Dr Meducci vet's blog

Carbohydrate metabolism

3:45 PM | Labels: ATP, carbohydrate metabolism, ETC, glycolysis, kreb cycle

Maybe it's easy for some of you because all we talked about is just the fate of carbohydrate that eventually becomes ATP, energy, or fat. But not all could remember the biochemical process of each product (unless if you're a nutritionist).

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In animals, the most important carbohydrate is glucose; so much so, that the level of glucose is used as the main control for the central metabolic hormone, insulin. Starch, and cellulose in a few organisms (e.g., termites, ruminants, and some bacteria), both being glucose polymers, are disassembled during digestion and absorbed as glucose. Some simple carbohydrates have their own enzymatic oxidation pathways, as do only a few of the more complex carbohydrates. The disaccharide lactose, for instance, requires the enzyme lactase to be broken into its monosaccharides components; many animals lack this enzyme in adulthood.

So here what's happen..

After the carbohydrate (CHO) are ingested by animals (or human), the polysaccharide chain of carbohydarte will be split into monoscharide form (glucose, fructose, galactose) by the action of pancreatic enzymes (pancreatic amylase) in the duodenum and this is only occur to all animals except ruminants (due to the lack of amylase instead of cellulase enzyme produced by bacteria during rumen fermentation) and glucose will be transported to the liver via blood. In the liver, they are stored as GLYCOGEN for later use. The glycogens are also stored in the muscle for rapid use. Whenever the body needs energy (ATP), the glycogens are split into glucose by the process of glycogenolysis.

Next, the glucose will enter cytoplasm of the cell to undergo GLYCOLYSIS, as the first part of CHO metabolism.

Glycolysis takes place in the cytoplasm and can occur without the presence of oxygen and is the primary energy source for most organisms. This process consumes two ATP molecules, and produces four ATP molecules and two NADH2+ molecules.

Glycolysis

1.Glucose 6-phosphate is formed when the 6th carbon on the glucose molecule is phosphorylated by an ATP molecule.

Phosphorilation (2ATP used-1 glucose molecule)

2.Glucose 6-phosphate is converted into a 5-carbon ring isomer, fructose 6-phosphate.

Isomerization

3.Fructose 6-phosphate is phosphorylated by another ATP to form fructose 1,6-diphosphate.

Phospholyration catalysed by pho(2ATP used)

4. Fructose 1, 6-diphosphate is processed by an enzyme into two glyceraldehyde 3-phosphate molecules.

Glycolysis. Cleavage to two Triose phosphates

5.Isomerization of dihydroxyacetone phosphate catalysed by Triose phosphate isomerase:

Glycolysis. Isomerization of dihydroxyacetone phosphate

Dihydroxyacetone phosphate <=> D-glyceroaldehyde-3-phosphate

6. Two molecules of glyceraldehyde 3-phosphate are oxidized, losing hydrogen atoms and gaining phosphate groups to form1, 3-diphosphoglycerate. Two molecules of NAD+ are converted into NADH2+ in the process.

Glycolysis. Generation of 1,3-Bisphosphoglycerate

7. Two 1,3-diphosphoglycerate molecules phosphorylate ADP (adenine diphosphate) to yield two molecules of 3-phosphoglycerate and two ATPs are produced.

Glycolysis. Substrate-level phosphorylation, 3-Phosphoglycerate

8. The phosphate groups on 3-phosphoglycerate move to the 2nd carbon, forming 2-phosphoglycerate.

Glycolysis. Phosphate transfer to 2-Phosphoglycerate

9. The two 2-phosphoglycerate molecules are dehydrated and forms two high-energy phosphoenolpyruvate molecules.

Glycolysis. Synthesis of Phosphoenolpyruvate

10. The two phospoenolpyruvate phosphorylates two ADPs and produces two more ATPs & two molecules of pyruvate

Glycolysis. Substrate-level phosphorylation. Pyruvate synthesis

Net ATP produce per glucose: 4ATP + 2NADH (later converted into ATP)

Net ATP used per glucose: 4ATP

Overview Glycolysis

Next, if enough oxygen supply, the pyruvates are sent to the mitochonria to be converted to more readily form to energy. This process is called aerobic respiration (Kreb cycle or citric acid cycle). If oxygen is not sufficient, the pyruvate will be converted to lactic acid. This process is called lactid acid fermentation.

