Action Potential and Ion Channels

Action Potential Events

• Depolarization, repolarization, and hyperpolarization are the key events in producing an action potential.
• An action potential is a rapid, transient change in membrane potential.
• Action potentials enable long-distance communication in neurons.

Action Potential Function and Properties

• Action potentials are "all-or-none" events.
• Action potentials exhibit a threshold, and refractory periods.
• Action potentials propagate along axons without decrement.

Structure and Properties of Ion Channels

• Ion channels are transmembrane proteins that allow specific ions to cross the cell membrane.
• Ion channels are essential for establishing resting membrane potential.
• Ion channels are essential for generating action potentials.
• Ion channels can be voltage-gated, ligand-gated, or mechanically gated.

Refractory Periods

• The absolute refractory period is the time during which another action potential cannot be generated.
• The relative refractory period is the time during which a stronger-than-normal stimulus is required to generate an action potential.

Na+/K+ Pump

• The Na+/K+ pump maintains resting membrane potential by pumping 3 Na+ ions out of the cell and 2 K+ ions into the cell.
• The Na+/K+ pump is an ATPase.
• The Na+/K+ pump requires energy to function.

Eukaryotic Cell Organelles

• Nucleus: Contains the cell's DNA/genetic material and controls cell activities.
• Endoplasmic Reticulum (ER): Involved in protein and lipid synthesis; rough ER has ribosomes, smooth ER does not.
• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
• Mitochondria: Produces ATP through cellular respiration, the powerhouse of the cell.
• Lysosomes: Contains enzymes for breaking down cellular waste and debris.
• Ribosomes: Synthesizes proteins.

Membrane Transport

• Passive transport: Includes diffusion, osmosis, and facilitated diffusion, which do not require energy.
  • Example: Oxygen enters cells through diffusion.
• Active transport: Requires energy (ATP) to move molecules against their concentration gradient.
  • Example: Sodium-potassium pump transports ions across the cell membrane.
• Vesicular transport: Includes endocytosis (bringing substances into the cell) and exocytosis (expelling substances from the cell).
  • Example: Neurotransmitters are released via exocytosis.

Membrane Potential

• Membrane potential is the difference in electrical charge across the cell membrane.
• Resting membrane potential is typically around -70mV in neurons.
• Resting membrane potential is maintained by the sodium-potassium pump and ion channels.
• Ion distribution contributes to resting potential (more K+ inside, more Na+ outside).

Neuronal Action Potential Phases

• Depolarization: Involves Na+ influx, membrane potential becomes more positive.
• Repolarization: Involves K+ efflux, membrane potential returns to a negative state.
• Hyperpolarization: Membrane potential briefly becomes more negative than resting potential.
• Ion movement contributes through ion channels opening and closing.

Homeostasis

• Homeostasis is the body's ability to maintain a stable internal environment.
• Temperature, pH, and blood glucose levels are all regulated to maintain homeostasis.
• Homeostasis is achieved through feedback loops.

Feedback Loops

• Negative feedback loops: Counteract changes to maintain stability.
  • Example: Body temperature regulation.
• Positive feedback loops: Amplify changes, often leading to a specific outcome.
  • Example: Blood clotting.

Neuron Components and Functions

• Dendrites: Receive signals from other neurons.
• Cell body (soma): Integrates signals and contains the nucleus.
• Axon: Transmits signals to other neurons, muscles, or glands.
• Axon terminals: Release neurotransmitters.

Chemical vs. Electrical Synapses

• Chemical synapses: Use neurotransmitters to transmit signals, slower but allow for signal amplification and modulation.
• Electrical synapses: Use gap junctions to directly transmit electrical signals, faster but less versatile.

Neurotransmission Steps

• Synthesis of neurotransmitter.
• Release of neurotransmitter into the synaptic cleft.
• Binding of neurotransmitter to receptors on the postsynaptic cell.
• Termination of neurotransmitter activity to prevent continuous signaling, occurs via reuptake, enzymatic degradation, or diffusion.

