Neural Signaling Chapter 40

Learning Objective 1 Describe the processes involved in neural signaling: reception, transmission, integration, and action by effectors

Neural Signaling 1 (1) Reception of information by a sensory receptor (2) Transmission by an afferent neuron to the central nervous system (CNS) (3) Integration by interneurons in the central nervous system (CNS)

Neural Signaling 2 (4) Transmission by an efferent neuron to other neurons or effector (5) Action by effectors the muscles and glands

Peripheral Nervous System (PNS) Made up of sensory receptors neurons outside the CNS

Response to Stimulus

(e.g., vibration, movement, light, odor) Internal stimulus External stimulus (e.g., vibration, movement, light, odor) Internal stimulus (e.g., change in blood pH or blood pressure) RECEPTION Detection by external sense organs Detection by internal sense organs Figure 40.1: Response to a stimulus. Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response. TRANSMISSION Sensory (afferent) neurons transmit information Fig. 40-1a, p. 846

Central Nervous System (brain and spinal cord) INTEGRATION Interneurons sort and interpret information TRANSMISSION Motor (efferent) neurons transmit impulses ACTION BY EFFECTORS (muscles and glands) Figure 40.1: Response to a stimulus. Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response. e.g., animal runs away e.g., espiration rate increases; blood pressure rises Fig. 40-1b, p. 846

(e.g., vibration, movement, light, odor) Internal stimulus External stimulus (e.g., vibration, movement, light, odor) Internal stimulus (e.g., change in blood pH or blood pressure) RECEPTION Detection by external sense organs Detection by internal TRANSMISSION Sensory (afferent) neurons transmit information Central Nervous System (brain and spinal cord) INTEGRATION Interneurons sort and interpret information TRANSMISSION Motor (efferent) neurons transmit impulses Figure 40.1: Response to a stimulus. Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response. ACTION BY EFFECTORS (muscles and glands) e.g., animal runs away e.g., espiration rate increases; blood pressure rises Stepped Art Fig. 40-1, p. 846

KEY CONCEPTS Neural signaling involves reception, transmission, integration, and action by effectors

Learning Objective 2 What is the structure of a typical neuron? Give the function of each of its parts

Neurons Specialized to Cell body receive stimuli transmit electrical and chemical signals Cell body contains nucleus and organelles

Dendrites Many branched dendrites extend from cell body of neuron specialized to receive stimuli and send signals to the cell body

Axons 1 A single long axon Transmits signals into terminal branches extends from neuron cell body forms branches (axon collaterals) Transmits signals into terminal branches which end in synaptic terminals

Axons 2 Myelin sheath Schwann cells surrounds many axons insulates form the myelin sheath in the PNS

Axons 3 In the CNS Nodes of Ranvier sheath is formed by other glial cells Nodes of Ranvier gaps in sheath between successive Schwann cells

Neuron Structure

Dendrites covered with dendritic spines Cytoplasm of Schwann cell Synaptic terminals Axon Axon collateral Cell body Nucleus Myelin sheath Nucleus Figure 40.2: Structure of a multipolar neuron. The cell body contains most of the organelles. Many dendrites and a single axon extend from the cell body. Schwann cells form a myelin sheath around the axon by wrapping their own plasma membranes around the neuron in jelly-roll fashion. Axon Nodes of Ranvier Schwann cell Terminal branches Fig. 40-2, p. 847

Nerves and Ganglia Nerve Ganglion several hundred axons wrapped in connective tissue Ganglion mass of neuron cell bodies in the PNS

Nerve Structure

Ganglion Cell bodies Myelin sheath Vein Axon Artery (a) Figure 40.3: Structure of a nerve. Vein Axon Artery (a) Fig. 40-3a, p. 848

Figure 40.3: Structure of a nerve. 100 µm (b) Fig. 40-3b, p. 848

Learn more about the structure of neurons and nerves by clicking on the figures in ThomsonNOW.

