The Neuroscience Of Motor Control

Front 1

Cerebral Peduncles

Back 1

Prominent structures (peduncle means "stalk") observed on the ventral surface of the midbrain comprising the tegmentum and the more ventrally located white matter tracts (basis pedunculi). In common usage, the term cerebral peduncle denotes these white matter tracts, which contain the efferent axons of the cerebral cortex that project to the brainstem and spinal cord.

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Corticospinal system

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pathway from the cerebral cortex to the brainstem and spinal cord. Components of this pathway include axons originating in the posterior frontal lobe and projecting to lower motor neurons in the brainstem (corticobulbar) and ventral horn of the spinal cord (corticospinal), and axons of the corticopontine tract, which convey command signals from the cerebral cortex to nuclei in the base of the pons.

Front 3

Rubrospinal tract

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Descending motor pathway by which upper motor neurons in the red nucleus project to lower motor neurons in the lateral ventral horn of the cervical spinal cord; however, this projection is vestigial and variable in humans and its functional significance is uncertain.

Front 4

Stretch Reflex ex knee jerk

Back 4

sensory organ muscle spindle responds to muscle stretch...
ending in tendons, (response to tension) if muscle becomes to tight muscle will relax
the muscle spindle - have sensory ending fires AP when muscle stretch
also have gamma motor neurons
intrafusal muscle fiber- tightens up the fiber

Front 5

Lateral pathways

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control both proximal and distal muscles and are responsible for most voluntary movements of arms and legs. They include the
lateral corticospinal tract
rubrospinal tract

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medial pathways

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control axial muscles and are responsible for posture, balance, and coarse control of axial and proximal muscles. They include the
vestibulospinal tracts (both lateral and medial)
reticulospinal tracts (both pontine and medullary)
tectospinal tract
anterior corticospinal tract

Front 7

Corticospinal tracts

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The corticospinal tract (along with the corticobulbar tract) is the primary pathway that carries the motor commands that underlie voluntary movement. The lateral corticospinal tract is responsible for the control of the distal musculature and the anterior corticospinal tract is responsible for the control of the proximal musculature. A particularly important function of the lateral corticospinal tract is the fine control of the digits of the hand. The corticospinal tract is the only descending pathway in which some axons make synaptic contacts directly onto alpha motor neurons. This direct cortical innervation presumably is necessary to allow the powerful processing networks of the cortex to control the activity of the spinal circuits that direct the exquisite movements of the fingers and hands. The percentage of axons in the corticospinal tract that innervate alpha motor neurons directly is greater in humans and nonhuman primates than in other mammals, presumably reflecting the increased manual dexterity of primates. Damage to the corticospinal tract results in a permanent loss of the fine control of the extremities. Although parallel descending pathways can often recover the function of more coarse movements, these pathways are not capable of generating fine, skilled movements. In addition to the fine control of distal muscles, the corticospinal tract also plays a role in the voluntary control of axial muscles.

Front 8

Rubrospinal tract

Back 8

The rubrospinal tract is an alternative by which voluntary motor commands can be sent to the spinal cord. Although it is a major pathway in many animals, it is relatively minor in humans. Activation of this tract causes excitation of flexor muscles and inhibition of extensor muscles. The rubrospinal tract is thought to play a role in movement velocity, as rubrospinal lesions cause a temporary slowness in movement. In addition, because the red nucleus receives most of its input from the cerebellum, the rubrospinal tract probably plays a role in transmitting learned motor commands from the cerebellum to the musculature. The red nucleus also receives some input from the motor cortex, and it is therefore probably an important pathway for the recovery of some voluntary motor function after damage to the corticospinal tract

Front 9

Vestibulospinal tracts

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The vestibulospinal tracts mediate postural adjustments and head movements. They also help the body to maintain balance. Small movements of the body are detected by the vestibular sensory neurons, and motor commands to counteract these movements are sent through the vestibulospinal tracts to appropriate muscle groups throughout the body. The lateral vestibulospinal tract excites antigravity muscles in order to exert control over postural changes necessary to compensate for tilts and movements of the body. The medial vestibulospinal tract innervates neck muscles in order to stabilize head position as one moves around the world. It is also important for the coordination of head and eye movements.

