Chapter 5. Motor Systems
5.1 Neural Control of Posture & Movements: A Survey

운동과 활동의 분석: 눈, 활동사진, 고속사진, EMG---> 단순 및 복잡한 운동

A. Phenomenology of motor acts

1. Reflex-controlled Movements

Knee Jerk, 뜨거운 것을 만졌을 때 피함: reflex
Stimulus-Response theory of behavior: 더 복잡한 활동을 reflex의 조합으로 봄
자극---> 자동적, 반복적, 고정화된, 특정 목적을 띈 stereotyped---> 반응
Spinal Cord가 중요한 역할
예: decerebrate frog: a) pinch to hind foot---> pulled away
b) noxious stimulation to frog's back
---> response with hind foot
reflex-controlled movements의 예
Corneal reflex, coughing, swallowing, food propell,
breathing, blood flow etc.
Newborn baby에서: 8-9주: 자극--->반응, 11-12주: localized reflex

2. Program-controlled (automatic) movments

Program theory of behavior without any external sensory inflow
(stimulus independent, innate, inherited, spontaneous,
voluntary (initiation & termination), rhythmic motor pattern)

예:
walking (newborn baby: 12-18 month, independent), running, crawling
breathing, chewing,
species specific fixed action pattern (nesting, tunneling, urination, etc)
learned programs (습득된 기술): sport, work, typing, drawing, driving
enough practice--->automatic.
central programs can be influenced by sensory feedback.

3. Voluntary & involuntary movments

Voluntary: Purposeful, goal directed, learned
운동기술의 습득, 말하기, 글쓰기, 테니스 치기, 전화걸기
습득후에 automatic (program-controlled movements로)
Involuntary: 고도로 고정화된 reflex, 학습 불필요

4. Postural & goal-directed functions

Intrapersonal: posture (standing, sitting, lying, balance)
orientation in space
Extrapersonal: goal-directed
intimate interlinkage between postural & goal-directed movements


B. Locations & Functions of Motor Centers

1. Hierarchy & Partnership

Motor centers: distributed throughout the CNS (spinal cord--cortex)
계층구조를 나타냄: 진화과정에서 더 복잡한과제를 수행하기 위해 적응
: old system위에 new system을 첨가
: 또한 특정 motor center의 specialization
: 전체에 기능하며 동시에 개체적 기능 (partnership)

Fig. 5.1. Hierarchial representation of the central nervous flow of
excitation related to posture & movement
preparation phase vs. executaion phase

2. Spinal motor systems
Spinal Cord
Sensory afferent--->Interneuron--(+/-)-->Motor Neurons
------------------------>Motor Neurons
(final common path)
:reflex arcs, spinal reflexes
:library of elementary postural &
movement programs
Higher CNS Ctr-------------------------> Spinal Cord
(modulation)
Other spinal Ctrs----------------------> Spinal Cord


3. Higher motor systems

Brainstem: postural functions & their coordination with
goal-directed movment의 조절
(integration of visual, vestibular, somatosensory input)
Higher Centers: perfomance of the directed movements

Fig. 5.1 : Drives to Act-->Movement Design-->Movement Programs-->Selection
(subcortical (Associative & (Premotor, Supp. of spinal
motivation sensory cortex) motor cortex) neurons.
areas) modulation of
spinal networks
(motor cortex, brainstem)
Fig. 5.2 Partnership of motor centers
Corticocortical information processing
Cortex-to-cortex loops by way of extracortical structures
a) pontine nuclei + cerebellum---> motor thalamus
b) basal ganglia-----------------> motor thalamus
------>associative thalamus-->assoc. cortex

