Michael Beierlein PhD Department of Neurobiology and Anatomy MSE R442 McGovern Medical School Houston TX Email michaelbeierleinuthtmcedu Cognition Action Environment Senses Motor control ID: 916061
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Slide1
Motor systems I
Spinal cord circuits and motor control
Michael Beierlein, PhD
Department of Neurobiology and Anatomy, MSE R442
McGovern Medical School
Houston, TX
Email: michael.beierlein@uth.tmc.edu
Slide2Cognition
Action
Environment
Senses
Motor control
Slide3Components of Proper Motor Control Volition
transformation of abstract goals into activation of specific set of muscles Coordination of signals to many muscle groups
Proprioception
constant updates about muscle length, force, joint position
Postural adjustments
Sensory feedback
Compensation for body and muscles Unconscious processingWalking, postural adjustments, etc.
Adaptability acquisition of motor skills
Slide4Hierarchical Organization and Functional Segregationof Central Motor Structures
Slide5Major components involved in motor control
Nolte
Slide6From J. Nolte (2002)
The Human Brain, 5
th
Edition
Alpha
(lower) motor neurons – the “final common pathway”
Alpha motor neuron
Slide7Motor Unit:
a single motor neuron and all of the muscle fibers that it
innervates. Each muscle fiber only receives input from
one
motor neuron
Innervation ratio:
number of muscle fibers which are part of a motor unit (~10
to more than 1000)
Slide8Motor n
euron pool and motor unitsMotor
neuron pool (motor
n
ucleus):
The
group of motor neurons
(MNs) that innervate a single muscle. Muscles used for finer movements have a larger MN pool
Motor unit:
An individual motor neuron and all the muscle fibers that it
innervates
Slide9Motor nuclei (motor neuron pools) - rod-like clusters
of
alpha
motor
neurons in the ventral
horn of the spinal
cord
Slide10Somatotopic organization of alpha motor neurons
Slide11Control Of Muscle Force
Rate code:
F
orce generated by a given muscle depends on
firing rate
of alpha motor neuron
Slide12Size principle: Small motor neurons are recruited first, then larger neurons
Control Of Muscle Force
Slide13Size principle: Small motor neurons are recruited first, then larger neurons
Control Of Muscle Force
Slide14Size principle: Small motor neurons are recruited first, then larger neurons
Control Of Muscle Force
Slide15From G. E. Loeb & C.
Ghez (2000), in Principles of Neural Science, 4th
Edition
(
Kandel
, Schwartz, &
Jessel
, Eds.)Size
principle: Small motor neurons are recruited first, then larger neurons
smaller neurons have larger input resistance (same synaptic current evokes larger excitatory postsynaptic responses)
Fewer synaptic inputs are needed to generate action potential in smaller neuron
Control Of Muscle Force
Slide16Size principle: 1. Small motor neurons are recruited first, then larger neurons
Control Of Muscle Force
Slide17Control Of Muscle Force
Size principle:
2
. Motor neurons of different size contact muscle fibers with distinct properties
-
small
motor neurons innervate
slow-twitch, fatigue-resistant fibers
- intermediate-sized
motor neurons innervate fast-twitch, fatigue-resistant fibers
- large motor neurons innervate fast-twitch, fatigable muscle fibers.
Slow twitch fibers (type I):
rely on aerobic metabolism, contract more slowly and generateless force, resistant to
fatigueFast twitch fibers (type II): rely on anaerobic metabolism, contract rapidly and generatelarge force, fatigue slowly (type
IIa) or quickly (type IIb)
slow-twitch
fast-twitch
fatigue-resistant
fast-twitch
fatigable
Slide18Control Of Muscle Force via motor unit with distinct size and properties
Slide19Control Of Muscle Force
Size principle:
3
. Larger motor neurons are part of larger motor units
Slide20Control of muscle force - Gradual recruitment of distinct types of motor units under different behavioral conditions
Slide21Summary - size principle: o
rderly recruitment of motor neurons (according to their size) with increases in afferent activity
slow-twitch, fatigue-resistant muscle fibers are recruited first, then fast-twitch
,
fatigue-resistant fibers, then fast-twitch fatigable fibers
Average size of motor
unit
increases with increases in muscle force production
Physiological Functions:Minimizes amount of muscle fatigue
Precise control of muscle force at all levels of muscle output
Control Of Muscle Force
Slide22Proprioception
Specialized receptors provide
information about muscle:
Length
velocity
(change in length)
load
(force
)Receptors
involved in proprioception
muscle spindles
golgi
tendon organs
