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Chapter 13 Spinal Control of Movement

Answers to Chapter Review Questions

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Question 1: What did Sherrington call the final common pathway, and why?

nswer: Sherrington called the lower motor neurons of the spinal cord thefinal common pathway that controls !eha"ior# $hese motor neurons,also called the somatic motor neurons, directly command musclecontraction# $hey are the output of the motor system# %nputs to lowermotor neurons include the sensory afferents entering the dorsal horn&pro"iding information a!out muscle length', the upper motor neurons inthe motor corte(, and the interneurons within the spinal cord thatparticipate in spinal motor programs# )egardless of the source of the

input, the output is the lower motor neurons, the final common path#Question *: +efine, in one sentence, motor unit# ow does it differ from motor neuronpool?

nswer: motor unit consists of one alpha motor neuron and all the musclefi!ers that the alpha motor neuron inner"ates# $his is the elementarycomponent of motor control# motor

neuron pool consists of all the alpha motor neurons that inner"atea single muscle# Question 3: Which is recruited first, a fast motorunit or a slow motor unit? Why?

nswer: -ost muscles ha"e a range of motor unit si.es# $hese motor units are

recruited in order

of si.e/the smallest !eing recruited first and the largest last# $his e(plains whyfiner control

is possi!le when muscles are under light loads than when they are undergreater loads# Small motor units ha"e small alpha motor neurons and largemotor units ha"e large alpha motor neurons# Small neurons are moreeasily e(cited !y signals descending from the !rain#

Question 0: When and why does rigor mortis occur?nswer: $he stiffening of muscles after death is a condition nown as

rigor mortis# -uscle contraction occurs !ecause of the interaction!etween myosin, the ma2or thic filament

protein, and actin, the ma2or thin filament protein, during e(citation contractioncoupling#

$he heads of myosin filaments !ind to actin filaments and undergo a

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conformational change# $his causes the thic filament to mo"e withrespect to the thin filament, shortening the

muscle fi!er during muscle contraction# $P is reuired to release themyosin heads from the actin filament# When no $P is a"aila!le !ecausethe tissue is dead, the attachment !etween the thic and thin filaments

!ecomes permanent#Question 4: 5our doctor taps the tendon !eneath your neecap and your lege(tends# What is the neural !asis of this refle(? What is it called?

nswer: When your doctor taps the tendon !eneath your neecap, thetendon attached to the uadriceps muscle of your thigh is stretched# Whenthis muscle is stretched, the muscle spindle afferents deli"er sensoryfeed!ac a!out the muscle length# $his causes the muscle to contract andyour leg to e(tend# $his is a monosynaptic refle( arc in"ol"ing the spindleafferents that enter the dorsal horn and the motor neurons that control themuscle# $he nee6 2er refle( tests the intactness of the ner"es andmuscles in this refle( arc#

Question 7: What is the function of gamma motor neurons?

nswer: lpha motor neurons inner"ate e(trafusal muscle fi!ers, causing musclecontraction#

8n the other hand, gamma motor neurons inner"ate the intrafusal musclefi!er at the two ends of the muscle spindle# $he acti"ation of these fi!erscauses a contraction of the two poles of the muscle spindle, pulling thenoncontractile euatorial region and eeping the %a a(ons acti"e# $hegamma motor acti"ity eeps the muscle spindle during muscle contractionunder control# 8therwise, during muscle contraction, the muscle spindles

would !ecomeslac and insensiti"e to muscle length#

Question 9: enny, a character in Stein!ec;s classic !oo Of Mice and Men,lo"ed ra!!its, !ut when he hugged them, they were crushed to death#Which type of propriocepti"e input might enny ha"e !een lacing?

nswer: enny might ha"e !een lacing the propriocepti"e input of re"erse myotacticrefle(#

$he normal function of the refle( arc is to regulate muscle tension withinan optimal range# %n e(treme circumstances, the refle( arc protects themuscle from !eing o"erloaded# $his type of propriocepti"e input is

particularly important for the proper e(ecution of fine motor acts, such asthe manipulation of fragile o!2ects with hands, which reuires a steady, !utnot too powerful, grip#

Question 1: ist the components of the lateral and "entromedial descending spinalpathways#

Which type of mo"ement does each path control?

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nswer: $he components of the lateral descending spinal pathways are thecorticospinal tract and the ru!rospinal tract# $he components of the"entromedial descending spinal pathways are the "esti!ulospinal tract, thetectospinal tract, the pontine reticulospinal tract, and the

medullary reticulospinal tract# $he lateral pathways are in"ol"ed in the

"oluntary mo"ement of the distal musculature# $he lateral pathways controlthe fine mo"ements of arms and fingers# $he "entromedial pathways controlthe posture of the head and nec#

Question *: 5ou are a neurologist presented with a patient who has the followingsymptom: an ina!ility to independently wiggle the toes on the left foot, !utwith all other mo"ements &waling, independent finger mo"ement' apparentlyintact# 5ou suspect a lesion in the spinal cord# Where?

