Neurons, Synapses, and Signaling Chapter 48 Neurons, Synapses, and Signaling

Overview: Lines of Communication 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) 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

Many animals have a complex nervous system which consists of: Fig. 48-3 Sensory input Integration Sensor Motor output Figure 48.3 Summary of Information Processing Nervous systems process information in three stages: sensory input, integration, and motor output Sensors detect external stimuli and internal conditions and transmit information along sensory neurons Sensory information is sent to the brain or ganglia, where interneurons integrate the information Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity 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 A peripheral nervous system (PNS), which brings information into and out of the CNS Effector Peripheral nervous system (PNS) Central nervous system (CNS)

Dendrites Stimulus Presynaptic Nucleus cell Axon hillock Cell body Fig. 48-4 Dendrites Stimulus Nucleus Presynaptic cell Axon hillock Cell body Axon Synapse Synaptic terminals Figure 48.4 Neuron Structure and Organization 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 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 Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell) Most neurons are nourished or insulated by cells called glia Structure fits function many entry points for signal one path out transmits signal Postsynaptic cell Neurotransmitter

Dendrites Axon Cell body Portion of axon Cell bodies of Fig. 48-5 Dendrites Axon Cell body Portion of axon Figure 48.5 Structural diversity of neurons INFORMATION PROCESSING: Sensory neurons transmit information from sensors that detect external stimuli and internal conditions Sensory information is sent to the CNS where interneurons integrate the information Motor output leaves the CNS via motor neurons, which communicate with effector cells (muscles or glands) 80 µm Cell bodies of overlapping neurons Sensory neuron Interneurons Motor neuron

Concept 48.2: Membrane & Resting Potential Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential Messages are transmitted as changes in membrane potential The resting potential is the membrane potential of a neuron not sending signals Ion pumps and ion channels maintain the resting potential of a neuron

The Resting Potential The resting potential is the membrane potential of a neuron that is not transmitting signals In all neurons, the resting potential depends on the ionic gradients that exist across the plasma membrane CYTOSOL EXTRACELLULAR FLUID [Na+] 15 mM [K+] 150 mM [Cl–] 10 mM [A–] 100 mM [Na+] 150 mM [K+] 5 mM [Cl–] 120 mM – + Plasma membrane Figure 48.10

[Na+] 150 mM [Cl–] 120 mM [K+] 140 mM [A–] 100 mM Fig. 48-6 Key Na+ Sodium- potassium pump Potassium channel Sodium channel K+ OUTSIDE CELL OUTSIDE CELL [K+] 5 mM [Na+] 150 mM [Cl–] 120 mM INSIDE CELL [K+] 140 mM [Na+] 15 mM [Cl–] 10 mM [A–] 100 mM Figure 48.6 The basis of the membrane potential In a mammalian neuron at resting potential, the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside the cell Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane These concentration gradients represent chemical potential energy The opening of ion channels in the plasma membrane converts chemical potential to electrical potential A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell Anions trapped inside the cell contribute to the negative charge within the neuron In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steady INSIDE CELL (a) (b)

Concept 48.3: Action Potentials Action potentials are the signals conducted by axons Neurons contain gated ion channels that open or close in response to stimuli Membrane potential changes in response to opening or closing of these channels

Action potentials are signals that carry information along axons Fig. 48-9 http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf Stimuli Stimuli Strong depolarizing stimulus +50 +50 +50 Action potential Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) –50 Threshold –50 Threshold –50 Threshold Resting potential Resting potential Resting potential Figure 48.9 Graded potentials and an action potential in a neuron (A) When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative. This is hyperpolarization, an increase in magnitude of the membrane potential (B) Other stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus (C) Voltage-gated Na+ and K+ channels respond to a change in membrane potential When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open A strong stimulus results in a massive change in membrane voltage called an action potential An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane Action potentials are signals that carry information along axons Hyperpolarizations Depolarizations –100 –100 –100 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6 Time (msec) Time (msec) Time (msec) (a) Graded hyperpolarizations (b) Graded depolarizations (c) Action potential