Kreb Cycle

Prior to entering the Krebs Cycle, pyruvate must be converted into acetyl CoA (pronounced: acetyl coenzyme A). This is achieved by removing a CO2 molecule from pyruvate and then removing an electron to reduce an NAD+ into NADH. An enzyme called coenzyme A is combined with the remaining acetyl to make acetyl CoA which is then fed into the Krebs Cycle. The steps in the Krebs Cycle are summarized below:

1.Citrate is formed when the acetyl group from acetyl CoA combines with oxaloacetate from the previous Krebs cycle.

2.Citrate is converted into its isomer isocitrate

3.Isocitrate is oxidized to form the 5-carbon α-ketoglutarate. This step releases one molecule of CO2 and reduces NAD+ to NADH2+.

4.The α-ketoglutarate is oxidized to succinyl CoA, yielding CO2 and NADH2+.

5.Succinyl CoA releases coenzyme A and phosphorylates ADP into ATP.

6.Succinate is oxidized to fumarate, converting FAD to FADH2.

7.Fumarate is hydrolized to form malate.

8.Malate is oxidized to oxaloacetate, reducing NAD+ to NADH2+.

9. The cycle is repeated.Because glycolysis produces two pyruvate molecules from one glucose, each glucose is processes through the kreb cycle twice.

For each molecule of glucose, Kreb cycle produces;

6 NADH2+,

2 FADH2,

2 ATP.

Opss...is that it? only little ATP from the previous processes?? (kreb cycle and glycolysis) but the muscle needs more energy to sustain life!! How about NADH2+ and FADH2??

Now, we have come to the third and final process of making ATP which can use directly by the cell. This process is called Electron Transport Chain which transported electron carriers NADH2+ and FADH2 for more ATPs.The molecules have been reduced, receiving high energy electrons from the pyruvic acid molecules that were dismantled in the Krebs Cycle. Therefore, they represent energy available to do work. These carrier molecules transport the high energy electrons and their accompanying hydrogen protons from the Krebs Cycle to the electron transport chain in the inner mitochondrial membrane.

Electron Transport Chain

The electron transport chain occurs at the inner membrane of the mitochondria

The steps ETC are as below;

1. NADH2+ is oxidized to NAD+, and FADH2 to FAD.

2. The high energy electrons are transferred to ubiquinone (Q) and cytochrome c molecules, the electron carriers within the membrane. The electrons are then passed from molecule to molecule in the inner membrane of the mitochondron, losing some of their energy (H+ ions) to the intermembrane at each step.

3. The final transfer involves the combining of electrons and H2 atoms with oxygen to form water. The molecules that take part in the transport of these electrons are referred to as the electron transport chain.

How does it contributed to ATP? Let's look how it does

Chemiosmosis and ATP synthesis

The intermembrane space is impermeable to H+ ions. Pumping H+ ions out of the matrix and into them intermembrane space creates an electrochemical gradient. Electron free energy is transferred to potential energy in the electrochemical gradient. This causes higher positive charge [H+ ions] in the intermembrane space than in the matrix and higher concentration of H+ ions in the intermembrane space than in the matrix.

The proton motive force is created by (H+ ion) to repel one another and need to diffuse back into the inner membrane of mitochondria due to low (H+ ions) concentration. This causes H+ ions to move around in the intermembrane space. Since it is unable to move through the inner membrane, H+ ions are forced to move through special protein channels called ATP synthase. The term chemiosmosis is used due tothe free energy of the electrochemical gradient drives the synthesis of ATP from ADP and Pi in the matrix.

In summary, for 1 NADH molecule will produce 3 ATP, due to the 3 times electron transport and for 1 FADH molecule will produce 2 ATP (2 times electron transport).

