5.0 Neuroendocrine System

Neuroendocrine System

HOMEOSTASIS is regulated by

• nervous & endocrine systems regulate the internal environment of the body → to confront internal

  & external changes

Homeostasis allow it to be

• modified appropriately when the internal & external environment require it.

Hormones

• A chemical messenger secreted by specialized cells into the bloodstream that can act at greater or lesser distances, often slowly, on specific organs & tissues.

Hormones allow

• cells in different parts of the body to coordinate their functioning

Regulatory mechanisms

• Nervous system

• Endocrine system

Nervous system

• Release of neurotransmitters

Endocrine system (homoestasis)

• Releasing of hormones into the bloodstream from glands & specialized tissues

Thymus function

• Release hormones that have local effects on cells of the immune system - lymphocytes

Distribution of the endocrine system

Endocrine

• Specialized endocrine cells secrete hormones, which travel through the bloodstream & act over target cells which are widely distributed throughout the body.

→ Ex: insulin, thyroid hormones, pituitary hormones.

Neuroendocrine

• Neurohormones are secreted by neurons, & travel through the bloodstream until they reach the target cell.

→ Ex: hormones released by the hypothalamus

Hormones bind to specific 

• receptors on the target cells

Hormone-receptor modifies

• the target cell

Response after the hormone has modified the target cell

• Changes in protein synthesis (increasing or decreasing expression of specific genes)

• Enzyme activation or inactivation (making reactions faster or slower)

• Regulating membrane transport (e.g. opening or closing ion channels)

A single hormone can act on 

• different tissues

The response to the same hormone can be 

• Different in different tissues (depends on the receptor)

Action mechanisms of hormones

• have different action mechanisms depending on whether they are HYDROPHOBIC/APOLAR or HYDROPHILIC/POLAR

HYDROPHOBIC/APOLAR

• Thyroid & steroid hormones

• They activate or repress genes.

• Hormone-receptor interaction → Transcription factors

• Much slower effect than hydrophilic hormones → 45 min to several days for a full effect

HYDROPHILIC/POLAR

• Peptides & amino acids

• G-protein-coupled receptors

• Receptors with enzymatic activity (Tyrosine kinase)

• Receptors are always on the surface of the plasma membrane

Hormone interactions

• Different hormones can cooperate to increase their influence on the target cell

• Synergism

• Antagonism

Synergism

• When a combination of several hormones has a greater effect than the simple sum (addition) of the individual effects (1+1=3), as opposed to an additive effect (1+1=2).

Antagonism (hormone)

• A hormone that has the opposite effect from another, so their effects cancel each other.

• Opposite physiological effects and they counterbalance each other.

Synergism: Example

• There are several hormones increasing circulating glucose through different molecular mechanisms:

  • Glucagon, cortisol, epinephrine

  • The effect of glucagon + epinephrine is higher than the addition of both individual effects

HYPOTHALAMUS

• Brain area specialized in the maintenance of homeostasis

Hypothalamus Regulates several physiological functions

• Hunger and thirst

• Body temperature

• Circadian rhythms

• Sleep and arousal

• Endocrine system

Hypothalamus dysfunction

• Eating & metabolic disorders

• Narcolepsy

• Mood changes & depression

HYPOTHALAMUS segregation (anterior)

• HYPOTHALAMUS → Pituitary portal system → ANTERIOR PITUITARY/ Adenohypophysis (Glandular tissue)

HYPOTHALAMUS segregation (posterior)

• HYPOTHALAMUS → POSTERIOR PITUITARY/ Neurohypophysis (Nervous tissue)

The anterior pituitary

• a true endocrine gland

• derived from embryonic epithelial tissue & produces six hormones (GH, TSH, ACTH, LH & FSH and Prolactin)

The posterior pituitary 

• is an extension of a nerve tissue

• stores two neurohormones (oxytocin & ADH) in hypothalamus.

