BIOL 1216 Week 3: The Nervous System and Neuronal Excitability (Student Notes) PDF

Last class: Cell Signaling Signal transduction ◦ Second messenger Receptor Second ◦ Kinase messenger ◦ Phoshorylation Inactive re...

Last class: Cell Signaling Signal transduction ◦ Second messenger Receptor Second ◦ Kinase messenger ◦ Phoshorylation Inactive relay  protein Extracellular chemical messenger (first messenger) Active relay ☺ protein (protein  Effector kinase) protein Effector protein P (phosphorylated) ☺ Cellular response Last Class: Cell Signaling Lipid soluble ECMs can bind in the cell Receptor-hormone complex Gene expression ◦ Transcription and translation Other types of receptors ◦ Ligand gated ion channels ◦ Enzyme receptors ◦ Enzyme-coupled receptors ◦ G-protein coupled receptors Signal amplification and termination Last Class: Blood Erythrocytes (Red blood cells) ◦ Erythropoiesis (feedback loop) ◦ Blood types Leukocytes (white blood cells) ◦ Neutrophils ◦ Basophils ◦ Eosinophils ◦ Monocytes ◦ Lymphocytes Section 1: https://forms.gle/YnVxZtxWVfPJB2Gc6 Section 2: https://forms.gle/vAvDeAEa4S2CJH2x8 Review Questions Which of the following is false about cell signaling? ◦ Lipid soluble hormones can pass through the cell membrane ◦ The transcription of a gene involves making a copy of a DNA sequence (called mRNA) ◦ Signal termination always occurs once the first messenger dissociates from the receptor ◦ The binding of a single messenger to a receptor could result in the production of hundreds of second messengers A person with type A blood might be able to receive a transfusion containing which of the following blood types? (select all that are correct) ◦ Type A ◦ Type B ◦ Type AB ◦ Type O The Nervous System and Neuronal Excitability BI OL 1 2 1 6 – W K 3 CHA PT ER 7 DR. T R E VOR KI N G Learning Objectives Describe how membrane potential is produced and maintained Predict how a change in the system might impact membrane potential Recall the organization of the nervous system Differentiate the roles of different cells of the nervous system Describe how electrical signals in neurons are generated and propagated (graded and action potentials) Outline the process of signal transmission at synapses Identify the functions of neurotransmitters Distinguish the different neural circuits Divisions of the nervous system Central Nervous system (CNS) ◦ Brain and Spinal cord Peripheral Nervous System (PNS) ◦ Afferent division ◦ Efferent division Organization of the Nervous System Central Nervous system (CNS) ◦ Brain and Spinal cord Peripheral Nervous System (PNS) ◦ Afferent division ◦ Input into the CNS ◦ Efferent division ◦ Info from the CNS to effectors ◦ Somatic (voluntary): skeletal muscles ◦ Autonomic (involuntary): Smooth muscle, cardiac muscle, glands ◦ Sympathetic and parasympathetic Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Functions of the Nervous system Sensory ◦ detect external or internal stimuli, and relay sensory information to the brain and spinal cord Integrative ◦ CNS analyzes sensory information, and makes decisions for appropriate responses Motor ◦ Motor information is conveyed from the CNS through cranial and spinal nerves of the PNS to appropriate effectors (muscles and glands). Feedback loops! Cells of the nervous system By the end of this part, you will be able to: Discuss the structure and function of neurons List the roles of neuroglia Explain the importance of myelination Describe the ability to repair neurons of the CNS and PNS Neuron Structure: An overview Nerve cells/Neurons Soma (Cell body) ◦ Control centre ◦ Dendrite receives signals Axon ◦ Generate and propagate action potentials Axon Terminal ◦ communicates with other neurons/cells at synapse Neuron Function Sensory neuron Interneuron Motor neuron Compare this organization to a feedback loop Electrical signals in Neurons Many cells in the nervous system display ‘Electrical Excitability’ E.g. Follow the path of action potentials ◦ Sensory neuron ◦ Interneuron ◦ Cerebral cortex ◦ Motor cortex ◦ Motoneurons ◦ Muscle (also excitable) Neuroglia Astrocyte Make up half the volume of the CNS Nourish and protect neurons Neuroglia cannot generate or Oligodendrocyte propagate action potentials If injured, neuroglia fill spaces Astrocytes Microglial cell Ependymal cell Neuroglia CNS and PNS have different neuroglia E.g. formation and maintenance of myelin sheath CNS – Oligodendrocytes PNS – Schwann cell Myelin Sheath Electrical insulation ◦ Increases speed of conduction Nodes of Ranvier ◦ Gaps in the myelination Neuron Repair Plasticity ◦ Ability to change throughout life ◦ E.g. sprout new dendrites, change synaptic contacts Repair ◦ Regeneration after damage ◦ Neurons have limited ability to repair PNS repair occurs if… ◦ Cell body is still intact ◦ Schwann cell remains active (guides and stimulates regrowth) CNS has little or no ability to repair ◦ Typically permanent Clinical Connection: Multiple Sclerosis Disease that causes a progressive destruction of myelin sheaths of neurons in the CNS ◦ Multiple regions of myelin sheath