Net ATP yield for 1 glucose molecule

From Glycolysis: 8ATP

** total ATP produced is 10 but minus 2ATP's used in the phosphorylation of glucose to fructose-6-phosphate.

Net ATP yield for each pyruvate

From 1 GDP (From kreb cycle) = 1 ATP X 2 pyruvates = 2 ATP

From ETC NADH: 3ATP x 4 (from kreb cycle) = 12 ATP x 2 pyruvates = 24 ATP

FADH: 2ATP X 1 (from kreb cycle) = 2 ATP x 2 pyruvates = 4 ATP

Total ATPs produced per 1 glucose: 38 ATP

** The liver and cardiac cells as well as many other cell produce 38 ATP molecules from the complete oxidation of 1 glucose molecule

**However, muscle cells and neurones produce only 36 ATP molecules for each glucose molecules because in these cells. 2 molecules of NADH+H produced in glycolysis cannot enter ETC directly (need shuttle)

Summary of glucose metabolism

How does the ATP works?

Sources: Glycolysis.co.uk, cellular respiration; oracle thinkquest.org, energy flow in living system, schoolworkhelper.net

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Just a brief.. explanation (adrenal medulla)

8:40 AM | Labels: adrenal medulla origin

All these while, I was wondering why the adrenal medulla only has direct stimulation (PREganglionic neuron) from the brain (CNS) without having POSTganglionic neuron when transmitting impulses unlike other organs.

Look at the figure below

Only PREganglionic neuron transmitting impulses to adrenal medulla (sympathetic)

Why?? so here is the answer

Medullary cells are derived from the embryonic neural crest and, as such, are simply modified neurons.In particular, they are modified postganglionic cells of the sympathetic nervous system that have lost their axons and dendrites, receiving innervation from corresponding preganglionic fibers. Moreover, as the synapses between pre- and postganglionic fibers are called ganglia, the adrenal medulla is actually a ganglion of the sympathetic nervous system.

Sources: Lecture notes and wikipedia

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Cranial nerves

12:10 AM | Labels: cranial nerves

Here we go again, nervous system. I was browsing through the net and found a quiz that challenge me to guess the cranial nerves of the brain. Surprisingly, I flunked :( . It was frustrating but it gave me a lesson not to give and take easily while reading.

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The Cranial nerves

The cranial nerves are composed of twelve pairs of nerves that emanate from the nervous tissue of the brain. In order reach their targets they must ultimately exit/enter the cranium through openings in the skull. Hence, their name is derived from their association with the cranium.

The function of the cranial nerves is for the most part similar to the spinal nerves, the nerves that are associated with the spinal cord. The motor components of the cranial nerves are derived from cells that are located in the brain. These cells send their axons (bundles of axons outside the brain = a nerve) out of the cranium where they will ultimately control muscle (e.g., eye movements) , glandular tissue (e.g., salivary glands) or specialized muscle (e.g., heart or stomach). The sensory components of cranial nerves originate from collections of cells that are located outside the brain. These collections of nerve cells bodies are called sensory ganglia. They are essentially the same functionally and anatomically as the dorsal root ganglia which are associated with the spinal cord.

In general, sensory ganglia of the cranial nerves send out a branch that divides into two branches: a branch that enters the brain and one that is connected to a sensory organ. Examples of sensory organs are pressure or pain sensors in the skin and more specialized ones such as taste receptors of the tongue. Electrical impulses are transmitted from the sensory organ through the ganglia and into the brain via the sensory branch that enter the brain. There are two exceptions to this rule that should be noted when the special senses of smell and vision are discussed. In summary, the motor components of cranial nerves transmit nerve impulses from the brain to target tissue outside of the brain. Sensory components transmit nerve impulses from sensory organs to the brain.

i- Olfactory nerve

The olfactory nerve is actually a collection of sensory nerve rootlets that extend down from the olfactory bulb and pass through the many openings of the cribriform plate in the ethmoid bone. These specialized sensory receptive parts of the olfactory nerve are then located in the olfactory mucosa of the upper parts of the nasal cavity. During breathing air molecules attach to the olfactory mucosa and stimulate the olfactory receptors of cranial nerve I and electrical activity is transduced into the olfactory bulb. Olfactory bulb cells then transmit electrical activity to other parts of the central nervous system via the olfactory tract.

