Motor units and muscle force Flashcards
(29 cards)
What is a motor neuron?
efferent neuron
motor cortex
brainstem
spinal cord
innervate muscles and glands to enable voluntary and involuntary motions
What is a Motor unit?
1 motor neuron and its innervated muscle fibers
What is a motor nucleus?
cluster of cell bodies of (hundred) motor neurons that innervate the same muscle
What is an afferent/efferent impulse?
afferent: from sensory organ to nervous system
efferent: from nervous system to effector organs
What are the different kinds of motor neurons?
alpha motorneuron: send info to skeletal muscles
gamma: send info to the muscle spindle fibers –> adjust the sensitivity of muscle spindle
what is an agonist muscle?
contract to create force and enable a movement
what is an antagonist muscle?
oppose the action of an agonist to control and module movements
How does stepping spinal reflex/stretch-reflex work?
Stimulus: muscle is stretched –> muscle spindle –> afferent impulse –> spinal cord
Coordination of alpha MN and gamma MN
Alpha MN: Efferent impulses –> contraction of the stretched muscles + inhibit contraction of antagonistic muscles
gamma MN just the sensitivity of muscle spindles
What is an innervation number?
number of muscle fibers innervated by a single motor neuron
What is the innervation number used for?
indicate the finest control of the muscle
small number –> finer control
What is the distribution of innervation numbers across the population of motor units?
Innervation number increases exponentially within a population of MUs –> non-linear
Most motor units innervate fewer fibers, while fewer large ones innervate many fibers.
How much of the fibers have low innervation numbers?
within a muscle e.g. in the first dorsal interosseous muscle
50%
innervation nr = 0-900
smaller MUs innervate type I fibers, have low innervation numbers
What are the 2 mechanisms used to control muscle forces, when the muscle develops a force?
recruitment - when
rate coding - speed
what is the recruitment threshold?
level of stimulation required to activate a motor unit
what is the discharge rate?
rate coding
the rate at which each of the active MN discharges APs
What are the relationships between motorneurons’ size and the size of the muscle fibers that they innervate?
smaller MN – smaller, slow-twitch muscle fibers – lower forces but more fatigue-resistant
larger MN - larger, fast-twitch muscle fibers - higher forces but fatigue more quickly
What is the Henneman’s size principle?
the size principle of MN recruitment
order: smaller - recruited before larger ones
What is twitch contraction?
the menchanical response to a single AP
What is contraction time?
time it takes the twitch to reach its peak force
1 measure of the contraction speed of the muscle fibers that compose a MU
What happens with the action potential in the fast twitch and slow twitch at the motor unit?
regarding the contraction time and the force production
slow twitch MUs - weak + long contraction e.g. long-distance running, maintaining posture
fast-twitch MUs - strong + shorter contraction e.g. sprinting, weightlifting
What is the difference between the peak force in fast- and slow-twitch motor units?
Absolute force: greater for fast-twitch MU at all frequency
What is the relationship between the recruitment threshold of a motor unit and the twitch force it produces?
linear correlation
What is explosive strength?
the ability to increase force or torque as quickly as possible during a rapid voluntary contraction realized from a low or resting level
e.g. athlete - sprinting, weightlifting, jumping
What is the Rate of force development (RFD)?
is derived from the force- or torque-time curves recorded during explosive voluntary contractions (rapid or ballistic actions)