deteriorate to scleroses (hardened scars) Slows then short-circuits action potential conduction Autoimmune disease Appears between ages 20 and 40 First symptoms ◦ Heaviness or weakness in muscles ◦ Abnormal sensations ◦ Double vision Brief remission, then attack every year or two ◦ Progressive loss of function interspersed with remission Checkpoint: Nervous System Sam senses the pressure of the pen using a ________ receptor/neuron, which is a component of the ______ division of the _______ nervous system. This information is relayed to the ______ nervous system, where it is passed to an ____ neuron. The brain _______ this information and decides that Sam must increase pressure on the pen using the ______ division of the nervous system. A _______ neuron tells the muscles of the hand to squeeze harder. All of this information is relayed via _____ potentials, which move quickly through the body because many axons are are _______ Electrical signals in Neurons: Ion channels Ion channels cause changes in membrane permeability ◦ Ions can flow down electrochemical gradients ◦ Resting membrane potential (difference in charges) changes ◦ Allows for electrical communication Remember: electrochemical gradient 4 types of ion channels that allow neurons to function: ◦ Leak channels ◦ Ligand-gated channels ◦ Mechanically gates channels ◦ Voltage gated channels Ion channels: Leak Leak channels have gates that randomly alternate between open and closed positions Ion channels: Ligand-gated A ligand-gated channel opens or closes in response to a specific ligand (chemical) stimulus. Ion channels: Mechanically-gated Mechanically-gated channels open or close in response to mechanical stimulation ◦ Mechanical stimulation may take on the forms of touch, pressure, tissue stretching, and vibration Ion channels: Voltage-gated A voltage-gated channel opens in response to a change in membrane potential (voltage) ◦ K+, Ca2+, Na+ ◦ Important for action potentials Membrane potential Voltage that exists across the plasma membrane ◦ The presence of the plasma membrane creates an unequal distribution of positive and negative changes. ◦ The unequal distribution of charge is measured in volts or millivolts Unstimulated neuron cells + + + -+ display a resting membrane + - - + - - potential of around -70 mV + - + - - + - - The inside of the cell - membrane is more This difference in charge across the - negatively charged membrane is called polarization than outside Membrane potential The difference in charges across a membrane Inside Cell Outside Cell Simplification: + ◦ Charge on inside + + ◦ 5 positive, 3 negative + + ◦ 5-3 = +2 + - + ◦ Charge on Outside + - ◦ 6 positive, 3 negative + ◦ 6-3 = +3 - - - - ◦ Membrane potential is difference in charges + ◦ Inside charge minus outside charge + ◦ (+2) – (+3) = -1 ◦ The membrane is more positive outside the cell +2 +3 than inside, therefore the membrane potential is negative ◦ Most neuron cells -70 mV Inside is more negative (less positive) than outside Determinants of resting membrane potential 1. Unequal distribution of ions in the ECF and cytosol 2. Action of the Na+/K+ ATPase ◦ 3 Na+ out and 2 K+ in 3. Differences in membrane permeability ◦ More K+ leak channels than Na+ leak channels Membrane potential: + Hypothetical K leak channel only cell 1. Na+/K+ pump creates K+ concentration gradient Hypothetical cell begins with no membrane potential 2. K+ would flow out (down concentration Concentration gradient) K+ leak Outside cell + Electrical channel Voltage: -90mV K+ 3. electrical gradient would increase, - wanting to flow back into the cell Inside cell K+ K+ K+ 4. Eventually, the electrical gradient and K+ K+ concentration gradient would be equal: equilibrium potential K+ K+ K+ K+ 5. K+ equilibrium potential close to resting membrane potential What does that indicate? Membrane potential: + Hypothetical Na leak channel only cell 1. Na+/K+ pump creates Na+ concentration Na+ Na+ Na+ gradient Hypothetical cell begins with no Na+ membrane potential Na+ 2. Na+ would flow in (down concentration Na+ Na+ Na+ Concentration Na+ gradient) Na+ leak Outside cell - Electrical channel Voltage: +60mV 3.Na+ electrical gradient would increase, + wanting to flow back out of the cell Inside cell 4. Eventually, the electrical gradient and concentration gradient would be equal: equilibrium potential 5. Na+ equilibrium potential is NOT close to resting membrane potential What does that indicate? Membrane potential: Typical cell Membrane is more permeable to K+ ◦ Still leak of Na+ into cell Na+ Na+ Electrochemical gradient Na+ K+ Electrochemical gradient Na+ Na+ Na+ More net movement of positive charge out the cell + Outside cell - Inside cell Na+/K+ ATPase balances the Na+ leak in and K+ leak out K+ K+ K+ K+ K+ ◦ Maintains electrochemical gradient of K+ each NOT at equilibrium potential What is required? Checkpoint What would happen to membrane potential in the following hypothetical situations? ◦ A cell has more Na+ leak channels than usual ◦ Increased K+ leak channels ◦ The Na+/K+ pump was blocked (stopped working)