Physiology No 7

‫بســـم الله الرحمن الرحيم‬

‫المحاضرة‬ ‫السابعة‬ ‫صـــفحة ‪137 -136‬‬ ‫صفحة ‪142 – 141‬‬ ‫صفحة ‪154 – 150‬‬ ‫صفحة ‪169 – 166‬‬ ‫صفحة ‪183 – 179‬‬

The reflex arc Many neurons work in groups called reflex arcs. The parts of a typical reflex arc are: 1- A receptor: A sense organ in skin, muscle, other organs, …. etc. 2- An afferent (sensory) neuron: Which carries nerve impulses toward the CNS. 3- The CNS: Where the synaptic connections are made between the sensory neurons and the interneurons. 4- The efferent (motor) neuron: Which makes a synaptic connection with the interneuron and carries the nerve impulses out from (away) CNS. .The effector: By which the animal responds to environmental changes -5 Muscles, glands, ciliated cells, nematocytes of the radiate animals, electric organs of fish .and pigmented cells (chromatophores) are examples of effectors

Notes: ! The simplest reflex arc consists of only 2 neurons. ! A single afferent neuron may make synaptic connections with many efferent neurons. ! In a similar way an efferent neuron may receive impulses from many afferent neurons. Some reflex acts are inherited and innate, others are acquired trough learning. ! The response is autonomic and predictable (‫(( متنبأ به‬e.g. breathing, heartbeats, diameter of blood vessels, sweat gland secretion, … etc).

(Autonomic nervous system (ANS It governs the involuntary, internal functions of the body that are do not ordinarily affect consciousness such as the movements of the alimentary canal, heartbeats, breathing, diameter of blood vessels, urinary bladder, iris of the eye (‫قزحية العين‬.secretion of various glands, … etc ,( It is characterized by the presence of two motor neurons (while in somatic nervous system there is only one motor neuron). The two motor neurons synapse, after leaving the cord and before arriving the effector, in ganglia. Nerve fibers passing from the cord to ganglia are called preganglionic autonomic fibers. Fibers passing from the ganglia to the effectors are called postganglionic fibers. ANS is subdivided into sympathetic & parasympathetic systems. Most organs are innervated by both sympathetic & parasympathetic fibers where their actions are antagonistic i.e. if one fiber speeds up an activity the other slows it down. However, neither kind of nerve is exclusively excitatory or inhibitory. For example, parasympathetic fibers inhibit heartbeat but excite peristaltic movements of the intestine.

Parasympathetic division is active under resting conditions while the sympathetic is active under conditions of physical activity and stress i.e. in fightflight response. In parasympathetic system, motor neurons emerge from the brain stem by certain cranial nerves & sacral region of the spinal cord. In sympathetic system, preganglionic fibers are located in the thoracic and upper lumber area of the spinal cord. The parasympathetic postganglionic fibers release acetylcholine at their endings, whereas the sympathetic postganlionic fibers with few exceptions release norepinephrine (noradrenaline)

NB: All preganglionic fibers rlease acetylcholine at the synapses with the postganglionic cells.

Endocrine system

Endocrine system (continue) Chemical structure of the hormones: There are 4 groups of hormones according to the chemical structure which are:

1- Protein or peptide hormones:

@ They represent the majority of hormones. ^ They may be short (3 amino acids) or long (made up of 180 amino acids). ^ They may be composed of a linear chain or may contain a ring structure. Some of the larger protein hormones are composed of the two chains. ^ They are hydrophilic (soluble in water) and lipophobic. ^ They have short-life due to their free state.

2- Steroid hormones:

^ Their precursor is cholesterol (either from ingested with the food or self synthesized by steroid-producing glands). ^ They are hydrophobic & lipophilic. So, simply diffuse across the plasma membrane of the steroid-producing cell and enter the blood. ^ The half-life of steroid hormones is apparently enhanced (‫ يعزز‬، ‫يزيد‬due to their ability( to be bound to the plasma proteins. They are synthesized only in the adrenal gland, the gonads (testes & ovaries) and placenta during pregnancy (‫المشيمة اثناء الحمل‬.( 3- Tyrosine-derived hormones: ^ They are synthesized from the amino acid, tyrosine.

