Chapter 36
Disorders of Neuromuscular
Function
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Motor Unit
upper motor neurons
• Lower motor neuron
send axons down
spinal cord tracts
• Lower motor neuron’s axon
running through peripheral
nerves
• The muscles it innervates
lower motor
neurons in spinal
cord
peripheral nerves
muscles
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
b. Lower
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.
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Muscle Tone
• Muscle stretches
• Afferent neuron carries impulse to spinal cord
• Motoneurons cause muscle to contract
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Alterations in Muscle Tone
• Hypotonia
• Hypertonia
• Rigidity
• Clonus
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
The Motor Unit
• One lower motor
neuron
(motoneuron)
• The
neuromuscular
junction
• The muscle fibers
it innervates
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Tell whether the following statement is true or false:
To increase the strength of a contraction, more motor
neurons must be recruited.
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
True
A motor unit consists of branches of a neuron and the
skeletal muscles fibers that they innervate. For stronger
contractions, more motor units are required.
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Possible Problems
With the Motor Unit
• Lower motor
neuron lesions or
infections;
peripheral nerve
injury
• Neuromuscular
junction disorders
• Muscle atrophy or
dystrophy
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Tell whether the following statement is true or false:
Acetylcholinesterase stimulates the release of acetylcholine
(ACh).
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
False
Acetylcholinesterase breaks down ACh, resulting in
relaxation of the skeletal muscle.
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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)
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Peripheral Nerve Injuries (cont.)
• Mononeuropathies
– Damage to one peripheral nerve
– For example, carpal tunnel
syndrome
• Polyneuropathies
– Damage to many peripheral nerves
– For example, Guillain-Barre
syndrome
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Back Pain
• Peripheral nerve injury at the spinal nerve roots
• Often due to compression of nerve root by vertebrae
or vertebral disk
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Basal Ganglia Dysfunction Can Increase
Patterned Movement
• Tremors
• Tics
• Hyperkinesia
– Choreiform: jerky movements
– Athetoid: continuous twisting movements
– Ballismus: violent flinging movements
– Dystonia: rigidity
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Which disease is a result of basal ganglia dysfunction?
a. Myasthenia gravis
b. Multiple sclerosis
c. Polio
d. Tourette syndrome
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
d. Tourette syndrome
The tics and hyperkinesia that often accompany Tourette
syndrome are typical of basal ganglia dysfunction (the
function of the basal ganglia is movement control).
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Parkinsonism
• Tremor
• Rigidity
• Bradykinesia (slow movement)
• Loss of postural reflexes
• Autonomic system dysfunction
• Dementia
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Cerebellum Damage
• Vestibulocerebellar disorders
– Difficulty maintaining posture
• Cerebellar ataxia
– Movements divided into separate components
• Cerebellar tremor
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Multiple Sclerosis
• Destruction of myelin coating on axons
• Demyelinated or sclerotic patches develop
through white matter of CNS
• Decreased conduction velocity
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Which disorder is caused by damage to both upper and
lower motor neurons?
a. ALS
b. MS
c. Myasthenia gravis
d. Parkinson
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
a. ALS
Also known as Lou Gehrig’s disease, ALS is the result of
damage to both upper and lower motor neurons.
Typical S/S include weakness, lack of motor control,
denervation atrophy, and hyporeflexia.
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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-Sequard syndrome: damage to one side of cord
– Motor function lost on that side; pain/temperature
sensation lost from other side
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins