0519 Spinal Reflex and Muscle Control

Question 1

Classify the types of neurons that are responsible for muscle control?

Answer 1

• Upper: commanding neurons from cortex, commands lower motor neurons
• lower: lower motor neurons control somatic muscles, from ventral horn
  - alpha motor neurons: innervates striated muscles
  - gamma motor neurons: innervation to contractile fibers in muscle spindle
  - contracts muscle spindle to remove slack

Question 2

Name the types of proprioceptive inputs to the spinal cord (hint: type of information and their carriers)

Answer 2

• proprioceptors: spindles, golgi tendon organs, mechanoreceptors
  - spindles: feedback of strength/stretch from muscle
  - golgi tendon: load/force on tendons, joint positions
  - mechanoreceptors: on joint capsules and ligaments, gives angle, velocity, and movement (dynamic input)
• increased rate of fire = increased stretch, force and dynamic info

Question 3

name the 4 groups of neurons and identify what they innervate.

Answer 3

• 4 Groups: I II III IV
• group I: Ia and Ib, innervates muscle spindles (fast, myelinated, large)
  - Ia: all muscle spindles, detects rate of and degree of stretch (dynamic)
  - Ib: golgi tendon organs, detects muscle strain, decreases force
• group II: mechanoreceptors from skin (myelinated, medium size + speed)
  - innervates bag2 + chain spindles, extra degree of stretch info (static)
• group III: pain receptors, (small, myelinated, lowest of myelinated)
• group IV: pain, temperature, itch (small, unmyelinated, persistent, slow)

Question 4

What are gamma motor neurons and their functions? what are the two different types responsible for?

Answer 4

• innervation to contractile fibers in muscle spindle
• contracts muscle spindle to remove slack
• allows detection at optimal range during muscle contraction
• activated in parallel with alpha motor neurons
• two types: gamma static, gamma dynamic
  - gamma static: increased sensitivity to I and II fibers
  - on Bag2 and chains (more static info than dynamic)
  - gamma dynamic: group Ia, on Bag1 fibers

Question 5

What is the organization of intrafusal fibers (name the types and their functions)

Answer 5

• two types: chain and bag fibers
• all bag fibers are innervated by group Ia fibers
  - responds to degree and rate of stretch (dynamic information)
  - Bag2 and chain fibers innervated by group II fibres
  - responds to degree of stretch only (static/positional)
  - group II will not respond to fast stretch such as taps
• Intrafusal fibers have gamma motor neuron innervation
  - keeps spindle working at optimal range
  - must contract at the same rate as extrafusal fibers (parallel)
  - maintains sensitivity
• Bag1 fibers increase sensitivity to group Ia fibers
  - gamma dynamic fibre innervation
• bag2 and chain increase sensitivity to group Ia and II fibres
  - gamma static fibre innervation

Question 6

What is the gamma loop? Describe its function.

Answer 6

• loop between muscle spindle afferents, gamma motor neurons, and alpha motor neurons
  - regulates tension during normal activity
  - causes involuntary contraction of muscle
• during muscle contraction by alpha motor neurons, muscle spindles slack
  - causes gamma motor neurons to innervate spindles
  - brain perceives this as stretch
  - stimulates alpha motor neurons to contract

Question 7

Name the 4 types of reflexes, their organization, and their function.

Answer 7

Read Image

Question 8

What are central pattern generators?

Answer 8

structured motor programs that generated patterned movement

     - can produce rhythmic movement despite constant input from          upper motor neurons
     - NMDA channels play a role
     - through calcium influx and potassium channel                                  hyperpolarization of cells - done by pre-programed interneuron wiring - these inter-neurons can have pace-maker abilities or produce rhythmic movements by the nature of their circuitry - interneurons process several types of inputs: primary sensory, descending axons from brain and spinal cord, other interneurons, lower motor neurons