Wk 7 Biomedical Science PT 1

Topic 15 - Communication, Integration and Homeostasis 

• List the four basic methods of cell-to-cell communication 

• Describe the general pattern of a signal pathway 

• Describe how signalling molecules interact with target-cell receptors 

• Understand the difference and relationship between chemical and electrical signals 

• List the differences between endocrine and neural control  

• Diagram control pathways for neural, neuroendocrine, and endocrine reflexes. 

Cell to Cell communication  

The two basic types of physiological signals in cell-to-cell communication are: 

1. Chemical Signals 

Definition: Molecules secreted by signaling cells that bind to specific receptors on target cells. 

Examples: 

• Hormones (e.g., insulin, adrenaline) 

• Neurotransmitters (e.g., dopamine, acetylcholine) 

• Local mediators (e.g., cytokines, growth factors) 

• Gaseous signals (e.g., nitric oxide, CO) 

 Mechanism: Can act over short (synaptic, paracrine) or long distances (endocrine). 

2. Electrical Signals 

Definition: Rapid changes in membrane potential (voltage) that transmit signals along excitable cells.  

Examples: 

•  Action potentials in neurons and muscle cells. 

•  Gap junction-mediated electrical coupling (e.g., in cardiac muscle). 

• Mechanism: Depolarization and ion flow (e.g., Na⁺, K⁺, Ca²⁺) trigger fast responses. 

Key Difference: 

• Chemical signals rely on ligand-receptor binding (slower, diverse responses). 

•  Electrical signals use ion fluxes for rapid, direct communication (e.g., nerve impulses). 

Examples: 

1. Nervous System (Neuron-to-Neuron or Neuron-to-Muscle) 

Electrical Signal: An action potential travels down the axon of a neuron. 

Chemical Signal: At the synapse, the electrical signal triggers neurotransmitter release (e.g., acetylcholine, glutamate), which binds to receptors on the next cell. 

 Result: Fast electrical transmission + targeted chemical signalling. 

 Example: Muscle contraction requires an action potential (electrical) leading to acetylcholine release (chemical) at the neuromuscular junction. 

2. Heart Function (Cardiac Muscle) 

Electrical Signal: Pacemaker cells generate rhythmic action potentials that spread via gap junctions (electrical coupling). 

 Chemical Signal: Adrenaline (hormone) from the adrenal glands binds to cardiac cells, altering their electrical activity to increase heart rate. 

 Result: Electrical synchronization + hormonal modulation. 

3. Neuroendocrine System (Brain-to-Body Coordination) 

Electrical Signal: Hypothalamic neurons fire action potentials. 

 Chemical Signal: These neurons release neurohormones (e.g., oxytocin, ADH) into the bloodstream to affect distant organs. 

 Result: Nervous system controls endocrine output. 

 Example: Stress triggers electrical signals in the brain, leading to cortisol (chemical) release from the adrenal glands. 

4. Smooth Muscle Control (e.g., Blood Vessels) 

Electrical Signal: Some smooth muscle cells depolarize spontaneously. 

Chemical Signal: Nitric oxide (NO) from endothelial cells causes relaxation by altering electrical activity. 

Result: Local chemical signals fine-tune electrically excitable tissue. 

Why Both Systems? 

Speed: Electrical signals are near-instantaneous (neural reflexes). 

Precision: Chemical signals allow diverse, specific responses (e.g., one hormone can have different effects on different organs). 

Integration: Systems like the adrenal medulla convert neural signals (electrical) into hormonal ones (chemical) for body-wide effects. 

Long distance communication  

Long-distance communication allows cells to coordinate functions across large distances in the body, often through the circulatory system (bloodstream) or neuronal pathways. The primary mechanisms include: 

1. Endocrine (Hormonal) Signaling 

How it works: Specialized cells (endocrine glands) release hormones into the bloodstream, which travel to distant target cells with matching receptors. 

Examples: 

•  Insulin (pancreas → regulates glucose uptake in muscles/liver). 

