Chapter 8 Neurons, Synapses, and Signaling Overview: Lines of Communication The cone snail kills prey with venom that disables neurons Neurons are nerve cells that transfer information within the body Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance) Concept 8.: Neuron organization and structure reflect function in information transfer The transmission of information depends on the path of neurons along which a signal travels Processing of information takes place in simple clusters of neurons called ganglia or a more complex organization of neurons called a brain Nervous systems process information in three stages: sensory input Integration and motor output Fig. 8- Sensor Sensory input Integration Many animals have a complex nervous system which consists of: A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord Motor output A peripheral nervous system (PNS), which brings information into and out of the CNS Effector Peripheral nervous system (PNS) Central nervous system (CNS)
Neuron Structure and Function Nerve tissues are made of neurons and glia Most neurons are nourished or insulated by cells called glia Most of a neuron s organelles are in the cell body Most neurons have dendrites, highly branched extensions that receive signals from other neurons The axon is typically a much longer extension that transmits signals to other cells at synapses An axon joins the cell body at the axon hillock Sensory Neurons Sensors detect external stimuli and internal conditions and transmit information along sensory neurons Integration Neurons Sensory information is sent to the brain or ganglia, where interneurons integrate the information Fig. 8-b Fig. 8-a Motor Neurons Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity A synapse is a junction between an axon and another cell The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters Fig. 8-d
Fig. 8- Nucleus Cell body Dendrites Stimulus Presynaptic cell hillock Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell) Synapse Synaptic terminals Neurotransmitter Postsynaptic cell Concept 8.: Ion pumps and ion s maintain the resting of a neuron Every cell has a voltage (difference in electrical charge) across its plasma called a Messages are transmitted as changes in The resting is the of a neuron not sending signals Formation of the Resting Potential In a mammalian neuron at resting, the concentration of is greater inside the cell, while the concentration of is greater outside the cell -potassium pumps use the energy of ATP to maintain these and gradients across the plasma These concentration gradients represent chemical energy Fig. 8-6a The opening of ion s in the plasma converts chemical to electrical A neuron at resting contains many open s and fewer open s; diffuses out of the cell Anions trapped inside the cell contribute to the negative charge within the neuron OUTSIDE [ ] CELL mm INSIDE CELL [ ] mm [ ] [Cl ] mm mm [ ] mm [Cl ] mm [A ] mm Animation: Resting Potential (a)
Fig. 8-6b OUTSIDE CELL potassium pump Modeling of the Resting Potential Resting can be modeled by an artificial that separates two chambers The concentration of KCl is higher in the inner chamber and lower in the outer chamber diffuses down its gradient to the outer chamber Negative charge builds up in the inner chamber INSIDE CELL (b) At equilibrium, both the electrical and chemical gradients are balanced Fig. 8-7a Inner chamber 9 mv Outer chamber mm mm KCI KCI Cl (a) Membrane selectively permeable to E K = 6 mv(log mm mm) = 9 mv The equilibrium (E ion ) is the voltage for a particular ion at equilibrium and can be calculated using the Nernst equation: E ion = 6 mv (log[ion] outside /[ion] inside ) The equilibrium of (E K ) is negative, while the equilibrium of (E Na ) is positive Fig. 8-7b +6 mv In a resting neuron, the currents of and are equal and opposite, and the resting across the remains steady Cl mm NaCI mm NaCI (b) Membrane selectively permeable to ) mm E Na = 6 mv (log mm = +6 mv
Concept 8.: s are the signals conducted by axons Neurons contain gated ion s that open or close in response to stimuli Membrane changes in response to opening or closing of these s Graded s are changes in polarization where the magnitude of the change varies with the strength of the stimulus When gated s open, diffuses out, making the inside of the cell more negative This is hyperpolarization, an increase in magnitude of the Production of Potentials Other stimuli trigger a depolarization, a reduction in the magnitude of the For example, depolarization occurs if gated s open and diffuses into the cell Voltage-gated and s respond to a change in When a stimulus depolarizes the, s open, allowing to diffuse into the cell The movement of into the cell increases the depolarization and causes even more s to open A strong stimulus results in a massive change in voltage called an action Fig. 8-9c Membrane (mv) + Strong depolarizing stimulus Threshold Resting An action occurs if a stimulus causes the voltage to cross a particular threshold An action is a brief all-or-none depolarization of a neuron s plasma s are signals that carry information along axons Time (msec) 6 (c)
