Nervous System - textbook Flashcards

Question

True or False? Voltage-gated Na+ channels open more slowly than voltage-gated K+ channels.

Answer 1

FALSE! Voltage-gated K+ channels open more slowly than voltage-gated Na+ channels. After the repolarization phase, K+ channels may remain open slightly longer than necessary to simply return to the resting potential. This over-shoot leads to hyperpolarization, where the membrane potential becomes more negative than the resting potential. This phase helps to prevent the neuron from immediately firing another action potential, thus helping to clearly demarcate the signaling events and ensuring that the action potentials do not overlap.

Answer 2

Schwann cells are in the Peripheral Nervous System (PNS). Oligodendrocytes are in the Central Nervous System (CNS). Both are types of Glial cells that wrap myelin sheath around axons in the nervous system.

Answer 3

The membrane potential becomes positive during the depolarization phase of an action potential primarily due to the rapid influx of sodium ions (Na+) into the neuron. The membrane's resting potential is negative so we have an influx of positive ions in order to depolarize it.

Answer 4

Because action potentials are constantly regenerated as they propagate. Unlike action potentials, graded potentials are not regenerated along the membrane. As they spread away from the point of origin, their amplitude decreases until the signal diminishes completely.

Answer 5

Temporal Summation: The process by which increased frequencies of action potentials lead to higher calcium levels and therefore more neurotransmitter release is a form of temporal summation. This means that the effects of individual action potentials are summed over time, leading to a greater overall response. Signal Intensity Coding: The frequency of action potentials is a way for neurons to code the intensity of a stimulus. A more intense stimulus might generate a higher frequency of action potentials, which, in turn, results in more neurotransmitter release and a stronger response in the post-synaptic neuron. Modulation of Synaptic Strength: The frequency-dependent release of neurotransmitters can modulate synaptic strength either in the short term (via temporary increases in neurotransmitter levels) or in the long term (through mechanisms such as synaptic plasticity, where sustained changes in action potential frequency can lead to structural changes in the synapse).

Answer 6

1.Type of Neurotransmitter: Different neurotransmitters have inherent properties that tend to cause specific effects. For example, glutamate typically acts as an excitatory neurotransmitter, causing depolarization, whereas GABA typically acts as an inhibitory neurotransmitter, causing hyperpolarization. 2. ype of Receptor: The effect of a neurotransmitter is not solely inherent to the neurotransmitter itself but also depends on the type of receptor it binds to on the postsynaptic cell. Receptors can be broadly classified into two categories: Ionotropic Receptors: These are ligand-gated ion channels that open in response to neurotransmitter binding, allowing specific types of ions to flow across the membrane. For example, if the channel allows positively charged ions like sodium (Na+) to enter the cell, it will cause depolarization. If the channel allows negatively charged ions like chloride (Cl-) to enter or positive ions like potassium (K+) to exit, it will cause hyperpolarization. Metabotropic Receptors: These receptors are part of a signaling complex that can activate second messengers and affect various ion channels indirectly or initiate other cellular responses that modulate the cell's excitability. The overall effect depends on the specific signaling pathways triggered by the receptor activation. 3. Ion Flow: The direction of ion flow across the membrane is crucial. The opening of ion channels that lead to an influx of Na+ or Ca2+ ions typically results in depolarization. Conversely, the influx of Cl- ions or efflux of K+ ions generally causes hyperpolarization. 4. Reversal Potential: Each type of ion channel associated with a neurotransmitter has a reversal potential, which is the membrane potential at which no net flow of ions occurs through the channel. The effect of the neurotransmitter depends on how this reversal potential compares to the resting membrane potential and the threshold potential for firing an action potential. If the channel brings the membrane potential closer to the threshold for firing an action potential, it is excitatory. If it moves the potential further from this threshold, it is inhibitory. 5. Local Membrane Environment: The local conditions of the postsynaptic membrane, including the density and distribution of receptors and ion channels, can also influence how a neurotransmitter affects the neuron. Different parts of the neuron might respond differently to the same neurotransmitter due to variations in these local conditions.

Answer 7

Receptor Binding: When neurotransmitters are released from the presynaptic neuron, they diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron. The more neurotransmitter molecules released, the more likely they are to find and bind to receptors, leading to an increased activation of those receptors. Increased Response: As more neurotransmitters bind to their receptors, more ion channels are activated (in the case of ionotropic receptors) or more intracellular signaling pathways are stimulated (in the case of metabotropic receptors). This leads to a greater change in the postsynaptic membrane potential or cellular activity. For ionotropic receptors, this might mean more ions flowing into or out of the neuron, altering its membrane potential more significantly and thereby increasing the likelihood or frequency of action potentials. Saturation: Receptors on the postsynaptic neuron can become saturated, which means that nearly all available receptors have neurotransmitter molecules bound to them. Once saturation is reached, adding more neurotransmitter will not increase the response because there are no additional receptors available to be activated. This saturation results in a maximum level of response that the postsynaptic neuron can achieve under the current conditions. Receptor Dynamics: Postsynaptic responses can also be limited by receptor dynamics such as desensitization, where receptors become temporarily insensitive to neurotransmitter binding despite the presence of neurotransmitters. Additionally, some receptors may be internalized or degraded, reducing the number of functional receptors available on the cell surface over time. Response Plateau: The maximum response (plateau) reached is also determined by the intrinsic properties of the postsynaptic cell, such as the number of receptors, the efficiency of the signal transduction pathways, and the cell's ability to handle ionic changes and return to baseline conditions. In summary, increasing the amount of neurotransmitter initially increases the postsynaptic response due to higher rates of receptor activation. However, this response will reach a maximum and plateau when all available receptors are occupied or when other cellular limitations prevent further increases in response, regardless of additional neurotransmitter presence.