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Reticulospinal tracts

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The reticulospinal tracts are a major alternative to the corticospinal tract, by which cortical neurons can control motor function by their inputs onto reticular neurons. These tracts regulate the sensitivity of flexor responses to ensure that only noxious stimuli elicit the responses. Damage to the reticulospinal tract can thus cause harmless stimuli, such as gentle touches, to elicit a flexor reflex. The reticular formation also contains circuitry for many complex actions, such as orienting, stretching, and maintaining a complex posture. Commands that initiate locomotor circuits in the spinal cord are also thought to be transmitted through the medullary reticulospinal tract. Thus, the reticulospinal tracts are involved in many aspects of motor control, including the integration of sensory input to guide motor output

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explain the modulation of decending pathways on spinal circuits in reflex modulation

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Another critical function of the descending motor pathways is to modulate the reflex circuits in the spinal cord. The adaptiveness of spinal reflexes can change depending on the behavioral context; sometimes the gain (strength) or even the sign (extension vs. flexion) of a reflex must be changed in order to make the resulting movement adaptive. The descending pathways are responsible for controlling these variables. For example, consider the flexor reflex under two conditions.

Imagine a situation in which you want to pick up a dish from the stove top, but you are uncertain whether it is hot or cold. You may attempt to lightly touch the surface, and this will often lower the threshold of the flexor reflex, making you more likely to pull your hand away even if the dish is not particularly hot. (You may even withdraw your hand numerous times before even touching the dish!) Descending pathways have lowered the threshold for producing the reflex in this case, making it easier for a weaker nociceptive input to trigger the reflex; these pathways can also change the gain of the reflex, making the withdrawal response greater than usual.
Imagine now picking up the dish in order to move it to the table. As you hold the dish, more of its heat begins to transfer to your hand, and it starts to get quite hot. Rather than dropping the dish and spilling your dinner all over the floor, you rush to the table to put it down, before withdrawing your hand and wishing you had used an oven mitt. In this case, the descending pathways inhibited the flexor response.

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alpha-gamma coactivation

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Alpha motor neurons innervate extrafusal muscle fibers, which provide the force for a muscle contraction. Gamma motor neurons innervate the ends of intrafusal fibers and help to maintain the tautness of muscle spindles, such that they are sensitive to changes of muscle length over a wide range. In order to work adaptively, the activity of alpha and gamma motor neurons must be coordinated. Thus, whenever motor commands are sent by descending pathways to alpha motor neurons, the appropriate compensating commands are sent to gamma motor neurons. This coordination of alpha-gamma motor commands is called alpha-gamma coactivation, and the adjustment of spindle sensitivity by gamma activation is called gamma bias.

Front 13

what does gamma bias mean

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Alpha-gamma coactivation solves this problem by relaxing the contraction of the intrafusal fibers of the antagonist muscle, allowing the muscle to be stretched without triggering the stretch reflex during a voluntary movement.

Front 14

The lateral corticospinal tract...

A. Undergoes a 50% decussation in the caudal medulla.

B. Arises exclusively from the primary motor cortex.

C. Is an uncrossed pathway.

D. Plays a major role in the fine control of distal musculature.

E. Terminates primarily in the posterior (dorsal) horn.

Back 14

Plays a major role in the fine control of distal musculature.

Front 15

In reciprocal excitation of the Golgi tendon reflex, stimulation of

Back 15

Ib afferent fibers causes excitation of antagonist muscles

Front 16

What is the primary motor cortex general function

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does not generally control individual muscles directly, but rather appears to control individual movements or sequences of movements that require the activity of multiple muscle groups. Alpha motor neurons in the spinal cord, in turn, encode the force of contraction of groups of muscle fibers using the rate code and the size principle. Thus, in accordance with the concept of hierarchical organization of the motor system, the information represented by motor cortex is a higher level of abstraction than the information represented by spinal motor neurons.

Front 17

Primary motor cortex neurons fire 5-100 msec before the onset of a movement

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Thus, rather than firing as the result of muscle activity, these neurons are involved in relaying motor commands to the alpha motor neurons that eventually cause the appropriate muscles to contract.

