Chapter 13 - Hormones and Hormonal Systems

Glands

Divided into two types;

Exocrine Glands - Release chemicals into ducts, which carry them into targets (mostly on the surface of the body)

-Ex; sweat glands

Endocrine Glands - Ductless glands, release chemicals (hormones) directly into the circulatory system

-Pituitary, Thyroid, Parathyroid, Thymus, Adrenal, Pancreas, Ovary, Testis, Pineal

-Hormone travels via the circulatory system to its target (other glands, nervous system, etc)

Gonads

Endocrine glands (Testes and Ovaries)

-Primary function is the production of sperm cells and ova, with the single sperm cell fertilizing an ovum to for a Zygote

-Each sex cell contains 23 chromosomes, to form a zygote with 23 pairs of chromosomes

-22 pairs of a zygote are non-sex (autosomal), 23rd pair are sex chromosomes

Sex Chromosomes

Contain genetic programs that direct sexual development

-Cells of females have two large sex chromosomes (X chromosomes) whereas males have one X and one Y chromosome

-Y chromosome is far smaller (controls synthesis of only 66 proteins vs 615 proteins of X chromosome)

Hormones

1. Amino-Acid Derivative Hormones - Synthesized in a few simple steps from an amino acid molecule (ex; epinephrine synthesized from tyrosine, released from Adrenal Medulla)

2. Peptide/Protein Hormones - Chains of amino acids (peptides are short chains like oxytocin, proteins are long chains like Human Growth Hormone)

3. Steroid Hormones - Synthesized from cholesterol (a fat molecule)

Function of Steroid Hormones

Influence sexual development and the activation of adult sexual behaviour

-Small and fat-soluble, penetrate cell membrane and affect cells by binding to receptors in cytoplasm/nucleus

-Directly influence gene expression, producing diverse/long-lasting effects

-Peptide/protein hormones can also affect gene expression, but less commonly and less directly

Gonadal Hormones (Sex Steroids)

Released by both the testes and ovaries

1. Androgens - Testosterone is the most common type

2. Estrogen - Estradiol is the most common type

3. Progestins - Progesterone is the most common type (involved in preparing uterus/breasts for pregnancy, but also in neuroplasticity)

Adrenal Cortex

Endocrine gland, outer layer of the adrenal glands, regulates glucose and salt levels in the blood

-Releases small amounts of all of the sex steroids released by the gonads

Pituitary Gland

“Master gland” due to the release of various Tropic Hormones (which stimulates the release of hormones from other glands)

-Ex; Gonadotrophin is a tropic hormone that stimulates release of gonadal hormones

Pituitary gland divided into Posterior and Anterior pituitary which fuse during embryological development

Posterior Pituitary

Develops from small outgrowth of hypothalamic tissue, dangles from the hypothalamus on the end of the Pituitary Stalk

-Thus it has a neural connection between the posterior pituitary and hypothalamus

Anterior Pituitary

Pinches off from embryonic tissue that forms the roof of the mouth, migrates towards its position next to posterior pituitary

-Releases tropic hormones (therefore, the “master gland”), stimulate release of sex steroid hormones from testes and ovaries

Endocrine Function; Male vs Females

-In females, levels of gonadal and gonadotropic hormones go through a cycle that repeats itself every 28 days (these hormone fluctuations control the Menstrual Cycle)

-Males levels of gonadal and gonadotropic hormones change little from day to day (non-cyclical)

Anterior Pituitary and Male/Female Patterns Study

Was believed that differences in anterior pituitary (being the master gland) between males/females is the basis for the difference in gonadotropic/gonadal hormone release between males/females

-Anterior pituitary from female was transplanted into male, and vice-versa

-Cyclical pituitary gland became a steady glad in male rats

-The steady pituitary gland became a cyclical pituitary gland in female rats

Thus it was controlled by another part of the nervous system

Control of the Posterior Pituitary Gland

-Oxytocin and vasopressin are synthesized in the paraventricular and supraoptic nuclei of the hypothalamus

-Oxytocin and vasopressin are carried by axonal transport down the pituitary stalk

-Oxytocin and vasopressin are released into general circulation from terminal buttons in posterior pituitary

Control of the Anterior Pituitary Gland

-Releasing and inhibiting hormones are released from hypothalamic neurons into the hypothalamopituitary portal system

-Hypothalamic-releasing and hypothalamic-inhibiting hormones carried down the pituitary stalk by the hypothalamopituitary portal

-Hypothalamic-releasing/inhibiting hormones increase or decrease (respectively) the release of anterior pituitary hormones into general circulation

Hypothalamic Releasing Hormones

Releasing Hormones - Stimulates the release of an anterior pituitary hormone (ex; Gonadotropin-Releasing Hormone)

Release-Inhibiting Hormone - Inhibits the release of an anterior pituitary hormone

Gonadotropin-Releasing Hormone

Stimulates the release of both of the anterior pituitary gonadotropins (Follicle-Stimulating Hormone and Luteinizing Hormone)

Regulation of Hormone Levels - Neural Signals

All endocrine glands (except anterior pituitary gland) are directly regulated by signals from the nervous system

-Those in the brain are regulated by cerebral neurons while

-Those outside the CNS are regulated by autonomic nervous system (by both sympathetic/parasympathetic branches)

Regulation of Hormone Levels - Hormonal Signals

Circulating hormones often provide feedback to the very structures that influence their release (pituitary gland, hypothalamus, sites in the brain)

-Hormonal feedback functions to maintain stable blood levels of hormones

Ex; High gonadal hormone levels → effects hypothalamus/pituitary → decrease in gonadal hormone release → Low levels → Stimulates increase in hormone release

Regulation of Hormone Levels - Nonhormonal Chemicals

Glucose, calcium and sodium levels in the blood influence the release of particular hormones

Ex; Increases in blood-glucose levels increase release of insulin, which in turn reduces blood-glucose levels

Regulation of Hormone Levels - Pulsatile Hormone Release

Hormones are often released in pulses, discharged several times per day in surges

-A consequence is that there can be large, minute-to-minute fluctuations in levels of circulating hormones

Brain Structures Associated with Sexual Activity

1. Cortex

2. Hypothalamus

3. Amygdala

4. Ventral Striatum

Cortex and Sexual Activity

Widespread cortical activation routinely recorded during brain imaging of volunteers exposed to sexually arousing stimuli

-Activation of the occipitotemporal, inferotemporal, parietal, orbitofrontal, medial prefrontal, insular, cingulate and premotor cortices

-Activation in secondary visual cortex even if eyes are closed

Cortical activation mediates most complex aspect of sexual experience (self-awareness, release/loss of control, love)

Hypothalamus and Sexual Activity

Sexually Dimorphic Nucleus - Nucleus in the Medial Preoptic Area, several times larger in males, growth of the dimorphic nuclei is triggered by estradiol that has been aromatized from testosterone

-Medial preoptic area plays a key role in male sexual behaviour

-Destruction of which leads to abolishment of sexual behaviour, medial preoptic lesions do not eliminate sexual behaviour in females

In contrast, the Ventromedial Nuclei (VMN) is critical for sexual behaviour in females

Amygdala and Sexual Activity

Plays a role in the indentification of potential mating partners

Kluver-Bucy Syndrome - bilateral amygdala lesion (in humans), results in humans displaying flat affect, hypersexuality and complete inhibit to focus sexual advance to appropriate partners or locations

-Lesions in male rats disrupted copulatory behaviour since they were incapable of limiting advances to receptive females

Ventral Striatum and Sexual Activity

Activity in the Ventral Striatum is associated with anticipation and experience of sex and other forms of pleasure