Neurons

Interneurons

1) A simple nervous system A) must include chemical senses, mechanoreception, and vision. B) includes a minimum of 12 effector neurons. C) has information flow in only one direction: toward an integrating center. D) has information flow in only one direction: away from an integrating center. E) includes sensory information, an integrating center, and effectors. Answer: E Neurons in the brain or ganglia integrate the sensory input, taking into account the immediate context and the animal's experience. The vast majority of neurons in the brain are interneurons, which form the local circuits connecting neurons in the brain. This information is sent to the brain or ganglia, where interneurons integrate the information This information is sent to the brain or ganglia, where interneurons integrate the information very compact

Simple Nervous System (fig. 37.4)

1) A simple nervous system A) must include chemical senses, mechanoreception, and vision. B) includes a minimum of 12 effector neurons. C) has information flow in only one direction: toward an integrating center. D) has information flow in only one direction: away from an integrating center. E) includes sensory information, an integrating center, and effectors. Answer: E Information processing by a nervous system occurs in three stages: sensory neurons (input), interneurons (integration), motor neurons (output). The cone snail's siphon acts as a sensor and transfers sensory information to neural circuits in the snail's heads. If the scent of prey is detected, these circuits issue motor commands that trigger release of a harpoon-like tooth from the proboscis.

Hyperpolarization (Causes)

10) For a neuron with an initial membrane potential at -70 mV, an increase in the movement of potassium ions out of that neuron's cytoplasm would result in A) the depolarization of the neuron. B) the hyperpolarization of the neuron. C) the replacement of potassium ions with sodium ions. D) the replacement of potassium ions with calcium ions. E) the neuron switching on its sodium-potassium pump to restore the initial conditions. Answer: B 18) A graded hyperpolarization of a membrane can be induced by A) increasing its membrane's permeability to Na+. B) decreasing its membrane's permeability to H+. C) decreasing its membrane's permeability to Cl-. D) increasing its membrane's permeability to Ca++. E) increasing its membrane's permeability to K+. Answer: E Increase in the magnitude of the membrane potential Makes the inside of the membrane more negative Results from any stimulus that increases the outflow of positive ions or the inflow of negative neurons in a resting neuron 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 Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+ Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold

Resting Neuron

11) Although the membrane of a "resting" neuron is highly permeable to potassium ions, its membrane potential does not exactly match the equilibrium potential for potassium because the neuronal membrane is also A) fully permeable to sodium ions. B) slightly permeable to sodium ions. C) fully permeable to calcium ions. D) impermeable to sodium ions. E) highly permeable to chloride ions. Answer: B 16) Two fundamental concepts about the ion channels of a "resting" neuron are that the channels A) are always open, but the concentration gradients of ions frequently change. B) are always closed, but ions move closer to the channels during excitation. C) open and close depending on stimuli, and are specific as to which ion can traverse them. D) open and close depending on chemical messengers, and are nonspecific as to which ion can traverse them. E) open in response to stimuli, and then close simultaneously, in unison. Answer: C 21) A "resting" motor neuron is expected to A) release lots of acetylcholine. B) have high permeability to sodium ions. C) be equally permeable to sodium and potassium ions. D) exhibit a resting potential that is more negative than the "threshold" potential. E) have a higher concentration of sodium ions on the inside of the cell than on the outside. Answer: D one that is not sending a signal membrane potential is -60--80mV Na+ K+ not at equilibrium the plasma membrane has many open potassium channels but few sodium open channels. Diffusion of ions, principally K+ through channels generates a resting potential, with he inside more positive than the outside. A resting neuron has many open potassium channels, allowing K+ to flow out In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steady

Sodium-Potassium Pump

12) The operation of the sodium-potassium "pump" moves A) sodium and potassium ions into the cell. B) sodium and potassium ions out of the cell. C) sodium ions into the cell and potassium ions out of the cell. D) sodium ions out of the cell and potassium ions into the cell. E) sodium and potassium ions into the mitochondria. Answer: D Maintains Na+ and K+ concentration, Na+ higher outside, K+ higher inside This ion pump uses the energy of ATP hydrolysis to actively transport Na+ out of the cell and K+ into the cell There are also concentration gradients for Cl- and other anions. Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane

Action Potential

19) Self-propagation and refractory periods are typical of A) action potentials. B) graded hyperpolarizations. C) excitatory postsynaptic potentials. D) threshold potentials. E) resting potentials. Answer: A 23) Action potentials move along axons A) more slowly in axons of large than in small diameter. B) by the direct action of acetylcholine on the axonal membrane. C) by activating the sodium-potassium "pump" at each point along the axonal membrane. D) more rapidly in myelinated than in non-myelinated axons. E) by reversing the concentration gradients for sodium and potassium ions. Answer: D 27) Immediately after an action potential passes along an axon, it is not possible to generate a second action potential; thus, we state that the membrane is briefly A) hyperexcitable. B) refractory. C) fully depolarized. D) above threshold. E) at the equilibrium potential. Answer: B 28) An action potential can start in the middle of an axon and proceed in both opposite directions when A) the neuron is an inhibitory neuron and operating normally. B) only the middle section of the axon has been artificially stimulated by an electrode. C) the dendritic region fires an action potential. D) it is in its typical refractory state. E) its membrane potential is above the threshold. Answer: B 29) The primary means by which a neuron can communicate to a second neuron is by A) the frequency of its action potentials. B) the peak of the depolarization phase of an action potential. C) the peak of the undershoot/hyperpolarization of an action potential. D) varying how much neurotransmitter it releases for a given action potential. E) remaining in the depolarization phase of the action potential for an extended interval. Answer: A 34) The fastest possible conduction velocity of action potentials is observed in A) thin, non-myelinated neurons. B) thin, myelinated neurons. C) thick, non-myelinated neurons. D) thick, myelinated neurons. Answer: D 42) Adjacent neurons with direct (non-neurotransmitter) action potential transfer are said to have electrical synapses, based on the presence of A) tight junctions at their point of contact. B) gap junctions at their point of contact. C) leaky junctions at their point of contact. D) anchoring junctions at their point of contact. E) desmosomes at their point of contact. Answer: B 69) A common feature of action potentials is that they A) cause the membrane to hyperpolarize and then depolarize. B) can undergo temporal and spatial summation. C) are triggered by a depolarization that reaches the threshold. D) move at the same speed along all axons. E) require the diffusion of Na+ and K+ through ligand-gated channels to propagate. Answer: C 72) Why are action potentials usually conducted in one direction? A) The nodes of Ranvier conduct potentials in one direction. B) The brief refractory period prevents reopening of voltage-gated Na+ channels. C) The axon hillock has a higher membrane potential than the terminals of the axon. D) Ions can flow along the axon in only one direction. E) Voltage-gated channels for both Na+ and K+ open in only one direction. Answer: B If a depolarization shifts the membrane potential sufficiently, the result is a massive change in membrane voltage called an AC Unlike graded potentials , action potential shave a constant magnitude and can regenerate in adjacent regions of the membrane. AC can therefore spread along axons, making them well suited for transmitting a signal over long distance. If a depolarization shifts the membrane potential sufficiently, it results in a massive change in membrane voltage, called an action potential Action potentials have a constant magnitude and transmit signals over long distances Action potentials occur whenever a depolarization increases the membrane potential to a particular value, called the threshold Action potentials are all or none Action potential triggered by a depolarization that reaches the threshold During the refractory period after an action potential, a second action potential cannot be initiated For most neurons, the interval between the start of an action potential and the end of the refractory period is only 1-2 msec 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 behind the zone of depolarization prevent the action potential from traveling backward The speed of an action potential increases with the axon's diameter In vertebrates, axons are insulated by a myelin sheath, which enables fast conduction of action potentials Myelin sheaths are produced 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 selective advantage of myelination is space efficiency The arrival of the action potential causes the release of the neurotransmitter A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron The combination of EPSPs through spatial and temporal summation can trigger an action potential The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential

Threshold

22) The "threshold" potential of a membrane A) is the point of separation from a living to a dead neuron. B) is the lowest frequency of action potentials a neuron can produce. C) is the minimum hyperpolarization needed to prevent the occurrence of action potentials. D) is the minimum depolarization needed to operate the voltage-gated sodium and potassium channels. E) is the peak amount of depolarization seen in an action potential. Answer: D Action potentials occur whenever a depolarization increases the membrane voltage to a particular value, called the threshold. For many mammalian neurons, the threshold is a membrane potential of about -55mV. Once initiated, the action potential has a magnitude that is independent of the strength id the triggering stimulus. Action potentials occur whenever a depolarization increases the membrane potential to a particular value, called the threshold Action potential triggered by a depolarization that reaches the threshold During the rising phase, the threshold is crossed, and the membrane potential increases The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential

Depolarization (Causes)

24) A toxin that binds specifically to voltage-gated sodium channels in axons would be expected to A) prevent the hyperpolarization phase of the action potential. B) prevent the depolarization phase of the action potential. C) prevent graded potentials. D) increase the release of neurotransmitter molecules. E) have most of its effects on the dendritic region of a neuron. Answer: B 25) After the depolarization phase of an action potential, the resting potential is restored by A) the opening of sodium activation gates. B) the opening of voltage-gated potassium channels and the closing of sodium channels. C) a decrease in the membrane's permeability to potassium and chloride ions. D) a brief inhibition of the sodium-potassium pump. E) the opening of more voltage-gated sodium channels. Answer: B 68) What happens when a resting neuron's membrane depolarizes? A) There is a net diffusion of Na+ out of the cell. B) The equilibrium potential for K+ (EK) becomes more positive. C) The neuron's membrane voltage becomes more positive. D) The neuron is less likely to generate an action potential. E) The cell's inside is more negative than the outside. Answer: C A reduction in the magnitude of the membrane potential Involves gated sodium channels, if a stimulus causes gated sodium channels in a resting neuron to open, the membrane's permeability to Na+ increases. Na+ diffuses into the cell along it's concentration gradient, causing a depolarization as the membrane potential shifts toward 63mV. Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold Action potential triggered by a depolarization that reaches the threshold Opening other types of ion channels triggers 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

Voltage-Gated Ion Channels

26) The "undershoot" phase of after-hyperpolarization is due to A) slow opening of voltage-gated sodium channels. B) sustained opening of voltage-gated potassium channels. C) rapid opening of voltage-gated calcium channels. D) slow restorative actions of the sodium-potassium ATPase. E) ions that move away from their open ion channels. Answer: B 30) In the sequence of permeability changes for a complete action potential, the first of these events that occurs is A) the activation of the sodium-potassium "pump." B) the inhibition of the sodium-potassium "pump." C) the opening of voltage-gated sodium channels. D) the closing of voltage-gated potassium channels. E) the opening of voltage-gated potassium channels. Answer: C Action potentials arise bc some of the ion channels in neurons are voltage gated ion channels, opening or closing when the membrane potential passes a particular level. If a depolarization opens voltage gated sodium channels, the flow of Na+ into the neuron result sin further depolarization. Bc the sodium channels are voltage gated, the increased depolarization causes more sodium channels to open, leading to an even greater flow of current. The result is a process of positive feedback that triggers a very rapid opening of many voltage gated sodium channels and the marked temporary change in membrane potential that defines an action potential. They arise because some ion channels are voltage gated, opening or closing when the membrane potential passes a certain level

Neuron Structure

3) The nucleus and most of the organelles in a neuron are located in the A) dendritic region. B) axon hillock. C) axon. D) cell body. E) axon terminals. Answer: D Diagram The neuron is a cell type that exemplifies the close fit of form and function that often arises over the course of evolution 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 single axon, 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 The branched ends of axons transmit signals to other cells at a junction called the synapse At most synapses, chemical messengers called neurotransmitters pass information from the transmitting neuron to the receiving cell Neurons of vertebrates and most invertebrates require supporting cells called glial cells In the mammalian brain, glia outnumber neurons 10- to 50-fold

Saltatory Conduction

31) Saltatory conduction is a term applied to A) conduction of impulses across electrical synapses. B) an action potential that skips the axon hillock in moving from the dendritic region to the axon terminal. C) rapid movement of an action potential reverberating back and forth along a neuron. D) jumping from one neuron to an adjacent neuron. E) jumping from one node of Ranvier to the next in a myelinated neuron. Answer: E Opening and closing ion channels occurs at only limited number of positions along the axon. This mechanism for propagating action potentials is called sc bc the action potential appears to jump along the axon from node to node. Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

