Motor System 2.0 Flashcards
Motor unit = ?
Motor System
Motor Unit: An alpha motor neuron AND the muscle fibers it innervates.
Types of Muscle Fibers
- Extrafusal fibers are innervated by = ?
- Intrafusal fibers are innervated by = ?
Motor System
Types of Muscle Fibers
(1) Extrafusal:
- Constitute majority of muscle fiber
- Innervated by α motorneuron
- Used to generate force
(2) Intrafusal:
- Encapsulated in sheaths, forming muscle spindles that run parallel to extrafusal muscle fiber.
- Innervated by Υ motor neuron
- Too small to generate significant force
- No or few actin/ myosin filaments
- Innervated by sensory and motor fibers
Two types of lower motor neurons = ?
Motor System
Two Types of Lower Motor Neuron’s
(1) Alpha:
- Axons of alpha motor neurons project to extrafusal skeletal muscle
- Normally, an alpha motor neuron releases enough ACh that all of the muscle fibers it innervates contract.
(2) Gamma:
- Axons of gamma motor neurons project to intrafusal fibers in the muscle spindle.
Upper Motor Neurons
Four UMN Medial tracts = ?
Motor System
Upper Motor Neurons - Medial tracts:
- Medial corticospinal tracts = Neck, shoulder and trunk muscles
- Reticulospinal tract = Bilateral postural muscles and gross limb movements
- Lateral vestibulospinal tract = Postural muscles
- Medial vestibulospinal tract = Neck muscles
Upper Motor Neurons
Two UMN Lateral tracts = ?
Motor System
Upper Motor Neurons - Lateral tracts:
- Lateral corticospinal tracts = Fine distal movements
- Rubrospinal tract = Extension of wrist and fingers
Upper Motor Neurons
UMN Lateral tracts = ?
Motor System
Upper Motor Neurons - Lateral tracts:
- Lateral corticospinal tracts = Fine distal movements
Lower Motor Neurons
What are the four LMN pools located in the ventral horn/anterior horn = ?
Motor System
Lower Motor Neurons
- LMN pools are groups of cell bodies in the spinal cord whose axons project to a single muscle.
- LMN pools located in the ventral horn/anterior horn
- Actions of these pools are arranged in the order:
- Medially located pools innervate axial and proximal muscles
- Laterally located pools innervate distal muscles
- Anteriorly located pools in ventral horn innervates extensors
- Posterior pools innervate flexors
Lower Motor Neurons
Posteriorly located pools in ventral horn innervate = ?
Motor System
(LMN) Posteriorly located pools in ventral horn innervate = Flexors
Lower Motor Neurons
Laterally located pools in ventral horn innervate = ?
Motor System
(LMN) Laterally located pools in ventral horn innervate = Distal Muscles
Lower Motor Neurons
Anteriorly located pools in ventral horn innervate =
Motor System
(LMN) Anteriorly located pools in ventral horn innervate = Extensors
Lower Motor Neurons
Medially located pools in ventral horn innervate = ?
Motor System
Medially located pools in ventral horn innervate = Axial and Proximal Muscles
Upper Motor Neurons
Corticobrainstem tract fibers arise in the cerebral cortex then project to ?
Motor System
Upper Motor Neurons
- Corticobrainstem tracts:
- Fibers arise in the cerebral cortex then project to the cranial nerve nuclei in the brainstem.
From Decision → Skeletal Muscles
Decision
↓
(1) = ?
↓
(2) =?
↓
(3) =?
↓
(4) =?
↓
(5) =?
↓
Skeletal Muscles
Motor System
(1) Neural activity begins with a decision made in the anterior part of the frontal lobe.
(2) Next, motor planning areas are activated, followed by control circuits.
(3) Control circuits, consisting of the cerebellum and basal ganglia, regulate the activity in upper motor neuron tracts.
(4) Upper motor neuron tracts deliver signals to spinal interneurons and lower motor neurons (LMNs).
