Bi/CNS 150 Lecture 15
Monday November 3, 2014
Motor Systems
Kandel, Chap. 14:p. 309-310 35, 37
Bruce Cohen
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Overview
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Motor neurons can be divided into two classes, “lower” and
“upper”
“Lower” motor neurons are located in the spinal cord and
brainstem
They initiate skeletal muscle contraction and are
cholinergic (release acetylcholine)
Lower motor neurons send axons directly to muscle fibers
Local circuits in the brainstem and spinal cord primarily
determine the spatial and temporal activation of lower
motor neurons
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Upper motor neurons
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“Upper” motor neurons are located in the brainstem and
cerebral cortex, and are glutamatergic (release glutamate)
Their axons form descending pathways such as the
corticospinal tract that modulate the activity of the lower
motor neurons
Upper motor neurons govern voluntary motor movements
such as locomotion
Damage to the descending pathways of the upper motor
neurons can cause weakness, spasticity (increased muscle
tone), and the loss of the ability to perform fine motor
movements
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Motor Areas of Cortex
Frontal Eye Fields
BA 8
Premotor/supplementary
Motor cortex
BA 6
Primary Motor Cortex
BA 4
Prefrontal Cortex
(Frontal
Association Areas)
Broca’s Area
(left side)
BA 44, 45
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Layer 5 is particularly prominent in primary motor cortex
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Flow chart of motor system hierarchy
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Corticospinal tract:
A key motor tract
Decussation in hindbrain
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Motor Unit
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Motor unit consists of a single  motor neuron and all the muscle fibers
that it innervates
Most muscle fibers in mature mammals are innervated by a single 
motor neuron
When the motor neurons fires, all innervated muscle fibers contract
because the endplate potential typically exceeds threshold voltage
Motor unit is the smallest unit of force that can be activated to produce
movement
To increase the force of a muscle contraction additional motor units are
recruited
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Fewer Myelinated Fibers in Lower Spinal Cord
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Motor neuron in spinal cord cross section
Dorsal Horn
Sensory
Motoneuron
Myelin
Ventral Horn
Motor
Ventral Root
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Motor
Size principle
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Force exerted by muscle contraction is increased by recruiting more motor units
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The size of motor units varies from small to large
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Small  motor neurons innervate relatively few muscle fibers, large  motor neurons
innervate many muscle fibers
Small motor units generate fine motor movements, large motor units generate gross
motor movement
Synaptic input to the pool of motor neurons excites small ones first because they
have the greatest input resistance
The size principle states that, during voluntary and reflexive movements, the
smallest motor units are recruited first and larger motor units are recruited later
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Orderly recruitment of motor units
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Spinal Reflexes
•Stretch reflex is a monosynaptic
spinal reflex triggered by
stretching a muscle
•Muscle spindles sense, and
signal, changes in muscle length
•Axons from spindle contact motor
neurons that drive the muscle they
are in (homonymous), synergist
muscles, and inhibitory
interneurons
•Inhibitory interneurons inhibit
motor neurons driving the
antagonist muscles
• They inhibit extensors when
flexors are commanded, and viceversa
Figure 35-2B
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The stretch reflex acts as a negative feedback loop to maintain
muscle length
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Fine structure of muscle spindle
Intrafusal fibers in parallel with extrafusal muscle fibers
Two types of sensory fibers – primary (Group Ia fibers)
and secondary (Group II fibers) spindle afferents
Group Ia – change in length (dynamic)
Group II – length (static)
Golgi tendon organ measures tension of muscle
contraction (not shown)
Extrafusal
fibers
Sensory information goes to spinal cord segment,
dorsal column nuclei (proprioception), and cerebellum
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Gamma motor neurons regulate muscle tone
Gamma motor neurons are small MNs that
project from ventral roots to intrafusal fibers
Activity in gamma-MNs contracts the intrafusal
muscles and makes the spindle apparatus more
sensitive
In turn, the group Ia and II fibers become more
active
Gamma-bias impacts muscle tone
Extrafusal
fibers
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Amyotrophic lateral sclerosis (ALS)
“Lou Gehrig’s Disease”
“Upper” motor
neurons also
degenerate
ALS symptoms
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Loss of motor unit innervation leads to
weakness or paralysis of muscle
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Fasciculations (spontaneous contractions
of muscle fibers); detected with
electromyography (EMG)
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Atrophy of muscles, due to loss of trophic
factors from motoneuron
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Hyporeflexia or areflexia
Average time from diagnosis to death ~ 3
yr
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Effects of damage to the upper and lower motor neurons
Lower Motor Neuron
Upper Motor Neuron
Paralysis
Paresis (weakness)
Muscle atrophy
No atrophy
Areflexia & atonia
Hyperreflexia, hypertonia, spasticity
Ipsi-lateral deficit in spinal cord
Contra-lateral deficit above decussation;
Ipsilateral deficit below decussation
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Stimulation in human motor
cortex.
An array is implanted . . .
to localize an epileptic focus
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Anterior Cingulate Cortex
Lesions in ACC cause impair one of the hierarchically highest levels of the motor system:
the will to act .
Patients with ACC lesions can exhibit "akinetic mutism": they are not paralyzed and are
conscious but respond poorly to their surroundings.
They sometimes exhibit conditioned responses, like picking up a phone that rings next to their
bedside (but then say nothing).
They often recover, and then explain that while in this state, they were fully conscious but just
lacked motivation to do anything and so did not respond or act on their surroundings.
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End of Lecture 15