Chapter 11 The Nervous System (Part B)

Electrochemical Gradient Ions flow along their chemical gradient: From high concentration to low concentration Ions flow along their electrical gradient: Toward an area of opposite charge In a cell the charge imbalance occurs only in the immediate membrane region. Electrochemical gradient: Electrical & chemical gradients together

Resting Membrane Potential (Vr) A potential difference of (–70 mV) Across the membrane of a resting neuron Generated by the different concentrations (intra & extracellular) of: Na+ & Cl (extracellular) K+ & protein anions (A) (intracellular) Ionic concentration differences are the result of: Differential membrane permeability to Na+ & K+ Operation of the sodium-potassium pump

Resting Membrane Potential (Vr) Figure 11.8

Membrane Potentials: Signals Used for information: Receiving Integration, and Sending Types of signals Graded potentials (travel short distances) Action potentials (travel long distances)

Changes in Membrane Potential Changes are manifested by three events Depolarization: The inside of the membrane becomes less negative Repolarization: The membrane returns to its resting potential Hyperpolarization: The inside of the membrane becomes more negative than the resting potential

Changes in Membrane Potential Figure 11.9

Graded Potentials Short-lived, local changes in membrane potential Decrease in intensity with distance Their magnitude varies directly with the strength of the stimulus Can initiate action potentials if sufficiently strong

Graded Potentials Figure 11.10

Graded Potentials Voltage changes in graded potentials are decremental Current is quickly dissipated due to the leaky plasma membrane Can only travel over short distances

Graded Potentials Figure 11.11

Action Potentials (APs) An AP is a brief reversal of membrane potential with a total amplitude of 100 mV An AP in the axon of a neuron is a nerve impulse APs are only generated by: muscle cells and neurons (nerve cells) They do not decrease in strength over distance Are the principal means of neural communication

Action Potential: Resting State Na+ and K+ gated channels are closed Leakage accounts for small movements of Na+ and K+ K+ move out faster than Na+ moving in This creates membr-ane resting potential Figure 11.12.1

Action Potential: Depolarization Phase Na+ permeability increases; membrane potential reverses Na+ gates are opened; K+ gates are closed Threshold – a critical level of depolarization (-55 to -50 mV) At threshold, depolarization becomes self-generating Figure 11.12.2

Action Potential: Repolarization Phase Sodium gates close Membrane permeability to Na+ declines to resting levels As sodium gates close, voltage-sensitive K+ gates open K+ exits the cell and internal negativity of the resting neuron is restored Figure 11.12.3

Action Potential: Hyperpolarization Potassium gates remain open, causing an excessive efflux of K+ This efflux causes hyperpolarization of the membrane (undershoot) Figure 11.12.4

Action Potential: Role of the Sodium-Potassium Pump Repolarization: Restores only the resting electrical conditions of the neuron Does not restore the resting ionic conditions Ionic restoration to resting conditions: Is achieved by the sodium-potassium pump Three Na+ are pumped outside the cell and two K+ are pumped inside

Phases of the Action Potential 1 – resting state 2 – depolarization phase 3 – repolarization phase 4 – hyperpolarization

Threshold and Action Potentials membrane is depolarized by 15 to 20 mV (-70 to -55 mV) Weak (subthreshold) stimuli: are not relayed into action potentials Strong (threshold) stimuli: are relayed into action potentials All-or-none phenomenon: action potentials either happen completely, or not at all

Absolute Refractory Period Is the time from the opening of Na+ gates until they begin to close The absolute refractory period: Prevents the neuron from generating a simultaneous action potential Ensures that each action potential is separate Enforces one-way transmission of nerve impulses

Absolute Refractory Period Figure 11.15

Relative Refractory Period The interval following the absolute refractory period when: Sodium gates are closed Potassium gates are open Repolarization is occurring The threshold level is elevated, allowing: Strong stimuli to increase the frequency of action potential events

Saltatory Conduction Current passes through a myelinated axon only at the nodes of Ranvier Voltage-gated Na+ channels are oncentrated at these nodes Action potentials are triggered only at the nodes They jump from one node to the next Saltatory conduction is much faster than continuous conduction occurring along unmyelinated axons

Saltatory Conduction Figure 11.16

Multiple Sclerosis (MS) An autoimmune disease Mainly affects young adults Symptoms include, among others: Weakness Loss of muscular control Myelin sheaths in the CNS are: Destroyed gradually

Synapses Junctions that mediate information transfer Information transfer is from: One neurone to nother neurone A neurone to an effector cell (muscle or gland) Presynaptic neuron: Conducts impulses toward the synapse Postsynaptic neuron: Transmits impulses away from the synapse