Learning Objective 3 Name the main types of glial cells Describe the functions of each

Glial Cells Support and nourish neurons Are important in neural communication

Glial Cell Types 1 Astrocytes physically support neurons regulate extracellular fluid in CNS (by taking up excess potassium ions) communicate with one another (and with neurons) induce and stabilize synapses

Glial Cell Types 2 Oligodendrocytes Schwann cells form myelin sheaths around axons in CNS Schwann cells form sheaths around axons in PNS

Glial Cell Types 3 Microglia Ependymal Cells Phagocytic cells line cavities in the CNS contribute to formation of cerebrospinal fluid serve as neural stem cells

KEY CONCEPTS Neurons are specialized to receive stimuli and transmit signals; glial cells are supporting cells that protect and nourish neurons and that can modify neural signals

Learning Objective 4 How does the neuron develop and maintain a resting potential?

Neural Signals Electrical signals transmit information along axons Plasma membrane of resting neuron (not transmitting an impulse) is polarized Inner surface of plasma membrane is negatively charged relative to extracellular fluid

Resting Potential Potential difference of about -70 mV across the membrane Magnitude of resting potential (1) differences in ion concentrations (Na+, K+) inside cell relative to extracellular fluid (2) selective permeability of plasma membrane to these ions

(a) Measuring the resting potential of a neuron. Axon 40 20 –20 –40 –60 –70 mV –80 Time Amplifier Figure 40.4: Resting potential. Plasma membrane Electrode placed inside the cell Electrode placed outside the cell + + + – + – + + – – – – – – – + – + – + – + + + (a) Measuring the resting potential of a neuron. Fig. 40-4a, p. 850

Ions Pass through specific passive ion channels K+ leak out faster than Na+ leak in Cl- accumulate at inner surface of plasma membrane Large anions (proteins) cannot cross plasma membrane contribute negative charges

Sodium–Potassium Pumps Maintain gradients that determine resting potential transport 3 Na+ out for each 2 K+ in Require ATP

+ + + + + + – – – – – – Extracellular fluid 3 Na+ CI– Na+ Na+ CI– Na+ Diffusion out K+ Na+ K+ K+ Na+ Plasma membrane + + + + + + – – – – – – Na /K pump K+ K+ Na+ Figure 40.4: Resting potential. K+ K+ K+ CI– Diffusion in K+ 2 K+ Na+ A_ A_ A_ CI– CI– CI– Cytoplasm (b) Permeability of the neuron membrane. Fig. 40-4b, p. 850

KEY CONCEPTS The resting potential of a neuron is maintained by differences in concentrations of specific ions inside the cell relative to the extracellular fluid and by selective permeability of the plasma membrane to these ions

Learning Objective 5 Compare a graded potential with an action potential Describe the production and transmission of each

Membrane Potential Membrane is depolarized Membrane is hyperpolarized if stimulus causes membrane potential to become less negative Membrane is hyperpolarized if membrane potential becomes more negative than resting potential

Graded Potential A local response Varies in magnitude Fades out depending on strength of applied stimulus Fades out within a few millimeters of point of origin

Action Potential 1 Action potential is a wave of depolarization that moves down the axon Generated when voltage across the membrane declines to a critical point (threshold level) voltage-activated ion channels open Na+ ions flow into the neuron

Voltage-Activated Ion Channels

(b) Potassium channels. Extracellular fluid Activation gate Figure 40.6: Voltage-activated ion channels. Cytoplasm Inactivation gate (a) Sodium channels. (b) Potassium channels. Fig. 40-6, p. 852

Voltage-Activated Ion Channels During an Action Potential

Spike (a) Action potential. Depolarization Repolarization Membrane potential (mV) Threshold level Resting state Figure 40.7: State of voltage-activated ion channels during an action potential. Time (milliseconds) (a) Action potential. Fig. 40-7a, p. 853

Return to resting state. Repolarization. Axon Extracellular fluid Sodium channel Potassium channel Figure 40.7: State of voltage-activated ion channels during an action potential. Cytoplasm 1 Resting state. 2 Depolarization. Return to resting state. 3 Repolarization. 4 (b) The action of the ion channels in the plasma membrane determines the state of the neuron. Fig. 40-7b, p. 853

