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.

1. Automatic control (sensory) of respiration is in - brainstem (midbrain) 

2. Behavioral/voluntary control is in - the cortex

3. Alveolar ventilation -the amount of atmospheric air that actually reaches the alveolar per breath and that can participate in the exchange of gasses between alveoli and blood

4. Only way to increase gas exchange in alveolar capillaries - perfusion-limited gas exchange 

5. Pulmonary ventiliation not effected by - concentration of bicarbonate ions

6. Central chemoreceptors - medulla -  CO2, O2 and H+ concentrations

7. Peripheral chemoreceptors - carotid and aortic bodies- PO2, PCO2 and pH 

8. Major stimulus for respiratory centers - arterial PCO2 

9. Rhythmic breathing depends on 
1. continuous (tonic) inspiratory drive from DRG (dorsal respiratory group)
2. intermittent (phasic) expiratory input from cerebrum, thalamus, cranial nerves and ascending spinal cord sensory tracts

10. Primary site for gas exchange - type I epithelial cells for alveoli

Blood is a liquid tissue. Suspended in the watery plasma are seven types of cells and cell fragments.

• red blood cells (RBCs) or erythrocytes
• platelets or thrombocytes
• five kinds of white blood cells (WBCs) or leukocytes
  • Three kinds of granulocytes
    • neutrophils
    • eosinophils
    • basophils
  • Two kinds of leukocytes without granules in their cytoplasm
    • lymphocytes
    • monocytes

Alveolar Ventilation: is the volume of air of new air , entering the alveoli and adjacent gas exchange areas each minute . It equals to multiplying of respiratory rate by ( tidal volume - dead space).
Va = R rate X (TV- DsV)
     = 12 X ( 500-150)
     = 4200 ml of air.

An anti-diruetic is a substance that decreases urine volume, and ADH is the primary example of it within the body. ADH is a hormone secreted from the posterior pituitary gland in response to increased plasma osmolarity (i.e., increased ion concentration in the blood), which is generally due to an increased concentration of ions relative to the volume of plasma, or decreased plasma volume.

The increased plasma osmolarity is sensed by osmoreceptors in the hypothalamus, which will stimulate the posterior pituitary gland to release ADH. ADH will then act on the nephrons of the kidneys to cause a decrease in plasma osmolarity and an increase in urine osmolarity.

ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water. This effect causes increased water reabsorption and retention and decreases the volume of urine produced relative to its ion content.

After ADH acts on the nephron to decrease plasma osmolarity (and leads to increased blood volume) and increase urine osmolarity, the osmoreceptors in the hypothalamus will inactivate, and ADH secretion will end. Due to this response, ADH secretion is considered to be a form of negative feedback.

The nephron of the kidney is involved in the regulation of water and soluble substances in blood.

A Nephron

A nephron is the basic structural and functional unit of the kidneys that regulates water and soluble substances in the blood by filtering the blood, reabsorbing what is needed, and excreting the rest as urine.

 Its function is vital for homeostasis of blood volume, blood pressure, and plasma osmolarity.

It is regulated by the neuroendocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.

The Glomerulus

The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. Here, fluid and solutes are filtered out of the blood and into the space made by Bowman's capsule.

A group of specialized cells known as juxtaglomerular apparatus (JGA) are located around the afferent arteriole where it enters the renal corpuscle. The JGA secretes an enzyme called renin, due to a variety of stimuli, and it is involved in the process of blood volume homeostasis.

The Bowman's capsule surrounds the glomerulus. It is composed of visceral (simple squamous epithelial cells; inner) and parietal (simple squamous epithelial cells; outer) layers.

Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances. However, in some injuries they may be able to pass through and can cause blood and protein content to enter the urine, which is a sign of problems in the kidney.

Proximal Convoluted Tubule

The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place. Water reabsorption in the proximal convoluted tubule occurs due to both passive diffusion across the basolateral membrane, and active transport from Na+/K+/ATPase pumps that actively transports sodium across the basolateral membrane.

Water and glucose follow sodium through the basolateral membrane via an osmotic gradient, in a process called co-transport. Approximately 2/3rds of water in the nephron and 100% of the glucose in the nephron are reabsorbed by cotransport in the proximal convoluted tubule.

Fluid leaving this tubule generally is unchanged due to the equivalent water and ion reabsorption, with an osmolarity (ion concentration) of 300 mOSm/L, which is the same osmolarity as normal plasma.

The Loop of Henle

The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed, which increases fluid osmolarity to about 1200 mOSm/L. In contrast, the ascending limb of Henle's loop is impermeable to water but highly permeable to ions, which causes a large drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L.

Distal Convoluted Tubule and Collecting Duct

The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH.

Normally, it is impermeable to water and permeable to ions, driving the osmolarity of fluid even lower. However, anti-diuretic hormone (secreted from the pituitary gland as a part of homeostasis) will act on the distal convoluted tubule to increase the permeability of the tubule to water to increase water reabsorption. This example results in increased blood volume and increased blood pressure. Many other hormones will induce other important changes in the distal convoluted tubule that fulfill the other homeostatic functions of the kidney.

The collecting duct is similar in function to the distal convoluted tubule and generally responds the same way to the same hormone stimuli. It is, however, different in terms of histology. The osmolarity of fluid through the distal tubule and collecting duct is highly variable depending on hormone stimulus. After passage through the collecting duct, the fluid is brought into the ureter, where it leaves the kidney as urine.