Antidiuretic hormone (ADH) / vasopressin (secreted by posterior pituitary) is involved in

• Blood volume → Increases reabsorption of water at the renal level

• Participates in the regulation of blood pressure, it is secreted in response to increased blood osmolarity or low blood pressure

Oxytocin (secreted by posterior pituitary)

• females → contractions during childbirth & ejection of breast milk

• social behaviour & relationships (males/females)

• attachment bonds

• possible role in anti-depressant & anxiolytic role

Oxitocin & reproduction In women

• Labor: Stimulates uterine contractions via a positive feedback loop

• Breastfeeding: Enhanced by nipple stimulation, triggers milk ejection

• Sexual activity: Released during arousal & orgasm → muscle contractions, pleasure

Oxitocin & reproduction In men

• Released during sexual arousal & orgasm

• Supports ejaculation through rhythmic contractions

Oxytocin: the “tend-and-befriend” hormone → in both sexes

• Oxytocin is released as a response to physical touch, sexual activity, comforting experiences, & positive social stimuli.

• Promotes emotional bonding and intimacy

• Promotes empathy, cooperation & altruistic behavior.

• Enhances pleasure & reward (via interaction with dopamine & endorphins).

• Helps reduce stress & anxiety (multiple mechanisms)

Neurohormones from the hypothalamus

• control release of the anterior pituitary hormones

portal system

• The hypothalamic hormones reach the anterior pituitary through a specialized region of the circulation

Sequence of Hypothalamus & pituitary ANTERIOR PITUITARY / ADENOHYPOPHYSIS

• Hypothalamic neurons secrete TROPHIC NEUROHORMONES →

• ENDOCRINE CELLS in the anterior pituitary release the HORMONES to the blood →

• Hormones affect specific TARGET ORGANS

Hypothalamus

• neurons → portal veins → endocrine cells

Neurons

• Synthesizing trophic neurohormones release them inot capillaries of the portal system.

Portal veins

• Carry the trophic neurohormones directly to the anterior pituitary, where they act on the endocrine cells.

Endocrine cells

• release their peptide hormone into the second set of capillaries for distribution to the rest of the body

TROPHIC HORMONES

• Hormones that control the secretion of other hormones.

Negative feedback loops

• The hormones released inhibit their source glands decreasing their own synthesis

• It is the most common regulatory mechanism in the body

What are the steps of the neuroendocrine (hypothalamic–pituitary–endocrine gland) axis, including feedback loops?

• Stimulus

• → Hypothalamus (IC₁) → releases trophic hormone (H₁)

• → Anterior pituitary (IC₂) → releases trophic hormone (H₂)

• → Endocrine gland (IC₃) → releases hormone (H₃)

• → Target tissue → produces response

• Feedback loops:

  • Short-loop negative feedback: Pituitary hormones inhibit the hypothalamus

  • Long-loop negative feedback: Final hormone (H₃) inhibits both hypothalamus & pituitary

What are the steps of the hypothalamic–pituitary–adrenal (HPA) axis?

• Hypothalamus → releases CRH (corticotropin-releasing hormone)

→ Anterior pituitary → releases ACTH (adrenocorticotropic hormone)

→ Adrenal cortex → releases cortisol

→ Cortisol acts on target tissues → produces stress response

• Feedback:

  • Cortisol provides long-loop negative feedback to both hypothalamus & anterior pituitary

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Anterior pituitary hormones (H2) includes

• Growth hormone

• Corticotropin

• Thyrotropin

• Prolactin

• Gonadotropins

Growth hormone

Growth of bones & muscles

Corticotropin

• Act on adrenal gland to release glucocorticoids & adrenal androgens

Thyrotropin

• Act on thyroid gland to secrete the thyroid hormones T3 & T4

Prolactin

• Promotes the growth of mammary glands & breast milk production

Gonadotropins

• Act on reproductive organs & regulates reproductive cycles

  • LH: Luteinizing hormone

  • FSH: Follicle stimulating hormone