Action Potential 2 An all-or-none response no variation in strength of a single impulse either membrane potential exceeds threshold level or it does not Once begun, an action potential is self-propagating

Repolarization As an action potential moves down an axon, repolarization occurs behind it

Transmission of an Action Potential

Area of depolarization Stimulus Axon Area of depolarization Potassium channel Sodium channel Figure 40.8: Transmission of an action potential along the axon. When the dendrites (or cell body) of a neuron are stimulated sufficiently to depolarize the membrane to its threshold level, an action potential is generated. Action potential (1) Action potential is transmitted as wave of depolarization that travels down axon. At region of depolarization, Na+ diffuse into cell. Fig. 40-8a, p. 854

Area of repolarization Area of depolarization Action potential Figure 40.8: Transmission of an action potential along the axon. When the dendrites (or cell body) of a neuron are stimulated sufficiently to depolarize the membrane to its threshold level, an action potential is generated. (2) As action potential progresses along axon, repolarization occurs quickly behind it. Fig. 40-8b, p. 854

Refractory Periods During depolarization, the axon enters an absolute refractory period when it can’t transmit another action potential When enough gates controlling Na+ channels have been reset, the neuron enters a relative refractory period when the threshold is higher

Learn more about ion channels and action potentials by clicking on the figures in ThomsonNOW.

KEY CONCEPTS Depolarization of the neuron plasma membrane to threshold level generates an action potential, an electrical signal that travels as a wave of depolarization along the axon

Learning Objective 6 Contrast continuous conduction with saltatory conduction

Continuous Conduction Involves entire axon plasma membrane Takes place in unmyelinated neurons

Saltatory Conduction Depolarization skips along axon from one node of Ranvier to the next more rapid than continuous conduction takes place in myelinated neurons Nodes of Ranvier sites where axon is not covered by myelin Na+ channels are concentrated

Saltatory Conduction

Area of action potential Saltatory conduction Nodes of Ranvier Axon Figure 40.9: Saltatory conduction. This sequence of diagrams (steps ●1 through ●4) illustrates transmission of an action potential in a myelinated neuron. Axon 1 Schwann cell 2 Fig. 40-9a, p. 855

Direction of depolarization 3 Figure 40.9: Saltatory conduction. This sequence of diagrams (steps ●1 through ●4) illustrates transmission of an action potential in a myelinated neuron. 4 Direction of depolarization Fig. 40-9b, p. 855

Learning Objective 7 Describe the actions of the neurotransmitters identified in the chapter

Synapses Junctions between two neurons Most synapses are chemical or between a neuron and effector Most synapses are chemical some are electrical synapses

Synaptic Transmission A presynaptic neuron releases neurotransmitter (chemical messenger) from its synaptic vesicles

Neurotransmitters 1 Acetylcholine Biogenic amines triggers contraction of skeletal muscle Biogenic amines norepinephrine, serotonin, dopamine important in regulating mood dopamine is also important in motor function

Neurotransmitters 2 Some amino acids Neuropeptides (opioids) glutamate (excitatory neurotransmitter in brain) GABA (widespread inhibitory neurotransmitter) Neuropeptides (opioids) endorphins (e.g. beta-endorphin) enkephalins

Neurotransmitters 3 Nitric oxide (NO) gaseous neurotransmitter transmits signals from postsynaptic neuron to presynaptic neuron (opposite direction from other neurotransmitters)

Learning Objective 8 Trace the events that take place in synaptic transmission Draw diagrams to support your description

Synaptic Transmission Calcium ions cause synaptic vesicles to fuse with presynaptic membrane releases neurotransmitter into synaptic cleft Neurotransmitter diffuses across the synaptic cleft combines with specific receptors on a postsynaptic neuron

Synaptic Transmission

Plasma membrane of postsynaptic neuron Synaptic vesicles Plasma membrane of postsynaptic neuron Figure 40.10: Synaptic transmission. 0.25 µm (a) The TEM shows synaptic terminals filled with synaptic vesicles. Fig. 40-10a, p. 858

