Power Point 18 & 19 Flashcards
Under the boutons lies specialized portion of muscle membrane termed
end plate
Motor axon branches at its terminal, each branch terminates in a
synaptic bouton
Neuromuscular synapses are
cholinergic (ACh)
One action potential fired by motor neuron produces one ________ in the muscle fiber
twitch
The time it takes the twitch to reach its peak force is termed the
contraction time
Depending on biochemical properties, there are ______ twitch motor units
slow and fast
A rapid series of action potentials (resulting in temporal summation) elicits superimposed twitches, resulting in sustained contraction is known as
tetanus
Biochemical properties of a muscle fiber influence what?
contraction speed, force, and fatigability
Type I muscle fibers are what type of twitch muscles
slow twitch - produce small forces but can sustain contraction longer
Type II muscle fibers are what type of twitch muscles
fast twitch - produce large forces but are easy to fatigue
The _________ is defined as a single motor neuron and the group of muscle fibers which it innervates.
motor unit
one muscle fiber is innervated by one ______
motor neuron
fine control = few muscle fibers per neuron – type _______ fibers (motor units of less than 100 for control of eye movements)
1
large force = many muscle fibers per neuron – type _______ fibers (>1000 for single motor unit controlling leg muscles)
2
all the muscle fibers innervated by the (Same/Different) motor neuron are of the same type
same
muscle fibers in one muscle work in parallel and are subdivided into _______
motor units
motor neurons in the ventral horn of the spinal cord that control one muscle are clustered into a column termed the __________________
motor nucleous
one motor nucleus = one _______
muscle
axons from a motor nucleus exit the spinal cord in several ____________, but cluster into one nerve bundle en rout to the target muscle.
ventral roots
the force exerted by a muscle depends on:
- the number of motor units activated during contraction
* the rate at which motor neurons discharge action potentials
Force is increased during a muscle contraction by the activation of additional motor units which are recruited progressively from the __________ to the __________.
weakest, strongest
the force during the contraction at which the motor unit is activated.
Motor unit recruitment threshold
Size Principle Of Motor Neuron Recruitment
recruitment proceeds in ascending order of motor neuron size
smaller motor neurons get recruited first
sequence of motor unit recruitment is determined by spinal mechanisms (brain cannot selectively activate specific motor units)
motor units are activated in order of increasing fatigability
Charles Sherrington recognized the importance of ________ in regulating movements.
sensory information
In 1906 he proposed that simple reflexes, elicited by activation of receptors in skin or muscle, are the ____________
basic units of movement.
Ibeta proprioceptive fibers (activated by muscle tension) synapse on Ibeta ________ interneurons in the spinal cord
inhibitory
the __________ slows muscle contraction as muscle tension increases, limiting the time muscles stay contracted in stationary animal. In the moving animals (when limbs are moving and muscle length is changing) inputs from other types of proprioceptive fibers modulate activity of Ibeta interneurons
reverse myotactic reflex
Ialpha proprioceptor is activated by muscle stretch, activates motor neuron that excites the same muscle, causing it to contract.
monosynaptic feed-forward excitation:
at the same time Ia also synapses on inhibitory interneurons that inhibit the motor neurons innervating antagonist muscles. This inhibition prevents muscle contraction that might otherwise resist the movement produced by stretch reflex
disynaptic feed-forward inhibition:
what reflex ark is controlled entirely by spinal cord circuitry, since it remains intact even if spinal cord has been transected from the brain.
stretch reflex
sensory feedback is required: if _____________
dorsal roots are transected reflex is abolished
Divergent Innervation
Opposite effect in the opposite leg: to support the body weight
Sensory afferent fibers excite both:
• motor neurons innervating the same (homonymous) muscle,• and motor neurons innervating other (heteronymous) muscles with a similar action
In all reflex spinal cord circuits sensory neurons form extensive divergent connections, allowing to produce a widely distributed, ___________
coordinated motor pattern.
Both the spatial extent and the force of muscle contraction depends on stimulus intensity. Diversity of sensory information – from Golgi tendon organ, muscle spindles, cutaneous receptors, and joints converges on interneurons in the spinal cord. In addition, they receive both excitatory and inhibitory inputs from descending pathways
Reflex adaptability:
Reflex Adaptability
Both the spatial extent and the force of muscle contraction depends on stimulus intensity.
Diversity of sensory information – from Golgi tendon organ, muscle spindles, cutaneous receptors, and joints converges on interneurons in the spinal cord.
Both excitatory and inhibitory inputs from descending pathways converge on interneurons in the spinal cord.
however the neural mechanism that produces stepping involves a set of spinal cord ________ that do not require any sensory feedback to produce activity
pacemaking circuits
for baseline activity spinal pattern-generating networks do not require sensory input, however their activity ________________ by both the inputs from the proprioceptors and by the descending signals from the brain
can be modulated
Properties of motor neurons are strongly influenced by ____________, so the excitability of motor neurons innervating a single muscle is under control of the brainstem.
monoamines