Generation of Action Potentials: A Closer Look A neuron can produce hundreds of action potentials per second The frequency of action potentials can reflect the strength of a stimulus An action potential can be broken down into a series of stages

Fig. 48-10-5 Key Na+ K+ 3 Rising phase of the action potential 4 Falling phase of the action potential +50 Action potential 3 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential 2 Depolarization Figure 48.10 The role of voltage-gated ion channels in the generation of an action potential (1) RESTING STATE - The activation gates on the Na+ and K+ channels are closed, and the membrane’s resting potential is maintained. (2) DEPOLARIZATION - A stimulus opens the activation gates on some Na+ channels. Na+ influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. (3) RISING PHASE OF ACTION POTENTIAL - Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respect to the outside. (4) FALLING PHASE OF ACTION POTENTIAL - The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on most K+ channels open, permitting K+ efflux which again makes the inside of the cell negative. (5) UNDERSHOOT - Both gates of the Na+ channels are closed, but the activation gates on some K+ channels are still open. As these gates close on most K+ channels, and the inactivation gates open on Na+ channels, the membrane returns to its resting state. During the refractory period after an action potential, a second action potential cannot be initiated. The refractory period is a result of a temporary inactivation of the Na+ channels. –100 Time Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop 5 Undershoot 1 Resting state

Figure 48.11 Conduction of an action potential Axon Plasma membrane Action potential Na+ Cytosol Action potential K+ Na+ Figure 48.11 Conduction of an action potential An action potential can travel long distances by regenerating itself along the axon - action potentials travel in only one direction: toward the synaptic terminals. 1) An action potential is generated as Na+ flows inward across the membrane at one location. 2) The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 3) The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon. K+ Action potential K+ Na+ K+

Figure 48.12 Schwann cells and the myelin sheath Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Nodes of Ranvier Nucleus of Schwann cell Axon Myelin sheath Figure 48.12 Schwann cells and the myelin sheath The speed of an action potential increases with the axon’s diameter In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction A myelin sheath is a many-layered coating that wraps around the axon of a neuron and very efficiently insulates it. At nodes of Ranvier, the axonal membrane is uninsulated and therefore capable of generating electrical activity. 0.1 µm

Schwann cell Depolarized region (node of Ranvier) Cell body Myelin Fig. 48-13 Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon Figure 48.13 Saltatory conduction Structure fits function in the vertebrate neuron: many entry points for signal (dendrites) one path out – signal travels in one direction only (axon) transmits signal quickly (myelin sheath) and effectively (nodes of Ranvier)

Synapses – Communication Between Neurons Neurons communicate with other cells at synapses The vast majority of synapses Are chemical synapses In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters, which are stored in the synaptic terminal

Synaptic vesicles containing Presynaptic neurotransmitter membrane Fig. 48-15 http://bcs.whfreeman.com/thelifewire/content/chp44/4402003.html 5 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 6 Figure 48.15 A chemical synapse When an action potential depolarizes the plasma membrane of the synaptic terminal, it opens volted-gated Ca2+ channels in the membrane triggering an influx of Ca2+ The elevated Ca concentration in the terminal causes synaptic vesicles to fuse with the presynaptic membrane The vesicles release neurotransmitter into the synaptic cleft The neurotransmitter binds to the ion-channels in the postsynaptic membrane, opening the channels and allowing the flow of both Na and K When the neurotransmitter releases from the receptors, the channels close and synaptic transmission ends. Synaptic cleft 3 Ligand-gated ion channels

Synapses – Communication Between Neurons After release, the neurotransmitter May diffuse out of the synaptic cleft May be taken up by surrounding cells May be degraded by enzymes

There are five major classes of neurotransmitters: 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

Can be inhibitory or excitatory Biogenic amines Table 48-1 Acetylcholine Is one of the most common neurotransmitters in both vertebrates and invertebrates Can be inhibitory or excitatory Biogenic amines Include epinephrine, norepinephrine, dopamine, and serotonin Are active in the CNS and PNS Various amino acids and peptides Are active in the brain Gases such as nitric oxide and carbon monoxide Are local regulators in the PNS