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Primary motor cortex encodes the direction of movement

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. Many neurons in the primary motor cortex are selective for a particular direction of movement. For example, one cell may fire strongly when the hand is moved to the left, whereas it will be inhibited when the hand is moved to the right

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Primary motor cortex encodes the force of a movement.

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The amount of force required to raise the arm from one location to another is much greater if one is holding a bowling ball than if one is holding a balloon. Many neurons in primary motor cortex encode the amount of force that is necessary to make such a movement (Figure 3.7). Note the distinction between movement force and muscle force. Whereas a minority of primary motor cortex neurons encodes individual muscle force, a larger number encodes the amount of force necessary for a particular movement, regardless of which individual muscles are used. Alpha motor neurons, in turn, translate the commands of the motor cortex neurons and control the amount of force generated by individual muscles to accomplish that movement, under the principles of the rate code and the size principle.

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Pre motor cortex general function

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The premotor cortex sends axons to the primary motor cortex as well as to the spinal cord directly. It performs more complex, task-related processing than primary motor cortex. Stimulation of premotor areas in the monkey at a high level of current produces more complex postures than stimulation of the primary motor cortex. The premotor cortex appears to be involved in the selection of appropriate motor plans for voluntary movements, whereas the primary motor cortex is involved in the execution of these voluntary movements.

Front 21

Premotor cortex neurons signal the preparation for movement

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Monkeys were trained to make a particular movement in response to a visual signal, with a variable delay between the onset of the signal and the onset of the movement (Figure 3.10). Recordings from premotor cortex have shown that many neurons fire selectively in the delay interval, for many seconds before the onset of the movement. A particular neuron will fire when the monkey is preparing to make a movement to the left, for example, but will be silent when the monkey is preparing to make a movement to the right. Thus, the firing of this type of neuron does not cause the movement itself, but appears to be involved in preparing the monkey to make the correct movement when the “Go” signal is given. This type of neuron is called a motor-set neuron, as it fires when the monkey is preparing, or getting set, to make a movement.

Front 22

Premotor cortex neurons signal various sensory aspects associated with particular motor acts.

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Some premotor neurons fire when the animal is performing a particular action, such as breaking a peanut (Figure 3.11). Interestingly, the same neuron fires selectively when the animal sees another monkey or person breaking the peanut. It also fires selectively to the sound of a peanut shell being broke, even without any visual or motor activity. These neurons are called “mirror” neurons, because they respond not only to a particular action of the monkey but also to the sight (or sound) of another individual performing the same action

Front 23

Premotor cortex is sensitive to the behavioral context of a particular movement.

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The premotor cortex of human subjects was imaged with functional MRI as they observed video of a hand grasping a cup (Figure 3.12). In one condition, the cup was full and surrounded by full plates of food; the implication was that the person was grasping the cup to take a drink. In the other condition, the cup was empty and surrounded by dirty dishes; the implication was that the person was grasping the cup to clear the table. In this experiment, the premotor cortex was more active when subjects viewed the former video than the latter, even though the movements were the same. Thus, premotor cortex neurons are sensitive to the inferred intentions of a movement, not just the movement itself, as deduce from the behavioral context in which the movement occurred.

Front 24

supplementary motor area (SMA

Back 24

is involved in programming complex sequences of movements and coordinating bilateral movements. Whereas the premotor cortex appears to be involved in selecting motor programs based on visual stimuli or on abstract associations, the supplementary motor area appears to be involved in selecting movements based on remembered sequences of movements.

Front 25

A corticospinal neuron in primary motor cortex can do all of the following EXCEPT:

A. Project to multiple motor neuron pools in the spinal cord.

B. Participate in the initiation of movement.

C. Code for the amount of force of individual muscles.

D. Code for the direction of movement.

E. Code for the extent of movement

Back 25

C. Code for the amount of force of individual muscles. This answer is CORRECT!

This is a FALSE statement. Motor cortex neurons code for the force of individual movements, not individual muscles. Lower motor neurons (alpha motor neurons) encode the force of individual muscles.

Front 26

Basal Ganglia Afferents

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The striatum is the main recipient of afferents to the basal ganglia

Front 27

Basal Ganglia Efferents

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The major output structures of the basal ganglia are the globus pallidus internal segment (GPint) and the substantia nigra pars reticulata (SNr) (Figure 4.3). Both of these structures make GABAergic, inhibitory connections on their targets.