4. Coupling of sensory & motor systems
Sensory information & motor actions are intricately intermeshed.
Control: feed back: slow movement, object touch
feed forward: rapid movement, catching ball, playing piano
지체될 시간적 여유가 없는 기계적인 운동
5.2. Sensors for Motor Functions: Muscle spindle & Tendon Organs
A. Morphological Aspects
1. Structure of the muscle spindles (근방추)
근섬유와 평행, 근육이 stretch시 활동
every muscle contains stretch receptors (or sensors): muscle spindle
Fig. 5.3A
a) connective tissue로된 capsule내에
intrfusal muscle fiber (15-30 um 직경, 4-7 mm long)
b) afferent innervation (2 종류)
Ia fiber-annulospinal endings (primary muscle spindle endings)
nuclear bag과 nuclear chain fiber를 감쌈
group-II fibers (9 umm 직경, thinner than annulospinal endings)
secondary muscle-spindle endings
nuclear chain fiber를 감쌈
c) efferent innervation
gamma fiber (2-8 um)에 의해 innervation 됨
Intrafusal fiber의 끝쪽에서 endplate로 synapse함 (2종류)
gamma-endplate (nuclear bag fiber에)
gamma-trail endings (nuclear chain fiber에)
2. Structure of the tendon organs
Fig. 5B.C.
근섬유와 연속되어 있다.
근육 stretch시 일시적으로 활동
Golgi tendon organ (골기건)
약 10개의 extrafusal muscle fiber의 tendon fascicle이
connetive tissue capsule에 enclosed
Ib afferent fibers (thick myelinated, 10-20 um diameter)
thinner branch--> terminal arborization
3. Distribution of muscle spindles & tendon organs
spindle density: the No of muscle spindles/gm of muscle tissue
세밀한 운동에 관여하는 small muscle에서 특히 spindle density가
높다. (예, 손: 130/g, 반면 trunk: 1/g)
tendon organ의 수: 50-80/ every 100 muscle spindles
B. Receptor Function of the Muscle Spindles & Tendon Organs
Fig. 5-4
a) Resting 상태:
Ia fibers (primary muscle-spindle endings): discharge
Ib fibers (tendon organ): silent
b) Stretching시: Ia and Ib 모두 discharge
c) Isotonic contraction시
Ia fiber shows decreased firing
---> decrease the tension in the muscle spindle
Ib fiber의 discharge는 그대로 (increases transiently0
d) gamma-motorneuron에 의한 intrafusal fiber의 수축시
Ia fiber shows increased firing, but Ib fiber is silent.
Intrafusal prestretching: serves to avoid silencing during active
shortening of the muscle: 따라서 preserving the receptor's
ability to signal small perturbations.
Secondary muscle-spindle endings: group II afferent, stretch receptors
priamry spindle endings보다 높은 threshold,
intrafusal muscle fiber의 수축에 의해 threshold 변화.
signaling mainly muscle length.

5.3 Spinal Motor Reflexes

Elements of a reflex arc, reflex time
Fig. 5.5 Reflex arc

Stimulus--->Sensors---(afferent pathway, afferent fibers)--->
Central Neurons---(efferent pathway,
motor axons, postganglionic fibers)--->
Effectors (skeletal, smooth musculatures, heart, glands)
----> Action
Reflex arc를 거치는 시간: reflex time

A. Reflex Arc with primary muscle spindle afferents

1) Stretch reflex elicited by muscle stretch
Fig. 5.5
Stretching muscle--->
Activation of the primary muscle-spindle endings (Ia fibers)--->
Excitation of the homonymous motorneurons---> muscle contracts

예: patellar tendon ("knee-jerk") reflex
quadriceps muscle stretch--->raising freely hanging lower leg
2) Induction of H-reflexes
Fig. 5.6
Stimulation: 20-30V electrical, Ia afferent fibers of the popliteal nerve
Response: from the triceps surae muscle
H wave: latency 30-35 ms
stronger stimulation (35V 이상)---> alpha-motoneurons이 흥분되어
latency 5-10 ms의 M wave가 나타난다.
H와 M wave가 같이 커지다가 나중에 M만 남는다 (자극강도 올림에 따라)
이유:
1) Golgi tendon organs의 Ib fiber가 흥분함에 따라
homonymous motoneurons의 inhibition
2) alpha-motor axon의 자극에 의해 antidromic impulse생성
a)---> Renshaw inhibition
b)---> invade soma & dendrites of motoneurons--->
collide with impulses from Ia fiber
3) Stretch reflex for the control of muscle length
Reflex maintenance of muscle length
Preservation of maintained tone in postural muscles.
Length of the muscle is kept constant
4) Reciprocal antagonist inhibition by Ia afferents
Fig. 5.7
Ia fibers--excite->Interneuron--inhibit-->antagonistic motoneurons
disynaptic reciprocal antagonist inhibition


B. Functions of the gamma-spindle loop

1. contractions initiated by intrafusal activation

Fig. 5.8
gamma-motoneuron의 흥분--->
intrafusal contraction--->
priamry muscle-spindle endings의 흥분--->
homonymous motoneuron의 흥분

gamma-spindle loop
muscle spindle---> extrafusal muscle의 contraction
via 1) muscle stretch
2) activation of the gamma motor axons
Activation of the gamma loop---> shortening of the muscle with little or
no change in the discharge rate of the muscle spindle afferents.

