Neurophysiology Part 1 PDF

1 PRINCIPLES OF NEUROPHYSIOLOGY, PART 1 Dr. Samantha Solecki, DC, MS Instructor, Biology Thinker. Learner. Motivator. Lover of Anatomy & Physiology [email protected] © 2019 Pearson Education, Inc. ...

1 PRINCIPLES OF NEUROPHYSIOLOGY, PART 1 Dr. Samantha Solecki, DC, MS Instructor, Biology Thinker. Learner. Motivator. Lover of Anatomy & Physiology [email protected] © 2019 Pearson Education, Inc.  2 Learning Objectives *Acquired from the Human Anatomy and Physiology Society (HAPS) with personal additions Describe the major functions of the nervous system. Describe the nervous system as a control system identifying nervous system elements that are sensory receptors, the afferent pathway, control centers, the efferent pathway and effector organs. With respect to the neuron: Identify soma, axon and dendrites. State which parts of each type of neuron receive information, and which parts conduct the output signal of the neuron. Define permeability Explain how ion channels affect neuron selective permeability. Contrast the relative concentrations of sodium, potassium and chloride ions inside and outside the cell. Differentiate between a concentration gradient and an electrical potential. Define electrochemical gradient. With respect to ion channels: Differentiate between passive and active ion channels. Explain how passive ion channels cause development of the resting membrane potential in neurons. Differentiate between voltage-gated and chemically-gated ion channels. Describe the voltage-gated ion channels that are essential for development of the action potential. Discuss the sequence of events that must occur for an action potential to be generated. Describe the role of the sodium-potassium exchange pump in maintaining the resting membrane potential and making continued action potentials possible.  3 Learning Objectives *Acquired from the Human Anatomy and Physiology Society (HAPS) with personal additions Define threshold. Discuss the role of positive feedback in generation of the action potential. Interpret a graph showing the voltage vs. time relationship of an action potential, and relate the terms depolarize, repolarize and hyperpolarize to the events of an action potential. With respect to refractory periods: Define absolute and relative refractory periods. Explain the physiological basis of the absolute and relative refractory periods. Discuss the consequence of a neuron having an absolute refractory period. With respect to impulse conduction: Describe how local circuit currents cause impulse conduction in an unmyelinated axon. Describe saltatory conduction.  4 The Nervous System Master controlling and communicating system of body Cells communicate via electrical and chemical signals Rapid and specific Usually cause almost immediate responses Figure 11.2 Levels of organization in the nervous system. 5 Central nervous system (CNS) Peripheral nervous system (PNS) Brain and spinal cord Cranial nerves and spinal nerves Integrative and control centers Communication lines between the CNS and the rest of the body Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory Motor nerve fibers nerve fibers Conducts impulses from the CNS Conducts impulses from to effectors (muscles and glands) receptors to the CNS Somatic nervous Autonomic nervous Somatic sensory fiber Skin system system (ANS) Somatic motor Visceral motor (voluntary) (involuntary) Conducts impulses Conducts impulses from the CNS to from the CNS to skeletal muscles cardiac muscles, smooth muscles, Visceral sensory fiber and glands Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic Mobilizes body systems division during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Parasympathetic motor fiber of ANS Bladder Motor (efferent) division of PNS  6 NEURONS  7 Neurons 90 billion neural cells 360 billion non-neural cells Structural units of nervous system Large, highly specialized cells that conduct impulses Extreme longevity ( 100 years or more) Amitotic—with few exceptions High metabolic rate—requires continuous supply of oxygen and glucose All have: cell body and, one or more processes – Dendrites & Axon  8 Neuron Cell Body (Perikaryon or Soma) Biosynthetic center of neuron 1. Synthesizes proteins, membranes, and other chemicals 2. In most, plasma membrane part of receptive region Rough ER (chromatophilic substance or Nissl bodies) Most active and best developed in body Spherical nucleus with nucleolus Some contain pigments Most