The Nerve Impulse

When a nerve is stimulated the resting potential changes. Examples of such stimuli are pressure, electricity, chemicals, etc. Different neurons are sensitive to different stimuli(although most can register pain). The stimulus causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is called the "action potential." In order for an action potential to occur, it must reach threshold. If threshold does not occur, then no action potential can occur. This moving change in polarity has several stages:

Depolarization

The upswing is caused when positively charged sodium ions (Na+) suddenly rush through open sodium gates into a nerve cell. The membrane potential of the stimulated cell undergoes a localized change from -55 millivolts to 0 in a limited area. As additional sodium rushes in, the membrane potential actually reverses its polarity so that the outside of the membrane is negative relative to the inside. During this change of polarity the membrane actually develops a positive value for a moment(+30 millivolts). The change in voltage stimulates the opening of additional sodium channels (called a voltage-gated ion channel). This is an example of a positive feedback loop.

Repolarization

The downswing is caused by the closing of sodium ion channels and the opening of potassium ion channels. Release of positively charged potassium ions (K+) from the nerve cell when potassium gates open. Again, these are opened in response to the positive voltage--they are voltage gated. This expulsion acts to restore the localized negative membrane potential of the cell (about -65 or -70 mV is typical for nerves).

Hyperpolarization

When the potassium ions are below resting potential (-90 mV). Since the cell is hyper polarized, it goes to a refractory phrase.

Refractory phase

The refractory period is a short period of time after the depolarization stage. Shortly after the sodium gates open, they close and go into an inactive conformation. The sodium gates cannot be opened again until the membrane is repolarized to its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and potassium ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized. This refractory area explains why action potentials can only move forward from the point of stimulation.

Factors that affect sensitivity and speed

Sensitivity

Increased permeability of the sodium channel occurs when there is a deficit of calcium ions. When there is a deficit of calcium ions (Ca+2) in the interstitial fluid, the sodium channels are activated (opened) by very little increase of the membrane potential above the normal resting level. The nerve fiber can therefore fire off action potentials spontaneously, resulting in tetany. This could be caused by the lack of hormone from parathyroid glands. It could also be caused by hyperventilation, which leads to a higher pH, which causes calcium to bind and become unavailable.

Speed of Conduction

This area of depolarization/repolarization/recovery moves along a nerve fiber like a very fast wave. In myelinated fibers, conduction is hundreds of times faster because the action potential only occurs at the nodes of Ranvier (pictured below in 'types of neurons') by jumping from node to node. This is called "saltatory" conduction. Damage to the myelin sheath by the disease can cause severe impairment of nerve cell function. Some poisons and drugs interfere with nerve impulses by blocking sodium channels in nerves. See discussion on drug at the end of this outline.

Characteristics of Facilitated Diffusion & Active Transport - both require the use of carriers that are specific to particular substances (that is, each type of carrier can 'carry' one type of substance) and both can exhibit saturation (movement across a membrane is limited by number of carriers & the speed with which they move materials

Heart is a hollow muscular organ , that is located in the middle mediastinum  between the two bony structures of the sternum and the vertebral column ( a very important location for applying Cardiopulmonary Resuscitation - CPR- ) .
It has a shape of clenched fist , which weighs about 300 grams ( with mild variation between male and female ).
  Heart has an apex that is anteriorly , inferiorly , and leftward oriented , and a base , that is posteriorly , superiorly and rightward oriented   .
 In addition to its apex and base the heart has anterior , posterior and left surfaces.
 
 The wall of the heart is composed of three layers :
 
1. Endocardium : The innermost layer , which lines the heart chambers and is in direct contact with the blood . It is composed of endothelial cells that are similar to those , that line the blood vessels , and of connective tissue too. 
 Endocardium has a smooth surface that prevents blood clotting, as it ensures laminar blood flow .

 Clinical Physiology 
 Endocarditis is the inflammation of the endocardium , which is resistant to antibiotic treatment and difficult to cure.Endocarditis usually involves heart valves and chordae tendineae too.

 2. Myocardium  : The middle layer of the cardiac wall . It is the thickest among the three layers , and is composed of two types of cardiac muscles :
a. contractile muscle cells (form about 98-99% of the cardiac muscle ) .
 b- non-contractile muscle cells ( form about 1-2 % of the cardiac muscles and are the cells that form excitatory-conductive system of the heart).
 The cardiac muscle cells are similar to the skeletal muscles in that they are striated , but similar to the smooth muscles in being involuntary and connected to each others via gap junctions , that facilitate conduction of electrical potential from one cell to the others. Desmosomes adhere cardiac muscle cells to each others .

 3- Epicardium :  is the outermost and protective layer of the heart . It is composed of connective tissue , and form the inner layer of the pericardium ( visceral pericardium - see bellow).

 Pericardium: 
The heart is surrounded by a fluid-fill sac , which is known as pericardium . Pericardium is composed of two layers ( doubled layer membrane ) , between which a fluid-fill pericardial cavity exist .

 The outer layer is called fibrous pericardium , while the inner layer is called serous pericardium , which is subdivided into parietal pericardium and visceral pericardium . The visceral pericardium is the previously mentioned outermost layer of heart ( epicardium) .
Pericardial sac plays an important role in protection of heart from external hazards and infections , as it fixes the heart and limits its motion. It also prevents excessive dilation of the heart.

Clinical physiology: 

When there is excessive fluid in the pericardial cavity as a result of pericardial effusion , a cardiac tamponade will develop . cardiac tamponade means compression of the heart within the pericardial sac , which will prevent the relaxation of the heart ( heart will not be able to fully expand ) , and thus the circulating blood volume will be decreased (obstructive shock) . This is a life threatening situation which has to be urgently cured by  pericardiocentesis . 