IPSP Inhibitory Post-Synaptic Potential

39) An inhibitory postsynaptic potential (IPSP) occurs in a membrane made more permeable to A) potassium ions. B) sodium ions. C) calcium ions. D) ATP. E) all neurotransmitter molecules. Answer: A 48) When several IPSPs arrive at the axon hillock rapidly in sequence from a single dendritic location, hyperpolarizing the postsynaptic cell more and more and thus preventing an action potential, this is an example of A) temporal summation. B) spatial summation. C) tetanus. D) the refractory state. E) an action potential with an abnormally high peak of depolarization. Answer: A 49) Assume that a single IPSP has a negative magnitude of -0.5 mV at the axon hillock, and that a single EPSP has a positive magnitude of +0.5 mV. For a neuron with an initial membrane potential of -70 mV, the net effect of the simultaneous arrival of six IPSPs and two EPSPs would be to move the membrane potential to A) -72 mV. B) -71 mV. C) -70 mV. D) -69 mV. E) -68 mV. Answer: A At other synapses, the ligand-gated ion channel is selectively permeable for only K+ or Cl-. When such a channel opens, the post synaptic membrane hyperpolarizes. A hyperpolarization produced in this manner is an IPSP because it moves the membrane potential further from threshold Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold 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 potential

Sensory Neurons

4) In certain large animals, this type of neuron can extend beyond 1 meter in length. A) glial cell in the brain B) a sensory neuron C) an interneuron D) a glial cell at a ganglion E) a neuron that controls eye movements Answer: B Sensory neurons, like those in the snail's siphon, transmit information processing centers in the brain or ganglia. Some sensors detect external stimuli such as light, sound, touch, or smell, whereas others monitor internal conditions such as blood pressure or muscle tension. Sensory neurons transmit information from eyes and other sensors that detect external stimuli or internal conditions

Ion Gated Channels

43) Ionotropic receptors are found at synapses operated via A) ligand-gated ion channels. B) second-messenger-gated ion channels. C) electrical synapses. D) inhibitory, but not excitatory, synapses. E) excitatory, but not inhibitory, synapses. Answer: A 44) An example of ligand-gated ion channels is A) the spreading of action potentials in the heart. B) acetylcholine receptors at the neuromuscular junction. C) cAMP-dependent protein kinases. D) action potentials on the axon. E) graded hyperpolarization. Answer: B 45) An example of the action of metabotropic receptors is when A) voltage-gated ion channels open. B) voltage-gated ion channels close. C) acetylcholine-gated sodium channels open. D) cAMP-linked ion channels open. E) the undershoot/after-hyperpolarization occurs. Answer: C 1. Voltage - na+ in 2. Ligand - neurotransmitters 3. Stretch receptors Changes in membrane potential occur because neurons contain gated ion channels that open or close in response to stimuli

EPSP Excitatory Post-Synaptic Potential

47) When several EPSPs arrive at the axon hillock from different dendritic locations, depolarizing the postsynaptic cell to threshold for an action potential, this is an example of A) temporal summation. B) spatial summation. C) tetanus. D) the refractory state. E) an action potential with an abnormally high peak of depolarization. Answer: B At some synapses, the ligand-gated ion channel is permeable to both K+ and Na+. When this channel opens, the membrane potential depolarizes toward a value roughly midway between Ek and ENa, bc such depolarization brings the membrane potential toward threshold, it is called EPSP Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold 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 potential When acetylcholine released by motor neurons binds to this receptor, the ion channel opens and an EPSP is generated A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron 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

Spatial Summation

47) When several EPSPs arrive at the axon hillock from different dendritic locations, depolarizing the postsynaptic cell to threshold for an action potential, this is an example of A) temporal summation. B) spatial summation. C) tetanus. D) the refractory state. E) an action potential with an abnormally high peak of depolarization. Answer: B ESPSs produced nearly simultaneously by different synapses on the same postsynaptic neuron can also add together, an effect celled spatial summation In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

