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Nervous System: Unit III > Motor Systems I-III > Flashcards

Flashcards in Motor Systems I-III Deck (25)
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Motor unit definition

  • alpha motor neuron (LMN) + muscle fibers that it innervaties
  • one muscle fiber is innervated by a single motor neuron


Characteristics of muscle spindles

  • muscle spindles = sensory receptors = "proprioceptors" = embedded /in muscle and detects muscle stretch
  • composed of specialized muscle fiber cells = "intrafusal muscle fibers" running in parallel to main (extrafusal) muscle fibers
  • signal stretch through group Ia and II sensory afferents (large, fast axons)
  • innervated by gamma motor neurons 


Gamma motor neuron fxn

  • innervate muscle spindle fibers
  • acts to contract muscle spindle fibers during voluntary contraction ==> system maintains the fxn of the stretch receptor


Characteristics of Golgi Tendon Orgams

  • = proprioceptor
  • collagen structures @ muscule-tendon jxn
  • signal via type Ib sensory afferent
  • situated in series w/muscle and tendon ==> preferentially sensitive to muscle tension ==> can signal muscle contraction
  • used by nervous system to regulate force
    • GTO Ib afferents contact inhibitory neurons for the homonymous muscle AND 
    • excitatory neurons for the antagonist muscle
    • ==> prevents overexertion


Characteristics of stretch reflex

  • = "monosynaptic reflex arc"
  1. muscle stretched @ muscle spindles
  2. stimulates activity @ Ia sensory axons
  3. sensory info relayed to alpha motor neurons of homonymous muscle (same muscle being stretched) @ spinal cord (ventral horn)
    1. Ia afferts also contact inhibitory interneurons controlling the antagonist muscle
  4. alpha motor neurons signal muscle to contract


Characteristics of the "size principle"

  • size principle = smaller motor units are recruited before larger motor units
    • allows for delicate grasp/movement
  • smaller motor neurons have high input resistance 
    • V=IR
    • thus, for a given current, smaller neurons ==> higher voltage 
    • smaller neurons can be brought to threshold with less synaptic input


Muscle tone definition

muscle tone = resistance of a muscle to stretch


Neurons innervated by muscle spindle afferents

  • Ia ==> excitatory alpha motor neurons @ homonymous muscle
  • Ia ==> inhibitory interneurons for antagonist muscle
  • II ==> dorsal column ==> neurons @ cuneate/gracile nuclei (?)


Characteristics of crossed extensor reflex

  • e.g. stepping on a tack ==> automatic recoil and weight shifting to other leg
  • = elaboration of knee-jerk reflex
  • cutaneous nociceptors innervate spinal interneuronal motor networks
  • step on tack ==> ipsilateral extensor relaxation and flexor contraction + contralateral extensor contraction and flexor relaxation
    • occurs via recprical innervation


Characteristics of central pattern generators

  • circuits w/in spinal cord capable of generating complex coordinated movements (i.e. locomotion, swimming)
  • can be modified by descending input from higher motor sensors
  • DO NOT require sensory input


CPG experiments on cats

  1. cats spinal cord transected at thoracic level
  2. placed over treadmill in sling ==> hind legs continued to have coordinated, alternating movements w/speed of treadmill
  3. movements possible even when dorsal root (sensory input) was transected


CPGs and future of spinal cord injury tx

  • some experiments have shown the role of CPGs control over locomotion in humans (though there is more descending control)
  • electrical stim @ lumbar spinal cord ==> locomotion-like EMG activity @ parapalegics
  • also, loads @ hip ==> CPG locomotor activity
  • research of CPGs ==> therapeutic advances in spinal cord injury tx


Reticular formation fxn in regulating movement

  • inputs from mesencephalic locomotor region (=role in regulation of locomotor speed)
  • RF is responsible for anticipatory responses to voluntary movement
    • e.g. flexes leg muscle before engaging in lifting w/biceps in anticipation of center of gravity change


Upper motor centers @ brainstem/midbrain

  • vestibular complex
  • reticular formation
  • superior colliculus (tectum)


Fxn of the vestibular complex/nuclei

  • receive inputs from semicircular canals via CN VIII
  • descending projections:
    • 1. axons ==> medial vestibulospinal tract ==> regulate head orientation and neck muscle activation
    • 2. axons ==> lateral vestibulospinal tract ==> regulate proximal limb musculature
  • axons ==> CN III, CN IV, and CN VI to regulate eye movements ==> vestibulo-ochlear reflex (VOR)


Fxn of superior colliculus

  • coordinates axial/neck musculature in response to synthesis of auditory and visual stimuli
    • e.g. turning towards a siren
  • descending projections via colliculospinal tract


General fxn of motor cortex

  • motor cortex = cerebral motor control = volunatry motor control
  • pyramidal neurons ==> corticospinal tract
    • axons ==> internal capsule ==> cerebral peduncle ==> pyramids @ medulla
    • some axons deccussate ==> lateral CS tract ==> distal limb interneurons/alpha motor neurons @ ventral horn
    • remaining axons ==> ventral CS ==> innervate axial/proximal limb muscles


Motor actions resulting from primary motor cortex microzone stimulation

  • ==> recruitment + suppression of several muscles
  • ==> organized movements (NOT just individual muscles) are represented @ M1 microzones
  • disproportionate amount of primary motor cortex surface is devoted to fine motor tasks


Characteristics of premotor cortex

  • lies anterior to primary motor cortex
  • makes up 25% of CS tract
  • involved in initiating movement in response to external cue
  • stimulation usually ==> complex, compuond movement (e.g. multiple joints)


Characteristics of supplementary motor cortex

  • part of premotor cortex
  • involved in self-cued movements
    • highly active during mental rehearsal of movement
  • stimulation ==> complex, multi-joint movement


Major components of motor cortex

  • primary motor cortex
  • premotor cortex
    • supplementary motor cortex


UMN syndrome

  • = lesion of premotor neurons (CS, brainstem)
  • ==> contralateral muscle faccidity
  • ==> weakness
  • ==> spasticity
    • increased tone
    • hyperreflexia
      • Babinski sign
    • clonus
  • ==> loss of voluntary movements


LMN syndrome

  • = loss/degeneration of motor neurons @ spinal cord
  • ==> paralysis
  • ==> weakness
  • ==> loss of deep refexes
  • ==> decreased muscle tone
  • ==> muscle atrophy
  • ==> fasciculations


Evidence that cortical motor representation is dynamic

  • Stroke
    • stroke victims have an acute loss of motor fxn ==> partial or complete recovery due to plasticity
  • Practice
    • repeated performance of a motion ==> expansion of region devoted to that action
      • e.g. musicians or blind reading Braille ==> expansion of finger control areas
  • Dynamic changes mostly occuring @ cortex