2. Alpha-gamma coactivation during movements

Fig. 5.9 A-C.

alpha & gamma motoneurons activated simultaneously.
alpha-gamma coactivation (alpha-gamma linkage)
a)gamma motoneurons의 낮은 conduction velocity
b)time required for intrafusal contraction
a)b)의 이유 때문에
EMG가 muscle spindle afferent 보다 빨리 활성을 보임
Main role of the gamma innervation:
to prevent relaxation of the muscle spindle during extrafusal
contraction, in order to ensure that the accuracy of the
muscle spindle as a sensor.

C. Reflex Arcs of Secondary Muscle-Spindle Afferents
Monosynaptic excitation of homonymous motoneurons.
Entire limbs에 대해:
a) excitatory influence on all the flexors
b) inhibitory influence on all the extensors

D. Motor Reflex Arcs with Tendon-Organ Afferents
1. Segmental connections of the Ib fibers
Fig. 5.10
di-or trisynaptic inhibitory--->
homonymous & agonist motoneurons
(autogenic inhibition, self-inhibition)
disynaptic excitatory----> antagonist motoneurons
other joint에도 영향










BRAIN FACTS

2. Functions of the tendon organs
to keep the tension of the muscle constant.
muscle spindle: length control system: single muscle & antagonist
tenson organ: tension-control system: whole limb

Force developed by a muscle depends on
a) efferent discharge of the alpha-motoneurons
b) prestretching
c) contraction velocity
d) degree of muscular fatigue

E. Polysynaptic Reflexes

all reflexes are polysynaptic.
Sensor가 effector와 다른 몸의 부위에.
autonomic reflexes, polysynaptic somatic reflexes.
1) Characteristics of polysynaptic reflexes
예: coughing
a) presence of delay period
b) Summation of the subthreshold stimulus
in central neurons & motorneurons of the reflex arc.
c) Reflex time & intensity of response depend on
the stimulus intensity.
d) plasticity of the reflex response
habituation, dishabituation, sensitization
conditioning (long-term changes in the reflex response)
2) Flexor reflex & crossed extensor reflex
Fig. 5.11
Spinal animal에서
Pinching, heat, strong electrical stimulation--->hindpaw
===> stimulated leg pulled away (flexor excited, extensor inhibited).
flexon of the (ankle, knee, hip joints)
flexor reflex, protective reflex
===> extension of the opposite limb (flexors inhibited, extensor excited)
(crossed extensor reflex)

Role of the group III & IV afferents from the muscles
Group I & II: motor control
Group III & IV: muscular pain, autonomic control
Role of the joint afferents
Group III & IV:
a) chief function may be nociception & reflex inhibition
b) small contribution to the conscious perception of the position
and movments of the joints.
3) Recurrent inhibition & presynaptic inhibition in spinal motor systemsFig. 5.12.
Motor neuron--->excite muscle
--->excite Renshaw cells (inhibitory interneuron)--->
feedback inhibition on the same Motor neuron.
Function: to prevent an uncontrolled oscillation of motor neuron
activity (increased muscle tone (spasticity) may
be caused by Renshaw cell malfucnction)

F. The Propriospinal System & the Capabilities of the Isolated Spinal Cord

1) Intersegmental reflex connections
Propriospinal neurons & propriospinal tracts에 의해
spinal cord의 여러 level에서 시작된 운동들의 coordination.
(fore- & hindlimbs, neck & limb movements)

2) Spinal Locomotion
Basic pattern of locomotion:
programmed at the level of the spinal cord.

3) Spinal Shock
Reversible motor & autonomic areflexia following spinal cord 절단
(local cooling, local anesthesia)
many months in humans
few minutes in frogs
hours in carnivores
days or weeks in monkeys
weeks & months in 유인원

Mechanisms responsible for the return of certain spinal function
: not konwn


5.4 Motor Centers in the Brainstem

1. Hierarchical position of the brainstem centers; methods of study

Motor centers가 motoneuronal activity를 coordination
(finely graded spatial and temporal pattern of excitation)
==> 자세의 조정과 더불어 적절한 운동의 수행

Mammals (특히, primates): cerebral cortex가 motor control의 주된 기능을 함
그러면 brainstem centers의 독자적인 기능은?

Lower vertebrates: forebrain을 lesion한 후에도 상대적으로 미미한 운동장애
==> 따라서 독자적인 운동 조절기능이 있다.
In primates: forebrain lesion하면 심한 운동장애 유발
==> brainstem의 운동조절 기능이 subordinate to the cerebral cortex

연구방법: superordinate parts of the brain를 제거(lesion)해서 조사
lesion technique의 결함:
1) motor centers are elements in a system that can be disrupted as
a whole when only a part has been destroyed.
Interaction and coordination of many centers arranged
hierarchically and in parrallel.
2) acute lesion is followed by long-term processes of
reorganization in the CNS.

