Chapter 16: The Endocrine System

Chapter 16: The Endocrine System

• The endocrine system uses hormones in the bloodstream to affect distant target cells with slow, long-lasting effects.
• The nervous system uses neurotransmitters at synapses to affect adjacent target cells with quick, short-lived effects.
• Neural signaling involves neurotransmitters acting on adjacent cells at synapses
• Paracrine signaling involves local hormones affecting nearby cells.
• Autocrine signaling involves hormones affecting the same cell that secreted them
• Endocrine signaling involves hormones in the bloodstream affecting distant target cells.
• Primary endocrine organs have hormone secretion as their main function i.e., pituitary, thyroid, adrenal glands.
• Secondary endocrine organs have other functions, also secrete hormones i.e., heart, kidneys, pancreas.
• Neuroendocrine organs release hormones in response to neural stimulation. Examples include the hypothalamus, posterior pituitary, and adrenal medulla.
• Amino acid-based hormones are produced by the hypothalamus, thyroid gland, and adrenal medulla
• Peptide/protein hormones are produced by the hypothalamus, anterior pituitary, thyroid gland (parafollicular 'C' cells), pancreas, parathyroid gland, and kidneys.
• Steroid hormones are produced by the adrenal cortex, testes (Leydig cells), and ovaries
• Peptide/protein hormones are hydrophilic, transported unbound in blood plasma, and have shorter half-lives.
• Steroid and thyroid hormones are hydrophobic, transported bound to transport proteins, and have longer half-lives.
• Upregulation is an increase in hormone receptors on target cells, increasing sensitivity.
• Downregulation is a decrease in hormone receptors, reducing sensitivity to maintain homeostasis.
• The steps of a 2nd messenger system are:
  • Hormone binds to receptor on cell surface.
  • Receptor activates a G-protein.
  • G-protein activates an enzyme.
  • Enzyme produces a second messenger.
  • Second messenger activates protein kinases.
• Second messenger activating hormones are: Catecholamines, peptide/protein hormones.
• Steroid and thyroid hormones diffuse through the plasma membrane, bind to intracellular receptors, and form hormone-receptor complexes that bind to DNA to regulate gene expression.
• Hormone-response element is a specific DNA region where steroid/thyroid hormone-receptor complexes bind.
• Hormones cause changes in gene expression, enzyme activity, membrane permeability, and cell growth/differentiation.
• Hormones with humoral stimulus: insulin, glucagon, parathyroid hormone (PTH), calcitonin, ADH, aldosterone.
• Hormones with neural stimulus: epinephrine, norepinephrine, oxytocin, melatonin.
• Hormones with a hormonal stimulus: anterior pituitary hormones, thyroid hormone, cortisol, testosterone.
• Negative feedback involves rising hormone levels inhibiting further secretion.
• Positive feedback involves rising hormone levels stimulating further secretion.
• The hypothalamic-hypophyseal portal system transports releasing/inhibiting hormones from the hypothalamus to the anterior pituitary which regulates anterior pituitary hormone release.
• The hypothalamus produces hormones (ADH, oxytocin) stored/released by the posterior pituitary.
• Tropic hormones stimulate the secretion of other hormones i.e., TSH, ACTH, FSH, LH.
• CRH targets corticotropic cells of the anterior pituitary, stimulating ACTH secretion.
• TRH targets thyrotropic and lactotropic cells, stimulating TSH and prolactin secretion.
• GHRH targets somatotropic cells, stimulating GH secretion.
• PRH targets lactotropic cells, stimulating prolactin secretion.
• GnRH targets gonadotropic cells, stimulating FSH and LH secretion.
• PIH (Dopamine) targets lactotropic cells, inhibiting prolactin secretion.
• GHIH (Somatostatin) targets somatotropic, lactotropic, and thyrotropic cells, inhibiting GH, prolactin, and TSH secretion.
• GnIH targets gonadotropic cells, inhibiting FSH and LH secretion.
• Somatotrophs release growth hormone (GH).
• Corticotrophs release adrenocorticotropic hormone (ACTH).