ii-Optic nerve

The optic nerve originates from the bipolar cells of the retina which are connected to the specialized receptors in the retina (rod and cone cells). Light strikes the rod and cone cells and electrical impulses are transduced and transmitted to the bipolar cells. The bipolar cells in turn transmit electrical activity to the central nervous system through the optic nerve. The optic nerve exits the back of the eye in the orbit and enters the optic canal and exits into the cranium. It enters the central nervous system at the optic chiasm (crossing) where the nerve fibers become the optic tract just prior to entering the brain.

iii- Oculomotor nerve

The oculomotor nerve originates from motor neurons in the oculomotor (somatomotor) and Edinger-Westphal (visceral motor) nuclei in the brainstem. Nerve cell bodies in this region give rise to axons that exit the ventral surface of the brainstem as the oculomotor nerve.

iv- Trochlear nerve

The trochlear nerve is purely a motor nerve and is the only cranial nerve to exit the brain dorsally. The trochlear nerve supplies one muscle: the superior oblique. The cell bodies that originate the fourth cranial nerve are located in ventral part of the brainstem in the trochlear nucleus. The trochlear nucleus gives rise to nerves that cross (decussate) to the other side of the brainstem just prior to exiting the brainstem. Thus, each superior oblique muscle is supplied by nerve fibers from the trochlear nucleus of the opposite side.

v-Trigeminal nerve

The trigeminal nerve as the name indicates is composed of three large branches. They are the ophthalmic (V1, sensory), maxillary (V2, sensory) and mandibular (V3, motor and sensory) branches.

It is distributed to the muscles of mastication, the mylohyoid muscle and the anterior belly of the digastric. The mandibular nerve also innervates the tensor veli palatini and tensor tympani muscles. The three sensory branches of the trigeminal nerve emanate from the ganglia to form the three branches of the trigeminal nerve.

vi- Abducent

The abducens nerve originates from neuronal cell bodies located in the ventral pons. These cells give rise to axons that course ventrally and exit the brain at the junction of the pons and the pyramid of the medulla. The abducens nerve passes through the common tendonous ring of the four rectus muscles and then enters the deep surface of the lateral rectus muscle. The function of the abducens nerve is to contract the lateral rectus which results in abduction of the eye.

vii- Facial nerve

The facial nerve is mixed nerve containing both sensory and motor components.

The facial nerve has four components with distinct functions:

Brancial motor (special visceral efferent)	Supplies the muscles of facial expression; posterior belly of digastric muscle; stylohyoid, and stapedius.
Visceral motor (general visceral efferent)	Parasympathetic innervation of the lcrimal, submandibular, and sublingual glands, as well as mucous membranes of nasopharynx, hard and soft palate.
Special sensory (special afferent)	Taste sensation from the anterior 2/3 of tongue; hard and soft palates.

General sensory

(general somatic afferent)

General sensation from the skin of the concha of the auricle and from a small area behind the ear.

**Branchial motor fibers constitute the largest portion of the facial nerve.

viii- Vestibulocochlear Nerve

The vestibulocochlear nerve is a sensory nerve that conducts two special senses: hearing (audition) and balance (vestibular). The receptor cells for these special senses are located in the membranous labyrinth which is embedded in the petrous part of the temporal bone. There are two specialized organs in the bony labyrinth, the cochlea and the vestibular apparatus. The cochlear duct is the organ that is connected to the three bony ossicles which transduce sound waves into fluid movement in the cochlea. This ultimately causes movement of hair cells which activate the auditory part of the vestibulocochlear nerve. The vestibular apparatus is the organ that senses head position changes relative to gravity. Movement causes fluid vibration resulting in hair cell displacement that activates the vestibular part of the eighth nerve.

ix- Glossopharyngeal nerve

The glossopharyngeal nerve as its name suggests is related to the tongue and the pharynx. The ninth cranial nerve exits the brain stem as a the most rostral of a series of nerve rootlets that protrude between the olive and inferior cerebellar peduncle. These nerve rootlets come together to form the ninth cranial nerve and leave the skull through the jugular foramen.

