Chemical Messengers of the Brain: Understanding the Role of Hormones in Behavior and Emotion


Hormones are chemical messengers released from endocrine glands that influence the nervous system to regulate the physiology and behavior of individuals. It regulates various body functions such as growth, reproduction, digestion, etc.

There are 2 types based on their chemical composition

  • Protein hormone: These hormones are soluble in water. Eg; Adrenaline, Noradrenaline.
  • Steroid hormone: These hormones are not soluble in water. Eg: Thyroid hormone.



Each hormone has receptors found on the target organ’s cell membrane. Once the hormone bind to its designated receptor, a series of actions are initiated to release secondary messengers inside the cell. These secondary messengers are responsible for relaying (passing one to another) information to the nucleus or other organelles.

Based on the structure, receptors are of different types

Internal receptors

  • They can be either nuclear or cytoplasmic
  • Nuclear receptors are found on the nuclear membrane
  • Cytoplasmic receptors are found in the cytoplasm of the cell
  • These receptors are of steroid hormones.

External receptors

  • These are the transmembrane (that exist or occur across the cell membrane) receptors that are embedded in the lipid layer of the cell membrane. These receptors are for the protein ones.

The mechanism of action hormones can be of 2 types

  1. Fixed Receptor Mechanism

The mechanism of action hormone is seen in the protein hormones such as Adrenaline, Insulin, ADH, TSH, etc.

They are water-soluble, they cannot pass through the cell membrane as it is made up of a lipid layer. Once the protein hormone binds to the receptor, a series of reactions occur beginning with the production of the adenyl cydase enzyme. This leads to the production of cyclic AMP or CAMP which is a secondary messenger.

2. Motor Receptor Mechanism

This kind is seen in steroid hormones. Insoluble in water. It is made up of fats, therefore can freely cause the lipid layer of the cell membrane. Their receptors are intracellular and not extracellular like those for Protein ones. These extracellular receptors can be floating in the cytoplasm, on the nuclear membrane, or inside the nucleus.

These receptors are also known as mobile receptors.


Hormones cause cellular changes by binding to receptors on target cells. The number of receptors on the target cell can increase or decrease in response to hormone activity.

The hormone can affect cells directly through intracellular hormone receptors or indirectly through plasma membrane hormone receptors.

Intracellular Hormone receptor

  • Receptor proteins are found on the inside of the cell, typically in the cytoplasm or Nucleus. Ex: Thyroid Hormone

Plasma Membrane Hormone Receptor

  • Lipid-insoluble hormone bind to receptors on the outer surface of the plasma membrane.

Hormones mediate changes in target cells by binding to specific hormone receptors. Hormone circulates throughout the body and comes in contact with many different cell types, they only affect the cells that possess the necessary receptors.

The hormone binds to the receptor proteins, resulting in the activation of Signal Transduction ( it is the transmission of molecule signals from the cell exterior to its interior.) mechanism which leads to cell type-specific response.

Receptor binding alters cellular activity and results in an increase or decrease in the normal body process, depending on the location of the protein receptor on the target cell and the chemical structure of the hormone.

Hormones can meditate changes directly by binding to intracellular hormone receptors and modulating Gene transcription (the process of making an RNA copy of a gene’s DNA sequence, this copy is called Messenger RNA -mRNA-carries gene’s protein information encoded in DNA) or indirectly by binding to cell surface receptors and stimulating signal pathways. Lipid soluble hormone (steroid) diffuses across the lipid layer membrane of endocrine cells.

Once outside the cells, they bind to transport proteins that keep them soluble in the bloodstream. At the target cell, the hormones are released from carrier protein and diffuse across the lipid layer of the plasma membrane of target cells. They then adhere to intracellular receptors residing in the cytoplasm or nucleus. The cell signaling pathways induced by the steroid hormone regulate specific genes within the cell’s DNA.

The endocrine system is a very important and very involved system within our body, it allows the body to communicate to larger distances and is a major player in regulating a stable environment or homeostasis. Hormone bind to receptor cells with the release of the secondary messenger. peptides and catecholamines use secondary messenger to communicate the receptor


Although the hormones have different chemical structures and are produced by different categories of organs, they all influence growth and development by stimulating protein synthase

The growth and activity of an individual is influenced by 4 major sources of hormones

  • The central nervous system
  • The thyroid gland
  • The adrenal gland
  • The gonal gland

Hormones play a significant role in the development and overall health of the individual. Abnormal functioning of any endocrine gland in the body has dramatic implication. The main hormone concerned with growth is the pituitary, growth hormone, thyroid hormones, the sex hormones- testosterone and estrogen, and pituitary gonadotropic hormones.