•  Adrenaline (Epinephrine) (adrenal glands → prepares body for "fight or flight"). 

•  Thyroid hormones (regulate metabolism throughout the body). 

Key Features: 

•  Slow but long-lasting effects (seconds to hours). 

•  Affects multiple tissues simultaneously. 

2. Neuronal Signaling (Long-Range Electrical + Chemical) 

How it works: Electrical signals (action potentials) travel along axons to distant cells, where they trigger neurotransmitter release at synapses. 

 Examples: 

•  Motor neurons (spinal cord → skeletal muscles for movement). 

•  Sympathetic nerves (release adrenaline-like signals directly onto organs). 

Key Features: 

•  Extremely fast (milliseconds). 

•  Precise, point-to-point communication. 

Signal pathways  

• A cell cannot respond to a chemical signal if the cell acks the appropriate receptor proteins for that signal  

Receptor proteins  

Receptor proteins are molecular "locks" on the surface or inside cells that bind specific signalling molecules (ligands) to trigger a cellular response. 

• They are essential for communication between cells and their environment. 

Binding of the signal to a receptor protein will initiate a response by:  

• Activation of the receptor  

• Subsequent activation of one or more intracellular signal molecules lead to => modification of existing proteins or synthesis of new proteins, resulting in a response   

Chemical signals can be converted to electrical signals  

Imagine your cells are like smartphones, and chemical signals are text messages sent between them. But to "read" these messages, the phone (cell) needs to convert them into something it understands—electrical signals (like a ringtone or vibration). Here’s how it works: 

1. The "Text Message" (Chemical Signal) 

Example: A neurotransmitter like dopamine or adrenaline is released by one cell. 

2. The "Receiver" (Receptor Protein) 

The chemical binds to a receptor on another cell (like a text notification popping up). 

 Two main types of receivers: 

Ion channel receptors (direct effect). 

GPCRs (indirect effect, like forwarding the message). 

3. The "Vibration" (Electrical Signal) 

Option 1: Direct Ion Channel Opening 

The receptor is a gate for ions (e.g., Na⁺, K⁺). 

Chemical binds → gate opens → ions rush in → cell "vibrates" (depolarizes) → electrical signal starts.  

Example: Acetylcholine making muscles move. 

Option 2: GPCR "Message Forwarding" 

Chemical binds → receptor activates a G protein → sends a "second message" (e.g., cAMP) → opens ion channels later. 

Example: Adrenaline making your heart race 

Modulation of signal pathways  

• Different cells can respond differently to the same signal 

Control pathways: 

• Two control systems are involved – the Endocrine and nervous system 

Receptor – different meanings  

Control systems  

• Control systems can vary 

Simplified: 

Examples 

Neural Reflex: 

A neural reflex is your body’s instant, automatic reaction to something—like pulling your hand away from a hot stove before you even feel the pain. No thinking required! 

How It Works (3-Step Chain Reaction): 

1. Sensor Detects Danger: 

Example: Heat sensors in your skin scream "HOT!" 

2. Spinal Cord Decides: 

 The signal zips to your spinal cord (like a shortcut—no brain needed yet!). 

3.  Muscles React: 

 Your arm yanks back before your brain processes "ouch! 

Knee-jerk test: Hammer tap → sensory neuron → spinal cord → motor neuron → quadriceps contract (no brain involvement!). 

Endocrine Reflex:  

An endocrine reflex is your body’s delayed but powerful response to changes—like a thermostat adjusting your house temperature over minutes or hours, instead of instantly. 

How It Works (3 Simple Steps): 

1. Detect a Problem: 

 Example: Your blood sugar spikes after eating candy. 

2. Hormone Messenger Released: 

 Special glands (like the pancreas) send hormones (like insulin) into your blood—like a "text message" to your whole body. 

3.  Organs Respond Slowly: 

 Liver/muscles absorb sugar over minutes to hours, lowering blood sugar. 