Motor Neurons

5) The somatic nervous system can alter the activities of its targets, the skeletal muscle fibers, because A) it is electrically coupled by gap junctions to the muscles. B) its signals bind to receptor proteins on the muscles. C) its signals reach the muscles via the blood. D) its light pulses activate contraction in the muscles. E) it is connected to the internal neural network of the muscles. Answer: B 37) The release of acetylcholine from the terminal of a motor neuron is most directly linked to A) the entry of potassium into the axon terminal. B) the exit of potassium from the axon terminal. C) the entry of sodium into the axon terminal. D) the exit of sodium from the axon terminal. E) the entry of calcium into the axon terminal. Answer: E 38) The observation that the acetylcholine released into the junction between a motor neuron and a skeletal muscle binds to a sodium channel and opens it is an example of A) a voltage-gated sodium channel. B) a voltage-gated potassium channel. C) a ligand-gated sodium channel. D) a second-messenger-gated sodium channel. E) a chemical that inhibits action potentials. Answer: C Neurons that extend our of the processing centers trigger output in the form of muscle or gland activity. Motor neurons transmit signals to muscle cells causing them to contract For example, motor neurons transmit signals to muscle cells, causing them to contract When acetylcholine released by motor neurons binds to this receptor, the ion channel opens and an EPSP is generated

Axon Hillock (Role)

51) Functionally, which cellular location is the neuron's "decision-making site" as to whether or not an action potential will be initiated? A) axonal membranes B) axon hillocks C) dendritic membranes D) mitochondrial membranes E) presynaptic membranes Answer: B 66) Action potentials are normally carried in only one direction: from the axon hillock toward the axon terminals. If you experimentally depolarize the middle of the axon to threshold, using an electronic probe, then A) no action potential will be initiated. B) an action potential will be initiated and proceed only in the normal direction toward the axon terminal. C) an action potential will be initiated and proceed only back toward the axon hillock. D) two action potentials will be initiated, one going toward the axon terminal and one going back toward the hillock. E) an action potential will be initiated, but it will die out before it reaches the axon terminal. Answer: D The cone shaped base of an axon called the axon hillock is typically where signals that travel down the axon are generated. Near its other end, an axon usually divides into many branches At the site where the action potential is initiated (usually the axon hillock), an electrical current depolarizes the neighboring region of the axon membrane The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential

Synaptic Vesicles

6) The point of connection between two communicating neurons is called A) the axon hillock. B) the dendrite. C) the synapse. D) the cell body. E) the glia. Answer: C 40) The following steps refer to various stages in transmission at a chemical synapse. 1. Neurotransmitter binds with receptors associated with the postsynaptic membrane. 2. Calcium ions rush into neuron's cytoplasm. 3. An action potential depolarizes the membrane of the axon terminal. 4. The ligand-gated ion channels open. 5. The synaptic vesicles release neurotransmitter into the synaptic cleft. Which sequence of events is correct? A) 1 → 2 → 3 → 4 → 5 B) 2 → 3 → 5 → 4 → 1 C) 3 → 2 → 5 → 1 → 4 D) 4 → 3 → 1 → 2 → 5 E) 5 → 1 → 2 → 4 → 3 Answer: C At each terminal the presynaptic neuron synthesizes the neurotransmitter and packages it in multiple membrane-enclosed compartments called synaptic vesicles. The arrival of an action potential at a synaptic terminal depolarizes the plasma membrane, opening voltage gated channels that allow Ca2+ to diffuse into the terminal. The resulting Ca2+ concentration in the terminal causes some of the synaptic vesicles to fuse with the terminal membrane, releasing the neurotransmitter. The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal Some neurotransmitters are recaptured into presynaptic neurons to be repackaged into synaptic vesicles

Neurotransmitter Release (don't memorize neurotransmitter types)

