Flashcards in Motor Systems I –III Deck (11):
What is a motor unit and how does it relate to the “size principle” for recruitment of muscles?
A motor unit is defined as an α motor neuron and the muscle fibers it innervates. A given muscle fiber is
innervated by A SINGLE motor neuron. Since there are many more muscle fibers than motor neurons,
motor neurons branch to innervate multiple muscle fibers
Grade tension in a skeletal muscle → change the number of active motor neurons that innervate the muscle
One motor neuron →motor unit size that produces approx 5% increase in tension
Smallest motor neurons are recruited first and the largest ones last. Think of "picking up an egg"
This allows the force of a contraction/movement to be varied based upon the specifics of a particular situation/movement.
Where within the motor systems are alpha motor
neurons organized somatotopically?
α motor neurons, which innervate striated skeletal muscle, are known as the final common pathway,
since they represent the output of numerous upstream convergent pathways. Their cell bodies reside in
the ventral horn of the spinal cord and they are organized somatotopically, with lateral musculature
innervated by laterally situated motor neurons and medial musculature innervated by medially situated
Compare and contrast muscle spindles and Golgi tendon organs in terms of what sensory information they encode, where they are located, and their innervation of motor neurons and interneurons.
Muscle Spindles: Arranged in parallel with extra fusal fibers. Info transmitted in LENGTH.
Golgi Tendon Organs: Arranged in series with muscles (when a muscle contracts or is passively stretched, 1b afferents fire APs). Info transmitted is FORCE. (GTO = "Chinese finger traps")
Describe the basic stretch reflex circuit
•Muscle is stretched ==> Ia afferent endings are distorted ==> initiate action pot.
•Aps propogate in spinal cord ==> synapse on dendrites on that muscle’s motor neuron
*α-motor neurons fire aps and produce muscle contraction
*relieves stress so Iα neurons silenced
•Reflex is simultaneous activation of all muscle spindle sensory neurons and activation of all motor neurons.
*DRG neurons innervating muscle spindles = largest of DRG neurons fastest conducting
*Stretch receptor neurons = Ia afferents
•Each afferent in particular muscle innervates all motor neurons in that muscle
How does coactivation of alpha and gamma motor neurons lead to rapid error correction in movements?
Voluntary movements triggered from descending cortical or brainstem motor centers coactivate α and ɣ motor neurons to co-contract extrafusal and intrafusal (spindle) muscle fibers, keeping the spindle sensitive to stretch even in a shortened muscle.
The Ia afferents maintain a low, but non-zero
firing rate under baseline conditions when the spindle is not stretched and not flaccid. This means that the Ia afferent can signal both passive stretch and
passive shortening by either increasing (as in stretch) or decreasing (as in shortening) its firing rate.
Since ɣs are activated by descending command axons executing voluntary movements, mismatches between expected and actual muscle stretch can be detected rapidly and used to correct errors in motor output. For example, one expects a box labeled “Books” to be heavy. You may anticipate this and exert a good amount of force to lift the box, thus your α and ɣ motor neurons would be highly active, shortening the muscle and the spindle. However, finding it empty your muscles would shorten
too quickly relative to the spindle (they would not remain
balanced) and the mismatch will cause the Ia afferent to drop its firing rate, rapidly reducing α motor neuron drive and reducing muscle contraction.
Describe extensor-flexor coupling circuits and the crossed-extensor reflex.
Extensor-flexor: Ib afferents innervating the GTOs directly contact inhibitory and excitatory interneurons in the spinal cord. In contrast to the jerk that is produced during the stretch reflex, this reflex protects the musculature
from over exertion by relaxing the synergist (homonymous) muscle and contracting the antagonist. The key take home message is that activity of synergist and antagonist musculature is coordinated in part by spinal cord interneuronal circuitry.
Crossed-extensor: Like when you step on a tack! Cutaneous (nociceptors in this case) sensory receptors innervate spinal interneuronal motor networks. These coordinate extensor relaxation and flexor contraction on the same side as the stimulus and a converse extensor contraction and flexor relaxation on the contralateral side
What is a central pattern generator? Describe a behavior that uses one and where it resides.
CPGs: central pattern generators, generate the intrinsic rhythmicity for coordinated movement through these command centers for movement intrinsic to the spinal cord. Allow for limbs to alternate when walking.
They are located in the SPINAL CORD.
Alternating rhythmic patter of flexor motor neurons and extensor neurons. Consist of flexor and extensor motor neuron to fire out of phase with eachother. On CPG for each leg.
What is meant by the ‘hierarchical’ organization of the motor system?
•Organized by the central pattern generators in the spinal cord that control the relative timing of the output.
•Central pattern generators are subject to control from “projection regions” of Brainstem or Motor cortex
•Above these central regions areas providing volitional intent:
1). Premotor cortical area
2). Supplemental cortical area.
Reason for organization: basic patterns of movement can take place automatically and modificatios of movement can occur w/ little or no conscious thought. Allows brain to be directed to more important goals. Cerebellum helps to make motor movements automatic.
Which descending pathways control finger movement? Which control axial musculature? Where are these pathways situated within the spinal cord?
Brodmann’s Area 4 -- is located in the precentral gyrus of the cerebral cortex.
Corticospinal axons course through the internal capsule then gather into the cerebral peduncle on the ventral surface of the midbrain. At the level of the medulla the tract is now called the ‘pyramids’ which continue to course through the brainstem and collateralize.
Most fibers in the pyramids cross the midline at the level of the caudal medulla and these form the LATERAL CORTICOSPINAL TRACT which in part makes direct synapses on α motor neurons that control hand and FINGER movement.
Mainly, however, these axons also target spinal cord local circuit interneurons that control motor neurons in the ventral horns. A small number of uncrossed fibers make up the ventral corticospinal tract, which innervate motor neuron pools that control axial and proximal limb muscles.
What is the organizational principle of motor cortex?
The somatotopic organization of the primary motor cortex is arranged such that the feet are most medial and head most lateral and ventral. There is distortion based upon the density of innervation each body part receives (face and hands receiving the most innervation).
Lateral/Ventral premotor cortex: involved in motor planning of actions that are to be executed with a delay (like making the hand conform to the shape of an object to grasp)
Supplementary motor cortex: involved in motor planning of movements that are initiated internally, ie, mentally rehearsing a pattern of movements