4- Eicosanoids: ^ They are synthesized from fatty acids derived from the arachidonic acid within the plasma membranes of most cells. ^ Prostaglandins, prostacyclins, thromboxanes and leukotrienes are members of this group. ^ They may be act on: 1- The cell that producing them → autocrine. 2- The neighboring cell → paracrine. ^ The major physiological role is: 1- The control of vascular smooth muscle activity (i.e. adjust local blood flow). 2- Platelet aggregation. 3- Induce inflammatory (‫ )التهابى‬or allergic (‫حساسية‬.responses (

Thyroid gland Location: It is at the base of the neck in front of the trachea. ^ It is composed of asymmetric lobes which are connected by thin isthmus (‫برزخ‬.( ^ It is the largest gland in the human body (10-20 g). ^ It is larger in women than men. $ It secretes thyroid hormones which are: * Thyroxine (tetraiodothyronine (T4): It is composed of two tyrosine molecules and 4 iodine atoms. * Triiodothyronine (T3): It is composed of 2 tyrosine molecules and 3 iodine atoms. & * calcitonin. isthmus

Regulation of thyroid hormones: ^ The thyroid hormone synthesis & secretion are regulated by the anterior pituitary thyrotrophin, thyroid-stimulating hormone (TSH). ^ The output of TSH is influenced by: 1- Thyrotrophinreleasing hormone (TRH) and the inhibitory hormone called somatostatin (SS) which are secreted from hypothalamus. 2- The blood level of free thyroid hormones (T4 & T3).

Actions (Effects) of thyroid hormones Target Most cells of the body

Effect * Increase the basal metabolic rate (BMR) * Rise in body temperature. * Increase mitochondrial oxidative phosphorylation (O2 consumption).

CNS

* Vital for mental (‫ﺫهنى‬، ‫ عقلى‬.development (

Bone

* Stimulate both resorption & synthesis of bone. NB: Bone resorption is the process by which osteoclasts break down bone and release the minerals, resulting in a transfer of calcium from bone to the blood.

Cardiovascular * Increase in force of heart contraction & cardiac output. system Carbohydrate metabolism

* Increase rate of glucose absorption. * Increase uptake of glucose by peripheral cells. * Enhance glycolysis, gluconeogenesis & glycogenolysis.

Protein metabolism

* Play a role in normal growth, maintenance & development of tissues.

Fat metabolism

* Stimulate lipolysis, increasing plasma free fatty acids & their oxidation.

Vitamin metabolism

* Stimulate vitamin A synthesis from carotenes in the liver. * Increased utilization of vitamins.

* Stimulate both protein anabolism & catabolism. * General depletion (‫ استنزاف‬، ‫استهلك‬of the body (’s stores of fat.

Calcium-regulating hormones To maintain Ca2+ levels, 3 main organs are involved which are: 1) Bone: It is the principle store (over 99% in the body found in the bones) & the immediate source of Ca2+. 2) Gut: The pathways by which Ca2+ can enter the body. 3) Kidneys: The pathways by which Ca2+ can leave the body. Calcium levels are kept within optimal levels by the effect of: 1- Calcitonin (CT): It is the only calcium-lowering hormone in the body i.e. hypocalcemic hormone. 2- Parathromone & 3- Vitamin D: They cause rising calcium levels. 4- Parathromone-related peptide: Calcitonin (CT): @ It is secreted from parafollicular cells in the thyroid gland. @ A rise in plasma Ca2+ causes a rapid rise in CT concentration. @ CT has 3 effects which are: 1- It causes more Ca2+ to be deposited in the bones. 2- It makes the intestine absorb less Ca2+ from the diet. 3- It makes the kidneys reabsorb less Ca2+ as they form urine.

