Motor System Flashcards
How does graded recruitment of motor neurons work?
small neurons have high input resistances (R) –> given current will cause a larger change in these neurons –> will be brought to threshold with less synaptic input than the larger motor neuron
A motor unit is defined as ________.
an α motor neuron and the muscle fibers it innervates
Corticospinal axons course through the ______ then gather into the cerebral peduncle on the ventral surface of the midbrain.
internal capsule
____ is highly activated during mental rehearsal of movement.
The supplementary motor cortex
Activation of α motor neurons ____.
maintains muscle tone
What are the 2 general functions of the reticular formation?
- modulatory functions
- premotor functions
During ______, α and ɣ motor neurons fire together, shortening both the extrafusal (normal skeletal muscle) and intrafusal (spindle) muscle fibers together.
voluntary contraction
The ______ is responsible for many anticipatory responses to a voluntary movement that help maintain postural control and balance.
reticular formation
What is mirror neuron activity?
selective neural responses are observed in response to the subject simply watching an action with an intended consequence
Degeneration of ______ is the hallmark of amyotrophic lateral sclerosis (ALS) which is characterized by progressive weakness and wasting of skeletal muscles.
α motor neurons
The vestibular nuclei project bilaterally to the _____.
abducens nuclei
______ is communicated to the spinal cord through group Ia and group II sensory afferents.
Stretch of the muscle spindle
During voluntary contraction, ___ and ____ motor neurons fire together, shortening both the extrafusal (normal skeletal muscle) and intrafusal (spindle) muscle fibers together.
α and ɣ
Ia sensory afferents contact ____ motor neurons in the spinal cord, which trigger muscle contraction of the homonymous muscle fiber in response to stretch.
α
What is a muscle spindle?
a specialized type of sensory receptor, known as a proprioceptor, that is embedded within a muscle and detects muscle stretch
Do intrafusal muscle fibers detect stretch in a contracted or relaxed muscle?
both
Descending projections of the ______ tract target motor neurons that control the axial musculature of the neck to generate coordinated orienting responses.
colliculospinal (aka tectospinal)
Explain the steps in the knee-jerk reflex.
hammer tap stretches the muscle spindles –> 1a sensory axons stimulated –> α motor neurons in the spinal cord activated –> contraction of the stretched muscle
fall –> vestibular system detects downward acceleration –> arms push out and head pulls up
the vestibulocervical/vestibulospinal reflex
The _____ informs the brain of head position, orientation, and motion.
vestibular system
Stretch of the muscle spindle is communicated to the spinal cord through ______ sensory afferents.
group Ia and group II
These are circuits in the spinal cord that contribute to bilateral organized rhythmic behaviors.
central pattern generators (CPGs)
1a spindle afferents branch and diverge in the spinal cord to contact large populations of motor neurons which innervate ______ of muscle fibers to create the reflex motion.
populations
Slow twitch muscle fibers generate action potentials to twitch, but fatigue very slowly owing to _____.
high concentrations of myoglobin and many mitochondria
The vestibular system informs the brain of _____.
head position, orientation, and motion
The ______ is a collection of loosely connected areas that lie within the midbrain tegmentum.
reticular formation
Explain the crossed extension reflex.
cutaneous/nociceptive sensory receptors activated –> spinal interneurons activated –> extensor relaxation with flexor contraction on the same side AND flexor relaxation with extensor contraction on the contralateral side
What are the modulatory functions of the reticular formation?
- cardiovascular control
- respiratory control
- some sensorimotor reflexes
- eye movement coordination
- sleep wake regulation
- coordination of limb and trunk movements
______ muscle fibers of the spindle are innervated by a special type of motor neurons called ɣ motor neurons.
Intrafusal
Corticospinal axons course through the internal capsule then gather into the ______ on the ventral surface of the midbrain.
cerebral peduncle
______ cell bodies reside in the ventral horn of the spinal cord and brainstem cranial motor nuclei.
α motor neuron
A _____ is defined as an α motor neuron and the muscle fibers it innervates.
motor unit
What is the Golgi tendon organ sensitive to?
muscle tension
Tonic muscle fibers are a type of non-spiking muscle fibers that shorten _____ and efficiently generate isometric tension with _____ fatigability.
shorten extremely slowly; low fatigue
A small α motor neuron will innervate ______ muscle fibers, forming a motor unit that generates small forces.
a small number of
1a sensory afferents directly contact _____ motor neurons in the spinal cord.
α
Give an example of a vestibulocervical/vestibulospinal reflex.
fall –> vestibular system detects downward acceleration –> arms push out and head pulls up
What are central pattern generators (CPGs)?
circuits in the spinal cord that contribute to bilateral organized rhythmic behaviors
A given muscle fiber is innervated by how many motor neurons?
one
α motor neuron cell bodies reside in the _____ of the spinal cord and brainstem cranial motor nuclei.
ventral horn
The vestibular nuclei project to which cranial nuclei?
3 (oculomotor), 4 (trochlear), and 6 (abducens)
GTOs are situated _____ with the muscle and tendon, in contrast to the _____ arrangement of the muscle spindles.
