Download Ch9. Motor System

Ch9. Motor System
Normal control of movement
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Skeletal muscle
Lower motor neurons
Spinal region connections
Descending tracts
Control circuits
Motor planning areas
• Motor neurons:
– nerve cells that control the activity of skeletal
muscles (low motor neuron directly innervate
skeletal muscle fibers)
• Descending tracts:
– from the brain to lower motor neurons in the spinal
cord or brain stem
– postural/gross movement tracts, controlling
automatic skeletal muscle activity, fine movement
tracts, controlling skilled, voluntary movements
• Control circuits:
– basal ganglia, cerebellum (adjust activity in the
descending tracts, resulting in excitation or
inhibition of the lower motor neurons, muscle
contraction)
Ant. Part of frontal lobe
Association cortex
Cerebellum and BG; regulate
the activity in descending motor
tract
Motor cortex
Brain stem
• Upper motor neuron
– Cortex 나 brain stem으로부터 시작되어
descending tract 을 통해 axon이 lower
motor neuron 이나 spinal interneuron으로
전달됨
• Lower motor neuron
– Cell bodies in the spinal cord or brain stem
and synapse with skeletal muscle fiber
– Spinal interneuron 과 brain stem으로부터 들
어오는 여러 input 을 받아 lower motor
neuron activity level 이 조절된다.
• Sensory function in regulating
movement:
– 수행하고자 하는 동작이 지금 처한 환경에 가
장 적합하게 조절되게 하기 위해 필수적임.
– 여러 sensory mechanism 을 통해 들어온 정
보들은 feed forward 나 feedback
mechanism 을 통해 운동 조절에 사용됨.
• 예>얼음판을 걸을 때나 울퉁 불퉁한 길을 걸을 때
• 상대방의 공격으로부터 방어하거나 피하기 위한
어떤 동작을 할 때 등
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Muscle fiber structure
Lower motor neuron
spinal region
Descending motor tract
1. Muscle structure and function
Length tension relationship
Adaptation of Number of Sarcomeres to Muscle
Length (Structural adaptation)
-Prolonged immobilization in a shortened length: decreased number of
sarcomeres muscle can generate optimal force at the new resting length.
-Also decreased amount of titin (elastic protein in muscle) will limit the
extensibility of the muscle
2. Lower motor neuron
• Alpha motor neuron
– Extrafusal muscle fiber(Skeletal muscle)
-Large myelinated axons
• Gamma motor neuron
– Intrafusal muscle fiber(Muscle spindle)
-medium-sized myelinated axons
Motor unit
• A single alpha motor neuron and the
muscle fibers(cells) the alpha motor
neuron innervates
• The activity of a motor unit depends on the
convergence of information from peripheral
sensors, spinal connections, and descending
tracts onto the cell body and dendrites of the
alpha motor neuron
• Hanneman’s size principles:
– the order of recruitment from smaller to larger
alpha motor neuron.
• Fast twitch motor neuron
– Fast twitch muscle fiber: phasic contraction, fast
and powerful movement
• Slow twitch motor neuron
– Slow twitch muscle fiber: postural and slowly
contracting muscles.
• Fine motor control muscle as small
number of muscle fibers per motor
neuron to allow precise control, and
postural muscles has large number of
muscle fibers per motor neuron
– Hand intrinsic muscle 10:1
– Gastrocnemius 1000:1
• Alpha-Gamma coactivation
(simultaneously)
– Important to maintain spindle sensitivity
– Most of input to the alpha motor neuron
has collaterals to gamma.
3. Spinal Region
• In the spinal region:
– Somatosensory information is integrated
with descending motor commands to
generate motor output
• Reflex:
– involuntary responses to external stimuli
• Spinal region coordination of voluntary
movement
– Central pattern generator (CPG)
• Motor neuron pools in the spinal cord
Motor Neuron pools in the spinal
cord
• Located in the ventral horn
• The actions of these pools correlate
with their anatomical position
Spinal motor neuron pool
• Single muscle 로 공급되는 motor neuron
들의 cell body 들의 모임. Anterior horn
에 정렬 되어있는데, 하나의 근육의 여러
spinal level 에 걸쳐 모여 있기도 하다.