Types of Synapses Axodendritic synapses: Axosomatic synapses: Between a neuron axon and a dendrite of another neuron Axosomatic synapses: Between a neuron axon and the soma (cell body) of another neuron Other less common types: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrites to soma)

Figure 11.17

Electrical Synapses Electrical synapses: Are less common than chemical synapses Correspond to gap junctions found in other cells Direct cytoplasmic connection between neurons Direct flow of ions Are important in the CNS in: Mental attention Emotions Memory

Chemical Synapses Specialized for the release and reception of neurotransmitters Typically composed of two parts: Axonal terminal of the presynaptic neuron that contains synaptic vesicles Receptor region on the dendrite(s) or soma of the postsynaptic neuron

Synaptic Cleft A fluid-filled space Separates pre and postsynaptic neurons Prevents direct passage of nerve impulses between neurons Transmission across the synaptic cleft: Is a chemical event Ensures unidirectional communication between neurons

Synaptic Cleft: Information Transfer Nerve impulses reach presynaptic axonal terminal These impulses open Ca2+ channels causing Neurotransmitter release into synaptic cleft (via ..?) Neurotransmitter crosses the synaptic cleft It binds to receptors on the postsynaptic neuron Postsynaptic membrane permeability changes Permeability changes lead to: An excitatory effect or An inhibitory effect

Synaptic Cleft: Information Transfer Figure 11.19

Termination of Neurotransmitter Effects Removal of neurotransmitters occurs by: Degradation by: Enzymes Reabsorption by: Astrocytes, or Presynaptic terminals Diffusing away from the synaptic cleft

Synaptic Delay Neurotransmitter must be: Synaptic delay is: Released Diffuse across the synapse Bind to receptors Synaptic delay is: The time needed for all phases (0.3-5.0 ms)

Postsynaptic Potentials Neurotransmitter receptors mediate local changes in membrane potential. Mediation is accomplished according to: The amount of neurotransmitter released The neurotransmitter-receptors binding time The two types of postsynaptic potentials are: EPSP: Excitatory Postsynaptic Potentials IPSP: Inhibitory Postsynaptic Potentials

Excitatory Postsynaptic Potentials Are graded potentials Can initiate an action potential in an axon Use only chemically gated channels (no voltage gated channels) Simultaneous Na+ and K+ flow in opposite directions Postsynaptic membranes do not generate action potentials (at dendrites/soma)

Excitatory Postsynaptic Potentials Figure 11.20a

Inhibitory Synapses & IPSPs A neurotransmitter-receptor binding at inhibitory synapses It causes: Increased membrane permeability to K+ (out) and Cl- (in) ions Increased negative charge on the inner surface of the membrane Reduced postsynaptic neuron’s ability to produce an action potential

Inhibitory Synapses & IPSPs Figure 11.20b

Neurotransmitters Chemicals used for neuronal communication Fifty different types have been identified Are classified as: Chemical neurotransmitters Functional neurotransmitters

Chemical Neurotransmitters Acetylcholine (ACh) Biogenic amines Amino acids Peptides Novel messengers: ATP Dissolved gases (NO and CO)

Neurotransmitters: Acetylcholine First neurotransmitter identified Best understood Synthesized & enclosed in synaptic vesicles Released at the neuromuscular junction Degraded enzymatically by: Cetylcholinesterase (AChE) Released by the following neurons: All neurons stimulating skeletal muscle Some neurons in the autonomic nervous system Some neurons found in the CNS

Neurotransmitters: Biogenic Amines Biogenic amines include: Catecholamines: Dopamine Norepinephrine (NE) Epinephrine (E) Indolamines: Serotonin Histamine They are broadly distributed in the brain They play roles in: Emotional behaviors Our biological clock

Neurotransmitters: Amino Acids Amino acid neurotransmitters include: Glutamate GABA – Gamma ()-aminobutyric acid Glycine Aspartate They are found only in the CNS

Functional Neurotransmitters Functional neurotransmitters are classified into: Excitatory Inhibitory Excitatory neurotransmitters: Cause depolarizations Ex. Glutamate Inhibitory neurotransmitters: Cause hyperpolarizations Ex. GABA & glycine

Functional Neurotransmitters Some neurotransmitters have both: Excitatory effects Inhibitory effects The neurotransmitter effect is determined by: The receptor type of the postsynaptic neuron Example: Acetylcholine: Excitatory effect: At the neuromuscular junctions (skeletal muscle) Inhibitory effect: In the cardiac muscle

Break Slide Biol2401.