Answer 8

Multipolar Neurons Structure: Multipolar neurons have one axon and multiple dendrites extending from the cell body. This is the most common type of neuron. Function: These neurons are typically involved in motor functions and the integration of sensory information within the central nervous system. They are commonly found in the brain and spinal cord. Example: Most of the neurons in the brain and spinal cord are multipolar. Bipolar Neurons Structure: Bipolar neurons have one axon and one dendrite attached to the cell body. They are structurally simpler than multipolar neurons. Function: These neurons are primarily involved in sensory pathways, such as in the retina of the eye or the olfactory epithelium. Example: Retinal cells that transmit visual signals from the eye to the brain are bipolar neurons. Unipolar Neurons (Pseudounipolar) Structure: Unipolar neurons have a single process that extends from the cell body and divides into two branches far from the cell body. One branch runs towards the central nervous system (acting as the central axon), and the other runs towards the periphery (acting as the peripheral axon). Technically, these are called pseudounipolar neurons because they start as bipolar neurons during development but their two poles fuse into a single process. Function: Unipolar neurons are primarily found in sensory ganglia of the peripheral nervous system and are involved in the transmission of sensory information from the body to the spinal cord. Example: The sensory neurons in dorsal root ganglia that carry touch and pain signals are typically unipolar. Summary Multipolar neurons: Multiple processes, primarily in the central nervous system, involved in complex integrative functions. Bipolar neurons: One dendrite, one axon, typically sensory neurons involved in sight and smell. Unipolar neurons: A single process that splits into two branches, found in sensory ganglia, conveying sensory information to the central nervous system.

Answer 9

Sensory (or afferent) neurons convey sensory information from the body to the central nervous system (which consists of the brain and spinal cord in vertebrates). Interneurons are located within the central nervous system, and convey signals from one neuron to another. Efferent neurons convey signals from the central nervous system to effector organs.

Answer 10

Astrocytes have large stellate (star-shaped) cell bodies and many processes. They are located in the central nervous system and play a variety of roles, including transporting nutrients to neurons, removing debris, guiding neuronal development, and regulating the contents of the extracellular space around neurons.

Answer 11

Microglia are involved in neuronal maintenance. Microglia are the smallest glial cells. They are similar to the macrophages of the immune system, and they function to remove debris and dead cells from the central nervous system. Microglia are most active following trauma or during disease.

Answer 12

Ependymal cells line the fluid-filled cavities of the central nervous system. They often have cilia, which they use to circulate the cerebrospinal fluid that bathes the central nervous system of vertebrates.

Answer 13

Satellite cells are a specific type of glial cell that are found in the ganglia of the peripheral nervous system (PNS), and enteric glia are associated with the neurons of the gut. These glial cells are thought to perform functions similar to those of astrocytes in the CNS. Radial glia are found in the central nervous system during development and play an important role in structuring the developing nervous system.

Answer 14

Myelination and increasing the diameter of the axon. The fastest nerve conduction is always observed in either large-diameter or myelinated neurons.

Answer 15

the properties of the axon that dictate current flow along the axon are often called the _cable properties_ of the axon.

Answer 16

The effects of membrane resistance, extracellular resistance, and intracellular resistance on the distance an electrical signal can travel are summarized by a parameter termed the _length constant (λ)_ of the membrane. The length constant is defined as the distance over which a change in membrane potential will decrease to 37 percent of its original value.

Answer 17

Giant Axons Size: Giant axons, as found in squid, are significantly larger in diameter than typical axons. The large diameter reduces the internal resistance to the flow of ionic currents, allowing faster propagation of action potentials. Simplicity: Giant axons are unmyelinated, which simplifies their structure but also limits their maximum conduction speed compared to myelinated axons. The absence of myelination makes them less energy-efficient in vertebrates. Speed Mechanism: The increased diameter facilitates a faster conduction speed by allowing more ions to flow more quickly due to the reduced resistance. Energy Use: These axons generally use more energy per action potential compared to myelinated axons, as the entire axon membrane needs to be depolarized and repolarized during each action potential. Function: In squid, the giant axon is used to rapidly transmit signals to muscles for escape responses, demonstrating a specialized adaptation where high speed is crucial over short distances. Myelinated Axons Myelination: Myelinated axons are wrapped in myelin sheaths, which are made of layers of lipid-rich membrane produced by glial cells (oligodendrocytes in the CNS and Schwann cells in the PNS). Myelin acts as an electrical insulator. Saltatory Conduction: Myelin sheaths are interrupted at regular intervals by nodes of Ranvier, where the axon membrane is exposed. Action potentials "jump" from node to node in a process called saltatory conduction, greatly increasing the speed of conduction. Speed Mechanism: The saltatory conduction mechanism allows rapid signal transmission by reducing the membrane area that needs to be depolarized, and by effectively "skipping" over parts of the axon. Energy Use: Myelinated axons are more energy-efficient, as the ionic exchanges required for action potential generation occur only at the nodes of Ranvier, reducing the metabolic cost associated with pumping ions in and out of the cell. Function: Myelination is particularly advantageous in vertebrates, where long-distance signal transmission is required, such as in the nerves extending from the spinal cord to the limbs.

Answer 18

Axon Diameter: Larger axons can support longer internodal distances. The diameter of an axon affects its internal resistance to the passive flow of electric current. In larger axons, the current can travel a longer distance before it dissipates too much to trigger an action potential at the next node. Therefore, larger axons can have nodes that are spaced further apart without losing the ability to propagate the action potential efficiently. Myelin Sheath Thickness: The thickness of the myelin sheath is also a critical factor. Thicker myelin provides better insulation, which improves the efficiency of electrical insulation and reduces the leakage of charged ions from the axon. This enhanced insulation can allow for longer internodes, as the myelin effectively preserves the electrical signal over a greater distance, enabling it to jump between more widely spaced nodes. Electrical Properties of the Axon: The specific electrical properties of the axon, such as its membrane capacitance and resistance, play a significant role in determining how far an action potential can travel along an axon before too much signal is lost. Lower capacitance and higher resistance along the axonal membrane favor longer internodal distances by reducing the rate at which charge leaks out of the axon. Speed of Signal Transmission Required: The biological function of the neuron can dictate the necessary speed of signal transmission. Neurons that need to transmit signals very quickly, such as those involved in reflex actions or fine motor control, might benefit from optimal spacing that balances signal speed and metabolic efficiency. Closer nodes can increase speed but at a higher metabolic cost, whereas longer internodes can reduce metabolic demand but potentially slow down signal transmission. Energy Efficiency: Maintaining and restoring the ion gradients after an action potential consumes metabolic energy, primarily through the activity of ion pumps like the sodium-potassium pump. Longer internodal distances can reduce the frequency of action potentials along the axon, thereby reducing metabolic energy demands.

