11 Fundamentals of the Nervous System and Nervous Tissue: Part 1

Figure 11.1 The nervous system’s functions. Sensory input Integration Motor output 6/27/2012 MDufilho

Divisions of the Nervous System The Tale of Two Brains Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves carry messages to and from the CNS MDufilho 6/27/2012

Peripheral Nervous System (PNS) Two functional divisions Sensory (afferent) division Somatic sensory fibers—convey impulses from skin, skeletal muscles, and joints to CNS Visceral sensory fibers—convey impulses from visceral organs to CNS Motor (efferent) division Transmits impulses from CNS to effector organs Muscles and glands Two divisions Somatic nervous system Autonomic nervous system 6/27/2012 MDufilho

Motor Division of PNS: Somatic Nervous System Somatic motor nerve fibers Conducts impulses from CNS to skeletal muscle Voluntary nervous system Conscious control of skeletal muscles 6/27/2012 MDufilho

Motor Division of PNS: Autonomic Nervous System Visceral motor nerve fibers Regulates smooth muscle, cardiac muscle, and glands Involuntary nervous system Two functional subdivisions Sympathetic Parasympathetic Work in opposition to each other 6/27/2012 MDufilho

Figure 11.2 Levels of organization in the nervous system. 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 nerve fibers Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Conducts impulses from receptors to the CNS Somatic nervous system Autonomic nervous system (ANS) Somatic sensory fiber Skin Somatic motor (voluntary) Visceral motor (involuntary) Conducts impulses from the CNS to skeletal muscles Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic division Mobilizes body systems 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 6/27/2012 MDufilho

Histology of Nervous Tissue Highly cellular; little extracellular space Tightly packed Two principal cell types Neurons (nerve cells)—excitable cells that transmit electrical signals Neuroglia – small cells that surround and wrap delicate neurons Astrocytes (CNS) Microglial cells (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS) 6/27/2012 MDufilho

Supporting Cells: Neuroglia The supporting cells (neuroglia or glial cells): Provide a supportive scaffolding for neurons Segregate and insulate neurons Assist with repair after damage Guide young neurons to the proper connections Promote health and growth MDufilho 6/27/2012

Resting Membrane Potential (Vr) Potential difference across the membrane of a resting cell Approximately –70 mV in neurons (cytoplasmic side of membrane is negatively charged relative to outside) Generated by ????? MDufilho 6/27/2012

Role of Membrane Ion Channels: Gated Channels Three types Chemically gated (ligand-gated) channels Open with binding of a specific 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 6/27/2012 MDufilho

Figure 11.6 Operation of gated channels. Chemically gated ion channels Voltage-gated ion channels Open in response to binding of the appropriate neurotransmitter Open in response to changes in membrane potential Receptor Neurotransmitter chemical attached to receptor Membrane voltage changes Chemical binds Closed Open Closed Open 6/27/2012 MDufilho

Resting Membrane Potential: Differences in Ionic Composition - Review ECF has higher concentration of ___than ICF Balanced chiefly by ________________ ICF has higher concentration of _____than ECF Balanced by _________________________ ___plays most important role in membrane potential PLAY A&P Flix™: Resting Membrane Potential 6/27/2012 MDufilho

Differences in Plasma Membrane Permeability - Review Impermeable large ______________ Slightly permeable to _____(through leakage channels) ________diffuses into cell down concentration gradient 25 times more permeable to ____than sodium (more leakage channels) _________diffuses out of cell down concentration gradient Quite permeable to _____ 6/27/2012 MDufilho

Resting Membrane Potential – Review More potassium diffuses out than sodium diffuses in Cell more ________inside Establishes resting membrane potential ___________________stabilizes resting membrane potential Maintains concentration gradients for Na+ and K+ __Na+ pumped out of cell; two ___pumped in 6/27/2012 MDufilho

Membrane Potential Changes Used as Communication Signals Membrane potential changes when Concentrations of ions across membrane change Membrane permeability to ions changes Changes produce two types signals Graded potentials Incoming signals operating over short distances Action potentials Long-distance signals of axons Changes in membrane potential used as signals to receive, integrate, and send information 6/27/2012 MDufilho

