Neuro 54: Muscles and motor neurons

Question 1

Muscle fiber types

Answer 1

• 2: fast and slow twitch
• composition of muscles will vary in the amnt of each type depending on the person and the fctn of the muscle
• the muscle fiber type is NOT changed by training!!

Question 2

Motorneuron

Answer 2

• gives efferent innervation to muscles
• muscles are activated by them
• cell bodies are found in the ventral horn of the SC
• divergence = a single mn can innvervate more than one muscle fiber
• BUT there is NO CONVERGENCE, each muscle fiber is only innervated by one mn

Question 3

Alpha motoneurons

Answer 3

• LOWER mn
• lg in diameter
• myelinated
• conduct rapidly

Question 4

Trophic factors

Answer 4

• released by the motorneurons onto the muscles

- necessary for the health of the muscle –> if the mn dies the muscle fibers it supplies will die

Question 5

Motorneuron pool

Answer 5

• consists of all the mn innervating one muscle
• distributed over several cord segments
• pool consists of a pop of mns of different sizes

Question 6

Motor unit

Answer 6

• single mn and all of the muscle fibers it innervates
• the fibers in a motor unit are scattered about in the muscle
• all of the fibers in a motor unit are the SAME type (fast or slow)

Question 7

Largest motor untis

Answer 7

• fast twitch
• fast = fatigable
• white muscle
• more force

Question 8

Smallest motor units

Answer 8

• slow twitch
• slow = resistant to fatigue
• red muscle

Question 9

Motor unit size

Answer 9

• AVERAGE size varies with the different muscles, smaller for muscles for fine control and larger for muscles for force and speed
• genetically determined, NOT effected by training –> can change with some diseases though (polio)

Question 10

The size principle

Answer 10

• motor units are recruited in order of increaseing size & drop out in reverse order
• so the fibers least resistant to fatigue are used most of the time
• allows for optimal control of force adjustment

Question 11

Two types of info recieved from the muscle fibers

Answer 11

1. muscle length - stretch receptor gives info on the joint angle and length changes
2. muscle force - golgi tedon organ is the receptor

Question 12

Golgi tendon organs

Answer 12

• gives feedback on the muscle force
• attach in series with the muscle = they are @ the junction of the muscle and tendon
• form a collagen “basket” that surrounds the sensory fiber
• the increase their force –> crunch down on the axons –> mechanically causes the depolarization of the axons –> cause APs

Question 13

Muscle spindle

Answer 13

• connective tissue sheath containing 2-12 specialized muscle fibers called intrafusal fibers
• imbeded in with the muscle fibers and they are scattered throughout
• found in parallell with the extrafusal fibers
• more are found in muscles that need more fine control

Question 14

Intrafusal fibers

Answer 14

• = in the spindle
• specialized muscle fibers found in the muscle spindle
• ordinary muscle fibers are called extrafusal fibers
• have contractile poles and noncontractile centers
• 2 types: nuclear bag and nuclear chain (differ on where the nuclei are found in the fiber)

Question 15

Sensory fibers

Answer 15

• 2 types: group Ia and group II - differ anatomically and functionally
• when a muscle is stretched these fibers depolarize and generate APs
• attach to the intrafusal fibers, when they stretch so do the sensory fibers

Question 16

Group Ia sensory fibers

Answer 16

• larger
• gives info on static (muscle length maintenance) and dynamic changes (rt of change of length)
• innervate EVERY intrafusal fibers
• form annulospiral endings (=wrap around the intrafusal fibers)

Question 17

Group II sensory fibers

Answer 17

• smaller
• give static info only = length maintenance
• innervate the nuclear chain intrafusal fibers ONLY
• form flower spray endings

Question 18

Gamma motoneurons

Answer 18

• innervate the contractile poles of the muscle spindles
• w/out them the spindle fibers would go slack as the muscle fibers shorten with contraction
• their cell bodies are found in the ventral horn of the sc too!
• they get a signal to contract when the alpha motor neurons get a signal, so the contraction of the muscle occurs smoothly