Neurons, Synapses, and Signaling 37 Neurons, Synapses, and Signaling

do now: label the parts- hw q #1 Stimulus Axon hillock A Cell body Presynaptic cell C Signal direction D Synaptic terminals Figure 37.2 Neuron structure Synaptic terminals Postsynaptic cell E 2

Neuron Structure and Function Most of a neuron’s organelles are in the cell body (A) Most neurons have dendrites (B), highly branched extensions that receive signals from other neurons The single axon (C), a much longer extension, transmits signals to other cells The cone-shaped base of an axon, where signals are generated, is called the axon hillock Video: Dendrites 3

The branched ends of axons transmit signals to other cells at a junction called the synapse (D) At most synapses, chemical messengers called neurotransmitters (E) pass information from the transmitting neuron to the receiving cell 4

Introduction to Information Processing Nervous systems process information in three stages Sensory input Integration Motor output 5

Processing center (CNS) Figure 37.4 Sensory input Integration Sensor(PNS) Motor output Figure 37.4 Summary of information processing Processing center (CNS) Effector(PNS) 6

(information processing) Figure 38.5 Central Nervous System (information processing) Peripheral Nervous System Afferent neurons Efferent neurons Autonomic nervous system Motor system Sensory receptors Control of skeletal muscle Figure 38.5 Functional hierarchy of the vertebrate peripheral nervous system Internal and external stimuli Sympathetic division Parasympathetic division Enteric division Control of smooth muscles, cardiac muscles, glands 7

Sensory neurons transmit information from eyes and other sensors that detect external stimuli or internal conditions (PNS) This information is sent to the brain or ganglia, where interneurons integrate the information (CNS) Neurons that extend out of the processing centers trigger muscle or gland activity For example, motor neurons transmit signals to muscle cells, causing them to contract (PNS) 8

Workbook: page 235 Dendrites Axon Cell body Portion of axon ??? neuron Figure 37.5 Structural diversity of neurons Portion of axon ??? neuron ???neuron ??? neuron 9

Structure of neurons: quick check What is true regarding neurons: 1.Motor neurons have a cell body in the middle of the axon 2.Nerve signals travel from an axon to a dendrite 3.The interneuron in vertebrates are located in the CNS 4.The gap between two neurons is called a neurotransmitter

The communication between these neurons occurs because of concentration gradients… The inside of a cell is negatively charged relative to the outside This difference is a source of potential energy, termed membrane potential The resting potential is the membrane potential of a neuron not sending signals Hw q #2:Which ion is in high concentration outside the cell? Inside the cell? Changes in membrane potential act as signals, transmitting and processing information 11

Key OUTSIDE Na OF CELL K Sodium- potassium pump Potassium channel Figure 37.6 Key OUTSIDE OF CELL Na K Sodium- potassium pump Figure 37.6 The basis of the membrane potential Potassium channel Sodium channel INSIDE OF CELL 12

Formation of the Resting Potential K and Na play an essential role in forming the resting potential In most neurons, the concentration of K is highest inside the cell, while the concentration of Na is highest outside the cell Sodium-potassium pumps use the energy of ATP to maintain these K and Na gradients across the plasma membrane Why is this important? Animation: hw Q #3 Animation: Resting Potential 13

Ions Change in membrane potential (voltage) Ion channel Figure 37.9 Ions Change in membrane potential (voltage) Ion channel Figure 37.9 Voltage-gated ion channel (a) Gate closed: No ions flow across membrane. (b) Gate open: Ions flow through channel. 14

Hw q: 4,5 and 6: Based on these graphs: what is the definition of hyperpolarization and depolarization? Stimulus Stimulus Strong depolarizing stimulus 50 50 50 Action potential Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) Threshold Threshold Threshold −50 −50 −50 Resting potential Resting potential Resting potential Hyperpolarizations Depolarizations −100 −100 −100 Figure 37.10 Graded potentials and an action potential in a neuron 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6 Time (msec) Time (msec) Time (msec) Which gates are open? Sodium or potassium? How do you know? (b) Which gates are open? Sodium or potassium? How do you know? © why is this the only situation Where you have an action Potential? 15

