Homeostasis and Control Pathways

Homeostasis

to maintain body functions within specific livable ranges, adjusting to internal and external changes

2 Physiological signals

1. Electrical signals = changes in membrane potential

2. Chemical signals = molecules secreted into extracellular fluid

Local cell to cell communication

1. Gap junctions: direct cytoplasmic connections between adjacent cells

2. Juxtracrine: contact dependent signals 

3. Autocrine/paracrine: signals act on same cell that secreted or secreted by one cell and diffuse to adjacent cells 

Long cell to cell communication 

1. endocrine: chemical (hormones) secreted into bloodstream

2. nervous: electrical and chemical, neurotransmitters secreted across synapse or neurohormones released by neurons into blood 

Why do some cells respond to a chemical signal while others ignore it?

Specific receptor proteins on target cell

Receptors are what type of proteins

Integral: so can be intracellular or cell membrane

Intracellular receptors must be:

• Cytosolic or nuclear

• Bind steroid or thyroid hormones

• Act as transcription factors

Cell membrane receptors must be: 

• Integral membrane proteins

• Bind peptide hormones and other lipophobic signals 

• Usually activate secondary messenge 

ALL receptors participate in

signal transduction

Local control systems include

Paracrine = restricted to tissue or cell involved

Reflex or systemic control systems

Response Loop

Feed Forward

Feedback Loop: negative and positive

All control systems (local or long) have 3 components 

1. Input signal

2. Controller/integrating center

3. Output signal for response 

Long reflex control: response loop

• Can be divided into input and output

• Stimulus —> sensor —> input signal —> integrating center —> output signal —> target —> response

• Ex. Simple Nervous Reflex: knee jerk

• Once response has occurred, ends there

Negative feedback loop

• Regulates response loop

• Response becomes stimulus and inhibits integrating center

• Ex of homeostasis

• Ex. Baroreceptor reflex which is used when blood pressure is out of range

1. Baroreceptors detect changes in arterial pressure

2. Signals send to medulla of brain stem

3. Heart rate adjusted

Positive feedback loop 

NOT HOMEOSTASIS

Response feeds into integrating center

Causes to keep producing 

Ex. During childbirth, oxytocin keep releasing 

Feed Forward Loop

• Anticipating change

• No stimulus —> still causes physiological response

• Ex. thinking of food —> cause salivation OR feeling scared —> prepare body response even no stimulus

Afferent path

Efferent path

Effector

Afferent path = input signal

Efferent path = output signal

Effector = target

Simple endocrine reflex 

Stimulus —> binds to receptor which is on endocrine gland/cell —> NO AFFERENT PATH —> endocrine gland/cell is integrating center —> release hormone (efferent path) —> target (effector) —> tissue or systemic response 

Ex. Insulin Pathway 

Insulin Pathway

Stimulus: increase high plasma glucose

Receptor: beta cell

Afferent path = none

Integrating center = beta cell

Efferent path = insulin

Effector = muscle or adipose tissue

Response = increase glucose uptake in cells and decrease plasma glucose

Simple Neural Pathway

Stimulus —> bind to sensory receptor —> input signal (afferent neuron) —> integrating center (CNS = brain or spinal cord) —> output signal (efferent nerve, motor nerve) —> target —> response

Has afferent signal!

Motor neuron can be somatic or autonomic

Ex. Knee Jerk Reflex

Parasympathetic release and sympathetic release_

Acetylcholine (ACh) 

Norepinephrien (NE) 

Knee Jerk Reflex

Stimulus = tap patellar ligament

Receptor = mechanoreceptor 

Afferent path = sensory neuron

Integrating center = CNS (spinal cord)

Efferent path = somatic motor neuron

Effector = skeletal muscle

Response = muscle contraction

Trophic hormone

A hormone that targets endocrine glands to secrete another hormone

Amine Hormones

Synthesized from tyrosine or tryptophan

• Catecholamines and thyroid hormones

• Ex. Tetraidothyronine, Norepinephrine

Steroid hormones 

• Made from cholesterol 

• Adrenal cortex and gonads 

• Ex. Aldosterone, estrogen, progesterone

Peptide hormones

• Synthesized by linking amino acids

• Most hormones

• Ex. Secretin, CRH 

Steroid vs Peptide Hormones 

Steroid is cholesterol based (lipophobic), so can diffuse through the cell membrane, but needs carrier protein to take into cell when traveling through blood

Go to nucleus and upregulate transcription

Peptide hormones are hydrophilic so cannot diffuse through the cell membrane, do not require carrier in blood 

Cause signal transduction

Catecholamines behave like… and thyroid hormones behave like…

peptides ; steroids

How can hormones be classified in connection to hypothalamus

Controlled by hypothalamus/anterior pituitary

Synthesized in hypothalamus and released from posterior pituitary

Independent of hypothalamus

Anterior Pituitary 

• Has vascular connection to hypothalamus by capillaries and portal system

• Is endocrine tissue —> can make its own hormones 

• FLAT PG 

Posterior Pituitary

• Connected to hypothalmus via neurons

• Is neural tissue

• Vasopressin, ADH, oxytocin

Long loop feedback vs short feedback

Long feedback: dominant; final hormone inhibits secretion of hormone from integrating centers

Short feedback: inhibit secretion of hormone from hypothalmus

Pathologies 

Primary: If originated from third integrating center, ex adrenal cortex

Secondary: If originated from secondary integrating center, anterior pituitary

Tertiary: If originated from first integrating center, hypothalmus

Gonadal dysfunction

Adrenal cortex does not work = low E and P

No long feedback loop = high GnRH, FSH, LH

Primary hypogonadism

Kallman syndrom / hypothalmic dysfunction

• Hypothalmus inhibited = no GnRH = no FSH and LH = no E and P

• Tertiary Hypogonadism

Oral contraceptives 

Mimic E and P 

Long loop negative feedback is activated, decrease GnRH and LH and FSH 

Primary hypergonadism 

Synergism

effect of 2 or more hormones on same parameter is greater than additive

Permissiveness

one hormone is needed for another to exert its full effect, first hormone has no direct efficient on parameter

Antagonisms 

Hormones have opposing effects