Excitability ( Bathmotropism ) : Excitability means the ability of cardiac muscle to respond to signals. Here we are talking about contractile muscle cells that are excited by the excitatory conductive system and generate an action potential.

Cardiac action potential is similar to action potential in nerve and skeletal muscle tissue , with one difference , which is the presence of plateau phase . Plateau phase is unique for cardiac muscle cells .
The  resting membrane potential for cardiac muscle is about -80 mV.
When the cardiac muscle is stimulated an action potential is generated . The action potential in cardiac muscle is composed of four phases , which are :

1. Depolarization phase (Phase 0 ) :

A result of opening of sodium channels , which increase the permeability to sodium , which will lead to a rapid sodium influx into the cardiac muscle cell.

2. Repolarization : Repolarization in cardiac muscle is slow and triphasic :

a. Phase 1 (early partial repolarization ) : A small fast repolarization , results from potassium eflux and chloride influx.
b. Phase 2 ( Plateau ) : After the early partial depolarization , the membrane remains  depolarized , exhibiting a plateau , which is a unique phase for the cardiac muscle cell. Plateau is due to opening of slow calcium-sodium channels , delay closure of sodium channels , and to decreased potassium eflux.
c. Phase 3  ( Rapid repolarization) :  opening of potassium channels and rapid eflux of potassium.
d. Phase 4 ( Returning to resting level) in other words : The phase of complete repolarization. This due to the work of sodium-potassium pump.

Absolute refractory period:

Coincides wit phase 0,phase1 , and phase 2 . During this period , excitability of the heart is totally abolished . This prevents tetanization of the cardiac muscle and enables the heart to contract and  relax to be filled by blood ..

Relative refractory period : 

Coincides with the rapid repolarization and allows the excitability to be gradually recovered .
Excitation contraction relationship : Contraction of cardiac muscle starts after depolarization and continues about 1.5 time as long as the duration of the action potential and reaches its maximum at the end of the plateau. Relaxation of the muscle starts with the early partial repolarization.

Factors , affecting excitability of cardiac muscle:

I. Positive bathmotropic effect :

1. Sympathetic stimulation : It increase the heart , and thus reduces the duration of the action potentia; . This will shorten the duration of the absolute refractory period , and thus increase the excitability .
2.  Drugs : Catecholamines and  xanthines derivatives .
3. Mild hypoxia and mild ischemia
4. Mild hyperkalemia as it decreases the K+ efflux and opens excess Na+ channels .
5. Hypocalcemia

II. Negative bathmotropic effect :

1. Parasympathetic stimulation: The negative bathmotropic effect is limited to the atrial muscle excitability , because there is no parasympathetic innervation for the ventricles. Parasympathetic stimulation decreases the heart rate , and thus increases the duration of cardiac action potential and thus increases the duration of the absolute refractory period.
2. moderate to severe hypoxia
3. hyponatremia , hypercalcemia , and severe hyperkalemia.

Clinical Physiology : Extrasystole is a pathological situation , due to abnormal impulses , arising from ectopic focus .It is expressed as an abnormal systole that occur during the early diastole .
Extrasystole  is due to a rising of excitability above the normal , which usually occurs after the end of the relative refractory period ( read about staircase phenomenon of Treppe)

Glomerular filtration

Kidneys receive about 20% of cardiac output , this is called Renal Blood Flow (RBF) which is approximatley 1.1 L of blood. Plasma in this flow is about 625 ml . It is called Renal Plasma Flow (RPF) .
About 20 % of Plasma entering the glomerular capillaries is filtered into the Bowman`s capsule .
Glomerular filtration rate is about 125 ml/min ( which means 7.5 L/hr and thus 180 L/day) This means that the kidney filters about 180 liters of plasma every day.

The urine flow is about 1ml/min ( about 1.5 liter /day) This means that kidney reabsorbs about 178.5 liters every day .

