GENERAL VISCERAL AFFERENT (GVA) PATHWAYS

Pain and Pressure Sensation via the Spinal Cord

Visceral pain receptors are located in peritoneal surfaces, pleural membranes, the dura mater, walls of arteries, and the walls of the GI tube.

Nociceptors in the walls of the GI tube are particularly sensitive to stretch and overdistension.

General visceral nociceptors conduct signals into the spinal cord over the monopolar neurons of the posterior root ganglia. They terminate in laminae III and IV of the posterior horn as do the pain and temperature pathways of the GSA system , their peripheral processes reach the visceral receptors via the gray rami communicantes and ganglia of the sympathetic chain

Second-order neurons from the posterior horn cross in the anterior white commissure and ascend to the thalamus in the anterior and lateral spinothalamic tracts,

Projections from the VPL of the thalamus relay signals to the sensory cortex.

The localization of visceral pain is relatively poor, making it difficult to tell the exact source of the stimuli.

Blood Pressure, Blood Chemistry, and Alveolar Stretch Detection

The walls of the aorta and the carotid sinuses contain special baroreceptors (pressure receptors) which respond to changes in blood pressure. These mechanoreceptors are the peripheral endings of GVA fibers of the glossopharyngeal (IX) and vagus (X) nerves

The GVA fibers from the carotid sinus baroreceptors enter the solitary tract of the brainstem and terminate in the vasomotor center of the medulla (Fig-14). This is the CNS control center for cardiovascular activity.

Stretch receptors in the alveoli of the lungs conduct information concerning rhythmic alveolar inflation and deflation over GVA X fibers to the solitary tract and then to the respiratory center of the brainstem. This route is an important link in the Hering-Breuer reflex, which helps to regulate respiration.

Carotid body chemoreceptors, sensitive to changes in blood PO2 and, to a lesser extent, PCO2 and pH, conduct signals to both the vasomotor and respiratory centers over GVA IX nerve fibers

GVA X fibers conduct similar information from the aortic chemoreceptors to both centers

Biological Functions are Extremely Sensitive to pH

• H+ and OH- ions get special attention because they are very reactive
• Substance which donates H+ ions to solution = acid
• Substance which donates OH- ions to solution = base
• Because we deal with H ions over a very wide range of concentration, physiologists have devised a logarithmic unit, pH, to deal with it
  • pH = - log [H⁺]
  • [H⁺] is the H ion concentration in moles/liter
  • Because of the way it is defined a high pH indicates low H ion and a low pH indicates high H ion- it takes a while to get used to the strange definition
  • Also because of the way it is defined, a change of 1 pH unit means a 10X change in the concentration of H ions
    • If pH changes by 2 units the H+ concentration changes by 10 X 10 = 100 times

• Human blood pH is 7.4
  • Blood pH above 7.4 = alkalosis
  • Blood pH below 7.4 = acidosis
• Body must get rid of ~15 moles of potential acid/day (mostly CO₂)
  • CO₂ reacts with water to form carbonic acid (H₂CO₃)
  • Done mostly by lungs & kidney
• In neutralization H⁺ and OH^- react to form water
• If the pH changes charges on molecules also change, especially charges on proteins
  • This changes the reactivity of proteins such as enzymes
• Large pH changes occur as food passes through the intestines.

The hypothalamus is a region of the brain. It secretes a number of hormones.

• Thyrotropin-releasing hormone (TRH)
• Gonadotropin-releasing hormone (GnRH)
• Growth hormone-releasing hormone (GHRH)
• Corticotropin-releasing hormone (CRH)
• Somatostatin
• Dopamine

All of these are released into the blood, travel immediately to the anterior lobe of the pituitary, where they exert their effects.

Two other hypothalamic hormones:

• Antidiuretic hormone (ADH) and
• Oxytocin

travel in neurons to the posterior lobe of the pituitary where they are released into the circulation.

Carbohydrates:

• about 3% of the dry mass of a typical cell
• composed of carbon, hydrogen, & oxygen atoms (e.g., glucose is C₆H₁₂O₆)
• an important source of energy for cells
• types include:
  • monosaccharide (e.g., glucose) - most contain 5 or 6 carbon atoms
  • disaccharides
    • 2 monosaccharides linked together
    • Examples include sucrose (a common plant disaccharide is composed of the monosaccharides glucose and fructose) & lactose (or milk sugar; a disaccharide composed of glucose and the monosaccharide galactose)
  • polysaccharides
    • several monosaccharides linked together

Examples include starch (a common plant polysaccharide made up of many glucose molecules) and glycogen (commonly stored in the liver)

CNS PROTECTION

- Bones of the Skull       Frontal, Temporal, Parietal, Sphenoid, Occipital

- Cranial Meninges         Dura mater, Arachnoid Space, Pia mater

- Cerebrospinal Fluid

Secreted by Chroid Plexi in Ventricles

Circulation through ventricles and central canal

Lateral and Median apertures from the 4th ventricle into the subarachnoid space

Arachnoid villi of the superior sagittal sinus return CSF to the venous circulation

Hydrocephalic Condition, blockage of the mesencephalic aqueduct, backup of CSF, Insertion of a shunt to drain the excess CSF

• it's the individual pressure exerted independently by a particular gas within a mixture of gasses. The air we breath is a mixture of gasses: primarily nitrogen, oxygen, & carbon dioxide. So, the air you blow into a balloon creates pressure that causes the balloon to expand (& this pressure is generated as all the molecules of nitrogen, oxygen, & carbon dioxide move about & collide with the walls of the balloon). However, the total pressure generated by the air is due in part to nitrogen, in part to oxygen, & in part to carbon dioxide. That part of the total pressure generated by oxygen is the 'partial pressure' of oxygen, while that generated by carbon dioxide is the 'partial pressure' of carbon dioxide. A gas's partial pressure, therefore, is a measure of how much of that gas is present (e.g., in the blood or alveoli). 
   
• the partial pressure exerted by each gas in a mixture equals the total pressure times the fractional composition of the gas in the mixture. So, given that total atmospheric pressure (at sea level) is about 760 mm Hg and, further, that air is about 21% oxygen, then the partial pressure of oxygen in the air is 0.21 times 760 mm Hg or 160 mm Hg.