Action Potential Generation and Propagation

• Generated by the influx of sodium ions into the cell, causing depolarization.
• Propagated along the axon via sequential opening of voltage-gated sodium channels.

Myelin and Saltatory Conduction

• Myelin is an insulating layer around axons formed by glial cells (Schwann cells in PNS, oligodendrocytes in CNS).
• Saltatory conduction is the "jumping" of action potentials between Nodes of Ranvier (gaps in myelin), which speeds up conduction velocity.

Key Neurotransmitters

• Acetylcholine
• Norepinephrine
• Dopamine
• Serotonin
• GABA
• Glutamate

Ionotropic vs. Metabotropic Receptors

• Ionotropic receptors: Ligand-gated ion channels, fast and direct effects.
• Metabotropic receptors: G protein-coupled receptors, slower and indirect effects via second messengers.

Divisions of the ANS

• Sympathetic division: "Fight or flight" responses.
  • Anatomical differences include ganglia location (close to the spinal cord)
• Parasympathetic division: "Rest and digest" responses.
  • Anatomical differences include ganglia location (close to or within target organs)

Basic Organization of the ANS

• Sympathetic division neurotransmitters and receptors: Preganglionic neurons release acetylcholine (ACh), postganglionic neurons release norepinephrine (NE).
• Parasympathetic division neurotransmitters and receptors: Both pre- and postganglionic neurons release acetylcholine (ACh).

Functional Outcomes of ANS Activation

• Sympathetic activation: Increased heart rate, dilated pupils, and inhibited digestion.
• Parasympathetic activation: Decreased heart rate, constricted pupils, and stimulated digestion.

GIT Smooth Muscle Organization

• Smooth muscle in the GIT is organized in circular and longitudinal layers.
• This organization enables peristalsis and segmentation for mixing and propulsion of food.

Enteric Nervous System (ENS)

• The ENS is the "brain" of the gut, regulates GIT function independently of the CNS.
• Regulates GIT through neural circuits and neurotransmitters.

Neural and Hormonal Control of the GIT

• Neural control via the autonomic nervous system (ANS) and enteric nervous system (ENS).
• Hormonal control via hormones like gastrin, secretin, and cholecystokinin (CCK).

GIT Reflexes

• Gastrocolic reflex: Increased colonic motility in response to food entering the stomach.
• Ileogastric reflex: Decreased gastric motility in response to distension of the ileum.

GIT Hormones

• Gastrin: Stimulates gastric acid secretion in response to stomach distension.
• Secretin: Stimulates bicarbonate secretion from the pancreas in response to acidic chyme in the duodenum.
• Cholecystokinin (CCK): Stimulates gallbladder contraction and pancreatic enzyme secretion in response to fats and proteins in the duodenum.

Functional Movements of the GIT

• Peristalsis: Propulsive movements that move food along the digestive tract.
• Segmentation: Mixing movements that break down food and mix it with digestive enzymes.

Oral Cavity Anatomy and Physiology

• Structures include teeth, tongue, and salivary glands
• Involved in mechanical digestion (chewing) and chemical digestion (salivary amylase breaking down starches)

Stomach Anatomy and Physiology

• Structures include the fundus, body, antrum, and pylorus.
• Involved in mechanical digestion (churning) and chemical digestion (gastric acid and pepsin breaking down proteins).

Regulation of Gastric Secretions

• Cephalic phase: Stimulated by the sight, smell, or taste of food.
• Gastric phase: Stimulated by stomach distension and the presence of proteins.
• Intestinal phase: Regulated by hormones released from the small intestine.

Small Intestine Anatomy and Physiology

• Structures include the duodenum, jejunum, and ileum.
• Primary site of nutrient absorption.

Chemical Digestion and Absorption

• Carbohydrates: Digested into monosaccharides, absorbed into the bloodstream.
• Proteins: Digested into amino acids, absorbed into the bloodstream.
• Lipids: Digested into fatty acids and glycerol, absorbed into the lymphatic system.