Stress response: Brain detects stress → pituitary releases ACTH → adrenal glands secrete cortisol → long-term metabolic changes. 

Neurohormonal control  

A teamwork system where your brain (nervous system) and hormones (endocrine system) work together to control your body. 

How it works: 

Step 1: Your brain detects something important (like stress or thirst). 

Step 2: Special brain cells release "neurohormones" (brain-made hormones) into your blood. 

 Step 3: These hormones travel like messengers to organs, telling them what to do. 

Real-life examples: 

 Stress: Brain → releases stress hormones → body stays alert. 

 Breastfeeding: Baby sucking → brain releases milk-flow hormone. 

 Thirst: Dry mouth → brain tells kidneys to save water. 

Comparison of Endocrine & Neural control  

 The endocrine system utilizes hormones for longer-lasting effects, while neural control provides rapid responses to immediate stimuli.

Topic 16 - Introduction to Endocrinology 

• Distinguish between endocrine and exocrine glands 

• Compare the two chemical classes of hormones based on their solubility  

• Describe the two general mechanisms of hormone action 

• Describe the locations of and relationships between the hypothalamus and pituitary gland 

Communication systems 

Multicellular organisms must coordinate and integrate the activity of body cells ot preserve homeostasis 

Nervous system 

• Rapid (msec) 

• Mediated by neurotransmitters 

Endocrine system 

• Slower (seconds to days) 

• sustained 

• Mediated by hormones  

Hormone activity  

Local hormones  

Paracrine 

• Act on nearby cells 

• Not via circulatory system 

Autocrine 

• Act on the same cell that secretes them 

Major Endocrine Organs  

Hormone groups  

1. Lipid derivatives 

2. Peptide hormones 

3. Amino acid derivatives  

Amino acid derivatives 

• Structurally related to tyrosine or tryptophan  

An amino acid derivative is a modified version of a single amino acid (protein building block) that now works as: 

•  A hormone (e.g., adrenaline, thyroid hormone) 

• A neurotransmitter (e.g., dopamine, serotonin) 

 How It Works: 

1. Start with 1 Amino Acid (e.g., tyrosine or tryptophan). 

2. Add/Remove a Few Atoms. 

3.  It becomes a whole new signalling molecule. 

Key Examples: 

Original Amino Acid Derivative Role 

Tyrosine Adrenaline Emergency "fight-or-flight" hormone 

Tyrosine Thyroxine (T4) Metabolism speed controller 

Tryptophan Serotonin Mood stabilizer ("happy chemical") 

Tryptophan Melatonin Sleep regulator 

Peptide or Protein hormones  

Peptide hormones are short chains of amino acids (like tiny protein pieces) that act as chemical messengers. They’re your body’s fast, precise instructions sent through the bloodstream. 

 Made of: 

•  Small proteins (just 3–50 amino acids long). 

•  Examples: Insulin, oxytocin, growth hormone. 

How They Work: 

1.  Stick to cell surfaces like a key in a lock (they can’t enter cells). 

2. Trigger rapid changes (seconds to minutes). 

 Where They Come From: 

 Mostly made in the pituitary gland, pancreas, and hypothalamus 

Lipid-derived hormones  

These hormones are made from fats (lipids) like cholesterol. They’re slow but powerful, and can slip directly into cells to turn genes on/off. 

Key Traits: 

• Slow but lasting (hours to days) 

• Can enter cells directly (no surface receptor needed) 

• Made in: Adrenal glands, ovaries/testes, placenta 

Water vs lipid soluble hormones 

Lipid soluble hormones: 

• Steroid hormones, thyroid hormones, nitric oxide 

• Circulate bound to transport proteins 

Water-soluble hormones 

• Amine hormones, peptide and protein hormones, eicosanoid hormones 

• Circulate freely in the plasma  

Mechanisms of hormone action 

Responses vary depending on the hormone itself and the target cell 

The response may be: 