Front 28

Two pathways process signals in the basal ganglia

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There are two distinct pathways that process signals through the basal ganglia: the direct pathway and the indirect pathway. These two pathways have opposite net effects on thalamic target structures. Excitation of the direct pathway has the net effect of exciting thalamic neurons (which in turn make excitatory connections onto cortical neurons). Excitation of the indirect pathway has the net effect of inhibiting thalamic neurons (rendering them unable to excite motor cortex neurons). The normal functioning of the basal ganglia apparently involves a proper balance between the activity of these two pathways. One hypothesis is that the direct pathway selectively facilitates certain motor (or cognitive) programs in the cerebral cortex that are adaptive for the present task, whereas the indirect pathway simultaneously inhibits the execution of competing motor programs. An upset of the balance between the direct and indirect pathways results in the motor dysfunctions that characterize the extrapyramidal syndrome

Front 29

Functions of the Basal Ganglia

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It appears that the basal ganglia is involved in the enabling of practiced motor acts and in gating the initiation of voluntary movements by modulating motor programs stored in the motor cortex and elsewhere in the motor hierarchy (Figure 4.6). Thus, voluntary movements are not initiated in the basal ganglia (they are initiated in the cortex); however, proper functioning of the basal ganglia appears to be necessary in order for the motor cortex to relay the appropriate motor commands to the lower levels of the hierarchy.

Front 30

What is the function of the tonic inhibitory output of the basal ganglia

Back 30

Recall from the Motor Cortex chapter that stimulating the motor cortex of monkeys at various locations results in stereotyped sequences of movements, such as bringing the hand to the mouth or adopting a defensive posture. It appears that a number of “primitive” motor programs are stored in the cortex, and motor control may require the activation of these elemental motor programs in the precise temporal order to accomplish a sophisticated motor plan. It is important that only one motor program be active at a given time, however, such that one motor act (e.g., use hand to bring food to the mouth) is not competing with a conflicting motor act (e.g., use hand to shield face from dangerous object). It is thought that the basal ganglia is normally active in suppressing inappropriate motor programs, and that activation of the direct pathway temporarily releases one motor program from inhibition, enabling it to be executed by the organism. Thus, the basal ganglia act as a gate that enables the execution of automatic programs in the hierarchy.

Front 31

Nigrostriatal pathway and Parkinson’s disease

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Parkinson’s disease is characterized by slowness or absence of movement (bradykinesia or akinesia), rigidity, and a resting tremor (especially in the hands and fingers). Patients have difficulty initiating movements, and once initiated the movements are abnormally slow. The cause of Parkinson’s disease is the loss of the dopaminergic neurons in the substantia nigra pars compacta (Figure 4.10). From one’s knowledge of the effects of the nigrostriatal pathway on the direct and indirect pathways, it becomes straightforward to see why the loss of this pathway results in the poverty of movement symptomatic of Parkinson’s disease. Because the nigrostriatal pathway excites the direct pathway and inhibits the indirect pathway, the loss of this input tips the balance in favor of activity in the indirect pathway. Thus, the GPint neurons are abnormally active, keeping the thalamic neurons inhibited. Without the thalamic input, the motor cortex neurons are not as excited, and therefore the motor system is less able to execute the motor plans in response to the patient’s volition.

Front 32

Indirect pathway and Huntington’s disease

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The symptoms of Huntington’s disease are in many respects the opposite of the symptoms of Parkinson’s disease. Huntington’s disease is characterized by choreiform movements: involuntary, continuous movement of the body, especially of the extremities and face. Often these movements resemble pieces of adaptive movements, but they occur involuntarily and without behavioral significance. Huntington’s disease results from the selective loss of striatal neurons in the indirect pathway (Figure 4.10). Thus, the balance between the direct and indirect pathways becomes tipped in favor of the direct pathway. Without the normal inhibitory influence on the thalamus that is provided by the indirect pathway, thalamic neurons can fire randomly and inappropriately, causing the motor cortex to execute motor programs with no control by the patient.

Front 33

Which of the basal ganglia nuclei receive direct cortical input?