Large Intestines, Liver, Gall Bladder and Pancreas

• Large intestines: Absorption of water and electrolytes, formation and storage of feces.
• Liver: Produces bile, metabolizes drugs, and stores glycogen.
• Gall bladder: Stores and concentrates bile.
• Pancreas: Secretes digestive enzymes and hormones (insulin and glucagon).

Large Intestine Digestion

• Mechanical digestion via haustral churning.
• Chemical digestion via bacterial fermentation.

Absorption and Feces Formation

• Absorption of water and electrolytes.
• Undigested material, bacteria, and dead cells contribute to feces formation.

Defecation Reflex

• Stimulated by distension of the rectum.
• Involves relaxation of the internal anal sphincter and voluntary relaxation of the external anal sphincter.

Liver Anatomy

• Anatomical organization includes lobes, lobules, hepatocytes, and sinusoids.
• Bile canaliculi collect bile produced by hepatocytes.

Liver Metabolic Functions

• Carbohydrate metabolism: Glycogenesis, glycogenolysis, and gluconeogenesis.
• Protein metabolism: Synthesis of plasma proteins, deamination of amino acids.
• Lipid metabolism: Synthesis of lipoproteins, cholesterol, and triglycerides.

Bile Formation and Regulation

• Bile is produced by hepatocytes and stored in the gallbladder.
• Bile emulsifies fats to aid in digestion and absorption.
• Secretion is stimulated by CCK.

Pancreatic Secretions

• Exocrine secretions: Digestive enzymes (amylase, lipase, proteases).
• Endocrine secretions: Insulin and glucagon.
• Secretion of digestive enzymes is stimulated by CCK.

GI System Functions

• Ingestion
• Secretion
• Mixing and propulsion
• Digestion
• Absorption
• Excretion

GIT Structures and Functions

• Mouth: Mechanical and chemical digestion.
• Pharynx: Swallowing.
• Oesophagus: Transports food to the stomach.
• Stomach: Mechanical and chemical digestion, storage of food.
• Small Intestines: Digestion and absorption.
• Large Intestines: Absorption of water and electrolytes, formation and storage of feces.

Mechanical and Chemical Digestion Along the GIT

• Mouth: Mechanical digestion via chewing, chemical digestion via salivary amylase.
• Stomach: Mechanical digestion via churning, chemical digestion via gastric acid and pepsin.
• Small intestine: Mechanical digestion via segmentation, chemical digestion via pancreatic enzymes and brush border enzymes.

Catabolic and Anabolic Reactions

• Catabolic reactions: Break down complex molecules into simpler ones, releasing energy.
  • Example: Glycolysis.
• Anabolic reactions: Build complex molecules from simpler ones, requiring energy.
  • Example: Protein synthesis.

Glycolysis, Krebs Cycle, ETC

• Glycolysis: Breaks down glucose into pyruvate in the cytoplasm.
• Krebs cycle: Oxidizes pyruvate to produce ATP, NADH, and FADH2 in the mitochondrial matrix.
• Electron transport chain (ETC): Uses NADH and FADH2 to generate a proton gradient, which drives ATP synthesis in the inner mitochondrial membrane.

Metabolic Terms

• Glycolysis: Breakdown of glucose.
• Glycogenesis: Synthesis of glycogen from glucose.
• Glycogenolysis: Breakdown of glycogen into glucose.
• Lipolysis: Breakdown of triglycerides into glycerol and fatty acids.
• Lipogenesis: Synthesis of triglycerides from glycerol and fatty acids.
• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources.
• Ketogenesis: Production of ketone bodies from fatty acids.

Hormones Regulating Metabolism

• Insulin: lowers blood glucose, produced by the pancreas.
• Glucagon: Increases blood glucose, produced by the pancreas.
• Thyroid hormones: Regulate metabolic rate, produced by the thyroid gland.
• Cortisol: Increases blood glucose and stress response, produced by the adrenal gland.