Brainstem Centers: fundamentally involved in the control of body posture.
Spinal cord에 descending fibers를 보내 영향을 미친다.


2. Static and statokinetic reflexes

Animals with forebrain lesions, particulary thalamic animals에서
과장된 postural & righting reflexes를 나타낸다.
a) to respond to certain stimuli by correcting the distribution of
tone in the musculature (reflexes to maintain
a certain body posture)
b) to return its body to the normal orientation (righting reflexes)

Static Reflexes:
occur in resting position.
to ensure that parts of the body are kept stably in place.
Statokinetic Reflexes:
are assocaited with changes in position.
adjust the limb positions when the orientation of the body changes.

Postural reflexes는 주로
trunk와 limbs의 proximal parts의 근육의 조정으로 나타난다.
afferents: from the neck musculature & the labyrinth
(그래서, tonic neck & labyrinth reflexes라고도 함)
또한 cutaneous & visual afferents도 참여
(mutisensory convergence)


3. Problems Associated with the Erect Stance of Humans and
its Modification during Movements


upright waling, standing on feet: marvel of regulation
(while breathing, active manipulation, trunk movements==>
continual shifting of the body's center of gravity: disturbance)

Postural synergies: platform paradigm을 사용해서 clinic에서
impaired postural function을 분석
Electromyographically measurable activity patterns of the leg
and trunk muscles in response to platform platform movements.
===> latency of 100-150 ms, a long reflex time
===> complex processing in the CNS involving supraspinal structures

Context-dependent adaptation
(experience + expectation + reflexes)
(involvement of higher motor centers)
Three clases of sensory inputs for triggering posutal responses
a) muscle proprioceptors (length or tension)
b) vestibular inputs
c) visual afferents

Postural motor system 또한 hierarchically organized
local reflex mechanism: subordinate
long functional loop (long-loop reflexes)

Postural compensation to
a) disturbances of external origin
b) disturbances by one's own movements (e.g., breathing)

Posturographic analysis of the compensatory adjustments to
goal-directed movements===> no delay
why?: anticipatory postural synergies (Fig. 5.15).
Both reflex- and program-controlled.

4. Vestibular & Neck Reflexes Stabilizes the Head & Eyes

1) Vestibular reflex
: evoked by changes in the position of the head
: evoked by otolith organs
(utricle, saccule in the vestibule of the inner ear)
---> signal a) the direction of gravity
b) the acceleration produced during head movements.
: gravity에 대해 머리를 수직으로 서게함

a) Vestibulocollic reflexes (act on the neck)
counteract head movements, keeping the head stable
b) Vestibulospinal reflexes (act on the limbs)
: extension of arms, flexion of the lower limbs
: 넘어질 때 준비, impact을 줄임
c) Vestibulo-occular reflexes: stabilize images on the retina.


2) Neck Reflexes
: triggered by tilting (bending) or turning the neck
a) Cervicocollic reflexes:
머리를 한방향으로 밀으면 반대쪽 목근육이 늘어난다
cervicocollic reflex에 의해 이 반대쪽의 목근육을 수축시켜
머리를 원위치 시키도록 한다.
: synergistic with vestibulocollic reflexes
b) Cervicospinal reflexes
bending the neck forward
==> flexion of the upper & lower extremities
tilting the neck backward
==> extension of the upper & lower extremities
turning to right
==> extension of right arm & leg (flexion of left limb)

Vestibular and neck afferents converge on the
Vestibular Nuclei & Propriospinal Neurons


5. Hierarchical Influences on Program-Controlled Automatisms:
Locomotion & Chewing


Automatisms (자동운동) : subprograms of the motor system,
under the influence of higher motor centers.
"Stepping generators"
spinal networks provide only stereotyped stepping rhythms.
Supraspinal centers: provide modulatory influences to suit the terrain
and other circumstances.
Contiunuous alteration of Goal-drected locomotion in open field
:according to the tactile, olfactory, visual and acoustic inputs
:==> stalking, pouncing

In humans: a paraplegic(하반신 불수), (no supraspinal influences)
no longer make stepping movements.
Locomotion에 관한 cortex의 기능 잘 모름
Locomotor center in the brainstem
:extending from diencephalon to the midbrain (locomotor strip)
: promote locomotor activity

Proprioceptive and cutaneous stimuli 또한 locomotion에 영향을 미친다.
: induction & sutaining of stepping rhythms
: feedback from the peripheral sensors is not a prerequisite
(deafferentation후에도 locomotor activity가 관찰됨으로)
curare로 운동의 실행을 막은후에도 muscle nerve에서 rhythmic
discharge pattern을 관찰할 수 있다 (fictive locomotion)

Chewing: another rhythmic automatism
trigger: a) by the oral intake of solid food
b) can be initiated voluntarily
decerebrate animals: 입에 음식을 넣으면 rhythmic chewing movements
Rodents: chewing center in the brainstem
Humans: brainstem의 chewing center가 frontal & temporal cerebral
cortex에 의해 조정 받는다.