Chambers of the heart : 

Heart has four chambers : two atria and two ventricles . The two right and left atria are separated from the two ventricles by the fibrous skeleton , which involves the right ( tricuspid ) and left ( bicuspid ) valves. Right and left atria are separated from each other by the interatrial  septum .
The two ventricles are separated by the interventricular septum.Interventricular septum is muscular in its lower thick part and fibrous in its upper thin part.
The two atria holds the blood returning from the veins and empty it only in a given right moment into the ventricles. Ventricles pump the blood into the arteries . 

Heart valves : 

There are four valves in the heart : Two atrioventricular valves and two semi-lunar valves:
1. Atrioventricular ( AV ) valves: These valves are found between the atria and ventricles , depending on the number of  the leaflets , the right atrioventricular valve is also called tricuspid valve (has three leaflets ) , while the left one is called bicuspid valve (has two leaflets ) . The shape of the bicuspid valve is similar to the mitre of bishop , so it is also called the mitral valve.
The leaflets of the valves are attached to fibrous threads (composed of collagen fibers ) , known as chordae tendineae , which from their side are attached to papillary muscles in the ventricles. These valves prevent backward flow of blood from ventricles during the systole. 

2. Semi-lunar valves : 

These valves are located on the base of the arteries ( aorta and pulmonary artery ) . They prevent the backward flow of blood from the arteries into ventricles.
The structure of the semilunar valves is quite different from that of the AV valves , as they have crescent-shaped cusps that do not have chorda tendinea , instead these cusps are like pockets which are filled of blood when it returns to the ventricles from the lumen of arteries during the diastole  , so they get closed and prevent the backward flow of blood.

The Posterior Lobe

The posterior lobe of the pituitary releases two hormones, both synthesized in the hypothalamus, into the circulation.

• Antidiuretic Hormone (ADH).
  ADH is a peptide of 9 amino acids. It is also known as arginine vasopressin. ADH acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood.
  • A deficiency of ADH
    • leads to excessive loss of urine, a condition known as diabetes  nsipidus.
• Oxytocin
  Oxytocin is a peptide of 9 amino acids. Its principal actions are:
  • stimulating contractions of the uterus at the time of birth
  • stimulating release of milk when the baby begins to suckle

Reflexes

A reflex is a direct connection between stimulus and response, which does not require conscious thought. There are voluntary and involuntary reflexes.

The Stretch Reflex:

The stretch reflex in its simplest form involves only 2 neurons, and is therefore sometimes called a 2-neuron reflex. The two neurons are a sensory and a motor neuron. The sensory neuron is stimulated by stretch (extension) of a muscle. Stretch of a muscle normally happens when its antagonist contracts, or artificially when its tendon is stretched, as in the knee jerk reflex. Muscles contain receptors called muscle spindles. These receptors respond to the muscles's stretch. They send stimuli back to the spinal cord through a sensory neuron which connects directly to a motor neuron serving the same muscle. This causes the muscle to contract, reversing the stretch. The stretch reflex is important in helping to coordinate normal movements in which antagonistic muscles are contracted and relaxed in sequence, and in keeping the muscle from overstretching.

Since at the time of the muscle stretch its antagonist was contracting, in order to avoid damage it must be inhibited or tuned off in the reflex. So an additional connection through an interneuron sends an inhibitory pathway to the antagonist of the stretched muscle - this is called reciprocal inhibition.

The Deep Tendon Reflex:

Tendon receptors respond to the contraction of a muscle. Their function, like that of stretch reflexes, is the coordination of muscles and body movements. The deep tendon reflex involves sensory neurons, interneurons, and motor neurons. The response reverses the original stimulus therefore causing relaxation of the muscle stimulated. In order to facilitate that the reflex sends excitatory stimuli to the antagonists causing them to contract - reciprocal activation.

The stretch and tendon reflexes complement one another. When one muscle is stretching and stimulating the stretch reflex, its antagonist is contracting and stimulating the tendon reflex. The two reflexes cause the same responses thus enhancing one another.

The Crossed Extensor Reflex -

The crossed extensor reflex is just a withdrawal reflex on one side with the addition of inhibitory pathways needed to maintain balance and coordination. For example, you step on a nail with your right foot as you are walking along. This will initiate a withdrawal of your right leg. Since your quadriceps muscles, the extensors, were contracting to place your foot forward, they will now be inhibited and the flexors, the hamstrings will now be excited on your right leg. But in order to maintain your balance and not fall down your left leg, which was flexing, will now be extended to plant your left foot (e.g. crossed extensor). So on the left leg the flexor muscles which were contracting will be inhibited, and the extensor muscles will be excited

1 - Passive processes - require no expenditure of energy by a cell:

• Simple diffusion = net movement of a substance from an area of high concentration to an area of low concentration. The rate of diffusion is influenced by:
  • concentration gradient
  • cross-sectional area through which diffusion occurs
  • temperature
  • molecular weight of a substance
  • distance through which diffusion occurs
• Osmosis = diffusion of water across a semi permeable membrane (like a cell membrane) from an area of low solute concentration to an area of high solute concentration

• Facilitated diffusion = movement of a substance across a cell membrane from an area of high concentration to an area of low concentration. This process requires the use of 'carriers' (membrane proteins). In the example below, a ligand molecule (e.g., acetylcholine) binds to the membrane protein. This causes a conformational change or, in other words, an 'opening' in the protein through which a substance (e.g., sodium ions) can pass.

2 - Active processes - require the expenditure of energy by cells:

• Active transport = movement of a substance across a cell membrane from an area of low concentration to an area of high concentration using a carrier molecule
• Endo- & exocytosis - moving material into (endo-) or out of (exo-) cell in bulk form