neuron cell bodies are in CNS Nuclei – clusters of neuron cell bodies in CNS Ganglia – lie along nerves in PNS  9 Dendrites In motor neurons 100s of short, tapering, diffusely branched processes Same organelles as in body Receptive (input) region of neuron Convey incoming messages toward cell body as graded potentials (short distance signals) In many brain areas fine dendrites specialized Collect information with dendritic spines Appendages with bulbous or spiky ends  10 The Axon: Structure One axon per cell arising from axon hillock Cone-shaped area of cell body In some axon short or absent In others most of length of cell Some 1 meter long Long axons called nerve fibers Occasional branches (axon collaterals) Branches profusely at end (terminus) Can be 10,000 terminal branches Distal endings called axon terminals or terminal boutons Figure 11.4a Structure of a motor neuron. 11 Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleus Axon (impulse- Myelin sheath gap Nucleolus generating Impulse (node of Ranvier) Chromatophilic and -conducting direction substance (rough region) Axon endoplasmic terminals reticulum) Schwann cell (secretory region) Axon hillock Terminal branches  12 Transport Along the Axon Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements Movement in both directions Anterograde—away from cell body Examples: mitochondria, cytoskeletal elements, membrane components, enzymes Retrograde—toward cell body Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins  13 NEUROPHYSIOLOGY  14 Membrane Potentials Like all cells, neurons have a resting membrane potential Unlike most other cells, neurons can rapidly change resting membrane potential Neurons are highly excitable  15 Basic Principles of Electricity Opposite charges are attracted to each other Energy is required to keep opposite charges separated across a membrane Energy is liberated when the charges move toward one another When opposite charges are separated, the system has potential energy  16 Basic Principles of Electricity Definitions Voltage: a measure of potential energy generated by separated charge Measured between two points in volts (V) or millivolts (mV) Called potential difference or potential Charge difference across plasma membrane results in potential Greater charge difference between points = higher voltage  17 Basic Principles of Electricity Definitions (cont.) Current: flow of electrical charge (ions) between two points Can be used to do work Flow is dependent on voltage and resistance Resistance: hindrance to charge flow Insulator: substance with high electrical resistance Conductor: substance with low electrical resistance  18 Basic Principles of Electricity Definitions (cont.) Ohm’s law: gives relationship of voltage, current, resistance Current (I)  voltage (V)/resistance (R) Current is directly proportional to voltage Greater the voltage (potential difference), greater the current No net current flow between points with same potential Current is inversely proportional to resistance The greater the resistance, the smaller the current  19 Basic Principles of Electricity Role of membrane ion channels Large proteins serve as selective membrane ion channels K+ ion channel allows only K+ to pass through Two main types of ion channels Leakage (nongated) channels, which are always open Gated channels, in which part of the protein changes shape to open/close the channel Three main gated channels: chemically gated, voltage—gated, or mechanically gated  20 Basic Principles of Electricity Chemically gated (ligand-gated) channels Open only with binding of a specific chemical (example: neurotransmitter) Voltage-gated channels Open and close in response to changes in membrane potential Mechanically gated channels Open and close in response to physical deformation of receptors, as in sensory receptors  When gated channels are open, ions diffuse quickly: 1. Along chemical concentration gradients from higher concentration to lower concentration 2. Along electrical gradients toward opposite electrical charge  21 Operation of Gated Channels Figure 11.7 Operation of gated channels.  22 Basic Principles of Electricity Electrochemical gradient: electrical and chemical gradients combined Ion flow creates an electrical current, and voltage changes across membrane Expressed by rearranged Ohm’s law equation: V = IR  23 Generating the Resting Membrane Potential  Potential generated by: Differences in ionic composition of ICF and ECF Differences in plasma membrane permeability  24 Measuring Membrane Potential in Neurons Figure 11.8 Measuring membrane potential in neurons.  