51 Skeletal muscles and reflexes Flashcards
What is a motor unit?
The muscle fibers and the motor neuron that innervate it
________ motor units are preferentially recruited in general. This is because?
Smaller
- membrane resistance is higher (V=IR), thus same current can lead to a higher voltage change
- Smaller soma, thus less decremental effects (越來越細) when graded potential travels across the soma
________motor units are fatigue resistant.
Smaller
_________ motor units are not fatigue resistant.
Larger
What is the Henneman’s size principle?
Smaller motor units are recruited before the larger ones. (Larger motor units are recruited only when activation level is high)
State the factors determining the force generated by skeletal muscles.
- Activation level (the number of small and larger motor units recruited)
- Muscle length (too short/too long/ optimal length of overlap)
In the cross-bridge cycle, active site on actin is exposed when _______ bind to ______.
Ca2+; troponin
____________forms a cross-bridge cycle with actin.
Myosin head (by binding to the active site)
During the power stroke, myosin head bends and ____________ are released.
ADP and phosphate
New molecule of ______ attaches to the myosin head, causing the cross bridge cycle to detach.
ATP
The process of __________ causes the myosin head to return to the ‘cocked’ position’.
ATP > ADP + P
> stored potential energy, waiting for next Ca2+ to bind to troponin
The greater/lesser the overlapping between thin (actin) and thick (myosin) filaments, the greater the force of contraction/
Greater
if already all contracted > all overlapped > no contraction available
_________ are the basic units of muscles. They are wrapped by _______________ which stores _____ for muscle contraction.
Sarcomeres in each muscle fiber; Sarcoplasmic reticulum ; Ca2+
What are the involvements of brainstem and spinal cords in motor coordination?
- voluntary actions
- locomotion (e.g. midbrain locomotor center, subthalamic locomotor region, pontine RF, CPG)
- reflexes (stereotypical motor patterns elicited by sensory stimuli)
What are the 4 functions of reflexes?
- Stereotypical motor responses
- Limb stability
- Prevention of muscle damage from high muscle tension
- Facilitate muscle activation in locomotion
Give examples of the stereotypical motor response function of reflexes.
- Plantar reflex (aid walking)
- Withdrawal reflexes
- Righting reflexes (flap cat > automatically turn around, flip back)
What is the normal and abnormal presentation in plantar reflexes?
Normal: plantarflexion (downward motion of toe)
Abnormal: UMNL: dorsiflexion of big tow, fanning out other toes = positive Babinski sign
Limb stability is a function of reflex which can be illustrated in __________ reflex.
Stretch
someone pushes you > won’t fall due to resist changes in muscle length
______________ are the sensor in monosynaptic stretch reflexes. It senses the _________ and _________ of muscle.
Muscle spindles;
change in length and speed of length change
Muscle spindles wrap around the non-contractile unit of the _________ muscles and sudden stretch of the muscle will activate them.
intrafusal muscles
Intrafusal muscles are __________ while extrafusal muscles are __________.
non-contractile;
contractile, for contraction
Briefly describe the monosynaptic stretch reflex pathway.
afferent: 1a afferent fibers
1. agonist activation
2. inhibition of antagonist by “Ia inhibitory interneuron”
Other than monosynaptic stretch reflex, what are the other mechanism contributing to limb stability?
- Muscle elasticity
2. Co-contraction of agonist and antagonist muscles
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
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