• Thyrotrophs release thyroid-stimulating hormone (TSH).
• Lactotrophs release prolactin.
• Gonadotrophs release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• Prolactin targets mammary glands, stimulating milk production.
• ACTH targets the adrenal cortex, stimulating cortisol secretion.
• TSH targets the thyroid gland, stimulating thyroid hormone secretion.
• GH targets liver, muscle, and bone, stimulating growth and metabolism.
• LH targets ovaries/testes, stimulating ovulation/testosterone production.
• FSH targets ovaries/testes, stimulating follicle development/sperm production.
• Feedback loops, affect anterior pituitary hormones and include the hypothalamus, anterior pituitary, and target organs.
• Insulin-like growth factor (IGF) promotes growth and development of bones and tissues.
• IGF is secreted by the liver, stimulated by growth hormone (GH).
• T3 and T4 from the thyroid gland target most cells, increasing metabolism and growth.
• Calcitonin from the thyroid gland targets bones and kidneys to decrease blood calcium levels.
• Thyroid hormone synthesis involves iodide uptake, oxidation to iodine, iodination of tyrosine residues, and coupling to form T3/T4.
• Thyroid hormone synthesis requires iodine (trace element), and tyrosine (amino acid).
• TRH stimulates TSH release from the anterior pituitary, which stimulates thyroid hormone secretion, high thyroid hormone levels inhibit TRH/TSH release (negative feedback).
• Parathyroid cells secrete PTH in response to low blood calcium levels, targeting bones and kidneys, increasing blood calcium levels.
• Thyroid C cells secrete calcitonin in response to high blood calcium levels, targeting bones and kidneys, decreasing blood calcium levels.
• Aldosterone (mineralocorticoid) is secreted from the zona glomerulosa of the adrenal gland cortex.
• Aldosterone targets kidneys, stimulates sodium retention and potassium excretion, increasing blood volume/pressure and is stimulated by angiotensin II/low blood sodium, inhibited by high blood sodium.
• Cortisol (glucocorticoid) is secreted by the zona fasciculata of the adrenal gland cortex.
• Cortisol targets most cells, increases blood glucose, suppresses immune response, aids in stress response, and stimulated by ACTH, inhibited by high cortisol levels (negative feedback).
• Dehydroepiandrosterone (DHEA) (androgens) are secreted from the zona reticularis of the adrenal gland cortex.
• Chromaffin cells are neuroendocrine cells in the adrenal medulla and secrete catecholamines (epinephrine/norepinephrine).
• Sympathetic preganglionic neurons stimulate the adrenal gland medulla to secrete epinephrine and norepinephrine.
• The neurohormones increase heart rate, blood pressure, and blood glucose, and dilate airways.
• Alpha cells of the pancreatic islets secrete the protein hormone, glucagon.
• Glucagon targets the liver, increases blood glucose by stimulating glycogen breakdown and is stimulated by low blood glucose and inhibited by high blood glucose.
• Beta cells of the pancreatic islets secrete the protein hormone, insulin.
• Insulin targets liver, muscle, adipose tissue, decreases blood glucose by promoting glucose uptake/storage, and is stimulated by high blood glucose and inhibited by low blood glucose.
• Insulin lowers blood glucose by promoting glucose uptake/storage.
• Glucagon raises blood glucose by stimulating glycogen breakdown.
• The steroid hormone testosterone targets reproductive organs, muscle, and bone.
• Testosterone promotes male secondary sexual characteristics/muscle growth and is secreted by Leydig cells in the testes, stimulated by LH from the anterior pituitary.
• GnRH stimulates LH secretion, stimulates testosterone production, high testosterone inhibits GnRH/LH secretion (negative feedback)..
• Estrogen and progesterone target reproductive organs, breast, bone, promote female secondary sexual characteristics/regulate the menstrual cycle and are secreted by the ovaries, stimulated by FSH/LH from the anterior pituitary.
• Melatonin is secreted by the pineal gland in response to darkness, targeting the brain, regulating sleep-wake cycles.