The glossopharyngeal nerve consists of five components with distinct functions:

Brancial motor (special visceral efferent)	Supplies the stylopharyngeus muscle.
Visceral motor (general visceral efferent)	Parasympathetic innervation of the smooth muscle and glands of the pharynx, larynx, and viscera of the thorax and abdomen.
Visceral sensory (general visceral afferent)	Carries visceral sensory information from the carotid sinus and body.
General sensory (general somatic afferent)	Provides general sensory information from the skin of the external ear, internal surface of the tympanic membrane, upper pharynx, and the posterior one-third of the tongue.
Special sensory (special afferent)	Provides taste sensation from the posterior one-third of the tongue.

x- Vagus nerve

The vagus nerve is the longest of the cranial nerve.

The vagus nerve consists of five components with distinct functions:

Brancial motor (special visceral efferent)	Supplies the voluntary muscles of the pharynx and most of the larynx, as well as one extrinsic muscle of the tongue.
Visceral motor (general visceral efferent)	Parasympathetic innervation of the smooth muscle and glands of the pharynx, larynx, and viscera of the thorax and abdomen.
Visceral sensory (general visceral afferent)	Provides visceral sensory information from the larynx, esophagus, trachea, and abdominal and thoracic viscera, as well as the stretch receptors of the aortic arch and chemoreceptors of the aortic bodies .
General sensory (general somatic afferent)	Provides general sensory information from the skin of the back of the ear and external auditory meatus, parts of the external surface of the tympanic membrane, and the pharynx.
Special sensory (special afferent)	A very minor component of CN X. Provides taste sensation from the epiglottic region. This component will not be discussed further.

xi- Accessory nerve

The spinal accessory nerve originates from neuronal cell bodies located in the cervical spinal cord and caudal medulla. Most are located in the spinal cord and ascend through the foramen magnum and exit the cranium through the jugular foramen. They are branchiomotor in function and innervate the sternocleidomastoid and trapezius muscles in the neck and back.

xii- Glossopharyngeal nerve

The hypoglossal nerve as the name indicates can be found below the tongue. It is a somatomotor nerve that innervates all the intrinsic and all but one of the extrinsic muscles of the tongue. The neuronal cell bodies that originate the hypoglossal nerve are found in the dorsal medulla of the brain stem in the hypoglossal nucleus. This nucleus gives rise to axons that exit as rootlets that emerge in the ventrolateral sulcus of the medulla between the olive and pyramid. The rootlets come together to form the hypoglossal nerve and exit the cranium via the hypoglossal canal.

The nerve passes laterally and inferiorly between the internal carotid artery and internal jugular vein. The twelfth cranial nerve travels lateral to the bifurcation of the common carotid and loops anteriorly above the greater horn of the hyoid bone to run on the lateral surface of the hyoglossus muscle. It then travels above the edge of the mylohyoid muscle. The hypoglossal nerve then separates into branches that supply the intrinsic muscles and three of the four extrinsic muscles of the tongue.

Summary of Cranial nerves

Click here

Ps: If you ready, then answer the quiz!

http://www.gwc.maricopa.edu/class/bio201/cn/cranqzr.htm

http://www.gwc.maricopa.edu/class/bio201/cn/cranqzr2.htm

Sources: The cranial nerves, loyola univ medical education network, cranial nerves, Yale Univ school of medicine

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Bone formation

1:19 AM | Labels: bone formation, EO, IO, Types of bone

Alas, I knew I have to come to this part. You see, studying histology isn't my interest. However, to start Pathology next sem needs a new refreshment on this inter-linking subject with histology. So I randomly choose this topic from my old lecture notes.

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Bone formation

Bone is formed either by direct ossification of embryonic connective tissue (intramembranous ossification) or by replacement of hyaline cartilage (intracartilaginous or endochondral ossification). Intramembranous ossification takes place in the so-called membrane bones of the skull, while endochondral ossification is characteristic of the bones of the trunk and extremities.

Intramembranous Ossification

Look at the slide above. This is the histological section of intramembranous ossification. This type of bone formation allows for the building of flat bones like the scapula, skull, and pelvic bones.