Filtration occurs through the filtration unit , which includes :

1- endothelial cells of glomerular capillaries , which are fenestrated . Fenestrae are quite small so they prevent filtration of blood cells and most of plasma proteins .

2- Glomerular basement membrane : contains proteoglycan that is negatively charged and repels the negatively charged plasma proteins that may pass the fenestrae due to their small molecular weight like albumin . so the membrane plays an important role in impairing filtration of albumin .

3- Epithelial cells of Bowman`s capsule that have podocytes , which interdigitate to form slits .

Many forces drive the glomerular filtration , which are :

1- Hydrostatic pressure of the capillary blood , which favours filtration . It is about 55 mmHg .

2- Oncotic pressure of the plasma proteins in the glomerular capillary ( opposes filtration ) . It is about 30 mm Hg .

3- Hydrostatic pressure of the Bowman`s capsule , which also opposes filtration. It is about 15 mmHg .

The net pressure is as follows :

Hydrostatic pressure of glomerular capillaries - ( Oncotic pressure of glomerular capillaries + Hydrostatic pressure of the Bowman capsule):
55-(35+10)
=55-45
=10 mmHg .

Te glomerular filtration rate does not depend only on the net pressure , but also on an other value , known as filtration coefficient ( Kf) . The later depends on the surface area of the glomerular capillaries and the hydraulic conductivity of the glomerular capillaries.
 

Events in Muscle Contraction - the sequence of events in crossbridge formation:

1) In response to Ca²⁺ release into the sarcoplasm, the troponin-tropomyosin complex removes its block from actin, and the myosin heads immediately bind to active sites.

2) The myosin heads then swivel, the Working Stroke, pulling the Z-lines closer together and shortening the sarcomeres. As this occurs the products of ATP hydrolysis, ADP and Pi, are released.

3) ATP is taken up by the myosin heads as the crossbridges detach. If ATP is unavailable at this point the crossbridges cannot detach and release. Such a condition occurs in rigor mortis, the tensing seen in muscles after death, and in extreme forms of contracture in which muscle metabolism can no longer provide ATP.

4) ATP is hydrolyzed and the energy transferred to the myosin heads as they cock and reset for the next stimulus.

Excitation-Contraction Coupling: the Neuromuscular Junction  

Each muscle cell is stimulated by a motor neuron axon. The point where the axon terminus contacts the sarcolemma is at a synapse called the neuromuscular junction. The terminus of the axon at the sarcolemma is called the motor end plate. The sarcolemma is polarized, in part due to the unequal distribution of ions due to the Sodium/Potassium Pump.

1) Impulse arrives at the motor end plate (axon terminus) causing  Ca²⁺ to enter the axon.

2) Ca²⁺ binds to ACh vesicles causing them to release the ACh (acetylcholine) into the synapse by exocytosis. 

3) ACH diffuses across the synapse to bind to receptors on the sarcolemma. Binding of ACH to the receptors opens chemically-gated ion channels causing Na⁺ to enter the cell producing depolarization.

4) When threshold depolarization occurs, a new impulse (action potential) is produced that will move along the sarcolemma. (This occurs because voltage-gated ion channels open as a result of the depolarization -

5) The sarcolemma repolarizes:

a) K⁺ leaves cell (potassium channels open as sodium channels close) returning positive ions to the outside of the sarcolemma. (More K⁺ actually leaves than necessary and the membrane is hyperpolarized briefly. This causes the relative refractory period) (b) Na⁺/K⁺ pump eventually restores resting ion distribution.  The  Na⁺/K⁺ pump is very slow compared to the movement of ions through the ion gates. But a muscle can be stimulated thousands of times before the ion distribution is substantially affected.

6) ACH broken down by ACH-E (a.k.a. ACHase, cholinesterase). This permits the receptors to respond to another stimulus. 

