Muscle Innervation & Motor Unit

Question 1

describe Ia, Iband II fibers

Answer 1

• Ia = largest, myelinated
  
  • receptor: primary spindle endings
  • sensitive to: length and rate of change in stretch
• Ib = largest, myelinated
  
  • receptor: Golgi tendon organ
  • sensitive to: tension
• II = large, myelinated
  
  • receptor: secondary spindle endings
  • sensitive to: length

Question 2

describe the muscle spindle

Answer 2

• muscle spindles are encapsulated structures found within skeletal muscles that inform the CNS about the length and rate of change in skeletal muscle
• the motor innervation via gamma motor neurons adjusts sensitivity of the spindle to stretch
• muscles involved in fine movements (hand) have a denser distribution of spindles that those involved in coarse movements (back)

Question 3

contrast the static and dynamic properties of the muscle cells

Answer 3

• the sensory info. arising from the primary endings and relaed by group Ia afferents relates to both static and dynamic properties of the muscle cells
• the sensory info. arising from the secondary endings and relayed by group II afferents relates only to static properties, as monitored by nuclear chain and static nuclear bag fibers

Question 4

describe the role of the muscle spindle in the stretch reflex

Answer 4

• when the muscle spindle is suddenly stretched by the striking of a tendon with a reflex hammer, Ia afferents conduct a train of action potentials towards excitatory syanpses on alpha motor neurons innervating the stretched muscle and its synergists
• collaterals of the excited La afferents also excite interneurons that inhibit firing of the alpha motor neurons of the antagonist muscle, contributing to a a process called reciprocal innervation

Question 5

describe the role of the Golgi tendon organ

Answer 5

• high tension in the muscle vigorously activates Ib afferent
• Ib afferents activate interneurons that inhibit alpha motor neurons communicating with overactive extrafusal fibers
• reduced activity in alpha motor neurons relaxes overactive extrafusal fibers, thus reducing chances of injury and/or redistributing load to less strained extrafusal fibers
• at lower muscular tensions, the apparatus exerts excitatory influences that aid in rhythmic muscular contraction

Question 6

describe a motor unit

Answer 6

• each muscle fiber receives only the ending of one branch of a motor axon
  
  • motor unit = alpha motor neuron and innervated extrafusal fibers
• fine movements (extraocular muscles) require small motor units (about 10 extrafusal fibers)
• coarsely controlled muscles utilize large motor units (about 1000 extrafusal muscle fibers per alpha motor neuron)

Question 7

describe the types of motor units (type I, IIA, IIB)

Answer 7

• type I: slow-twitch fibers, low tension, fatigue-resistant, aerobic, innervated by relatively small motor neuron and axon
• type IIA: fast, relatively fatigue-resistant, large tension, some aerobic capacity, innervated by relatively large motor neuron and axon
• type IIB: fast, fatigue-susceptible, large tension, anaerobic, innervated by relatively large motor neuron and axon

Question 8

describe lower motor neuron syndrome

Answer 8

• arises from damage to cells derived from the basal plate that communicate with somatic muscle cells
• features:
  
  • paresis/paralysis
  • hyporeflexia
  • hypotonia
  • fasciculations
  • fibrillations
  • muscular wasting/atrophy

Question 9

describe hypotonia (as seen in LMN lesion)

Answer 9

• hypotonia is detectable by palpation and testing resistance to passive stretch of the limb
• damage to either the Ia afferent pathway or the LMNs will reduce muscle tone, yielding hypotonia and hence flaccid paralysis
  
  • slight hypotonia can also arise from cerebellar lesions