2. Muscles and Motoneurons Flashcards by K L

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

smooth, useful movement requires

coordination

How well did you know this?

Not at all

Perfectly

as muscles work, there are changes in __ __ and __ __

joint angle, muscle length

How well did you know this?

Not at all

Perfectly

flex: __ joint angle; flexor __ and __; extensor is __ and __

decrease, contracts, shortens, stretched, lengthens

How well did you know this?

Not at all

Perfectly

extend: __ joint angle; extensor __ and __; flexor is __ and __

increase, contracts, shortens, stretched, lengthens

How well did you know this?

Not at all

Perfectly

muscle with opposite action at the joint

antagonist

How well did you know this?

Not at all

Perfectly

all muscles with the same action at a joint

synergists

How well did you know this?

Not at all

Perfectly

posture definition

resisting forces of gravity

How well did you know this?

Not at all

Perfectly

posture antigravity muscles

extensors

How well did you know this?

Not at all

Perfectly

posture antagonist muscles

flexors

How well did you know this?

Not at all

Perfectly

differences in the ___ control of flexors and extensors reflect the role of __ in resisting gravity

descending, extensors

How well did you know this?

Not at all

Perfectly

postural fixation

elimination of unwanted movement at a joint (necessary for fine movement)

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

slow twitch muscle fibers

red muscle
S fibers: least force, slowest contraction time but most resistant to fatigue

How well did you know this?

Not at all

Perfectly

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.

How well did you know this?

Not at all

Perfectly

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

1. muscle length: receptor is the muscle spindle - maintained length (static information) - rate of change of length (dynamic information) 2. 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