Figure 11.9a Depolarization and hyperpolarization of the membrane. Depolarizing stimulus +50 Inside positive Inside negative Membrane potential (voltage, mV) Depolarization –50 –70 Resting potential –100 1 2 3 4 5 6 7 Time (ms) Depolarization: The membrane potential moves toward 0 mV, the inside becoming less negative (more positive). 6/27/2012 MDufilho

Changes in Membrane Potential Terms describing membrane potential changes relative to resting membrane potential Hyperpolarization An increase in membrane potential (away from zero) Inside of cell more negative than resting membrane potential) Reduces probability of producing a nerve impulse 6/27/2012 MDufilho

Figure 11.9b Depolarization and hyperpolarization of the membrane. Hyperpolarizing stimulus +50 Membrane potential (voltage, mV) –50 Resting potential –70 Hyper- polarization –100 1 2 3 4 5 6 7 Time (ms) Hyperpolarization: The membrane potential increases, the inside becoming more negative. 6/27/2012 MDufilho

Graded Potentials Short-lived, localized changes in membrane potential Magnitude varies with stimulus strength Stronger stimulus  more voltage changes; farther current flows Either depolarization or hyperpolarization Triggered by stimulus that opens gated ion channels Current flows but dissipates quickly and decays Graded potentials are signals only over short distances 6/27/2012 MDufilho

Figure 11.10 The spread and decay of a graded potential. Stimulus Depolarized region Plasma membrane Depolarization: A small patch of the membrane (red area) depolarizes. Depolarization spreads: Opposite charges attract each other. This creates local currents (black arrows) that depolarize adjacent membrane areas, spreading the wave of depolarization. Active area (site of initial depolarization) Membrane potential (mV) –70 Resting potential Distance (a few mm) Membrane potential decays with distance: Because current is lost through the “leaky” plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental). Consequently, graded potentials are short-distance signals. 6/27/2012 MDufilho

Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents. The big picture The key players Voltage-gated Na+ channels Voltage-gated K+ channels Resting state Depolarization 1 2 Outside cell Outside cell +30 3 3 Repolarization Inside cell Activation gate Inactivation gate Inside cell Membrane potential (mV) Action potential Closed Opened Inactivated Closed 2 Hyperpolarization Opened 4 The events –55 Threshold Potassium channel –70 Sodium channel 1 1 4 0 1 2 3 4 Time (ms) Activation gates The AP is caused by permeability changes in the plasma membrane: Inactivation gate Resting state 1 +30 Action potential 3 Membrane potential (mV) Na+ permeability Relative membrane permeability 2 4 Hyperpolarization Depolarization 2 K+ permeability –55 –70 1 1 4 0 1 2 3 4 Time (ms) Repolarization 3 6/27/2012 MDufilho

Not all depolarization events produce APs Threshold Not all depolarization events produce APs For axon to "fire", depolarization must reach threshold That voltage at which the AP is triggered At threshold: Membrane has been depolarized by 15 to 20 mV Na+ permeability increases Na influx exceeds K+ efflux The positive feedback cycle begins 6/27/2012 MDufilho

Figure 11.12 Propagation of an action potential (AP). Voltage at 2 ms +30 Voltage at 0 ms Voltage at 4 ms Membrane potential (mV) –70 Recording electrode Time = 0 ms. Action potential has not yet reached the recording electrode. Time = 2 ms. Action potential peak reaches the recording electrode. Time = 4 ms. Action potential peak has passed the recording electrode. Membrane at the recording electrode is still hyperpolarized. Resting potential Peak of action potential Hyperpolarization 6/27/2012 MDufilho

Action potentials Stimulus voltage Membrane potential (mV) +30 –70 Figure 11.13 Relationship between stimulus strength and action potential frequency. Action potentials +30 Membrane potential (mV) –70 Stimulus Threshold Stimulus voltage Time (ms) 6/27/2012 MDufilho

Figure 11.14 Absolute and relative refractory periods in an AP. Absolute refractory period Relative refractory period Depolarization (Na+ enters) +30 Membrane potential (mV) Repolarization (K+ leaves) Hyperpolarization –70 Stimulus 1 2 3 4 5 6/27/2012 Time (ms) MDufilho