25 Generating the Resting Membrane Potential  26 Resting Membrane Potential Focus Figure 11.1-1 Resting Membrane Potential.  27 Generating the Resting Membrane Potential Figure 3.15 The key role of K + in generating the resting membrane potential. 28 1 K+ diffuse down their steep Extracellular fluid concentration gradient (out of the cell) via leakage channels. Loss of K+ results in a negative charge on the inner plasma membrane face. 2 K+ also move into the cell because they are attracted to the + + + + negative charge established on the + + inner plasma membrane face. + + – – – – 3 A negative membrane potential – – (–90 mV) is established when the – Potassium – movement of K+ out of the cell equals leakage K+ movement into the cell. At this channels point, the concentration gradient promoting K+ exit exactly opposes the electrical gradient for K+ entry. Protein anion (unable to follow K+ through the Cytoplasm membrane)  29 Generating the Resting Membrane Potential  30 Generating a Resting Membrane Potential Depends on (1) Differences in K+ and Na+ Concentrations Inside and Outside Cells, and (2) Differences in Permeability of the Plasma Membrane to these Ions Focus Figure 11.1 Resting Membrane Potential.  31 Changing the Resting Membrane Potential Membrane potential changes when: Concentrations of ions across membrane change Membrane permeability to ions changes Changes produce two types of signals Graded potentials Incoming signals operating over short distances Action potentials Long-distance signals of axons Changes in membrane potential are used as signals to receive, integrate, and send information  32 Changing the Resting Membrane Potential Terms describing membrane potential changes relative to resting membrane potential: 1. Depolarization: decrease in membrane potential (moves toward zero and above) Inside of membrane becomes less negative than resting membrane potential Probability of producing impulse increases 2. Hyperpolarization: increase in membrane potential (away from zero) Inside of membrane becomes more negative than resting membrane potential Probability of producing impulse decreases  33 Depolarization and Hyperpolarization of the Membrane Figure 11.9 Depolarization and hyperpolarization of the membrane.  34 GRADED POTENTIALS  35 Graded Potentials Short-lived, localized changes in membrane potential The stronger the stimulus, the more voltage changes and the farther current flows Triggered by stimulus that opens gated ion channels Results in depolarization or sometimes hyperpolarization Named according to location and function Receptor potential (generator potential): graded potentials in receptors of sensory neurons Postsynaptic potential: neuron graded potential EPSP IPSP  36 The Spread and Decay of a Graded Potential Figure 11.10a The spread and decay of a graded potential.  37 Graded Potentials Once gated ion channel opens, depolarization spreads from one area of membrane to next Current flows but dissipates quickly and decays Graded potentials are signals only over short distances  38 The Spread and Decay of a Graded Potential Figure 11.10b The spread and decay of a graded potential.  39 The Spread and Decay of a Graded Potential Figure 11.10c The spread and decay of a graded potential.  Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. 40 Ion movement Graded potential 5 Binding of neurotransmitter open on channels, resulting in graded otentials.  41 ACTION POTENTIALS  42 Action Potentials Principal way neurons send signals Means of long-distance neural communication Occur only in muscle cells and axons of neurons Brief reversal of membrane potential with a change in voltage of ~100 mV Action potentials (APs) do not decay over distance as graded potentials do Involves opening of specific voltage-gated channels  43 Action Potential Focus Figure 11.2 Action Potential.  44 Generating an Action Potential  45 Action Potential Focus Figure 11.2 Action Potential.  46 Generating an Action Potential  47 Action Potential Focus Figure 11.2 Action Potential.  48 Generating an Action Potential  49 Action Potential Focus Figure 11.2 Action Potential.  50 Generating an Action Potential  51 Action Potential Focus Figure 11.2 Action Potential.  52 Action Potential Focus Figure 11.2 Action Potential.  53 Action Potential Focus Figure 11.2 Action Potential.  