Pancreas It is made of: 1- The exocrine pancreas: The major digestive gland of the body. 2- The endocrine pancreas: * It consists of about 1 million groups of cells (clusters) (‫عناقيد‬which scattered( within the exocrine pancreas. * It is called islets of Langerhans. * It represents 2% of the pancreatic tissue. * It’s cells differentiated into: 1- A (α)-cells that synthesize, store & secrete glucagon hormone. 2- B (β)-cells that synthesize, store & secrete insulin hormone. 3- D (δ)-cells secrete somatostatin hormone. 4- F-cells which produce pancreatic polypeptide.

Insulin It is synthesized, stored & secreted by B (β)-cells of islets of Langerhans. Each molecule consists of a 21 amino acid α-chain and a 30 amino acid β-chain which are linked by disulphide bridges.

It is involved in the control of the blood glucose concentration. It’s secretion is stimulated by an increase of the blood glucose concentration. In the liver & skeletal muscle cells (as a target cells) , insulin stimulates glucose uptake & glycogenesis. In the body cells (as a target cells), insulin stimulates glucose uptake for immediate energy use, for the storage of energy & for the synthesis of proteins. Glucagon It is synthesized, stored & secreted by A (α )-cells ofislets of Langerhans. It is a polypeptide. It.’s principle effect is to rise the blood glucose concentration :The target organs are the liver (the main target) and adipose tissues where 1- Stimulates breakdown of stored glycogen (glycogenolysis). 2- Stimulates gluconeogenesis. 3- Stimulates ketogenesis. 4- Stimulates lipolysis in adipose tissues.

(Somatostatin (SS It is synthesized by D(δ)-cells of islets of Langerhans. It is an inhibitory paracrine hormone. i.e. It inhibits the release of both insulin and glucagon. Also, it regulates the movement of nutrients from the gut to the blood. (Pancreatic polypeptide (PP It is a peptide found in the F-cells of islets of Langerhans. It is released after a high protein meal and in case of hypoglycemia. It inhibits the secretion of pancreatic juice and the bile. Amylin It is a peptide secreted by B (β)- cells of islets of Langerhans. It was found to have opposing metabolic effects of insulin.

Diabetes mellitus It is a disease characterized by a high blood glucose level & excessive excretion of urine. It is a disease in which the body cells are unable to absorb glucose from the blood. It occurs when there is not enough insulin in the blood or when the body cells do not respond normally to blood insulin. So, the body cells starved for fuel, and they forced to burn the body’s supply of fats and proteins.

There are two types of diabetes: Type I: Insulin-dependent diabetes mellitus (IDDM)

* It is an auto-immune disease where the body's immune system destroys the insulin-producing β-cells of islets of Langerhans in the pancreas. * This type of diabetes, also known as juvenile-onset diabetes, accounts for 10-15% of all people with the disease. * It can appear at any age, although commonly under 40, and is triggered by environmental factors such as viruses, diet or chemicals in people genetically predisposed (‫جعله ميال‬.(

* The three main target tissues of insulin (liver, muscle & fat) fail to take up absorbed nutrients and continue to spew (‫يرشح كالمطر‬glucose, amino acids & ( fatty acids into bloodstream from their storage depots. * The continued fat breakdown lead to the production and accumulation of ketones. * To live, people with type 1 diabetes must inject themselves with insulin several times a day and follow a careful diet and exercise plan.

(Type II: Noninsulin-dependent diabetes mellitus (NIDDM * Type 2 diabetes is the most common form of diabetes, affecting 85-90% of all people with the disease. * This type of diabetes, also known as late-onset diabetes, is characterized by insulin resistance and relative insulin deficiency. * The disease is strongly genetic in origin but lifestyle factors such as excess weight, inactivity, high blood pressure and poor diet are major risk factors for its development. * Symptoms may not show for many years and, by the time they appear, significant problems may have developed. * People with type 2 diabetes are twice as likely to suffer cardiovascular disease. * Type 2 diabetes may be treated by dietary changes, exercise and/or tablets. * Insulin injections may later be required.

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