(5) LMNs transmit signals directly to skeletal muscles, eliciting a contraction.
Signs of Motor Neuron Lesions = ?
Motor System
Signs of Motor Neuron Lesions:
- Paresis
- Paralysis
- Involuntary muscle contractions
- Muscle atrophy
- Abnormal muscle tone
- Abnormal reflexes
- Disturbances of movement efficiency and speed
- Impaired postural control
Signs of Motor Neuron Lesions
Paresis vs. Paralysis
Motor System
- Paresis is the partial loss of voluntary contraction.
- Paralysis is the complete loss of voluntary contraction (such as complete spinal cord injury - SCI).
- BOTH UMN and LMN
Muscle Atrophy
Disuse atrophy vs. Denervation
Motor System
- Disuse atrophy is the result of a lack of muscle use (UMN).
- Denervation of skeletal muscle produces the most severe atrophy, because frequent neural stimulation, even at a level inadequate to produce muscle contraction, is essential for the health of skeletal muscle (LMN).
- Atrophy is the loss of muscle bulk.
“A motor disorder, characterized by a velocity-dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyper-excitability of the stretch reflex as one component of the upper motor neuron (UMN) syndrome” = ?
Motor System
Spasticity - Velocity-Dependent Hypertonia:
- “A motor disorder, characterized by a velocity-dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyper-excitability of the stretch reflex as one component of the upper motor neuron (UMN) syndrome”
Abnormal Muscle Tone
hypertonia vs. hypotonia
BONUS: Flaccidity = ?
Motor System
- Muscle tone is the resistance to stretch in the resting muscle
- Spasticity = hypertonia (UMN)
- Usually more resistance to stretch in one direction (leads to posturing)
- Hypotonia is an abnormally low resistance to passive stretch (LMN).
Flaccidity is the lack of resistance to passive stretch
- Complete loss of muscle tone
- Initially in UMN and also in LMN
Involuntary Muscle Contractions
Fasciculations vs. Fibrillations
Motor System
- Spontaneous involuntary contractions include:
Fasciculations = Quick twitches of muscle fibers of a single motor unit visible on the skin surface (LMN).
Fibrillations (always pathologic) = Brief contractions of single muscle fibers not visible.
- EMG can detect.
- Muscular dystrophy, peripheral nerve injuries, conditions in which muscle fiber regeneration and /or denervation are often or very often present.
Abnormal Muscle Tone
Hypotonia or flaccidity can be caused by = ?
Motor System
- LMN lesions
- ACUTE UMN lesions like stroke, SCI
hypotonia will usually be temporary
Abnormal Muscle Tone
Hypertonia can be caused by = ?
Motor System
- Chronic UMN lesions
- Velocity-dependent: The amount of resistance to passive movement depends on the velocity of movement.
- Resistance during slow stretch is low, with greater resistance occurring with faster stretch.
Velocity-Dependent hypertonia limits = ?
Motor System
Spasticity - Velocity-Dependent Hypertonia:
- Limits joint range of motion, interferes with function, and may cause deformity.
Example:
- Significant bilateral plantarflexor hypertonia may allow only toe-walking because lack of ankle dorsiflexion prevents the heels from touching the ground.
What is the Clasp-Knife response = ?
Motor System
Clasp-Knife Response:
- Occurs when a paretic muscle is slowly and passively stretched and resistance drops at a specific point in the range of motion.
- Change in resistance is similar to the opening of a pocketknife.
- The initial strong resistance to opening the knife blade gives way to easier movement.
- Clasp knife phenomenon or response/spasticity
Normal Reflex Order
*click to view**
Motor System
Myotatic/ Stretch Reflex/DTRs:
- Whenever a muscle is stretched, excitation of the spindle causes reflex contraction of the muscle.
- Stimulus: stretch of the muscle (mechanical);
- Sensor: spindles
- Involving alpha (and gamma) motor neurons, local circuit interneurons, and afferent somatic sensory input.
