Lecture 8: Lower Motor Systems Flashcards
The four systems of motor
control
- Local spinal cord and
brainstem
• “Final common path” - Descending control pathways
• Upper motor neurons
• Voluntary and skilled movements - Basal ganglia
• Suppresses unwanted movements - Cerebellum
• “Tweaking” motor commands
Describe the three ways that motor neurons are orderly arranged at the level of the spinal cord.
- Spatial map of the body’s musculature
- Along the length
- Distinct clusters in the ipsilateral ventral horn
- Cylindrically shaped and distinct distribution of different motor pools
• Across the medial to lateral
- Axial musculature is in the medial cord
- Distal musculature is farthest from the midline
Alpha motor neurons
Innervate extrafuscal muscle fiers that generate force and movement.
Beta motor neurons (very basic)
innervate extra and intrafuscal muscle fibers
just know that they exist
What do gamma motor neurons do?
innervate intrafuscal muscle fibers, which generate no actual movement, but set the gain of the muscle fibers by changing the length of the intrafuscal muscle fibers.
Describe the local circuit neurons in the spinal cord. What do they do? (talk about long and short circuits)
- Interneuron processes remain in the spinal cord
- Mediate simple reflexes
- Contribute to the coordination of motor neurons involved in fine motor behaviors, balance, and locomotion
- Long distance local circuit neurons are medial, and control posture/balance.
- Short distance local circuit neurons are lateral in the ventral horn and control fine motor behaviors.
Motor unit
A motor neuron, and all the muscle
fibers it innervates
• Most extrafusal muscle is innervated
by a single alpha motor neuron
• Motor neuron must branch within
muscles to synapse onto many fibers
What are the differences between small and large alpha-motor units?
Motor units and alpha motor neurons can vary in size
• Small alpha-motor neurons innervate few muscle
fibers = small force
• Large alpha-motor neurons innervate many muscle
fibers = greater force
Red muscle fibers
• “Red” (S) muscle fibers make up most of the skeletal
muscles used for long exertions like standing
- Smaller alpha-motor neurons
- Contract slowly, relatively low force, resistant to fatigue (hours)
Pale muscle fibers (fast fatiguing)
“Pale” (FF) muscle fibers are used for brief exertions
(like jumping)
• Large alpha-motor neurons
• Contract quickly, high force, fatigue quickly (<1 to 5 min)
due to fewer mitochondria, higher activation threshold
“also red” or FFR muscle fibers
“Also Red” (FFR) muscle fibers used for moderate for
exertions (like running)
Describe motor unit plasticity
Training improves muscle performance primarily through changes in the
motor unit
• Increases in firing rate and decrease in firing latency
What the size principle of the motor unit.
Increases in muscle tension are
brought about through increases in
the number of motor units recruited
• Order in which motor units are
recruited based on stimulation level
• S units –> FFR –> FF
• Muscle tension is also controlled by
frequency
- Individual motor unit 8-25Hz
- Multiple motor units firing asynchrously
• Allows for smooth motionIncrea
increase force production
What three factors lead to the most force?
More motor units
Higher frequency
Asynchronous
Describe the monosynaptic stretch reflex?
Monosynaptic Reflex
• Best known is the patellar tendon
tap (knee-jerk reflex)
• Reflex arc involves only the primary
afferent neuron, a synapse, and the
efferent neuron
• Tap causes muscle stretch detected
by muscle spindle and conveyed to
spinal cord
• Results in contraction of the muscle
that was stretched and inhibition of
the antagonist muscle
• Occurs in addition to sensory
information asecending the dorsal
column
• Almost all muscles can exhibit a
stretch reflex
How do the Ia afferents associate with the muscle to mediate stretch reflex? (muscle spindle)
Large diameter (Ia afferents) coiled around intrafusal fibers (either nuclear bag or nuclear chain)
• Forms ‘annulospiral’ primary endings
How do gropu II afferents play into the muscle spindle reflexes?
Group II afferents form secondary endings
• Combined
• Group Ia afferents respond to small
stretches
• Group II afferents respond to sustained
fiber stretch
Group Ia afferents respond to _______ stretches
small
rapidly adapting.
Group II afferents respond to _________ fiber stretch.
sustained. *slow adapting*
Golgi tendon organs
Golgi Tendon Organs (GTOs)
• Encapsulated afferent nerve endings
• Located at the junction of a muscle and
tendon\
• Sensitive to muscle tension, but relatively
insensitive to muscle stretch
• Protects the muscle from damage
How are GTOs part of a negative feedback loop?
- GTO afferent Ib fibers stimuluate inhibitory neurons that suppress activity in the alpha motor neurons that innervate homonymous muscle
- Can also activate antagonistic muscle
- Prevents muscle damage and maintains a steady level of muscular tone
Flexion-withdrawl reflex pathways
used to withdraw a limb from a painful stimulus
involves slowly conducting afferent axons and multiple synpatic links
leads to withdrawl of the limb
Excitation of ipsilateral flexion muscles, inhibition of ipsilateral extensor muscles.
Cross extension reflex
Happens with flexion withdrawl reflex
opposite reaction in the contralateral limb
Pain stimul leads to flexion on one side, extension on opposite.
Sources of converging input in flexion reflex pathways and how they can modify a reflex
other spinal cord neurons
upper motor neuron pathways
functional purpose is unclear
seems to be involved in modulating the responsiveness of the local circuitry
can even inhibit/suppress activity
Central pattern generators
local circuits with the spinal cord
can produce repetitive or cyclic movements without sensory feedback
ex. locomotion, rhythmic movements of the tongue, respiration, heartbeat
control the timing adn coordination of complex patterns of movement and can even adjust them to altered circumstances.
stance phase
limb in contact with ground and pushing forward (extension)
swing phase
limb is lifted and brought forward (flexion)
What happens to the pattern from the CPG with different types of locomotion?
Order of swing/stance (flexion/extension) is important
increases in speed increase the cycle frequency and decrease the cycle length.
When transitioning from different types of locomotion, pattern changes in CPG
How are multiple limbs controlled with CPG?
each limb has its own
connected to one another through local interneurons
receive input from descending sources and from sensory neurons, but are not required.
Can decerebrate and spinal transected cats still walk?
Yep. no descending input needed
can still be modified by sensory input
can be pharmacologically activated (local injection causes walking)