GTO = in series; muscle spindle = in parallel
The knee-jerk reflex is an example of a _____ reflex arc.
monosynaptic
Fast twitch oxidative muscle fibers activate quickly, have many mitochondria and fatigue ______ because they contain _____ mitochondria.
fatigue slowly; lots of mitochondria
What does the vestibular ocular reflex (VOR) do?
produces eye movements to counter head movements to keep the gaze fixed
Degeneration of α motor neurons is the hallmark of _____ which is characterized by progressive weakness and wasting of skeletal muscles.
amyotrophic lateral sclerosis (ALS)
What activates ɣ motor neurons?
descending command axons executing voluntary movements
Fast twitch glycolytic fibers activate _____ and fatigue ______ owing to few mitochondria and a dependence of anaerobic glycolysis ATP generation.
quickly; rapidly
Intrafusal muscle fibers of the spindle are innervated by a special type of motor neurons called ____ motor neurons.
ɣ
______ muscle fibers activate quickly, have many mitochondria, and fatigue moderately slowly because they contain lots of mitochondria.
Fast twitch oxidative
Skeletal muscle contraction is driven by _____ neurons that innervate skeletal muscle cells.
α motor neurons
_____ muscle fibers generate action potentials to twitch, but fatigue very slowly owing to high concentrations of myoglobin and many mitochondria.
Slow twitch
Stimulation of premotor cortex elicits _____.
more complex, compound movements that involve multiple joints
What is a muscle spindle made of? Where are they located?
a specialized muscle fiber called intrafusal muscle fibers; they run in parallel with the main “extrafusal” muscle fibers
What is the size principle?
the systematic recruitment of smaller-to-larger motor units
_____ muscle fibers are a type of non-spiking muscle fibers that shorten extremely slowly and efficiently generate isometric tension with low fatigability.
Tonic
Fast twitch oxidative muscle fibers activate _____, have ______ mitochondria and fatigue moderately slowly because they contain lots of mitochondria.
activate quickly; many mitochondria
cutaneous/nociceptive sensory receptors activated –> spinal interneurons activated –> extensor relaxation with flexor contraction on the same side AND flexor relaxation with extensor contraction on the contralateral side
the crossed extension reflex
The ______ is critical for orienting gaze and body position.
superior colliculus (aka tectum)
_____ sensory afferents contact α motor neurons in the spinal cord, which trigger muscle contraction of the homonymous muscle fiber in response to stretch.
Ia
Fast twitch glycolytic fibers activate quickly and fatigue rapidly owing to ____ mitochondria and a dependence of ______.
few; anaerobic glycolysis ATP generation
A large α motor neuron will innervate ______ muscle fibers, forming a motor unit that generates large forces.
a large number of
This is a specialized type of sensory receptor, known as a proprioceptor, that is embedded within a muscle and detects muscle stretch.
a muscle spindle
The reticular formation is responsible for many anticipatory responses to a voluntary movement that help maintain ______.
postural control and balance
The superior colliculus (aka tectum) is critical for ______.
orienting gaze and body position
Ia sensory afferents contact α motor neurons in the spinal cord, which trigger _____.
muscle contraction of the homonymous muscle fiber in response to stretch
Activation of _____ maintains muscle tone.
α motor neurons
The population of α motor neurons that innervates the muscle fibers within a single muscle is called the _____ for that muscle.
‘motor neuron pool’
During voluntary contraction, α and ɣ motor neurons fire together, shortening both the ____ and _____ muscle fibers together.
extrafusal (normal skeletal muscle) and intrafusal (spindle)
What are the targets of the vestibular nuclei?
- the medial vestibulospinal tract –> medial spinal cord –> head and neck orientation
- lateral vestibulospinal tract –> lateral motor pools –> proximal limb musculature
What allows for rapid error-correction in muscle movement and contraction?
α and ɣ motor neuron coactivation
What is the target of the 8th cranial nerve?
the vestibular nuclei
Force increases the tension on collagen strands and pinches the intertwined afferent fibers, causing ____.
them to fire
What is the Golgi tendon organ made of? Where are they found?
collagen; at the junction of a muscle and a tendon
This produces eye movements to counter head movements to keep the gaze fixed.
the vestibular ocular reflex (VOR)
This is when selective neural responses are observed in response to the subject simply watching an action with an intended consequence.
mirror neuron activity
The reticular formation is a collection of loosely connected areas that lie within the _____.
midbrain tegmentum
1a spindle afferents branch and diverge in the spinal cord to contact ______ of motor neurons which innervate populations of muscle fibers to create the reflex motion.
large populations
Slow twitch muscle fibers generate action potentials to twitch, but fatigue ____ owing to high concentrations of myoglobin and many mitochondria.
very slowly
Is motor unit type plastic?
yes- exercise and chronic stimulation can shift motor unit phenotype from fast to slow
What innervates the Golgi tendon organ?
type Ib sensory afferents that wind around and within the collagen strands
_______ fibers activate quickly and fatigue rapidly owing to few mitochondria and a dependence of anaerobic glycolysis ATP generation.
Fast twitch glycolytic
Descending projections of the colliculospinal (aka tectospinal) tract target motor neurons that control the ______ to generate coordinated orienting responses.
axial musculature of the neck
Gradients from small to large motor units exist, generating _____.
graded forces
Are small or large motor units recruited first?
small- think about grabbing something delicate
