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.

SPECIAL SOMATIC AFFERENT (SSA) PATHWAYS

Hearing

The organ of Corti with its sound-sensitive hair cells and basilar membrane are important parts of the sound transducing system for hearing. Mechanical vibrations of the basilar membrane generate membrane potentials in the hair cells which produce impulse patterns in the cochlear portion of the vestibulocochlear nerve (VIII)

Special somatic nerve fibers of cranial nerve VIII relay impulses from the sound receptors (hair cells) in the cochlear nuclei of the brainstem

These are bipolar neurons with cell bodies located in the spiral ganglia of the cochlea.

Vestibular System

The vestibulocochlear nerve serves two quite different functions.

The cochlear portion, conducts sound information to the brain,

The vestibular portion conducts proprioceptive information.

It is the central neural pathways

Special somatic afferent fibers from the hair cells of the macula utriculi and macula sacculi conduct information into the vestibular nuclei on the ipsilateral side of the pons and medulla.

These are bipolar neurons with cell bodies located in the vestibular ganglion.

 Some of the fibers project directly into the ipsilateral cerebellum to terminate in the uvula, flocculus, and nodulus, but most enter the vestibular nuclei and synapse there.

Vision

The visual system receptors are the rods and cones of the retina.

Special somatic afferent fibers of the optic nerve (II) conduct visual signals into the brain

Fibers from the lateral (temporal) retina of either eye terminate in the lateral geniculate body on the same side of the brain as that eye.

SSA II fibers from the medial (nasal) retina of each eye cross over in the optic chiasm to terminate in the contralateral lateral geniculate body.

Area 17 is the primary visual area, which receives initial visual signals.

Neurons from this area project into the adjacent occipital cortex (areas 18 and 19) which is known as the secondary visual area. It is here that the visual signal is fully evaluated.

The visual reflex pathway involving the pupillary light reflex - in which the pupils constrict when a light is shined into the eyes and dilate when the light is removed.

Some SSA II fibers leave the optic tract before reaching the lateral geniculates, terminating in the superior colliculi instead.

From here, short neurons project to the Edinger­Westphal nucleus (an accessory nucleus of III) in the midbrain, which serves as the origin of the preganglionic parasympathetic fibers of the oculomotor nerve (GVE III).

The GVE III fibers in turn project to the ciliary ganglia, from which arise the postganglionic fibers to the sphincter muscles of the iris, which constrict the pupils.

Normal Chemical Composition of Urine

Urine is an aqueous solution of greater than 95% water, with a minimum of these remaining constituents, in order of decreasing concentration:

Urea 9.3 g/L.

Chloride 1.87 g/L.

Sodium 1.17 g/L.

Potassium 0.750 g/L.

Creatinine 0.670 g/L .

Other dissolved ions, inorganic and organic compounds (proteins, hormones, metabolites).

Urine is sterile until it reaches the urethra, where epithelial cells lining the urethra are colonized by facultatively anaerobic gram-negative rods and cocci. Urea is essentially a processed form of ammonia that is non-toxic to mammals, unlike ammonia, which can be highly toxic. It is processed from ammonia and carbon dioxide in the liver.

Bronchitis = Irreversible Bronchioconstriction
 .    Causes - Infection, Air polution, cigarette smoke

a.    Primary Defect = Enlargement & Over Activity of Mucous Glands, Secretions very viscous
b.    Hypertrophy & hyperplasia, Narrows & Blocks bronchi, Lumen of airway, significantly narrow
c.    Impaired Clearance by mucocillary elevator
d.    Microorganism retension in lower airways,Prone to Infectious Bronchitis, Pneumonia
e.    Permanent Inflamatory Changes IN epithelium, Narrows walls, Symptoms, Excessive sputum, coughing
f.    CAN CAUSE EMPHYSEMA

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.

Phases of cardiac cycle :

1. Early diastole ( also called the atrial diastole , or complete heart diastole) : During this phase :

- Atria are  relaxed
- Ventricles are relaxed
- Semilunar valves are closed
- Atrioventricular valves are open
During this phase the blood moves passively from the venous system into the ventricles ( about 80 % of blood fills the ventricles during this phase.

2. Atrial systole : During this phase :

- Atria are contracting
- Ventricles are relaxed
- AV valves are open
- Semilunar valves are closed
- Atrial pressure increases.the a wave of atrial pressure appears here.
- P wave of ECG starts here
- intraventricular pressure increases due to the rush of blood then decrease due to continuous relaxation of ventricles.

The remaining 20% of blood is moved to fill the ventricles during this phase , due to atrial contraction.

3. Isovolumetric contraction : During this phase :

- Atria are relaxed
- Ventricles are contracting
- AV valves are closed
- Semilunar valves are closed
- First heart sound
- QRS complex.
The ventricular fibers start to contract during this phase , and the intraventricular pressure increases. This result in closing the AV valves , but the pressure is not yet enough to open the semilunar valves , so the blood volume remain unchanged , and the muscle fibers length also remain unchanged , so we call this phase as isovolumetric contraction ( iso : the same , volu= volume , metric= length).

4. Ejection phase : Blood is ejected from the ventricles into the aorta and pulmonary artery .

During this phase :

- Ventricles are contracting
- Atria are relaxed
- AV valves are closed
- Semilunar valves are open
- First heart sound
- Intraventricular pressure is increased , due to continuous contraction
- increased aortic pressure .
- T wave starts.

5. Isovolumetric relaxation:  This phase due to backflow of blood in aorta and pulmonary system after the ventricular contraction is up and the ventricles relax . This backflow closes the semilunar valves .

During this phase :

- Ventricles are relaxed
- Atrial are relaxed
- Semilunar valves are closed .
- AV valves are closed.
- Ventricular pressure fails rapidly
- Atrial pressure increases due to to continuous venous return. the v wave appears here. 
- Aortic pressure : initial sharp decrease due to sudden closure of the semilunar valve ( diacrotic notch) , followed by secondary rise in pressure , due to elastic recoil of the aorta ( diacrotic wave)  .
- T wave ends in this phase