motor function

Question 1

define motor unit

Answer 1

single motor neurone together with all muscle fibres it innervates (smallest functional unit with which to produce force)

Question 2

how many muscle fibres does one motor neurone supply

Answer 2

average 600 (motor neurone axon branches)

Question 3

what does stimulation of one motor unit cause

Answer 3

contraction of all muscle fibres in that unit

Question 4

3 types of motor unit (in order of recruitment; de-recruitment is reverse order)

Answer 4

slow (S, type I); fast, fatigue resistant (FR, type IIA); fast, fatiguable (FF, type IIB)

Question 5

diameter of cell bodies of motor unit types

Answer 5

type I has smallest diameter cell bodies vs type IIA and IIB

Question 6

size of dendritic trees of motor unit types

Answer 6

type I have smallest dendritic trees vs type IIA and IIB

Question 7

diameter of axon of motor unit types

Answer 7

type I has thinnest axons vs type IIA and IIB

Question 8

conduction velocity of motor unit types

Answer 8

type I has slowest conduction velocity (unmeylinated) vs IIA and IIB

Question 9

how to determine distribution of fibre types

Answer 9

different amount of ATPase so can stain

Question 10

how are the 3 motor unit types classified

Answer 10

by amount of tension generated (force), speed of contraction, and fatiguability of motor unit

Question 11

twitch speed of motor unit types

Answer 11

type I has slow twitch, IIA and IIB have fast twitch

Question 12

tension of motor unit types

Answer 12

type I has low tension, type IIA has moderate tension, type IIB has high tension

Question 13

fatiguability of motor unit types

Answer 13

type I and IIA are fatigue resistant (type I much longer), type IIB has high fatigue (rapid fatigue)

Question 14

two mechanisms by which brain regulates force that a single muscle can produce

Answer 14

recruitment, rate coding

Question 15

recruitment of motor units (size principle)

Answer 15

not randomly recruited; governed by size principle, so smaller units with slow twitch units recruited first, but as more force is required, more units are recruited to allow fine control, under which low force levels are required (e.g. writing)

Question 16

rate coding of motor unit (summation)

Answer 16

slow units fire at lower frequency, but as firing rate increases, force produced by unit increases; summation occurs when units fire at frequency too fast to allow muscle to relax between arriving action potentials

Question 17

what are neutrophic factors

Answer 17

type of growth factor that affect motor units by preventing neuronal death and promoting growth of neurones after injury

Question 18

effect of neutrophic factors: normal innervation of SOL, FDL and FHL

Answer 18

motor unit and fibre characteristics are dependent on innervating nerve (SOL is slow and FDL and FHL are fast)

Question 19

effect of neutrophic factors: crossed innervation

Answer 19

if fast twitch and slow twitch muscle are cross innervated, SOL becomes fast and FDL becomes slow; motor neurone has some effect on properties of muscle fibres which it innervates

Question 20

plasticity of motor unit: what is the most common motor unit type change following training

Answer 20

type IIB to IIA (more fatigue resistant muscle fibres to maintain performance)

Question 21

plasticity of motor unit: when is type I to II possible

Answer 21

in cases of severe deconditioning or spinal cord injury e.g. microgravity during spaceflight also results in shift from slow to fast muscle fibre types

Question 22

plasticity of motor unit: what is aging associated with and effect

Answer 22

loss of type I and II fibres, but also preferential loss of type II fibres, resulting in a larger proportion of type I fibres in aged muscle (evidence from slower contraction times)