7) In a simple synapse, neurotransmitter chemicals are released by A) the dendritic membrane. B) the presynaptic membrane. C) axon hillocks. D) cell bodies. E) ducts on the smooth endoplasmic reticulum. Answer: B 8) In a simple synapse, neurotransmitter chemicals are received by A) the dendritic membrane. B) the presynaptic membrane. C) axon hillocks. D) cell bodies. E) ducts on the smooth endoplasmic reticulum. Answer: A 33) Neurotransmitters are released from axon terminals via A) osmosis. B) active transport. C) diffusion. D) transcytosis. E) exocytosis. Answer: E 46) Neurotransmitters categorized as inhibitory are expected to A) act independently of their receptor proteins. B) close potassium channels. C) open sodium channels. D) close chloride channels. E) hyperpolarize the membrane. Answer: E 50) Receptors for neurotransmitters are of primary functional importance in assuring one-way synaptic transmission because they are mostly found on the A) axonal membrane. B) axon hillock. C) dendritic membrane. D) mitochondrial membrane. E) presynaptic membrane. Answer: C 52) Neurotransmitters affect postsynaptic cells by A) initiating signal transduction pathways in the cells. B) causing molecular changes in the cells. C) affecting ion-channel proteins. D) altering the permeability of the cells. E) All of these options are correct. Answer: E 53) The primary neurotransmitter from the parasympathetic system that influences its autonomic targets is A) acetylcholine. B) adenosine. C) norepinephrine. D) adrenaline. E) dopamine. Answer: A 54) The major inhibitory neurotransmitter of the human brain is A) acetylcholine. B) epinephrine. C) glutamate. D) nitric oxide. E) GABA. Answer: E 55) The major excitatory neurotransmitter of the human brain is A) acetylcholine. B) epinephrine. C) glutamate. D) nitric oxide. E) GABA. Answer: C 70) Where are neurotransmitter receptors located? A) the nuclear membrane B) the nodes of Ranvier C) the postsynaptic membrane D) synaptic vesicle membranes E) the myelin sheath Answer: C The arrival of an action potential at a synaptic terminal depolarizes the plasma membrane, opening voltage gated channels that allow Ca2+ to diffuse into the terminal. The resulting Ca2+ concentration in the terminal causes some of the synaptic vesicles to fuse with the terminal membrane, releasing the neurotransmitter. Once relaesed the neurostransmitter diffuses across the synaptic cleft, the gap that separates the presynaptic neuron from the presynaptic cell. The neurotranmitter binds to and activates a specific receptor in the membrane, which in turn triggers a resposne in the postsynapic cell. At most synapses, chemical messengers called neurotransmitters pass information from the transmitting neuron to the receiving cell Most synapses are chemical synapses, in which a chemical neurotransmitter carries information from the presynaptic neuron to the postsynaptic cell 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 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 The duration of postsynaptic potential is limited by rapidly clearing neurotransmitter molecules from the synaptic cleft Some neurotransmitters are recaptured into presynaptic neurons to be repackaged into synaptic vesicles Some are recaptured into glia to be used as fuel or recycled to neurons Others are removed by simple In some synapses, a neurotransmitter binds to a receptor that is metabotropic Binding of a neurotransmitter to a metabotropic receptor activates a signal transduction pathway in the postsynaptic cell involving a second messenger 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 Acetylcholine is one of more than 100 known neurotransmitters

Temporal Summation

71) Temporal summation always involves A) both inhibitory and excitatory inputs. B) synapses at more than one site. C) inputs that are not simultaneous. D) electrical synapses. E) multiple inputs at a single synapse. Answer: E Two EPSPs occure at a single synapse in such rapid succession that thepostsynaptic neuron's membrane potential has not returned to the resting potential before the arrival of the second EPSP. When that happens the ESPSs add together and effect called temporal summation If two EPSPs are produced in rapid succession, an effect called temporal summation occurs The combination of EPSPs through spatial and temporal summation can trigger an action potential

