Hormonal Communication

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Hormonal Communication

The Endocrine System

Made up of endocrine glands, a group of cells specialised to secrete chemicals (hormones) that are secreted straight into the bloodstream

Examples of endocrine glands are the pancreas and adrenal gland

Hormones

Chemical messengers that carry information from one part of the body to another

Can be steroids, proteins, glycoproteins, polypeptides, amines or tyrosine derivatives

Hormones are secreted directly into the bloodstream when a gland is stimulated, can occur as a result of a change in the concentration of substances such as blood glucose, can also occur as the result of another hormone or nerve impulse

Once secreted hormones are transported in the blood plasma all over the body. The hormones diffuse out of the blood and bind to specific receptors for that hormone, found on the membranes, or in the cytoplasm of cells in the target organ. These are known as target cells. Once the hormones have bound the hormones stimulate the target cells to produce a response

Steroid hormones: lipid soluble, they pass through the lipid component of the cell membrane and bind to steroid hormone receptors to form a hormone-receptor hormone. The hormone-receptor complex formed acts as a transcription factor which in turn facilitates or inhibits the transcription of a specific gene e.g. oestrogen

Non-steroid hormones: hydrophilic so cannot pass directly through the cell membrane, the bind to specific receptors on the cell surface membrane of the target cell, this triggers a cascade reaction mediated by chemicals called second messengers e.g. adrenaline

Adrenal glands

Two small glands that are located on top of each kidney and are made up of 2 distinct parts surrounded by a capsule:

The adrenal cortex- outer region of the glands, produces hormones that are vital to life e.g. cortisol and aldosterone

The adrenal medulla- inner region of the glands, produces non essential hormones e.g. adrenaline

Adrenal cortex

Produces hormones and is controlled by hormones released by the pituitary gland in the brain

Glucocorticoids- e.g. cortisol which helps to regulate metabolism by controlling how the body converts fats, proteins and carbohydrates to energy, also helps to regulate blood pressure and cardiovascular function in response to stress. Another example is corticosterone, this works with cortisol to regulate immune response and suppress inflammatory reactions. The release of these hormones is controlled by the hypothalamus

Mineralocorticoids- main hormone is aldosterone which helps control blood pressure by maintaining the balance between salt and water concentrations in the blood and body fluids, release is mediated by signals triggered by the kidney

Androgens- small amounts of male and female sex hormones are release, impact is relatively small compared to testosterone and oestrogen but they're still important

Adrenal medulla

Released when the sympathetic nervous system is stimulated which occurs when the body is stressed (fight or flight)

Adrenaline- increases heart rate by sending blood quickly to the muscles and brain, rapidly raises blood glucose concentration levels by converting glycogen to glucose in the liver

Noradrenaline- works with adrenaline in response to stress, producing effects such as increased heart rate, widening of pupils, widening of air passages in lungs, narrowing of blood vessels in non essential organs which results in higher blood pressure

Structure and function of the pancreas

Function

Exocrine gland- to produce enzymes and release them via a duct into the duodenum

Endocrine- to produce hormones and release them into the blood

Role as an exocrine gland

Responsible for producing digestive enzymes and pancreatic juice (alkaline fluid), secreted into ducts and released into the duodenum

Amylases- breaks down starch into simple sugars, e.g. pancreatic amylase

Proteases- break down proteins into amino acids e.g. trypsin

Lipases- break down lipids into fatty acids and glycerol e.g. pancreatic lipase

Role as an endocrine gland

Responsible for producing insulin and glucagon

The islets of langerhans are responsible for producing insulin and glucagon, and secreting hormones directly into the bloodstream

alpha cells- produce and secrete glucagon

beta cells- produce and secrete insulin

Alpha cells (stain pink) are larger and more numerous than beta cells (stain blue) within the islet

Regulation of blood glucose concentration

Increasing blood glucose concentration

Diet- eating carbohydrates which are then broken down to release glucose into the bloodstream and the blood glucose concentration rises

Glycogenolysis- glycogen stored in the liver and muscle cells is broken down into glucose which is released into the bloodstream, increasing blood glucose concentration

Gluconeogenesis- the production of glucose from non carbohydrate sources, e.g. the liver making glucose from glycerol (lipids) and amino acids, the glucose is released into the bloodstream and causes an increase in blood glucose concentration

Decreasing blood glucose concentration

Respiration- some of the glucose in the blood is used by cells to release energy, during exercise more energy is demanded and the blood glucose concentration decreases