• Leptin is secreted by adipose tissue in response to fat storage, targeting the hypothalamus, regulating appetite.
• Ghrelin is secreted by the stomach in response to fasting, targeting the hypothalamus, stimulating hunger.
• ANP is secreted by the heart in response to high blood volume, targeting kidneys, promoting sodium and water excretion.
• EPO is secreted by the kidneys in response to low oxygen, targeting bone marrow, stimulating red blood cell production.
• ADH increases to promote water retention which occurs when blood volume or pressure falls below normal
• Aldosterone increases to promote sodium retention which occurs when blood volume or pressure falls below normal
• ANP decreases to reduce sodium and water excretion which occurs when blood volume or pressure falls below normal.
• When blood volume/pressure rises above normal: ADH decreases to reduce water retention.
• Aldosterone decreases to reduce sodium retention when blood volume/pressure rises above normal.
• ANP increases to promote sodium and water excretion when blood volume/pressure rises above normal.
• Insulin decreases during the fasting state.
• Glucagon increases during the fasting state to raise blood glucose.
• Insulin increases during the post-prandial state to lower blood glucose.
• Glucagon decreases during the post-prandial state.
• Aldosterone and ADH both increase blood volume/pressure (Synergistic relationship)
• Insulin decreases blood glucose, while glucagon increases it (Antagonistic relationship).
• Thyroid hormone is necessary for growth hormone to exert its effects (Permissive relationship)
• Thyroid hormone enhances the effects of epinephrine (Potentiation relationship)

Chapter 11: Introduction to the Nervous System & Nervous Tissue

• The central nervous system (CNS) consists of the brain and spinal cord and functions in sensory integration, motor control, and higher cognitive functions.
• The peripheral nervous system (PNS) consists of cranial nerves, spinal nerves, ganglia, sensory receptors, and motor neuron synapses.
• Afferent neurons transmit sensory information from receptors to the CNS.
• Efferent neurons transmit motor information from the CNS to target organs.
• The sensory division of the PNS includes somatic sensory (skin, joints, muscles) and visceral sensory (internal organs).
• The motor division of the PNS includes somatic motor (voluntary movement) and visceral motor (involuntary functions).
• The somatic division of the PNS controls sensory information from the skin, joints, muscles, and special senses, and motor control of skeletal muscles for voluntary movement.
• The autonomic division of the PNS controls cardiac muscle, smooth muscle, and glands for involuntary functions.
• Neurons contain Dendrites, a Cell Body, an Axon and Axon terminals.
• The receptive region includes the dendrites and cell body.
• The conductive region includes the axon.
• The secretory region includes the axon terminals.
• Interneurons relay information within the CNS and make up 99% of neurons.
• Multipolar neurons are found in the CNS (interneurons and motor neurons) and PNS (motor neurons).
• Bipolar neurons are found in the PNS (special sensory pathways).
• Pseudounipolar (unipolar) neurons are found in the PNS (general sensory pathways).
• A nucleus is a collection of neuron cell bodies in the CNS.
• A ganglion is a collection of neuron cell bodies in the PNS.
• A nerve is a bundle of axons in the PNS.
• A tract is a bundle of axons in the CNS.
• Astrocytes in the CNS support and maintain the blood-brain barrier.
• Oligodendrocytes in the CNS form myelin sheaths.
• Microglia in the CNS provide immune defense.
• Ependymal cells, produce cerebrospinal fluid.
• Schwann cells in the PNS form myelin sheaths.
• Satellite cells in the PNS support and protect neuron cell bodies.
• Oligodendrocytes form myelin sheaths in the CNS.
• Schwann cells form myelin sheaths in the PNS.
• An internode is the myelinated segment of an axon between two nodes of Ranvier.
• A node of Ranvier is the unmyelinated gap between internodes where action potentials are generated.
• White matter is composed of myelinated axons and transmits information between different areas of the CNS.
• Gray matter is composed of neuron cell bodies and dendrites, and processes and integrates information in the CNS.
• Myelin sheaths increase the speed of action potential propagation by allowing saltatory conduction.