Pre-Synaptic and Post-Synaptic

9) In the communication between a motor neuron and a skeletal muscle, A) the motor neuron is considered the presynaptic cell and the skeletal muscle is the postsynaptic cell. B) the motor neuron is considered the postsynaptic cell and the skeletal muscle is the presynaptic cell. C) action potentials are possible on the motor neuron but not the skeletal muscle. D) action potentials are possible on the skeletal muscle but not the motor neuron. E) the motor neuron fires action potentials but the skeletal muscle is not electrochemically excitable. Answer: A 32) The surface on a neuron that discharges the contents of synaptic vesicles is the A) dendrite. B) axon hillock. C) node of Ranvier. D) postsynaptic membrane. E) presynaptic membrane. Answer: E 36) One possible disadvantage to a nerve net is that it might conduct impulses in two directions from the point of the stimulus. Most of the synapses in vertebrates conduct information in only one direction A) as a result of the nodes of Ranvier. B) as a result of voltage-gated sodium channels found only in the vertebrate system. C) because vertebrate nerve cells have dendrites. D) because only the postsynaptic cells can bind and respond to neurotransmitters. E) because the sodium-potassium pump moves ions in one direction. Answer: D 41) The activity of acetylcholine in a synapse is terminated by A) its active transport across the presynaptic membrane. B) its diffusion across the presynaptic membrane. C) its active transport across the postsynaptic membrane. D) its diffusion across the postsynaptic membrane. E) its degradation by a hydrolytic enzyme on the postsynaptic membrane. Answer: E 60) A chemical that affects neuronal function but is not stored in presynaptic vesicles is A) acetylcholine. B) epinephrine. C) endorphin. D) nitric oxide. E) GABA. Answer: D 73) Which of the following is a direct result of depolarizing the presynaptic membrane of an axon terminal? A) Voltage-gated calcium channels in the membrane open. B) Synaptic vesicles fuse with the membrane. C) The postsynaptic cell produces an action potential. D) Ligand-gated channels open, allowing neurotransmitters to enter the synaptic cleft. E) An EPSP or IPSP is generated in the postsynaptic cell. Answer: A The majority of synapses are chemical synapses, which involve the release of a chemical neurotranmitter by the presynaptic neiron. At each terminal, the presynaptic neuron synthesizes the seurotransmitter and packages it in multiple membrane enclosed compartments called synaptic vesicles. The arrival of an action potentila at a synaptic terminal depolarizes the plasma membrane, opening voltage gated channels that allow Ca2+ to diffuse into the terminal. The reulting rise in Ca2+ concentration in the terminal causes some of the synaptic vesicles to fuse with the terminal membrane, releasing the neurotransmitter. Once released the neurotranmitter diffuses across the synaptic cleft, the gap that separates the presynaptic neuron from the postsynaptic cell. Diffusion time is very short because the gapis less than 50nm across. Upon reaching the postsynaptic membrane, the neurotransmitter binds to and activates a specific receptor in th epostsynapic cell. Most synapses are chemical synapses, in which a chemical neurotransmitter carries information from the presynaptic neuron to the postsynaptic cell The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal Some neurotransmitters are recaptured into presynaptic neurons to be repackaged into synaptic vesicles 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 The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell 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 Postsynaptic potentials fall into two categories The duration of postsynaptic potential is limited by rapidly clearing neurotransmitter molecules from the synaptic cleft 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 Binding of a neurotransmitter to a metabotropic receptor activates a signal transduction pathway in the postsynaptic cell involving a second messenger

Graph of Membrane Potential

A nerve impulse travels from the axon hillock to the synaptic terminals by propagating a series of action potentials along the axon. The speed of conduction increases with the diameter of the axon ans, in many vertebrate axons, with myelination. Action potentials in myelinated axons jump between the nodes of ranvier a process called saltatory conduction. At resting potential Most voltage-gated sodium (Na+) channels are closed; most of the voltage-gated potassium (K+) channels are also closed 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 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

Summation

Sum of two excitatories Added together More painful 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 potential

Membrane Potential

Voltage across the membrane IONs in neuronal signaling: In a neuron, as in other cells, ions are unequally distributed between the cell interior and the surrounding extracellular fluid. As a result, the inside of a cell is negatively charged relative to the outside. Because the attraction of opposite charges across the plasma membrane is a source of potentila enegry, this charge difference, or voltage, is called the membrane potential. For a resting neuron- one not sending a signal- the membrane potentila is called the resting potential and is typically between -60 and -80 mV. Inputs and stimuli cause changes in membrane potential- enable info to transmit 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 Changes in membrane potential act as signals, transmitting and processing information The resulting buildup of negative charge within the neuron is the major source of membrane potential Researchers can record the changes in membrane potential when a neuron responds to a stimulus Changes in membrane potential occur because neurons contain gated ion channels that open or close in response to stimuli This is hyperpolarization, an increase in magnitude of the membrane potential Opening other types of ion channels triggers a depolarization, a reduction in the magnitude of the membrane potential If a depolarization shifts the membrane potential sufficiently, it results in a massive change in membrane voltage, called an action potential They arise because some ion channels are voltage gated, opening or closing when the membrane potential passes a certain level Action potentials occur whenever a depolarization increases the membrane potential to a particular value, called the threshold During the rising phase, the threshold is crossed, and the membrane potential increases+