Generation of Potentials: A Closer Look Fig. 8-- A neuron can produce hundreds of action s per second The frequency of action s can reflect the strength of a stimulus An action can be broken down into a series of stages Depolarization Extracellular fluid Inactivation loop Resting state Membrane (mv) + Time Threshold Resting. Most voltage-gated Na+ and K+ s are closed, but some K+ s (not voltage-gated) are open Fig. 8--. Depolarization Voltage-gated Na+ s open first and Na+ flows into the cell Membrane (mv) + Threshold Fig. 8-- Rising phase of the action Membrane (mv) + Threshold. During the rising phase or the threshold is crossed, and the increases Depolarization Time Resting Depolarization Time Resting Extracellular fluid Extracellular fluid Inactivation loop Inactivation loop Resting state Resting state Fig. 8-- Fig. 8-- Rising phase of the action Falling phase of the action Rising phase of the action Falling phase of the action Membrane (mv) + Threshold Membrane (mv) + Threshold Depolarization Extracellular fluid Inactivation loop Resting state Time Resting. During the falling phase, voltage-gated Na+ s become inactivated; voltagegated K+ s open, and K+ flows out of the cell Depolarization Extracellular fluid Inactivation loop Resting state Time Resting Undershoot 6
. During the undershoot, permeability to is at first higher than at rest, then voltagegated s close; resting is restored During the refractory period after an action, a second action cannot be initiated The refractory period is a result of a temporary inactivation of the s BioFlix: : How Neurons Work Animation: Potential Conduction of Potentials An action can travel long distances by regenerating itself along the axon At the site where the action is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon Inactivated s behind the zone of depolarization prevent the action from traveling backwards s travel in only one direction: toward the synaptic terminals Fig. 8-- Fig. 8-- 7
Fig. 8-- Conduction Speed The speed of an action increases with the axon s diameter In vertebrates, axons are insulated by a myelin sheath, which causes an action s speed to increase Myelin sheaths are made by glia oligodendrocytes in the CNS and Schwann cells in the PNS s are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated s are found s in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction Fig. 8-a Fig. 8- Concept 8.: Neurons communicate with other cells at synapses Cell body Schwann cell Depolarized region (node of Ranvier) Myelin sheath At electrical synapses, the electrical current flows from one neuron to another across the gap junction At chemical synapses, a chemical neurotransmitter carries information. Most synapses are chemical synapses 8
Fig. 8- The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal Synaptic vesicles containing neurotransmitter Presynaptic Na+ The action causes the release of the neurotransmitter The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell Voltage-gated Ca + Ca + Synaptic cleft Ligand-gated ion s Postsynaptic 6 Animation: Synapse Generation of Postsynaptic Potentials Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion s in the postsynaptic cell Neurotransmitter binding causes ion s to open, generating a postsynaptic Postsynaptic s fall into two categories: Excitatory postsynaptic s (EPSPs) are depolarizations that bring the toward threshold Inhibitory postsynaptic s (IPSPs) are hyperpolarizations that move the farther from threshold Summation of Postsynaptic Potentials After release, the neurotransmitter May diffuse out of the synaptic cleft May be taken up by surrounding cells May be degraded by enzymes Unlike action s, postsynaptic s are graded and do not regenerate Most neurons have many synapses on their dendrites and cell body A single EPSP is usually too small to trigger an action in a postsynaptic neuron If two EPSPs are produced in rapid succession, an effect called temporal summation occurs 9
Fig. 8-6ab E Postsynaptic neuron E I Terminal branch of presynaptic neuron E E I hillock In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together Membrane (mv) 7 Resting Threshold of axon of postsynaptic neuron The combination of EPSPs through spatial and temporal summation can trigger an action E E E E (a) Subthreshold, no summation (b) Temporal summation Fig. 8-6cd E E Membrane (mv) 7 E I E + E (c) Spatial summation E E I I E + I (d) Spatial summation of EPSP and IPSP Through summation, an IPSP can counter the effect of an EPSP The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action Modulated Synaptic Transmission Neurotransmitters In indirect synaptic transmission, a neurotransmitter binds to a receptor that is not part of an ion This binding activates a signal transduction pathway involving a second messenger in the postsynaptic cell The same neurotransmitter can produce different effects in different types of cells There are five major classes of neurotransmitters: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases Effects of indirect synaptic transmission have a slower onset but last longer
Table 8-a Table 8-b