As seen under a microscope, membranous bones first appear as flat, membrane-like layers of early connective tissue (mesenchyme). These layers are provided with a constant flow of nutrient blood supply by networks of blood vessels (Bv) formed in between the layers. Early connective tissue cells first arrange themselves among the layers and then differentiate into bone-forming cells called osteoblasts (OB). The osteoblasts then remove calcium from the blood and deposit it among the bone matrix (the cartilage). As a result, layers of spongy bone are formed around the original cartilage. Later in development, spaces among the spongy bone are filled with bone matrix and become compact bone.

Osteoblasts continue to deposit calcium supplements into the matrix until it is totally surrounded by it. Once this occurs the osteoblasts are considered to be encased in a lacunae and now called osteocytes (Os). The original connective cells first formed around the network of blood vessels are now called the periosteum. Osteoblasts still not isolated in a lacunae can emerge from beneath the layer of compact bone and form layers of spongy bone over compact bone.

Two dense connective tissue "membranes", "P", separated by a thin space form early periosteum. Special fibroblasts within the dense CT differentiate into osteoblasts,"Ob", which begin to lay down bone martix and fibers, but not on a cartilage model. Bone matrix is built into trabeculae, "BS", which are shaped by dissolving osteoclasts, not shown. Within the trabeculae are osteocytes, "OC", in lacuna. When the spongy bone (trabeculae) is mature, the two CT "membranes" build a two layers of compact bone, covering and connecting the spongy bone on both sides with a smooth surface. Most of compact bone are derived from IO.

Endochondral Bone Formation

Endochondral ossification forms bone by replacing a cartilaginous model (hyaline cartilage), or precursor, that appeared there earlier in embryonic development. The cartilaginous models first undergo quick changes as the connective tissue cells enlarge which in turn destroys the surrounding matrix. Soon after, the connective tissue cells die. While the cells disintegrate, a periosteum is formed on the outside of the developing structure (a membrane with many blood vessels). Next blood vessels and undifferentiated cells raid on into the disintegrating tissue. Certain connective tissue cells differentiate and form spongy bone around the previous template of cartilage.

There are 5 stages os EO

Resting zone
Zone of proliferation (multiplication of cell)
Zone of hypertrophy (cell hypertrophy and blood vessels embedded)
Zone of degeneration (calcification and erosion)
Zone of ossification (contain large no of osteoclast, bone absorbing cells)

The hyaline cartilage is the top half of the photo. "RC" is the label for the Reserve Zone. The chondrocytes are evenly spaced in the matrix (gel). The "PC" is the Proliferation Zone, where the chondrocytes divide so rapidly that they stack up from too little time between divisions to secrete much matrix and fibers (collagen). The "HC" is the Hypertrophic or Maturation Zone where the lacunae are enlarging and the chondrocytes are dying. The line between light and dark just below the "HC" is the Calcification Zone, where dissolved minerals are seeping into the hyaline cartilage matrix andcalcifying it. From here and below we call the matrix calcified hyaline cartilage matrix. Everything below the calcification zone is the Ossification or Resorption Zone. Osteoclasts, "Ocl", (blood cells) dissolve some calcified hayline cartilage matrix and leave spikes leftover called trabeculae (purple spikes). Osteoblasts, "Ob", (from the osteo-progenitor cells in the endosteum) sit on the surface of the calcified hyaline cartilage matrix and begin to secrete bone matrix (solid) and fibers (collagen), which stains pink around the edges of the original purple spike. When the osteoblasts have secreted enough matrix and fibers to surround themselves in lacunae, they are called osteocytes, "Oc".