Glycogenesis- the production of glycogen to reduce blood glucose concentration, excess glucose taken in through diet is converted into glycogen stored in the liver

Role of insulin

Produced by the beta cells in the islets of Langerhans in the pancreas, if the blood glucose concentration is too high, the beta cells secrete insulin directly into the bloodstream

When insulin binds to its glycoprotein receptor it causes a change in the tertiary structure of the glucose transport protein channels causing them to open and allow more glucose to enter the cell, insulin also activates enzymes within some cells to convert glucose to glycogen and fat

How insulin lowers blood glucose concentration:

Increasing the rate of glucose absorption by cells e.g. skeletal muscle cells

Increasing the respiratory rate of cells

Increasing the rate of glycogenesis

Increasing the rate of glucose to fat conversion

Inhibiting the release of glucagon from the alpha cells of the islets of Langerhans

Insulin is broken down by enzymes in the cells of the liver so to maintain its effect has to be constantly secreted

As blood glucose concentration returns to normal the beta cells in the pancreas detect this and when it falls below a set level the beta cells reduce their secretion of insulin (negative feedback)

Negative feedback ensures that in any control system changes are reversed and returned back to the set level

Role of glucagon

Produced by the alpha cells in the islets of langerhans in the pancreas

If the blood concentration levels are too low the alpha cells detect this fall and respond by secreting glucagon directly into the bloodstream

The only cells in the body that have glucagon receptors are the liver and fat cells, therefore these are the only cells that can respond to glucagon

How glucagon raises blood glucose concentration:

Glycogenolysis- the liver breaks down its glycogen store into glucose and releases it back into the bloodstream

Reducing the amount of glucose absorbed by the liver cells

Increasing gluconeogenesis- increasing the conversion of amino acids and glycerol into glucose in the liver

As blood glucose concentration returns to normal, this is detected by the alpha cells in the pancreas, when it rises above a set level the alpha cells reduced their secretion of glucagon (negative feedback)

Control of insulin secretion

When blood glucose concentration rises above the set level, the beta cells in the islets of langerhans detect this change and insulin is secreted

1. At normal concentration levels, potassium channels in the plasma membrane of beta cells are open and potassium ions diffuse out of the cell, the inside of the cell has a potential of -70mV

2. When the concentration rises, glucose enters the cell by a glucose transporter

3. The glucose is metabolised inside the mitochondria resulting in the production of ATP

4. The ATP binds to potassium channels and causes them to close (ATP sensitive potassium channels)

5. As potassium ions can no longer diffuse out of the cell, the potential difference reduces to around -30mV and depolarisation occurs

6. Depolarisation causes the voltage-gated calcium channels to open

7. Calcium ions enter the cell and cause secretory vesicles to release the insulin they contain by exocytosis

Diabetes and its control

Types of diabetes

Unable to produce insulin, the beta cells in the islets of Langerhans don't produce insulin (cannot be prevented or cured) Evidence suggests it arises as a result of an autoimmune response where the body's own immune system attacks the beta cells

Cannot effectively use insulin and control their blood sugar levels. Either because the persons beta cells don't produce enough insulin or the persons body cells don't respond properly to insulin. Often because the glycoprotein insulin receptor on the cell membrane doesn't work properly and the cells lose their responsiveness to to insulin and don't take up enough glucose, leaving it in the bloodstream. Caused by excess body weight, physical inactivity and symptoms are similar to type 1 diabetes

Diabetes is currently not a curable disease but can be controlled successfully

Medically produced insulin

Originally insulin was obtained from the pancreas of cows and pigs, this was difficult and expensive and the insulin extracted could cause allergic reaction because it differs slightly from human insulin

Human insulin can now be produced from genetically modified bacteria and is better because it is produced in a pure form so less likely to cause an allergic reaction, can be produced in much higher quantities and at a cheaper price

Potential use of stem cells in diabetes treatment

There has been an attempt to cure diabetes by injecting patients with pancreatic beta islet cells but fewer than 8% of cell transplants performed have been successful

The immunosuppressant drugs used to prevent rejection of these cells increases the metabolic demand on insulin-producing cells which eventually exhausts their capacity to produce insulin

As type 1 diabetes results from the loss of a single cell type there is also evidence that a small number of islet cells can restore insulin production the disease is a perfect candidate for stem cell therapy