nswer: esions in the descending motor tracts, which originate in the uppermotor system, can cause an a!normal oseph

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neurons are lost and dopa treatment is no longer effecti"e# +opa also hassome trou!lesome side effects#

Question 4: %ndi"idual

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of the !odyGs response to the situation, the person !ecomes afraid#ccording to the Fannon6

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do not now whether or not the animals feel angry, !ecause feelings aresu!2ecti"e e(periences that can !e reported "er!ally !y humans !ut not!y rats#

Question 0: What changes in emotion were o!ser"ed following temporal lo!ectomy!y KlL"er

and aco!sen of the 5ale Cni"ersity reported thatfrontal lo!e

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lesions had a calming effect in chimpan.ees# %t has !een suggested that frontal

lesions ha"e

this effect !ecause of the destruction of lim!ic structures, particularly in connectionwith frontal and cingulate corte(# &$his surgery is also associated with !lunted

emotions, inappropriate !eha"ior, difficulty in planning and woring toward goals,and difficulty in

concentrating#' %n addition, reduced aggression in amygdalectomi.ed animals led

some

neurosurgeons to use this method in humans# Flinical reports claimconsidera!le success in reducing aggressi"e asocial !eha"ior,increasing the a!ility to concentrate, decreasing hyperacti"ity, andreducing sei.ures with this type of !rain surgery#

Question 9: $he drug nown as Pro.ac is a serotonin6selecti"e reuptae inhi!itor#ow does this

drug affect a person;s le"el of an(iety and aggression?

nswer: $he neurotransmitter serotonin is in"ol"ed in regulating aggressionHdecreased serotonin is associated with an increase in aggression, andincreased serotonin is associated with decreased aggression# $he lin!etween aggression and an(iety is not perfectly clear, !ut it is nown thatserotonin antagonists increase aggressi"eness and agonists of the 46$1< and 46$1< serotonin receptors decrease an(iety andaggressi"eness in mice# %t is also nown from e(perimental wor inanimals that an(iety and aggression increase and decrease together#Pro.ac is a selecti"e serotonin reuptae inhi!itor &SS)%' that effecti"ely

increases the amount of serotonin in the synaptic cleft !y pre"enting itsreuptae into the presynaptic element# $his increase in serotonina"aila!ility is associated with decreased an(iety, so in addition to its useas an antidepressant, Pro.ac and other SS)%s are used as

antian(iety agents#

Question 1: Why do AADs with relati"ely fast freuencies tend to ha"e smalleramplitudes than

AADs with slower freuencies?

nswer: $he amplitude of the AAD signal depends on the synchroni.ation of theacti"ity of the underlying neurons# %f a group of cells are e(citedsimultaneously, the tiny signals sum to generate one large surface signal#owe"er, when each cell recei"es the same amount of e(citation, !utspread out in time, the summed signals are meager and irregular# %n thiscase, the number of acti"ated cells and the total amount of excitation has notchangedH howe"er, the timing of the acti"ity has changed# $herefore, AADswith relati"ely fast freuencies tend to ha"e smaller amplitudes than AADs

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with slower freuencies#

Question *: $he human cere!ral corte( is "ery large and must !e foldede(tensi"ely to fit within the sull# What do the foldings of the cortical surfacedo to the !rain signals that are recorded !y an AAD electrode at the scalp?

nswer: =or the most part, an AAD measures "oltages generated !y the currents

that flow during synaptic e(citation of the dendrites of many pyramidalneurons in the cere!ral corte(# $he signal must penetrate se"eral layers ofnon6neural tissue, including the meninges, fluid, !ones of the sull, and sin,to reach the electrodes# $he population of cells deep within the folds of thecortical surface contri!utes "ery little to the recorded AAD, which measuresacti"ity only in the superficial layers of corte( close to the sull#

Question 3: Sleep seems to !e a !eha"ior of e"ery species of mammal, !ird,and reptile# +oes this mean that sleep performs a function essential for thelife of these higher "erte!rates? %f

you do not thin so, what might !e an e(planation for the a!undance of sleep?

nswer: Mo single theory of the function of sleep is widely accepted, !ut the mostreasona!le ideas fall into two categories: theories of restoration and theoriesof adaptation# $he theory of restoration states that we sleep in order to restand reco"er, and to prepare to !e awae again# $he theory of adaptationstates that we sleep to eep out of trou!le, to hide from predators when weare most "ulnera!le or from other harmful features of the en"ironment, or toconser"e energy# A"en animals that ne"er rest, such as dolphins, gi"e eachhemisphere a nap: a!out * hours asleep on one side, then 1 hour awae on!oth sides, * hours asleep on the

other side, and so on for 1* hours e"ery night# Similarly, the !lind %ndus )i"erdolphin uses microsleeps of 067 seconds in duration, adding up to 9 hours in

a *06hour day to rest its !rain# $his reinforces the importance of sleep !ut thereason for its importance remains unnown# nd it is possi!le that sleep issimply a !yproduct of some other "ital process# Monethless,

rats depri"ed of sleep lose weight in spite of increased food intae, !ecomewea, accumulate stomach ulcers and internal hemorrhages, and in se"erecases e"en die# $hey are una!le to regulate !ody temperature and meta!olicneeds#