Slide23Motor control and proprioceptive input
The case of Ian Waterman (“IW – the man who lost his body”)
at age 19 had never-diagnosed
fever that is
believed to
have set off an auto-immune
reaction
l
ost all somatosensory and proprioceptive input from the neck down, no paralysisinitially unable to make coordinated movements
trained himself to make movements under visual guidance
Requires total concentration to move and even maintain postureCollapses when blindfolded
Clip here
https://www.youtube.com/watch?v=pMEROPOK6v8
Slide24Proprioception
Receptors
muscle spindles
-
in parallel with
muscle
- signal length and velocity
g
olgi
tendon organs
- in series with muscle
- signal tension (force)
Slide25Muscle spindle
Slide26Three types of intrafusal muscle fibers in a muscle spindle
Nuclear chain fibers - signal static
length
of
muscle
Static nuclear bag fibers
- signal static length of muscleDynamic
nuclear bag fibers - signal rate of change
of muscle length
A typical muscle spindle is composed of
1 dynamic
nuclear bag fiber, 1 static nuclear bag fiber, and ~5 nuclear chain fibers
Slide27Innervation of muscle spindles
Sensory
Type
Ia
:
innervate all fibers via
annulospiral
endings
, report both muscle velocity
and
length
Type II: innervate nuclear chain and static nuclear bag fibers via
flower spray endings, report only muscle lengthMotor
Gamma motor neurons: innervate only intrafusal muscle fibers
Slide28Gamma motor neurons – control sensitivity of muscle spindles
innervate intrafusal fibers only
ensure that muscle spindle stays taut and responsive during changes in muscle length
alpha-gamma
coactivation
(coordinated contraction of extra- and
intrafusal fibers
Gamma motor neurons can be independently targeted by descending pathways, thereby regulating muscle spindle sensitivity
Slide29Golgi Tendon Organ
- in series with muscle
-
a
ctivated following increases in muscle tension
- innervated by Group
Ib
fibers, no motor neuron innervation
Slide30Muscle spindles: respond to changes in length and velocity
of muscle Golgi tendon organs: respond to changes in load or force applied muscle
Spinal
cord circuits
Motor control of limbs and
body
Rhythmic Movements (e.g. walking)
Reflexes
Slide32Myotatic
reflex (stretch reflex)
Activated by:
muscle spindle (in e.g. flexor), in response to stretched muscle
Mediated by:
Ia
afferent fibers
+
alpha motor neuronsActivation of:
alpha motor
neuronsResults in: homonymous muscle contraction (e.g. flexor)
Function: control of posture, e
tc.
Slide33Activated by:
muscle spindle (in e.g. flexor
)
Mediated by:
Ia
afferent fibers
+
Ia
inhib. interneuron + alpha motor neuronInhibition of:
alpha motor neuron of opposing muscleResults in:
inhibition of opposing muscle contraction (e.g. extensor)Function: minimizes co-contraction of opposing muscles
Inhibition
in the myotatic reflex (stretch reflex)
Slide34From K. Pearson & J. Gordon (2000), in
Principles of Neural Science, 4
th
Edition
(
Kandel
, Schwartz, &
Jessel, Eds.)
Reflex modulation (in this case enhancement) by descending inputs
Slide35Stretch reflex
Hyporeflexia:
problem with the reflex arc itself
Hyperreflexia
: problem with descending control of reflex
Slide36Activated by:
golgi
tendon
organ, in response to muscle tension
Mediated by:
Ib
afferent fibers
+ Ib
inhib. interneuron + alpha motor neuronInhibition of:
alpha motor neuron
Results in: inhibition of homonymous muscle contraction
Function: limits excessive tensions in muscles, spread of workload
across many muscle fibers Autogenic inhibition reflex
Slide37Flexor reflex
Activated by:
c
utaneous or pain receptors
Mediated by:
g
roup III afferent fibers
+
excitatory interneurons +
alpha motor neuronsExcitation of:
alpha motor neurons for separate flexor muscles in thigh and hip
Results in:
contraction of thigh and hip flexors Function: coordination of muscle activityover multiple joints
Reciprocal inhibition in the flexor reflex:Mediated by:
group III afferents + inh. interneurons +
alpha motor neurons in extensor muscle
Slide38Crossed extension reflex (part of flexor reflex)
Activated by:
c
utaneous and pain receptors
Mediated by:
g
roup III afferent fibers
+
excitatory interneurons crossing midline
+ alpha motor neurons on contralateral side
Excitation of:alpha motor
neurons in extensor ino
pposite legResults in:contraction of extensor musclesFunction: m
aintain balance and posture duringflexor reflex
Slide39Reading:
Neuroscience onlinehttps://
nba.uth.tmc.edu/neuroscience/s3/chapter01.html
https://
nba.uth.tmc.edu/neuroscience/s3/chapter02.html
Purves
Chapter 16, Lower motor neuron circuits and motor control