neural control of contraction

where are UMNs located?

CNS

where are LMNs located?

PNS

where do UMNs originate & project to?

originate in the cerebal cortex & project to the brainstem & spinal cord

where do LMNs originate & project to?

originate in the brainstem (cranial nerves) & project to anterior gray matter of the spinal cord (anterior horn cells)

areas that produce fine motor movements have a ___________ (smaller/larger) representation in the primary motor cortex & require a ___________ (lower/higher) level of dexterity

larger representation; require a higher level of dexterity

primary motor cortex (M1) plays a large role in the generation of _________________

motor commands

where do motor cortex neurons project?

motor neurons innervating muscles

interdigitation

neurons to a single muscle are widely distributed & interspersed with neurons supplying other nearby muscles

anterior horn cells

large motor neuron located in the front grey matter of the spinal cord that sends signals to control voluntary skeletal muscles

interneurons

receive information from the brain, process it, & send signals to motor neurons or target regions to regulate body functions & maintain homeostasis

motor unit

a single alpha motor neuron & all the muscle fibers it innervates

all-or-none principle

an AP generated by a motor neuron affects all the muscle fibers it innervates

innervation ratio

number of muscle fibers innervated by a single motor neuron

innervation ratio

a single motor neuron & the amount of muscle fibers it innervates

what types of muscles have small innervation ratios?

muscles that produce fine motor movements (ie: eye, hand, mouth)

what types of muscles have large innervation ratios?

muscles that produce gross movements / force (ie: quads, gastroc)

small (slow) motor units

contain slow oxidative muscle fibers

what types of events utilize small motor units?

long endurance events (ie marathons)

medium motor units

contain fast fatigue-resistant muscle fibers

what types of events utilize medium motor units?

medium-distance running (ie 800m)

large motor units

contain fast fatiguable muscle fibers

what types of events utilize large motor units?

short distance explosive running (ie: sprinting)

characteristics of small motor units

• smaller cell bodies

• low IR

• lower twitch force

• longer time to peak force

• slower to fatigue

characteristics of large motor units

• larger cell bodies

• higher IR

• greater twitch force

• shorter time to peak force

• faster to fatigue

as contraction force increases, firing frequency ____________ (decreases/increases) & # of motor units recruited ____________ (decreases/increases)

firing frequency increases & # of motor units recruited increases

smaller cell bodies have a decreased area & a ____________ resistance (decreased/increased)

↓ area, ↑ resistance; EPSP is larger & reaches threshold sooner, resulting in an AP

larger cell bodies have a increased area & a ____________ resistance (decreased/increased)

↑ area, ↓ resistance; EPSP is smaller & takes longer to reach threshold for an AP

does it take more or less of a stimulus to reach threshold in larger neurons?

more stimulus to reach threshold in larger neurons

sequence of events for force production

• UMNs create the drive to increase force

• LMNs increase firing rate & recruit motor units (smaller → larger)

what sensation does the DCML control?

pressure & touch

what sensation does the ALS control?

pain & temperature

divergence of sensory signals

information from a single neuron spreads to multiple other neurons

example of neural divergence

touch a hot surface → pain & thermal sensory signals enter the spinal cord & diverge across multiple neural pathways → rather than causing a single muscle twitch, that single stimulus activates multiple muscle groups at once (flexing your elbow to pull away, engaging shoulder muscles, & adjusting posture to lean back)

convergence of sensory signals

sensory information from multiple neurons come together to one neuron

example of neural convergence

to stay upright & stable, the brain combines signals from three distinct sensory systems, the visual system, the vestibular system & the somatosensory/proprioceptive system

what is the main goal of neural convergence?

fine tune movement

what is the role of spinal reflexes?

to protect tissue, maintain posture & improve movement efficiency

segmental stretch reflex

a fast, local reflex confined to a single spinal cord segment that uses muscle spindles to automatically contract a stretched muscle & maintain its length

example of a segmental stretch reflex

knee tapping reflex

multi-segment reflexes

a broader reflex that uses divergent pathways to spread sensory signals across multiple spinal levels, coordinating actions across several muscle groups & joints simultaneously

example of a multi-segment reflex

touching a hot stove

long-loop reflexes

a flexible reflex that loops up through the motor cortex before returning to the spinal cord, allowing for adaptable postural control & real-time load adjustments during movement

where does the sensory input for come from tendon reflexes?

muscle spindles

sensory information from muscle spindles help facilitate muscle force _____________

stabilization

where are muscle spindles located?

embedded within the muscle & arranged parallel to the skeletal muscle fibers

• skeletal muscle = extrafusal fiber

• spindle = intrafusal fiber

muscle spindles detect _____________ & _____________

• muscle length (static)

• rate of change of muscle length (dynamic)

what do nuclear chain fibers detect? (muscle spindles)

static length

what do static nuclear bag fibers detect? (muscle spindles)

static length

what do dynamic nuclear bag fibers detect? (muscle spindles)

rate of change of muscle length

Ia afferent nerves of muscle spindles

large primary sensory nerves that receive input from all intrafusal fibers (bag & chain) to provide feedback on static length & dynamic velocity

IIa afferent nerves of muscle spindles

secondary sensory nerves that receive input from static bag & chain fibers to provide feedback strictly on static length (not velocity)

alpha motor neurons

LMNs that synapse directly onto extrafusal skeletal muscle fibers to drive muscle contraction & force generation

gamma motor neurons

motor neurons that innervate the contractile ends of intrafusal spindle fibers, adjusting spindle tension to maintain length sensitivity during active contraction

alpha-gamma coactivation

during physical activation, alpha & gamma motor neurons are activated simultaneously, which facilitates the spindle’s sensitivity to muscle length during activity

static & dynamic activity of the fusimotor system is dependent on what?

the task

GTOs

consist of strands of collagen fibers interwoven with type Ib sensory nerve fibers in serial arrangement

autogenic inhibition

too much force causes the GTO to cause an inhibitory effect

how does autogenic inhibition happen?

↑ muscle tension = GTO activation → Ib afferent signal to spinal cord → Ib afferent synapses on Ib interneuron → Ib interneuron inhibits the motor neuron, reducing tension in the same muscle

physiological basis of stretch reflex

crossed extensor reflex

occurs when a person has a painful stimulus when the foot is on the ground

• response: flexion in ipsilateral LE with extension of contralateral LE