Advantages of stem cells over current therapies:

Donor availability wouldn't be an issue- stem cells could produce an unlimited source of new beta cells

Reduced likelihood of rejection problems as embryonic stem cells are generally not rejected by the body, can also be made by somatic nuclear transfer

People no longer have to inject themselves with insulin

Because our ability to control growth and differentiation is limited stem cells transplanted into the body might induce the formation of tumours as a result of unlimited cell growth

Coordinated responses

Fight or flight response

When a potentially dangerous situation is detected, the body automatically triggers a series of physical responses which are intended to increase the mammals survival rate

Once a threat is detected by the autonomic nervous system, the hypothalamus communicates with the sympathetic nervous system and the adrenal-cortical system. The sympathetic nervous system uses neuronal pathways to initiate body reactions whereas the adrenal-cortical system uses hormones in the bloodstream

The sympathetic nervous system send out impulses to glands and smooth muscles and tells the adrenal medulla to release adrenaline and noradrenaline into the bloodstream

The release of other stress hormones which have a longer-term action from the adrenal cortex is controlled by hormones produced by the pituitary gland in the brain

They hypothalamus stimulates the pituitary gland to secrete ACTH which travels in the bloodstream to the adrenal cortex, where it activates the release of many hormones

Action of adrenaline

Triggers liver cells to undergo glycogenolysis so that glucose is released into the bloodstream, this allows respiration to increase so more energy is available for muscle contraction

Adrenaline is a hydrophilic hormone so cannot pass through the cell membrane, instead it binds with receptors on the surface of a liver cell membrane and triggers a chain reaction inside the cell

When adrenaline binds to its receptor the enzyme adenylyl cyclase is activated

This triggers the conversion of ATP into cAMP on the inner surface of the cell membrane in the cytoplasm

The increase in cAMP levels activates specific enzymes called protein kinases which phosphorylate and hence activate other enzymes

This model of hormone action is known as the second messenger model, adrenaline is the first messenger and cAMP is the second messenger

One hormone molecules can cause many cAMP molecules to be formed and at each stage the number of molecules involved increases so the process is said to have a cascade effect

Controlling heart rate

Involuntary and controlled by the autonomic nervous system

The medulla oblongata is responsible for controlling heart rate and making any necessary changes

There are 2 centres within the medulla oblongata linked to the SAN in the heart by motor neurones

One centre increases heart rate by sending impulses through the sympathetic nervous system, these impulses are transmitted by the accelerator nerve

One centre decreases heart rate by sending impulses through the parasympathetic nervous system, these impulses are transmitted by the vagus nerve

Which centre is stimulated depends on the information received by receptors in the blood vessels the two types of receptors affecting heart rate are:

Baroreceptors- (pressure receptors) detect changes in blood pressure, present in the aorta, vena cava, and carotid arteries

Chemoreceptors- (chemical receptors) detect changes in the level of particular chemicals in the blood e.g. carbon dioxide, present in the aorta, carotid artery and the medulla

Chemoreceptors

Sensitive to pH changes in the blood, if the carbon dioxide levels increase, the pH of the blood decreases because carbonic acid is formed when the carbon dioxide interacts with water in the blood. If the chemoreceptors detect a decrease in pH a response is triggered to increase heart rate and blood therefore flows quicker to the lungs so the carbon dioxide can be exhaled

When the carbon dioxide level in the blood decreases, the pH of the blood rises, this is detected by the chemoreceptors in the wall of the carotid arteries and the aorta. This results in a reduction of the frequency of impulses being sent to the medulla oblongata and in turn reduces the impulses being sent to the SAN via the sympathetic nervous system and heart rate decreases to its normal level

Baroreceptors

Present in the aorta and carotid artery wall detect changes in pressure. If blood pressure is too high, impulses are sent to the medulla oblongata centre which decreases heart rate and the medulla oblongata sends impulses along the parasympathetic neurones to the SAN which decreases the rate at which the heart beats, reducing blood pressure back to normal

If blood pressure is too low, impulses are sent to the medulla oblongata centre which increases heart rate and then impulses are sent along sympathetic neurones to the SAN which increases the rate at which the heart beats and blood pressure increases to return to normal levels

Hormonal control

Heart rate can also be influenced by hormones such as adrenaline and noradrenaline, these affect the pacemaker region of the heart and speed up heart rate by increasing the frequency of impulses produced by the SAN