Chapter 35
Disorders of Neuromuscular
Function
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Upper Motor Neurons Are in the
Brain and Spinal Cord
• Upper motor neuron
cell bodies are in
the motor cortex
• They send their
axons down through
the internal capsule
• The axons then run
down the white
matter of the spinal
cord
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Two Motor Systems
• Extrapyramidal
– Most go to
same side of
body
Motor cortex neurons
Internal capsule
• Pyramidal
– Most cross to
other side of
body
Pons
Extrapyramidal
system
Pyramidal system
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Upper motor neurons
Motor Unit
• Lower motor neuron
• Lower motor neuron’s
axon running through
peripheral nerves
• The muscles it innervates
Send axons down
spinal cord tracts
Lower motor
neurons in spinal
cord
Peripheral nerves
Muscles
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Question
Which motor neurons are damaged in patients who have
neuromuscular disorders that directly affect skeletal
muscle?
a. Upper
b. Lower
c. Both upper and lower
d. Neither upper nor lower
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Answer
b. Lower
Rationale: The axons of lower motor neurons pass
through peripheral nerves to effector tissue in skeletal
muscle. Upper motor neurons’ axons travel down the
spinal cord.
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Muscle Tone
• Muscle stretches
• Afferent neuron
carries impulse
to spinal cord
• Motoneurons
cause muscle to
contract
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Alterations in Muscle Tone
• Hypotonia
• Hypertonia
• Rigidity
• Clonus
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Terms to Describe Motor Dysfunction
• -plegia = stroke or paralysis
• Paralysis = loss of movement
• -paresis = weakness
• Mono- = one limb
• Hemi- = both limbs on one side
• Di- or para- = both upper limbs or both
lower limbs
• Quadri- or tetra- = all four limbs
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Discussion
What would be the terms for the following?
• A defect causing weakness in both arms
• A weakness in the right arm and leg
• Inability to move one leg
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Upper vs. Lower
Motor Neurons
• Upper motor neurons
– In the brain and spinal
cord
• Lower motor neurons
– Send axons out of the
spinal cord
Upper motor neurons
Send axons down
spinal cord tracts
Lower motor
neurons in spinal
cord
Peripheral nerves
Muscles
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Upper Motor Neuron Damage
• Weakness and loss of voluntary motion
• Spinal reflexes remain intact but cannot be
modulated by the brain
– Increased muscle tone
– Hyperreflexia
– Spasticity
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Lower Motor Neuron Damage
• Neurons directly innervating muscles are affected
• Irritated neurons
– Spontaneous muscle contractions: fasciculations
• Death of neurons
– Spinal reflexes are lost
– Flaccid paralysis
– Denervation atrophy of muscles
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The Motor Unit
• One lower motor
neuron
(motoneuron)
• The
neuromuscular
junction
• The muscle fibers
it innervates
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Question
Tell whether the following statement is true or false.
To increase the strength of a contraction, more motor
neurons must be recruited.
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Answer
True
Rationale: A motor unit consists of branches of a neuron
and the skeletal muscle fibers that they innervate. For
stronger contractions, more motor units are required.
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Possible Problems
With the Motor Unit
• Lower motor
neuron lesions or
infections;
peripheral nerve
injury
• Neuromuscular
junction disorders
• Muscle atrophy or
dystrophy
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Skeletal Muscle Problems
• Disuse atrophy
• Denervation atrophy
• Muscular dystrophy
– Contractile
proteins not
properly
attached to
cytoskeleton of
muscle cell
– Protein
movement does
not effectively
contract muscle
cell
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Neuromuscular Junction Problems
• Decreased acetylcholine release
– Botulism
• Decreased acetylcholine effects on muscle cell
– Curare
– Myasthenia gravis
• Decreased acetylcholinesterase activity;
acetylcholine has a stronger effect on the muscle
cell
– Organophosphates
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Question
Tell whether the following statement is true or false.
Acetylcholinesterase stimulates the release of acetylcholine
(ACh).
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Answer
False
Rationale: Acetylcholinesterase breaks down ACh,
resulting in relaxation of the skeletal muscle.