Summary of bone formation

Transformed into compact bone
Bone development occurs within connective tissue membrane (mesenchyme)
Formed most flat bones

Transformed Hyaline to spongy or bone marrow (during embryonic formation)
Formed most skeletal element from neck down to certain cranial bones
Growth of log bones depends upon the no pf cartilaginous growth plates found at tissue end

Classes of bone

1. Compact Bone

Derived from IO
Supportive and weight-bearing role
Shaft long bones and cranial bones

2. Cancellous Bone

Derived from EO
Form marrow cavities at the end of long bones

Types of bone

1. Long bones

These bones typically have an elongated shaft and two expanded ends one on either side of the shaft. The shaft is known as diaphysis and the ends are called epiphyses. Normally the epiphyses are smooth and articular. The shaft has a central medullary cavity where lies the bone marrow. Example of long bones; humerus, ulna, tibia,fibula

2. Short bones

These bones are short in posture and can be of any shape. Most of them are named according to their shape. Examples of this class of bones include cuboid, cuneiform, scaphoid, trapezoid etc. In fact all the carpal and tarsal bones are included in this category.

3. Irregular bone

The shape of these bones is completely irregular and they do not fit into any category of shape. Examples of this type of bones are vertebrae, hip bone and bones in the base of skull.

4. Flat bone

scapula

These bones are flat in appearance and have two prominent surfaces. They resemble shallow plates and form boundaries of certain body cavities. Examples include scapula, ribs, sternum etc

5. Sesamoid bone

These are not like the other types of bones because they are in the form of nodules embedded in tendons and joint capsules. They do not possess any periosteum and their ossification also takes place after birth. Examples of this type of bones are patella, pisiform and fabella.

Sources: projectskeleton.tripod.com, types of bone;A man anatomy,

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Autonomic nervous system

3:58 AM | Labels: autonomic nervous system

I always like nervous system. It's kind of complicated, relating to all parts of the body, need gallons of memorizing and understanding yet mystery, still I'd love to learn them all. Maybe I should consider to a vet neurologist, don't I? hehe

Note: Before you read the below post, make sure to have a little knowledge regarding nervous system first. Nervous system is quite confusing though.

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Autonomic nervous system

The autonomic nervous system (ANS or visceral nervous system) is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions.The ANS affects heart rate, digestion,respiration rate, salivation, perspiration, diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind.

Division of nervous system

The ANS is predominantly an efferent system transmitting impulses from the Central Nervous System (CNS) to peripheral organ systems. Its effects include control of heart rate and force of contraction, constriction and dilatation of blood vessels, contraction and relaxation of smooth muscle in various organs, visual accommodation, pupillary size and secretions from exocrine and endocrine glands. Autonomic nerves constitute all of the efferent fibres which leave the CNS, except for those which innervate skeletal muscle.

There are some afferent autonomic fibres (i.e. transmit information from the periphery to the CNS) which are concerned with the mediation of visceral sensation and the regulation of vasomotor and respiratory reflexes, for example the baroreceptors and chemoreceptors in the carotid sinus and aortic arch which are important in the control of heart rate, blood pressure and respiratory activity. These afferent fibres are usually carried to the CNS by major autonomic nerves such as the vagus, splanchnic or pelvic nerves, although afferent pain fibres from blood vessels may be carried by somatic nerves.

The ANS is primarily involved in reflex arcs, involving an autonomic or somatic afferent limb, and then autonomic and somatic efferent limbs. For instance, afferent fibres may convey stimuli from pain receptors, or mechanoreceptors and chemoreceptors in the heart, lungs, gastrointestinal tract etc.

There may then be a reflex response to this involving autonomic efferent fibres causing contraction of smooth muscle in certain organs (e.g. blood vessels, eyes, lungs, bladder, gastrointestinal tract) and influencing the function of the heart and glands. The efferent limbs of these reflexes may also involve the somatic nervous system (e.g. coughing and vomiting). Simple reflexes are completed entirely within the organ concerned, whereas more complex reflexes are controlled by the higher autonomic centres in the CNS, principally the hypothalamus.

The ANS is divided into two separate divisions called the Parasympathetic and Sympathetic Systems, on the basis of anatomical and functional differences. Both of these systems consist of myelinated preganglionic fibres which make synaptic connections with unmyelinated postganglionic fibres, and it is these which then innervate the effector organ. These synapses usually occur in clusters called ganglia. Most organs are innervated by fibres from both divisions of the ANS, and the influence is usually opposing (e.g.the vagus slows the heart, whilst the sympathetic nerves increase its rate and contractility), although it may be parallel (e.g. the salivary glands). The responses of major effector organs to autonomic nerve impulses are summarised in table below;

Summary of ANS

Sympathetic Nervous System

The cell bodies of the sympathetic preganglionic fibres are in the lateral horns of the spinal segments T1-L2, the so called thoraco-lumbar outflow. The preganglionic fibres travel a short distance in the mixed spinal nerve, and then branch off as white rami (myelinated) to enter the sympathetic ganglia. These are mainly arranged in two paravertebral chains which lie anterolateral to the vertebral bodies and extend from the cervical to the sacral region. They are called the sympathetic ganglionic chains. The short preganglionic fibres which enter the chain make a synapse with a postsynaptic fibre either at the same dermatomal level, or at a higher or lower level, and then the longer postganglionic fibres usually return to the adjacent spinal nerve via grey rami (unmyelinated) and are conveyed to the effector organ.

The sympathetic system enables the body to be prepared for fear, flight or fight. Sympathetic responses include an increase in heart rate, blood pressure and cardiac output, a diversion of blood flow from the skin and splanchnic vessels to those supplying skeletal muscle, increased pupil size, bronchiolar dilation, contraction of sphincters and metabolic changes such as the mobilisation of fat and glycogen.

Some preganglianic fibres do not synapse in the sympathetic chains but terminate in separate cervical or abdominal ganglia, or travel in the greater splanchnic nerve and directly synapse with chromaffin cells in the adrenal medulla. As discussed above, Ach is the neurotransmitter via a nicotinic receptor at the preganglionic synapse. The adrenal medulla is innervated by preganglionic fibres and therefore adrenaline is released from the gland by stimulation of nicotinic Ach receptors.

At most postganglionic sympathetic endings, the chemical transmitter is noradrenaline, which is present in the presynaptic terminal as well as in the adrenal medulla. In sweat glands, however, postganglionic sympathetic fibres release Ach and this transmission is nicotinic.

Parasympathetic nervous system

The preganglionic outflow of the parasympathetic nervous system arises from the cell bodies of the motor nuclei of the cranial nerves III, VII, IX and X in the brain stem and from the second, third and fourth sacral segments of the spinal cord. It is therefore also known as the cranio-sacral outflow.

Preganglionic fibres run almost to the organ which is innervated, and synapse in ganglia close to or within that organ, giving rise to postganglionic fibres which then innervate the relevant tissue. The ganglion cells may be either well organised (e.g. myenteric plexus of the intestine) or diffuse (e.g. bladder, blood vessels).The cranial nerves III, VII and IX affect the pupil and salivary gland secretion, whilst the vagus nerve (X) carries fibres to the heart, lungs, stomach, upper intestine and ureter.

The sacral fibres form pelvic plexuses which innervate the distal colon, rectum, bladder and reproductive organs.In physiological terms, the parasympathetic system is concerned with conservation and restoration of energy, as it causes a reduction in heart rate and blood pressure, and facilitates digestion and absorption of nutrients, and consequently the excretion of waste products.

The chemical transmitter at both pre and postganglionic synapses in the parasympathetic system is Acetylcholine (Ach). Ach is also the neurotransmitter at sympathetic preganglionic synapses, some sympathetic postganglionic synapses, the neuromuscular junction (somatic nervous system), and at some sites in the CNS. Nerve fibres that release Ach from their endings are described as cholinergic fibres.

The synthesis of Ach occurs in the cytoplasm of nerve endings and is stored in vesicles in the presynaptic terminal. The arrival of a presynaptic action potential causes an influx of calcium ions and the release of the contents of several hundred vesicles into the synaptic cleft. The Ach then binds to specific receptors on the postsynaptic membrane and increases the membrane permeability to sodium, potassium and calcium ions, which results in an excitatory post-synaptic potential. The action of Ach is terminated by hydrolysis with the enzyme Acetyl Cholinesterase.

The preganglionic and post ganglionic neurons of SNS ans PNS

Ps: Just a little introduction of ANS. For better understanding, please visit

http://itc.gsw.edu/faculty/gfisk/anim/autonomicns.swf

Sources: The autonomic nervous system, Dr S Bakewell Addenbrooke's hospital cambridge

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