6. The Brainstem Motor Centers as the origin of Descending Pathways, and Cortical influences upon Them

1) Medial System
:descends in the ipsilateral ventral columns of the spinal cord.
:termination: a) interneurons, b) long propriospinal neurons
c) medial motor neurons (axial & proximal muscles)
A) Vestibulospinal tract (2종류: medial & lateral)
Origin: vestibular nuclei
Function: reflex control of balance & posture
B) Reticuospinal tract (2종류: medial & lateral)
Origin: reticular formations in pons & medullar
Function: maintenance of posture
integration of vetibular input and cortical input
(cortico-reticulospinal pathway: important for
the suppression of spinal reflexes)
C) Tectospinal tract
Origin: superior colliculus of the midbrain
Projection: contralaterally to the cervical segment
Function: coordination of head & eye movements.
(cortico-tectospinal pathway)
2) Lateral Pathways (Rubrospinal tract)

Medial system보다 진화상 최근에 생긴 motor system
Origin: Red Nucleus (magnocellular portion) in the midbrain.
Termination: propriospinal neurons,
lateral motor neurons, lateral interneurons
Function: fine movements, distal muscles (flexors)
reaching, manipulating objects with fingers & hand
유인원과 사람에서는 rubrospinal tract은 흔적만 있으며
corticospinal system에 의해 기능이 수행됩니다. 여러분!

3) Aminergic Pathways Modulate the excitabiltiy of Spinal Neurons

A) Ceruelospinal system
Origin: Locus ceruleus (noradrenergic)
Termination: lateral colum의 ventrolateral part

B) Raphe-spinal system (serotonergic)
Origin: brainstem의 raphe nuclei
Termination: lateral & ventral column
dorsal horn의 outer layers (pain control)


7. The Decerebrate Preparation and Decerebrate Rigidity

brief anoxia(무산소증)후==>
selective and irreversible damage to the forebrain.
-->all intellectual abilities are lost
vital circulatory and respiratory centers of the brainstem
often remain intact--> vegetative state.
Patient: Brain Dead (forebrain death)

1) Decerebrate rigidity
Fig. 5.13
transection 1: rostral half of the midbrain에서 짜름 (decerebrate animal)
(above vestibular nuclei and below the Red nucleus)
a) ==> Decerebrate Rigidity 발생: immediate,
a massive muscular stiffness (elevated muscle tone)develops,
especially in the muscles that serve to oppose gravity.
exaggerated standing
four limbs tonically extended
b) Reflex의 특징을 보임: 왜냐? dorsal roots를 짜르면 없어짐으로 보아
: (reflex arc의 중단)
: interruption of stretch reflex
: preventing spindle endings from providing tonic facilitation
to motor neurons.
: vestibular nerve를 짜르면--->reduce decrebrate regidity
c) Disinhibition현상
: Lateral vestibulospinal tract과
Medial reticulospinal tract의 dominant action은 alpha 및 gamma
motorneurons을 활성화하여 extensor muscle을 수축시킨다.
: Reticulo
정상동물에서 superordinate motor center가
brainstem center에 억제적으로 작용.
: anterior lobe of the cerebellum (inhibition to lat. vesti. N).
자극: reduce decrebrate rigidity
파괴: increase "
d) 사람의 경우: by cerebral hemorrhage or large tumors
===> less pronounced increases in extensor tone,
intermittent extensor posture

2) Motor patterns following other cerebral lesions
Fig. 5.13
transection 2: midbrain animal (the whole midbrain remains intact)
decrebrate animal 보다는 덜 심한 운동장애
스스로 일어나 앉는다. 덜 심한 rigidity (why?: red nucleus가 intact)
Red nucleus: oppose that of the vestibulo- & reticulospinal
systems of the lower brainstem
(facilitate flexors & inhibits extensors)
transection 3: thalamic animal (intact diencephalon)
rhythmic stepping movements가 자발적 또는 인위적으로 유발됨
locomotor pattern은 기계적이다.(not like normal gait)
transection 4: basal ganglia intact (after extensive decortication)
Movement repertorie is preserved remarkably well
in rodents & carnivores.
Movement sequence는 기계적 특징을 보임










BRAIN FACTS