to keep glycolysis going (in the absence of NAD+ glycolysis cannot occur).

• Other pathways of pyruvate include pyruvate carboxylase (oxaloacetate synthesis)
• Pyruvate decarboxylase reaction - (NAD+ generation to replenish stores) 
• Amination of pyruvate to form alanine
• Pyruvate can be converted to glucose (reverse of glycolysis)
• Pyruvate can be converted to malate and then oxaloacetate

The oxaloacetate formed from pyruvate can in turn be supplied to the TCA cycle to form ATP

Sources or synthesis of pyruvate
Glycolysis
Oxidation of lactate
Deamination of alanine
Decarboxylation of oxaloacetate
From glucogenic amino acids

Carbohydrate Metabolism - Glycolysis

Students should know the most common pathways that occur in metabolism of carbohydrates. The pathways include the following

• Glycolysis  - breakdown of glucose 
• Pyruvate metabolism
• Glycogenesis  - synthesis of glycogen
• Glycogenolysis - breakdown of glycogen
• Pentose phosphate pathway
• Metabolism of fructose
• Metabolism of galactose
• Gluconeogenesis - synthesis of glucose from non-carbohydrate sources
• Cori cycle

Glycolysis

• Glycolysis is the breakdown of glucose to produce energy and occurs in all living cells
• Site of glycolysis in the cell - cytoplasm (important to remember this)
• Glycolysis can occur in the presence of oxygen (aerobic) or in the absence of oxygen (anaerobic) but the end products are different
• Products of aerobic glycolysis - Two pyruvate + 2 ATP + 2 NADH following glycolysis of 1 glucose molecule
• Product of anaerobic glycolysis - 2 ATP + 2 lactate from glycolysis of 1 glucose molecule

The lactate thus formed is utilized to regenerate NAD+ to keep glycolysis going (in the absence of NAD+ glycolysis cannot occur).

Applied Concepts:

• Deficiency of any of the enzymes (most commonly pyruvate kinase) in the glycolytic pathway can lead to hemolysis of RBC as red cells depend on glucose for their energy and will breakdown if their energy requirements are not met
• Skeletal muscle is adapted for anaerobic glycolysis and ATP is provided in the absence of oxygen
• Blood glucose values in a test tube can be lower than actual values of the patient since anaerobic glycolysis can still occur within the test tube with formation of lactate. This can be prevented by adding fluoride which inhibits glycolysis to the test tube 
• Effect of arsenate on glycolysis - prevents ATP formation by uncoupling the GAPDH and PGK reactions. Interestingly it does not inhibit any of the enzymes involved

Axillary artery

Axillary artery

• Continuation of subclavian artery
• Starts at the outer border of the first rib and ends at the inferior of teres major muscle
• It is divided into three parts by pectoralis minor muscle (see figure)

first part





second part (under pectoralis minor)


third part

• The branches of the axillary artery supply the shoulder and the chest wall
• Easily remembered, the first, second and third parts give rise to one, two and three branches respectively.
• First part - superior thoracic A
• Second part - acromiothoracic trunk, lateral thoracic A
• Third part - subscapular A, anterior circumflex A and posterior circumflex A

Applied Anatomy

All the axillary artery branches except the circumflex vessels are seen during axillary dissection of a radical mastectomy for breast cancer

Events In Early Pregnancy

Formation of Blastocyst 

• Fertilization of the ovum by sperm occurs in the fallopian tube. The fertilized ovum is called the zygote. 
• The zygote divides rapidly by mitosis. The rapidly dividing cells of the fertilized ovum resemble a mulberry and this stage is referred to as morula (12-16 cell stage).
• Around the 5th to 8th day of fertilization, the morula is propelled into the uterine cavity by peristalsis and by the cilia that line the inner surface of the fallopian tube. 
• Once in the uterine cavity, secretions of the endometrial glands diffuse through the membrane of the zygote and accumulate in the morula resulting in tiny fluid filled cavities.
• At the same time, the cells of the morula differentiate into a central mass and a peripheral layer (trophoblast)

