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Endocrine System

1. Hormone Basics

• Hormones are chemical messengers that travel in the blood and regulate body functions.

• Peptide hormones are water-soluble and bind to receptors on the cell membrane. They trigger internal signaling cascades (second messengers).

• Steroid hormones are lipid-soluble, pass through cell membranes, and bind to receptors inside the cell, directly altering gene expression.

• Amine hormones are derived from amino acids and can behave like either type.

2. Major Glands and Hormones

• The hypothalamus controls the pituitary gland by secreting releasing hormones.

• The anterior pituitary releases hormones like growth hormone, ACTH (stimulates adrenal cortex), TSH (stimulates the thyroid), LH and FSH (stimulate gonads).

• The posterior pituitary releases ADH (antidiuretic hormone) and oxytocin.

• The thyroid gland produces thyroxine (T4) and triiodothyronine (T3) to increase metabolism.

• The parathyroid glands secrete parathyroid hormone to raise blood calcium levels.

• The pancreas secretes insulin to lower blood sugar and glucagon to raise it.

• The adrenal cortex makes cortisol and aldosterone; the adrenal medulla makes epinephrine and norepinephrine.

• The gonads produce estrogen, progesterone, and testosterone for reproductive functions.

3. Feedback Control

• Negative feedback keeps hormone levels stable. For example, high thyroid hormone levels inhibit TSH and TRH production.

• Positive feedback amplifies changes, as in the case of oxytocin during labor.

📞 Cell Communication

1. Types of Cell Signaling

• Autocrine signaling happens when a cell targets itself.

• Paracrine signaling occurs between nearby cells.

• Endocrine signaling uses the bloodstream to reach distant cells.

• Juxtacrine signaling involves direct contact between neighboring cells.

2. Signal Transduction Steps

• Reception: A signaling molecule (ligand) binds to a specific receptor.

• Transduction: The signal is converted into a form that can bring about a response, often through a cascade of proteins.

• Response: The cell does something — like making a protein, activating an enzyme, or changing its behavior.

3. Receptor Types

• G-protein-coupled receptors (GPCRs) activate a G protein that turns on enzymes to produce second messengers like cAMP.

• Receptor tyrosine kinases (RTKs) activate by dimerization and phosphorylation, leading to multiple signal pathways.

• Ion channel receptors open to let ions in or out, affecting membrane potential.

• Intracellular receptors bind steroid hormones and act directly on DNA to regulate transcription.

4. Second Messengers

• cAMP is common in GPCR pathways and activates protein kinases.

• Calcium ions are released from the endoplasmic reticulum and activate cellular processes.

• IP₃ is a molecule that helps release calcium from storage.

🛡 Immune and Lymphatic System

1. Innate Immunity (Nonspecific)

• Provides immediate defense against infection.

• Barriers include skin, mucus, and enzymes in tears and saliva.

• Internal defenses include phagocytic cells (macrophages, neutrophils), natural killer cells, inflammation, fever, and antimicrobial proteins like interferons.

2. Inflammation

• Damaged cells release histamine.

• Blood vessels dilate and become permeable.

• White blood cells and proteins enter tissue to destroy pathogens and begin healing.

3. Adaptive Immunity (Specific)

• Involves lymphocytes: B cells and T cells.

• B cells mature in bone marrow and produce antibodies.

• T cells mature in the thymus and either help other immune cells or kill infected cells directly.

• Adaptive responses are slower but specific and form memory cells for faster responses next time.

4. Antibody-Mediated (Humoral) Response

• Activated B cells become plasma cells that secrete antibodies.

• Antibodies neutralize pathogens, clump them together, and mark them for destruction.

5. Cell-Mediated Response

• Helper T cells (CD4+) recognize antigens presented by antigen-presenting cells on MHC class II molecules.

• Cytotoxic T cells (CD8+) recognize infected cells presenting antigens on MHC class I and kill them using perforin and enzymes.

6. Major Histocompatibility Complex (MHC)

• MHC class I: Found on all nucleated cells; displays to cytotoxic T cells.

• MHC class II: Found on immune cells; displays to helper T cells.

7. Lymphatic System Role

• Returns leaked fluid to the blood.

• Transports lipids from the digestive system.

• Filters lymph in lymph nodes, where immune responses are initiated.

• Includes spleen (filters blood), thymus (T cell maturation), and tonsils (immune surveillance).

Biology – Nervous System (Highly Specific, No Charts)

1.

Overview of the Nervous System

The nervous system is the body’s fast, electrochemical communication network that:

• Detects stimuli (internal and external),

• Processes information (in the CNS),

• Generates responses (muscular or glandular),

• Works closely with the endocrine system, but operates much faster.

2.

Major Divisions

Central Nervous System (CNS)

• Includes the brain and spinal cord.

• Responsible for integration, processing, and decision-making.

Peripheral Nervous System (PNS)

• Consists of nerves and ganglia (clusters of neuron cell bodies outside the CNS).