54 Generating an Action Potential Repolarization resets electrical conditions, not ionic conditions After repolarization, Na+/K+ pumps (thousands of them in an axon) restore ionic conditions  55 Resting Membrane Potential Focus Figure 11.1-1 Resting Membrane Potential.  56 Threshold and the All-or-None Phenomenon Not all depolarization events produce APs For an axon to “fire,” depolarization must reach threshold voltage to trigger AP At threshold: Membrane is depolarized by 15 to 20 mV Na+ permeability increases Na+ influx exceeds K+ efflux The positive feedback cycle begins All-or-None: An AP either happens completely, or does not happen at all  57 Propagation of an Action Potential Propagation allows AP to be transmitted from origin down entire axon length toward terminals Na+ influx through voltage gates in one membrane area cause local currents that cause opening of Na+ voltage gates in adjacent membrane areas Leads to depolarization of that area, which in turn causes depolarization in next area  58 Propagation of an Action Potential Once initiated, an AP is self-propagating In nonmyelinated axons, each successive segment of membrane depolarizes, then repolarizes Propagation in myelinated axons differs Since Na+ channels closer to the AP origin are still inactivated, no new AP is generated there AP occurs only in a forward direction  59 Propagation of an Action Potential Figure 11.11 Propagation of an action potential (AP). Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity CNS tells difference between a weak stimulus and a strong one by frequency of impulses Frequency is number of impulses (APs) received per second Higher frequencies mean stronger stimulus Relationship Between Stimulus Strength and Action Potential Frequency Figure 11.12 Relationship between stimulus strength and action potential frequency.  62 Refractory Periods Refractory period: time in which a neuron cannot trigger another AP Voltage-gated Na+ channels are open, so neuron cannot respond to another stimulus, independent of strength of frequency Two Types: 1. Absolute – another AP is not possible until the resetting of the VG Na+ Channels Enforces the All-or-None Principle 2. Relative – Follows the absolute refractory; most Na+ are already reset Coincides with repolarization Think of a disobedient (refractory) dog – if he is absolutely refractory he will never come when called, but if he is relatively refractory, he may come but only if you call loud enough Absolute and Relative Refractory Periods in an AP Figure 11.13 Absolute and relative refractory periods in an AP.  64 PRINCIPLES OF THE AP Conduction Velocity APs occur only in axons, not other cell areas AP conduction velocities in axons vary widely Rate of AP propagation depends on two factors: 1. Axon diameter Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction 2. Degree of myelination Two types of conduction depending on presence or absence of myelin Conduction Velocity Continuous conduction: slow conduction that occurs in nonmyelinated axons Saltatory conduction: occurs only in myelinated axons and is about 30 times faster Myelin sheaths insulate and prevent leakage of charge Voltage-gated Na+ channels are located at myelin sheath gaps APs generated only at gaps Electrical signal appears to jump rapidly from gap to gap Clinical–Homeostatic Imbalance Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence Treatment: drugs that modify immune system activity May not be able to prevent, but maintaining high blood levels of vitamin D may reduce risk of development Conduction Velocity Nerve fibers are classified according to diameter, degree of myelination, and speed of conduction Fall into three groups: Group A fibers Largest diameter Myelinated somatic sensory and motor fibers of skin, skeletal muscles, and joints Transmit at 150 m/s (~300 mph) Conduction Velocity Group B fibers Intermediate diameter Lightly myelinated fibers Transmit at 15 m/s (~30 mph) Group C fibers Smallest diameter Unmyelinated Transmit at 1 m/s (~2 mph) B and C groups include ANS visceral motor and sensory fibers that serve visceral organs  70 Clinical Correlate Impaired AP impulse propagation can be caused by a number of chemical and physical factors Local anesthetics act by blocking VG Na+ channels Cold temperatures (persistent or extreme) or continuous pressure interrupts blood flow to neurons Cold fingers become numb Foot “falls asleep” Table 11.2 Comparison of Graded Potentials and Action Potentials (4 of 4) 71