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.
Discuss the consequences of lesions of the spinal cord and brainstem on locomotion in experiments in cats, explain the midbrain locomotory center, and describe the experiment in which this center was stimulated in a cat on a treadmill. In general, describe the role of sensory input in the performance of rhythmic, automatic motor behavior such as walking.
Spinal prep:
Lesions in thoracic SC; cats’s legs move in coordinated fashion on treadmill.
Deafferented prep:
Transected dorsal roots: limbs continued to behave in coordinated fashion even without sensory input if stimulated with L-Dopa (SO: locomotor activity not dependent on sensory input)
Decerebrate preparation: transect midbrain; spontaneous locomotor activity if transected rostral to mamillary bodies, not if transected caudal to MB
(Mesencephalic locomotor center stimulation necessary for locomotion)
Passive vs active stretch: when is GTO vs muscle spindle activated (afferent activity)
Passive stretch:
spindle activated
GTO not
Active contraction:
spindle not
GTO activated
Compare and contrast Muscle spindle with GTO
Muscle spindle: in parallel with muscle preferentially signals muscle STRETCH Imp for maintaining muscle tone composed of muscle fibers signals via 1a afferents Innervated by gamma motor neurons Feedback system for maintaining muscle length
GTO in SERIES with muscle preferentially signals muscle TENSION Imp for stabilizing contractions Composed of collagen fibers and capsule Signals via 1b afferent fibers Not contractile Feedback system for maintaining muscle force
Diseases the lead to lower motor neuron syndrome
Syphilis (DRG)
Herniated disc (sensory nerve root)
Polio or ALS (motor neuron)
Guillan-Barre: demyelination of peripheral motor nerve
Lambert-Eaton synd: peripheral Ca channel attack hits NMJ
Muscular dystrophy (can affect muscle directly)
reticulospinal tract (general)
important for posture, balance, and anticipatory movements; locomotor initiation
Vestibulospinal tract
imp for posture, balance, and orienting movements; mediate protective responses to falls
Medial vestibulospinal tract to medial SC where axons regulate head orientation and neck muscle activation
Lateral vestibulospinal tract to lateral motor pools controlling proximal limb musculature.
Adjust head and torso in vestibulocervical and vestibulospinal reflexes (ex. falling forward–>arms out, head up)
superior colliculus/tectospinal tract
imp for orienting movements (gaze, body position) toward unexpected stimulus
superior colliculus maps diverse sensory modalities onto external space (visual and auditory maps)
Vestibular ocular reflex (VOR)
Projections from vestibular nuclei to cranial nerves III (oculomotor), IV (trochlear), and VI (abducens). VOR produces eye movements to counter head movements to keep gaze fixed.
Reticular formation
loosely connected areas in midbrain tegmentum.
Modulatory (CV control, resp, sensorimotor, eye movement coord, sleep wake regulation, and coordination of limb and trunk movements) and premotor functions (input from mesencephalic locomotor region; also anticipatory responses to voluntary movement–balance etc)
Key principles from Penfield stimulation and motor fields
- disproportionate M1 surface area is devoted to controlling musculature used in fine motor tasks
- movements (not muscles) are mapped (so no true homunculus for motor cortex)
Premotor cortex
Neurons here are involved when movement is initiated by external cue
Contains supplementary motor cortex: activated in self cued movements; esp in mental rehearsal of movement
mirror neuron activity
Plasticity in motor cortex
stroke recovery
practice
brain machine interfaces
UMN syndrome
lesion of premotor neurons (corticospinal; brainstem)
-contralateral muscle flaccidity
-weakness
-spasticity (increased tone, hyperactive reflexes, clonus, or large involuntary rhythmic movements because of loss of inhibitory tone mediated by descending projections)
Babinski sign,
loss of voluntary movements
LMN syndrome
loss or degen of motor neurons in the spinal cord
- paralysis
- weakness
- loss of DTRs
- decreased muscle tone
- muscle atrophy
- spontaneous twitches due to changes in muscle excitability after denervation.
