Motor System

Question 1

Coordination and Posture: Major motor functions

Where does movement occur? How does it occur?

Answer 1

Most movement occurs at joints –> where 2 bones come together.

Movement occurs by muscles contracting and pulling. To get full range of motion around a joint, there must be at least one muscle attached on either side of joint. Smooth, useful movement requires coordination.

As muscles work there are changes in joint angle and changes in muscle length.

Question 2

Two types of muscle: Flexors and Extensors

Answer 2

FLEX: decreases joint angle. Flexor contraction shortens the muscle, while it lengthens the extensor muscle.
EXTEND: Increases joint angle. Extensor contracts and shortens, while its contraction lengthens the flexor muscle.

Question 3

Muscle Definitions: Agonist, Antagonist, Synergists

Answer 3

-Agonist: “actor” –> Which muscle is acting, flexor or extensor?
-Antagonist: the muscle with the opposite action at the joint
-Synergists: all muscles with the same action at the joint. Additive effect.

Question 4

Posture: Definition, muscles involved

Answer 4

-2nd major function of the motor system
-posture definition = Resisting the force of gravity
-Antigravity muscles = Extensors
-Their antagonists = Flexors
Differences in descending control of flexors and extensors reflect the role of extensors in resisting gravity.

Question 5

Postural fixation

Answer 5

Elimination of unwanted movement at a joint which is necessary for useful movement. For example when doing a root canal (fine motor movement skill) you need complete control.

Question 6

4 Main Parameters of muscle action

Answer 6

Different muscles in the body vary in properties.
Parameters:
1) Force
2). Speed of contraction (how quickly muscle will respond)
3) Resistance to fatigue (how long muscle can work without getting weaker)
4) Fineness of control (small changes in force, small changes in joint angle) –> ex) important for hands and fingers fine motor control

Question 7

What do the 4 parameters of muscle action each depend on?

Answer 7

1) Force: result from the structure of muscle
2) Speed of contraction: results from the structure of muscle
3) Resistance to fatigue: results from the structure of muscle
4) Fineness of control: results from both the structure of muscle as well as the pattern of innervation of muscle

Question 8

The two Types of muscle fibers

Answer 8

Slow and fast twitch. Named for how quickly they respond.

Question 9

Fast twitch muscle fibers

Answer 9

-White muscle
-FF fibers: fast, fatigable, large force, fast contraction time, fatigue readily. Most vulnerable to fatigue.
-FR fibers: also fast but more resistant to fatigue, also large force.

Question 10

Slow twitch muscle fibers

Answer 10

-Red muscle
-S fibers: least force, slowest contraction time BUT most resistant to fatigue.

Question 11

How does training effect muscle fibers?

Answer 11

CANNOT increase the number of muscle fibers or change the muscle fiber type with training; Properties of individual fibers are changed.

Question 12

Muscle comoposition

Answer 12

Composition of different muscles reflect their function.
-Each muscle is composed of a mixture of these fiber types
-% different fiber types varies with different muscles, with how muscle is used.
-Composition of a particular muscle may vary among individuals, and may correlate with athletic ability
-% different types DO NOT change by training

Question 13

Marathon runner vs. sprinter

Answer 13

Long distance runner likely has more S fibers (red muscle) where as a sprinter most likely has more F fibers (white muscle).

Question 14

Efferent innervation of muscle

Answer 14

-done by Alpha motoneurons (located in ventral horn of spinal cord).
-Alpha MN cell bodies are in the ventral horn and axons leave in the ventral roots.
-The axons are large in diameter and myelinated. The Aalpha fibers have a known range of diameter and conduction velocity which is important because these numbers can change with various disorders

Question 15

Each alpha MN branches and innervates muscle fibers, how many?

Answer 15

Number varies, 2-1000’s, depends on size od MN

Question 16

Motoneuron pool

Answer 16

ALL MN’s innervating a particular muscle are the motoneuron pool; they are distributed over several cord segments (Not just at one level)

Question 17

Motoneurons in a MN pool vary in size. How?

Answer 17

1) Size (diameter) of soma/ cell body
2) Diameter of axon (correlated with the cell body)
3) Number of branches of axons (Function of size, bigger MN’s more branches)
4) Number of muscle fibers innervated (function of number of branches and there for of size) {each mn pool contains a population of mn’ of a range of sizes}

Question 18

Trophic factors

Answer 18

Motoneurons release chemicals called trophic factors which are essential for the health of muscles

-if motoneurons die their axons also die and the muscles die or ATROPHY therefore non of these trophic factors being released

Question 19

Polio and ALS-lou Gehrigs disease

Answer 19

Both diseases that are caused by loss/ death of alpha motoneurons and therefore an absence of trophic factors for muscle.

-in ALS –> stands for amyotrophic lateral sclerosis
–> Amyotrophic= absence of trophic factors for muscles
–> Lateral sclerosis = pattern of scar tissue in the spinal cord and cortex

Question 20

Poliomyelitis (aka polio)

Answer 20

Viral infection, highly contagious, human to human contact. Major health problem in the USA until vaccine. MN’s die, muscles atrophy (paralysis). It was gone in the usa since 1979 because of vaccine; still present in Afghanistan, Nigeria, and Pakistan

-look like skin and bones

Question 21

Motoneurons and motor units

Answer 21

-Size of cell body (and axon diameter) is correlated with the number of branches the axon makes in muscle, therefore the number of muscle fibers innervated.