• Resting membrane potential is -70 mV, maintained by sodium-potassium pump and leak channels maintaining ion gradients.
• The sodium-potassium exchange pump maintains ion gradients by pumping 3 Na+ out and 2 K+ into the cell.
• The receptive region of a neuron contains ligand-gated or mechanically-gated Na+, K+, and Cl- channels.
• The conductive region of a neuron contains voltage-gated Na+ and K+ channels.
• A local potential is a temporary change in membrane potential in response to a stimulus.
• Local potentials are graded because their strength varies with the strength of the stimulus.
• Local depolarization involves Na+ ions entering the cell through ligand-gated or mechanically-gated channels.
• Excitatory postsynaptic potential (EPSP) is a subthreshold depolarization that brings the neuron closer to firing an action potential.
• Local hyperpolarization involves K+ ions leaving the cell or Cl- ions entering the cell.
• Inhibitory postsynaptic potential (IPSP) is a subthreshold hyperpolarization that inhibits the neuron from firing an action potential.
• A generator (receptor) potential is a graded potential that activates a sensory receptor.
• Local potential is graded, reversible, and short-distance, while an action potential is all-or-none, long-distance, and uniform strength.
• Action Potential involves Depolarization, Repolarization and Hyperpolarization
  • For Depolarization, Na+ channels open, Na+ enters the cell
  • For Repolarization, K+ channels open, K+ leaves the cell
  • For Hyperpolarization, K+ channels remain open, cell becomes more negative
• Continuous conduction occurs in unmyelinated axons, is slower, while saltatory conduction occurs in myelinated axons, is faster, and action potentials "jump" between nodes of Ranvier.
• The 'all or none' principle is that, once threshold is reached, an action potential of the same magnitude is generated.
• Absolute refractory period is the period during which no new action potential can be generated, regardless of stimulus strength.
• The relative refractory period is the period during which a stronger-than-normal stimulus is required to generate an action potential.
• Type A fibers are myelinated with fast conduction while Type B fibers are lightly myelinated with a moderate conduction. Type C fibers are unmyelinated with slow conduction.
• The presynaptic neuron releases neurotransmitters into the synaptic cleft.
• The postsynaptic neuron receives neurotransmitters from the presynaptic neuron.
• An electrical synapse: Electrical signals pass directly between cells via gap junctions.
• Electrical synapses are found in the CNS and cardiac muscle.
• A chemical synapse consists of a presynaptic terminal, synaptic cleft, and postsynaptic membrane.
• The events at a chemical synapse:
  • Action potential reaches presynaptic terminal
  • Ca2+ enters
  • Neurotransmitters are released
  • neurotransmitters bind to receptors on the postsynaptic membrane
  • ion channels open or close
• Types of Chemical Synapses: Neural, Neurotransmitters and Neuro glandular.
• Synaptic delay is the time it takes for neurotransmitters to be released, diffuse across the synaptic cleft, and bind to receptors.
• Spatial summation: Multiple presynaptic neurons fire simultaneously to reach threshold.
• Temporal summation: A single presynaptic neuron fires repeatedly to reach threshold.
• Synaptic transmission is terminated in chemical synapses via neurotransmitter reuptake, enzymatic degradation, and diffusion away from the synapse.
• Acetylcholine (ACh) stimulates muscle contraction.
• Norepinephrine increases heart rate and blood pressure.
• Dopamine regulates mood and movement.
• Serotonin regulates mood and sleep.
• GABA inhibits neural activity.
• Glutamate excites neural activity.
• Substance P mediates pain.
• Opioids reduce pain perception.
• Diverging neural circuit: One neuron synapses with multiple neurons, amplifying the signal (motor pathways).
• Convergent neural circuit: Multiple neurons synapse with one neuron, integrating signals (sensory pathways).
• Serial neuronal pool: Neurons relay information in a sequence (pain pathways).
• Reverberating neuronal pool: Neurons form a feedback loop, maintaining activity (breathing).
• Parallel after-discharge neuronal pool: Multiple neurons process information simultaneously, leading to a single output (complex reflexes).