Figure 11.15 Action potential propagation in nonmyelinated and myelinated axons. Stimulus Size of voltage In bare plasma membranes, voltage decays. Without voltage-gated channels, as on a dendrite, voltage decays because current leaks across the membrane. Stimulus Voltage-gated ion channel In nonmyelinated axons, conduction is slow (continuous conduction). Voltage-gated Na+ and K+ channels regenerate the action potential at each point along the axon, so voltage does not decay. Conduction is slow because it takes time for ions and for gates of channel proteins to move, and this must occur before voltage can be regenerated. Stimulus Myelin sheath Myelin sheath gap Myelin sheath 1 mm In myelinated axons, conduction is fast (saltatory conduction). Myelin keeps current in axons (voltage doesn’t decay much). APs are generated only in the myelin sheath gaps and appear to jump rapidly from gap to gap. 6/27/2012 MDufilho

Nervous system works because information flows from neuron to neuron The Synapse Nervous system works because information flows from neuron to neuron Neurons functionally connected by synapses Junctions that mediate information transfer From one neuron to another neuron Or from one neuron to an effector cell 6/27/2012 MDufilho

Figure 11.16 Synapses. 6/27/2012 MDufilho Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonal synapses Axon Axon Axosomatic synapses Cell body (soma) of postsynaptic neuron 6/27/2012 MDufilho

Varieties of Synapses: Chemical Synapses Specialized for release and reception of chemical neurotransmitters Typically composed of two parts Axon terminal of presynaptic neuron Contains synaptic vesicles filled with neurotransmitter Neurotransmitter receptor region on postsynaptic neuron's membrane Usually on dendrite or cell body Two parts separated by synaptic cleft Fluid-filled space Electrical impulse changed to chemical across synapse, then back into electrical 6/27/2012 MDufilho

Varieties of Synapses: Electrical Synapses Less common than chemical synapses Neurons electrically coupled (joined by gap junctions that connect cytoplasm of adjacent neurons) Communication very rapid May be unidirectional or bidirectional Synchronize activity More abundant in: Embryonic nervous tissue Nerve impulse remains electrical 6/27/2012 MDufilho

Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron Action potential arrives at axon terminal. 1 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 Mitochondrion Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis 3 Synaptic cleft Axon terminal Synaptic vesicles Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. 4 Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse Binding of neurotransmitter opens ion channels, resulting in graded potentials. 5 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. 6 6/27/2012 MDufilho

Postsynaptic Potentials Neurotransmitter receptors cause graded potentials that vary in strength with Amount of neurotransmitter released and Time neurotransmitter stays in area Types of postsynaptic potentials EPSP—excitatory postsynaptic potentials IPSP—inhibitory postsynaptic potentials 6/27/2012 MDufilho

Table 11.2 Comparison of Graded Potentials and Action Potentials (1 of 4) 6/27/2012 MDufilho

Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory. +30 An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass through simultaneously. Membrane potential (mV) Threshold –55 –70 Stimulus 10 20 30 Time (ms) Excitatory postsynaptic potential (EPSP) 6/27/2012 MDufilho

Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory. +30 An IPSP is a local hyperpolarization of the postsynaptic membrane that drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels. Membrane potential (mV) Threshold –55 –70 Stimulus 10 20 30 Time (ms) Inhibitory postsynaptic potential (IPSP) 6/27/2012 MDufilho

Table 11.2 Comparison of Graded Potentials and Action Potentials (4 of 4) 6/27/2012 MDufilho

Synaptic Integration: Summation A single EPSP cannot induce an AP EPSPs can summate to influence postsynaptic neuron IPSPs can also summate Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons Only if EPSP's predominate and bring to threshold  AP 6/27/2012 MDufilho

Figure 11.19 Neural integration of EPSPs and IPSPs. Threshold of axon of postsynaptic neuron Membrane potential (mV) Resting potential –55 –70 E1 E1 E1 E1 E1 + E2 l1 E1 + l1 Time Time Time Time No summation: 2 stimuli separated in time cause EPSPs that do not add together. Temporal summation: 2 excitatory stimuli close in time cause EPSPs that add together. Spatial summation: 2 simultaneous stimuli at different locations cause EPSPs that add together. Spatial summation of EPSPs and IPSPs: Changes in membane potential can cancel each other out. Excitatory synapse 1 (E1) Excitatory synapse 2 (E2) Inhibitory synapse (I1) 6/27/2012 MDufilho