Excitation-Contraction Coupling:

1) The impulse (action potential) travels along the sarcolemma. At each point the voltaged-gated Na+ channels open to cause depolarization, and then the K+ channels open to produce repolarization.

2) The impulse enters the cell through the T-tublules, located at each Z-disk, and reach the sarcoplasmic reticulum (SR), stimulating it.

3) The SR releases Ca²⁺ into the sarcoplasm, triggering the muscle contraction as previously discussed. 

4) Ca²⁺ is pumped out of the sarcoplasm by the SR and another stimulus will be required to continue the muscle contraction.

Urine is a waste byproduct formed from excess water and metabolic waste molecules during the process of renal system filtration. The primary function of the renal system is to regulate blood volume and plasma osmolarity, and waste removal via urine is essentially a convenient way that the body performs many functions using one process. Urine formation occurs during three processes:

Filtration

Reabsorption

Secretion

Filtration

During filtration, blood enters the afferent arteriole and flows into the glomerulus where filterable blood components, such as water and nitrogenous waste, will move towards the inside of the glomerulus, and nonfilterable components, such as cells and serum albumins, will exit via the efferent arteriole. These filterable components accumulate in the glomerulus to form the glomerular filtrate.

Normally, about 20% of the total blood pumped by the heart each minute will enter the kidneys to undergo filtration; this is called the filtration fraction. The remaining 80% of the blood flows through the rest of the body to facilitate tissue perfusion and gas exchange.

Reabsorption

The next step is reabsorption, during which molecules and ions will be reabsorbed into the circulatory system. The fluid passes through the components of the nephron (the proximal/distal convoluted tubules, loop of Henle, the collecting duct) as water and ions are removed as the fluid osmolarity (ion concentration) changes. In the collecting duct, secretion will occur before the fluid leaves the ureter in the form of urine.

Secretion

During secretion some substances±such as hydrogen ions, creatinine, and drugs—will be removed from the blood through the peritubular capillary network into the collecting duct. The end product of all these processes is urine, which is essentially a collection of substances that has not been reabsorbed during glomerular filtration or tubular reabsorbtion.

SPECIAL VISCERAL AFFERENT (SVA) PATHWAYS

Taste

Special visceral afferent (SVA) fibers of cranial nerves VII, IX, and X conduct signals into the solitary tract of the brainstem, ultimately terminating in the nucleus of the solitary tract on the ipsilateral side.

Second-order neurons cross over and ascend through the brainstem in the medial lemniscus to the VPM of the thalamus.

Thalamic projections to area 43 (the primary taste area) of the postcentral gyrus complete the relay.

SVA VII fibers conduct from the chemoreceptors of taste buds on the anterior twothirds of the tongue, while SVA IX fibers conduct taste information from buds on the posterior one-third of the tongue.

SVA X fibers conduct taste signals from those taste cells located throughout the fauces.

Smell

The smell-sensitive cells (olfactory cells) of the olfactory epithelium project their central processes through the cribiform plate of the ethmoid bone, where they synapse with mitral cells. The central processes of the mitral cells pass from the olfactory bulb through the olfactory tract, which divides into a medial and lateral portion The lateral olfactory tract terminates in the prepyriform cortex and parts of the amygdala of the temporal lobe.

These areas represent the primary olfactory cortex. Fibers then project from here to area 28, the secondary olfactory area, for sensory evaluation. The medial olfactory tract projects to the anterior perforated sub­stance, the septum pellucidum, the subcallosal area, and even the contralateral olfactory tract.

Both the medial and lateral olfactory tracts contribute to the visceral reflex pathways, causing the viscerosomatic and viscerovisceral responses.

(RDS) Respiratory distress of Newborn
1.    hyaline membrane disease of the new born
2.    decrease in surfactant, Weak, Abnormal complience of chest wall
3.    Small alveoli, difficult to inflate, Alveoli tent to collapse, many of varied sizes
4.    decrease in O2 diffusion area, lung difficult to expand, in compliance