Action potentials are all or none Action potentials occur whenever a depolarization increases the membrane potential to a particular value, called the threshold (-55mV) Action potentials are all or none 16

Membrane potential: quick check: Select the statement that is false: At rest, there is a high concentration of Na+ outside the neuron Ion channels are specific: for example K+ channels will only allow K+ to pass through When sodium channels open the membrane potential becomes more positive If there is a depolarization, this means that potassium channels must be open

Hw Q#7 Key Na K Rising phase of the action potential 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 −100 Time Figure 37.11 The role of voltage-gated ion channels in the generation of an action potential OUTSIDE OF CELL Sodium channel Potassium channel 5 Undershoot INSIDE OF CELL Inactivation loop 1 Resting state 18

Key Na K Resting state OUTSIDE OF CELL Sodium channel Potassium Figure 37.11a Key Na K OUTSIDE OF CELL Sodium channel Potassium channel Figure 37.11a The role of voltage-gated ion channels in the generation of an action potential (part 1: resting state) INSIDE OF CELL Inactivation loop 1 Resting state 19

When stimulus depolarizes the membrane Some gated Na+ channels open first and Na flows into the cell During the rising phase, the threshold is crossed, and the membrane potential increases During the falling phase, voltage-gated Na channels become inactivated; voltage-gated K channels open, and K flows out of the cell 20

Key Na K Depolarization 2 Figure 37.11b Figure 37.11b The role of voltage-gated ion channels in the generation of an action potential (part 2: depolarization) 2 Depolarization 21

Rising phase of the action potential Figure 37.11c Key Na K Figure 37.11c The role of voltage-gated ion channels in the generation of an action potential (part 3: rising phase) 3 Rising phase of the action potential 22

Falling phase of the action potential Figure 37.11d Key Na K Figure 37.11d The role of voltage-gated ion channels in the generation of an action potential (part 4: falling phase) 4 Falling phase of the action potential 23

During the undershoot, membrane permeability to K is at first higher than at rest, and then voltage-gated K channels close and resting potential is restored 24

Key Na K Undershoot 5 Figure 37.11e Figure 37.11e The role of voltage-gated ion channels in the generation of an action potential (part 5: undershoot) 5 Undershoot 25

Membrane potential (mV) Figure 37.11f 50 Action potential 3 Membrane potential (mV) 2 4 Threshold −50 1 1 Figure 37.11f The role of voltage-gated ion channels in the generation of an action potential (part 6: graph) 5 Resting potential −100 Time 26

During the refractory period after an action potential, a second action potential cannot be initiated Why might this be beneficial? The refractory period is a result of a temporary inactivation of the Na channels For most neurons, the interval between the start of an action potential and the end of the refractory period is only 1–2 msec 27

Conduction of Action Potentials At the site where the action potential is initiated (usually the axon hillock), an electrical current depolarizes the neighboring region of the axon membrane Action potentials travel only toward the synaptic terminals Inactivated Na channels are behind the zone of movement Why is this beneficial? 28

Axon Plasma Action membrane potential Cytosol 1 1 Na Figure 37.12-1 Figure 37.12-1 Conduction of an action potential (step 1) 29

Axon Plasma Action membrane potential Cytosol Action potential 1 2 1 Figure 37.12-2 Axon Plasma membrane Action potential 1 1 Na Cytosol Action potential K 2 Na Figure 37.12-2 Conduction of an action potential (step 2) K 30

Axon Plasma Action membrane potential Cytosol Action potential Action Figure 37.12-3 Axon Plasma membrane Action potential 1 Na Cytosol Action potential K 2 Na Figure 37.12-3 Conduction of an action potential (step 3) K Action potential K 3 Na K 31