Answer 19

Synapses in which the presynaptic and postsynaptic cells are connected via gap junctions are termed _electrical synapses_, because the electrical signal in the presynaptic cell is directly transferred to the postsynaptic cell through the gap junctions. Most neurons, however, do not form gap junctions with their target cells. Instead, these neurons form _chemical synapses_.

Answer 20

Electrical Synapses Mechanism: Electrical synapses directly connect the cytoplasm of two neurons through gap junctions, which are composed of connexin proteins forming channels called connexons. These channels allow ions and small molecules to flow directly from one neuron to another. Characteristics: Speed: Transmission at electrical synapses is very fast, almost instantaneous, because it involves the direct passage of electrical current between cells. Bidirectionality: Electrical synapses are typically bidirectional, allowing signals to pass in either direction between neurons. Synchronization: They are effective in synchronizing the activity of a group of neurons, making them important in behaviors that require simultaneous action of multiple neurons, such as rhythmic activities in the heart and brain. Simplicity: Electrical transmission does not involve complex neurotransmitter release and receptor mechanisms, which simplifies the transmission process. Chemical Synapses Mechanism: Chemical synapses involve the release of neurotransmitters from the presynaptic neuron into the synaptic cleft. These neurotransmitters then bind to receptors on the postsynaptic neuron, causing changes in the postsynaptic cell that may result in an excitatory or inhibitory post-synaptic potential. Characteristics: Speed: Chemical transmission is slower than electrical transmission due to the time required for neurotransmitter release, diffusion, and receptor binding. Unidirectionality: Chemical synapses are generally unidirectional; the presynaptic neuron releases neurotransmitters that influence only the postsynaptic neuron. Amplification: Chemical synapses can amplify neuronal signals because the binding of neurotransmitters to receptors can activate second messengers or open many ion channels. Versatility and Plasticity: Chemical synapses can be modified (synaptic plasticity), which is critical for learning and memory. They also allow for a more varied and complex range of responses due to different types of neurotransmitters and receptors involved. Functional Implications Electrical synapses are typically found where rapid, synchronized responses are necessary. They are less versatile than chemical synapses but provide the reliability and speed required for certain physiological functions. Chemical synapses allow for a broad diversity of responses and integrative capabilities, which is essential for most brain functions, including processing complex information and maintaining flexibility and adaptability in neural circuitry.

Answer 21

An increase in neurotransmitter release in response to repeated action potentials is termed synaptic facilitation. _Synaptic facilitation_ occurs because the accumulation of intracellular following each action potential allows more neurotransmitter to be released by subsequent action potentials. In contrast, _synaptic depression_, which is a decrease in neurotransmitter release with repeated action potentials, occurs because of the progressive depletion of the readily accessible pool of synaptic vesicles that is available for fusion and exocytosis of neurotransmitter.

Answer 22

_Synaptic plasticity_, or the ability of a synapse to change its function in response to patterns of use, underlies many important brain functions, including learning and memory.

Answer 23

Adrenergic receptors are a class of G protein-coupled receptors that are targeted by the catecholamines, norepinephrine, and epinephrine. There are two main types of adrenergic receptors, alpha (α) and beta (β). Alpha Adrenergic Receptors Alpha-1 (α1) Receptors: Location: Predominantly found in the smooth muscles of blood vessels, the eye (iris), and other tissues. Mechanism: These receptors typically couple to G_q proteins, leading to activation of phospholipase C and an increase in intracellular calcium levels. Effects: Contraction of smooth muscle, such as vasoconstriction (narrowing of blood vessels), pupil dilation, and increased peripheral resistance. Alpha-2 (α2) Receptors: Location: Found in both the central and peripheral nervous systems, often presynaptically. Mechanism: These receptors usually couple to G_i proteins, which inhibits adenylate cyclase, decreasing cAMP levels. Effects: Inhibition of neurotransmitter release (especially of norepinephrine), reduction of insulin secretion, and contraction of smooth muscles. Beta Adrenergic Receptors Beta-1 (β1) Receptors: Location: Mainly located in the heart and kidneys. Mechanism: These receptors typically couple to G_s proteins, leading to an increase in cAMP. Effects: Increase in heart rate (positive chronotropic effect), increase in heart muscle contraction strength (positive inotropic effect), and increased renin release from the kidneys. Beta-2 (β2) Receptors: Location: Primarily found in the lungs, arterial blood vessels, uterus, liver, and skeletal muscle. Mechanism: Also couple to G_s proteins, increasing cAMP. Effects: Relaxation of smooth muscles (such as dilation of bronchial tubes and vasodilation), enhanced muscle and liver glycogenolysis, and increased insulin secretion. Beta-3 (β3) Receptors: Location: Mainly in adipose tissue and also found in the bladder. Mechanism: Like other beta receptors, they couple to G_s proteins to increase cAMP. Effects: Lipolysis in adipose tissue, relaxation of the bladder detrusor muscle.

Answer 24

Gap Junctions Structure: Direct connections formed by channels that link the cytoplasm of two adjacent cells. Mechanism: Allow the direct passage of ions and small molecules, enabling rapid and bidirectional communication. Function: Important for quick synchronization in tissues like cardiac muscle. Chemical Synapses Structure: Comprise a presynaptic terminal, a synaptic cleft, and a postsynaptic neuron. Mechanism: Use neurotransmitters to transfer signals across the synaptic cleft, initiating responses in the postsynaptic cell. Function: Enable complex signal modulation and plasticity, crucial for processes like learning and memory.

Answer 25

Axodendritic Synapses Location: These synapses occur between the axon terminal of one neuron and the dendrites of another. Function: Axodendritic synapses are the most common type of synapse in the nervous system. They are typically involved in transmitting signals toward the cell body of the postsynaptic neuron, often contributing to excitatory or inhibitory inputs that are integrated in the dendrites. Signal Integration: Because dendrites can have many such synapses along their length, they play a key role in integrating multiple synaptic inputs from various sources. Axosomatic Synapses Location: These synapses occur between the axon terminal of one neuron and the soma (cell body) of another. Function: Axosomatic synapses directly influence the postsynaptic neuron's ability to generate an action potential. They are particularly effective at modulating the neuron's activity because they apply their effects closer to the neuron's axon hillock, where action potentials are initiated. Impact on Neuronal Firing: Due to their proximity to the action potential initiation site, changes in membrane potential at axosomatic synapses can have a strong and immediate effect on the neuron’s firing rate.

