Motor control
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Proprioception and movement
• in general we are not aware of information
coming from proprioceptors though they
belong to somatosensory receptors
• these receptors detect stretching of the
muscles and tension in tendons and induce
reflexes as well as provide information for
the control of movement
• receptors in joints detecting the angle of the
joints also belong to this category
• they also provide input for the control of
movement
• these facts justify treatment of these
receptors in the framework motor control
1
Final common pathway
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• somatic and visceral motor systems represent
the output of the CNS
• in the somatic system the final common pathway
is the motor neuron in the ventral horn of the
spinal cord or in the motor nucleus of cranial
nerves in the brainstem
• in the visceral system the final common pathway
means the motor neuron in the lateral horn of
the spinal cord or in the vegetative nucleus of
cranial nerves in the brainstem
• final common pathway means (Sherington) that
executive organs can be only accessed through
these motor neurons, integration occurs at this
or at a higher level
• in the somatic system, fibers innervate targets
(skeletal muscle) directly, in the visceral (smooth
muscle and gland cells) through an intercalated
neuron
Hierarchical organization
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• areas where stimulation causes movements are
classified as motor areas
• these areas receive somatosensory input as
well, thus they are sometimes referred to as
somatomotor areas and systems
• regulation is done on several levels – the higher
the level the more complicated movements are
controlled, though lower levels are influenced by
descending effects as well
• cortical areas are able to control motor neurons
in the spinal cord and brainstem directly, but
they also exert their effects on the spinal cord
indirectly through the brain stem
• in addition to these pathways, cerebellum and
basal ganglia also participate in motor control
by influencing brain stem and cortical areas
• somatotopy is present at every level
2
Organization of motor system
cortex
basal ganglia
thalamus
brain stem
cerebellum
motor neuron sensory pathway
brain stem and spinal cord
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Muscle spindle
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• muscle spindle is 4-10 mm long, it is made up by
6-8 modified muscle fibers, and is located in
parallel with regular muscle fibers
• muscle spindle is encased in a connective tissue
capsule
• within the muscle spindle, the middle part of
intrafusal fibers is modified for receptor
function, terminal parts are able to contract
• usually there are 2 nuclear bag (one dynamic,
one static) and 5-6 nuclear chain fibers (all
static) in a spindle
• primary terminals form spirals around the middle
part of all three types of receptors annulospiral terminal - Ia (Aα) afferent
• secondary terminals (flower spray endings)
target static nuclear bag and nuclear chain
fibers - II (Aβ) afferent
3
Operation of the muscle spindle
• when extrafusal (regular) fibers contract,
intrafusal fibers are shortened and relax
• when extrafusal fibers are stretched, then
intrafusal fibers are also stretched –
elongation of receptor terminals increase the
discharge rate
• in case of continuous stretch polar regions of
dynamic nuclear bag receptors lengthen due to
their viscoelastic properties, thus excitation
of the nerve terminal decreases
• regular muscle fibers are innervated by Aα
α
axons, intrafusal fibers by Aγγ axons –
excitation is parallel, thus sensitivity of the
receptor remains constant
• previously a servo-mechanism was suggested
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The tendon organ
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• tendon organs are in series with muscles
• they are 1 mm long structures surrounded
by a capsule
• collagen fibers of the tendon penetrate the
muscle
• perpendicularly Ib afferents, branches run
between fibers or wrap around them
• they are deformed with increasing tension
– excitation
• tendon organs inform about the contraction
and passive extension of muscles, but they
are more sensitive to the former
• thus, they provide information about forces
developing in the muscles
4
Spinal reflexes I.
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• the lowest level in the hierarchy of the motor
control is represented by the spinal reflexes
• the myotatic (stretch) reflex is monosynaptic:
afferents terminate on the motoneuron of the
same muscle – proprius (Latin): one’s own
• stretching of muscle spindle induces contraction
of the same muscle
• patella reflex, Achilles tendon reflex – mostly in
extensors, though in flexors as well (biceps)
• collateral of Ia afferent inhibits antagonist
motoneuron through an inhibitory interneuron –
reciprocal innervation is characteristic for the
spinal cord – simultaneous contraction is
organized always at a higher level
• myotatic reflex can be dynamic or tonic, in the
second type secondary terminals also participate
Spinal reflexes II.
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• diagnostic value: motoneuron excitability – direct
stimulation: H or Hoffman reflex
• its function is to keep posture, thus it is
stronger in extensors than in flexors, but sloth
• divergence and convergence in respect of agonist
muscles are both present – though the reflex
remains segmental
• muscle tone (resistance against passive moving) is
due to this reflex: a subset of motor units are
always slightly contracted
• myotatic reflex, thus muscle tone is continuously
modified by descending effects through setting
the sensitivity of the motoneuron (e.g. REM)
• collateral of Ia afferent terminates on neurons
belonging to the column of Clarke:
spinocerebellar tract
5
Spinal reflexes III.
