Lecture 10 Spinal Cord ,Brainstem , Cortex control of Motor Function

Question 1

Anterior motor neurons

Answer 1

Alpha motor neurons give rise to A alpha(Aa)fibers

Question 2

Interneurons

Answer 2

• 30X as numerous as anterior motor neurons
• Small and excitable
• Capable of spontaneous activity
• Responsible for most of spinal cord integrative function

Question 3

Motor unit

Answer 3

• Composed of a single motor nueron and muscle fibers it innervates
• composed of extrafusal fibers

Question 4

Motor Neuron pool

Answer 4

-Group of motor neurosn taht innervate fibers within the same muscle

Question 5

Recruitment

Answer 5

• Increase in tension of muscle contractions by the activation of additional motor units(size principle)
• -size principle- states that under load, motor units are recruited from smallest to largest. In practice, this means that slow-twitch, low-force, fatigue-resistant muscle fibers are activated before fast-twitch, high-force, less fatigue-resistant muscle fibers

Question 6

Small vs Large motor neurons

Answer 6

• small is first to fire , lowest threshold , innervates few muscle fibers , and generate smallest force
• large is last fire , highest threshold, innervates many fibers , generates largest force

Question 7

Renshaw cells

Answer 7

• Inhibitory cells in anterior horns of spinal cord
• receive collateral branches from alpha motor neurons
• transmit inhibiotry signals to surrounding motor neurosn
• transmit inhibitory signals to same motor neuron–>recurrent inhibition

Question 8

What type of muscle sensor is muscle spindle?

Answer 8

• Group Ia and II afferents
• arranged in parallel with extrafusal fibers
• detect both dynamic and static changes in muscle length

Question 9

What type of muscle sensor is Golgi tendon organ

Answer 9

• Group 1b afferents
• arranged in series with extrafusal fibers
• detect muscle tension

Question 10

What type of muscle sensor is Pacinian corpuscles

Answer 10

• Group II afferents

- detect vibration

Question 11

What type of muscle sensor are free nerve endings?

Answer 11

• Groups III and IV afferents

- Detect noxious stimuli

Question 12

Which nerve fiber type is not myelinated?

Answer 12

Group IV

-1,2,3 are myelinated

Question 13

Details about Muscle spindle

Answer 13

• 3 to 10mm long
• 3 to 12 intrafusal fibers(make up the muscle fibers , which are innervated by alpha motor neurons)
• -innervated by small gamma motor neurons(group II afferents)
• -run parallel to extrafusal fibers
• Central region of spindle has no contractile fibers; functions as a sensory receptor
• -stretching of central region of intrafusal fiber stimuatle sensory fibers
• detect changes in muscle length
• with finer movements ,the nubmer of muscle spindles requried increases

Question 14

Types of intrafusal fibers in muscle spindle?

Answer 14

Nuclear bag fibers

Nuclear chain fibers

Question 15

Nuclear bag fibers

Answer 15

• detect rate of chagne in muscle length
• innervated by group 1a afferents and dynamic gamma efferents
• Multiple nuclei located in a central “bag like”configuration

Question 16

Nuclear chain fibers

Answer 16

• Detect static change in muscle length
• innervated by group II afferents and static gamma efferents
• more numerous than nuclear bag fibers
• multiple nuclei arranged in a single row

Question 17

Sensory fibers from central region of intrafusal fiber?(muscle spindle)

Answer 17

• Stimulation results from:
• -lengthening of entire muscle
• -contraction of ends of intrafusal fibers
• Type of sensory fibers
• -Ia(primary fibers
• -II(secondary fibers )
• –smaller then Ia and stimulation results in stimulation of a-motor neurons,resulting in contraction and shortening of muscle

Question 18

Muscle Spindle y motor neurons

Answer 18

• Innervate intrafusal fibers
• Adjust sensitivity of muscle spindle
• coacitvated with a-motor neurons
• -stimulation results from lengthening of entire muscle

Question 19

Gamma Motor Neurons

Answer 19

Equal to about half the number of alpha motor neurons
Types:
-Ay:supply small intrafusal fibers in middle of muscle spindle
-Gamma-dynamic:excite nuclear bag intrafusal fibers
-Gamma-static: excite nuclear chain intrafusal fibers