Answer 26

The term "metazoans" refers to a large group of organisms known collectively as animals. Chara is not a multicellular animal but rather a multicellular algae. Metazoans In metazoans, the action potential primarily involves the movement of sodium (Na+) and potassium (K+) ions across the neuronal membrane: Depolarization: When an action potential is initiated, voltage-gated Na+ channels open, allowing Na+ ions to flow into the cell. This influx of Na+ causes the membrane potential to rapidly rise (depolarize). Repolarization: Shortly after the peak of the depolarization, the Na+ channels close and voltage-gated K+ channels open. K+ then flows out of the cell, driving the membrane potential back down (repolarization). Hyperpolarization: The overshoot of K+ efflux can sometimes lead to the membrane potential becoming more negative than the resting potential (hyperpolarization). The membrane potential then returns to resting levels as ion channels reset and ion gradients are restored by ion pumps. Chara The action potential in Chara, while also dependent on ion movement, involves different ions, particularly calcium (Ca2+) and chloride (Cl-): Initiation and Depolarization: The action potential in Chara is triggered by an influx of Ca2+ ions rather than Na+. This influx depolarizes the cell membrane, similar to the effect of Na+ in metazoans. Repolarization and Hyperpolarization: Following depolarization, Cl- channels open, allowing Cl- ions to leave the cell (since the internal concentration of Cl- is higher in Chara compared to the external environment), contributing to the repolarization of the membrane. Additionally, K+ channels also open to facilitate repolarization. Restoration: As in metazoans, ion pumps ultimately restore the original ion gradient to reset the cell’s membrane potential to its resting state.

Answer 27

Having action potentials with a depolarization phase based on Na+ may be advantageous for systems requiring rapid and frequent firing, such as the nervous system. The speed and efficiency of Na+ channels enable quick and precise communication between neurons, critical for processing and responding to complex information rapidly.

Answer 28

The unidirectional propagation of action potentials along an axon is primarily due to the behavior of voltage-gated sodium and potassium ion channels. Sodium channels open quickly in response to a threshold voltage, allowing Na+ ions to rush in and depolarize the membrane. These channels then inactivate rapidly, preventing the flow of ions and contributing to the absolute refractory period, during which no new action potential can be initiated in the same section of the axon. Potassium channels open with a slight delay and close slowly, repolarizing and slightly hyperpolarizing the membrane, ensuring that the action potential moves forward along the axon rather than backward. This sequential opening and inactivating/closing of ion channels along the axon prevents the reversal of the action potential direction, effectively directing it from the axon hillock toward the synaptic terminals

Answer 29

Reduced Neurotransmitter Release: Voltage-gated calcium channels are crucial for the release of neurotransmitters. When an action potential reaches a synaptic terminal, Ca²⁺ channels open, allowing Ca²⁺ to enter the neuron. This influx of calcium triggers the fusion of synaptic vesicles with the presynaptic membrane and the subsequent release of neurotransmitters into the synaptic cleft. Blocking these channels would inhibit calcium entry, thereby reducing or completely preventing neurotransmitter release. Impaired Synaptic Transmission: Since neurotransmitter release is reduced, the synaptic transmission would be significantly impaired. This means that the signal from the presynaptic neuron would not be effectively transmitted to the postsynaptic neuron, leading to diminished or absent postsynaptic responses. Potential Therapeutic Applications: Drugs that block voltage-gated calcium channels may be useful in conditions where reducing neurotransmitter release is beneficial, such as in the treatment of certain types of neurological disorders like epilepsy, where excessive neuronal activity needs to be controlled. Side Effects: Depending on where the drug acts (i.e., which synapses and parts of the nervous system are affected), there could be a range of side effects due to the widespread role of Ca²⁺ in many cellular processes beyond neurotransmission, such as muscle contraction, hormone secretion, and other cellular signaling pathways.

Answer 30

Interneurons are likely to have more dendrites compared to afferent (sensory) neurons. This is because interneurons play a central role in integrating and processing information within the central nervous system. They often receive input from multiple sources, necessitating a more complex dendritic architecture to accommodate and integrate various synaptic inputs. In contrast, afferent neurons are primarily responsible for transmitting specific types of sensory information from peripheral sensors to the central nervous system, typically having fewer dendrites as their role is more focused on conveying specific sensory data rather than broad integration.

Answer 31

Norepinephrine, acting as a neurotransmitter and hormone, binds to alpha (α) and beta (β) adrenergic receptors, each triggering distinct signal transduction pathways. Alpha receptors, subdivided into α1 and α2, generally mediate constrictive and inhibitory effects through mechanisms involving calcium release or inhibition of cyclic AMP (cAMP), respectively. Beta receptors, including β1, β2, and β3 subtypes, predominantly stimulate cardiac function, smooth muscle relaxation, and lipid breakdown by increasing cAMP levels via activation of the Gs protein. These pathways elucidate the diverse physiological responses influenced by norepinephrine, from vascular tone to metabolic processes, highlighting their therapeutic relevance in conditions like asthma, heart failure, and hypertension.

Answer 32

A single neurotransmitter can have excitatory or inhibitory effects depending on the type of receptor it binds to on different cells. For instance, the neurotransmitter acetylcholine binds to nicotinic receptors to cause excitatory responses by triggering depolarization through the influx of cations. Conversely, when acetylcholine binds to muscarinic receptors, it can cause inhibitory responses by opening potassium channels, leading to hyperpolarization, or by inhibiting adenylate cyclase, reducing cyclic AMP levels. This variation allows the neurotransmitter to have diverse and context-dependent effects, contributing to the complex regulation of physiological processes across different cell types and tissues.

Answer 33

The length constant of a neuron's membrane indicates how far an electrical signal can travel along an axon before it significantly attenuates. It is influenced by the membrane's resistance to current leakage across it and the internal resistance to current flow within the axon. When the diameter of an axon increases, its internal resistance decreases due to a larger cross-sectional area that allows more ions to flow freely. Additionally, a thicker axon has increased membrane resistance because there is less membrane area per unit length for current to leak out. These changes result in an increased length constant, meaning the electrical signal can travel further without losing intensity. This effect contributes to faster signal propagation along larger axons, as the signal degrades less over distance, enabling quicker transmission of nerve impulses.

