69 - Motor Systems Flashcards
Alpha motor neuron
Alpha motoneuron - also called lower motoneuron; can be called anterior or ventral horn cell when in spinal cord
Motor neuron pool
The group of motor neurons that innervate a muscle
Motor unit
Motor unit definition - one alpha motor neuron plus all the skeletal muscle fibers it innervates
Fibrillation
Fibrillations - spontaneous activity within single muscle fibers. They are not visible clinically since they are too small to be seen as movement through the skin.
Fibrillations can be recognized only by electromyography.
Fasciculation
Fasciculations - visible twitches of muscle that can be seen as ripples under the skin. Can involve one or more motor units.
Benign fasciculations are not a sign of lower motor neuron disorder. An example of a benign fasciculation is the eyelid twitching often seen during periods of fatigue or eye strain.
What size motor units would you find in extra ocular eye muscles
Small
What size motor units would you find in muscles controlling individual fingers?
Small
What size motor units would you find in large postural muscles?
Large
What size motor units would you find in limb muscles that have been affected by polio?
Large
How can you determine if a motor unit for a particular muscle is large or small?
Many small motor units when need fine control of a muscle
Large motor units (~1,000 muscle fibers) can be found in large, strong muscles
Lower motor neuron
Lower motor neurons -
They are the motor neurons of the spinal cord and brain stem that directly innervate skeletal muscle
Upper motor neuron
Upper motor neurons (UMN)
An UMN is not actually a motor neuron since it does not synapse on skeletal muscle fibers. An upper motor neuron is a neuron in a higher motor area (e.g. motor cortex, premotor cortex, brainstem center) that synapses on a lower motor neuron or on an interneuron which synapses on a lower motor neuron.
LMN lesion
- Atrophy
- Decreased muscle tone
- Weak or absent reflexes (hyporeflexia)
- Fasiculations
- Fibrillations
- Paralysis of muscle if too many LMNs to that muscle are damaged
Paralysis vs paresis
Paralysis means complete loss of voluntary contraction of a muscle while paresis refers to partial loss of voluntary contraction or voluntary muscle weakness.
UMN lesion
- Spasticity
- Overactive deep tendon reflexes (DTR)
- Positive Babinski sign (extensor plantar reflex)
When would a patient with an UMN lesion experience flaccid vs spastic paralysis?
The patient would have flaccid paralysis due to spinal shock immediately after the lesion and then later develop spastic paralysis
Spasticity
Spasticity is defined as “ a motor disorder characterized by a velocity- dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex, and is one component of the upper motor neuron syndrome.”
Partial vs total UMN loss
- If there is a total loss of descending UMN axons due to a total transection of the spinal cord there will be paralysis of all muscles innervated by lower motor neurons below the level of the spinal cord transection.
- If only some upper motor neurons controlling a muscle are damaged then the patient might have voluntary muscle weakness and develop spastic paresis.
What happens immediately after a spinal cord transection?
Initial effect of transection is spinal shock
Spinal shock
Spinal shock is the temporary loss of spinal cord reflex activity that occurs below a total (or near total) spinal cord injury.
Effects of spinal shock
a. flaccid paralysis - joints offer no resistance to being bent; no stretch reflexes
b. areflexia includes loss of all spinal reflexes below level of transection (so micturition reflex lost)
c. loss of autonomic function below level of transection; (can cause hypotension, loss of temperature control)
Permanent effects of complete transection of the spinal cord
- Permanent total paralysis for all muscles innervated by spinal segments below level of cut. Paralysis means loss of voluntary movement . The paralysis is permanent in that voluntary movement is lost forever. The paralysis is initially a flaccid paralysis and later changes to a spastic paralysis.
- Permanent anesthesia (lack of feeling below level of transection)
Abnormal reflexes following a recovery from spinal shock
Patient with upper motor neuron lesion (such as a patient who has recovered from spinal shock) can show certain abnormal reflexes. (Clonus and the clasp-knife reflex)
This is NOT voluntary movement
