Section 7: Nervous and endocrine system (C8853502)

Getting started

Cells can be held together either by a matrix or cell junction. Matrix is material in which other structures are embedded. For example, the extracellular matrix along with collagen and elastin embedded within the matrix, are synthesized by fibroblasts. The extracellular matrix provides structural support to animal cells. Tissue with extensive matrix is known as connective tissue.

“Classically, the Holy Spirit would reside in the Jewish Temple (2 Chronicles 7:1-3), specifically in the Holy of Holies,” Mandy commented, “now, he can permanently reside in our hearts (1 Corinthians 6:19-20), following baptism in the Holy Spirit (1 Corinthians 12:13).”

“Indwelling of the Holy Spirit is SO special! It’s like being Kim Kardashian !!” Blaire remarked.

“Isn’t that the Gnostic heresy of dualism?” Mandy asked, “who tried to appear more spiritual by distancing themselves from flesh, when in actuality, God created both flesh and spirit. That’s mob thus believed Jesus wasn’t truly human, because to be truly God, was not to be human.”

“Pre-Pentecost, He would come upon judges (Judges 3:9-10), prophets (Numbers 11:16-25), and other leaders for a specific purpose (Exodus 35:30-35) to anoint them with a special power to carry out their ministry, but He wasn’t a permanent possession, which was why David could say ‘Take not your Holy Spirit from me’ in Psalm 51:11.”

“Totes exclusive, residing in the hearts of all believers, but yet never discovered,” Mandy replied, “He really does seem like the forgotten person of the Holy Trinity, with the post-apostolic fathers seeming almost Bin-itarian, making only a single concluding statement in the 325 Nicene Creed, ‘And in the Holy Spirit’.”

“That’s why unlike the Old Testament Jews who were marked by impotence and terrible Sin, we have the Holy Spirit to empower us to live an abundant life coursing with power and vitality, like springs of living water, which Jesus notes should be the normality (John 7:37-39) and not the exception. Romans 8:9 notes the proof of belonging to Christ is the mark of the Holy Spirit, and 1 Corinthians 12:4-11 notes ‘to each one’ some manifestation is given. Though not the absence of problem, He provides a source of strength to live Victoria-ously over them.”

“See what I did ?” Mandy giggled, “Victoria ?”

Cell junction is contact between neighboring cells, important in enabling communication between neighboring cells. The three major types of cell junctions are:

• Gap junction, which are direct connections between the cytoplasm of two cells, allowing molecules and ions to freely pass between cells. Gap junctions are used in cardiac and smooth muscle, to permit action potential to spread from one cell to another
• Tight junctions, which is a water-tight [or near water-tight] barrier between cells. Also, whereas proteins can generally traverse laterally through the membrane, tight junctions prevent lateral movement of these proteins between the apical and basolateral surfaces, permitting specialized functions of each surface to be preserved. Apical means facing the lumen, and basolateral means the rest (basal meaning facing the rear, and lateral meaning facing the side)
• Desmosomes, which are localized spot adhesions, which binds cells to one another

Single-celled organism is an organism consisting only of one cell, and a multicellular organism consists of multiple cells. Whereas, single-celled organisms carry out all life processes to survive, cells in multicellular organisms are so specialized they depend on each other in order to survive.

“Following Israel’s apostasy, the Holy Spirit withdrew (Ezekiel 9:3, 11:23) in preparation for judgment on Israel,” Blaire replied, “analogously, we should not ‘grief’ (Ephesians 4:30) or ‘quench’ (1 Thessalonians 5:19) the Spirit through Sin or disobedience.”

But that’s exactly what happened. Despite Jamie and Mandy were both “on fire” for God ; their disobedience to the church’s regulation on boy/girl relationships was going to soon burn them .

“Tap that ,” Mandy remarked to Jamie, “HARD !!!” as he tried to hammer a steel tip into the cadaver’s skull.

“There is FAR too much sexual innuendo between the two of you ,” Blaire commented.

Despite the diminishing voice of the Holy Spirit, and self-justification, Mandy knew deep down what she was doing with Jamie was wrong, and she needed to repent.

Because of this inter dependability, levels of organization from small to large are, atoms, molecules, organelles, cells, tissues, organs, and organ systems, organisms. Tissue is a collaboration of different cells which carry out the same specific function. Tissue can be divided into:

• Epithelial tissue, which are cellular barriers, separating cavities from each other. For example, epithelium is found in skin and blood vessel
• Connective tissue, which is tissue with an extensive matrix (mentioned ). For example, connective tissue is found in adipose tissue, cartilage, tendon, ligament, bone, blood, and lymphoid tissue
• Nervous tissue (discussed )
• Muscle tissue (discussed )

Organ is a collection of different tissues to serve a common function. Organ system is a group of organs working together to form a certain task. Organisms are a collection of organ systems.

1 Nervous system

Cells in multicellular organisms communicate with each other by:

• Neurotransmitters, which travel from nerve cell to nerve cell over very short distances known as a synapse
• Paracrine hormones, which are released into the interstitial fluid (fluid between cells) and diffuse to local cells, in the vicinity of where the signal was released. For example, prostaglandin, which has a short half-life, is a paracrine hormone
• Endocrine hormones, which travel through the blood to reach all parts of the body (discussed in further depth )

The nervous system is faster and more specific than the endocrine system. For example, a neural request to move a finger occurs essentially immediately and directs a very specific change. In contrast, a hormonal request for puberty occurs over a period of time and results in body wide changes. The functional unit of the nervous system is the neuron (aka nerve cell). Neurons are such specialized cells they are unable to undergo cell division. Glucose is the only fuel normally used by neurons, transported across the blood-brain barrier, which separates circulating blood from the neurons, by facilitated diffusion. Unlike most cells in the body however, the brain (like the liver) doesn’t require insulin for efficient uptake of glucose, as an insulin-independent facilitated diffusion transporter is used to import glucose. Electrical signals from other neurons are received by the dendrites, transmitted through the cell body known as soma, down a single axon, to the branching axon terminal. The axon hillock is where the soma connects to the axon, and has the greatest density, voltage-gated sodium channels, meaning it is the most easily excited part of the neuron, hence is where the action potential initiates.

But it wasn’t just Mandy who was behaving promiscuously. It takes two to tangle, right? Jamie would, for example, place his hand around Mandy, play with her hair, to the extent, that as Blaire said, “people are going to think she [Mandy] is your girlfriend.”

But do you think tangentially sprung Jamie or Mandy listen to this wise Godly counsel? Nooooooooooooooo.

Action potential is the electrical potential associated with an impulse along a neuron (as stated, in particular, the axon hillock). Resting potential is an equilibrium established by a balance between pumping and leaking. Sodium-potassium pump (a protein) transports three sodium ions out [of the neuron], and two potassium ions in [to the neuron], at the expense of one ATP molecule. Because both sodium ($\ce{Na+}$) and potassium ($\ce{K+}$) have the same charge, this transport would make a neuron more positive outside the cell than within. Defined relative to the outside (set at $0mV$), the inside is more negative, and thus electrical potential is negative. As the membrane is semipermeable to sodium and potassium ions, these leak across the membrane in the direction of their electrochemical gradient, proportional to their build up concentration. Therefore, an equilibrium will be reached between the pumping and leaking actions. Action potentials are initiated by:

• Sodium voltage-gated channels (a protein) are activated by action potentials, permitting a flow of sodium ions [which were previously pumped out of the neuron] back into the neuron, known as depolarization. As sodium is positively charged, notice that [the previously negative] interior of the neuron becomes positive. If sufficient sodium voltage-gated channels open to permit a potential difference greater than the threshold potential, an action potential is initiated. Known as the all-or-nothing property, if the potential difference is not greater than the threshold, an action potential is not initiated. Also, irrespective of greater numbers of sodium voltage-gated channels open, the action potential is not greater
• As action potential increases, less sensitive potassium voltage-gated channels are activated, permitting a flow of potassium ions [which were previously pumped into the neuron] back out of the neuron, known as repolarization. Notice that this causes [the previously positive] interior of the neuron to become negative. Additionally, [the previously activated] sodium voltage-gated channels are deactivated
• Because potassium voltage-gated channels are less sensitive, they are also slower to close, leading to an undershoot, driving the membrane potential to below the resting potential, known as hyperpolarization
• The combination of the deactivation of potassium voltage-gated channels, and the leakage of potassium back into the neuron, brings the membrane potential back to the resting potential, known as the refractory period. Until after the absolute refractory period, the neuron is unable to evoke another action potential, despite that strength of stimulus. During the relative refractory period, a stronger than usual stimulus is required to evoke another action potential. The absolute refractory period ensures the action potential only travels in one direction along the axon

“All or nothing: that reminds me of High School Musical,” Mandy remarked, “that song ‘Now or Never’.”

“The analogy should’ve been, the photoelectric effect.”

At the axon terminal is a synapse, which permits a neuron to transmit a signal to another cell. Synaptic cleft is the space between neurons. There are two types of synapses:

• Chemical synapse, where the presynaptic neuron contains synaptic vesicles packaged with neurotransmitters. In membrane near the synapse are calcium voltage-gated channels, which activate as a result of the action potential. Calcium voltage-gated channels permit a flow of calcium ions to flow into the neuron, causing the synaptic vesicles to migrate to the synaptic surface and release its content (the neurotransmitters) into the synaptic cleft via exocytosis. The neurotransmitter then binds and therefore activates a protein receptor on the postsynaptic cell. To prevent further excitation, the neurotransmitter is degraded by enzymes in the synaptic cleft. Chemical synapses pass information uni-directionally from presynaptic to postsynaptic cell

• Electrical synapse, which utilize gap junctions (direct connections, mentioned ). Electrical synapses are distinct from chemical synapse, as they pass information bi-directionally, and are faster

Along with neurons, the nervous system contains neuroglia (aka glia cells), which provide support for neurons. The glial cells that contain myelin are Schwann cells and oligodendrocytes. Myelin is an electrically insulating material, which wraps around the axon [of a neuron], permitting the action potential to jump from node to node [rather than the slower method of travelling down the entire axon neuron by neuron], known as saltatory conduction. Myelinated axons are white in appearance, whilst neural components are generally grey in appearance. Therefore, white matter are myelinated components, and grey matter are unmyelinated components.

“Oh, I love that word ‘Myelination’, it sounds like Miley,” Mandy remarked before Jamie looked at her funny, “it’s a good mnemonic because Miley ‘quickly’ changes from Miley to Hannah.”

The nervous system can be divided into a central and peripheral system. Central nervous system (CNS), consists of the brain and spinal cord. The CNS is composed of interneurons, which is a neuron connecting two other neurons, thereby helping the CNS fulfill its role of integrating information between the sensory and motor neurons, to coordinate the activities of the body. Notable regions of the brain include:

• Medulla oblongata, which controls heart rate, breathing, other somatic (voluntary, ) and autonomic (involuntary, ) functions
• Cerebellum, which controls coordination, equilibrium, balance
• Prefrontal cortex, which controls decision making and thought
• Hypothalamus, which controls body temperature, hunger, thirst, pleasure, pain, libido, blood pressure, water balance, etc.

Whilst Mandy was changing for something more appropriate to wear to Young Adults, Blaire noticed that Mandy was sporting apparel that was male, and well, Jamie’s.

“Don't you think it's a little bit weird to wear his underwear?” Blaire remarked, “Do you like him?”

"OoOoOOo, our very own sexy scandal ,” Mandy giggled.