Back 33

Caudate and putamen. This answer is CORRECT!

The caudate and putamen are the only parts of the basal ganglia that receive direct cortical input

Front 34

All of the following statements about the basal ganglia are correct EXCEPT:

A. The net effect of excitation of the direct pathway is to inhibit cortex.

B. Dopaminergic neurons of the substantia nigra signal unexpected reward or unexpected absence of reward.

C. The basal ganglia have both motor and cognitive functions.

D. The subthalamic nucleus is the origin of the only purely excitatory pathway within the basal ganglia intrinsic circuitry.

E. Parkinson's disease results from damage to the basal ganglia.

Back 34

All of the following statements about the basal ganglia are correct EXCEPT:

A. The net effect of excitation of the direct pathway is to inhibit cortex.

Front 35

1 Function of the Cerebellum

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Maintenance of balance and posture. The cerebellum is important for making postural adjustments in order to maintain balance. Through its input from vestibular receptors and proprioceptors, it modulates commands to motor neurons to compensate for shifts in body position or changes in load upon muscles. Patients with cerebellar damage suffer from balance disorders, and they often develop stereotyped postural strategies to compensate for this problem (e.g., a wide-based stance).

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2 function of the cerebellum

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Coordination of voluntary movements. Most movements are composed of a number of different muscle groups acting together in a temporally coordinated fashion. One major function of the cerebellum is to coordinate the timing and force of these different muscle groups to produce fluid limb or body movements.

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3 function of the cerebellum

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Motor learning. The cerebellum is important for motor learning. The cerebellum plays a major role in adapting and fine-tuning motor programs to make accurate movements through a trial-and-error process (e.g., learning to hit a baseball).

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4 function of the cerebellum

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Cognitive functions. Although the cerebellum is most understood in terms of its contributions to motor control, it is also involved in certain cognitive functions, such as language. Thus, like the basal ganglia, the cerebellum is historically considered as part of the motor system, but its functions extend beyond motor control in ways that are not yet well understood.

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Vestibulocerebellum

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The vestibulocerebellum comprises the flocculonodular lobe and its connections with the lateral vestibular nuclei. Phylogenetically, the vestibulocerebellum is the oldest part of the cerebellum. As its name implies, it is involved in vestibular reflexes (such as the vestibuloocular reflex; see below) and in postural maintenance.

Front 40

Spinocerebellum.

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As its name implies, it receives major inputs from the spinocerebellar tract. Its output projects to rubrospinal, vestibulospinal, and reticulospinal tracts. It is involved in the integration of sensory input with motor commands to produce adaptive motor coordination.

Front 41

Cerebrocerebellum.

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The cerebrocerebellum is the largest functional subdivision of the human cerebellum, comprising the lateral hemispheres and the dentate nuclei. Its name derives from its extensive connections with the cerebral cortex, via the pontine nuclei (afferents) and the VL thalamus (efferents). It is involved in the planning and timing of movements. In addition, the cerebrocerebellum is involved in the cognitive functions of the cerebellum.

Front 42

Damage to Cerebellum Produces Movement Disorders

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Most of our movements involve the coordinated activity of many muscle groups and different joints to produce a smooth trajectory of the body part through space. Patients with cerebellar dysfunction are unable to produce these coordinated, smooth movements. Instead, they often break the movements down into their component parts in order to execute the desired trajectory. For example, touching one’s finger to one’s nose requires the coordinated activity of shoulder, elbow, and wrist joints. Cerebellar patients must first perform the shoulder movement, then the elbow movement, and finally the wrist movement in sequence, rather than as one, uniform motion

Front 43

The lateral vestibular nuclei are functionally analogous to the...

Back 43

The lateral vestibular nuclei, although not contained within the cerebellum, are considered to be functionally analogous to the deep cerebellar nuclei because of their functional connectivity with the cerebellum.

Front 44

Following a strenuous workout with his neighborhood team, a right-handed, 52-year-old former professional basketball player awoke the next morning with paralysis of the right lower extremity. A neurological exam revealed an exaggerated stretch reflex. There was no disturbance of position sense, pain sensation or tactile discrimination. Where is the problem localized?

