CHAPTER 36DISORDERS OF NEUROMUSCULAR FUNCTION

Essentials of Pathophysiology

PRE LECTURE QUIZ (TRUE / FALSE)

  Paralysis refers to weakness or incomplete loss of

muscle function.   Carpal tunnel syndrome is an example of a

polyneuropathy. All levels of spinal cord injury will require assistance

to maintain breathing. Autonomic dysreflexia represents an acute episode

of exaggerated sympathetic reflex responses that occur in persons with some types of spinal cord injuries.

The pathophysiology of multiple sclerosis involves the demyelination and subsequent degeneration of nerve fibers in the central nervous system.

F

F

F

T

T

PRE LECTURE QUIZ1. Myasthenia gravis, an autoimmune disorder, is caused by an

antibody-mediated destruction of ______________________ receptors in the neuromuscular junction.

2. Guillain-Barré syndrome is characterized by progressive ascending muscle _____________________ of the limbs, producing a symmetric flaccid paralysis.

3. Parkinson disease is a degenerative disorder of the basal ___________________ that results in variable combinations of tremor, rigidity, and bradykinesia.

4. A __________________ is a an irregularly occurring, brief, repetitive movement such as winking, grimacing, or shoulder shrugging.

5. Muscular dystrophy is a term applied to a number of genetic disorders that produce progressive degeneration and necrosis of skeletal muscle __________________, which are eventually replaced with fat and connective tissue.

Acetylcholine

Fibers

Ganglia

Tic

Weakness

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

TWO MOTOR SYSTEMS

Extrapyramidal Most go to

same side of body

Pyramidal Most cross to

other side of body

Motor cortex neurons

Internal capsule

Pons

Extrapyramidal system Pyramidal system

MOTOR UNIT

Lower motor neuron

Lower motor neuron’s axon running through peripheral nerves

The muscles it innervates

Upper motor neurons

Send axons down spinal cord tracts

Lower motor neurons in spinal

cord

Peripheral nerves

Muscles

QUESTION

Which motor neurons are damaged in patients who have neuromuscular disorders that directly affect skeletal muscle?

a. Upperb. Lowerc. Both upper and lowerd. Neither upper nor lower

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.

MUSCLE TONE

Muscle stretches

Afferent neuron carries impulse to spinal cord

Motoneurons cause muscle to contract

ALTERATIONS IN MUSCLE TONE

Hypotonia Hypertoni

a Rigidity Clonus

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

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

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

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

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

THE MOTOR UNIT

One lower motor neuron (motoneuron)

The neuromuscular junction

The muscle fibers it innervates

QUESTION

Tell whether the following statement is true or false.

To increase the strength of a contraction, more motor neurons must be recruited.

ANSWER

TrueRationale: 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.

POSSIBLE PROBLEMS WITH THE MOTOR UNIT

Lower motor neuron lesions or infections; peripheral nerve injury

Neuromuscular junction disorders

Muscle atrophy or dystrophy

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

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

QUESTION

Tell whether the following statement is true or false.

Acetylcholinesterase stimulates the release of acetylcholine (ACh).

ANSWER

FalseRationale: Acetylcholinesterase breaks

down ACh, resulting in relaxation of the skeletal muscle.

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

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)

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

BACK PAIN

Peripheral nerve injury at the spinal nerve roots

Often due to compression of nerve root by vertebrae or vertebral disk

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

BASAL GANGLIA DYSFUNCTION CAN INCREASE PATTERNED MOVEMENT Tremors Tics Hyperkinesia

Choreiform: jerky movements Athetoid: continuous twisting

movements Ballismus: violent flinging

movements Dystonia: rigidity

QUESTION

Which disease is a result of basal ganglia dysfunction?

a. Myasthenia gravisb. Multiple sclerosisc. Poliod. Tourette syndrome

ANSWER

d. Tourette syndromeRationale: The tics and hyperkinesia

that often accompany Tourette syndrome are typical of basal ganglia dysfunction (the function of the basal ganglia is movement control).

PARKINSONISM

Tremor Rigidity Bradykinesia (slow movement) Loss of postural reflexes Autonomic system dysfunction Dementia

CEREBELLUM DAMAGE Vestibulocerebellar disorders

Difficulty maintaining posture Cerebellar ataxia

Movements divided into separate components

Cerebellar tremor

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

MULTIPLE SCLEROSIS Destruction of myelin coating on axons Demyelinated or sclerotic patches

develop through white matter of CNS Decreased conduction velocity

QUESTION

Which disorder causes damage to both upper and lower motor neurons?

a. ALSb. MSc. Myasthenia gravisd. Parkinson disease

ANSWER

a. ALSRationale: 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.

SPINAL CORD INJURY Immediate damage causes:

Spinal cord shockº Temporary complete loss of function

below injury Primary neurologic injury

º Irreversible damage to neurons

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

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

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