Thyroid Hormone
  • Play an important part in metabolism, growth, and development
  • Increase protein synthesis
Growth Hormone
  • Protein hormone
  • Increase in weight gain, and feed conversion while decreasing food intake
  • The role of growth hormone include influencing height, helping build our bones and muscles
  • Facilitate fat deposition
  • Responsible for growth and maturation
  • Growth and development of bones and muscles

These hormones help coordinate your body’s function from metabolism to growth and development of emotions, mood, sexual function, and even sleep.


  • The feedback mechanism is defined as the mechanism that is used to maintain the hormone balance in the blood/body.
  • The increase or decrease in the concentration of hormones can sometimes stimulate and increase the secretion of hormone or inhibit the secretion of hormones. It is called feedback.
  • The feedback mechanism controls the most hormone that are secreted by endocrine glands
  • The feedback mechanism is a loop with the help of which a product feeds to regulate its own production.

2 Types of Feedback Mechanisms

  • Positive Feedback
  • Negative Feedback

Positive feedback- Stimulates and enhances the secretion and production of the hormone

Negative feedback- Prevents and inhibits the hormone from being secreted

Most mechanism of hormone feedback constitutes negative loops. Negative feedback helps and maintains the hormone concentration within a limited range.

Eg: Glucose level in the blood increases when we eat food that is rich in carbohydrate. The pancreas synthesizes and releases insulin as the amount of blood glucose rises. It will stimulate blood glucose absorption by the cells. This is the process of how the glucose level drops.


The endocrine system is made up of glands that make hormones. Hormones are the body’s chemical messenger. They carry information and instructions from one set of cells to another.

  • The endocrine gland release hormones into the bloodstream, which lets the hormones travel to cell in other parts of the body.
  • The endocrine hormones help control mood growth and development, the way our organs work, metabolism, and reproduction
  • The endocrine system regulates how much of each hormone is released
  • This can depend on levels of hormones already in the blood, or on levels of other substances in the blood, like calcium

While many parts of the body make hormones, the major gland that makes up the hormone system are

  • Hypothalamus
  • Pituitary
  • Thyroid
  • Parathyroid
  • Adrenal
  • Pineal body
  • The ovaries
  • The testes

The pancreas is part of the endocrine system and the digestive system. It secretes hormones into the bloodstream and makes and secretes enzymes into the digestive tract.


  • The lower central part of the brain
  • It links the endocrine system and the nervous system
  • Nerve cells in the hypothalamus make chemicals that control the release of hormones secreted from the pituitary gland
  • The hypothalamus gathers information sensed by the brain and sends it to the pituitary


  • The pituitary gland is at the base of the brain and is no bigger than a pea
  • The pituitary gland is also called the Master Gland
  • GH, growth hormone, stimulates the growth of bone and other body tissues and plays a role in the body’s handling of nutrients and minerals
  • Prolactin- activates milk production
  • Thyrotropin– stimulates the thyroid gland to make thyroid hormones
  • Corticotropin- stimulates the adrenal gland to make the certain hormone
  • Antidiceretic- helps control body water balance through its effects on the kidney
  • Oxytocin– triggers the contractions of the uterus that happen during labor
  • The pituitary also secretes endorphins, chemicals that act on the nervous system and reduce feelings of pain


  • It is in the frontal part of the lower neck
  • Shaped like butterfly
  • The thyroid hormone secretes thyroxine and triiodothyronine
  • These hormones control the rate at which cells burn fuel from food to make energy
  • Bone growth and development of the nervous system and brain


  • Attached to the thyroid are 4 tiny glands that work together called parathyroids
  • Parathyroid control the level of calcium in the blood with the help of calcitonin


Two triangular adrenal glands sit on the top of each kidney. The adrenal gland has 2 parts

Outer Part – Adrenal Cortex

  • Corticosteroids help in control salt and water balance in the body. The body’s response to stress, metabolism, immune system and sexual development and function.

Inner part – Adrenal Medulla

  • It makes catecholamines, such as epinephrine also called adrenaline, epinephrine increases blood pressure, and heart rate, when the body is under stress.


  • It is located in the middle of the brain, it secretes melatonin, a hormone that may help regulate when you sleep at night and when you wake in the morning


  • It makes insulin and glucagon hormones regulate or control the level of glucose or sugar in the blood
  • Insulin helps keep the body supplied with a store of energy
  • The body uses this stored energy for exercise and activity

Reproductive glands

The gonads are the main source of sex hormone


  • Male gonads or testes are in scrotum
  • Secretes androgen
  • Testosterone (puberty hormone)
  • Penis and height growth deepening voice, growth in facial pubic hair
  • Working with hormones from the pituitary gland, testosterone also tells a guy’s body when it’s time to make sperm in the testes.


  • Ovaries are in her pelvis
  • They make eggs and secrete female hormones and progesterone
  • Estrogen is involved when the girl starts puberty
  • During puberty, the girl will have breast growth, start to accumulate body fat around her hips and thighs, and have a growth spurt
  • Estrogen and progesterone in regulating the menstrual cycle
  • It also plays a role in pregnancy

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