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• inverse myotatic reflex starts from tendon
organs
• incoming fibers target agonist motoneuron
through inhibitory interneurons, antagonist
through excitatory ones
• main function is to protect muscle and tendon
against overstretching
• it also supplements myotatic reflex: when tension
in tendon decreases – weaker inhibition –
contraction
• flexion withdrawal reflex starts from nociceptors
(exteroceptive), its function is to remove
extremity from the noxious stimulus source
• it is polysynaptic, in addition to activate
antagonist muscles, it also induces crossed
extension reflex
• the reflex is intersegmental, many muscle groups
can participate
Motor stereotypes
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• spinal cord is able to organize simple motor
stereotypes – these are intersegmental
• scratching reflex involves rhythmically
alternating movements, frequency is independent
from the strength of the stimulus, it only
increases duration
• afterdischarge is characteristic (reverberating
circuits)
• stereotypes are generated by a central rhythm
generator no feedback from proprioceptors is
needed – based on mutual inhibition, adaptation
and rebound
• walking has similar central organization, but
feedback from proprioceptors is needed and
central descending effects influence frequency
(walk, trot, canter, gallop)
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Spinal cord organization I.
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• location of motor neurons follows somatototopy
• motor neurons of proximal and distal muscles are
located medially and laterally, respectively
• axial muscles in the midline of the trunk belong
to the most medial motor neurons
• these motor neurons receive input from
interneurons on both sides – bilateral control –
posture
• motor neurons of extensors and flexors are
located ventrally and dorsally, respectively
• motor neurons controlling a given muscle are
found in 1-4 neighboring segments – motor
neuron pool
• within the pool muscle fibers innervated by 1
motor neuron form the motor unit – 10 (eye),
100 (hand), 2000 (foot) fibers
• all fibers in a unit are the same type
Types of muscle fibers
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• tonic fibers
–
–
–
–
postural muscles in amphibians, reptiles and birds
muscle spindles and extraocular muscles in mammals
no AP, motor axon forms repeated synapses
slow shortening – effective isometric contraction
• slow-twitch (type I) fibers
– mammalian postural muscles
– slow shortening, slow fatigue – high myoglobin
content, large number of mitochondria, rich blood
supply – red muscle
• fast-twitch oxidative (type IIa) fibers
– specialized for rapid, repetitive movements – flight
muscles of migratory birds
– many mitochondria, relatively resistant to fatigue
• fast-twitch glycolytic (type IIb) fibers
– very fast contraction, quick fatigue
– few mitochondria, relies on glycolysis
– breast muscles of domestic fowl – white muscle
7
Spinal cord organization II.
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• reflexes activate only part of the motor units
– fractionization principle
• reflexes and voluntary movements can be
graded – more and more units get involved –
recruitment
• activation follows size principle – first small
units are activated – motor neurons are also
small, EPSPs are more effective
• the largest units contain fast-twitch glycolytic
fibers (white muscle) – they are only activated
when really necessary
• in addition to recruitment, frequency can be
also increased - during voluntary movements
8-25 Hz causing incomplete tetanus – motor
units contract asynchronously
Inhibitory interneurons
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• α-motor neurons are innervated by three
types of inhibitory interneurons: Ia, Ib and
Renshaw
– Ia receives input from the muscle spindle in the
antagonist muscle, but it is also activated by
descending fibers targeting the antagonist motor
neuron
– it also receives inhibitory inputs – suspension of
reciprocal innervation – „column” function
– Ib receives input from the tendon organ, but it is
also influenced by descending pathways, receptors
in the skin and joints – these control the strength
of contraction: touching, stroking
– Renshaw receives input from collaterals of α-motor
neurons – feedback inhibition
– sensitivity is controlled by descending excitatory
and inhibitory pathways
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Brain stem reflexes and posture
• lesions of the neuraxis change the tone of
postural muscles – Sherington: tone in these
muscles is caused by reflexes
• tone is modified by descending effects: lifting
one leg increases the tone of the others
• transection between the n.ruber and the Deiters’
nucleus – decerebrate rigidity in tetrapods
• it can be abolished by cutting the reflex arch
• Deiters’ nucleus (tr. vestibulospinalis lat.) and
pontine RF (tr. reticulospinalis med.) strongly
enhances extensor tone
• inhibitory effects:
– cerebellum
– in tetrapods tr. rubrospinalis from n. ruber
– in primates it has only effect to the level of cervical
segments, cortex is more important
– medullary RF – tr. reticulospinalis lateralis
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Voluntary movements I.
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• the background for voluntary movements is
provided by muscle tone control
• appropriate rearrangement of muscle tone is
needed to preserve posture during voluntary,
reflex and stereotype movements
• common characteristic of voluntary movements
that they become automated through learning
and exercise – learning to walk in babies
sports, etc.
• Fritsch and Hitzig 1870: cortical stimulation
in dogs might lead to movements
• information about the organization of cortical
motor control was collected from five sources:
–
–
–
–
–
stimulation studies (Penfield human surgeries)
analysis of brain injuries
unit recording in monkeys
imaging, e.g. PET
anatomical studies
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Voluntary movements II.
