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Hormones

Hormones are chemicals messengers that travel in the blood.

Hormonal communication is much slower than neural impulses. Response times of target organs can range from minutes to hours. However, hormones can have more widespread effects than neural impulses:

  1. Hormone is produced by an endocrine gland (hormone-secreting organ)
  2. Gland secretes hormone into the blood
  3. Hormone travels to the target organ(s)
  4. Target organs produce a response

Hormones are removed from the blood and broken down by the liver.

Glands and hormones are effectors. Glands respond to stimuli by altering the release of hormones.

Before a meal, the gut secretes a hormone (ghrelin) which increases appetite!

Target cells (that respond to a hormone) have receptors that recognise that specific hormone.
Target cells (that respond to a hormone) have receptors that recognise that specific hormone.

Hormones are the chemical messengers of the endocrine system.

The endocrine system coordinates the release of hormones across all the glands in the body.

The endocrine system is spread across the whole body!
The endocrine system is spread across the whole body!

Cell signalling describes the ways in which cells communicate. Extracellular chemical messages released from one cell can result in changes within another cell.

There are several stages within cell signalling:

  1. Binding of the signal to the receptor (ligand-receptor interaction)
  2. Transduction of the signal - when a signalling molecule activates a receptor
  3. Cellular response

Tyrosine kinase receptors and G-protein coupled receptors are common types of receptors.

There are several common features of cell signalling pathways. One is the amplification of the signal and the other is the phosphorylation of intermediate reactants.

Amplification is important as it allows for low concentrations of a hormone to have large effects within the cell.

Insulin is a hormone. It lowers glucose levels in the blood.

Insulin is released from $$\beta$$ cells in the islets of Langerhans of the pancreas.

Insulin lowers glucose levels in the blood by:

  • Increasing the permeability of cells to glucose.
  • Inhibiting the breakdown of glycogen to glucose in the liver and muscle.
  • Increasing the rate of glycogen synthesis from glucose.

Insulin is unable to pass through cell membranes and so binds to receptors on the surface of cells. There are insulin receptors on the membrane of almost every cell.

Insulin works antagonistically (in an opposite way) with glucagon, which raises blood glucose levels. This relationship helps to maintain homeostasis.

Insulin signal transduction refers to the events that occur when the hormone binds to the receptor molecule.

Insulin is a lipid insoluble molecule (it is a protein) and therefore has to bind to extracellular receptors.

Receptor tyrosine kinases (RTKs) are common receptors for hormones. Kinases are a category of enzymes that can add phosphate groups (phosphorylate) to other molecules, in this case the amino acid tyrosine.

When insulin binds to the receptor, the receptor becomes phosphorylated. This activates further processes that amplify the signal.

An important molecule that is activated by this pathway is protein kinase B (PKB).

PKB moves glucose transporters to the plasma membrane, increasing the glucose permeability of the cell. It can also activate glycogen synthase to convert glucose to glycogen.

Glucagon is one of two major hormones involved in the control of blood glucose.

Glucagon raises glucose levels in the blood. It is released from $$\alpha$$ cells in the islets of Langerhans of the pancreas.

Glucagon binds to cell surface receptors present on liver cells. It stimulates the breakdown of glycogen in to glucose (glycogenolysis).

It also stimulates gluconeogenesis, the synthesis of glucose from amino acids. Glucagon levels rise between meals as levels of glucose in the blood begins to drop.

Glucagon works antagonistically with insulin, which lowers blood glucose levels.

The hormone glucagon works by a signal transduction pathway. This pathway is slightly different from the signal transduction pathway of insulin.

Glucagon binds to a G-protein coupled receptor (GPCR) on the plasma membrane. These are the most common cell surface receptors.

G proteins are proteins made up of three subunits, $$\alpha$$, $$\beta$$ and $$\gamma$$. GPRCs interact with these proteins and activate them.

A single GPCR can activate several G-proteins. This is an example of amplification.

The G-protein in the glucagon pathway activates the enzyme adenylate cyclase. Adenylate cyclase then sets off further cellular processes.

These processes involve several reactions and enzymes. The net result is the conversion of glycogen to glucose, which increases blood glucose concentration.

Glucose is the primary source of cellular energy. It is broken down during respiration to produce ATP. Glucose is transported around the body in the blood.

The level of glucose in the blood needs to be kept within narrow limits (80-120 mg cm$$^{-3}$$). Deviations from this level can have severe repercussions on the body, so it is kept under homeostatic control via a negative feedback loop.

  • Hypoglycaemia (low blood glucose levels) can lead to shaking, muscle weakness, tiredness and confusion.
  • Hyperglycaemia (high blood glucose levels) can lead to excessive thirst, vision problems, weight loss and fatigue. Without treatment, both conditions can be fatal.

The hormones insulin and glucagon are vital in the control of blood glucose. They are both produced in the pancreas, in groups of specialised endocrine cells called the islets of Langerhans.

The islets of Langerhans contain several cell types. These cell types include $$\alpha$$ and $$\beta$$ cells that release glucagon and insulin respectively.

Diabetes mellitus is a disease in which people are unable to regulate their blood glucose levels.

Diabetes is caused by problems with insulin, the hormone that lowers blood glucose levels. Classical symptoms of diabetes are:

  • High blood glucose levels, particularly after meal times.
  • Increased glucose in urine.

There are two types of diabetes:

Type of diabetes Causes Treatment
Type I

(Juvenile-onset diabetes)

Pancreas cannot produce enough insulin and/or

Pancreas produces defective insulin

Manual injection of insulin

Balancing insulin injection with diet

Type II

(Adult-onset diabetes)

Insensitivity of cells to insulin.

Poor diet

Does not initially require insulin injection

Regulating sugar intake

Roughly 90% of diabetics have type II diabetes. Type II diabetes is becoming a big problem in countries (such as the US) where obesity is high.

Diabetes patients need to regularly monitor and manually control their blood sugar levels.
Diabetes patients need to regularly monitor and manually control their blood sugar levels.

Blood glucose concentration is maintained within a safe range in the blood by homeostasis.

The blood needs a certain level of glucose to enable cells to respire. However, having too much glucose in the blood can be harmful.

Long term high blood sugar (hyperglycaemia) can lead to complications such as blindness and kidney damage.

The body has two hormones that have opposing effects on blood glucose. Both hormones are produced by specialised cells in the pancreas called islets of Langerhans.

  • Insulin is released when blood glucose is too high. It causes the removal of glucose from the blood by body tissue.
  • Glucagon is released when blood glucose is too low. It causes the release of glucose from cell stores into the blood.
The levels of blood glucose and insulin change through the day.
The levels of blood glucose and insulin change through the day.

Adrenaline is the hormone responsible for the group of changes associated with the fight or flight response.

This response occurs when you feel you are in danger or are scared. It prepares the body either to fight or to run away, hence the name!

  • Adrenal glands release adrenaline
  • Heart beats faster
  • Breathing becomes faster and deeper
  • Pupils dilate
  • Blood is diverted away from skin and gut to the muscles
  • Liver converts glycogen to glucose, providing more energy

These physiological changes provide more oxygen and glucose for muscles. This is used to produce energy for the animal to either fight or run away (take flight)!

Adventure sports like rock climbing cause the body to release adrenaline.
Adventure sports like rock climbing cause the body to release adrenaline.