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Lecture 16

CSB332H1 Lecture Notes - Lecture 16: Golgi Tendon Organ, Motor Neuron, Afferent Nerve Fiber


Department
Cell and Systems Biology
Course Code
CSB332H1
Professor
Melanie Woodin
Lecture
16

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CSB332H1S L16; March. 14, 2012
Motor Control
Limb muscles are controlled my motorneurons &
interneurons of the spinal motor apparatus
Interneurons w/i spinal cord & brainstem make u central
pattern generators that direct motor apparatus
Motor output planned & refined by motor cortex, basal
ganglia, & cerebellum
Movement either voluntary or unconscious reflexes
Use reflex to compensate – need sensory fdbk to CNS
(spinal or cortex)
oSimplest level is reflex
Need something to tell muscle that something is going
on & something to control output
Neural organization of motor control is hierarchical
1. Simplest level: sensory neurons synapse w
motoneurons in spinal cord to mediate simple reflexes
oBasic reflex – ex. Holding something steady
2. Central pattern generators: networks of interneurons in
spinal cord & brainstem coordinating interplay of
multiple motor grps (locomotion, respiration)
oNot in this lecture
3. Motor output that is planned & refined by motor cortex,
basal ganglia & cerebellum
The Motor Unit
Sherrington: spinal motoneuron = final common
path because all the neural influences that
concern movement or posture converge upon it
motoneuronsα – major motoneurons of spinal cord
ocause muscles to contract
motoneuronsγ – smaller motoneurons, regulate
sensitivity of muscle spindles
oMake sure don’t over or under contract muscle,
fine-tuning
Motor unit – a single motoneuron & all muscleα
fibres it innervates
Motor pool – all motor neurons supplying a
particular muscle
oSingle muscle innervated by multiple grps of
motoneurons
The Motor Unit
Muscle contraction begins w small motor units
being activated (or recruited) first.
When more contraction is required progressively
larger motor units recruited
oBecause small motoneurons = more easily
excited than large motoneurons, so more easily
activated
Single muscle spindle fibre (1a) has multiple inputs to
multiple alpha motoneurons
1 act pot in a single 1a
afferent synaptic pot in
alpha motoneuron – only a
fraction of a mV (blue)
oVery small depolarization
Presynaptic fiber fires 3 act
pots rapidly in succession
synaptic pots (brown) ride on
falling phase of previous one,
increasing the depolarization
– prob enough to have effect
in motoneuron
A muscle (e. soleus in cat)
may have 50 1a afferents
converging onto 1
motoneuron
A strong stretch of muscle
can activate all 1a afferents
o Indiv EPSPs will
spatially summate to
depolarize membrane
o activation of alpha
motoneuron
Agonists & Antagonists
Limb movements are produced by coordinated
contraction of grps of muscles that work together.
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