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Myasthenia Gravis
• Autoimmune disease
– Gradual destruction of acetylcholine receptors
– Associated with thymus tumor or hyperplasia
• Gradual development of weakness
– From proximal to distal portions of body
• Myasthenia crisis: respiration compromised
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Peripheral Nerve Injuries
• Damage to LMN cell bodies in the spinal cord
• Damage to axons in the spinal or peripheral
nerves
• Damage to myelin sheath (demyelination)
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Peripheral Nerve Injuries (cont.)
• Mononeuropathies
– Damage to one peripheral nerve
– E.g., carpal tunnel syndrome
• Polyneuropathies
– Damage to many peripheral nerves
– E.g., Guillain-Barré syndrome
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Back Pain
• Peripheral
nerve injury
at the spinal
nerve roots
• Often due to
compression
of nerve root
by vertebrae
or vertebral
disk
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Motor Impulses Are Modulated by the
Basal Ganglia
• Upper motor
neuron cell bodies
are in the motor
cortex
• They send their
axons down
through the
internal capsule
• The basal ganglia
inhibit and
modulate
movement
patterns
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Basal Ganglia Dysfunction Can Increase
Patterned Movement
• Tremors
• Tics
• Hyperkinesia
– Choreiform: jerky movements
– Athetoid: continuous twisting movements
– Ballismus: violent flinging movements
– Dystonia: rigidity
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Question
Which disease is a result of basal ganglia dysfunction?
a. Myasthenia gravis
b. Multiple sclerosis
c. Polio
d. Tourette syndrome
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Answer
d. Tourette syndrome
Rationale: The tics and hyperkinesia that often
accompany Tourette syndrome are typical of basal
ganglia dysfunction (the function of the basal ganglia is
movement control).
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Parkinsonism
• Tremor
• Rigidity
• Bradykinesia (slow movement)
• Loss of postural reflexes
• Autonomic system dysfunction
• Dementia
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Cerebellum Damage
• Vestibulocerebellar disorders
– Difficulty maintaining posture
• Cerebellar ataxia
– Movements divided into separate components
• Cerebellar tremor
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Amyotrophic Lateral Sclerosis
• Damages both upper and lower motor neurons
• UMN damage  weakness, lack of motor control
– Loss of control over spinal reflexes  stiffness,
spasticity
• LMN damage
– Irritation  fasciculations
– Decreased neuron firing  weakness, denervation
atrophy, hyporeflexia
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Multiple Sclerosis
• Destruction of myelin coating on axons
• Demyelinated or sclerotic patches develop
through white matter of CNS
• Decreased conduction velocity
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Question
Which disorder causes damage to both upper and lower
motor neurons?
a. ALS
b. MS
c. Myasthenia gravis
d. Parkinson disease
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Answer
a. ALS
Rationale: Also known as Lou Gehrig disease, ALS
destroys both upper and lower motor neurons. Typical
S/S include weakness, lack of motor control,
denervation atrophy, and hyporeflexia.
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Spinal Cord Injury
• Immediate damage causes:
– Spinal cord shock
º Temporary complete loss of function below injury
– Primary neurologic injury
º Irreversible damage to neurons
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Secondary Injury to the Spinal Cord
• Neurons and white matter in area of initial damage
are affected
• Possible causes include:
– Damage to blood vessels supplying the area
– Decreased vasomotor tone decreasing blood
supply
– Local release of substances that cause
vasospasm
– Release of digestive enzymes from damaged
cells
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Partial Spinal Cord Injury
• Central cord syndrome: damage to axons near the gray
matter
– Arms more affected than legs
• Anterior cord syndrome: damage to anterior section of
cord
– Motor functions affected; touch sensation not
affected
• Brown-Séquard syndrome: damage to one side of cord
– Motor function lost on that side; pain/temperature
sensation lost from other side
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Complete Spinal Cord Injury
• To upper motor neurons (T12 and above)
– Spinal reflexes still work
– No longer modulated by brain
– Hypertonia, spastic paralysis
• To lower motor neurons (T12 and below)
– Cells in spinal reflex arcs damaged
– Flaccid paralysis
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