Chapter 7 Nerve Cells and Electrical Signaling

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1 Chapter 7 Nerve Cells and Electrical Signaling 7.1. Overview of the Nervous System (Figure 7.1) 7.2. Cells of the Nervous System o Neurons are excitable cells which can generate action potentials o 90% cells in the nervous system are glial cells provide support to neurons. A Typical Neuron (Figure 7.2) Components of a Neuron a. cell body (soma) b. dendrites c. axon d. axon terminal e. axon hillock 1) Where axon originates from the cell body, for initiation of action potential 2) branch from the cell body, receive input signals and convey information to cell body. 3) contain nucleus and organelles, carry out cellular functions such as protein synthesis and cell respiration. 4) specialized to releases neurotransmitters upon arrival of an action potential. 5) nerve fiber, send out electrical information action potential. Location of Ion Channels in Neuron Plasma Membrane o The different functions of a neural component may be contributed by the different channels located in the plasma membrane Leak channels Voltage-gated channels Ligand-gated channels o Leak channels or called non gated channels Are always open Found throughout plasma membrane of neuron Are responsible for o Voltage-gated Channels Open or close in response to. Example: Sodium and potassium channels; throughout, but more in axon (especially axon hillock) Responsible for generate or conduct 1

2 Example: Calcium Channels Most located in axon terminal Trigger the release of neurotransmitter o Ligand-gated channels Ligand is a substance (atom, ion or a molecule) that binds specifically and reversibly with anther chemical. Open or close in response to a binding Found in most in the dendrites and cell body, regions that receive neurotransmitters Responsible for generating potentials Match the function of neurons (Figure 7.6) Neuron Function Afferent a. Output information from CNS to effectors in PNS Efferent b. Integrate information in CNS Interneuron c. Input information from receptors PNS to CNs 7.3. Establish of the Resting Membrane Potential Table 7.1 Types of Electrical Potentials Membrane Potential o Due to unequal distribution of anions and cations across cell membrane strength of force o Usually measured in millivolts Distribution of Ion across the Membrane o Potassium ions and fixed anions are more abundant outside or inside of the cell. o More sodium, chloride and calcium ions are located in outside or inside of the cell Equilibrium Potential (Ex) (Figure 7.6) o Ex is the membrane potential (Vm) when electrochemical driving force = o Computation of El: Nernst equation (page 99) E1 = (61 mv/z) log [I]o/ [I]i Where: El = equilibrium potential of ion l, Z = valence of ion l, [I]o = ECF concentration of ion l and [I]i = ICF concentration of ion l o EK+ = -90mV; ENa+ = +60mV Resting Membrane Potential o Resting member potential is the membrane potential when a cell is o The resting membrane potential is closer to (Ek or ENa) Factors Critical in Establishing Resting Vm o Differential membrane permeability to ions Membrane is more permeable to potassium ions than it is to sodium ions, membrane is not permeable to fixed anions o Sodium/Potassium pumps 2

3 Prevent the dissipating of sodium and potassium gradient across the membrane o Difference in ion concentration gradients across plasma membrane Establishing Resting Membrane Potential (Figure 7.8) Na + /K + Pump and Resting Vm o 20% of resting membrane potential directly due to Na/K-ATPase Electrogenic: 3 Na + out, 2 K + in; Net +1 out o Na + /K + pump establishes and maintains concentration gradient of both ions 7.4. Electrical Signaling Through Changes in Membrane Potential Changes in Membrane Potential o Resting potential: reference point o : Vm is less negative than resting Vm (less polarized) o : Vm returns to resting Vm following a depolarization o : Vm is more negative (more polarized) than resting Vm Changes in Membrane Potential :(Table 7.2) o potentials are small changes in Vm in response to a stimulus that triggers the opening or the closing of ions channels; magnitude proportional (graded) to the strength of a stimulus o potentials are large changes in Vm in response to a graded potential that has reached the threshold. Graded Potentials (Figure 7.12) o Location: Sensory receptors: Receptor potentials can be produced in response to a stimulus on a sensory receptor Dendrites and cell body: Synaptic potentials are produced when neurotransmitter molecules binding to the receptors at the post synaptic cells o Strength: Relatively weak; proportional to the strength of stimulus (graded); Spread by electronic conduction (Figure 7.13) Decremental (Figure 7.13): Travel for a short distance; decays as it spreads o Change in Membrane Potential: Hyperpolarization or Depolarization Figure 7.14 Which stimulus is excitatory? Which stimulus is inhibitory? 3

