Motor Systems I-III

Question 1

Motor unit definition

Answer 1

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

Question 2

Characteristics of muscle spindles

Answer 2

• 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

Question 3

Gamma motor neuron fxn

Answer 3

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

Question 4

Characteristics of Golgi Tendon Orgams

Answer 4

• = 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

Question 5

Characteristics of stretch reflex

Answer 5

• = “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

Question 6

Characteristics of the “size principle”

Answer 6

• 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

Question 7

Muscle tone definition

Answer 7

muscle tone = resistance of a muscle to stretch

Question 8

Neurons innervated by muscle spindle afferents

Answer 8

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

Question 9

Characteristics of crossed extensor reflex

Answer 9

• 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

Question 10

Characteristics of central pattern generators

Answer 10

• 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

Question 11

CPG experiments on cats

Answer 11

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

Question 12

CPGs and future of spinal cord injury tx

Answer 12

• 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

Question 13

Reticular formation fxn in regulating movement

Answer 13

• 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

Question 14

Upper motor centers @ brainstem/midbrain

Answer 14

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

Question 15

Fxn of the vestibular complex/nuclei

Answer 15

• 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)

Question 16

Fxn of superior colliculus

Answer 16

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

Question 17

General fxn of motor cortex

Answer 17

• 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

Question 18

Motor actions resulting from primary motor cortex microzone stimulation

Answer 18

• ==> 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

Question 19

Characteristics of premotor cortex

Answer 19

• 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)

Question 20

Characteristics of supplementary motor cortex

Answer 20

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

Question 21

Major components of motor cortex

Answer 21

• primary motor cortex
• premotor cortex
  
  • supplementary motor cortex

Question 22

UMN syndrome

Answer 22

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

Question 23

LMN syndrome

Answer 23

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

Question 24

Evidence that cortical motor representation is dynamic

Answer 24

• 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