• Synthesis of new molecules 

• Changing permeability of the cell membrane 

• Stimulating transport of a substance into or out of the cell 

• Altering the rate of metabolic actions 

• Causing contraction of smooth or cardiac muscle  

Lipid-soluble hormones bind to receptors within target cells:  

Water soluble hormones and to receptors on the exterior surface of the target cell: 

 Mechanism of hormone action  

Target cell response to a hormone is based on: 

• The hormone's concentration in the blood 

• The number of hormone receptors on the target cell 

• Influencers exerted by other hormones 

• Some hormones work more effectively when a second hormone is present to assist them (synergistic effect) 

• Some hormones oppose the action of the others (antagonistic effect)  

Control of hormone secretion 

Most hormone regulation is achieved via negative feedback 

EPO is an example of a hormone controlled by simple negative feedback regulation  

Control of hormone secretion 

• Hormones are secreted in short bursts when needed 

Secretion is regulated by: 

• Signals from the nervous system 

• Chemical changes in the blood 

• Other hormones 

• The hypothalamic-pituitary axis is of central importance  

Hypothalamic-pituitary axis 

Hypothalamic Control Of Endocrine Function 

Posterior pituitary gland 

• does not synthesize hormones 

• stores and releases from axon terminals two hormones produced by neurosecretory cells of the hypothalamus: 

• oxytocin (OT) 

• anti-diuretic hormone (ADH) 

Posterior pituitary hormones  

Antidiuretic hormone (ADH) 

Anterior pituitary gland 

• Controlled by specific regulatory factors from the hypothalamus 

• Released from neurons at median eminence 

• Carried by the hypophyseal portal system to the anterior pituitary 

• Links two capillary networks; direct communication before dilution in general circulation  

Topic 17 - Hormone Secretion & Actions  

• Describe the mechanism of control of hormone secretion 

• Describe the location, histology, hormones, and functions of the anterior and posterior pituitary 

• Describe the location, histology, hormones, and functions of the thyroid gland and adrenal gland 

• Describe how the body responds to stress 

Control of the anterior pituitary  

• The anterior pituitary is primarily regulated by hypothalamic hormones and negative feedback from target gland hormones 

Control of the anterior Pituitary Gland:  

1. The Hypothalamus  => (The Boss) 

The hypothalamus (a brain region) sends chemical signals (hormones) to the anterior pituitary to tell it what to release. 

Releasing Hormones (ON switches): 

•  TRH → Tells pituitary to release TSH (for thyroid). 

•  CRH → Tells pituitary to release ACTH (for stress hormones). 

•  GnRH → Tells pituitary to release FSH & LH (for sex hormones). 

•  GHRH → Tells pituitary to release GH (for growth). 

Inhibiting Hormones (OFF switches): 

•  Dopamine → Stops prolactin (milk hormone). 

•  Somatostatin → Stops GH & TSH. 

2. Feedback Loops (Like a Thermostat) 

Too much hormone? The body tells the hypothalamus & pituitary to slow down. 

• Negative feedback loops control the secretions of thyrotropes, gonadotrophs & corticotropes  

 Example: High thyroid hormone (T4) → Brain stops TRH & TSH. 

 Example: High cortisol → Brain stops CRH & ACTH. 

3. Principal actions of anterior pituitary hormones  

1. Growth Hormone (GH) 

→ Stimulates growth (bones, muscles). 

→ Boosts metabolism (burns fat, builds protein). 

Most plentiful anterior pituitary hormones=> released in bursts every few hours by somatotrophs  

Somatotroph activity is controlled by 2 hypothalamic hormones: 

• Growth hormone-releasing hormone (GHRH) 

• Growth hormone-inhibiting hormone (GHIH)  

2. Thyroid-Stimulating Hormone (TSH) 

→ Tells the thyroid gland to make thyroid hormones (T3/T4). 

→ Controls energy levels, metabolism, and body temperature. 