“Mandy,” Blaire replied with increased austerity, realizing Mandy was veiling the significance of her Sin with flippancy, “Jamie, has a girlfriend.”

Peripheral nervous system (PNS) is the nervous system outside the CNS. PNS neurons can be classified by the mnemonic SAME DAVE, such that sensory is afferent, motor is efferent; dorsal is afferent, ventral is efferent. Sensory neurons are neurons, which convey information from sensory receptors (discussed ) towards the CNS, which are afferent. Motor neurons are neurons which convey information away from the CNS to the target effectors, like tissue, gland, muscles and organs, which is efferent. This can be memorized with the mnemonic that to be “affected” is to be influenced by others; and to “effect” is to influence others. Whereas sensory (afferent) neurons enter the spinal cord dorsally (from the back), motor (efferent) neurons exit the spinal cord ventrally (from the belly/front). This can be memorized with the mnemonic that in a car, the motor is at the front. The PNS can be further divided into:

• Somatic nervous system, which is involved with controlling predominantly voluntary body movements via skeletal muscle
• Autonomic nervous system, which is involved with controlling predominantly involuntary body movements below the level of consciousness via smooth muscle, cardiac muscle, glands, and visceral organs. The motor neurons of the ANS can be further divided into sympathetic and parasympathetic, both of which simultaneously innervate. Sympathetic is mobilization of the nervous system’s fight or flight response. Parasympathetic is mobilization of the nervous system’s rest or digest response. For example, whereas sympathetic stimulation increases heart rate, parasympathetic stimulation decreases heart rate. Another example, sympathetic stimulation decreases intestinal movement, and parasympathetic stimulation increases intestinal movement. ANS requires a two-neuron efferent pathway, the preganglionic neuron which synapses to the postganglionic neuron before innervating the target organ. Ganglion is a mass of neurons in the PNS. Preganglionic is the length of the fiber from the CNS to the ganglion. Postganglionic is the length of the fiber from the ganglion to the effector organ. Sympathetic has a paravertebral ganglia, which is ganglia which runs along the length of the spine. Therefore, the preganglionic in sympathetic is shorter [than the preganglionic in parasympathetic] as the ganglion is located paravertebral. An exception is adrenal medulla, which are modified postganglionic sympathetic neurons, so the preganglionic neurons do not synapse at the paravertebral ganglia, and thus have longer preganglionic. In contrast, as parasympathetic has ganglion in or close to the effector organ, the postganglionic in parasympathetic is shorter (than the postganglionic in sympathetic). All neurons in the ANS use acetylcholine as the neurotransmitter, except for the postganglionic sympathetic neurons which use adrenaline (epinephrine) or noradrenaline (norepinephrine)

Unfortunately for Jamie and Mandy, their little affair was going to soon be uncovered by Sophie. Whilst waiting for the bus, Jamie and Mandy, engaged in their usual touchy feely play around. It had however, progressed from mere hand holding, perhaps as both were tired, to whilst Jamie was sitting, their pulling of arms against another, whilst Mandy was sandwiched between his legs.

Intending on catching another bus, Sophie initially failed to recognize Jamie or Mandy (as you don’t expect another girl to be gyrating your boyfriend), but when all recognized another, Sophie was visibly stunned.

“This is-,” Jamie began, before stumbling as his sympathetics kicked in, salivation decreasing.

“Hey, that’s my bus,” Sophie remarked, “I need to go.”

Sensory receptors include:

• Eye, whose lens focuses image on the retina. Retina is light sensitive tissue, serving the same function as film in camera, and is located at the back of the eyeball. Retina is composed of cone cells, which distinguish between wavelengths, and therefore support perception of color; and rod cells, which are more sensitive, but only able to support black and white vision. Iris is smooth muscles which control the size of pupils. Pupil is the hole in the eye permitting light to enter the eye
• Ear, which consists of the outer, middle and inner ear. Outer ear contains pinna, the visible ear flap; and ear canal, a tube running from the outer to middle ear. Middle ear contains three tiny bones: malleus, incus and stapes, which transfer sound energy from the ear drum to the inner ear at the oval window (of the cochlea). Inner ear contains cochlea, which is the organ of hearing, converting sound into electrical nerve signal for the brain; semicircular canal, position, orientation, and movement of the head

Although it wasn’t clear what Sophie was feeling (being tactful, she hid her feelings well), it became increasingly obvious they were over, as Jamie heard not from Sophie. It was like as if they were never friends.