Answer 34

A Paramecium likely could not generate a typical action potential in water lacking Ca2+ because its action potentials depend primarily on the influx of Ca2+ ions, rather than Na+ ions, which are more typical in vertebrate neurons. In Paramecium and other similar organisms, the depolarization phase of the action potential is driven by the entry of Ca2+ ions into the cell. Without Ca2+, the crucial depolarizing current would be absent, preventing the generation of an action potential. Thus, the absence of Ca2+ in the surrounding water would impair the cellular mechanisms necessary for initiating and propagating electrical signals in these organisms.

Answer 35

Increasing the density of voltage-gated sodium channels on a neuron’s membrane effectively lowers the threshold potential necessary for triggering an action potential. This is because having more sodium channels available means that fewer channels need to open to reach the critical amount of inward current required to depolarize the neuron to the threshold level. As a result, a smaller stimulus can result in sufficient sodium influx to reach the threshold potential, thus making the neuron more sensitive and responsive to smaller inputs. Essentially, the more sodium channels there are, the easier it is for the neuron to reach the depolarization needed to initiate an action potential, thereby lowering the threshold potential.

Answer 36

The after-hyperpolarization phase of an action potential occurs immediately following the repolarization phase, where the membrane potential becomes even more negative than the resting potential. This phase coincides with the relative refractory period, during which a neuron can still fire another action potential, but requires a stronger-than-normal stimulus to do so. This is because many sodium channels are still inactivated and potassium channels remain open, leading to increased potassium efflux that hyperpolarizes the cell. The relative refractory period is crucial for neural signaling as it helps to modulate the frequency and timing of action potentials, ensuring that neural responses are proportionate to stimulus strength. It also contributes to the unidirectional flow of action potentials along an axon, preventing the backward propagation of the signal and aiding in the clear transmission of neural messages across long distances.

Answer 37

If you stimulate an axon near both the axon hillock and the axon terminal simultaneously, you would initiate two action potentials moving towards each other. Normally, action potentials propagate unidirectionally from the axon hillock (where they are typically generated) to the axon terminal due to the refractory state of previously activated membrane segments, which prevents backward propagation. However, in this experimental setup, each action potential would propagate toward the midpoint of the axon. When these action potentials meet, no further propagation would occur beyond their point of collision because the membrane areas where they meet would already be in the refractory state due to the preceding action potential. Thus, each half of the axon would experience an action potential that abruptly stops at the meeting point, preventing the signal from passing beyond this junction.

Answer 38

If a drug were applied to an axon that caused voltage-gated K+ channels to remain open constantly, it would significantly impair the axon's ability to generate action potentials. Normally, the opening of K+ channels follows the opening of Na+ channels during an action potential, contributing to repolarization of the membrane by allowing K+ to flow out of the cell. If K+ channels are permanently open, the membrane potential would likely remain close to or below the resting potential, as constant K+ efflux would keep the membrane hyperpolarized. This hyperpolarization would prevent the membrane from reaching the threshold potential necessary to open voltage-gated Na+ channels, thereby inhibiting the generation of action potentials. This would result in a reduction or complete cessation of neural signaling along that axon.

Answer 39

When neuron A repeatedly fires, even if each individual input is slightly below the threshold potential needed to trigger an action potential, the proximity to the axon hillock means that the depolarizing currents have a shorter distance to travel and are less likely to dissipate before reaching this critical region. Therefore, the cumulative effect of repeated inputs can more effectively and efficiently depolarize the axon hillock to reach the threshold potential, thus initiating an action potential. In contrast, neuron B contacts a dendrite far from the axon hillock. Due to this greater distance, the depolarizing currents generated by its synaptic activity must travel a longer path to reach the axon hillock. This distance increases the likelihood of the currents attenuating or dissipating as they pass through the cellular medium. Additionally, dendrites typically contain fewer voltage-gated sodium channels compared to the axon hillock, making it harder for the depolarization caused by neuron B’s inputs to maintain their strength as they propagate towards the axon hillock. Therefore, even if neuron B fires at the same intensity and frequency as neuron A, its inputs are less likely to cumulatively reach the threshold potential at the axon hillock, thus failing to initiate an action potential.

Answer 40

Selective serotonin reuptake inhibitors (SSRIs) are a class of drugs that specifically inhibit the reuptake of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) from the synaptic cleft back into the presynaptic neuron. By blocking the serotonin transporter (SERT) responsible for this reuptake, SSRIs effectively increase the concentration of serotonin within the synaptic cleft. The primary effect of increasing serotonin levels in the synaptic cleft is to enhance and prolong the activation of serotonin receptors on the postsynaptic neuron. Since serotonin typically causes excitatory postsynaptic potentials (EPSPs), the increased availability of serotonin means that these excitatory signals are more frequent and last longer, leading to a stronger overall excitatory effect on the postsynaptic neuron. This enhanced excitatory signaling can help to correct the neurotransmitter imbalances often observed in conditions like depression, where reduced serotonergic activity is commonly implicated. Therefore, the administration of an SSRI would likely result in an increase in the frequency and magnitude of excitatory responses in the postsynaptic cells targeted by serotonin. This heightened excitatory activity can help to elevate mood, improve emotional responses, and contribute to the overall therapeutic effects of SSRIs in treating depression. This is because the increased excitatory signaling may help to stimulate neural circuits that have been underactive in depressive states, contributing to the improvement of symptoms.

Answer 41

Cnidarians, such as jellyfish, anemones, and corals, have a relatively simple nervous system compared to more complex organisms. They possess a nerve net, a decentralized network of neurons that lacks a central brain or major ganglia seen in more advanced animals. This nerve net allows cnidarians to respond to environmental stimuli through diffuse conduction pathways. The simplicity of their nervous system means that cnidarians do not have clearly defined afferent neurons, interneurons, and efferent neurons like those found in the central nervous systems of more evolved animals. Instead, their neurons are multipolar, capable of both receiving sensory information and sending motor commands, thus serving mixed functions without distinct specializations. This arrangement reflects their basic body plan and lifestyle, which require less complex neural processing.

Answer 42

a group of cells whose cytoplasms are functionally connected either directly or via gap junctions.