Action potential: quick check Identify the following statements as true or false 1. the threshold value for an action potential is -55mV 2. increased amounts of sodium entering causes the membrane potential to become negative. 3. potassium channels open at +35mv 4. the action potential moves forward and not backwards due to inactivated potassium channels

Label the parts: C A Axon ??? B cell B cell C Axon A Nucleus of B cell Figure 37.13 Schwann cells and the myelin sheath 0.1 m 33

Evolutionary Adaptations of Axon Structure The speed of an action potential increases with the axon’s diameter In vertebrates, axons are insulated by a myelin sheath Why is this beneficial? Myelin sheaths are produced by glia—oligodendrocytes in the CNS and Schwann cells in the PNS 34

A selective advantage of myelination is space efficiency 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 selective advantage of myelination is space efficiency 35

Schwann cell Depolarized region (node of Ranvier) Cell body Myelin Figure 37.14 Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon Figure 37.14 Saltatory conduction 36

Concept 37.4: Neurons communicate with other cells at synapses At electrical synapses, the electrical current flows from one neuron to another Most synapses are chemical synapses, in which a chemical neurotransmitter carries information from the presynaptic neuron to the postsynaptic cell 37

Hw q #8 K Ca2 Na Presynaptic cell Postsynaptic cell Axon Synaptic vesicle containing neurotransmitter 1 Synaptic cleft Postsynaptic membrane Presynaptic membrane 3 4 Figure 37.15 A chemical synapse K Ca2 2 Ligand-gated ion channels Voltage-gated Ca2 channel Na 38

The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal The arrival of the action potential causes the release of the neurotransmitter The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell Animation: Synapse Animation: How Synapses Work 39

Generation of Postsynaptic Potentials Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential Q #8 from hw: what are the two kinds of post synaptic potentials? 40

Postsynaptic potentials fall into two categories Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold 41

The neurotransmitters clear quickly, in the following ways: Some are recaptured into presynaptic neurons to be repackaged and reused Some are recaptured into glia to be used as fuel Others are removed by simple diffusion or hydrolysis of the neurotransmitter (ex: acetylcholinesterase for acetylcholine 42

Summation of Postsynaptic Potentials The cell body of one postsynaptic neuron may receive inputs from hundreds or thousands of synaptic terminals A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron 43

Integration What is the difference between the way these two neurons are generating an action potential?

If two EPSPs are produced in rapid succession, an effect called temporal summation occurs In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together The combination of EPSPs through spatial and temporal summation can trigger an action potential 45

Neurotransmitters Signaling at a synapse brings about a response that depends on both the neurotransmitter from the presynaptic cell and the receptor on the postsynaptic cell A single neurotransmitter may have more than a dozen different receptors Acetylcholine is a common neurotransmitter in both invertebrates and vertebrates 46

Acetylcholine Acetylcholine is vital for functions involving muscle stimulation, memory formation, and learning Vertebrates have two major classes of acetylcholine receptor, one that is ligand gated and one that is metabotropic Nicotine is a direct agonist. What does this mean? Atropine and curare is an antagonist. What does this mean? 47

Table 37.2 Table 37.2 Major neurotransmitters 48

Amino Acids Glutamate (rather than acetylcholine) is used at the neuromuscular junction in invertebrates Gamma-aminobutyric acid (GABA) is the neurotransmitter at most inhibitory synapses in the brain Glycine also acts at inhibitory synapses in the CNS that lies outside of the brain 49

Biogenic Amines Biogenic amines include Norepinephrine and the chemically similar ephinephrine Dopamine Serotonin They are active in the CNS and PNS Biogenic amines have a central role in a number of nervous system disorders and treatments 50

Synapses: quick check Which of the following statements is true regarding synapses: 1. a synapse can occur if two IPSP occur at a neuron 2. a synapse involves the release of Ca ions which cause the release of neurotransmitters 3. acetylcholine is a rare neurotransmitter 4. only a spatial summation can cause a synapse