2. Muscles and Motoneurons Flashcards
in motor control, you need to think about what the muscles are __, rather than about the goal of the __. motor control is not __!
doing, movement, intuitive
where does most movement occur
at joints, where 2 bones come together (tongue and eyes are exceptions). muscles contract and pull; to get full range of motion around joint, must have at least one muscle attached on each side of a joint
smooth, useful movement requires
coordination
as muscles work, there are changes in __ __ and __ __
joint angle, muscle length
flex: __ joint angle; flexor __ and __; extensor is __ and __
decrease, contracts, shortens, stretched, lengthens
extend: __ joint angle; extensor __ and __; flexor is __ and __
increase, contracts, shortens, stretched, lengthens
muscle with opposite action at the joint
antagonist
all muscles with the same action at a joint
synergists
posture definition
resisting forces of gravity
posture antigravity muscles
extensors
posture antagonist muscles
flexors
differences in the ___ control of flexors and extensors reflect the role of __ in resisting gravity
descending, extensors
postural fixation
elimination of unwanted movement at a joint (necessary for fine movement)
4 parameters of muscle action
- force
- speed of contraction
- resistance to fatigue
- fineness of control (small changes in force, small changes in joint angle)
- first three properties result from structure of muscle
- last property results from structure and pattern of innervation of muscle
fast twitch muscle fibers
- white muscle
- FF fibers (think fast, fatigueable): largest force, fastest contraction time, fatigue readily
- FR fibers (think fast, resistant): fast, more resistant to fatigue, large force
slow twitch muscle fibers
- red muscle
- S fibers: least force, slowest contraction time but most resistant to fatigue
true or false: you can increase the number of muscle fibers or change muscle fiber type with training
false, you cannot. however, properties of individual fibers are changed.
composition of different muscles reflects their __. each muscle is composed of a mixture of __ types. % of different fiber types varies with how muscle is used. composition of a particular muscle may vary among individuals.
function, fiber
axons in efferent innervation of muscle
- large diameter
- myelinated
- Aalpha fibers
- 12-20 um diameter
- 70-120 m/sec conduction velocity
- each axon branches and innervates muscle fibers (varies from 2-1000s, depends on size of mn)
all mn’s innervating a particular muscle are the __ __. they are distributed over several cord segments.
motoneuron pool
mns in a mn pool vary in size
- size (diameter) of soma
- diameter of axon (12-20 um)
- number of branches of axon (function of size, bigger mns more branches)
- number of muscle fibers innervated (function of number of branches and therefore of size)
*each mn pool contains a population of mns of a range of sizes
motoneurons release chemicals called __ factors which are essential for the health of muscles. if mn die, their axons also die and the muscles will __. give two clinical examples.
trophic, atrophy
1. polio
2. ALS/Lou Gehrig’s disease (amyotrophic lateral sclerosis; absence of trophic factors for muscle; spinal cord and cortex)
*lose alpha motoneurons in these diseases
size of cell body (and axon diameter) is correlated with the number of ___ the axon makes in muscle aka the number of muscle fibers __
branches, innervated
motor unit
single motoneuron and all muscle fibers it innervates
- each muscle fiber receives innervation from one motoneuron; mn fires –> muscle fibers contract –> unit of action of motor system)
true or false: all muscle fibers in a motor unit are of the same type
true
which fibers have largest motor units and which have smallest?
largest: FF fibers
smallest: S fibers
all muscles are composed of a population of motor units of different sizes. average motor size varies among muscles.
fine control: average motor unit size __; __ twitch
force, speed: average motor unit size __; __ twitch
smaller, slow
larger, fast
larger mn =
larger axon = more branches = more muscle fibers it innervates = larger motor unit = more force it can produce = faster response = faster fatigue
when brain wants a muscle to contract, how are the motor units used?
motor units are recruited in order of increasing size. smallest first, largest last, and drop out in reverse order.
functional consequences of size principle
S fibers (most resistant to fatigue) are in use most of the time, which allows optimal adjustment of force increments
(incrementing force gradually by calling in motor units from small to large size)
2 kinds of feedback information from muscle
- muscle length: receptor is the muscle spindle
- maintained length (static information)
- rate of change of length (dynamic information) - muscle force: receptor is the golgi tendon organ (GTO)
structure of muscle spindle
- connective tissue sheath containing 2-12 specialized muscle fibers called intrafusal fibers (ordinary muscle fibers are extrafusal)
- found in all muscles
- density varies among muscles (higher density with fine control)
- attached “in parallel” with extrafusal fibers
- have both sensory and motor innervation
intrafusal fibers
- have contractile poles (motor innervation) and noncontractile centers (sensory innervation)
- 2 types: have different structures, different sensory innervation and code different info
- nuclear bag fibers (dynamic)
- nuclear chain fibers (static)
group IA afferent
- both nuclear bag fiber and nuclear chain fiber
- primary ending
- larger diameter, faster
group II afferent
- nuclear chain fiber
- secondary ending
- smaller diameter, slower
sensory innervation of spindle: _ types of sensory fibers, different anatomically and functionally. like all sensory fibers innervating the body, cell bodies are in __, axons innervate __ __
2, drg, muscle spindles
when might CVs change?
diabetic neuropathy - assessed by measuring CVs
group IA fibers innervate every __ fiber, form primary or __ endings, convey both __ and __ info
intrafusal, annulospiral, dynamic, static
group II fibers innervate only __ __ fibers, secondary or __ __ endings, code __ info
nuclear chain, flower spray, static
when a muscle is __, the endings are __ (stretch-sensitive channels); if enough __, action potentials are generated
stretched, depolarized, stretch
what happens when a muscle contracts?
muscle shortens, spindle does not, and goes “slack”. no stretch on spindles, no sensory input, no action potentials. this is a problem
solution to motor innervation of spindle
motor innervation of contractile poles of intrafusal fibers, adjust the length of spindle in parallel with length of extrafusal fibers
gamma mn
- “fusimotor”: motor to the spindle
- somata in ventral horn, axons 5-12 um diameter, CV = 30-70 m/sec
- axons innervate the contractile poles of intrafusal fibers, both kinds (nuclear bag and nuclear chain fibers)
GTO and muscle force
- attached “in series” with muscle (instead of parallel), at the junction of the muscle and tendon
- Ib fibers: 12-20 um diameter, 70-120 m/sec CV
- active over entire range of tensions muscle can generate, not just at extremes of force