Answer 43

1. It is encased in bone or cartilage. 2. It is surrounded by a protective membrane called the meninges. 3. It floats in a cushioning fluid called cerebrospinal fluid (CSF). 4. It is physiologically separated from the rest of the body by the blood-brain barrier.

Answer 44

These three regions, which are found in all vertebrate brains, are called the: 1. hindbrain, or rhombencephalon. 2. the midbrain, or mesencephalon. 3. The forebrain, or prosencephalon.

Answer 45

Birds and mammals.

Answer 46

Mammalian brains have grey matter around the outside, forming the Isocortex. In other vertebrates, their brains have an outer layer of white matter surrounding an inner core of gray matter.

Answer 47

FALSE The midbrain is greatly reduced in mammals In fish and amphibians, the midbrain coordinates reflex responses to auditory and visual stimuli and is the primary center for coordinating and initiating behavioral responses. In contrast, in mammals it is much smaller relative to the rest of the brain and primarily serves as a relay center

Answer 48

True! In mammals, the forebrain is involved in processing and integrating sensory information, and in coordinating behavior. The forebrain consists of the cerebrum, the thalamus, the epithalamus, and the hypothalamus.

Answer 49

True. The hypothalamus is located at the base of the forebrain just below the thalamus. The hypothalamus controls the internal organs and interacts with the autonomic nervous system, which we discuss later in this chapter. In addition, it regulates the secretion of pituitary hormones (see Chapter 4: Cell Signaling and Endocrine Regulation). The hypothalamus plays an important role in regulating the endocrine system and thus serves as a crucial link between the nervous and endocrine systems. Indeed, the primary function of the hypothalamus is to maintain the body’s homeostatic balance. The hypothalamus regulates body temperature, fluid balance, blood pressure, body weight, and many bodily sensations such as hunger, thirst, pleasure, and sex drive.

Answer 50

Ataxia: This is perhaps the most characteristic symptom of cerebellar damage. Ataxia refers to a lack of muscle coordination during voluntary movements, such as walking or picking up objects, which may appear clumsy or imprecise. Dysmetria: This involves difficulty in judging distances or scaling movements. Individuals with dysmetria may overestimate or underestimate the movement needed to reach a target, resulting in movements that fall short or overshoot their goal. Tremor: Specifically, an intention tremor may occur, which is a shaking that becomes more pronounced when an individual moves towards a target or performs some task, contrasting with the resting tremor seen in Parkinson's disease. Nystagmus: This is an involuntary movement of the eyes, typically a rapid, repetitive oscillation that can affect the ability to see clearly and can impair balance and coordination. Poor Balance: The cerebellum is important for maintaining balance and posture. Damage can lead to a wide-based, staggering gait, and individuals may struggle to maintain stability when standing or walking, often swaying or tilting. Slurred Speech: Known as ataxic dysarthria, this condition involves irregular, jerky speech that can be hard to understand due to poor control of the rhythm and pitch of voice. Hypotonia: Muscle tone may decrease, leading to muscles that feel unusually soft and may be less responsive to reflex testing. Vertigo or Dizziness: Some individuals experience these symptoms, which are related to the disturbed spatial orientation and impaired balance.

Answer 51

Dominance and Specialization: The midbrain in mammals has evolved with more specialized regions like the substantia nigra, reflecting complex behavioral and neuromodulatory functions. In contrast, in other vertebrates, it tends to play a more central role in direct sensory processing and immediate motor responses. Relative Size: The relative size of the midbrain is generally smaller in mammals compared to that in other vertebrates, where it can be a major component of the brain. Functional Specialization: Mammals have a greater degree of functional specialization in the midbrain structures, particularly with components involved in advanced cognitive and emotional processing, such as those linked to the dopaminergic systems.

Answer 52

Many organs need to be controlled for rest and digest, and then stopped during times of fight or flight.

Answer 53

The neurosecretory chromaffin cells of the adrenal medulla primarily express nicotinic acetylcholine receptors. These receptors are ionotropic, meaning they are directly linked to ion channels and mediate fast synaptic transmission. The role of these receptors in chromaffin cells is crucial for the physiological function of the adrenal medulla, which involves the secretion of catecholamines (mainly adrenaline and noradrenaline) into the bloodstream in response to stress. When the sympathetic nervous system is activated during stress, preganglionic sympathetic neurons release acetylcholine. This neurotransmitter binds to the nicotinic acetylcholine receptors on the chromaffin cells, leading to depolarization of these cells. The depolarization triggers the opening of voltage-gated calcium channels, resulting in an influx of calcium ions. The increase in intracellular calcium then stimulates the exocytosis of vesicles containing adrenaline and noradrenaline, releasing these hormones into the circulation.

Answer 54

Most reflex arcs have a more complex structure, and are called _polysynaptic_ reflex arcs, because they contain synapses between more than two types of neurons.

Answer 55

Pattern generators govern many important physiological processes and simple rhythmic behaviors such as chewing, walking, swimming, and breathing. Pattern generators are groups of neurons that produce self-sustaining patterns of depolarization, independent of sensory input. The simplest form of organization involves a pacemaker cell. A pacemaker cell generates a spontaneous rhythmic depolarization, and thus controls the firing of all the cells in the network.

Answer 56

The main difference between a monosynaptic reflex arc and a polysynaptic reflex arc lies in the number of synapses involved in transmitting the neural signal. In a monosynaptic reflex arc, there is only one synapse between the sensory neuron and the motor neuron, resulting in a rapid and direct response. This simplicity allows for quicker reflex actions, as seen in the knee-jerk reflex. Conversely, in a polysynaptic reflex arc, the signal passes through one or more interneurons in the central nervous system before reaching the motor neuron, involving multiple synapses. This additional processing introduces a slight delay in the reflex response but allows for more complex and adaptive responses, as seen in withdrawal reflexes where multiple muscles are coordinated to remove a body part from a harmful stimulus.