• Transmits signals to and from the CNS.

• Two main branches:
  

  • Sensory (afferent): Carries information from receptors to CNS.

  • Motor (efferent): Carries commands from CNS to effectors (muscles/glands).
    

    • Somatic motor: Voluntary control (skeletal muscles).

    • Autonomic motor: Involuntary control (organs).
      

      • Sympathetic: “Fight or flight” (↑ HR, dilates pupils).

      • Parasympathetic: “Rest and digest” (↓ HR, stimulates digestion).

3.

Neuron Anatomy and Function

Structure:

• Dendrites: Short, branched extensions that receive signals from other neurons.

• Cell body (soma): Contains nucleus and organelles; integrates incoming signals.

• Axon: Long extension that conducts action potentials away from the cell body.

• Axon terminal: End of axon; releases neurotransmitters into synapse.

Myelination:

• Myelin sheath: Fatty layer that insulates axons; speeds up signal transmission.
  

  • Produced by Schwann cells in PNS and oligodendrocytes in CNS.

• Nodes of Ranvier: Gaps in the myelin where ion exchange occurs.
  

  • Enables saltatory conduction (AP “jumps” from node to node).

4.

Action Potentials (AP)

An action potential is a rapid, temporary change in membrane potential due to ion movement across the axon membrane.

Step-by-Step:

1. Resting potential (~ –70 mV):
   

   • Inside of neuron is negative compared to outside.

   • Maintained by the sodium-potassium pump:
     

     • Pumps 3 Na⁺ out and 2 K⁺ in (requires ATP).

   • K⁺ leak channels allow some K⁺ to exit, contributing to negative charge inside.

3. Depolarization:
   

   • When stimulus reaches threshold (about –55 mV), voltage-gated Na⁺ channels open.

   • Na⁺ rushes in → membrane becomes positive inside (up to +30 mV).

5. Repolarization:
   

   • Na⁺ channels inactivate.

   • Voltage-gated K⁺ channels open → K⁺ rushes out → inside becomes negative again.

7. Hyperpolarization:
   

   • K⁺ channels stay open too long → membrane becomes more negative than resting (~ –80 mV).

   • Resting potential restored by Na⁺/K⁺ pump.

9. Refractory period:
   

   • Brief time during and after AP when neuron can’t fire again.

   • Ensures one-way conduction.

5.

Synaptic Transmission

Electrical Signal → Chemical Signal → Electrical Signal

1. Action potential reaches axon terminal.

2. Voltage-gated Ca²⁺ channels open → Ca²⁺ enters terminal.

3. Ca²⁺ triggers vesicles to fuse with membrane and release neurotransmitter into synaptic cleft.

4. Neurotransmitter binds to receptors on postsynaptic cell (usually ligand-gated ion channels).

5. Postsynaptic response:
   

   • Excitatory (EPSP): Na⁺ channels open → depolarization.

   • Inhibitory (IPSP): Cl⁻ or K⁺ channels open → hyperpolarization.

7. Signal ends by:
   

   • Reuptake of neurotransmitter into presynaptic neuron.

   • Enzymatic degradation (e.g., acetylcholinesterase breaks down acetylcholine).

   • Diffusion out of the cleft.

6.

Common Neurotransmitters and Functions

• Acetylcholine (ACh): Activates skeletal muscles; used in parasympathetic pathways.

• Dopamine: Involved in pleasure/reward, motor control.

• Serotonin: Regulates mood, appetite, sleep.

• GABA (gamma-aminobutyric acid): Major inhibitory neurotransmitter in the brain.

• Glutamate: Main excitatory neurotransmitter in the CNS.

• Epinephrine / Norepinephrine: Fight-or-flight; also act as hormones via the adrenal medulla.

7.

Reflex Arcs (Simplified Neural Pathways)

• A reflex is a rapid, automatic response to a stimulus.

• Involves:
  

  • Receptor (detects stimulus),

  • Sensory neuron (sends signal to CNS),

  • Interneuron (in spinal cord),

  • Motor neuron (sends signal to muscle),

  • Effector (muscle contracts).

• Example: Patellar reflex (knee-jerk).

8.

Neural Plasticity and Development

• Neural plasticity refers to the brain’s ability to form new connections in response to learning or injury.

• Synapse formation and pruning happen during development and learning.

• Long-term potentiation (LTP): Strengthening of synapses due to repeated stimulation; important in memory.

9.

Disorders and Disruptions

• Multiple sclerosis: Immune system destroys myelin → slower conduction.

• Parkinson’s disease: Dopamine-producing neurons degenerate.

• Alzheimer’s disease: Accumulation of plaques and tangles leads to memory loss.

• Epilepsy: Uncontrolled, repetitive electrical activity (seizures).

• Depression and anxiety: Often linked to neurotransmitter imbalances (e.g., serotonin, GABA).