Download Zoran Đogaš

General
Organization of
Motor Systems
Zoran Đogaš
Motor Systems
•  Functions
–  movement
–  posture & balance
–  communication
•  Guided by sensory systems
–  internal representation of world & self
–  detect changes in environment
•  external & internal ~
Three Classes of Movement
•  Voluntary
–  complex actions
reading, writing, playing piano, etc.
–  purposeful, goal-oriented
–  Learned
• may improve with practice
Three Classes of Movement
•  Reflexes
–  involuntary, rapid, stereotyped
eye-blink, coughing, knee jerk
–  graded control by eliciting stimulus
•  Rhythmic motor patterns
–  combines voluntary & reflexive acts
chewing, walking, running
–  initiation & termination voluntary
–  once initiated, repetitive & reflexive
Movement & Muscles
•  Movement occurs at joints
•  Contraction & relaxation of opposing
muscles
–  agonists
•  prime movers
–  antagonists
•  counterbalance agonists
•  decelerate movement
Movement & Muscles
•  Movement control more than contraction &
relaxation
–  Accurately time control of many muscles
–  Make postural adjustment during movement
–  Adjust for mechanical properties of joints &
muscles
•  inertia, changing positions
Sensorimotor Integration
•  Sensory inputs guide movement
–  visual, auditory, tactile
• 
location of objects in space
–  Proprioceptive & vestibular
• 
position of our body
•  Critical for planning & refining movements
Error Correction: Feedback
•  During or after movement
•  Compare actual position with intended
position
–  if different ----> make correction
•  muscle contractions
•  Limited to slow movements
Error Correction: Feed-forward
•  Sensory events control movements in
advance
–  ballistic movements
•  Prediction
• internal model of events
•  e.g. catching ball
–  representation of ball trajectory
–  properties of musculoskeletal system
•  Reevaluation after response completed
SENSORY INPUTS GUIDE VOLUNTARY MOVEMENT THROUGH
FEED-BACK AND FEED-FORWARD MECHANISMS
EXAMPLE OF FEEDBACK AND FEEDFORWARD MOVEMENT CONTROL:
CATCHING A FALLING BALL
Visual input provides feed-forward
control of the task enabling us to:
1) Position hand under where ball is
anticipated to fall
2) Partially stiffen joints in anticipation
of ball’s impact on hand
Somatosensory and proprioceptive inputs
provide feed-back control used
to grasp ball.
Some aspects of feedback control
involve task-specified programming
of spinal reflexes
Sensorimotor Impairments
•  Feed-forward control
–  eyes open: ballistic movements OK
–  eyes closed:
• ballistic movements highly inaccurate
hand drifts at end of movement
•  Eyes open only prior to movement
–  errors greatly reduced
–  lack of info about starting position
Motor unit
α-motor neuron
+ all the muscle fibers to
which it connects
Motor units
Three types of motor units
• SF motor units (type I)
(slow, fatigable)
• FF motor units (type IIA)
(fast, fatigue resistant)
• FFR motor units (type IIB)
(fast, fatigable)
Distribution of Fiber Types
•  All muscle
composed of ST &
FT fibers
•  Distribution varies
from muscle to
muscle within an
individual
•  Most individuals
possess between 45
and 55% ST
Distribution of Fiber Types
•  Vastus lateralis on
average 52% SO,
33% FOG, 14% FG.
Same for deltoid,
biceps brachii.
•  Soleus may have as
much as 85% SO.
•  Triceps brachii may
have as few as 30%
SO.
Distribution of Fiber Types
•  Great variation between
individuals
•  Vastus lateralis of elite
distance runners had 79%
ST, untrained had 58%
•  Available evidence
indicates that the
distribution of slow and fast
twitch fibers is genetically
determined and not altered
by training
Large and small α-motoneurons
Strength of muscle contraction
• Recruitment
(Henneman’s Size Principle)
• Frequency code
More precise movements
Less precise movements
Motor Units and Muscle Force
Production
• 
The All-or-None Law (Bowditch’s Law) for motor units
– 
– 
Applies to individual motor units, but not the entire muscle.
The all-or-none law is based upon the difference between
graded potentials and action potentials
• 
• 
– 
– 
– 
Stimulation threshold
A motor unit is either activated completely or is not activated at all
If there is enough graded potential to create an action potential
that travels down the α-motor neuron of a motor unit, then all of
the fibers in that motor unit will contract.
The level of force production of a single motor unit is
independent of the intensity of the stimulus, but it is dependent
on the frequency of the stimulus
This law implies a stimulation threshold  important for the
Henneman’s Size Principle
Gradation of Muscle
Force
• 
Two neural mechanisms
responsible for force
gradations:
1.  Recruitment
  Spacial summation
2.  Rate coding
  Temporal summation
Recruitment
• 
Varying the number of motor units activated.
Rate Coding
Rate
coding
100
Larger
muscles
Smaller
muscles
% Maximal
Voluntary 50
Motor Unit
Recruitment
Motor unit
firing
frequency
0
0
50
% Maximal Voluntary Force
Production
100
Important!
•  Smaller muscles (ex: first dorsal
interosseous) rely more on rate coding
•  Larger muscles of mixed fiber types (ex:
deltiod) rely more on recruitment
Strength of muscle contraction
•  PRINCIPLE OF ORDERLY RECRUITMENT ACCORDING TO
SIZE: INTRACELLULAR, physiological stimulation of nerves, the
smallest axons are stimulated first (have the lowest threshold), and
largest axons are stimulated last.