Figure 40.10: Synaptic transmission. Fig. 40-10bc, p. 858

(b) How a neural impulse is transmitted across a synapse. Axon of presynaptic neuron Synaptic terminal Voltage-gated Ca2+ channel 1 Synaptic vesicle Ca2+ 2 3 Neuro- transmitter molecule Figure 40.10: Synaptic transmission. 4 Ligand-gated channels 5 Postsynaptic membrane (b) How a neural impulse is transmitted across a synapse. Receptor for neurotransmitter Postsynaptic neuron Fig. 40-10b, p. 858

Synaptic terminal Synaptic cleft Ca2+ Synaptic terminal Presynaptic membrane Synaptic cleft Na+ Postsynaptic membrane Figure 40.10: Synaptic transmission. (c) Neurotransmitter binds with receptor. Ligand-gated channel opens, resulting in depolarization. Fig. 40-10c, p. 858

Neurotransmitter Receptors Many are proteins that form ligand-gated ion channels Others work through a second messenger such as cAMP

Learn more about synaptic transmission by clicking on the figure in ThomsonNOW.

KEY CONCEPTS Neurons signal other cells by releasing neurotransmitters at synapses

Learning Objective 9 Compare excitatory and inhibitory signals and their effects

Binding of Neurotransmitter to a Receptor Binding causes either excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP) Depending on the type of receptor

EPSPs and IPSPs EPSPs IPSPs bring neuron closer to firing move neuron farther away from its firing level

Learning Objective 10 Define neural integration Describe how a postsynaptic neuron integrates incoming stimuli and “decides” whether or not to fire

Neural Integration Process of summing (integrating) incoming signals Summation process of adding and subtracting incoming signals

Summation Each EPSP or IPSP is a graded potential vary in magnitude depending on strength of stimulus applied Summation of several EPSPs brings neuron to critical firing level

Temporal Summation Occurs when repeated stimuli cause new EPSPs to develop before previous EPSPs have decayed

Spatial Summation Occurs when several closely spaced synaptic terminals release neurotransmitter simultaneously stimulating postsynaptic neuron at several different places

Neural Integration

Threshold level Resting potential Postsynaptic membrane potential (mV) Resting potential Figure 40.11: Neural integration. E1 and E2 are excitatory stimuli. I represents an inhibitory stimulus. Time (msec) (a) Subthreshold (no summation). (b) Temporal summation. (c) Spatial summation. (d) Spatial summation of EPSPs and IPSPs. Fig. 40-11, p. 860

KEY CONCEPTS During integration, incoming neural signals are summed; temporal and spatial summation can bring a neuron to threshold level

Learning Objective 11 Distinguish among convergence, divergence, and reverberation Explain why each is important

Neural Circuits Complex neural circuits are possible because of associations such as convergence and divergence

Convergence A single neuron is affected by converging signals from two or more presynaptic neurons Allows CNS to integrate incoming information from various sources

Divergence A single presynaptic neuron stimulates many postsynaptic neurons allowing widespread effect

Neural Circuits

Figure 40.12: Neural circuits. (a) Convergence of neural input. Several presynaptic neurons synapse with one postsynaptic neuron. (b) Divergence of neural output. A single presynaptic neuron synapses with many postsynaptic neurons. Fig. 40-12, p. 861

Reverberating Circuits Important in rhythmic breathing mental alertness short-term memory Depend on positive feedback new impulses generated again and again until synapses fatigue

Reverberating Circuits

1 2 (a) Simple reverberating circuit. An axon collateral of the second neuron turns back on its own dendrites, so the neuron continues to stimulate itself. Figure 40.13: Reverberating circuits. Fig. 40-13a, p. 861

Interneuron Axon collateral 1 2 3 (b) Reverberating circuit with interneuron. An axon collateral of the second neuron synapses with an interneuron. The interneuron synapses with the first neuron in the sequence. New impulses are triggered again and again in the first neuron, causing reverberation. Figure 40.13: Reverberating circuits. Fig. 40-13b, p. 861