4 o Summation A stimulus that repeated close together in time can be summed in summation (Figure 7.15b) summation: different stimuli from different sources overlap in time are summed (Figure 7.15c) Does an action potential occur (Figure 7.15 d)? o No refractory periods Action Potentials (APs) o When graded potentials reach a, a large, rapid depolarization occur in membranes of excitable cells (such as neuron and muscle cells) = action potentials o is the minimum depolarization necessary to induce the regenerative mechanism for the opening of sodium channels o principle: Action potentials from threshold and suprathreshold stimulus have the magnitude; if the membrane is not depolarized to threshold, no action potential occurs. Threshold Stimulus (Figure 7.19) Properties of Action Potentials (APs) o Location Axon o Strength: Communicate over long distances without decrease in strength (100 mv); all or- none o Depolarization only o Cannot be summed o Absolute and relative refractory periods Three Phases of an Action Potential (Figure 7.16 a) o Depolarization o Repolarization o After-hyperpolarization Ionic Base of an Action Potential (Figure 7.16 Table 6.3) o Depolarization: permeability of sodium (PNa) increases, sodium ions moves into cell rapidly. o Repolarization: Permeability of potassium increases (Pk), potassium ions moves out cell o After-hyperpolarization: Continued movement of K + out of the cell. Voltage-Gated Sodium Channel (Figure 7.17) o Two gates associated with channel Activation gate: Voltage dependent and positive feedback (Figure 7.18) Inactivation gate: Voltage and time dependent Table 7.3 Characteristics of a Neuron at Rest and During an Action Potential Block Action Potentials Formation o Local anesthetics such as lidocaine (Xylocaine) block the channels in neurons. No action potentials will be 4

5 produced therefore brain will not receive the pain stimuli. Refractory Periods (figure 7.20) o Period of time following an action potential marked by decreased excitability o Contribute to All-or-none principle of action potentials Frequency coding of the stimulus intensity Unidirectional propagation along an axon o Phases: Absolute and relative o Absolute refractory period Spans all of depolarization and most of the repolarization phase Second action potential can be generated regardless the strength of the stimulus Reason: Sodium gates are inactivated o Relative refractive period Spans last part of repolarization phase and hyperpolarization Second action potential can be generated with a stronger stimulus Some sodium gates closed, some are inactivated, and more potassium gates are open. How APs Convey the Intensity of a Stimulus (Figure 7.21)? o By coding. o The stronger of the supra threshold stimuli, the higher frequency of Aps o Based on the diagram provided in class, answer the following questions: Which stimulus is not strong enough to generate an action potential? Which stimulus is the strongest? Propagation of Action Potentials o Action potentials are propagated along the length of the axon to axon terminal o Mechanisms depend on presence or absence of myelin sheath Factors Affecting Propagation (Table 7.4) o Refractory period contributes to unidirectional propagation of an action potential along the axon o Axon diameter Large axon: Less resistance, speed Smaller: More resistance, slower o Myelination Myelinated fibers conduction: conduction: Leaping depolarization at the node of Ranvier Unmyelinated axon conduction: Action potentials are propagated by conduction Maintaining Neural Stability Graded potentials and action potentials tend to dissipate Na + and K + concentration gradients Na + /K + pump prevents dissipation 5