Answer 57

Pattern generators, also known as central pattern generators (CPGs), are neural networks in the nervous system that produce rhythmic patterned outputs without sensory feedback. They are crucial for generating coordinated patterns of motor activity, such as those required for various types of locomotion and repetitive movements. Behaviors that typically involve pattern generators include walking, swimming, flying in certain insects, breathing, and chewing. Each of these actions requires a rhythmic, repetitive motor pattern that can be initiated and maintained even in the absence of rhythmic sensory input or feedback. In vertebrates, the pattern generator responsible for governing walking is primarily located in the spinal cord. This neural circuitry in the spinal cord can generate the necessary rhythmic muscle contractions that control limb movements necessary for walking. The activation and modulation of this spinal pattern generator are influenced by signals from the brain, particularly from areas such as the brainstem and the motor cortex, which adjust the pattern based on environmental cues and higher-order commands. However, the core capability to produce a basic walking rhythm resides within the spinal network itself, allowing even some spinalized animals to display stepping movements under certain conditions.

Answer 58

Habituation is a decrease in response to a repeated benign stimulus. For example, if a person hears a sound repeatedly and learns it is not harmful, they may gradually stop paying attention to it. This process helps organisms to focus on novel or significant stimuli and ignore irrelevant, repetitive inputs. Sensitization is an increase in responsiveness to a stimulus, often following a strong or noxious stimulus. For example, after experiencing a painful shock, an organism might react more strongly to a light touch. Sensitization can make an organism more alert to stimuli that may be associated with danger or injury.

Answer 59

What is long-term potentiation? Long-term potentiation (LTP) is a persistent increase in synaptic strength following high-frequency stimulation of a chemical synapse, commonly associated with learning and memory in the brain, particularly in the hippocampus. What kinds of evidence suggest that long-term potentiation is involved in learning and memory in mammals? Evidence supporting the role of LTP in learning and memory includes: Neural Location: LTP occurs in brain regions like the hippocampus that are essential for memory. Behavioral Correlations: Animals with impaired LTP often show learning deficits, while enhancing LTP can improve learning. Molecular Similarities: The molecular mechanisms underlying LTP, such as changes in receptor activity and gene expression, are also activated during learning processes. Intervention Studies: Drugs that block LTP mechanisms can disrupt learning and memory, indicating a direct role in cognitive functions.

Answer 60

The hypothalamus can directly sense the osmolarity of the blood, an essential function for maintaining body fluid balance, despite the typically restrictive blood-brain barrier (BBB). This capability is facilitated by specialized regions within the hypothalamus called circumventricular organs, particularly the organum vasculosum of the lamina terminalis (OVLT) and the subfornical organ (SFO), which lack a typical BBB. These areas have more permeable capillaries, allowing them to directly monitor blood osmolarity without the barrier's usual impediments. When these regions detect high osmolarity, indicating dehydration, they trigger responses to conserve water. The hypothalamic neurons, particularly in the paraventricular and supraoptic nuclei, respond by releasing antidiuretic hormone (ADH, also known as vasopressin) from the posterior pituitary gland into the bloodstream. ADH acts on the kidneys to promote water reabsorption, thereby adjusting the concentration of urine and conserving water to help normalize blood osmolarity.

Answer 61

Cnidarians, such as jellyfish, have a comparatively simple nervous system known as a nerve net, which contrasts sharply with the more centralized nervous systems found in most other animals. Unlike vertebrates that have a central nervous system (CNS) consisting of a brain and spinal cord, cnidarians' nerve nets are diffuse arrangements of interconnected nerve cells (neurons) that extend throughout their bodies. This structure allows for basic responses to environmental stimuli from all directions, but lacks the centralization needed for complex processes. Additionally, cnidarians do not have a brain or true specialized sensory organs, but they can still perform essential functions like movement and feeding through this more primitive and decentralized nervous system.

Answer 62

Spinal nerves are the nerves that emerge directly from the spinal cord. There are 31 pairs of spinal nerves, each originating from a specific segment of the spinal cord. These nerves branch out to different parts of the body and are responsible for transmitting sensory information from the body to the spinal cord and motor information from the spinal cord to the muscles. Each spinal nerve is a mixed nerve, meaning it carries both afferent (sensory) and efferent (motor) fibers. They play a crucial role in the body's ability to react to the environment by facilitating both sensation and movement. These nerves are categorized into five groups based on where they emerge from the spine: cervical, thoracic, lumbar, sacral, and coccygeal nerves, each serving different regions of the body.

Answer 63

If you compare a reptile and a mammal of the same body size, the mammal is likely to have the larger brain. Mammals generally have larger brains relative to their body size compared to reptiles, a feature often associated with more complex behaviors, higher metabolic rates, and greater sensory processing capabilities. The part of the brain that would be most different between these two groups is the neocortex (or isocortex), which is far more developed in mammals. The neocortex is the outer layer of the cerebral hemispheres and is involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, spatial reasoning, and language. In mammals, the neocortex is typically larger and more convoluted, providing greater surface area and thus supporting more complex cognitive functions. Reptiles, on the other hand, have a simpler cerebral cortex that lacks the layered structure of the mammalian neocortex. Their brains emphasize other areas like the olfactory bulbs and the midbrain, reflecting different sensory and motor priorities. These differences underscore significant variations in brain architecture and functionality across these animal groups.

Answer 64

Efficient Processing: By arranging neurons that process similar types of information close to one another, the brain maximizes efficiency. For example, neurons that process visual information from adjacent parts of the visual field are located near each other in the visual cortex. This proximity allows for rapid and effective communication and integration of sensory data. Specialized Regions: The cortex is divided into areas that are specialized for different functions. For example, the visual cortex is responsible for processing visual information, the auditory cortex handles auditory data, and different parts of the motor cortex control different parts of the body. This specialization supports the complex processing needs required for different sensory inputs and motor outputs. Plasticity and Adaptation: Topographic organization allows for plasticity, meaning the brain can adapt based on experiences. For example, if one part of the body is used more frequently, the cortical area devoted to processing information from that part can expand. This plasticity is crucial for learning and adapting to new environments or after injury.

Answer 65

The phenomenon of basal tone in the autonomic nervous system (ANS) refers to the continuous, moderate level of activity maintained by the sympathetic and parasympathetic branches of the ANS, even in the absence of external stimuli. This basal tone is crucial because it provides a ready state of balance and readiness in the body, allowing rapid adjustments to maintain homeostasis. For instance, in the cardiovascular system, the basal tone of the sympathetic nervous system helps maintain a consistent blood pressure and heart rate under normal conditions. If a quick response is needed—such as during stress or to avoid danger—the system can quickly ramp up or decrease its activity based on immediate needs. By maintaining this basal activity, the ANS ensures that the body's internal environment remains stable and can adapt flexibly to changes, supporting overall health and well-being.

