51 Skeletal muscles and reflexes

Question 1

What is a motor unit?

Answer 1

The muscle fibers and the motor neuron that innervate it

Question 2

________ motor units are preferentially recruited in general. This is because?

Answer 2

Smaller

1. membrane resistance is higher (V=IR), thus same current can lead to a higher voltage change
2. Smaller soma, thus less decremental effects (越來越細) when graded potential travels across the soma

Question 3

________motor units are fatigue resistant.

Answer 3

Smaller

Question 4

_________ motor units are not fatigue resistant.

Answer 4

Larger

Question 5

What is the Henneman’s size principle?

Answer 5

Smaller motor units are recruited before the larger ones. (Larger motor units are recruited only when activation level is high)

Question 6

State the factors determining the force generated by skeletal muscles.

Answer 6

1. Activation level (the number of small and larger motor units recruited)
2. Muscle length (too short/too long/ optimal length of overlap)

Question 7

In the cross-bridge cycle, active site on actin is exposed when _______ bind to ______.

Answer 7

Ca2+; troponin

Question 8

____________forms a cross-bridge cycle with actin.

Answer 8

Myosin head (by binding to the active site)

Question 9

During the power stroke, myosin head bends and ____________ are released.

Answer 9

ADP and phosphate

Question 10

New molecule of ______ attaches to the myosin head, causing the cross bridge cycle to detach.

Answer 10

ATP

Question 11

The process of __________ causes the myosin head to return to the ‘cocked’ position’.

Answer 11

ATP > ADP + P

> stored potential energy, waiting for next Ca2+ to bind to troponin

Question 12

The greater/lesser the overlapping between thin (actin) and thick (myosin) filaments, the greater the force of contraction/

Answer 12

Greater

if already all contracted > all overlapped > no contraction available

Question 13

_________ are the basic units of muscles. They are wrapped by _______________ which stores _____ for muscle contraction.

Answer 13

Sarcomeres in each muscle fiber; Sarcoplasmic reticulum ; Ca2+

Question 14

What are the involvements of brainstem and spinal cords in motor coordination?

Answer 14

1. voluntary actions
2. locomotion (e.g. midbrain locomotor center, subthalamic locomotor region, pontine RF, CPG)
3. reflexes (stereotypical motor patterns elicited by sensory stimuli)

Question 15

What are the 4 functions of reflexes?

Answer 15

1. Stereotypical motor responses
2. Limb stability
3. Prevention of muscle damage from high muscle tension
4. Facilitate muscle activation in locomotion

Question 16

Give examples of the stereotypical motor response function of reflexes.

Answer 16

1. Plantar reflex (aid walking)
2. Withdrawal reflexes
3. Righting reflexes (flap cat > automatically turn around, flip back)

Question 17

What is the normal and abnormal presentation in plantar reflexes?

Answer 17

Normal: plantarflexion (downward motion of toe)

Abnormal: UMNL: dorsiflexion of big tow, fanning out other toes = positive Babinski sign

Question 18

Limb stability is a function of reflex which can be illustrated in __________ reflex.

Answer 18

Stretch

someone pushes you > won’t fall due to resist changes in muscle length

Question 19

______________ are the sensor in monosynaptic stretch reflexes. It senses the _________ and _________ of muscle.

Answer 19

Muscle spindles;

change in length and speed of length change

Question 20

Muscle spindles wrap around the non-contractile unit of the _________ muscles and sudden stretch of the muscle will activate them.

Answer 20

intrafusal muscles

Question 21

Intrafusal muscles are __________ while extrafusal muscles are __________.

Answer 21

non-contractile;

contractile, for contraction

Question 22

Briefly describe the monosynaptic stretch reflex pathway.

Answer 22

afferent: 1a afferent fibers
1. agonist activation
2. inhibition of antagonist by “Ia inhibitory interneuron”

Question 23

Other than monosynaptic stretch reflex, what are the other mechanism contributing to limb stability?

Answer 23

1. Muscle elasticity

2. Co-contraction of agonist and antagonist muscles

Question 24

Which of the following is true?
A. Each motor neuron innervate one muscle fiber
B. Motor neurons in the ventral horn innervates the skeletal muscles
C. Each muscle has 1000-1000000 muscle fibers
D. Motor unit consists of the muscle fibers only

Answer 24

B and C only
A: Each motor neuron innervates a few - thousands of muscle fibers

D: Motor unit = muscle fibers and the motor neuron innervating it

Question 25

Give an example of the monosynaptic stretch reflex.

Answer 25

Knee jerk reflex

Question 26

Example of reflex preventing muscle damage from high muscle tension?

Answer 26

Golgi tendon reflex/ autogenic inhibition reflex (polysynaptic)

Question 27

How does reflex prevent muscle damage from high muscle tension? What is the pathway involved?

Answer 27

by reflexive relaxation of muscle when muscle tension is high

Autogenic inhibition reflex pathway:

Afferent: 1b afferent fibers, receptors of GTO (sensor)
Efferent: act on motor neuron via 1b inhibitory interneuron

Question 28

Receptors of GTO (receptors) lies within ______________ of muscle tendon > DRG of spinal cord.

Answer 28

collagen fibers

Question 29

What is state-dependent reflex reversal?

Answer 29

Signals from other locomotor spinal interneurons are activated only during locomotion.

e.g. GTO reflex is exploited during walking, with an excitatory reflex in order to enhance the activation of extension muscles > can walk with more force (esp stance phase)

Question 30

Normally, spinal reflexes are under ___________________ by the cerebrum and brainstem.

Answer 30

Inhibitory modulation

Question 31

When do hyperreflexia and hyporeflexia happen respectively?

Answer 31

Hyperreflexia: UMNL
Hyporreflexia: 
1. LMNL
2. UMNL
3. muscle damage
4. sensory/ motor nerve lesion

Question 32

Different cortical neuron has a particular preferred direction, determined by ______________.
Motor cortical signals thus may encode the __________ of voluntary movement.

Answer 32

Population vector > Sum of vectors;
Direction

(movement of certain direction is greater > vector on that side is higher (?))