• Medially located pool: innervate axial
and proximal muscles
• Laterally located pool: distal muscle
• anteriorly located pool: extensors
• Posterior located pool: flexors
Spinal Region Coordination
• Reciprocal inhibition
Muscle Synergies
• Type 2 afferents contribute to synergies by
delivering information to spinal cord neurons
from tonic receptors in muscle spindles,
certain joint receptors, and cutaneous and
subcutaneous touch and pressure receptors
• Interneurons excites by type 2 afferents
project to motor neurons of muscles acting at
other joints, providing a spinal region basis
for muscle synergies.
Proprioceptive Body Schema
• The spinal cord interprets
proprioceptive information as a whole,
and computes a complete
proprioceptive image(schema) of the
body in time and space.
– Schema is essential for adapting
movements to the environment, based on
the proprioceptive feedback.
• This nonconscious schema is used to
plan and adapt movements.
A group of muscles innervated
by a single spinal nerve: myotome
Reflexs
• Proprioceptive Reflexs
– Muscle spindle
• Phasic stretch reflex
• Tonic stretch reflex
– Golgi tendon organ reflex
• Cutaneous Reflexs
1) Muscle spindle reflex:
quick stretch reflex, phasic stretch reflex, myotatic reflex,
muscle stretch reflex, deep tendon reflex, tendon jerk
2) Tonic stretch reflex:
secondary spindle ending
• Maintained stretch 가 muscle의 central
spindle 에 지속적으로 가해졌을 때
• Primary and secondary nerve ending 을 통해
Ia, II afferent fiber 흥분, interneuron 을 통해
alpha motor neuron으로 전달됨
• When slow or sustained stretch is applied to
the central spindle, the tonic stretch reflex
facilitates contraction in the muscle
3) Golgi tendon organ reflex
• Tendon tension is registered by GTO, the
information conveyed into the spinal cord by Ib
afferent  stimulate interneuron that inhibit the
alpha motor neurons to the same muscle,
autogenic inhibition
Cutaneous reflexes
withdrawl reflex
• Reflexes can be elicited by stimulation
of musculoskeletal or cutaneous
receptors
• Stimulation of the muscle spindle
receptors can result in phasic or tonic
stretch reflexes
• Activation of the Golgi tendon organ
can inhibit activity of the corresponding
muscle
• Noxious cutaneous information can
result in a withdrawl reflex
H-reflex : to quantify whether alpha motor
neurons are facilitated or inhibited
Spinal region coordination
1) Reciprocal inhibition
Spinal region coordination
2) Central pattern generator
• Many parts of the nervous system produce
patterns independent of either their sensory
input or supraspinal input.
• Neural circuits that produce self-sustaining
patterns of behavior are called central pattern
generators.
• Animal vs. human
CPG in the spinal cord probably contribute to
normal walking in humans but do not provide
adequate control to support walking without
descending input
Descending Motor Tracts
(upper motor neurons)
• Upper motor neuron project from supraspinal centers
to lower motor neurons(alpha and gamma) and to
interneuron in the brain stem and spinal cord
• Medial activation system : controls lower motor
neurons that innervate postural and girdle muscles
• Lateral activation system : controls lower motor
neuron that innervate distally located muscles used
for fine movements
• Nonspecific activating tracts : contributes to
background levels of excitation in the cord and
facilitates local reflex arcs
Postural and gross movement tracts :
Medial activation system
• Brain stem tracts arise in the tectum(posterior
midbrain), medial reticular formation, and vestibular
nuclei(brain stem nuclei concerned with equilibrium)
- Tectospinal : 갑작스러운 시각 또는 청각자극이 들어오
는 방향으로 머리를 돌리는 반응에 관여
- Medial reticulospinal : postural muscle and limb
extensor
- Medial and lateral vestibulospinal : 자세반사
• The tracts from the cortex : medial corticospinal :
control of neck, shoulder, trunk muscles
Limb and fine movements tracts:
lateral activation system
• Lateral corticospinal : provide fractionation of distal
movements
• Rubrospinal : innervate upper limb flexor muscle
• Lateral reticulospinal : facilitate flexor muscle motor
neurons and to inhibit extensor motor neuron, but
during movement, can be reversed
• Corticobulbar fibers : project to cranial nerve nuclei in
the brain stem and do not reach the spinal cord ->
control lower motor neurons innervating the muscles
of the face, tongue, pharynx, and larynx
Nonspecific activating tracts
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Ceruleospinal
Raphespinal
Not related to specific movements
Contribute of changes in motor
performance with varying levels of
motivation
Control Circuits
• Basal Ganglia
• Cerebellum