Answer 66

A pattern generator, also known as a central pattern generator (CPG), is a neural circuit within the central nervous system that produces coordinated patterns of rhythmic activity without requiring sensory feedback from the muscles or external stimuli. These circuits are crucial for generating repetitive motor activities such as walking, swimming, and breathing. A CPG is typically formed by a network of interconnected neurons that have intrinsic properties enabling them to generate rhythmic outputs. These neurons can interact through excitatory and inhibitory synapses to produce a balanced and coordinated output that drives motor behaviors. The output from a CPG can be modulated by sensory inputs and higher brain functions to adapt to changing conditions, but its core ability to generate rhythmic patterns operates independently of these inputs, allowing for consistent and efficient control of repetitive motor tasks.

Answer 67

The limbic system is a complex set of structures in the brain that includes the hippocampus, amygdala, and parts of the thalamus and hypothalamus, among others. It plays a crucial role in regulating emotions, memory, and behavior. This system is central to the formation and retrieval of memories, particularly emotional memories, which is essential for learning and survival. The amygdala, for instance, is key in processing fear responses and emotional reactions, while the hippocampus is involved in the formation of new memories and spatial orientation. The limbic system's connections with higher cortical areas enable it to influence decision-making and behavioral responses based on emotional and memory-related inputs, making it fundamental to emotional intelligence and the management of social behaviors.

Answer 68

The hypothalamus plays a central role in regulating sleep-wake cycles primarily through its interactions with various neurotransmitter systems and the control of circadian rhythms. It contains several key nuclei, including the suprachiasmatic nucleus (SCN), which is considered the body's master clock. The SCN coordinates daily physiological rhythms by responding to light cues sensed by the eyes and signaling other parts of the brain to promote wakefulness or sleepiness. Additionally, the hypothalamus regulates the release of hormones like melatonin from the pineal gland, which increases at night to promote sleep. Through these mechanisms, the hypothalamus helps maintain the synchronization of the body's internal clock with the external environment, facilitating consistent and restorative sleep patterns.

Answer 69

Nicotinic acetylcholine receptors, which are ligand-gated ion channels, are activated by acetylcholine and nicotine. When these receptors on muscle cells and postganglionic neurons in the sympathetic nervous system are stimulated by nicotine from gum, it mimics the action of acetylcholine. This activation increases the firing of neurons that release neurotransmitters which, among other effects, elevate heart rate and stimulate muscle contractions. In nonsmokers, who are not habituated to nicotine, this can result in marked physiological responses such as a rapid increase in heart rate and muscle tremors, particularly in the hands, as these muscles become inadvertently stimulated by the nicotine-induced neuronal activity.

Answer 70

If the autonomic nervous system utilized different neurotransmitters at the preganglionic level between the sympathetic and parasympathetic divisions, but the same neurotransmitters postganglionically, it would still function, albeit with altered dynamics. Preganglionic neurotransmitters mainly determine how signals are relayed from the central nervous system to the autonomic ganglia. Typically, both systems use acetylcholine at this stage. Differences here could affect how signals are initiated and modulated in each system. However, since the final action on target organs is determined by postganglionic neurotransmitters, if these remained consistent (e.g., norepinephrine for sympathetic and acetylcholine for parasympathetic), the ultimate effects on target organs would be less altered. The key impact would be on the efficiency and modulation of signal transmission from the CNS to the autonomic ganglia, possibly leading to differences in response speeds or intensities.

Answer 71

Nerve gases like sarin inhibit acetylcholinesterase, an enzyme responsible for breaking down acetylcholine in synapses. This leads to an accumulation of acetylcholine, which continuously stimulates neurons. In the context of the parasympathetic nervous system, where acetylcholine is a primary neurotransmitter, this results in overstimulation of parasympathetic functions. Symptoms of sarin poisoning would thus include excessive salivation, sweating, miosis (constriction of the pupils), bronchoconstriction, slowed heart rate, and potentially digestive disturbances, all indicative of heightened parasympathetic activity. Sarin would also affect other parts of the nervous system, including the somatic nervous system, leading to muscle twitching, paralysis, and possibly fatal respiratory failure due to prolonged contraction of respiratory muscles. Additionally, the sympathetic nervous system could be impacted, causing further complications in autonomic balance and function.

Answer 72

In the context of neural plasticity, habituation and sensitization involve distinct roles of presynaptic and postsynaptic mechanisms. Habituation, a decrease in response to a repetitive, benign stimulus, primarily involves presynaptic changes such as the reduced release of neurotransmitters. This reduction occurs because repetitive stimulation leads to a depletion of available neurotransmitter or a down-regulation of its release mechanisms. On the other hand, sensitization, an increased response to a stimulus following a potent or noxious stimulus, typically involves both presynaptic enhancements, such as increased neurotransmitter release, and postsynaptic changes, like receptor upregulation or increased postsynaptic sensitivity. This combination of mechanisms ensures a heightened responsiveness to stimuli perceived as significant or threatening.

Answer 73

The hypothalamus receives inputs from various brain regions, reflecting its central role in integrating bodily functions. Key inputs come from the limbic system, particularly the amygdala and hippocampus, which are crucial for emotional processing and memory, respectively. The thalamus relays sensory and motor signals to the hypothalamus. Additionally, the prefrontal cortex sends information related to higher cognitive functions, decision-making, and social behavior. The brainstem, including structures such as the reticular formation, transmits vital information regarding autonomic functions. These diverse inputs enable the hypothalamus to coordinate and regulate body temperature, hunger, thirst, fatigue, sleep, and circadian rhythms.

Answer 74

CaMKII (calcium/calmodulin-dependent protein kinase II) is a crucial enzyme in the brain's signaling pathways that are vital for learning and memory. This kinase is particularly important in the process of long-term potentiation (LTP), a phenomenon strongly associated with the strengthening of synapses and the formation of memories. In the hippocampus, a brain area essential for memory formation, CaMKII phosphorylates various proteins that are involved in strengthening synaptic connections by increasing the number and sensitivity of glutamate receptors at the synapse. When mice lack the gene encoding CaMKII, this crucial signaling pathway is disrupted, resulting in a failure to properly execute synaptic plasticity and, consequently, impairments in memory formation and retention.