• The fluid filled cavities in the morula join together to form a single large cavity (blastocele).
• The inner mass of cells is pushed to the periphery and becomes attached to the peripheral trophoblast layer of cells at one place
• This stage of the embryo is referred to as the blastula or blastocyst

Resting Membrane Potential and Action Potential

Resting membrane potential

and

action potential

are both the basis of electrical activity in the cell. It is important to learn these two topics well to get a grasp of cell, nerve and muscle physiology

• Within the body the intracellular compartment is separated from the extracellular compartment by means of the cell membrane.
• Normally, at rest there is a potential difference (measured by electrical charge) across the membrane; usually the inside of the cell is negative relative to the outside. This difference in ionic charges is the resting membrane potential (RMP).
• In excitable tissues such as nerve and muscle, at rest, the membrane potential varies between -60 to -90 mV. 
• When these tissues are excited, there is generation of an action potential due to movement of ions across the membrane and disturbance in the RMP.
• Generation of action potential is necessary to transmit impulses across nerve cells and for initiation of contraction in muscle cells

Creation and maintenance of the RMP - Important facts

•  As mentioned earlier, the intracellular fluid (ICF) is more negative relative to the extracellular fluid (ECF) and this difference creates the RMP. If the negative and positive ions on both sides were equally balanced, there would be no RMP.
• The ions chiefly responsible for creation of the RMP are Na+, K+ and negatively charged intracellular proteins.
• Concentration of Na+ is more in the ECF whereas the ICF has a higher concentration of K+; this concentration is maintained by the Na+ K+ ATPase pump which depends on energy for its activity.
• At resting potential, the cell membrane is naturally more permeable to K+ than Na+ (25-30 times more) by passive diffusion
• The activity of Na+ K+ ATPase pump results in transport of 3 Na+ outside the cell and 2 K+ into the cell; this phenomenon contributes in part to the higher negative of the ICF
• However, the major contribution of maintaining RMP occurs when Na+ and K+ ions diffuse passively along their concentration gradients - namely K+ moves outward and Na+ moves inward. 
• Recollect that at rest the membrane is 30 times more permeable to K+. Therefore at rest the K+ ion is more important in maintaining the RMP

Electrical gradient and Concentration gradient And Equilibrium Potential

• At rest, the concentration of K+ is more inside the cell is higher and thus K+ ions would tend to move outward along the concentration gradient. 
• This movement will continue until the ECF being more positive (electrical gradient) now would repel the K+ ions forcing it to move inwards and slowing down the outward movement of K+ ions. 
• At one point the concentration gradient of K+ will be neutralized by the electrical gradient and movement of K+ ion will cease. This is referred to as equilibrium potential for K+. 
• The equilibrium potential of K+ is -90 mV with the ICF being more negative than the ECF. 

• The same logic can be applied for Na+ also to derive its equilibrium potential. The equilibrium potential of Na+ is +60 mV with the ICF being more positive relative to the ECF.

• However, both Na+ and K+ exist together in the body fluids and in reality there is no equilibrium potential. It is a hypothetical value

• The RMP is closer to the equilibrium potential of K+ due to greater permeability of the membrane to K+ ions. It is normally about -70 mV.

Action Potential Generation

• When nerve or muscle is stimulated, an action potential is generated. During this phenomenon, there is a very brief rapid reversal in the membrane permeability being more permeable to Na+ ions.
• Thus the interior of the cell membrane becomes more positive relative to the outside.
• Action potential is associated with marked changes in the membrane permeability to Na+ and K+ ions. 
• Initially there is some movement of Na+ into the cell making the membrane depolarized just sufficiently to reach the threshold potential (-55 mV).
• At the threshold potential, voltage gated Na+ channels open and the membrane becomes more depolarized and there is a rapid surge of Na+ ions inward with the membrane potential approaching the equilibrium potential of Na+ namely -65 mV causing the spike of the action potential.
• As the membrane potential approaches -65 mV, the voltage gated Na+ channels become inactivated and the voltage gated K+ channels are activated 
• Opening of K+ channels increases the permeability of K+ with repolarization of the membrane. Thus the spike of the action potential is transient and followed by repolarization.
• Delayed closure of the K+ channels will result in a brief state of hyperpolarization where the membrane potential becomes even more negative