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.
____ 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.
The ______ is responsible for many anticipatory responses to a voluntary movement that help maintain postural control and balance.
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 _____.
______ 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.
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?
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.
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.
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.
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.
Corticospinal axons course through the internal capsule then gather into the ______ on the ventral surface of the midbrain.