Back 44

E. Left motor cortex, medial (superior) portion of motor map. This answer is CORRECT!

Lesions to the medial portion of the motor map produce contralateral paralysis of the lower parts of hte body.

Front 45

Cupula

Back 45

The cupula is a gelatinous vane that bends when fluid within the semicircular ducts moves because of angular rotation.

Front 46

cristae

Back 46

The cristae respond to angular acceleration (rotation of the head)

Front 47

Linear acceleration is transduced in the:

Back 47

macula

Front 48

Utricle

Back 48

Utricle is not the best answer. The utricle is most sensitive to gravity - changes in head position from an upright position.

Front 49

If a nerve membrane suddenly became equally permeable to both Na+ and K+, the membrane potential would:

Back 49

Approach a value of about 0 mV This answer is CORRECT!

Roughly speaking, the membrane potential would move to a value half way between EK and ENa. The GHK equation could be used to determine the precise value.

Front 50

what will happen if u developed a drug that Blocked the voltage-dependent Na+ permeability

Back 50

Blocking the voltage-dependent sodium permeability would decrease the amplitude of the action potential, but it would probably do nothing to the resting potential. If it did anything to the resting potential, it would lead to a hyperpolarization, not a depolarization as is the case with drug X.

Front 51

what will happen if u developed a drug that Blocked the voltage-dependent k+ permeability

Back 51

The voltage-dependent potassium channels are generally not activated unless the membrane potential is fairly depolarized. Thus, blocking the voltage-dependent potassium permeability would have very little, if any, effect on the resting potential. Also, blocking the voltage-dependent potassium permeability would have a tendency to perhaps increase the amplitude (and duration) of the action potential rather than decreasing it.

Front 52

Drug X, when applied to a nerve axon, results in both a gradual decrease in the amplitude of individual action potentials and a depolarization of the resting potential, both of which develop over a period of several hours. The drug is most likely:

Back 52

Blocking the sodium potassium pump leads to a gradual influx of sodium into the cell, and efflux of potassium out of the cell. These changes in concentration lead to a change in the equilibrium potential for potassium, as well as for sodium. As the equilibrium potential for potassium becomes more positive, the resting potential becomes more positive (i.e., more depolarized). Because of the sodium influx into the cell, the equilibrium potential for sodium is changed, namely, it is less positive. And because the peak amplitude of the action potential is dependent upon the value of the sodium equilibrium potential, the peak amplitude of the action potential would also decrease over time.

Front 53

Increasing the rate at which voltage-dependent changes in K+ permeability occur

Back 53

Increasing the rate at which voltage-dependent changes in K+ permeability occur

Front 54

An endplate potential in a skeletal muscle cell could in principle be produced by a decreased permeability to which of the following ions(s)? (Assume that there is a finite initial permeability to each of the ions listed below and that physiological concentration gradients are present.):

Back 54

K+ This answer is CORRECT!

A decrease in the potassium permeability would lead to a depolarization similar to an end-plate potential. This is so because there is at rest a tonic permeability to potassium and to sodium. The high permeability to potassium tends to keep the membrane potential near the potassium equilibrium potential. If that resting permeability is decreased, alpha in the Goldman equation would become a greater value, moving the membrane potential a bit closer to the sodium equilibrium potential (i.e., a depolarization).

Front 55

What are the other steps in the process of chemical synaptic transmission?

Back 55

A nerve action potential that is initiated in the cell body of a spinal motor neuron propagates out the ventral roots and eventually invades the synaptic terminals of the motor neurons. As a result of the action potential, the chemical transmitter acetylcholine (ACh) is released into the synaptic cleft. ACh diffuses across the synaptic cleft and binds to special receptors on the postsynaptic or the postjunctional membrane. The binding of ACh to its receptors produces a conformational change in a membrane channel that is specifically permeable to both Na+ and K+. As a result of an increase in Na+ and K+ permeability, there is a depolarization of the postsynaptic membrane. That depolarization is called the endplate potential or more generally the EPSP. If the EPSP is sufficiently large, as it normally is at the neuromuscular junction, it leads to initiation of an action potential in the muscle cell. The action potential initiates the process of excitation contraction coupling and the development of tension. The duration of the endplate potential is about 10 msec.