General Physiology Laboratory

Interplay of nerve cells and endocrine cells

Allows for controlling and coordinating all other cell types in the body

How do endocrine cells communicate?

Hormones secreted into the blood

How does endocrine control act?

• Slow

• Broadcasts to many organ systems

Hormonal signal can take seconds or minutes

Hormones are capable of acting on target tissues for minutes, hours, or days

Nerve and encodrine cell functions

• Nerve cells: acute response functions

• Endocrine cells: response functions that play a role in acclimation/acclimatization

Control systems affect different body functions

Impact one another and can exert control over the other’s systems in tandem

Animals have two types of glands that produce and secrete materials

• Exocrine glands: mammary, salivary, and tear glands that have secretion flow

• Endocrine glands: pituitary, hypothalamus, pancreas, and thyroid glands secrete products directly into extracellular fluid which diffuse into capillaries and travel through the circulatory system

hormones

• products of endocrine cells

• travel to distant target tissues via the blood and exert some kind of regulatory effect

• effective at very low concentrations

• initiate their effects by binding noncovalently to receptor proteins on the outer membranes of target cells

neurosecretory cells

• have cell bodies located in the central nervous systems and the axons extend outside

• generate action potentials but release a hormone that is carried through the circulation to reach target cells

• also called neurohormones

nonneural endocrine cells

• do not generate action potentials

• generally stimulated to release hormones by receiving another hormone

• present receptor proteins that bind a hormone substrate and cause the release of a new hormone product

paracrine cells

signal to neighborhood of cells surrounding the cell that first initiated a response by sending out a hormone that is bound to a receptor on those neighboring cells

autocrine cells

release hormones that are taken back into the same cell for self-regulation of functions

Two factors involved in how long it takes to initiate a response and how long that response lasts

• sensitivity: the more receptor molecules on a cell’s surface, the more sensitive it is to the specific hormone substrate that binds those receptors; sensitivity changes over time; cells can also have receptors for more than one hormone

• blood concentrations and hormone half-lives: depend on the rate of addition and the rate of removal of hormones from the blood

steroid hormones

made from cholesterol and are hydrophobic, can diffuse through the cell membrane to reach their receptor molecules within the target cell

peptide hormones

chains of amino acids and vary in size depending on the number of amino acids, soluble in aqueous solutions

amine hormones

modified amino acids

catecholamines

derived from tyrosine, synaptic transmitter molecules

iodothyronines

derived from tyrosine, thyroid hormones rich in iodinne and are lipid soluble

intracellular receptors

lipid-soluble hormones bind, diffuse through the lipid bilayer or are transported by a lipoprotein, receptors are in the cytoplasm or the nucleus and create a hormone-receptor complex that act as a transcription factor

G protein-coupled receptors

water soluble peptide and catecholamine hormones bind, receptors are in cell membrane that affect ion-channel permeability or activate intracellular second messenger system

enzyme-linked receptors

insulin and growth hormone bind, receptors in the cell membrane, act by changing teh functions of preexisting proteins

negative feedback

hormone causes changes in its control pathway that suppress its own secretion, stabilizes the levels of hormone rather than reducing them to zero

positive feedback

hormone causes changes in its control pathway to increase its own secretion, amplify or hasten the response of its target cells

synergism

one hormone amplifies the effect of another

permissiveness

requires the presense of one hormone for another hormone to exert its full effect on the target tissue

antagonism

hormone directly opposes the action of another

When does neural modulation occur?

When a stimulus provides input to the CRH-secreting pathway

Circadian rhythm

• Controlled by a neuronal biological clock

• Can affect hormone secretion based on diel cycle and our actions during certain times

How does the pulsatile nature of synaptic signaling modulate hormones?

• Cause pulses in hormone secretion

• Periods of high concentration separated by periods of low concentration prevent desensitization of target cells to hormones regularly circulating

axis

a system where the secretions of one endocrine gland act on another in a sequence

Axes include

• Hypothalamo-pituitary-adrenal cortex (HPA)

• Hypothalamo-pituitary-gonad axis (HPG)

• Hypothalamo-pituitary-thyroid axis (HPT)

• Hypothalamo-pituitary-liver axis (HPL)

hypothalamus

the primary control center of the brain that detects changes in the body and enacts processes to return to homeostasis

anterior pituitary gland

• made of nonneural endocrine tissue

• all hormone products are peptides, proteins, or glycoproteins, direct-acting or tropic hormones

Direct-acting hormones

• Growth hormone (GH)

• Prolactin

• Melanocyte-stimulating hormone (MSH)

Tropic hormones

• Thyroid-stimulating hormone (TSH)

• Adrenocorticotropic hormone (ACTH)

• Luteinizing hormone (LH)

• Follicle-stimulating hormone (FSH)

posterior pituitary gland

• made of bundles of neurons originating from the hypothalamus that terminate at a capillary bed

• releases peptide hormones like vasopressin (ADH) and oxytocin

HPG axis

1. Hypothalamas: produces gonadotropin-releasing hormone (GnRH) in response to cues for puberty, sexual development, and maintenance of reproductive organs

2. GnRH travels to anterior pituitary via hypothalamic-hypophysial-portal circulation that stimulates the production of LH and FSH

3. LH and FSH circulate until they reach the testes or ovaries

4. LH and FSH stimulate and sustain spermatogenesis and androgen synthesis in the testes or ovarian follicle maturation, ovulation, and corpus luteus formation in the ovaries

HPA axis

1. Hypothalamus secretes corticotropin releasing factor (CRF) into teh capillaries of the anterior pituitary

2. Anterior pituitary stimulates the release of adrenocorticotropic hormone (ACTH) into the systemic circulation

3. ACTH travels to the adrenal gland which secretes glucocorticoids

4. Glucocorticoids including cortisone, cortisol, and corticosterone promote increased blood glucose concentrations

HPT axis

1. Hypothalamus produces thyrotopin-releasing hormone (TRH)

2. TRH stimulates the synthesis of thyroid-stimulating hormone (TSH) in the anterior pituitary gland

3. TSH travels to the thyroid gland to promote thyroid hormone (TH) synthesis

4. Thyroxine (T4) and triiodothyronine (T3) are bound to transport proteins until the nucleus of the target cell, they unbind the transporter, and bind receptors that initiate transcription

Endocrine responses

1. Receive some stimulus

2. Process stimulus into a coordinated response

3. Intermediate and target organs receive the messages from one another in a sequence

4. The response

HP axis

1. Changes in osmolality detected in osmoreceptors in hypothalamus

2. supraoptic and paraventricular nuclei of the hypothalamus synthesize and release ADH from their terminals in the posterior pituitary

3. ADH released into systemic circulation

4. ADH travels to kidneys for water re-uptake, smooth muscle cells of blood vessels to increase blood pressure, sweat glands to decrease sweat production