L5 LMNs Flashcards
Lower motor neurons
Known as the final common path
- skeletal muscle contraction is initiated by them in the SC and brainstem
- located within the ventral horn of SC gray matter and motor nuclei in the brainstem
- Send axons to muscles via peripheral nerves or cranial nerves
- Influenced by location and timing of activation
- Help with reflexes, rhythmic, and stereotyped behaviors
- Receive input from UMNs
- Have Alpha and Gamma types
Sensorimotor reflexes
LMNs mediate them
automatic responses to stimuli
Rhythmic behaviors
actions that are repeated in a regular pattern, like walking or breathing
Stereotyped behaviors
actions that follow a fixed and predictable sequence, like coordinated movements involved in chewing food
Interactive systems that produce movement
- UMN
- LMN
- Cerebellum
- Basal Ganglia
Upper Motor Neurons
- Cell bodies are in the brainstem and cerebral cortex
- Axons synapse with local neurons or with LMNs
- Essential for voluntary movements and sequencing of skilled movements
- Help with planning, initiating, directing of voluntary movements
- Other purposes include speech (broca’s area), eyes/body/head in respect to sensory stimuli, expression
Cerebellum
- Detects the difference or motor error between an intended movement and what was performed
- Mediates reductions in errors
- Functions via efferent pathways to UMNs
Basal Ganglia
prevents UMNs from initiating unwanted movements and prepares motor cicuits for initiation of movements
Motor Neuron Pools
groups or clusters of motor neurons within the spinal cord that work together to control the contraction of a specific muscle or group of related muscles
groups of LMNs
remember that medial motor pools innervate proximal muscles
Alpha motor neurons
large and innervate striated muscles fibers, that generate the forces needed for posture and movement
Gamma motor neurons
innervate specialized muscle fibers, like spindles
sensory receptors
perform refining movements
Motor units
the distribution of multiple fibers to one motor neuron helps to ensure even spread of contractile forces, damage to a motor neuron won’t hugely impact a muscle
Slow motor units
contract slowly and generate small forces
rich myoglobin content, plentiful mitochondria make them resistant to fatigue
helps with sustained contraction like posture. have lower thresholds for activation
Fast Fatigable
larger alpha motor neurons innervating larger muscle fibers with fewer mitochondria
important for brief exertions requiring large forces
threshold reached only during rapid movements requiring a lot of force like jumping
Fast fatigue resistance
intermediate size
generate about twice force of a slow motor unit and are more resistant to fatigue
How can muscle force be regulated?
increasing/decreasing the number of motor units active
increasing/decreasing the firing rate
Size principle
- progressive increases in muscle tension are produced by progressively increasing the activity of axons that provide input to the LMN pool
- Smallest units are activated, and then as the input increases, FFR and then FF will be recruited
Unfused tetanus
individual twitches are still apparent
lower neural stimulation, allows muscle to partially relax
Fused tetanus
individual twitches are not longer apparent
muscle fibers are stimulated so rapidly taht there is not chance to fully relax between contractions
Not allowing for muscle relaxation
Necessary for actions like maintaining grip or posture
Temporal summation
repeated firing of APs in quick sucession, allowing muscle to partially relax, and then contract again
with each subsequent impulse, the force increases
Intrafusal fibers
make up muscle spindles
two classes –> nuclear bag and nuclear chain
each spindle contains 2-3 bag and 4-6 chain
large diameter sensory axons (Group 1a) are coiled around middle region of each intrafusal fiber
Group 2a axons form nuclear chain
Group 1a afferents
trans it rapid changes in muscle length and rate of stretch
fire more when muscle is stretched, providing info about speed and direction of movement
Group 2 afferents
respond to static changes in muscle length, contribute to proprioception
help with static fiber stretch, little “dynamic” sensitivity
Reciprocal innervation
allows for rapid contraction of the stretched muscle and relaxation of the antagonist muscle
innervation of the sensory neuron to alpha motor neuron and GABA local circuit neurons
also responsible for muscle tone because the muscles are always under some kind of stretch
Muscle gain
also referred to as the level of gamma motor neuron activity
Gain of myotatic reflex
the amount of muscle force generated in response to a given stretch of the intrafusal fibers
how sensitive and responsive the reflex is to changes in muscle length
Higher gain = small changes in length result in strong reflex response
Low gain = larger changes are needed to trigger a reflex
Changes in gain
Voluntary stretching = gain is reduced to facilitate lengthening
Standing on moving bus = gain is modulated by UMN to compensate for variable changes
level of gamma motor neuron activity can be modulated independently of alpha motor neuron to allow for fine adjustments in movements
Challenging, precise, or unpredictable conditions…
tend to lead to an increase in baseline level of gamma motor neurons
Golgi tendon organs
sensory receptor important in reflexive regulation of motor unit activity
encapsulated afferent nerve endings, located at the junction of a muscle and tendon
innervated by Ib sensory axons
in series w/extrafusal muscle fibers (not parallel)
What happens when a golgi tendon organ is stretched?
- Stretch causes the sensory nerve endings entertwined with GTO to become squeezed
- Ion channels in sensory nerve endings cause AP, which travel along 1b axon
- Signals reach the spinal cord, and synapse with inhibitory neurons (GABA interneurons).
- GABA is synapsed with alpha motor neurons, which puts a break on the muscle activity
GTO vs spindle
GTO = force
Spindle = length
Flexor withdrawal reflex
- Stimulation of nociceptive fibers
- Withdrawal of limb by excitation of flexor muscles, inhibition of extensor muscles (ipsilateral limb)
- Contralateral limb has the opposite stimulation to provide support
Central pattern generators
neural circuits found in the spinal cord or brain that are responsible for generating rhythmic and repetitive movements
allows us to perform movements without thinking
Areflexia
interruption of efferent limb of sensorimotor reflex arcs
Hypotonia
in part dependent on reflex arc
reduced muscle tone or tension
Fibrillations or fasciculations
spontaneous twitches characteristic of denervated muscle muscle fibers/motor units
Lower Motor Neuron Syndrome
Experience paralysis of affected muscles, areflexia, hypotonia, fibrillations, and atrophy
Type 1b Afferent Neurons
signal changes in muscle tension and force, helping avoid excessive force
Nuclear Bags
found within muscle spindle
nuclei are located within the central region, giving it the name of nuclear bag
stretching the muscle stretches these, causing a firing of the sensory neurons located within the bag
Contain type 1a
Nuclear Chain
found within muscle spindle
arranged in linear chain like fashion
sense changes in muscle length and transmit sensory information
contain Type 2 neurons