•  This is another way to increase the force with which a muscle
contracts: "Recruit" more alpha-neurons to fire on the muscle. In this
case, again, the smallest neurons will fire first (small twitch tension),
and larger neurons will fire later (larger twitch tension).
•  V = IR: Small neurons have a higher resistance, which means they
will show a stronger depolarization (V) for the same current (I).
•  That's why they fire first. This is the opposite of external stimulation,
where smaller neurons are stimulated last.
•  In physiological stimulation, smaller neurons have a smaller
threshold potential than larger neurons.
Strength of muscle contraction
FORCE TRANSDUCTION: •  Twitch Tension in a muscle is increased by
increasing the frequency with which alpha-motor
neurons are fired.
•  With high frequency firing, the muscle doesn't get a
chance to completely relax before the next action
potential.
•  Tetanus is maximal twitch tension.
Organization of Motor Control
•  Hierarchical & Parallel
•  Parallel
–  pathways active simultaneously
–  e.g. moving arm
1. muscles producing movement
2. postural adjustments during
movement
•  Recovery of function after lesion
–  overlapping functions
Hierarchical Control of Movement
•  3 levels of control
•  Spinal cord (SC)
•  Brainstem
•  Cortex
•  Division of responsibility
–  higher levels: general commands
–  spinal cord: complex & specific
•  Each receives sensory input
–  relevant to levels function
Hierarchical Control: Spinal Cord
•  Automatic & stereotyped responses
–  reflexes
–  rhythmic motor patterns
•  Can function without brain
•  Spinal interneurons
–  same circuits as voluntary movement
•  Pathways converge on α motor neurons
–  final common path
Hierarchical Control: Spinal Cord
•  Motor neurons in ventral horn
•  Topographical organization of motor nuclei
•  motor neuron pools
–  longitudinal columns across 1-4 spinal segments
–  according to 2 rules
Topographical organization of motor nuclei
• 
F
P
D
E
Flexor-Extensor rule
ventral: extensors
dorsal: flexors
•  Proximal-distal rule
– medial: proximal muscles
– lateral: distal muscles
•  Parallel control systems
– proximal: postural
– distal: manipulative
Somatotopic distribution
Hierarchical Control: Brain Stem
•  Modulates neurons in spinal cord
•  interneuerons & motor neurons
–  2 main parallel pathways
•  Medial
–  to ventromedial spinal cord
–  postural / proximal muscles
•  Lateral
–  to dorsolateral spinal cord
–  manipulative / distal muscles
Hierarchical Control: Cortex
•  2 tracts
•  Corticobulbar --->cranial nerves
–  facial muscles
•  Corticospinal ---> spinal nerves
–  Origin of axons
1/3 from primary motor cortex (M1)
1/3 from premotor areas
1/3 from somatosensory cortex
Descending
motor
pathways
Descending motor pathways
• Medial system pathways
(axial and proximal muscles)
• Lateral system pathways
(distal muscles)
• Monoamine descending pathway
(general level of excitability)
Descending motor pathways
Descending motor pathways
Positive Signs: Babinski Sign
•  Lesion of corticospinal tract
•  Plantar reflex
–  Stroke firmly stroke sole of foot
•  heel ---> toe
•  Normal: flexion
–  toe curl down
•  Lesion: Extension
–  toes curl up and fan
Babinski sign
Muscle tone
Unconscious nerve impulses maintain the muscles in a partially contracted state. If a sudden pull or stretch occurs, the body responds by automatically increasing the muscle's tension, a reflex which helps guard against danger as well as helping to maintain balance.
Such near-continuous innervation can be thought of as a
"default” or "steady state" condition for muscles. There is, for the most part, no actual "rest state" insofar as activation is concerned. Both the extensor and flexor muscles are involved in the maintenance of a constant tone while "at rest". In skeletal muscles, this helps maintain a normal posture.
Muscle tone
Physical disorders can result in abnormally low (hypotonia) or
high (hypertonia) muscle tone. Hypotonia can present clinically as muscle flaccidity, where the
limbs appear floppy, stretch reflex responses are decreased,
and the limbʼs resistance to passive movement is also
decreased. Hypertonia can present clinically as either spasticity or rigidity.
While spasticity is velocity-dependent resistance to passive
stretch (i.e. passively moving an elbow quickly will elicit
increased muscle tone, but passively moving elbow slowly may
not elicit increased muscle tone), rigidity is velocityindependent resistance to passive stretch (i.e. there is uniform
increased tone whether the elbow is passively moved quickly
or slowly). Muscle tone testing
Muscle tone
testing
Muscle Weakness
•  Lesions produce different syndromes
•  Lower motor neuron syndrome
•  spinal motor neurons
–  lesion: soma or axon
–  Symptoms
• weakness
• fasciculations
• atrophy
Muscle Weakness
•  Upper motor neuron syndrome
•  descending motor pathways
–  imbalance of excitatory/inhibitory interneurons
–  symptoms
spasticity occurs
↑ tonicity & deep tendon reflexes
atrophy is rare
no fasciculations
Parallel Control & Recovery
•  Fractionation of movement
–  independent control of single muscles
–  via direct input from corticospinal tract
•  Lesion in medullary pyramids
–  can no longer grasp objects
–  locomotion, posture unaffected
•  Parallel pathways assume control
Parallel Control & Recovery
•  Monkeys: If premotor outflow spared
•  indirect control via brainstem
–  strength returns
–  but movement slow
•  M1 ---> lateral brainstem intact
–  cortico-rubrospinal & cortico-reticulospinal tracts
assume control
–  humans: fewer fibers ---> less recovery