But in the same vein, Jamie also heard not from Mandy.

 Learning activity What is the nervous system?

2 Endocrine system

Endocrine system secretes hormones directly into the bloodstream [to reach all parts of the body, as mentioned ]. In contrast, exocrine glands excretes products into the external environment by ducts. Exocrine glands can be sweat glands (aka sudoriferous), which secrete sweat; sebaceous glands, which secrete oily/waxy matter; mucous glands, which secrete mucous; and digestive gland, which secrete digestive enzymes. Cells respond to a hormone if they express a specific protein receptor for that hormone. Hormones regulates growth, metabolism, reproduction and development. Hormones can be divided into:

• Peptide hormones, which are peptides or protein. As stated , peptides, specifically, preprohormones, are manufactured by ribosomes in the rough ER membrane. Preprohormones are precursor proteins to [one or more] prohormones, which in turn are precursors to the active peptide hormones. Preprohormone contain intervening segments, which are cleaved off in the ER lumen, to create the prohormone. As stated , the Golgi apparatus modifies and packages the prohormone into vesicles, which are secreted from the cell by exocytosis in response to specific stimuli. Peptide hormones are water soluble, so dissolve in blood [which is also water soluble], since “like dissolves like”. However, peptide hormones [which are water soluble] can’t cross the lipid bilayer membrane [which are fat soluble], meaning peptide hormone receptors must be on the surface of their target cells. When peptide hormone binds to cell surface receptor, the second messenger system is activated, which relays an amplified signal [via a signal cascade] from the cell surface receptor to the target molecules inside the cell. An example of a signal cascade is the cAMP-dependent pathway, where the binding of the peptide hormone to the cell surface receptor activates a G protein, which either activates or inhibits adenylate cyclase (another transmembrane protein). Adenylate cyclase converts several ATP into cAMP (cyclic AMP). cAMP in turn may activate several protein kinase, which phosphorylates or dephosphorylates an enzyme, which in turn alters protein activity. As peptide hormones do not enter a cell, they are not used up, and hence can be recycled
• Steroid hormones, which are derived from cholesterol, and therefore have a structure similar to cholesterol. Cholesterol is an example of a steroid, although not a hormone. Therefore, steroid hormones are four-ringed structures, which are fat soluble. [In contrast with proteins which were synthesized at the rough ER,] as steroid hormones are lipids, they are synthesized at the smooth ER and mitochondrion. [In contrast with peptide hormones which could be transferred directly in blood,] as steroid hormones are fat soluble, they require carrier proteins to travel in blood. [In contrast with peptide hormones which could not pass through the lipid bilayer membrane,] as steroid hormones are fat soluble, they can diffuse freely through the lipid bilayer membrane of the target cell, and bind to a protein receptor, which as a bound unit can now enter the nucleus. The steroid hormone can then alter DNA, and induce transcription [into mRNA], and therefore translation [into protein] as required. Steroids are synthesized in the adrenal cortex, gonads, and placenta. Gonad is an organ which makes gametes, which is testes in males, and ovaries in females
• Tyrosine derivatives, which are derived from tyrosine, an amino acid with a polar side group. These include:
• Catecholamines, which include adrenaline (epinephrine) and noradrenaline (norepinephrine). Catecholamines [though steroid hormones] behave like peptide hormones
• Thyroid hormones, which include T4 (thyroxine) and T3 (triiodothyronine). Thyroid hormones behave like steroid hormones. As a part of the HPA axis, the hypothalamus secretes TRH (thyrotropin-releasing hormone), which stimulates the anterior pituitary to secrete TSH. TSH (thyroid-stimulating hormone) is a hormone which stimulates the thyroid gland to produce more thyroid hormones. Thyroid hormones demonstrate negative feedback, meaning it causes both hypothalamus and anterior pituitary to produce less TRH and TSH respectively. Note that negative feedback doesn’t cause high thyroid concentration to reduce thyroid concentration directly, but rather, indirectly, through reducing the concentration of its stimulant hormone