Question 0: n AAD during )A- sleep is "ery similar to an AAD when awae#ow do the !rain and !ody in )A- sleep differ from the !rain and !ody whenawae?

nswer: )apid eye mo"ement sleep, or )A- sleep, is a state where the whole

!ody &e(cept for the eye and respiratory muscles' is immo!ili.ed, and "i"id,detailed illusions called dreams are con2ured up# $he o(ygen consumption ofthe !rain is higher in )A- sleep than when the !rain is awae andconcentrating on difficult mathematical pro!lems# Some areas, includingprimary "isual corte(, are eually acti"e in the two states# owe"er,e(trastriate cortical areas

and portions of the lim!ic system are significantly more acti"e during )A-

sleep#

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Fon"ersely, regions of the frontal lo!es are noticea!ly less acti"e during)A-# -ost of the !ody is incapa!le of mo"ing during )A- sleep, whereasthe !ody can !e mo"ed normally when awae# $he paralysis that occursduring )A- sleep is almost a total loss of seletal muscle tone# $he

muscles controlling eye mo"ement, the tiny muscles of the inner ear, andthe muscles of respiration are the e(ceptions, as these are striingly acti"e#+uring )A- sleep, the same core !rain systems that control the sleepprocesses of the fore!rain acti"ely inhi!it the spinal motor neurons,pre"enting the descending motor acti"ity from e(pressing itself as actualmo"ement#

Question 4: What is a liely e(planation for the !rainGs relati"e insensiti"ity tosensory input during )A- sleep, compared to the waing state?

nswer: $he control of )A- sleep, as with the other functional !rain states,deri"es from diffuse modulatory systems in the core of the !rain stem,particularly the pons# $he diffuse modulatory systems control the rhythmic

!eha"iors of the thalamus, which in turn controls many AAD rhythms of thecere!ral corte(H slow, sleep6related rhythms of the thalamus apparently !locthe flow of sensory information up into the corte(#

Question 7: $he SFM recei"es direct input from the retina "ia theretinohypothalamic tract, and this is how light6dar cycles can entraincircadian rhythms# %f the retinal a(ons were somehow disrupted, what would!e the liely effect on a personGs circadian rhythms of sleeping and waing?

nswer: %nput from the retina to the suprachiasmatic nucleus &SFM' of thehypothalamus is essential and sufficient to entrain sleeping and waingcycles to night and day# When retinal

a(ons are disrupted, and this essential input to the SFM is a!sent, the sleep6

wae cycles

cannot !e entrained !y light# Such an indi"idual would !e su!2ect to a free6running cloc, which would drift out of phase with the typical light or darcycle !ecause a free6running cloc runs on a longer day than normal &*4hours in the short6term, 3I637 hours in the long6 term'# Such an indi"idualwould !ecome sleepy during the day and waeful at night, until the cycledrifted !ac into phase with the normal light or dar cycle#

Question 9: What differences would there !e in the !eha"ioral conseuencesof a free6running circadian cloc "ersus no cloc at all?

nswer: free6running circadian cloc still has certain alternating phases of

sleep and waefulness, and other !eha"ioral and physiological cycles,such as !ody temperature, continue to alternate, although they may!ecome desynchroni.ed so that sleep6wae and !ody temperature cycleat their own pace, uncoupled# 8n the other hand, when the SFM is

lesioned, circadian rhythms are a!olishedEthe periodicity is lost# =ore(ample, suirrels and moneys with no SFM ha"e persistent high6freuencyrhythms of !oth !rain acti"ity and

temperature with no e"idence of regular cycling#

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Question 1: ow is it possi!le for a split6!rain person to spea intelligi!ly if theleft hemisphere controls speech? %sn;t this inconsistent with the fact that theleft hemisphere must direct motor corte( in !oth hemispheres to coordinatemo"ements of the mouth?

nswer: Some midline features are represented in !oth sides of the !rain, suchas the fo"ea, which is represented in !oth right and left hemispheres# -otorcontrol of the mouth and laryn( may !e similarly represented on !oth sidesof the !rain# %n addition, the motor system wors according to the populationcode rather than a strict one6to6one correspondence !etween neural acti"ityand neural output# $his may loosen the topographic relationships !etweenmotor corte( and motor output# =inally, the two hemispheres cancommunicate "ia the anterior commisure in split6!rain indi"iduals !ecausethis su!cortical fi!er tract remains intact when the corpus callosum isse"ered#

Question *: What can you conclude a!out the normal function of

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taught, to signify no"el stimuli#

Question 0: What does the Wernice6Deschwind language6processing modele(plain? What data are inconsistent with this model?

nswer: $he Wernice6Deschwind language processing model offers simplee(planations for ey elements of

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of the left hemisphere !eing language dominant# $he right hemisphere haslanguage comprehension and can read and understand num!ers, letters,and short words as long as the reuired response is non"er!al#

Question 7: What e"idence is there that