Motor unit: Single motoneuron and all muscle fibers it innervates
All muscle fibers in a motor unit are of the same type. (each muscle fiber receives innervation from one and only one motoneuron; mn fires, muscle fibers contract (all muscle fibers in that unit), unit of action of motor system).

Correlation between size of motor unit and muscle fiber type.

Question 22

Motor Unit size and muscle fiber type

What are the largest motor units?
what are the smallest?

Answer 22

Largest: FF fibers
Smallest: S fibers

Muscle fibers in a single motor unit are scattered about the muscle

Question 23

Relationship between motor unit size and muscle properties

Answer 23

All muscles are composed of a population of motor units of different sizes
The AVERAGE motor unit size varies among muscles
-Fine control: average motor unit size is smaller (activating a relatively small force which is necessary for control)
-Force, speed: average motor unit size is larger. (F twitch muscle fibers are found more in larger motor units)

Question 24

Summary: properties of motor units

Answer 24

The larger to motoneuron the more muscle fibers it innervates; the larger the motor unit the more force it can produce, the more quickly it will respond AND the more quickly it will fatigue.

Question 25

Small motor units

Answer 25

S fibers

Question 26

Larger motor units

Answer 26

FF fibers

Question 27

Size principle: When brain wants a muscle to contract how are the motor units used?

Answer 27

Motor units are recruited in order of increasing size. smallest first, largest last, and drop out in reverse order.

Functional consequences: S fibers (most resistant to fatigue, so fibers are called in only when needed) in use more of the time; also this allows optimal adjustment of force increments (incrementing force gradually).

Question 28

Two kinds of feedback information from muscle

Answer 28

1) Muscle length
2) Muscle force

Question 29

Muscle length as feedback information from muscle

Answer 29

-the receptor is the muscle spindle
-there is maintained length (static information –> how long is the muscle when it is not changing) and rate of change of length (dynamic information–> is it steady, is it changing, how long is it changing)

Question 30

Muscle force as feedback information from muscle (whats the receptor)

Answer 30

Receptor is the golgi tendon organ

Question 31

What is the structure of the muscle spindle? Where are they found?

Answer 31

• Muscle spindle is a connective tissue sheath containing 2-12 specialized muscle fibers called intrafusal fibers (meaning w/ in the spindle)
  (ordinary muscle fibers are “extrafusal” fibers)
  -Muscle spindles are found in all muscles
  -The density varies among muscles
• muscle spindles are attached in parallel with extrafusal fibers.

Question 32

What kind of innervation do muscle spindles have?

Answer 32

spindles have both sensory and motor innervation

Question 33

Intrafusal fibers

Answer 33

-intrafusal fibers have contractile poles (motor innervation) and noncontractile centers (sensory innervation)

Question 34

2 types of intrafusal fibers

Answer 34

-different structures, different sensory innervation and code different information: nuclear bag fibers (dynamic), nuclear chain fibers (static)

-Nuclear bag fibers (dynamic): all nuclei are centered in bag like formation
-Nuclear chain fibers (static): all nuclei spread out in a chain like formation

Question 35

Sensory innervation of spindle

Answer 35

• 2 types of sensory fibers, different anatomically and functionally.
  -Like all sensory fibers innervating the body, cell bodies are in dorsal root ganglion, axons innervate muscle spindles
  -Group IA 12-20 micrometers diameter, CV = 70-120
  -Group II 5-12 micrometers diameter, CV = 30-70 m/sec

Question 36

sensory innervation of spindle: Group IA fibers

Answer 36

Group IA fibers innervate every intrafusal fiber. They form primary or annulospiral endings and they convey both dynamic and static information

Question 37

Sensory innervation of spindle: Group II fibers

Answer 37

Innervate only nuclear chain fibers. They have secondary or flower spray endings, and code static information.
-when a muscle is stretched, the endings are depolarized, (stretch sensitive channels) –> if stretched enough, action potentials are generated

Question 38

Motor innervation of the spindle: the gamma motoneuron

Answer 38

Problem with the system so far: what happens when a muscle contracts? –> The muscle shortens, the spindle does not, and goes “slack.”
-No stretch on spindles, not sensory input (No opening of those stretch sensitive channels)
-Solution: motor innervation of contractile poles of intrafusal fibers, adjust the length of spindle in parallel with length of extrafusal fibers

Question 39

Gamma (Y) mns: “fusimotor”

Answer 39

Somata (cell body) in ventral horn, axons 5-12 micrometers diameter, CV= 30-70 m/sec
-axons innervate the contractile poles of intrafusal fibers, both kinds (nuclear bag and nuclear chain fibers)

Question 40

Golgi tendon origins and muscle force

Answer 40

Attached “in series” with muscle, at the junction of the muscle and tendon
-Ib fibers: 12-20 micrometer diameter
(again cell body in dorsal root ganglion. information to spinal cord, cerebellum).
Active over entire range of tensions muscle can generate, not just at extremes of force.