“Did Jesus experience stress?” Emily asked.

“Yes He did, His fully human nature whilst on earth was as ordinary weak as our limitations, relating to anxiety, stress, emotion, weariness, hunger, thirst, morality, and limitation in knowledge [and thus He could grow up],” Mandy replied, “otherwise, you fail to explain His evident lack of omniscience regarding the Second Coming (Matthew 24:36), in addition to requirement for fasting and prayer.”

“This emphasizes the mistake of replacing that Jesus is ‘truly God’ per the Nicene creed with the more modern ‘100% God’, and is a Gnostic heresy, attempting to exalt Christ by denying His humanity.”

“Rather, as a result of His humanity, Jesus was a charismatic who fulfilled His ministry only by reliance on the Holy Spirit, so how much more do we do!” Blaire exclaimed, “Jesus was conceived through the Holy Spirit (Luke 1:35), begun His ministry after infilling of the Holy Spirit at His baptism (Luke 3:21-22), conducted exorcisms (Matthew 12:28), healing (Acts 10:38), preaching (Luke 4:14-21) through the Holy Spirit, standing in the role of Jesus to continue His ministry in His absence (John 16:7,13), and even the ability to atone (Hebrews 9:14).”

Endocrine glands are glands that produce hormones. They include, going from the top to the bottom of the body–

Hypothalamus is located below the thalamus (which is at the center of the brain). Hypothalamus secretes hypothalamic-releasing or –inhibiting hormones into portal vessels (i.e. blood vessels that directly link the hypothalamus and the anterior pituitary). Hypothalamic-releasing or –inhibiting hormones in turn stimulate or inhibit the secretion of anterior pituitary hormones. As hypothalamic-releasing and –inhibiting hormones are intrinsically linked with their related anterior pituitary hormone, their behaviors can be predicted from their name, for example, gonadotropin-releasing hormone which stimulates the release of gonadotropins, namely FSH and LH; prolactin-inhibiting hormone which inhibits the release of prolactin; or thyrotropin-releasing hormone, which stimulates the release of thyrotropin, also known as the TSH.

In contrast, as the posterior pituitary consists of [neuron’s] axons, whose soma is in the hypothalamus, the hypothalamus produces the posterior pituitary’s hormones directly, as they are the same cell.

Pituitary gland, which only produces peptide hormones. Tropic hormone means it is a hormone that has another endocrine gland as its target. Pituitary can be divided by hormone production by the anterior and posterior

