Motor Systems I-III Flashcards
Define a motor unit. Relate the motor unit concept to the “size principle” for recruitment of muscles. Recognize how exercise influences the motor unit.
Motor unit: an alpha motor neuron and the muscle fibers it innervates. A given muscle fiber is innervated by a SINGLE motor neuron.
Slow (S) motor units:
small alpha motor neurons with fine caliber axons; innervated small # of slow oxidative muscle fibers; generate small forces, fatigue slowly, recruited FIRST due to high input resistance of neuron
Fast Fatigue resistant motor units: intermed sized alpha motor neurons with medium caliber axons; inn an intermed number of fast oxidative glycolytic muscle fibers; generate LARGE forces, fatigue slowly; recruited second
Fast Fatigable motor units: large alpha motor neurons, thick axons; inn a large number of fast glycolytic muscle fibers, generate large forces, fatigue quickly, recruited LAST
Exercise: endurance exercise slows the contractile properties of motor units and increase endurance and strength. High intensity strength training increases amount of contractile protein.
Effects not only on motor unit phenotype, also central changes may alter recruitment
Exercise alters distrib (endurance? more FR)
Illustrate what is meant by “somatotopy” of motor neurons. Identify where within the motor system alpha motor neurons are organized somatotopically
Cell bodies of motor neurons reside in ventral horn of spinal cord. Lateral musculature is innervated by laterally situated motor neurons and medial musculature is innervated by medially situated motor neurons.
Cervical
and lumbar enlargements represent the enlarged motor neuron
populations that innervate the upper and lower limb musculature respectively.
Muscle spindles, what they innervate, where they are located
Muscle spindles are a specialized type of sensory receptor (proprioceptor), embedded within a muscle that detects muscle STRETCH. They are specialized muscle fibers called “intrafusal muscle fibers”. PARALLEL to main extrafusal fibers. Stretch of spindle to SC through group 1a and group II sensory afferents (large, fast, fire APs in response to stretch). Ia afferents contact alpha motor neurons in SC which trigger muscle contraction of the homonymous muscle fiber in response to stretch.
The spindle is contractile. Intrafusal muscle fibers of the spindle are innervated by GAMMA motor neurons (in voluntary contraction, alpha and gamma neurons fire together, intra and extra fusal shorten and thus there is sensitivity to stretch in contracted and relaxed muscle.)
Describe the basic stretch reflex circuit.
“knee-jerk”; monosynaptic reflex arc. Hammer stretches muscle, 1a afferent fires (fast, fat), activates alpha motor neuron in SC, which contracts stretched muscle (ie the homonymous or synergist muscle).
–>1a branches in SC to hit large pops of motor neurons
Describe how coactivation of alpha and gamma motor neurons leads to rapid error correction in movements.
1a afferents maintain a low firing rate in baseline conditions. So, 1a afferents can signal both passive stretch and passive shortening by either increasing (in stretch) or decreasing (in shortening) its firing rate.
Since gamma MNs are activated in voluntary (descending control) actions, mismatches between expected and actual muscle stretch can be detected rapidly and used to correct errors in motor output. Ex lifting a supposedly heavy box: high activity of alpha and gammas. If empty box, muscle shortens faster than spindle. Mismatch leads to 1a afferent dropping its firing rate, reducing alpha motor neuron drive. (too rapid contraction of extrafusal muscle results in spindle going flaccid and reducing its firing rate.)
OR: heavier than expected: muscle shortens slower than spindle, increased 1a firing and feeding back to increase muscle contraction.
Describe extensor-flexor coupling circuits
1b afferents innervating GTOs directly contact inhibitory and excitatory interneurons in the spinal cord. Reflex protects musculature from over exertion by relaxing synergist and contracting antagonist. synergist and antagonist musculature is coordinated in part by spinal cord interneuronal circuitry
Define a central pattern generator. Describe a behavior that uses one and where it resides.
CPGs are neural networks that can produce patterned, rhythmic outputs in the absence of sensory or central input.
Experiments w/ cats w/ transected thoracic SCs (legs still produce coordinated alternating movement on treadmill).
Useful in swimming, locomotion (extension/flexion alteration).
CPG circuitry:
- part of locomotor CPG is the rhythm generator or “clock”
- The clock component innervates interneuronal networks that amplify the clock signal and distribute it appropriately to coordinate muscle contrac/relaxation.
- CPGs for both limbs interact via commisural fibers to coordinate between-limb use.
- CPGs are modulated by descending pathways that can affect clock rate and motor patterns.
- Some of the circuits in simple reflexes are flexibly engaged in locomotion.
* human locomotion depends more on depending commands, but CPGs exist (can apply loads to the hip and induce CPG activity)
Justify what is meant by the term ‘hierarchical organization’ of the motor system.
Motor neurons spinal cord brainstem motor cortex basal ganglia cerebellum
Compare descending pathways that control finger movement vs axial musculature. Identify where these pathways are situated within the spinal cord.
Primary motor cortex (BA 4) in precentral gyrus–>internal capsule–>cerebral peduncle (midbrain)–>pons–>medulla (pyramids) cross midline at caudal medulla and form lateral corticospinal tract. Synapse on alpha MNs to control hand/finger movement, also to ventral horns. Small number of UNCROSSED fibers make up the ventral corticospinal tract and innervate motor neuron pools that control axial and proximal limb muscles.
Name two ways in which motor cortical plasticity is advantageous for recovery and/or treatment of diseases or damage to the motor system.
stroke recovery (areas adjacent to damaged region can sprout new connections and contribute to functional recovery; Constraint induced movement therapy-- increase motor ability, increase primary motor cortex area devoted to deprived limb) Practice (musicians, Braille)
Do small neurons have low or high input resistance?
high (fewer channels in membrane) So: brought to threshold with lower synaptic input than larger motor neurons.
Smaller to larger recruitment =”size principle”
Golgi Tendon Organs
Another type of proprioceptor. Collagen. Location: muscle tendon junction. Innervated by and signal via type Ib sensory afferents that wind around and within collagen strands. GTOs are in series with muscle and tendon. Preferentially detect MUSCLE TENSION rather than passive stretch. Used by NS to regulate force.
Reciprocal innervation
Type Ia afferents in reflex arc contact alpha motor neurons that contract homonymous muscle. But they ALSO hit inhibitory interneurons that inhibit motor neurons controlling opposite “antagonist” muscle. (synergist and antagonist contract and relax simultaneously)
Crossed extension reflex
Ex: step on tac, weight shifts to other leg.
Cutaneous sensory receptors innervate spinal interneuronal motor networks. These coordinate extensor relaxation and flexor contraction on same side as stimulus and a converse extensor contraction and flexor relaxation on contralateral side.
Normal function of myotatic reflex (stretch reflex)?
maintaining muscle tone (tone=resistance of muscle to stretch)
Gamma motor neurons contribute to top-down regulation of muscle tone.