Thyroid gland: Butterfly shaped gland located inferior to the larynx and anterior to the trachea => produces hormones that regulate metabolism  

Follicular cells are stimulated by TSH to produce the thyroid hormones  

• Thyroxine (Tetraiodothyronine, T4) 

• Triiodothyronine (T3) 

Parafollicular cells produce the hormone calcitonin to help regulate calcium homeostasis 

Thyroid hormones:  

• Increase basal metabolic rate 

• Help maintain normal body temperature 

• Stimulate protein synthesis 

• Increase the use of glucose and fatty acids for ATP production  

Control of thyroid hormone secretion: 

3. Adrenocorticotropic Hormone (ACTH) 

→ Tells adrenal glands to release cortisol (stress hormone). 

→ Helps body handle stress, inflammation, and blood sugar. 

 Adrenal gland: located at the top of each kidney  

Divided into 2 regions: 

• Outer cortex 

• Medial medulla  

Covered by a connective tissue capsule 

Cortex is divided into 3 regions 

1. Zone glomerulosa => secretes mineralocorticoids 

2. Zona fasciculata => secretes glucocorticoids  

3. Zona reticularis => secretes (weak) androgens  

ALDOSTERONE = major mineralocorticoid secreted by the adrenal gland 

• Helps regulate sodium and potassium homeostasis 

• Secretion controlled by the renin-angiotensin-aldosterone (RAA) pathway  

GLUCOCORTICOIDS = cortisol, cortisone, & corticosterone 

• Secretion regulated by negative feedback 

• Helps control: protein breakdown, glucose formation, lipolysis, resistance to stress, inflammation & immune responses  

ADRENAL ANDORGENS = major androgen is dehydroepiandrosterone (DHEA) 

Males: after puberty testosterone is secreted I much larger quantities so DHEA has virtually no effect 

Females: adrenal androgens promote libido and are converted to oestrogens. After menopause, all female oestrogens come from adrenal androgens  

ADRENAL MEDULLA = stimulated by sympathetic neurons of the autonomic nervous system (ANS) 

• Chromaffin cells secrete adrenaline and noradrenaline – part of the stress response 

• Increases blood pressure & heart rate 

• Increases glycogenolysis & glycolysis, dilate bronchi, stimulate lipolysis  

4. Prolactin (PRL) 

→ Stimulates milk production in breastfeeding women. 

→ Also affects fertility and immune function. 

5. Follicle-Stimulating Hormone (FSH) 

→ In women: Helps eggs grow in ovaries & oestrogen production. 

→ In men: Helps make sperm. 

6. Luteinizing Hormone (LH) 

→ In women: Triggers ovulation & progesterone production. 

→ In men: Stimulates testosterone production. 

Summary Table 

Hormone   Main Job 

GH               Makes you grow & burns fat 

TSH             Controls thyroid (energy & metabolism) 

ACTH          Triggers stress hormone (cortisol) 

Prolactin     Makes breast milk 

FSH             Eggs & sperm production 

LH                Ovulation & testosterone 

These hormones keep your body running smoothly—growing, reproducing, handling stress, and staying energized! 

4. Other Factors That Influence Release 

Time of day (e.g., ACTH is highest in the morning). 

Stress response → More ACTH & cortisol. 

 Breastfeeding → More prolactin. 

Simple Summary 

Hypothalamus sends ON/OFF signals → Pituitary listens → Organs (thyroid, adrenals, ovaries/testes) respond. 

• Too much hormone? Body turns OFF production. 

• Too little? Body turns ON production. 

•  This keeps hormones balanced for growth, metabolism, stress, and reproduction 

Control of activity –Summary 

Regulation involves interaction (direct & indirect) between the nervous & endocrine systems 

• Posterior pituitary secretes oxytocin & ADH – under neural control 

• Anterior pituitary secretes major hormones & tropic hormones – tropic hormones control the release of other hormones   

• The hypothalmo-pituitary axis has a central role in controlling some of major endocrine glands => adrenal glands, thyroid glands & gonads