 Target Function Anterior pituitary, whose hormones can be remembered by the mnemonic “FLAT PiG”, including: FSH (discussed ) Gonads LH (discussed ) Gonads ACTH (adrenocorticotropic hormone) Adrenal cortex (hence the “adrenocortico” prefix) To release glucocorticoids. Note that although mineralocorticoid (such as aldosterone) is also produced by the adrenal gland, it is not stimulated by ACTH TSH (thyroid-stimulating hormone) Thyroid To release thyroid hormone, which as expressed , includes thyroxine and triiodothyronine. Note that although calcitonin is also produced by the thyroid gland, it is not stimulated by TSH Prolactin Mammary gland Involved with mammary gland development, and stimulation of mammary glands to produce milk GH (growth hormone, somatotropin) GH is unique amongst the anterior pituitary hormones, as it doesn’t have a specific target tissue, but just causes everything to grow Stimulates growth of the whole body, by increasing protein synthesis, and increasing use of fats for source of energy. Related hormones to somatotropin are somatostatin (aka GH-inhibiting hormone, which inhibits GH. It is produced by the hypothalamus; as well as delta cells of the pancreas), and somatomedin (aka insulin-like growth factors, IGF, are hormones produced when stimulated by GH, that increase bone growth) Posterior pituitary, which is located behind the anterior pituitary. Its hormones include: Oxytocin Mammary glands, uterus Causes milk letdown (movement of milk from the mammary glands to the nipple), and uterine contractionProlactin and oxytocin can be differentiated because prolactin is involved with milk production, whereas oxytocin and orgasm both start with an “O”, and cause uterine contraction Vasopressin (aka antidiuretic hormone, ADH) Kidneys Increases water retention by increasing water reabsorption at the distal convoluted tubule and collecting ducts of the [kidney’s] nephron. ADH also acts on the CNS, increasing thirst, and thus water intake

Thyroid gland, which is located just below the larynx, in front of and on either side of the trachea. Its hormones include:

• Thyroid hormones, including $T_{4}$ and $T_{3}$, mentioned . Thyroid hormones act on nearly every cell in the body, to increase basal metabolic rate, which is the energy expended at rest, and therefore increase metabolism. Thyroxine ($T_{4}$) is the prohormone (precursor) of triiodothyronine ($T_{3}$), its active form. $T_{4}$ contains four iodine atoms, and $T_{3}$ contains three iodine atoms. As mentioned , thyroid hormones are tyrosine derivatives. As also mentioned , as they behave like steroid hormones, they are fat soluble, and therefore require a carrier protein to transport in blood, can diffuse through cell membrane, are carried to the cytoplasm with another carrier protein, and alter protein synthesis
• Calcitonin, which is a peptide hormone, which reduces blood calcium, by causing more calcium to be stored in bone. This can be memorized with the mnemonic that calcitonin “tones down” blood calcium (i.e. reduces it). Calcitonin inhibits osteoclast activity. Osteoclast break down bone, and deposit the constituent minerals [including calcium and phosphate ions] into the blood stream

Parathyroid gland, which are a set of four small glands, located on the rear of the thyroid gland. Parathyroid gland secretes parathyroid hormone (PTH), a peptide hormone, which promotes osteoclast activity, therefore releasing calcium into the bloodstream, thereby increasing blood calcium [thereby acting in an opposite manner to calcitonin]. PTH also increases reabsorption of calcium in the kidney [from to-be urine back into the blood] and intestine [from to-be feces into the blood]. Although [like calcium], as bone is degraded, phosphate is released, PTH decreases reabsorption of phosphate in the kidney (contrary to calcium). Although PTH does increase reabsorption of phosphate in the intestine, this effect is not as great as the decreased reabsorption in the kidney, thereby causing a net loss of phosphate in blood

Pancreas, which is both an endocrine and exocrine gland, located behind and slightly below the stomach. The region of the pancreas that is endocrine is the Islets of Langerhans, which contains:

• Alpha cells, which secrete glucagon, which is a peptide hormone that is 29-amino acids long, which promotes glycogenolysis and gluconeogenesis in the liver. Glycogenolysis is the lysis of glycogen into glucose. Gluconeogenesis is the synthesis of glucose from non-glucose precursors, for example, proteins and fats. Both glycogenolysis and gluconeogenesis raise blood glucose levels, which is the effect of glucagon. Note that since glucagon is a peptide hormone, as mentioned , since they are water soluble, they transport in blood directly, activate a cell surface receptor, and activate the second messenger system. Specifically, the cascade is, one glucagon peptide hormone activates adenylate cyclase, which produces several cAMP, which activate several protein kinase; the net effect being, that one glucagon can be responsible for the release of many glucose molecules in to the blood
• Beta cells, which secrete insulin, which is also a peptide hormone that is 51-amino acids long, which causes about 80% of all cells to become highly permeable to glucose. As mentioned  however, the brain (like the liver) doesn’t require insulin for efficient uptake of glucose, as an insulin-independent facilitated diffusion transporter is used to import glucose. In contrast, the remaining other 80% of cells, have insulin receptors, which are transmembrane protein receptors, which when activated by insulin, form glucose facilitated diffusion transporter protein channels. Note that in both instances, the uptake of glucose is via facilitated diffusion. As permeability to glucose increases, tissue uptake glucose from blood, so that blood glucose is reduced, which is the effect of insulin [which is opposite to the effect of glucagon]. Insulin also achieves this by converting excess glucose into glycogen and fat [for storage], decrease protein breakdown [for use as source of energy], but increasing protein synthesis from amino acid
• Delta cells, which secrete somatostatin (as already mentioned, GH-inhibiting hormone, also released by the hypothalamus). In the pancreas, somatostatin inhibits release of glucagon and insulin, and decreases GI tract motility. Decreased GI tract motility helps to prolong digestion time, and therefore increase nutrient absorption
• It also contains F cells (PP cells), which although very few in number, secrete pancreatic peptide

Adrenal gland (aka suprarenal glands, from “supra” meaning “on top”, and “renal” meaning “kidney”), which are located on top of either kidney. The adrenal gland has two distinct structures:

• Adrenal cortex (from Greek “cortex” meaning “shell”), which is the outer shell of the adrenal gland. Adrenal cortex synthesizes steroid hormones, including:
• Mineralocorticoids, as mentioned , the most important being aldosterone, which conserves sodium, by acting on the distal tubule [of the kidney’s nephron] to reabsorb sodium [from the to-be urine back into the blood stream], and secrete potassium. As a result of increased sodium in bloodstream, the osmotic pressure in bloodstream increases. Osmotic pressure is the pressure caused by the presence of solutes (discussed ), which due to inability of the solute (sodium) to move across a semipermeable membrane (blood vessel), results in the requirement for water (from the interstitial fluid, which is water that bathes and surrounds cells) to cross the semipermeable membrane (blood vessel) to balance concentrations. Therefore, increased sodium in bloodstream, causes increased water retention, and therefore increased blood pressure
• Glucocorticoids (from “gluco” meaning “glucose” and “cort” meaning “cortex”), which increase blood glucose. The most important glucocorticoid is cortisol, which is released in response to physical or psychological stress, including illness, trauma, and temperature extremes. Cortisol responds to stress (as a glucocorticoid) by increasing blood glucose, by conducting gluconeogenesis (already described as creation of glucose from fats and proteins) in the liver. Cortisol also responds to stress, by suppressing the immune system, therefore also reducing inflammation, thereby diverting energy away from this low-priority activity
• Adrenal medulla (from Greek “medulla” meaning “middle”), which is the central portion of the adrenal gland, which secretes catecholamines, which as expounded , are tyrosine derivatives that behave like peptide hormones [acting through a second messenger system], and include adrenaline (epinephrine) and noradrenaline (norepinephrine). Remembering from  that catecholamines are also neurotransmitters, specifically, used as a neurotransmitter in the postganglionic sympathetic neurons, their effect as hormones are analogous, inducing the sympathetic fight-or-fight response, except that whereas neurotransmitters are quickly degraded, hormones are not, causing a longer lasting effect. Apart from the fight-or-flight response including increasing heart rate as stated , fight-or-flight also involves vasoconstriction of blood vessels generally to redirect blood to vessels supplying muscles which vasodilate, and liberate nutrients [particularly fat and glucose] for muscular action. Adrenal medulla is composed mainly of chromaffin cells, which as stated , are modified postganglionic sympathetic neurons (which as stated just , are the neurons which normally use the catecholamines as neurotransmitters)

(Specifically sexual endocrine glands, which will be discussed .)

 Learning activity What is the endocrine system?

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