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• based on these data we know what type of
damages impair control of voluntary movements,
and which areas are activated first –
organization of motor control is less clear
• primary motor area: Br.4 – gyrus precentralis
• somatotopy is similar to the somatosensory area:
leg medially, representation is proportional with
the sophistication of movements
• secondary motor cortex: Br.6 – in front of
primary
• it consists of two parts: supplementary motor
area and premotor cortex
• their role is in the preparation (premotor), and
in the planning (supplementary) of movements:
electrophysiological and blood flow changes
before the movements and during contemplating
of movements
Voluntary movements III.
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• the corticospinal or pyramidal tract is the most
important motor pathway
• most of the fibers originate from layer V
pyramidal neurons in Br.4 and 6, but from
other areas as well – upper motor neurons
• 90% of the axons cross over and run in the
„pyramids” on the surface of medulla (name),
then in the lateral corticospinal tract, 10%
cross in the spinal cord (ant. tr.) before ending
• direct effect on α-motor neurons (lower motor
neurons), indirect effect through interneurons
• motor cortices receive input from the VL
(thalamus) and from the somatosensory cortex
• VL transmits information from cerebellum and
putamen, no direct projection
• interaction goes both ways (see before)
• movements can be elicited from other cortical
areas as well, but with strong stimuli only
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Cerebellum I.
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• cerebellum coordinates motor activities, its
injuries impair coordination and execution of
voluntary movements
• motor learning is also lost
• cerebellum has more neurons than the other
parts of the CNS
• modular structure, János Szentágothai
contributed heavily to its description
• principal neurons are inhibitory Purkinje cells,
projecting to deep cerebellar nuclei that in turn
project to VL
• various excitatory (e.g. granule) and inhibitory
(e.g. Golgi) interneurons
• input: climbing fiber (contralateral oliva inferior)
and mossy fiber (cortex, spinal cord, brainstem)
• multiple somatotopical representation in the
cortex and deep cerebellar nuclei
Cerebellum II.
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• three parts (connections, evolution):
– vestibulocerebellum (archeocerebellum) – oldest,
caudal part (flocculus, nodulus)
• direct input from semicircular canals, utriculus, sacculus
• direct output to Deiters’ nucleus (it can be considered as a
deep cerebellar nucleus)
• balance and gait, coordination of eye movements, reflex
movements of the head
– spinocerebellum (paleocerebellum) – middle part of
cerebellum (vermis, central, intermediary parts of the
hemispheres)
• input through the dorsal spinocerebellar tract from sensory
afferents – information about the position of extremities and
about changes that occurred (external feedback)
• input through the ventral spinocerebellar tract about the
activity of interneurons: it reflects descending commands
(internal feedback)
• the cerebellum monitors the execution of motor commands
– cerebrocerebellum (neocerebellum) – lateral part of
hemispheres
• input from the cortex and n. ruber through the pons
• output to n. dentatus, thalamus, cortex
• planning, starting and stopping as well as learning of
movements
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Basal ganglia I.
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• basal ganglia consist of:
– neostriatum (n. caudatus + putamen)
– pallidum or globus pallidus (neostriatum+pallidum=
corpus striatum, putamen+pallidum= n.lentiformis)
– substantia nigra (pars compacta and pars reticularis)
– n. subthalamicus
• main functions are in the control of movements
and muscle tone – deducted from impairments
caused by lost of different cell groups
• Parkinson’s disease: muscle rigidity, tremor,
slowing or loss of physical movements (see
Awakenings) – caused by loss of dopaminergic
cell in substantia nigra pars compacta – MPTP
(methyl-phenyl-tetrahydropyridine)
• Huntington’s chorea: abnormal, jerky, random
movements (chorea: Greek word for dance) –
cholinergic and GABAergic neurons in the
neostriatum die – genetic background is known –
prenatal diagnosis – ethical issues
Basal ganglia II.
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• outdated conception: pyramidal – extrapyramidal
pathways
• extrapyramidal was supposed to originate from
the neostriatum – incorrect, the term is rarely
used now
• neurons of the basal ganglia are not activated
before cortical neurons – no role in initiating
movements
• they modify through multiple circuits involving
thalamic VA (ventralis anterior), VL (ventralis
lateralis) and CM (centre median) the
functioning of the motor cortex
• in Parkinson’s disease stimulation of the direct
and inhibition of the indirect pathway decrease
– less excitation on VA, VL
• in Huntigton’s chorea the indirect pathway is
affected more - VA, VL inhibition decreases
12
Muscle spindle
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 12-1
Afferents in the muscle spindle
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 12-2
13
Myotatic reflex
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 12-11
Inverse myotatic reflex
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 12-12
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Flexion withdrawal reflex
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 12-13
Somatomotor cortex
Blumenfeld, Sineauer Assoc. Inc., 2002, Fig. 2-13
15
Cerebellum
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 14-9
Divisions of the cerebellum
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 14-11
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Somatotopy in the cerebellum
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 14-12
Direct and indirect pathways
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 14-21
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