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Flashcards in motor systems Deck (53)
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skeletal muscle is innervated by what kind of neuron. Where do their cell bodies reside

alpha motor neurons (lower motor neurons) with cell bodies in ventral horn of spinal cord and brainstem cranial motor nuclei


functions of vestibular nucleus, resticular formation, superior colliculus and motor cortex

anticipatory motor control, rhythmic motor output such as locomotion, eye movements and voluntary motor control respectively


modulatory motor centers and functions

cerebellum acts as a feedback error correction circuit and computes short and long term corrections to these errors. Basal ganglia have roles in movement initiation and action selection as well as roles in motor learning, reinforcement and motivated behavior


somatotopic arrangement of alpha motor neurons

lower motor neurons are arranged somatotopically, with lateral musculature innervated by laterally situated motor neurons and medial muscles innervates by medially situated motor neurons.


What do the cervical and lumbar enlargements represent

enlarged motor neurons populatioins innervating the upper and lower limbs


What disease causes degeneration of alpha motor neurons



motor neuron pool

The population of α motor neurons that innervates the muscle fibers within a single muscle is called the ‘motor neuron pool’ for that muscle. The neurons of this neuron pool are within the spinal cord in rod like clusters running along the axis of the cord Each muscle fiber is innervated by only onemotor nueron, but each neuron can innervate multiple muscle fibers.


compare the motor units for large vs smal alpha motor neurons

small: innervates small number of muscle fibers, generating motor unit that forms small forces. Large: innervates a large number of muscle fibers forming a motor unit that generates large forces. Gradients btw small and large also exist


Size principle

Systematic recruitment of smaller motor units before larger motor units. This occurs b/c small neurons have high input resistance due to fewer membrane channels. A given synaptic current will induce a relatively larger voltage change in a small motor neuron compared to a large motor neuron, thus the small motor neuron will be brought to threshold faster, with less synaptic input


tonic muscle fibers

non spiking muscle fiber that shortens extremely slow and efficiently generates isometric tension with low fatigability. Found in spindles and extraocular muscles


slow twitch fibers

Have a high conc of myoglbin and mitochondria which aid in slow fatiguability


fast twitch oxidative fibers

Activate quickly, Have many mitochondria and fatigue moderately slowly


Fast twitch glycolytic fibers

activate quickly and fatigue rapidly due to few mitochondria and dependence on anaerobic glycolysis ATP generation


describe slow motor units

small motor neuron, small number of slow twitch muscle fibers, generates small forces, fatigue slowly, recruited first due to high input resistance, recruited during sustained activities such as maintaining posture


describe fast fatigue-resistant motor units

Intermediate sized motor neuron, intermediate number of fast twitch oxidative muscle fibers, generates large forces, fatigue slowly, recruited second


describe fast fatigable motor units

Large motor neurons, large number of fast twitch glycolytic muscle fibers, generate large forces, fatigue quickly, recruited last, recruited during running and jumping


What determines a muscles phenotypes (ie. Fast twitch vs slow) and can it be changed

The motor neuron activity governs muscle phenotype. Can be altered with chronic stimulation or exercise


What are muscle spindles

A special type of sensory receptor, called a proprioceptor, that is embedded within a muscle and detects muscle stretch. They are composed of specialized muscle fibes called intrafusal muscle fibers, which run in parallel with the main extrafusal muscle fibers.


How do muscle spindles signal stretch

through group 1a and group II sensory afferents (large, fast axons) which fire AP's in response to mechanical stretch of the intrafusal muscle fibers.


2. What is the function of gamma motor neurons?

Innervate intrafusal muscle fibers of the spindle. During voluntary contraction, gamma motor neurons fire with alpha motor neurons, shortening the extrafusal (normal skeletal muscle) and intrafusal (spindle) fibers together. This ensures that when the muscle is contracted it can still detect stretch


What are golgi tendon organs

Another type of proprioceptor- collagen structures that reside at the junction of a muscle and a tendon and signal via type Ib sensory afferents that wind around and within the collagen strands


3. Compare and contrast muscle spindles and Golgi tendon organs. Include parallel versus series arrangement, the type of information they transmit (force vs. length), and the type of innervation they make on motoneurons.

Golgi tendon organs: situated in series with muscle and tendon, preferentially signal muscle tension during muscle contraction, type Ib sensory afferents, maintains muscle force. Spindles: situated in parallel with muscle, preferentially signal passive stretch which lengthens the muscle before the tendon, type Ia sensory afferents, maintains muscle length


1. Describe the key elements of the stretch reflex. Include the sensory ending, the sensory neuron, and the motor neuron.

Muscle is stretched > 1a sensory neuros sense the stretch in the spindle > sensory info relayed to populations of alph motor neuron in spinal cord > alpha motor neuron contracts the stretched muscle (i.e. the “homonymous” or “synergist” muscle).


In addition to the motor neurons innervating the muscle in which the spindle resides (homonymous motor neurons), name two other sets of neurons innervated by the afferent endings in a stretch reflex

1. inhibitory interneurons 2. excitatory interneurons


Reciprocal innervation

Type Ia afferents in stretch reflex directly contact inhibitory interneurons within the spinal cord that inhibit motor neurons controlling the antagonist muscle. This causes relaxation of the antagonist muscle at exactly the same time as the synergist muscle contracts.


flexor extensor coupling

Ib afferents from the GTOs directly contact inhibitory and excitatory interneurons in spinal cord. This protects muscles from over exertion by relaxing the synergist (homonymous) muscle and contracting th antagonist.


Normal function of the stretch reflex

Maintains muscle tone- resistance of muscle to stretch.


Causes of hypotonia and hypertonia

hypotonia: damage to the Ia sensory afferents innvervating the spindles or the alpha motor neurons innervating the muscle. Hypertonia: damage to descending motor pathways that influence the spinal cord premotor circuits


Basal firing rate of Ia afferents

Even when the spindle is flaccid, there is a basal rate of firing. So, the Ia can signal both stretch and shortening by increasing or decreasing firing rate, respectively. This allows for maintenance of tone


How do our muscles correct for errors

Via co-activation of alpha and gamma motor neurons. Gamma fibers are activated with voluntary movements, so mismatches btw expected and actual muscle stretch can be detected rapidly and used to correct errors in motor output (ie. Anticipating that an item will be heavy, but lifting it and finding out it is actually not heave). If the spindles are overcontracted relative to the muscle fiber, there is a drop in Ia afferent firing which rapidly reduces alpha motor neuron drive and reduces muscle contraction.